ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-273-2015Global investigation of the Mg atom and ion layers using SCIAMACHY/Envisat observations between 70 and 150 km
altitude and WACCM-Mg model resultsLangowskiM. P.https://orcid.org/0000-0003-2674-5759von SavignyC.BurrowsJ. P.https://orcid.org/0000-0003-1547-8130FengW.PlaneJ. M. C.https://orcid.org/0000-0003-3648-6893MarshD. R.https://orcid.org/0000-0001-6699-494XJanchesD.SinnhuberM.https://orcid.org/0000-0002-3527-9051AikinA. C.LiebingP.Institute of Environmental Physics (IUP), University of Bremen,
GermanyInstitut für Physik, Ernst-Moritz-Arndt-Universität
Greifswald, GermanySchool of Chemistry, University of Leeds, UKNational Centre for Atmospheric Science, University of Leeds, UKNational Center for Atmospheric Research, Boulder, Colorado, USASpace Weather Lab, GSFC/NASA, USAInstitut für
Meteorologie und Klimaforschung – Atmosphärische
Spurengase
und Fernerkundung, KIT, Karlsruhe, GermanyThe Catholic University
of America, Washington D.C., USAM. Langowski (langowskim@uni-greifswald.de)12January201515127329519December201322January201429September201412November2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/15/273/2015/acp-15-273-2015.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/15/273/2015/acp-15-273-2015.pdf
Mg and Mg+ concentration fields in the upper
mesosphere/lower thermosphere (UMLT) region are retrieved from
SCIAMACHY/Envisat limb measurements of Mg and Mg+ dayglow
emissions using a 2-D tomographic retrieval approach. The time
series of monthly mean Mg and Mg+ number density and
vertical column density in different latitudinal regions are
presented. Data from the limb mesosphere–thermosphere mode of
SCIAMACHY/Envisat are used, which cover the 50 to
150 km altitude region with a vertical sampling of ≈
3.3 km and latitudes up to 82∘. The high
latitudes are not observed in the winter months, because there is no
dayglow emission during polar night. The measurements were
performed every 14 days from mid-2008 until April 2012. Mg
profiles show a peak at around 90 km altitude with a density
between 750 cm-3 and 1500 cm-3. Mg does not
show strong seasonal variation at latitudes below 40∘. For higher latitudes
the density is lower and only in the Northern Hemisphere a seasonal cycle with a summer minimum is observed.
The Mg+
peak occurs 5–15 km above the neutral Mg peak altitude.
These ions have a significant
seasonal cycle with a summer maximum in both hemispheres at mid and
high latitudes. The strongest seasonal variations of Mg+ are
observed at latitudes between 20 and 40∘ and the density at
the peak altitude ranges from 500 cm-3 to
4000 cm-3. The peak altitude of the ions shows
a latitudinal dependence with a maximum at mid latitudes that is up
to 10 km higher than the peak altitude at the equator.
The SCIAMACHY measurements are compared to other measurements and
WACCM model results. The
WACCM results show a significant seasonal variability for Mg with
a summer minimum, which is more clearly pronounced than for SCIAMACHY, and globally
a higher peak density than the SCIAMACHY results.
Although the peak density of both is not in agreement,
the vertical column density agrees well,
because SCIAMACHY and WACCM profiles have different widths.
The agreement between SCIAMACHY and WACCM
results is much better for Mg+ with both showing the same seasonality
and similar peak density. However, there are also minor differences, e.g. WACCM showing
a nearly constant altitude of the Mg+ layer's peak density for all latitudes and seasons.
Introduction
The amount of meteoric mass
deposited daily on the Earth upper atmosphere is estimated to range from 2 to
300 tons (see, e.g. Table 1 of , and
references therein, and and
).
Since the number of extraterrestrial particles encountering the
Earth's atmosphere is approximately inversely proportional, in
a log–log scale, to the particle's size, the bulk of the daily
meteoric mass flux is contributed mainly by particles in the 1–100
micrometre (in radius) size range
(). Furthermore, most of the daily mass input
originates from the sporadic meteor background, which is the
interplanetary dust forming the Zodiacal dust cloud. The orbits of the meteoroids from that source
are so evolved that they cannot be traced back to their original
parent body
(). The
constant bombardment of particles from the sporadic background
contributes far more input than meteor showers
(e.g. ), the latter being a time-limited
(days to several weeks) enhancement of particles entering the
atmosphere in parallel trajectories appearing to radiate from one
point in the sky ().
When meteoroids enter
the atmosphere at geocentric speeds between 11 to
72 kms-1, they collide with air molecules, leading to
frictional heating and deceleration. The meteoroids, consisting partly
of metals, melt at their surfaces and metals are ablated. The ablation
process depends on several factors, such as the entry velocity and angle
of the meteoroid, the boiling point of the different
constituents and the size of the meteoroid (see,
e.g. and for more details).
For the micron-size particles of interest to this work, the ablation
process occurs between 80 and 125 km altitude
(),
resulting in the deposition of metallic atoms such as sodium (Na), iron (Fe),
potassium (K) and magnesium (Mg) in the mesosphere and lower thermosphere
(MLT).
The observation technique, used in this study, measures the number density of
the metal atom and ion layers present in the MLT that are formed from the
ablated material from the meteoroids. Once ablated the meteoric metals
undergo chemical transformation (see, e.g. ). A fraction
of the ablated metals in the ablation region (80 to 105 km altitude) is already in
the ionized form. For the case of Mg+ this fraction is estimated to
be negligibly small (, Fig. 14). Mg+ is formed by
charge exchange of Mg with the main ion constituents at this altitude, i.e.
NO+ and O2+. The dominant loss process for Mg is the
reaction with O3, which leads to stable oxides, hydroxides and
carbonates as reservoir species. Mg+ is lost by reactions with
O3 but also by reactions with N2 and other trace gases
(CO2, H2O) – see, e.g. Fig. 9 in for
a schematic diagram of the most important reactions.
The metal molecules condense and form meteoric smoke particles
(e.g. ). These
meteoric smoke particles are thought to act as nucleation nuclei.
Heterogeneous nucleation of clouds is thought to play an important role in the
formation of noctilucent clouds (NLCs) (see, e.g. ) in the polar summer mesopause
region and also in the formation of polar stratospheric clouds (PSCs)
in the polar winter stratosphere (see, e.g. ; ). PSCs have an important role in the
ozone chemistry and the depletion of stratospheric ozone.
In a dynamical
equilibrium of meteoroid input and the above-mentioned loss reactions,
the neutral Mg layer is formed between 85 and 90 km altitude and the
Mg+ layer is formed slightly above the Mg layer (see,
e.g. and
).
The MLT is not readily probed by in situ instrumentation, as balloons
fly too low, and satellites have to fly higher because of the
atmospheric drag that reduces the satellite's operation time. As
a result, only rockets carrying in situ instrumentation are able to
access this region (see Sect. for references). These are, however, expensive and rare.
Metal atoms and ions have very strong absorption
coefficients and oscillator strengths (see,
e.g. ). As a result, although their density even
in the peak region is only a few thousand particles per cubic
centimetre at most (for Na, Fe, Mg, less for others), they are very strong emitters of resonance
fluorescence.
Mg and Mg+ spectral lines are observed at
wavelengths below 290 nm. This wavelength region is strongly
affected by ozone absorption in the stratosphere, so that observations
from ground are not possible. For this reason the knowledge of the Mg
and Mg+ content in the MLT is poor.
