ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-33-561-2015Ionization and NO production in the polar mesosphere during high-speed solar wind streams: model validation and comparison with NO enhancements observed by Odin-SMRKirkwoodS.sheila.kirkwood@irf.seOsepianA.BelovaE.https://orcid.org/0000-0002-6698-321XUrbanJ.PérotK.https://orcid.org/0000-0002-4267-8560SinhaA. K.Polar Atmospheric Research, Swedish Institute of Space Physics, P.O. Box 812, 98128 Kiruna, SwedenPolar Geophysical Institute, Halturina 15, 183 023 Murmansk, RussiaDepartment of Radio and Space Science, Chalmers University of Technology, Hörsalsvägen 11, 412 96 Gothenburg, SwedenIndian Institute of Geomagnetism, 410218 Navi Mumbai, IndiadeceasedS. Kirkwood (sheila.kirkwood@irf.se)26May201533556157227October201421April201523April2015This 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://angeo.copernicus.org/articles/33/561/2015/angeo-33-561-2015.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/33/561/2015/angeo-33-561-2015.pdf
Precipitation of high-energy electrons (EEP) into the polar middle atmosphere
is a potential source of significant production of odd nitrogen, which may
play a role in stratospheric ozone destruction and in perturbing large-scale
atmospheric circulation patterns. High-speed streams of solar wind (HSS) are
a major source of energization and precipitation of electrons from the
Earth's radiation belts, but it remains to be determined whether these
electrons make a significant contribution to the odd-nitrogen budget in the
middle atmosphere when compared to production by solar protons or by
lower-energy (auroral) electrons at higher altitudes, with subsequent
downward transport. Satellite observations of EEP are available, but their
accuracy is not well established. Studies of the ionization of the atmosphere
in response to EEP, in terms of cosmic-noise absorption (CNA), have
indicated an unexplained seasonal variation in HSS-related effects and have
suggested possible order-of-magnitude underestimates of the EEP fluxes by the
satellite observations in some circumstances. Here we use a model of
ionization by EEP coupled with an ion chemistry model to show that published
average EEP fluxes, during HSS events, from satellite measurements , are fully consistent with the published average CNA
response . The seasonal variation of CNA response
can be explained by ion chemistry with no need for any seasonal variation in
EEP. Average EEP fluxes are used to estimate production rate profiles of
nitric oxide between 60 and 100 km heights over Antarctica for a series of
unusually well separated HSS events in austral winter 2010. These are
compared to observations of changes in nitric oxide during the events, made
by the sub-millimetre microwave radiometer on the Odin spacecraft. The
observations show strong increases of nitric oxide amounts between 75 and 90 km heights, at all latitudes poleward of 60∘ S, about 10 days after the
arrival of the HSS. These are of the same order of magnitude but generally
larger than would be expected from direct production by HSS-associated EEP,
indicating that downward transport likely contributes in addition to direct
production.
Atmospheric composition and structure (middle atmosphere – composition and chemistry)Introduction
Production of nitric oxide (NO) in the polar upper atmosphere
by energetic particle precipitation is considered to be an important source
of NO in the polar winter stratosphere, where it contributes to ozone
destruction and, as a consequence, may affect radiative heating, modify
circulation patterns and possibly even affect climate for a recent
review see . Most odd nitrogen is produced in the
thermosphere, above 100 km altitude, by electrons with energies up to a few
keV, which are accelerated in the Earth's magnetosphere and precipitated in
the northern and southern auroral zones. From the thermosphere, NO can be
transported downward, particularly in polar winter, when its lifetime is long
(due to a lack of sunlight) and the residual circulation is downward. There
is also the possibility of direct production of NO below 100 km altitude by
solar protons with MeV energies, by solar X-rays and by energetic electrons
which are energized and precipitated from within the Earth's magnetosphere
with energies in excess of 10 keV. Solar proton events are the result of
coronal-mass-ejection (CME) events on the Sun. They are rare, typically
affecting about 100 days in any 10-year solar cycle, but the proton fluxes
can be high. Direct changes in middle-atmosphere composition down to 40–50 km
heights have been demonstrated during solar proton events see
e.g.and references therein. Since solar
proton events occur primarily close to the maximum of the solar sunspot
cycle, it has been suggested that these could be a source of climate forcing
in phase with the solar cycle.
