ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-9075-2018Production and transport mechanisms of NO in the polar upper mesosphere and lower thermosphere in observations and modelsSOFIE and WACCM NO comparisonHendrickxKoenkoen.hendrickx@misu.su.sehttps://orcid.org/0000-0003-3679-6744MegnerLindaMarshDaniel R.Smith-JohnsenChristineDepartment of Meteorology, Stockholm University, Stockholm, SwedenNational Centre for Atmospheric Research, Boulder, Colorado, USASection for Meteorology and Oceanography (MetOs), University of Oslo, Oslo, NorwayBirkeland Centre for Space Science, University of Bergen, Bergen, NorwayKoen Hendrickx (koen.hendrickx@misu.su.se)28June201818129075908915December201729January20188June201811June2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/9075/2018/acp-18-9075-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/9075/2018/acp-18-9075-2018.pdf
A reservoir of nitric oxide (NO) in the lower thermosphere efficiently cools
the atmosphere after periods of enhanced geomagnetic activity. Transport from
this reservoir to the stratosphere within the winter polar vortex allows NO
to deplete ozone levels and thereby affect the middle atmospheric heat
budget. As more climate models resolve the mesosphere and lower thermosphere
(MLT) region, the need for an improved representation of NO-related processes
increases. This work presents a detailed comparison of NO in the Antarctic
MLT region between observations made by the Solar Occultation for Ice
Experiment (SOFIE) instrument on-board the Aeronomy of Ice in the Mesosphere
(AIM) satellite and simulations performed by the Whole Atmosphere Community
Climate Model with Specified Dynamics (SD-WACCM). We investigate 8 years of
SOFIE observations, covering the period 2007–2015, and focus on the Southern
Hemisphere (SH), rather than on dynamical variability in the Northern
Hemisphere (NH) or a specific geomagnetic perturbed event. The morphology of
the simulated NO is in agreement with observations though the long-term mean
is too high and the short-term variability is too low in the thermosphere.
Number densities are more similar during winter, though the altitude of peak
NO density, which reaches between 102 and 106 km in WACCM and between 98 and
104 km in SOFIE, is most separated during winter. Using multiple linear
regression (MLR) and superposed epoch analysis (SEA) methods, we investigate
how well the NO production and transport are represented in the model. The
impact of geomagnetic activity is shown to drive NO variations in the lower
thermosphere similarly across both datasets. The dynamical transport from the
lower thermosphere into the mesosphere during polar winter is found to agree
very well with a descent rate of about 2.2 km day-1 in the 80–110 km
region in both datasets. The downward-transported NO fluxes are, however, too
low in WACCM, which is likely due to medium energy electrons (MEE) and
D-region ion chemistry that are not represented in the model.
Introduction
Nitric oxide (NO) is one of the major background constituents in the lower
thermosphere and its presence can have direct and indirect consequences to
Earth's radiation budget. NO acts as a natural thermostat in the lower
thermosphere and the cooling at 5.3 µm
infrared emission of excited NO is primarily dependent on variations in NO
number densities and kinetic temperature . During
polar winter, NOx species (NO+NO2) can
prevail for several days or weeks due to the absence of sunlight and can be
dynamically transported to mesospheric and stratospheric altitudes due to the
downward motion of the summer-to-winter general circulation
. Once in the
stratosphere, NOx catalytically destroys ozone, thereby
altering the radiation budget and atmospheric dynamics, and possibly having
an effect on surface temperatures. Observational and modelling evidence can
be found in this non-comprehensive list: ,
, ,
, ,
, ,
and .
An NO reservoir is present between 100 and 110 km altitude
and the main production
processes of NO involve the interaction of ground-state and excited nitrogen
with molecular oxygen, while destruction occurs primarily via ground-state
nitrogen, ionised molecular oxygen and solar UV radiation
. Several NO chemistry reactions are temperature
dependent and NO densities vary with solar and
geomagnetic activity. Solar radiation (soft X-rays and UV) is responsible for
dissociating the strong N2 and O2 bands, as are
subsequent photoelectrons, while at polar latitudes energetic particle
precipitation (EPP) during geomagnetic activity causes this dissociation
. EPP directly affects NO concentrations in the
upper mesosphere and lower thermosphere (MLT), while it can also indirectly affect
stratospheric NO densities via descent of aurorally produced NO
. Distinguishing between
the direct and indirect effects on NO production is difficult and the
relative contribution of each is still not determined.
NO is transported from reservoir altitudes into the mesosphere and
stratosphere with the downward residual circulation during polar winter, and
a strong 27-day periodicity in NO production and subsequent descent into the
mesosphere has been observed in Solar Occultation for Ice
Experiment (SOFIE) observations
. A similar response of NO production to
recurring geomagnetic forcing every 27 days is seen in SCIAMACHY observations
in the upper mesosphere . The downward transport is
especially prominent in connection to sudden stratospheric warmings (SSW) in
the Northern Hemisphere (NH) winter, after which stratospheric NOx is strongly
enhanced . NOx can further
also be locally produced in the stratosphere by solar proton events
, but these occur infrequently and their direct effect
on stratospheric ozone has been found to be half that of the indirect effect
. Ensuring a correct
representation of EPP effects and a dynamical pathway of NO is essential,
since otherwise the flux of NOx descending in the
stratosphere is underrepresented when compared to observations
.