In this study the dayglow
emissions of Mg and Mg+, detected by the satellite experiment
SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric
CHartographY) on Envisat (Environmental Satellite) are used. The Mg
and Mg+ densities are retrieved by mathematical inversion
of these measurements using a radiative transfer model. Mg and
Mg+ profiles had already been retrieved from SCIAMACHY by
. As a result of these investigations
a novel optimized measurement mode for the purpose of better covering the
peak region of Mg and Mg+ has been established, and the observations are used for
this study.
The resulting new data products are compared in this study with other Mg and
Mg+ measurements, as well as with WACCM-Mg simulations. WACCM-Mg
combines different models: a Meteor Input Function model is coupled to
a meteor differential ablation model, which provides the metal input in the
MLT. The metal release into the upper atmosphere is then used as one input to
simulations using the Whole Atmosphere Community Climate Model (WACCM)
() including a new chemistry model of Mg
and Mg+ ().
WACCM is a general circulation model that incorporates interactive chemistry
for both neutral and ion species.
In Sects. and
the SCIAMACHY/Envisat satellite observations and the Mg and
Mg+ density retrieval are briefly described. In Sects.
and the results obtained for Mg and Mg+ are presented
and discussed. In Sect. these findings are
compared to other measurements. In Sect.
the WACCM-Mg model simulations of Mg and Mg+ are introduced
and the model simulations are compared with the SCIAMACHY data set.
In Sect. a possible connection between Mg and Mg+ results and NLCs is discussed.
Finally,
Sect. summarizes the results obtained in this
study.
Local time at the tangent point of SCIAMACHY limb measurements at different latitudes for
different seasons of the year.
Instrument and algorithm
SCIAMACHY, a satellite-borne grating spectrometer on Envisat, was launched by ESA
(European Space Agency) on 28 February 2002 (see,
e.g. ; ). Envisat
flies in a sun-synchronous orbit with a 10:00 a.m. descending node. The duration of one
orbit is roughly 100 min (≈ 15 orbits a day). Note that the local
time of SCIAMACHY limb observations changes with latitude. It is
≈11 a.m. at 60∘ N and 9 a.m. at 60∘ S and, as shown in Fig. , varies strongly
in the vicinity of the poles. The highest latitude covered by
SCIAMACHY limb observations of scattered light is ≈82∘.
Data from SCIAMACHY's limb MLT mode with
tangent altitudes between ≈50 and ≈150 km covered in 30 consecutive
≈3.3 km steps are used in this study. Dependent on latitude,
consecutive limb measurements have a latitudinal separation of up to
8∘, because nadir measurements are performed between two
MLT measurements. However, there is also a small latitude offset between MLT
measurements in two consecutive orbits, to improve the meridional sampling for zonal averages.
MLT measurements were performed from
mid-2008 until the loss of contact to Envisat in April 2012.
They were performed roughly every 2 weeks for 15 consecutive
orbits, which corresponds to 1 day of consecutive measurements.
This means that measurements on 84 single days were performed, which is roughly a quarter year
of single day data. This is much less than 4 years of single day data, and
there are no continuous daily observations in the MLT mode.
Apart from
a few days, where some orbits are missing in the data, all longitudes are
equally covered (sampling every ≈24∘ of longitude).
The 2-D retrieval algorithm uses all measurements of one orbit
and retrieves the metal density from each emission line of the species on
a latitude and altitude grid. For averaged data, a reference orbit for
geolocations is used, which is formed from the geolocations of the data that
are averaged. Measurements with similar latitudes and local times as the
measurements of the reference orbit are averaged. For Mg the spectral line at
285.2 nm is used. For Mg+ the spectral lines at
279.6 nm and 280.4 nm are used. A comprehensive discussion of
the algorithm was provided by , who
also present an extensive discussion of systematic and statistical errors
and, e.g. also explain why only data for altitudes above 70 km altitude are
used in this study.
An improvement compared to the analysis described by is achieved by correcting the measured spectra for the polarization response of the instrument.
This is briefly discussed in Sect. .
As the main focus of this study is on the results of the retrieval algorithm, this discussion is kept short and more details can be found in .
The first measurements at the start of the sunlit part of each
orbit (high/mid northern latitudes) are contaminated by solar stray-light, which can be seen in the
dark signal measurement at 350 km tangent altitude. We, therefore, exclude
these measurements and also adjacent measurements, for which the contamination cannot be clearly excluded.
When using the remaining measurements, some of the features similar to the ones resulting from this error source are still observed. However, these features are not dependent on
the orbit phase like the ones resulting from the stray-light contamination, but only appear at latitudes >78∘ N, and it is not clear whether these features
are real or not (see discussion of
Fig. at the end of
Sect. ).
The actual latitude grid for calculating the density has
a larger extent than the shown results. This is because the outermost latitude intervals have been cut off
to reduce edge effects, which may, however, affect the neighbouring latitudes.
In Sect. , where near-polar effects are discussed, fewer data are cut off.
Polarization correction
SCIAMACHY is sensitive to the polarization of the incoming radiation.
This was known before launch and wavelength-dependent correction parameters were measured.
The majority of backscattered radiation from the Earth's atmosphere is produced by Rayleigh scattering,
which polarizes the incoming unpolarized solar radiation.
The resonance fluorescence process of metals results in non-zero polarization of radiation even if the incoming beam
is unpolarized, depending on the change of angular momentum between the electronic states that are involved in the fluorescence process.
The polarization of the resonance fluorescence process of metals
is well described by and can also be found in
with a slightly different nomenclature in the used formulas.
The incoming solar radiation is assumed to be unpolarized, which means that the parallel and perpendicular intensity components are equal in any observation plane.
Following (Eq. (259), p. 51), the incoming radiation components parallel and perpendicular to the scattering plane are transformed as follows:
I∥outI⟂out=32E1cos2θ001+12E21111I∥inI⟂in.
The parallel component is dependent on the scattering angle θ, which
leads to non-zero polarization, if E1≠0. The parameters (E1,E2)
are (1,0) for Mg at 285.2 nm, (0.5,0.5) for Mg+ at 279.6 nm and (0,1)
for Mg+ at 280.4 nm. The coordinate system independent degree of
polarization P is P=I∥out-I⟂outI∥out+I⟂out2,
and the polarization direction is perpendicular to the scattering plane.
The response of SCIAMACHY depends on the polarization state of the radiation,
so that an accurate absolute calibration requires the effect to be corrected
for. Until recently, this was not possible because the available set of
calibration data for the polarization sensitivity which is provided with the
Level 1 data suffered from severe errors, in particular for the limb
measurement geometry, and studies for an improvement of this effect in the
calibration are still being done (see, e.g. and
).
From in-flight data it is possible to derive polarization sensitivities in
the wavelength range below 300 nm by comparing measured reflectances with
the ones calculated by a vector radiative transfer model (SCIATRAN 3.1, see,
e.g. ) and then fitting the differences to the
expected polarization. The results of this fit have been applied here
together with Eq. () and a transformation to the
SCIAMACHY polarization reference frame.
To derive the polarization sensitivities an ozone profile has to be assumed.
The error due to the unknown actual ozone distribution is estimated to be
quite large. This is why a consistency check to this approach has been
performed here. Assuming the unpolarized Mg+ line at 280.4 nm (D1
line) to yield exact results, a correction factor for the line at 279.6 nm
(D2 line) can then be determined requiring that the density retrieved
from both Mg+ lines match. Assuming that the polarization sensitivity does
not change between 280 and 285 nm wavelength, a correction factor for Mg can
be calculated knowing that the degree of polarization PD2 of the
D2 line is just half of the degree of polarization PMg of the
Mg line.