It has been known since the beginning of the satellite age that there are
large numbers of high-energy particles trapped in the radiation belts in the
Earth's magnetosphere. It is also well known that their pitch angles can be
scattered into the loss cone so that they are precipitated into the
atmosphere when the magnetosphere is disturbed by changing conditions in the
solar wind e.g.. It has long been suspected that
energetic electrons precipitated during geomagnetic disturbances could be a
source of electron density and NO enhancements observed (by sounding rockets)
not only at high latitudes but, with a few days' delay, also at mid-latitudes
e.g.. The availability of direct
observations of the solar wind in recent decades has led to an understanding
that the arrival of high-speed solar wind streams (HSS) at Earth is a major
source of energization, pitch-angle scattering and precipitation of
high-energy electrons into the atmosphere see review
by. Statistical studies of energetic electrons using
instruments on the Polar Orbiting Environmental Satellites (POES) have shown
that there are large increases in the fluxes of both trapped and
precipitating electrons associated with HSS, affecting electron energies from
30 keV to some MeV (the limits of the observations) .
These can be expected to reach altitudes between 50 and 90 km in geomagnetic
latitude bands between about 55–70∘ N or S. Further, since
the solar coronal holes which cause HSS are more prevalent around the
declining phase of the solar sunspot cycle, and HSS occur much more
frequently than solar proton events, it has been suggested that the result
may be climate forcing which is not exactly in phase with the solar cycle. A
chemistry–climate simulation by suggested that
energetic electron precipitation could have as much or more of an effect on climate
than the changes in solar UV fluxes between solar maximum and solar minimum.
The accuracy of POES measurements of energetic electron precipitation (EEP)
for low fluxes has been questioned by , who found a
mismatch between measured fluxes and their expected effect in the middle
atmosphere. Rodger et al. (2013) used calculations of electron density profiles
based on measured EEP fluxes, then calculated the expected cosmic-noise
absorption (CNA) corresponding to the electron density profiles and compared
with CNA observations. They found that measured CNA was an order of magnitude
higher than expected on the basis of the EEP measurements when EEP fluxes
(for energies > 30 keV) were reported as < 106 cm-2 s-1 sr-1.
This would imply that EEP fluxes and NO production rates are underestimated
by 2 orders of magnitude when based on the POES measurements. Since
statistical average HSS-related EEP fluxes are
below 106 cm-2 s-1 sr-1, it is then not clear whether they
represent true conditions or are underestimated by a large factor.
The EEP–CNA comparison made by was not
specifically concerned with HSS-related disturbances. It was also based only
on short intervals of night-time observations at a single CNA measurement
site in the auroral zone, where the effects of auroral electron precipitation
(energies < 10 keV) could potentially have affected the results. In a
separate study, compiled a statistical view
of the CNA response specifically to HSS-related disturbances. The latter uses
a much larger number of measurement sites for CNA and includes all times of
the day and all seasons. found a strong response
of CNA to HSS, with systematic daily and seasonal variations. Although they
were qualitatively able to explain the daily variation by the expected daily
variation in EEP fluxes e.g., they could not find
any evidence of a seasonal variation in EEP and were unable to find an
explanation for the seasonal variation in CNA response. They did not attempt
to make a quantitative comparison between the CNA response and EEP fluxes.
In the current paper we use the statistical averages of HSS-related EEP
fluxes based on POES measurements to calculate
ionospheric electron density profiles and associated CNA and make a
quantitative comparison with the observed statistical response in CNA . We further compare NO production rates, calculated on
the basis of statistical EEP fluxes, to direct measurements of NO increases
in the Antarctic winter middle atmosphere associated with HSS events.