investigated the ability of SD-WACCM (Whole
Atmosphere Community Climate Model with Specified Dynamics) to reproduce
stratospheric NOx levels, as compared to observations from
the Halogen Occultation Experiment, during a strong SSW and an elevated stratopause (ES) event
in the boreal winter in 2003–2004. The NOx enhancements
produced by precipitating auroral electrons were of similar magnitude to the
observations, while the descending flux of this EPP-produced
NOx, though present in WACCM, was underestimated by a factor
of 4. From temperature measurements it was found that WACCM did not properly
simulate the SSW recovery and that descent from the MLT into the stratosphere
was underestimated. Based on this finding, together with the fact that the
simulations only included auroral electrons, the authors concluded that the
too-low NOx descent is a combination of missing medium
energy electrons (MEE) and insufficient transport from the MLT. The
study shows the difficulty in disentangling
the direct and indirect EPP effect on NO, especially during disturbed NH
winters.
The EPP indirect effect during the geomagnetically quiet NH winter in
2008–2009 has been studied by to investigate how
atmospheric models handle the dynamically active conditions and the
associated NO transport. Before the sudden stratospheric warming and elevated
stratopause event that winter, NOx descent was reproduced
within 20 % of observations, while discrepancies became
apparent after the SSW. High-top models, with
upper lid above 120 km and including WACCM4, were shown to typically
underestimate upper mesospheric temperatures after the elevated stratopause
onset, which manifests itself in a too-slow downward transport and too-low
descending NOx concentrations. Discrepancies of medium-top
models (upper lid around 80 km) with observations are on average smaller but
show a large spread, which can be traced back to either the implementation of
the gravity wave drag scheme or the prescribed NOx at the
uppermost model layers as constrained from observations. Overall, the authors
concluded that atmospheric models were able to represent the EPP indirect
effect during the geomagnetic quiet and dynamically active NH winter
conditions of 2008–2009 but that improvements could be made with a better
dynamical representation of ES events. They further note that during periods
of high geomagnetic activity the EPP representation may not be as accurate
and that inclusion of MEE could be important.
Similar results were found by , in which the ability
of three global chemistry–climate models to produce stratospheric
NOy in response to energetic particle precipitation was
investigated and compared to MIPAS observations during the period 2002–2010.
Even though the particle effect is implemented differently in the studied
models, the resulting NOy in the upper mesosphere agrees
well between the three simulations. The indirect particle effect, however, is
captured rather differently in each model and the resulting
NOy flux that descends into the lower mesosphere and upper
stratosphere is dependent on the timing of the downwelling and rate of
descent.
The occurrence of polar vortex breakups during SSW events and the accompanied
reformation of the stratopause region in the northern hemispheric winter
complicates the polar vortex descent and the contribution of MEE during
geomagnetic active conditions imposes further difficulties by impacting both
the direct and indirect EPP effect on NO densities.
disentangle the (in)direct EPP effects on
Antarctic NO during a 2010 geomagnetic storm by using a continuous energy
spectrum for precipitating electrons between 60 and 120 km. They found that
during that particular event NO variability above 90 km could be up to
95 % accounted for by the direct EPP effect, while only 35 % or less
could be attributed to direct EPP below 80 km.
In this work we study the general production and transport of NO. Since SSW
events during the NH winter complicate the typical polar vortex descent and
create an extra downward draft during the recovery phase, we choose to focus
on the Antarctic MLT region, where SSW generally do not occur. We first
compare the climatological NO observations from SOFIE and simulations from
SD-WACCM in the lower thermosphere and mesosphere (Sect. ).
The physical drivers of NO are investigated in Sect. for
both model and observations using multiple linear regression (MLR). We then
investigate the winter transport of NO enhancements after geomagnetic
disturbances in Sect. and derive a polar vortex descent
rate in the MLT region, from which we determine the contribution of MEE to
the NO fluxes. The results are discussed in Sect. and in
Sect. conclusions are given.
DatasetsAIM-SOFIE
Since May 2007, the SOFIE instrument on-board the Aeronomy of Ice in the Mesosphere (AIM) satellite has performed
atmospheric profile scans 15 times a day to obtain vertical distributions of
temperature, ice water content and trace gases (NO, CO2, CH4
and O3) . NO volume mixing ratios (VMRs) are
retrieved using the 5.3 µm absorption band, with an approximate
vertical resolution of 2 km. The AIM satellite is in a retrograde,
sun-synchronous, polar orbit. Since SOFIE uses the solar occultation
technique, the local sunrise and sunset measurements in the Southern
Hemisphere (SH) and Northern Hemisphere (NH), respectively, are limited to a
latitudinal coverage from 65 to 85∘, depending on the time of year.
Due to the orbital drift of AIM (from mid-2012 onward) the latitudinal
coverage is drifting towards lower latitudes with time. The effective
latitudes covered in this study range from 83 to 50∘ S with a
semi-annual periodicity and with the more poleward latitudes taken during the
equinoxes and the more equatorward latitudes during solstices.
The NO profiles are reported from 35 to 150 km on a 200 m altitude grid and
are available on the SOFIE website (http://sofie.gats-inc.com, Mission
Data File version 1.3, last access: 20 June 2016). In this study, daily
averaged NO values in both VMR and number density are used and a further
vertical smoothing of the NO data with a 2 km low pass filter is applied. An
empirical correction to the NO VMR data is applied as described by
. To investigate long- and short-term variations at
high latitudes, all available data from 20 May 2007 to 1 February 2015 are
used. During local summer, polar mesospheric clouds (PMC) influence the
observation at the 5.3 µm band and cause higher NO concentrations
at and below PMC height. No correction is available as of this writing and we
therefore neglect NO retrievals during PMC season (from day of year 315 to
53) in our comparison to WACCM.