Latitudinal variation of monthly (averaged over all January,
February, etc. results from 2008–2012) and zonally averaged Mg
density profiles retrieved from the 285.2 nm line. The Mg
layer peaks at around 90 km altitude and has a FWHM of about
15 km.
In cases where the Mg+ signal is not dominated by noise, the results from
both methods agree with each other well for Mg. Similarly, the Mg+
density retrieved from both lines, using the updated polarization correction
for SCIAMACHY, is in good agreement. We, therefore, conclude that the
polarization correction for the 280–285 nm region improved the calibration
of the data considerably.
Seasonal variations of Mg
The seasonal variation of Mg and Mg+ density on a latitude and
altitude grid is investigated. Prior to averaging, measurements at night or
those having noise peaks in the investigated spectral region, caused by
highly energetic particles hitting the detector (e.g. in the Southern
Atlantic Anomaly region), are removed from the data set by filtering. The
limb spectra of up to 15 daily single orbits are zonally averaged before
running the retrieval algorithm, to improve the signal to noise ratio. To
further reduce the noise in the results, data for individual months are
averaged for the entire measurement period.
Latitudinal variation of Mg profiles averaged over all
available data for the years 2008–2012. Note that only between
45∘ N and S do measurements from all 12 months contribute
to the averages, while for high latitudes there is only summer
coverage and/or near-terminator measurements.
The retrieval algorithm calculates the density on an altitude and latitude grid
with 40 equidistant day-side latitude intervals between
82∘ S and 82∘ N and 80 altitude intervals with 1 km
thickness between 70 and 150 km. Additionally, there are
latitude intervals for the part of the orbit,
where the satellite moves northwards (see Fig. ), to overcome ambiguities of identical
latitudes but different local time at high latitudes.
The monthly averaged results for Mg are shown in Fig. .
Mg shows a peak at around 90 km with densities up to
1500 cm-3. There is a strong month-to-month variability in
the latitudinal distribution. The annual mean results for Mg are shown
in Fig. . In the annual mean, the
density at low latitudes is higher than at the high latitudes
in the peak region. At the highest southern latitude covered there
an increased density above 100 km, which, however, is most
likely explained by the fact that at this latitude the number of measurements is smaller and the statistical error thus larger.
Seasonal variation of the vertical Mg profile for different
latitudinal zones (left: Northern Hemisphere (NH), right: Southern Hemisphere SH,
top: low latitudes, middle: mid latitudes, bottom:
high latitudes). The peak altitude is at 90 km for all
latitudes with variations of ±5 km. The month-to-month
variations in peak altitude and density are bigger than any seasonal
variations.
Figure shows the seasonal variation
of the vertical profile for low, mid and high latitudes, and
Fig. shows the seasonal and latitudinal variation of the vertical column density (VCD). The month-to-month variations are large, and thus the error
on the estimate of the amplitude of seasonal variations for
mid latitudes is relatively large. The seasonal variation of the Mg
peak altitude is estimated to be less than 5 km.
The VCD of Mg varies between 0.5×109cm-2 and
3.5×109cm-2 and is around
1.75-2.5×109cm-2 for most latitudes and times. The VCD
between 40∘ N and 40∘ S is higher than for higher
latitudes, which are only covered in the hemispheric summer and show a summer
minimum, which , however, is only clearly observed in the Northern Hemisphere.
The seasonal variation is small compared to the mean of the VCD. This relatively small variation is required to retrieve the
correct mean value, because of non-linearities in the retrieval, as
comprehensively discussed and explained by .
Seasonal variation of the Mg vertical column density (VCD)
between 70 km and 150 km for different latitudinal
regions. The VCD varies between 0.5×109cm-2 and
3.5×109cm-2. No clear seasonal cycle is
observed. Variations between consecutive days with measurements (at
least 14 days difference) are even higher than the inter-monthly
variations.
Seasonal variations of Mg+
For Mg+ the same approach for data averaging is used as described for
Mg in Sect. . For Mg+ there are two spectral lines,
one at 279.6 nm and the other one at 280.4 nm. Both emission
lines yield very similar results and only the results for the
280.4 nm line are shown here. This line is not affected by polarization-related issues, but has the larger statistical error. Mg+
density is
independently retrieved from both lines. The differences in the peak
region are lower than 25%.
The density on an altitude and latitude grid for the
monthly averaged results is shown in Fig. .
The Mg+ density peaks at altitudes in the range of 95–105 km
with peak values of 500–4000 cm-3. The peak density shows
a seasonal variation with a summer maximum between
25–45∘ in both hemispheres. In summer the highest peak
altitude is roughly at 45∘ (N and S). It occurs around
105 km, which is up to 10 km higher than the lowest
peak altitude at the equator and higher than that at the high-latitude region. There
is a minimum at the equator and also a second maximum in peak altitude
in the mid latitudes in the winter hemisphere, which, however, is more
variable than the one in the summer hemisphere.
Latitudinal variation of monthly and zonally averaged
Mg+ profiles (for the years 2008–2012) retrieved from the
Mg+ line at 280.4 nm. Mg+ shows a seasonal
cycle with a summer maximum, which is especially pronounced in the
region between 25∘ and 45∘ latitude in the
summer hemisphere. Furthermore, in this region the peak altitude is
about 105 km, which is up to 10 km higher than at
the equator or at the poles.
The latitudinal variation of Mg+ is also seen in the average over all
available measurements in 2008–2012, which is shown in
Fig. . The latitudinal dependence of the peak
altitude seems to be symmetric to the equator. However, there is an asymmetry
in the peak density, with larger density for the Northern Hemisphere (NH)
than for the Southern Hemisphere (SH). However, this effect is compensated by
a broader profile in the SH, so that the differences in the VCD are smaller.
A reason for this may be Lorentz force induced transport processes together
with the very different mapping of geomagnetic coordinates to geographic
coordinates (the magnetic south pole at around 65∘S is further away
from the geographic pole than the magnetic north pole at around
82∘ N).
At the upper vertical profile edge up to 110 km the density is higher at mid latitudes than at the equator, which can be explained by the density maximum
at mid latitudes being also at higher altitudes, than at the equator.
However, above 110 km altitude this behaviour is reversed and the equatorial
density is larger than the density at mid latitudes. This may be
explained by enhanced vertical upward transport in the equatorial region,
which will be discussed in Sect. .
Original sources and geolocations of plots in Fig. .
No.LatLongDateOriginal referenceTaken from1mid NU.S.S.R.(Eur)15 June 1960230 N86 W31 October 1963330 N86 W12 April 1967430 N86 W12 April 196758 N77 E19 March 197068 N77 E19 March 197078 N77 E9 March 197088 N77 E10 March 1970930 N86 W20 November 19701040 N9 E14 December 19711138 N75 W12 August 19761251 N93 W24 February 19791367 N20 E30 November 19801467 N20 E13 August 19781538 N75 W1 July 200316±20 Nall15 October–29 November 1999
Latitudinal variation of Mg+ profiles averaged over
all available data for 2008–2012 for the Mg+ line at
280.4 nm.
Figure shows the seasonal variation
of the vertical Mg+ profile for low, mid and high latitudes, and
Fig. shows the corresponding VCD. In
each of the latitude regions, a maximum in peak altitude is found
during summer in the corresponding hemisphere.