Ion and NO production rate model
The statistical characteristics of both precipitating and trapped energetic
electron fluxes associated with HSS have been comprehensively documented by
in terms of integral fluxes for energies > 30, > 100 and > 300 keV. In order to calculate ion (and hence NO)
production rate profiles, differential flux-energy spectra of the
precipitating electrons are needed. used a
pitch-angle scattering model for the kind of conditions expected during HSS,
together with comparisons between satellite measurements of trapped fluxes,
incoherent-scatter radar measurements of the resulting electron density
profiles in the atmosphere, and observations of CNA, to show that an
exponential form for the differential flux was consistent with the
measurements. However, the latter study covered only the energy range 30–200 keV. A simple exponential form for the differential flux-energy spectrum does
not give a good enough fit to the integral fluxes in , which include higher energies. Neither does a power law,
as proposed by e.g.. The exponential form gives too
high fluxes between 100 and 300 keV, and the power law gives too low fluxes, compared to
the 30–100 and > 300 keV intervals. Therefore here we use an exponential
form for energies below 100 keV, with a power-law tail covering the higher
energies. This allows a close fit to the integral fluxes, within the
precision with which they can be read from the figures in . The details of the fitted flux-energy spectra for the
day following HSS onset (for L=5, but there is no significant difference
for L=6) are shown in Table 1, where three alternatives are given,
corresponding to the upper quartile (UQ-HSS), mean (mean-HSS) and
lower quartile (LQ-HSS) levels of the integral fluxes. The statistical study
by does not provide any direct information on
fluxes for energies below 30 keV. Electrons with these relatively low
energies do not penetrate below 90 km altitude, so this part of the
distribution is not important for the ionization in the mesosphere. However,
any NO produced will have a much longer lifetime than the ionization and
might be redistributed by vertical transport, becoming important in polar
winter (when the large-scale circulation has a downward component) for NO
concentrations even at much lower heights. Thus we also illustrate the effect
of including a lower-energy source, a typical auroral flux , also listed in Table 1 (we cut off this flux at
30 keV, so there is no contribution to the integral flux at > 30 keV).
Finally we introduce a variation over magnetic local time in the form of a
smoothed fit to the variation documented in for
> 30 keV fluxes. This is illustrated in the top
panel of Fig. . We apply the same factor to all fluxes, so that the
same e-folding energy for the exponential and power-law coefficient for the
tail is used at all magnetic local times (MLTs). This may not be completely
accurate, but the uncertainties in this approximation will be less than the
very large differences between mean, upper and lower quartile fluxes.
Example model calculations for winter solstice, for the location of
Maitri Station, Antarctica, at geomagnetic latitude 63∘ S,
geographic latitude 71∘ S. Top panel: input fluxes of energetic
electrons, corresponding to the UQ-HSS model in Table 1. Second panel:
ionization rate by energetic electrons. Third panel: ionization rate of NO by
solar Lyman α radiation, including nightglow. Fourth panel: resulting
electron density. Timescale is in magnetic local time (MLT). Local solar
noon is at 10:04 MLT.
Mean daily downward differential flux models used to calculate
ionization rate profiles. Fluxes at energies E > 30 keV are
power-law fits to precipitating fluxes during the first day following the
arrival of high-speed solar wind streams according to
. Fits to the lower quartile (HSS-LQ), mean
(HSS-mean) and upper quartile (HSS-UQ) integral fluxes are shown (note that
mean fluxes are higher than UQ). “Aurora” is a representative auroral
spectrum (Kirkwood and Eliasson, 1990). Corresponding integral fluxes for
E > 30, 100 and 300 keV are shown in the last three columns for
comparison with Meredith et al., 2011.
Ionization rate and ion/electron density profiles are calculated using the
model documented in . This uses
ionization rate calculations based on the methods of .