SD-WACCM
This study uses the NCAR Community Earth System Model with WACCM
as its atmospheric component. The model has 88
pressure levels from the ground to about 5.9×10-6 hPa. For
comparison to observations, we determine for each geopotential height H the
geometric altitude Z, following
Z=rEarthHrEarth-H,
with rEarth being the Earth radius, and interpolate it onto a fixed
altitude grid up to 140 km with 2 km vertical resolution. The horizontal
resolution is 1.9 latitude by 2.5∘ longitude and the timestep is
30 min. Output is written as the simulation runs and represents the model
value at the nearest latitude, longitude and UT of the SOFIE observation
profile. The model provides volume mixing ratios NOVMR, which
are converted into number densities using the ideal gas law equation:
NOden=PkTNOVMR,
with P and T being the respective simulated pressure and temperature and
k the Stefan–Boltzmann constant. The simulations used in this work are
performed with specified dynamics (SD-WACCM), relaxing horizontal winds and
temperatures to data from the Modern-Era Retrospective Analysis for Research
and Applications in the troposphere and
stratosphere, with a free-running atmosphere above 60 km. The simulations
follow the reference chemistry–climate model initiative (REF-C1SD) forcing
scenario from the SPARC Chemistry–Climate Model Initiative
. Solar fluxes are from the Naval Research
Laboratory (NRLSSI v.1) empirical solar model and vary daily, while the
parametrised aurora varies with the daily Kp index. The model is run with
enhanced eddy diffusion (Prandtl number of 2, Pr2) as this enhances the rate of eddy diffusion
and improves trace species concentrations in the
MLT region . A control simulation with a Prandtl number
of 4 is used as a sensitivity test. The nitric oxide empirical model (NOEM)
is used as an upper boundary condition for modelled NO concentrations
and is based on 2.5 years of observations made by
the Student Nitric Oxide Explorer (SNOE) satellite during the inclining phase
of solar cycle 23 .
Seasonal climatology of Antarctic NO number density in
SOFIE (a) and WACCM (b). Data are smoothed with a 3-month
running average. Hashed areas represent the Antarctic PMC season, during which SOFIE data should not be compared to the
WACCM climatology (see more information in Sect. ).
Seasonal climatology of Antarctic NO volume mixing ratio, similar to
Fig. . The white contour line represents the
climatological mesopause altitude. Hashed areas represent the Antarctic
PMC season, during which SOFIE data should not be
compared to the WACCM climatology (see more information in
Sect. ).
Results
This section is divided into three parts, starting with similarities and
differences in MLT NO between SOFIE and WACCM. In Sect. the
relative importance of the physical drivers of NO is investigated while in
Sect. the dynamical aspect of EPP-produced NO is compared.
NO in the mesosphere–lower thermosphere
A seasonal climatology of the Antarctic NO in number density and volume
mixing ratio is shown in Figs. and
, respectively, for both SOFIE and WACCM data. The
observing latitude is closer to the polar regions during winter and summer
observations, as described in Sect. . In
Fig. the total number density of SOFIE observations
show the NO reservoir to be at approximately 100 km, with changes throughout
the year in the altitude of the maximum density. Typical polar vortex descent
can be seen in the Antarctic winter from March through September. The
enhanced NO densities around 85 km during summer are an artefact in the data
product due to enhanced radiation in the observed NO band in the presence of
noctilucent clouds. It is clear that WACCM simulates the NO reservoir at a
higher altitude and with an overall higher column density in the lower
thermosphere. Below the mesopause region, a strong seasonal cycle is present
and WACCM tends to underestimate the NO number densities, particularly during
winter, as compared to SOFIE. Figure shows a similar
climatology in NO VMR with a 6-order-of-magnitude change in the considered
altitude range. The climatological mesopause altitude in each dataset is also
shown with a white contour line Fig. . It varies between
86 and 98 km in SOFIE and between 78 and 100 km in WACCM data, while the SOFIE
mesopause is typically 4 km lower during winter and 4 km higher during
summer than the WACCM mesopause. During summer and winter the WACCM mesopause
is up to 10 K colder than SOFIE, while being warmer during the equinoxes
(not shown).
Altitude of the maximum NO number density obtained from the SOFIE
and WACCM seasonal climatologies in Fig. .
(a) Mean column density of NO in the lower thermosphere
region from 90 to 140 km. (b) Mean column density in the 10 km bin
centred around the altitude of maximum NO.
Figure shows in more detail how the altitude of the NO
maximum changes throughout the year. For SOFIE data the NO maximum ranges in
altitude from 100–102 km in summer and early winter to 96–100 km
during midwinter. At the end of winter and in early spring, the mesospheric
overturning circulating winds change direction and the altitude of the NO
maximum layer increases up to 104 km before restoring to around
100–102 km. This altitude is lower than the commonly accepted peak
altitudes of 105–110 km (see e.g. ) but is in agreement with NO
observations from for example the sounding rocket project ECOMA
, ACE-FTS satellite observations
, and the OSIRIS and SMR instruments on-board the Odin
satellite . During Antarctic summer, WACCM
simulates the peak density at similar altitude levels to SOFIE. However,
during winter the NO maximum is at an altitude of 104 km, down from 106 km,
where the NO peak densities are found during the equinoxes. NO descent during
spring to winter bridges about 4 km in altitude in SOFIE and 2 km in WACCM.