The VCD varies between 1×109cm-2 and
6×109cm-2. A seasonal cycle with a summer
maximum is observed and the strongest variations are observed at latitudes between
20∘ and 40∘.
Figure shows the
ratio of Mg+ and Mg VCD. The ratio is
in between 0.5 and 5. The seasonal variability of the ratio follows
the one of Mg+, as Mg (beside for northern high latitudes) does not show a strong seasonal
variability.
Seasonal variation of the vertical profile of Mg+
(280.4 nm line) for different latitudinal zones (left:
NH, right: SH, top: low latitudes,
middle: mid latitudes, bottom: high latitudes), averaged over all
available observations in 2008–2012. For all these latitudinal
zones a seasonal cycle in the peak altitude with a summer maximum
and a winter minimum can be observed. The seasonal variations are
similar or larger than the month-to-month variations. The maximum
peak altitude is 5–10 km higher than the minimum peak
altitude. For mid latitudes, the peak altitude in the winter
hemisphere is still higher than the peak altitude at the equator
(see also Fig. ).
Comparison to other measurements
Mg and Mg+ cannot be observed from the ground, because the
wavelengths of the lines are below 300 nm and the emission signal is
absorbed strongly by ozone in the stratosphere. As a result, only few
measurements are available and rely on observations from sounding rockets and
satellites. The first rocket-borne ion mass spectrometer measurement of metal
ions was reported by (May 1954, White Sands) and
enhanced ion signals between 93 and 124 km were found. According to
approximately 50 flights of rocket-borne mass
spectrometers had been made until 2002, probing the region between 80 and
130 km altitude. Results of these flights for Mg+ can, e.g. be found in
, , ,
, , ,
, ,
, and .
Date, local time, latitude and reference publications of these and other
rocket flights can also be found in Table 1 of
. Figure shows
reproduced plots of vertical number density profiles of Mg+. The original
sources of the data for these plots are listed in
Table .
A disadvantage for the comparison with the new data set presented in this study is
that most of these measurements were performed during special events
having Es layers (sporadic electron layers), aurora, meteor showers, stratospheric
warmings or NLCs present. Sporadic Mg+
layers often occurred between 105–110 km and/or at around 120 km altitude. The Mg+
peak altitude in most of these measurements can be found between 90
and 95 km altitude and the full width at half maximum (FWHM)
of the layers is on the order of 5–10 km, but sometimes the FWHM is
only 1 km.
Seasonal variation of the Mg+ vertical column
density (VCD) between 70 km and 150 km and for
different latitudinal regions, retrieved from the 280.4 nm
line. A clear
seasonal cycle with a summer maximum can be observed. The summer
maximum in the NH has higher values compared to
the one in the SH. The highest variability can be
found between 25∘ and 50∘ in both
hemispheres.
When comparing in situ mass spectrometer measurements with satellite remote
sensing results, it has to be noted that the in situ measurements are
localized and limited to the direct vicinity of the rocket. In contrast,
remote sensing techniques typically cover a large volume with horizontal
distances along and perpendicular to the viewing direction of several
hundreds of kilometres. This results in smoother layers with a larger FWHM
for the remote sensing method. Still, the width of the Mg+ layers as
well as the peak density presented in this study are in good agreement with
the in situ rocket measurements. However, the strong latitudinal dependence
of the peak altitude observed in Fig.
as retrieved from SCIAMACHY
measurements is not found in the in situ rocket data.
In addition to the in situ measurements with rockets, there are also airglow
measurements available from rockets, the space shuttle and satellites.
However, to retrieve density information from this method radiative transfer
models as well as inversion techniques and computational power are needed. As
a result, slant column information rather than profiles has been retrieved
and made available in the first remote sensing studies (see, e.g. or ). In
(summer, ≈40∘ N) the region
up to 106 km altitude is scanned during a sporadic Es layer
event, and the peak altitude of the Mg+ was not observed during this
flight, i.e. it was higher than 106 km. No Mg signal above the instrumental
noise was observed in these spectra.
Seasonal variation of the Mg+ to Mg ratio for different latitudes.
The ratio varies between 0.5 and 5 and shows
a summer maximum, which is in good agreement with
.
The region above the peak
altitude from 150 km up to the F-layer and above was investigated by
, , , and
and typically shows less than 100 cm-3Mg+ ions at
150 km altitude. This is in good agreement with the profiles described in
Sect. . In some cases a higher density is observed in
the profiles retrieved from the SCIAMACHY limb observations. This is
explained by retrieval artifacts on the edge of the retrieval grid
and vertical constraints tuned for the main peak. These effects result in small
oscillations, compared to the main peak, in regions with lower
density. Since these artifacts appear at the upper edge of the
profile it also implies that there is a significant density above the
highest tangent altitude for Mg+.
In
a combined NO and Mg+ retrieval for satellite limb
measurements is shown, which is in good agreement with the results in
Sect. in terms of the Mg+ concentrations at
peak altitude and at the upper edge of the profile. However, the
Mg+ peak altitude is at 90 km. Taking into account the
coarser sampling (every 7 km), higher statistical errors and
a tangent height offset (±4 km) in , the
agreement is reasonable. Accurate tangent height determination was an issue for SCIAMACHY, too.
Offsets of a similar magnitude were initially observed
for SCIAMACHY data products (see, e.g. ).
However, this error source was minimized and the tangent height knowledge was
improved to ±200 m (). The NO band
emission, which overlaps with the Mg+ lines and the Mg line, is of
the same order of magnitude as the Mg+ lines in the study by
and even bigger than the Mg emission, which made
a NO correction necessary. The SCIAMACHY MLT data set does not show these
strong NO lines at 280 and 285 nm. NO in this region is very
sensitive to solar activity. Results on NO retrievals from the same SCIAMACHY
level 1 data set as used in this study are reported by
. Only at high latitudes in summer and winter and from
late 2011 to 2012 do the results of show NO density of
the same magnitude as the equatorial density plot for NO in
. However, we did not observe clear NO signals in the
vicinity of the Mg/Mg+ lines during this period.
Measurements of the vertical Mg+ number density profile mentioned
in the text and listed in Table . Note that
the data points are redrawn from the original figures, and not all details of
the original figures may be captured.
A time series of Mg+ vertical columns covering several years and
retrieved from SBUV nadir measurements was presented by
. These measurements were performed approximately 1
day per month, with a spectral resolution of 1.13 nm and a spectral sampling
every 0.2 nm (compare to SCIAMACHY with ≈0.22 nm resolution and
sampling every ≈0.11 nm). The results in
are in very good agreement with the results obtained in this study,
especially when comparing Figs. 10 and 11 in with
Fig. in this study.
Figs. and show redrawn
VCD time series from different sources for a quick and easy comparison of the
results of Mg+ and Mg (see the original sources for more details).
Mg was also investigated in . However, the average
VCD for these profiles, where the signal was significant, is
4×1010cm-2, which is a factor of 10 more than the VCD
in Fig. . These large discrepancies must be
investigated in the future.
Satellite measurements with long time series and daily coverage are
available from GOME and GOME-2 in nadir mode and from SCIAMACHY in
nadir and nominal limb mode. All three instruments have a similar spectral
resolution.
The VCD of Mg and Mg+ was retrieved from the GOME data set
by and .