To give us the possibility to validate the model by comparison with other
observations, we also need to include other sources of ionization and we need
to compute electron density profiles from the ionization rates. This is
achieved using the positive-ion chemistry model of (with four representative ions: O2+, NO+, a simple
cluster ion and a complex cluster ion) and the negative-ion model of (two representative ions: O2-, X-). The
underlying neutral atmosphere model is MSIS00E
(http://ccmc.gsfc.nasa.gov/modelweb/atmos/msise.html; ). There are two important updates to the model as compared
to the description in – a correction for
energetic particle albedo which reduces
ionization rates by a factor of 0.62 and the inclusion of UV ionization sources
including nightglow following. At the heights of
interest, the main contribution is ionization of NO by Lyman α. The
ionization and ion-chemistry calculations require appropriate
minor-constituent models for NO and H2O, respectively. For H2O we have
made an analytical approximation to the climatologies reported by (from a decade of year-round measurements from Andenes,
northern Norway) and by (polar summer mesosphere in both
hemispheres observed by the SOFIE instrument on the AIM satellite). This is
illustrated in the right-hand panel of Fig. .
Our model of NO is based on measurements by the Sub-Millimeter Radiometer (SMR)
instrument on the Odin satellite . Trace-gas measurements,
including NO, have been made by Odin-SMR since October 2003 for a description
of the measurement technique see . Until May 2007, the relevant
height range was covered for only about 1 day per month, but since then the
coverage has increased to about 4 days per month. Odin travels in a quasi-polar
Sun-synchronous orbit which nominally crosses the Equator at 06:00 and 18:00 LT (in practice this
has varied between 06:00 and 07:00 LT and 18:00 and 19:00 LT between 2003 and 2014). There are about 15
orbits per day and measurements are made by limb scanning on both ascending and descending
nodes. Estimates are provided at 1 km height intervals but the true height resolution in
the mesosphere is about 7 km. Individual profile measurements show high variability, and
averages have to be used to give geophysically reliable results . For
this study, daily zonal averages have been computed for 5∘ bins of geomagnetic
latitude as in, each representing an average over, typically,
about 40 different measurements. Since HSS events can be expected to lead to changes in
the NO number density, we develop an empirical model which accounts for this, using
solar wind speed observations from the Wind spacecraft (http://omniweb.gsfc.nasa.gov/).
Measurement days corresponding to three different solar wind conditions are identified –
pre-HSS,
where solar wind speed is below 500 km s-1 for the entire day but increases to above 500 km s-1 sometime
the following day; onset-HSS, where solar wind speed was below 500 km s-1 for the entire previous
day but increases to above 500 km s-1 sometime during the current day; and post-HSS, where solar wind
speed is above 500 km s-1 for the entire day and has been above 500 km s-1 for the preceding 24 h. We
exclude any observations made within 20 days after a solar proton event (defined as proton
flux > 10 cm-2 s-1 sr-1 at 10 MeV) or before 2007. The number of suitable
observations from Odin is small. Out of altogether 337 observation days since 2007, 24 can be
identified as pre-HSS, 48 as onset-HSS and 20 as post-HSS. The resulting NO densities for the
geomagnetic latitude band 60–65∘ S for summer (November–December–January), autumn
(February–March–April), winter (May–June–July) and spring (August–September–October) are shown in
the left-hand panel of Fig. . The number of observation days in each category
and each season is small – for summer there are 6, 8 and 3 days in pre-, onset- and post-HSS conditions, respectively; in autumn there are 7, 13 and 5 days; in winter there are 7, 16 and
8 days; and in spring there are 4, 11 and 4 days. Despite the small numbers, the averages
show clearly the increased background NO densities in winter, and a
strong response to HSS, at all heights between 70 and 100 km, in that season. In the
first instance, these profiles will be used to estimate whether the increase in NO due to
HSS can give a significant signature in CNA. In this context, we note that comparison with
four other satellite instruments has shown a possible low bias for Odin-SMR NO measurements,
below 100 km altitude, by about 10 % compared to Odin-OSIRIS, and a high bias by up to 40 % compared to SCIAMACHY, MIPAS and ACE-FTS .