It can also be seen from Fig. that the WACCM total
density in the thermosphere is higher around the equinoxes in March and
September than during summer or winter. Equinoctial geomagnetic activity
maxima have long been recognised to occur and could be a possible reason for the NO enhancements in
WACCM during these periods. Therefore, the discrepancy of equinoctial NO
between SOFIE and WACCM in the lower thermosphere could be an indication that
the model is too sensitive to changes in geomagnetic activity.
Seasonal variability of the lower thermospheric NO number density
profile for SOFIE (diamonds) and WACCM (stars). Each season represents a
multi-year mean of a 90-day period centred on the solstice or equinox. The
September equinox and December solstice correspond to Antarctic spring and
summer, respectively, while the March equinox and June solstice correspond to
the Antarctic autumn and winter season, respectively.
Inter-annual variability of the mean SH winter profile for
SOFIE (a) and WACCM (b). A multi-year mean winter profile
for SOFIE (black diamonds) and WACCM (grey stars) is given in each
subfigure.
A key aspect of understanding differences between model and observations is
how much NO is present in the lower thermosphere throughout the year.
Figure therefore shows the mean NO density between 90 and
140 km altitude. WACCM NO densities are on average 1.6 times higher than in
SOFIE, whereas in summer they are twice as high. During winter the difference
becomes smaller (a factor of 1.2). Another approach to investigate the lower
thermosphere NO densities is to compare the mean density around the NO
maximum. The peak NO density in WACCM is situated between 102 and 106 km
while in SOFIE it is between 96 and 104 km altitude. By comparing the NO
average over a 10 km region centred around the altitude of peak NO one
minimises differences introduced by, for example, atmospheric dynamics. The
right-hand panel in Fig. shows the evolution of this
climatological 10 km average. One can see that WACCM still has more NO – on
average 1.4 times as much as SOFIE – ranging from similar winter values to
1.8 times the summer values. It should also be
noted that, apart from the higher NO column densities, the seasonal variation
within each dataset is different: in SOFIE observations winter values are 3.5
times larger than summer values, while the winter / summer ratio is a
factor of 2 in WACCM. Seasonal variability of the NO profiles are highlighted
in Fig. and reveal that above 100 km WACCM produces
too-high NO concentrations in the climatological mean.
Since we are interested in NO densities during the dynamical coupling of the
MLT region, we conclude this section by showing winter year-to-year
variability of NO profiles in Fig. , which highlights
structural differences between the observations and model. The winter is here
defined as a 90-day period centred at the June solstice. A large year-to-year
variation is present in the observations with NO values during winter 2013
being 3 times larger than during winter 2009. This in contrast to the
model data in which significantly less variation is found from year to year
with a maximum difference of about a factor of 1.25. In years with low
geomagnetic activity, NO concentrations are considerably overestimated by
WACCM, while they are underestimated in years with high geomagnetic activity.
The inter-annual variability of winter NO concentrations thus follows the
level of geomagnetic activity more closely in SOFIE data than in WACCM, with
an overall too-high background of WACCM NO in the lower thermosphere.
We have so far thus found that WACCM simulates higher NO values at higher
altitudes in the lower thermosphere and with less yearly and seasonal
variations when compared to SOFIE observations. Plausible reasons for the
obtained differences are as follows: a too-small NO flux transported downward during
the Antarctic winter, an incorrect meridional gradient of NO revealed by a
seasonal shift of the observing latitudes, too much NO production and/or too
little NO destruction in the lower thermosphere. The excess summertime NO as
compared to SOFIE indicates that the production or destruction mechanisms of
NO in WACCM may not be entirely correct. In the next section we will first
investigate the drivers of NO variability and how well they agree between
model and observation, while in Sect. we will investigate
the dynamical picture of winter NO.
Percentage of the total variance in SOFIE (black) and WACCM (grey)
data that can be explained by the seasonal climatology (dashed lines). Full
lines represent the combined explained variance of the seasonal climatology
and MLR model.
Results of the MLR performed on SOFIE (diamonds) and WACCM (stars)
data throughout the lower thermosphere. (a) Estimates for the
coefficients of geomagnetic activity (blue) and solar radiation (green),
which can directly be compared to each other in terms of magnitude.
(b) Total variation explained by the model for SOFIE (black) and
WACCM (grey).
Physical drivers of NO
As described in the introduction, solar radiation (soft X-rays and UV
irradiance) and photoelectrons ionise and dissociate the main constituents
present in the lower thermosphere (O, O2, N2), creating the
elements for NO chemistry to take place. At polar latitudes precipitating
energetic particles have a similar effect. The multiple linear regression
method can be used to determine coefficients for solar and geomagnetic
variability, which are related to NO concentrations
. The relative importance
and contribution of each physical driver to the NO budget in the lower
thermosphere can be determined in a similar MLR approach.