Figures 1 and 2 in show Mg and Mg+ VCD
for 1996 and 1997 as well as the Mg+ to Mg ratio for latitude
intervals from 0–10∘ and 30–40∘ for both
hemispheres. For the low latitudes, where there is less seasonality,
the VCD for Mg+ is about 6–7×109cm-2, which
is higher than in Fig. . The Mg density
is about 3×109cm-2, which is similar to
that in Fig. . In the equatorial region the
Mg column agrees better, and a higher VCD in the nadir results for
Mg+ can be explained by the thermospheric part of Mg+
which is not part of the VCD in Fig. . At
mid latitudes Mg+ shows a strong seasonal cycle with a summer
maximum in . This seasonality is quite symmetric
for both hemispheres with higher VCD in the SH. Furthermore, the summer maximum at mid latitudes does not
exceed the VCD at low latitudes. For Mg a summer maximum is observed,
which is more pronounced in the SH. The Mg+
to Mg VCD ratios in are in good agreement with
our results shown in Fig. .
Mg+ VCD results for different latitudinal regions and from
different sources. The left column shows the NH and the right column the SH.
From top to bottom results for low, mid and high latitudes are shown. Note
that different sources originally used different time periods. The data used
are from SCIAMACHY (blue) and WACCM (green) from this work
(0–20∘ N/S, 20–60∘ N/S, 60–90∘ N/S, WACCM data
co-located to SCIAMACHY coverage), from
(Figs. 7.3 and 7.4, red), from (Fig. 1, cyan) and
(Figs. 10 and 11, magenta).
The nominal limb mode data set, covering the tangent height range from the
surface up to 92 or 105 km (only until early 2003), respectively,
combined with the nadir mode data set from SCIAMACHY from 2002 to 2007 was
investigated in and
. The peak region for the ions could not
be fully resolved with the nominal limb mode. However, qualitatively the
agreement between the results by (page
69 and 70) and the results from this study for the ions is quite good,
showing a similar seasonal cycle in the NH, which, however, shows up
to a factor 3–4 larger density and the highest density at high
latitudes, and not at mid latitudes. In the SH the seasonal cycle was not
identified well in , which agreed with
the results similar to Fig. when using the
pre-flight polarization correction (not shown), which we now found to be
wrong (see Sect. ).
At the very beginning of the Scharringhausen VCD data set in 2002
(e.g. Figs. 69 and 70 in ) slightly
higher VCD for both species are observed than for the other years. This may
come from increased NO signals near the Mg and Mg+ lines
during solar maximum. Another reason for the higher Mg+ VCD in 2002
may also be related to the change of the maximum tangent altitude from 105 to
92 km.
Mg VCD results for different latitudinal regions and from different sources.
The left column shows the NH and the right column the SH. From top to bottom
results for low, mid and high latitudes are shown. Note that different
sources originally used different time periods. The data used are from
SCIAMACHY (blue) and WACCM (green) from this work (0–20∘ N/S,
20–60∘ N/S, 60–90∘ N/S, WACCM data co-located to
SCIAMACHY coverage), from (Figs. 7.3 and
7.4, red) and from (Fig. 1, cyan).
More differences between our retrievals and the results by
can be found in the Mg data. This is
because the data product retrieved using the Mg line is more affected by the
radiative transfer improvements made in this study than the product retrieved
using the Mg+ lines. For example, there was no correction of the Ring
effect, the filling in of Fraunhofer lines by inelastic Raman scattering in
the Earth's atmosphere (), in
, which led to high density below
80 km and a density maximum at the lower edge of the retrieval
boundaries at 70 km. This inelastic scattering contribution
additionally adds a seasonal variation to the data set.
Monthly mean WACCM results for Mg averaged over 7 yr from 2005 to 2011. The model results
show a clear seasonal cycle with a winter maximum, most
pronounced at high latitudes.
Furthermore, the Mg line at 285.2 nm is much more affected
by self-absorption of the emission, which was not considered in
, so the Mg VCD was smaller than
reported here. However, most of the important findings from the further analysis
of the Scharringhausen data set are only weakly affected by these
differences and are still valid.
In summary, it can be concluded that the Mg/Mg+
results presented here are often in good agreement with previous satellite
and rocket instruments. The most striking difference to previous
measurements is the strong latitudinal dependence of the peak altitude
of Mg+ with differences of up to 10 km for different
latitudes.
Comparison of Mg and Mg+ observed by SCIAMACHY and
modelled with WACCMWACCM-Mg model
The first global 3-D model of magnesium, which is similar to other metals
(e.g. Na in , Fe in and K in
),
is used to investigate several important questions:
for example what injection rate of Mg via the ablation of interplanetary dust
particles is needed to explain the absolute Mg and Mg+ densities; how well
can Mg+ – the only atomic ion to be observed on a global scale – be
modelled; and is the coupling of long-range transport and chemistry in the
MLT region correctly captured?
Monthly mean WACCM results for Mg+ averaged over 7 yr from 2005 to 2011. The model
results show a clear seasonal cycle with a summer maximum, most
pronounced at high latitudes. There is also a second
smaller maximum at high latitudes for the winter hemisphere.
WACCM is a high-top chemical-dynamical model, which simulates the
altitudes from the Earth's surface up to 140 km
().
Here we use the specified dynamics SD-WACCM, which is nudged by the
GEOS5 meteorological data set (including temperature, specific
humidity, horizontal winds, see Suarez et al., 2008) below 60 km. This is the same
version used by , and ,
who successfully incorporated iron, sodium and potassium chemistry into WACCM.
Here we develop a global model of meteoric magnesium in the atmosphere by
combining three components: a treatment of the injection of meteoric
constituents into the atmosphere (;
; ), a description of the
neutral and ion–molecule chemistry of magnesium in the mesosphere and lower
thermosphere (; ) and
WACCM. The Mg meteoroid injection function (MIF) has a similar seasonal
variability as those of Fe and Na. The minimum of the MIF occurs in the
spring with an ablation flux of 1300 atoms cm-2s-1, and the
maximum occurs in the autumn with 2400 cm-2s-1 (both at high
latitudes). The average flux is ≈ 1850 cm-2s-1,
which sums up to 32.5 kg d-1 over the entire planet. Magnesium
constitutes 9.6 % of chondritic meteorites by mass, so the equivalent
interplanetary dust particle mass is 0.338 t d-1. This is an extremely
low number, which indicates that Mg ablates relatively inefficiently from
meteoroids.
WACCM VCD timelines for different latitudes for Mg (left) and
Mg+ (right). Mg VCD varies between
0.5×109cm-2 and
8×109cm-2 and shows a clear seasonal cycle
with a winter maximum, which is most pronounced at the poles.
Mg+ VCD varies from 1×109cm-2 to
5×109cm-2, with most VCD values between
2×109cm-2 and
3×109cm-2. A clear seasonal cycle with
a summer maximum for Mg+ is observed. SCIAMACHY
results of VCD are shown in Figs.
and .
The peak ablation height is at around 95 km. The fluxes are obtained by scaling
the Mg MIF to match the observed Mg+ column density (not to the Mg column
density, as Mg+ is better known from earlier satellite studies). This
requires dividing the Mg ablation flux by a factor of 15, relative to that of
Na (). Since the relative chondritic abundance of
Mg is 16 times that of Na, this means that the ablation fluxes of Mg and Na
are similar in the model. Similarly, when modelling the Fe layer using WACCM,
had to reduce the Fe MIF relative to Na by a factor of
4. The most likely explanation is that more extreme differential ablation is
occurring. As shown in – see Fig. 12 – the more refractory
elements such as Mg and Fe ablate less efficiently than Na from slow
meteoroids with entry speeds below 20 km s-1.