Model profiles of NO number density and H2O volume mixing ratio
input into the ionization rate/ion-chemistry model. NO profiles are divided
according to their relation to the arrival of high-speed solar wind streams:
solid lines correspond to days including HSS arrival (onset-HSS), circles for
days immediately prior to arrival (pre-HSS), crosses at least 1 day
after arrival (post-HSS). See text for further details.
Example results of the modelled daily variation of ionization rate and
electron density for the UQ-HSS electron flux spectrum in Table 1, for winter
solstice, at an Antarctic location at L=5.0 (Maitri Station,
geographic coordinates 70.77∘ S 11.73∘ E), are shown in
Fig. . The variation of ionization rate over the day is
dominated by the prescribed variation of the precipitating electron flux. The
variation of the electron density is further strongly affected by ion
chemistry. At night, electron attachment leads to a build-up of negative ions
and a strong reduction in electron density. Increased UV radiation and
increased atomic oxygen density during daytime remove the electrons from the
negative ions and the electron density increases see e.g.. Together, the daily variations in precipitating
electron flux and ion chemistry lead to a morning maximum in the electron
density. Figure shows mean (over all MLTs)
profiles of ionization rate and electron density corresponding to
upper quartile (UQ), mean and lower quartile (LQ) HSS electron flux spectra,
and onset-HSS model for NO, calculated for each month of the year (on the 23rd of
each month, and then averaged for the 3 months of each season). Here it can
be seen that seasonal variations in ionization rate (due to changes in
atmospheric scale height following seasonal changes in temperature) are
fairly small (up to a factor of 4), while the seasonal changes in electron
density are greater (up to an order of magnitude). This is due to
ion chemistry. For example, around 80 km height, the ionization rate is
essentially the same in summer and winter, but electron densities are about 3
times lower in summer as a result of increased water vapour together with
lower temperature, which leads to the formation of positive cluster ions which
recombine with electrons more rapidly than molecular ions do. Around 60 km
height, ionization rates in spring, autumn and summer are 2–10 times less
than in winter, but electron densities are 2–5 times higher. This is due to
a lack of sunlight leading to more persistent negative ion formation in winter
see e.g.. (It can be noted that this
seasonal effect is also found in the IMAZ empirical model of the auroral-zone
lower ionosphere . Although not strictly
comparable, since IMAZ provides electron density as a function of CNA rather
than as a function of incident electron flux, IMAZ does show that, in
disturbed conditions (CNA at 27.6 MHz 0.8–2.0 dB), electron densities at 80 km are on average a few times higher in winter than in summer).
Seasonal averages of model calculations, for the location of Maitri
Station, Antarctica. Left panel: daily average ionization rate of NO by solar
Lyman α radiation, including nightglow. Middle panel: daily average
ionization rate by energetic electrons. Right panel: resulting daily average
electron density. Solid lines in the middle and right-hand panels are for
mean-HSS fluxes, dotted lines are for LQ-HSS and dashed lines are for UQ-HSS.
Averages of model calculations of cosmic-noise absorption at
38 MHz, for the location of Maitri Station, Antarctica. Top panel: annual
averages for different EEP models – solid line for mean-HSS fluxes, dash-dot
line for LQ-HSS and dashed line for UQ-HSS, with the addition of “aurora”
fluxes to the UQ-HSS model in the 4 h before magnetic midnight (solid lines
with +, visible only close to the right-hand edge of the plot). Lower
panel: seasonal averages for UQ-HSS fluxes (plain solid lines), with the
addition of “aurora” fluxes in the 4 h before magnetic midnight (solid
lines with +) and for the increase in absorption due to the HSS-associated
NO increase, without any energetic electron precipitation (dashed lines).
Computation of ionization rate profiles, and the ion-chemistry modelling
which is needed to calculate electron density profiles and CNA, requires
complex software, with the possibility of coding errors. Therefore, for the present
study, results have been carefully compared to the independently coded model
described in and
(which uses the same D-region ionization
sources, the same positive-ion model and a more complex negative-ion model
with four ions: O-, O2-, CO3-, NO3-), and no significant
differences have been found in the calculated electron density profiles.