obtained consistent results between NO
observations from SOFIE and SNOE even though observations were separated by
nearly a decade in time and the former instrument uses solar occultation
while the latter uses UV spectrometry. A similar analysis performed on SOFIE
and WACCM data can show whether the correct processes drive NO densities at
high latitudes in the model. Since the seasonal NO climatology represents a
mode of variation that we do not seek to explain, we deseasonalise the
datasets by subtracting the seasonal climatology and focus on the direct
production and destruction mechanisms. Figure
reveals that between 70 and 85 % of the NO budget can be explained by the
climatology shown in Fig. and that throughout the lower
thermosphere the WACCM climatology can explain a larger portion of the NO
density than the SOFIE climatology. This is a result of the low year-to-year
variability in the model. The remaining variations in the NO anomalies are
then driven by variability in geomagnetic activity and solar irradiance upon
which they are regressed:
ΔNO(z,AE,Lyα,t)=γAE(z)AE(t)+γLyα(z)Lyα(t)+ϵ(z,t),
where γAE and γLyα are the
estimated coefficients of the corresponding geomagnetic auroral electrojet
(AE) index and solar Lyman-α(Lyα)
irradiance regressors, ϵ is the residual error term and ΔNO
denotes the anomaly of NO from its climatological value. More information can be found in
.
The MLR output combined with the climatological contribution results in a
total explained NO variance larger than 90 % for both SOFIE and WACCM
(see Fig. ). The altitudinal profile of the MLR
estimated coefficients is shown in Fig. . Geomagnetic
activity impacts the NO variations in a similar way in both datasets with the
highest contribution above 110 km. The parametrised auroral input in WACCM
deposits most of the energy above 100 km and the larger difference between
the SOFIE and WACCM geomagnetic impact below 105 km is therefore likely due
to missing medium energy electrons. The estimated γAE
coefficient in WACCM shows a similar shape to the estimated coefficient in
SOFIE but is slightly smaller in value below 120 km, which can explain the
lower year-to-year variability in WACCM NO that was seen in
Fig. . Throughout the lower thermosphere a
small-to-negligible impact of solar irradiance is to be expected at high
latitudes as solar soft X-rays and EUV (extreme ultraviolet
radiation) are
most important for NO production at equatorial latitudes. Variations in polar
NO attributed to solar irradiance in SOFIE observations are small and
consistent with a value of zero below 115 km and become slightly negative above that altitude.
The effect of irradiance in WACCM data seems to be more pronounced at high
altitudes and differs significantly from the SOFIE irradiance impact,
suggesting that solar forcing due to soft X-rays or UV photolysis has a
stronger effect on WACCM NO than on what is observed.
To investigate the effect of solar irradiance further, one can rewrite
Eq. () to
NOmodel=NOclim+ΔNO=NOclim+ΔNO‾+γLyασΔNOσLyαLyα-Lyα‾+γAEσΔNOσAEAE-AE‾,
with ΔNO‾ and σΔNO being the
mean and standard deviation of NO variations to scale to zero mean and unit
variance (similar for AE and Lyα), and with
NOclim being the seasonal climatology. The sign of the
estimated coefficient needs to be considered together with the time evolution
of the regressor, as the AE and Lyα variations can be both
positive and negative. The contribution of radiation to the NO density can
thus be identified as the third term in Eq. () and is shown in
Fig. . At lower altitudes where γLyα>0 and when solar activity is below average (solar
minimum conditions) the contribution to NO will be negative. Above-average
solar activity (solar maximum) will contribute to more NO. At higher
altitudes γLyα is negative and the opposite is true:
during solar minimum years the effect of radiation is to enhance NO
concentrations, while at solar maximum years a lowering effect is seen. A
positive sign of the estimated coefficient does not necessarily
mean production at that altitude since the whole term needs to be considered:
in a time period when Lyα is below average it either means
destruction or less production than normally.
The contribution of Lyα radiation, as given by
Eq. (), to the NO budget for SOFIE (black) and WACCM (grey) at
100 km (dashed) and 130 km (solid) altitude. The solar contribution of NOEM
at 130 km is shown in red for comparison to WACCM, is offset by a factor of
5×106 cm-3 and is based on the solar F10.7 radio index.
The NO contribution due to solar radiation clearly has a larger effect on
WACCM NO than on SOFIE NO at 130 km. The impact, however, seems to be
dependent on the phase of the 11-year solar cycle. To test this assumption an
MLR is performed with the Lyα regressor replaced by its
third-order polynomial fit, without small day-to-day variations. A similar
profile of the estimated coefficient γLyα was
obtained throughout the lower thermosphere. This implies that it is not the
shorter-term smaller variations in Lyα that are causing the
NO variations, but rather the variations on long timescales, similar to the
11-year solar cycle. It could also imply that the high latitude NO densities
are not varying with irradiance changes, but rather with a process in the
lower thermosphere that follows the 11-year solar cycle, such as
temperature . This was also suggested by
to explain a negative contribution of solar
variability at high latitudes.
Figure also shows the NO contribution due to solar
radiation at 130 km in NOEM. This NOEM output is on similar magnetic
latitudes to SOFIE observations and is offset by a factor 5×106 cm-3 because it acts on a different climatological background
than the MLR. The solar-induced NO in NOEM behaves very similar to that in
WACCM, even though the radiation component in the MLR is linear with
Lyα and logarithmic with F10.7 in NOEM and shows the same
long-term trend. Because NOEM is used as an upper boundary condition for NO
at the WACCM model top, discrepancies between WACCM and SOFIE at this
altitude are likely caused by differences between NOEM and SOFIE. At 100 km,
the solar contributions to NO in WACCM and SOFIE agree very well, which
implies that the chemistry in WACCM reacts similarly to UV variability as in
the observations. The contribution of radiation in NOEM at 100 km is of
opposite sign (not shown) because the associated EOF (empirical orthogonal function) is negative
. This implies that, since SOFIE and WACCM show a
similar variation, NOEM did not properly capture the radiation impact at
these lower altitudes from the shorter SNOE dataset.