Annual means of Mg for SCIAMACHY (left) and WACCM
(right). SCIAMACHY measures smaller peak density
having a wider vertical profile than WACCM. The averaging for WACCM simulations is restricted to match
the time periods, where SCIAMACHY measurements are
available.
The MIF used in the present study was calculated from an astronomical dust model where the average
speed is around 30 km s-1 – see and for further details.
Hence, one implication of these relatively small injection rates of Mg and Fe is that a
significant fraction of meteoroids enter the atmosphere from near-prograde orbits,
with speeds close to the minimum of 12 km s-1.
Tables – list the Mg chemical reactions and
their rate coefficients added to the standard chemistry scheme in this study.
A schematic diagram of the Mg+/Mg chemistry appears as Fig. 9 in
. An important magnesium reservoir on the underside of
the Mg layer is Mg(OH)2. This is reduced back to Mg via MgOH by
reaction with H atoms (R12a). The subsequent reaction with H atoms (R12b) is
most likely faster than R12a, and so the reaction rate is not explicitly
listed in Table (). The polymerization
of MgO2H2 to form meteoric smoke is parametrized by a dimerization
rate coefficient (reaction R13 in Table ). This reaction should
be essentially at the high-pressure limit even in the upper mesosphere
because of the large number of atoms in the dimer and the large binding
energy (268 kJmol-1, calculated at the B3LYP/6-311+g(2d, p)
level of electronic structure theory using the Gaussian 09 program suite;
). The capture rate coefficient is then increased to
9×10-10cm3molecule-1s-1 to account for the
concentration of other metallic species (e.g. NaHCO3, FeOH) with
which MgO2H2 can also polymerize.
Annual means of Mg+ for SCIAMACHY (left,
280.4 nm) and WACCM (right). SCIAMACHY shows a wider vertical
profile, and a stronger latitudinal dependence of the peak
altitude. The peak density of both is in very good agreement. The averaging for WACCM simulations is restricted to match
the time periods, where SCIAMACHY measurements are
available.
The model was run for the period 2004 to 2011, when the GEOS5 analysis
data are available. To derive the modelled climatologies of
temperature, Mg and other chemical constituents, we use the model
output from 2005 to 2011.
Neutral chemistry of magnesium added into WACCM.
No.ReactionRate/cm3molecule-1s-1ReferenceR1Mg+O3→MgO+O2k1=2.3×10-10exp(-139/T)R2MgO+O→Mg+O2k2=6.2×10-10×(T/295)0.167R3MgO+O3→MgO2+O2k3=2.2×10-10exp(-548/T)R4MgO2+O→MgO+O2k4=7.9×10-11exp(T/295)0.167R5MgO+H2O+M→MgO2H2+Mk5=1.1×10-26×(T/200)-1.59R6MgO3+H2O→MgO2H2+O2k6=1.0×10-12R7MgO+O2+M→MgO3+Mk7=3.8×10-29×(T/200)-1.59R8MgO+CO2+M→MgCO3+Mk8=5.9×10-29×(T/200)-0.86R9MgCO3+O→MgO2+CO2k9=6.7×10-12R10MgO2+O2+M→MgO4+Mk10=1.8×10-26×(T/200)-2.5R11MgO4+O→MgO3+O2k11=8.0×10-14R12aMgO2H2+H→MgOH+H2Ok12=1.0×10-11×exp(-600/T)R12bMgOH+H→Mg+H2Ofaster than R12asee textR13MgOH+MgOH→Mg2O2H2k13=9.0×10-10see textR14MgO3+H→MgOH+O2k14=2.0×10-12R15MgO3+O→MgO2+O2k15=1.0×10-13Comparison of WACCM model and SCIAMACHY measurements
results
Figure shows the monthly mean results for 7 yr of
WACCM simulations of Mg from 2005 to 2011 and Fig.
shows the same for Mg+. Figure shows the
VCD for both Mg and Mg+. Mg shows a clear seasonal cycle
with a winter maximum which is most pronounced at the poles. The peak
altitude is nearly constant, but small seasonal variations can be
found. For example, in February the peak altitude at the poles is roughly
85 km, while it is 5 km higher at 90 km in the
equatorial region. Mg+ shows a seasonal cycle with a summer
maximum. In addition, there is an increased Mg+ density at the
poles, even in the winter hemisphere. The Mg+ peak altitude is
close to 95 km and shows no strong variation with latitude
and time. The seasonal variations in the VCD profiles for Mg and
Mg+ are very similar to Na and Na+ profiles (Fig. 3 in
) and Fe and Fe+ in .
The geophysical behaviour of the Mg and Mg+ layers can be explained as
follows: the abundance of Mg atoms in the layer is governed by the injection
flux of Mg atoms, as well as chemical loss and transport through diffusion
and advection. The column injection flux of Mg atoms has a seasonal cycle
with an autumn maximum most pronounced at the poles where the autumn Mg
column injection rate is roughly twice as large as in the spring minimum. The
vertical distribution of the metal's injection has a maximum at around 90 km
altitude (see, e.g. , Fig. 11). However, the actual height
and shape of this injection is not that relevant, as the neutral Mg layer is
governed by an ion–molecule cycle on the top-side, and a neutral cycle on the
bottom-side, which are fast compared to vertical transport.
Equatorial annual mean vertical profile of Mg for SCIAMACHY
and WACCM. The right plot shows the profiles normalized to the
maximum of each profile, to better compare the shape of the
profiles. The smoothed profile is smoothed with a triangular
function with a base width of 4 km. The VCD between 70 and
140 km is 2.3×109cm-2 for SCIAMACHY and
2.5×109cm-2 for WACCM. Note that the statistical error for SCIAMACHY estimated by
(Fig. 21) is ≈±200 cm-3 for the low-density region above 100 km and below 80 km and ≈±400 cm-3 at 90 km.
Ion–molecule chemistry of magnesium added into WACCM.
The strongest loss processes are the charge transfer reactions of neutral Mg
with NO+ and O2+ (R28 and R29 in Table ), which occur
mainly at the top of the layer above 90 km, where the concentrations of
NO+ and O2+ are large, as well as the reaction with O3 (R1 in
Table ), which has a nearly exponentially increasing density
towards lower altitudes, which leads to the largest loss rate at the centre
and the bottom of the Mg profile. The ozone reaction is fast and several
percent of the Mg atoms are converted into MgO within a second. However, this
is countered by the reaction of MgO and O to form Mg (R2) and also by
reaction R12 on the underside of the layer, which results in a nearly
constant amount of Mg atoms in the Mg layer over the diurnal cycle, although
the density of the reaction partners, e.g. ozone, varies by a factor of 10
between day and night.
There is also the dissociative recombination of Mg-containing ions (R22–R27)
on the upper side of the layer, which produces Mg, but is less important for
the formation of the Mg layer than the reactions of O and H with neutral
reservoir species. More details on the neutral chemistry of Mg, which in
summary is complex and does not have only one dominant reservoir species, are
discussed by . The seasonal cycle is thus a complex
coupling of photochemistry involving O, H, O3 and H2O and transport
(mainly vertical, but horizontal convergence/divergence is also significant
at high latitudes). For mid and low latitudes this complex chemistry leads to
a low seasonal variation of Mg, which is observed in both SCIAMACHY and WACCM
data. Horizontal transport processes are important at high latitudes. At
winter high latitudes, which are not observed by SCIAMACHY, WACCM shows a
maximum for Mg, which is caused by the convergence of the meridional
circulation over the polar vortex. At the summer pole the transport due to
the meridional circulation reduces the Mg density.