These models have been extensively tested in various different conditions
(auroral electron precipitation, solar proton events, quiet conditions) with
ionization sources according to satellite measurements and electron-density
measurements by sounding rockets, by partial-reflection radar and by the
EISCAT incoherent-scatter radar e.g..
These models use simplified ion-chemistry and prescribed trace-constituent
models to allow computational efficiency in calculating electron density
profiles, as well as their dependence on trace constituents, which can be readily
compared with observations. More complex ion-chemistry models such as the
Sodankyla or University of Bremen models e.g. use large numbers of individual ion species (up to 55 positive
ions, 49 negative ions) with the aim of calculating both electron and
individual ion densities and production rates of neutral trace constituents.
As demonstrated by the comparisons cited above, this level of complexity is
not needed to estimate electron density. Our model does not provide a direct
calculation of the production of NO. Recent work using the University of
Bremen model has shown that NOx production rates
should be about 1.25 times the ion production rate below 80 km, increasing to
about 1.7 times as height increases up to 110 km, with the ratio of NO / NOx
about 0.55 below 110 km. However, the partitioning depends on conditions so
here we estimate an “upper limit” NO production rate from the total
ionization rate by multiplying by a factor of 1.25, while noting that this may
still be an underestimate by up to 35 % between 80 and 110 km.
HSS model validation
The statistical study of observed cosmic-noise absorption in relation to HSS
by provides an excellent validation of our model
results. Figure shows the MLT variation in cosmic-noise
absorption (at 38 MHz) which would result from our modelled electron density
profiles. To calculate these, we have first calculated absorption for
electron density profiles with pre-HSS values of NO and no electron
precipitation and subtracted those absorption values from the results when
electron precipitation and high NO densities (onset-HSS) are present. The
upper panel shows the average (over all 12 months) for UQ-, mean- and LQ-HSS
electron flux spectra, the lower panel shows averages for each season, for
mean HSS fluxes. In the lower panel, for completeness, we also show how much
CNA would result from the increase in NO corresponding to the “post-HSS”
profile in Fig. , without any energetic electron
precipitation (dashed lines). It can be seen that the latter is very small.
The energetic electron precipitation is by far the dominant contribution to
the CNA. We also show the effect of adding our “aurora” precipitation in the
4 h preceding magnetic midnight (solid lines with crosses). There is a
clear contribution to CNA, by about 0.1 dB, even though these electrons do
not cause any ionization below 90 km altitude.
Comparing the upper panel of Fig. with the first day
after HSS onset in Fig. 6 of , we find generally
good agreement. In our case, the annual averages for UQ-HSS and LQ-HSS models
peak in the late morning hours at about 0.47 and 0.07 dB, respectively.
Corresponding observational results in Kavanagh et al. (2012) peak in the late
morning hours at about 0.65 and 0.1 dB. (We cannot compare our mean-HSS model
with Kavanagh et al. (2012) since only median rather than mean values are included
in the latter study.) The lower panel of Fig. can be
compared with Fig. 8 of Kavanagh et al. (2012). In both our model and in the
observations, maximum daytime CNA is higher during the spring equinox than
during winter, and the summer shows the lowest CNA values of all seasons. Note
that there is absolutely no seasonal change in the spectrum of particle
precipitation we have assumed – the seasonal differences in CNA are simply a
result of the seasonal changes in ion chemistry. There is a slight difference
between our model results and the observations in that our model predicts
lower absorption in autumn than in spring, whereas the observations show the
opposite. In the model, this is due to the asymmetry in the seasonal
variations in temperature and, to a lesser extent, H2O. The temperature
asymmetry is in the underlying neutral atmosphere model, and both temperature
and H2O vary rapidly during the weeks either side of the equinoxes. The
variation in peak absorption values is of the order of plus or minus 0.1 dB from the
equinox values, so we cannot expect a close fit for the spring/autumn
asymmetry between our model and the results of Kavanagh et al. (2012) without taking
into account the exact neutral atmosphere conditions for the observations
included in the statistical averages.