Dynamical transport of NO
In winter, dissipating gravity waves cause turbulent mixing and an effective
transport of air from the lower thermosphere into the mesosphere, thereby
creating a pathway for NO to descend from the thermospheric reservoir down
into the middle atmosphere where it can destroy ozone. Periods of perturbed
geomagnetic activity will create enhanced NO densities, which are transported
down into the polar vortex. Following , we
perform a superposed epoch analysis (SEA) on SD-WACCM winter data to compare
the model and observational response of NO after increased geomagnetic
activity.
A SEA was performed on dates on which geomagnetic activity, as represented by
the AE index, showed increases that were larger than 2 standard deviations of
the dataset. The dates are given in Table and correspond to
a doubling of normal geomagnetic activity. The resulting NO responses are
enhancements from a running monthly mean and reveal the 27-day periodicity of
NO production, shown in Fig. . On the central epoch date,
SOFIE observes NO increases up to 80 % while increases reached in
SD-WACCM are much smaller, up to 35 %. Similarly, SOFIE NO enhancements
are larger for the recurring dates 27 days earlier and later.
Epoch analysis performed every 2 km on winter hemispheric data in
SOFIE (b) and WACCM (c). Dates are selected when the AE
variation exceeds 2σ, resulting in 17 events. (a) Blue and red
lines represent the mean and standard errors of the AE variations, while full
and dashed green lines represent 1σ and 2σ significance levels.
(b, c) NO number density enhancements with the white contour line
and the grey background representing a 1σ significance level and
non-significant or negative NO variations, respectively.
To study the rate of downward transport we identify at which altitude the
maximum NO enhancement is situated. Figure reveals that
the NO increase starts at 105 km in SOFIE and 112 km in WACCM and that
progressively with time WACCM almost consistently places the NO enhancements
5 km higher than SOFIE. The descent rate of the NO peak enhancements is thus
about 2.2 km day-1 in both datasets. An epoch analysis on the WACCM
control run with standard diffusion (WACCM Pr4) shows that the NO
enhancements descend with a rate of about 2.1 km day-1. The increases
in absolute densities are shown in the right-hand side of
Fig. and indicate that the maximum enhancements are lagged
by 2 days from the geomagnetic onset and that SOFIE observes double the
increase as compared to WACCM. Maximum values exponentially decrease with
time and the difference between SOFIE and WACCM becomes progressively larger
lower in the atmosphere. After 13 days the difference reaches a factor of 4
with the enhanced diffusion run and a factor of 9 with the standard diffusion
run. Even though enhanced diffusion decreases the differences between SOFIE
and WACCM in descending NO fluxes, a factor of 4 difference remains, despite
the similar inferred rate of descent. This implies either missing NO
production, too much NO destruction or horizontal diffusion in the model. A
possible source of NO that is not included in the current model is ionisation
by medium energy electrons and D-region ion chemistry
.
Selected dates during Antarctic winter on which the AE index
increased more than 2 standard deviations.
YearDay – month200815 Jun, 13 Jul, 23 Jul, 10 Aug, 18 Aug20097 May, 22 Jul, 30 Aug20102 May, 29 May, 30 Jun, 4 Aug, 24 Aug201128 May20131 May, 14 Jul201427 Aug
(a) Altitude of the maximum NO enhancement after the onset
of geomagnetic activity for SOFIE and WACCM with enhanced diffusion (WACCM
Pr2, obtained from Fig. ) and for a control run with standard
eddy diffusion (WACCM Pr4). The slope of a linear regression fit (dashed
lines) represents the MLT descent rate. (b) The maximum NO
enhancement at each corresponding day after the epoch, and at the
corresponding altitude as shown in the left panel, that is transported
downward (also obtained from Fig. ). An exponentially
decreasing fit (dashed black and grey lines) is performed onward from day 2,
when the largest NO enhancement is reached. The ratio between SOFIE and WACCM
NO enhancements (fit) is shown by the full (dashed) red line.
Percentage of NO for each day after the epoch that remains as
calculated from the NO concentration at 100 km altitude for days
with (a) strong and (c) medium geomagnetic activity.
(b) Difference of NO percentages between SOFIE and WACCM for each
altitude.
Another way to study how much NO is being transported downward is to
calculate the percentage that remains from a specific altitude level. Because
WACCM places the NO enhancements 5 km higher than SOFIE and dynamics are
different at different altitudes, we study the percent NO that remains once
the enhancements passed the 100 km altitude level. Density enhancements in
SOFIE NO pass this level at day 2.4 and on day 4 72 % of the NO
enhancement at 100 km remains as can be seen in
Fig. . For WACCM the enhancements reach the 100 km
level at day 4.5, and on day 6 only 67 % remains. At about 97 km
altitude there is therefore an NO deficit of around 5 %. Extending this
process to lower altitudes gives an indication of how this deficit varies
throughout the upper mesosphere. The middle panel of
Fig. shows the inferred difference between SOFIE
and WACCM for every kilometre between 80 and 100 km, revealing that the
deficit ranges between 2 and 9 % and maximises around 90 km. This is an
indication that a process is missing in the model, which can produce
differences up to 9 % with the observations in the NO descent. Altering
the arbitrary altitude of 100 km up or down does not change the range of
deficit percentages nor the level where it maximises.