The seasonal cycle of the Mg+ layer is more easily explained than that of the Mg layer, and is mainly driven by the availability of the
charged main atmospheric constituents NO+ and O2+ for charge exchange reactions. The Mg+ maximum thus can be found in summer,
particular at high latitudes. A summarizing schematic diagram of the neutral and ionized magnesium chemistry is shown in Fig. 9 of .
Vertical profiles of Mg+ for SCIAMACHY and WACCM
at the equator. The right plot shows the profiles normalized to
the maximum of each profile, to better compare the shape of
the profiles. The VCD between 70 and
140 km at the equator is 2.7×109cm-2 for SCIAMACHY (both 279.6 nm and 280.4 nm)
and 2.4×109cm-2 for WACCM. Note that the statistical error for SCIAMACHY estimated by (Figs. 22 and 23)
is ≈±150 cm-3 for the 279.6 nm line and ≈±250 cm-3 for the 280.4 nm line.
In contrast to the SCIAMACHY
measurements, which are made at one particular local time, daily
averaged output is used for WACCM in Figs. –. We also co-located the WACCM data set
to SCIAMACHY local time and latitudinal coverage, but found only small
differences to the non co-located daily average mean of WACCM (not shown). There
is a diurnal variation of the vertical column density in the WACCM
data, but nearly no diurnal variation for the vertical profile
shape. Figures and
show the direct comparison of
SCIAMACHY and WACCM annual means for Mg and Mg+. We used the
co-located data for both plots, since these are the only plots with
significant differences between not co-located daily means and co-located
data, for latitudes higher than 40∘.
Figures and
show the comparison for the
equatorial vertical profile only.
The peak density of Mg for WACCM in the global annual mean profile is higher
than for SCIAMACHY. However, the peak width of the SCIAMACHY profile is
larger than for WACCM. Therefore, the VCD of SCIAMACHY and WACCM is globally similar,
which shows that determining the Mg input rate so that the Mg+ VCD is in
agreement also leads to an agreement in the neutral Mg VCD. The altitude of
the maximum number density of Mg at the equator is at 90 km for both
SCIAMACHY and WACCM. In the high-latitude region, which is only covered by
SCIAMACHY in summer, however, the maximum number density altitude of WACCM is
about 2–4 km lower than for SCIAMACHY. Considering the 3.3 km vertical
sampling in the SCIAMACHY measurements the ≈3.5 km vertical
resolution of WACCM in the MLT and the altitude grid step size of 2 km for
WACCM and 1 km for SCIAMACHY, this difference is small, but nevertheless
significant.
Mg+ profiles averaged over all available data for
2008–2012 for the Mg+ line at 280.4 nm. The stray-light
contaminated measurements at the start of the sunlit part of each orbit are also
used here extending the coverage to higher northern latitudes and
even the ascending node side. High Mg+ density is
observed at the north pole. See Fig.
and Sect. (second paragraph) for an explanation of the x-axis labels.
Note that this is an older plot, which shows data with a wrong polarization correction,
which is, however, unimportant for the effect shown here.
A possible retrieval issue, which might explain the wider SCIAMACHY peak, is
a strong Ring effect influence () for the Mg spectral
line. At the lower and upper peak edge, the SCIAMACHY number density is
not zero. This may be related to the requirement in the retrievals that the
density must be positive, which is needed for stability in the retrieval.
However, the density is smaller than the single day error there (error
estimation for the Mg and Mg+ data set has been done by
, e.g. Figs. 21–23). When averaging the data, it must be
kept in mind that the error for the close to zero density region does not
become smaller, as this is a systematic error induced by initial
statistical errors, rather than simply a statistical error.
The agreement of WACCM and SCIAMACHY is better for Mg+. The WACCM
Mg+ peak and column densities are of similar magnitude as the
SCIAMACHY results and also show the same seasonal variability with a summer
maximum. There are also small differences: the global density maximum, which
is found in the summer hemisphere and which moves over time to the other
hemisphere, is not reduced in the WACCM data when passing the equatorial
region during equinox and also has a stronger extension to the higher
latitudes in summer. In addition, a second maximum appears in the winter
hemisphere at high latitudes in the WACCM data set. SCIAMACHY makes no
measurements during polar night and so this second maximum is not observed.
The WACCM results do not show the strong seasonality and latitude dependence
of the peak altitude, that can be found in the SCIAMACHY results. The peak
altitude of 95 km in WACCM is in slightly better agreement with the rocket
data than with SCIAMACHY. The differences of the vertical profile of WACCM
and SCIAMACHY are small especially at the equator (see
Fig. ).
Vertical profile of Mg density (left) and Mg density normalized
to the monthly maximum (right) at 70∘ N, a latitude observed with SCIAMACHY before, during and after the period when NLCs occur.
NLCs occur at this latitude between about the end of May and mid-August.
The lower peak edge of Mg+ is in better agreement than for Mg. The
impact of the filling-in of the Fraunhofer lines in the atmosphere by
rotational Raman scattering by air molecules is much weaker for Mg+
than for Mg. The agreement of the peak edge becomes worse with larger
distance from the peak altitude. Nevertheless, the FWHM of the peak is in
good agreement.
A part of the differences between the WACCM results and the SCIAMACHY measurements may be attributed
to transport processes of charged particles in electromagnetic fields in the lower thermosphere.
The equatorial electron distribution with transport from the equator
to higher latitudes has been observed in ionospheric soundings. It is
known as the equatorial anomaly or Appleton anomaly
(). Typically there are fewer electrons
at the equator than at 20∘ on either side of the equator due
to this transport.
Similar transport processes have also been observed for the ions and in situ
satellite measurements of the metals above 120 km, e.g. have been carried
out using the Atmospheric Explorer satellite. A discussion of this data set
can be found in .
Averaged SCIAMACHY limb MLT signal between 458 and 552 nm showing a
NLC signature at 84 km and
a background fit. The NLC radiance is quantified as the summed up differences between the signal and
the background fit in the vicinity of the NLCs signal.
The vertical upward transport of Mg+ near the equator has been
discussed, e.g. in , which is followed here.
The sun is passing the equatorial region from east to west. The ionization in
the thermosphere is strong for low solar zenith angles which leads to
denser plasmas there. This leads to a strong electric field from east to
west, which results in a strong eastwards current, the so-called equatorial
electrojet (see, e.g. ) within ±3∘ around the
geomagnetic equator.
The motion of a single charged particle with charge q can be separated into
the gyration in the magnetic field B, and the motion of the guiding
centre. For a force F, which is perpendicular to the magnetic field,
the guiding centre moves perpendicular to B and F with the
drift velocity v=1qF×BB2. Using
the Lorentz force F=qE results in the drift velocity
v=E×BB2. This drift is called E×B-Drift (see, e.g. the textbook of ).
The magnetic field along the meridians and the electric field along the
equator lead to an E×B-Drift perpendicular to both
fields into the radial direction, which lifts both electrons and ions into
higher-altitude regions and even above the F-layer. However, because of the
limited extent of the equatorial electrojet, this effect can only explain an
upward transport of Mg+ within ±3∘N geomagnetic latitude.
showed that additionally neutral meridional winds have to
be taken into account in order to explain vertical transport also at higher
latitudes up to 30∘ and this was experimentally shown, e.g. in
and .
Vertical column density of Mg+ (left), Mg (mid) and NLC radiance (right) retrieved from SCIAMACHY.
A clear reduction of Mg+ and Mg VCD is observed in the vicinity of NLCs. However, this is also observed in neighbouring regions without
NLCs and can also be explained by other processes. (Note that the NLCs retrieval includes more latitudes from the ascending node side).
The annual mean of SCIAMACHY Mg+ density (see
Fig. ) shows a higher density above 110 km at the
northernmost and southernmost latitudes due to less coverage and, therefore,
higher statistical errors. However, at high northern latitudes and at the
ascending node side, which is cut off in the shown results, high density
is retrieved when also using the stray-light contaminated measurements. This
is shown in Fig. . The results as
well as the input raw data show low density below 90 km and at different
altitudes in between the high signal region, which makes a differentiation
between a stray-light effect and a true metal emission complicated. Should
this feature be real it could be explained by the cleft ion fountain found by
, which describes the transport of charged
particles along the magnetic field lines, which strongly converge at the
pole. This transport may lift charged particles up to several Earth radii
until they become neutralized and sink down if they are heavy enough and are
not quickly ionized again.
Mg, Mg+ and noctilucent clouds
Metal chemistry plays a role in the generation of nucleation nuclei required
for the formation of NLCs. Consequently it is interesting to investigate the
change of the Mg and Mg+ layers in the vicinity of NLCs. The vertical
profile shape of Mg at around 70∘ N shown in
Fig. changes during the summer and shows a
weak uplift and thinning of the layer (especially in June and July), which, however,
is hard to qualify as significant, as Mg shows strong variations and
in addition the latitudinal edge region of the retrieval bears potential for
systematic errors, since, e.g. the latitudinal smoothing constraint used does
not work well at the edge. An uplift of the layer, may also be attributed to
transport due to the upper mesospheric meridional circulation (Note that in
Figs. and fewer data are
cut out at the high latitudes, so that more months are covered, with the
tradeoff of more edge artifacts, especially in the south).
The VCD, however, is significantly reduced at polar latitudes during the
summer and the reduction is stronger in the NH than in the SH. In contrast to
the WACCM-Mg model results Mg+ does not show the highest density in
summer at high latitudes, but at mid latitudes. NLCs, which are not considered
in WACCM-Mg, occur at polar latitudes during a 3 month period in the summer
hemisphere.
NLCs can be detected in many different ways. For example they show an
increased amplitude of backscattered radiation at the altitude of their
appearance. This is used here to quantify the amount of NLCs. A two-step
algorithm is used (for details see ). First it is
determined whether NLCs are detected at all. Afterwards the amount of NLCs is
determined only for the cases where NLCs are identified, as otherwise this
quantification produces wrong results. The quantification is illustrated in
Fig. . At a certain wavelength window (here we use 458
to 552 nm), where the signal is dominated mainly by Rayleigh scattering, the
average signal is formed for each tangent altitude of the MLT measurements.
NLCs are typically observed as peaks at around 84 km. The background spectra,
which results only from Rayleigh-scattering without NLCs, is fitted and
subtracted from the limb radiance. The remaining signal is summed up and the
resulting quantity is called NLC radiance here.
The VCD of Mg and Mg+ and the NLC radiance are shown in
Fig. . The NLC radiance is well anticorrelated with both
the Mg and Mg+ density at high latitudes. However, this does not
unambiguously imply that the lower VCD of Mg and Mg+ is dominated by Mg
being taken up by NLC particles, as other effects may play a more dominant
role. For example, the seasonal variation with a summer minimum for Mg is
even more pronounced in the WACCM-Mg (also for Na and Fe) model, which does
not consider NLCs, than in the SCIAMACHY results, and in the SCIAMACHY results
the Mg density is also reduced at regions, e.g. 50∘N in April, where
no NLCs occur. The Mg+ layer is formed at the top of the NLC particles
region and only weakly overlaps with the region where NLCs are formed.
Furthermore, Lorentz-force-induced upward transport may reduce the Mg+
density at the poles.
In summary it is interesting to note that the VCD of both Mg and Mg+ is
lower in summer during NLCs episodes but further investigations are needed to
understand the origin of the lower Mg and Mg+ density to decide whether
loss to NLCs or other processes are dominant.
Summary and conclusions
Monthly averaged global vertical and latitudinal density distributions of Mg
and Mg+ retrieved from the SCIAMACHY limb MLT measurements
from 2008 to 2012 have been shown. The MLT measurements cover the
vertical region between 70 km and 150 km and have
a good vertical resolution in the peak region.
Mg shows highly variable results and variability is mostly affected by the
measurement error and no clear seasonal cycle can be observed for Mg, apart
from the northern high latitudes where a summer minimum is observed. The peak
altitude of Mg is nearly constant at around 90 km for all latitudes and
times.
Mg+ shows a clear seasonal cycle with
a summer maximum in the peak density, most pronounced at
mid latitudes. Mg+ shows a seasonal variation of
the peak altitude with higher altitudes in the summer, as well as
a latitudinal dependence of the peak altitudes with up to 5–10 km
higher altitudes at mid latitudes compared to the equatorial peak
altitude of 95 km.
The SCIAMACHY results are reasonably consistent with
previously published rocket profiles and significantly extend the amount of
data of Mg and Mg+. The peak density values and the observed seasonality
for Mg+ are in good agreement. However, the rocket measurements do
not show the latitudinal dependence of the peak altitude, but typically show
peak altitudes at slightly below 95 km.
The SCIAMACHY results were compared with WACCM-Mg model results.
For Mg WACCM shows a clearer seasonal cycle with a winter
maximum at the poles than SCIAMACHY. The WACCM Mg peak density is
roughly a factor 2–3 bigger than the SCIAMACHY peak density. The peak
altitude and peak shape are in good agreement with SCIAMACHY, which shows
a slightly wider profile. The combination of higher peak density
for WACCM, but wider peak profile for SCIAMACHY, leads to very similar
values for the VCD.
The agreement between WACCM and SCIAMACHY is generally
better for Mg+ than for Mg. Both SCIAMACHY and WACCM Mg+ results show
a clear seasonal cycle with a summer maximum and the peak density
is in good agreement. However, similar to the limited number of the rocket measurements, WACCM
does not show a latitudinal dependence of the peak altitude. The peak
shape of Mg+ agrees well for WACCM and SCIAMACHY in the
high-density region, where the SCIAMACHY error is low.
During the arctic summer Mg and Mg+ show a density minimum at high
latitudes. This is in coincidence with the occurrence of NLCs, but our results
do not allow the conclusion that the Mg and Mg+ reduction at high
latitudes during summer is related to the occurrence of NLCs. Further
investigations involving model simulations are required to address this
question.
Acknowledgements
We wish to thank AFOSR and EOARD for the financial support
of the project (grant#FA8655-09-3012).
SCIAMACHY is jointly funded by Germany, the Netherlands and
Belgium. This work was in part supported by the University of Bremen
and Ernst-Moritz-Arndt-University of Greifswald and by the ESA MesosphEO project. SCIAMACHY data were
kindly provided by the European Space Agency (ESA). The WACCM-Mg
modelling work was supported by the UK Natural Environment Research
Council (NERC grant NE/G019487/1) and the European Research Council (project number 291332-CODITA).
The National Center for
Atmospheric Research is operated by the University Corporation for
Atmospheric Research under sponsorship of the National Science
Foundation. Edited by: E. Kyrölä
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