A production mechanism of NO that is not included in this version of WACCM is
MEE. The selected events for the SEA occurred during strong geomagnetic
activity and can therefore be considered to include MEE. A similar SEA is
performed on 66 dates where geomagnetic activity was enhanced but not to its
most active levels (variations between 1σ and 2σ), ensuring NO
production but minimising MEE. The descent rate of the maximum NO
enhancements is similar for SOFIE (2.1 km day-1) and WACCM
(2.3 km day-1) in the 80–110 km altitude region (not shown). The
time evolution of the NO percentage after it passed the 100 km altitude
level is shown in the right panel of Fig. . The
inferred difference between SOFIE and WACCM for the medium storms is also
shown in the middle panel of Fig. and reveals that
the deficit now reaches up to 5 %. This implies that the EPP indirect
effect on NO can have a contribution of 4 % of direct NO production by
MEE. Because the epoch analysis was performed on dates with moderate
geomagnetic activity, the occurrence of MEE was minimised but not excluded:
the MEE contribution we determined is therefore an effective lower limit. The
remaining difference could be related to non-excluded MEE or D-region ion
chemistry.
Discussion
The simulated Antarctic NO densities in WACCM display the general features of
NO in the mesosphere and lower thermosphere as observed by SOFIE. However,
there are several differences. WACCM produces higher NO average
concentrations throughout the lower thermosphere, with a lower year-to-year
variability and higher altitude of peak NO density.
The results of the MLR indicated that NO variations are determined by
geomagnetic activity and solar radiation. The impact of solar radiation,
however, seems to be dependent on the phase of the 11-year solar cycle and it
affects WACCM NO more strongly than is observed by SOFIE. Since the
variations in NO as observed by SOFIE and SNOE behave in a consistent way
, the result shown in Fig.
indicates that the UV/EUV radiation, as represented by the Lyα
regressor, appears to have a stronger impact in WACCM NO than in the
observations. As argued above, this could be related to temperature changes.
WACCM uses the NO concentrations from NOEM as an upper boundary condition
. NOEM is a model which is based on 2.5 years of
SNOE measurements taken during the ascending phase of solar cycle 23 and is
able to reproduce about 50 % of the variance of all SNOE observations
. Climatological NO densities simulated by NOEM
and WACCM were compared (not shown), and it was found that both models vary
very similarly in concentration, altitude of NO peak, thermospheric NO
profile and year-to-year variation. Because the contribution of solar
radiation to the NO budget at 130 km behaves in a similar way in NOEM and
WACCM, it implies that WACCM at its upper altitudes is strongly constrained
by NOEM and that differences between WACCM and SOFIE at these altitudes are
likely caused by differences between NOEM and SOFIE.
Throughout the lower thermosphere and during all seasons, higher NO
concentrations are present in WACCM. NO concentrations are very sensitive to
the branching ratio of excited and ground-state nitrogen
P(N(2D)/N(4S)) during N2 dissociation .
WACCM has a constant branching ratio of 0.60, which means that 60 % of
atomic nitrogen is produced in the excited state .
As N(2D) is the primary source and N(4S) the primary loss of
NO, one possibility of the higher WACCM NO concentrations is that a too-high branching ratio
results in more NO production and less destruction.
Determining rates and branching ratios in several reactions of the NO
chemistry is challenging and large uncertainties remain: some studies, for
example, have suggested a ratio of 0.5 ,
while recent research advises an altitude-dependent ratio ranging from 0.50
at 90 km to 0.60 at 150 km . A second
possibility that further could alter the sensitivity of the NO chemistry to
solar radiation is the temperature in the lower thermosphere, which impacts
temperature-dependent reactions. Furthermore, simulating correct atomic
oxygen concentrations in the lower thermosphere is also of importance. Yet
another possible solution may be related to outdated values of reaction rates
or missing reactions (see , for a recent
update). A detailed analysis of which reactions could be updated is outside
the scope of this study but would be valuable future work to make
improvements in NO modelling.
The general features of the thermospheric response during the 5 April 2010
geomagnetic storm were rather accurately simulated by the coupled
ionosphere–thermospheric TIEGCM model, although differences with observations
remained in, for example, the NO cooling rate .
The authors found that the differences in NO cooling power between TIEGCM and
TIMED/SABER observations were improved by obtaining larger NO number
densities, which they accomplished via a new temperature-dependent reaction
rate for the N(2D)+O2→NO+O reaction. An excess of
thermospheric NO as compared to satellite observations is present in WACCM,
as found in this study. Given that the TIEGCM and WACCM models share a
similar implementation of the thermosphere, it is likely that TIEGCM also has
an NO excess. In that case an increase in NO densities would appear not to be
a solution to improve NO cooling rates.
Another key aspect is the NO descent in the MLT region during polar winter,
since the NOx flux that is transported into the lower
mesosphere and stratosphere is important for catalytic ozone destruction and
atmospheric dynamics. Differences in atmospheric dynamics and the size or
location of the polar vortex between observations and simulations could
introduce additional variation in the SOFIE–WACCM comparison. However, a
SEA performed on geomagnetic active dates revealed that NO enhancements
decrease in altitude with the same descent rate (about 2.2 km day-1)
in the 80 to 110 km altitude region in WACCM and SOFIE. The MLT descent in
the SH therefore does not seem to suffer from dynamical disturbances, as it
does in the NH. Eddy diffusion is the driving force of downward transport of
trace species and is enhanced in this version of WACCM by halving the Prandtl
number to 2. In previous versions, WACCM used a Prandtl number of 4 and
halving it was shown to improve the comparison of MLT region CO and CO2
between model and satellite observations . A control
run with a Prandtl number of 4 confirms that the descent rate is slightly lower
(2.1 km day-1) and that the descending NO flux is considerably less
(about half) after 2 weeks.
However, even though the rate of descent of the NO enhancements is the same,
the absolute increases in WACCM and SOFIE are different. The MLR shows that
the impact of geomagnetic activity on NO variations is similar in both
datasets, while the NO enhancements obtained after the SEA show a larger
increase in the observations. This is interesting and may seem contradicting
at first. The SEA shows the direct impact of geomagnetic activity and reveals
the NO response after 17 strong AE events. The MLR on the other hand
highlights the impact of drivers on a daily basis and therefore gives a
relatively high weight to the more commonly occurring small variations. The
different NO response is therefore most likely related to the intensity of
the geomagnetic events and could perhaps be linked to a non-linear response
to auroral input .
In the light of the HEPPA-II intercomparison project,
performed an evaluation of the dynamically active NH
winter of 2008–2009 as observed by seven satellites and simulated by eight
atmospheric models. The authors concluded that the EPP indirect effect was
adequately described in the models and that inclusion of MEE in one of the
models (HAMMONIA) did not introduce noticeable differences. However, it was
noted that geomagnetic activity during the studied period was very low and
that MEE could still be important during more perturbed periods. The SEA we
have performed was done on dates with strong geomagnetic activity, which is
representative of a doubling of normal activity. The AE index used here is,
however, only a proxy for particle precipitation and as such does not tell us
for certain whether MEE were present during these days. A similar epoch
analysis, performed on dates with only slightly enhanced geomagnetic
activity, is used to provide a lower limit of the MEE contribution to the
descending NO flux. Our analysis revealed that MEE can account for at least
4 % of the difference between descending NO levels.
Finally, one major aspect of the NO reservoir could play a key role in the NO
winter descent: the altitude of the NO maximum density. This layer in WACCM
is placed at a higher altitude throughout almost the entire year, with a
6 km difference as compared to SOFIE during winter. Auroral electron
precipitation in WACCM has a characteristic energy of 2 keV, corresponding
to a maximum energy deposition at an altitude of 110 km, but increasing this
characteristic energy does not sufficiently lower the NO peak layer
.
Conclusions
We investigated the ability of WACCM to simulate Antarctic NO concentrations
in the MLT region and compared the results to SOFIE observations. The general
features of the NO seasonal climatology are well captured by WACCM, though
differences remain. Above the mesopause region, the modelled NO is almost a
factor of 2 higher in concentration and shows less seasonal and inter-annual
variability than observations. The NO maximum in WACCM is up to 6 km higher
in altitude than in SOFIE. Using an MLR we have shown that a seasonal
climatology and the NO variations from that climatology can explain more than
90 % of the variance in both datasets. The variations in NO are driven
mainly by geomagnetic activity at high latitudes and the altitudinal profile
of the geomagnetic driver is similar in WACCM and SOFIE. On the other hand,
the impact of solar irradiance on NO, which is expected to be small at the
polar regions, appears to be too large at high altitudes in WACCM and is
linked to the use of NOEM as upper boundary condition.
While the day-to-day geomagnetic activity drives NO variations in a similar
way in WACCM and SOFIE, there are differences in the direct impact on
absolute NO densities during strong geomagnetic disturbances. The maximum
produced NO was found to be consistently placed 5 km higher in WACCM than in
SOFIE. During winter these NO enhancements descend with a remarkably
consistent rate of about 2.2 km day-1 in the 80–110 km altitude
region in both datasets, indicating that dynamical transport in the SH is
accurately described in WACCM. The impact on the descending NO flux, however,
is about twice as large in SOFIE and becomes progressively larger, up to a
factor of 4, lower in the MLT region, which indicates a missing NO production
process. We suggest three, possibly connected, mechanisms for the lower NO
fluxes descending into the mesosphere: a too simplified parametrisation of
D-region ion chemistry that can produce NO, excluded precipitation of medium
energy electrons that directly produce NO and a too-high altitude of the NO
reservoir.
The AIM-SOFIE observations were downloaded from
http://sofie.gats-inc.com/sofie/ (last access: 20 June 2016) and
represent the version 1.3 mission data files. The WACCM simulations analysed
in this work can be downloaded from 10.17632/rvyvk6j5yf.1
(). The geomagnetic AE index data were downloaded
from the World Data Center for Geomagnetism
(http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html, last access:
1 April 2016).
The authors declare that they have no conflict of
interest.
Acknowledgements
Linda Megner is supported by the Swedish Research Council under contract
621-2012-1648. Daniel R. Marsh is supported in part by NASA LWS grant
NNX14AH54G. The National Center for Atmospheric Research (NCAR) is sponsored
by the U.S. National Science Foundation (NSF). Christine Smith-Johnsen
is supported by the Norwegian
Research Council under project 222390. Edited
by: Franz-Josef Lübken Reviewed by: two anonymous referees
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