Overall, the agreement between the daily and seasonal variation of CNA in our
model, and the statistical averages of observations, is very good. This gives
confidence that the average fluxes published in ,
which are the input for our model, are representative of conditions during
HSS. Any systematic error in energetic electron fluxes (say by a factor of X)
in the energy ranges included in our model would result in a corresponding
error (by approximately X0.5) in CNA. For example, i.e. a factor of 4
increase (decrease) in flux for UQ-HSS conditions would increase (decrease)
peak CNA from 0.47 to 0.94 (0.23) dB, and seems to be ruled out by the
closeness of our model results to the observations. It is difficult to put an
exact figure on the uncertainty, but the comparison suggests the fluxes in our
UQ and LQ-HSS models might be underestimated, but by less than a factor of 4.
However, it should be remembered that the model results and the observations
represent average conditions. In practice, EEP is often intermittent and
spatially variable, so that at any particular place and time the measured CNA
and the ion production rates will sometimes be much higher or much lower than
the average values. In particular, it needs to be recognized that the
mean-HSS model has higher fluxes (and higher CNA) than the UQ-HSS model,
which indicates that there are a small but significant number of extremely
strong events at the high-flux end of the distribution, pushing the mean
fluxes above the upper quartile. It should also be noted that CNA is
relatively insensitive to ionization by lower-energy electrons, such as
auroral electrons, which can produce strong ionization and NO production at
heights above 90 km. The auroral flux in Table 1, for example, increases CNA
by less than 0.1 dB. Auroras are even more variable, both in time and space,
than HSS-related EEP, so that auroras may be severely under-sampled in median
(as opposed to mean) CNA averages such as presented in .
Comparison with NO observations in the Antarctic winter mesosphere
We would next like to compare our model results with observed increases in NO
densities associated with HSS. The amounts of NO produced each day are very
small and NO is rapidly destroyed in sunlight. The best time to do this is in
polar winter so that a sufficient amount of NO can be accumulated to give a
possibility of detection. It is also better to look in the Southern
Hemisphere, where wind systems are more zonally symmetric than in the north, so
that mixing between different geographic latitudes is minimized and NO
produced poleward of the polar circle can remain in darkness for several
weeks. One possibility is to use the average winter NO profiles in Fig. , subtracting the pre-onset profile from the later
profiles to give a measure of the increase. However, due to the fortunate
synchronization of Odin observations with a number of unusually well
separated HSS arrivals during the austral winter of 2010, it is also possible
to consider a number of discrete events. These are shown in Fig. , which covers the period 1 April to 1 September
2010. There are recurrent HSS arrivals, with the main peaks at about the
solar rotation period of 27 days, and only minor peaks from secondary
coronal holes in between, with a clear correlation to strong increases in NO
observed by Odin. Odin made mesospheric measurements on a number of days with
different intervals between the observation days – 2, 4, 10 or 14 days – and
as a result caught conditions just before, on the day of the onset or the day
after, and about 10 days after the onset for several of these HSS. Since we
want to be as close as possible to midwinter, and we need to avoid the
complication of additional ionization by solar protons, be study only those
events corresponding to the peaks in NO densities observed by Odin on 4 May,
1 and 29 June, and 27 July. The timing of the HSS arrivals, defined as the
time when the solar wind speed increased through 500 m s-1, and the related
Odin-SMR observations are summarized in Table 2. In Fig. , onset/post-onset days used here are marked by open
diamonds, pre-onset days by open circles and observations 10 days after onset
by asterisks (observations on 15 June and 6 August are affected by solar
protons, so they are not used here).
Time of HSS arrival, dates of available Odin-SMR measurements of NO
profiles, and mean CNA measured between 07:00 and 11:00 UT (approx. 08:00–12:00 LT,
06:00–10:00 MLT) by the 38.2 MHz riometer at Maitri, on the 2 days following the HSS
arrival, during austral winter 2010.
Observations of 24 h averaged NO number densities in the
60–65∘ S geomagnetic latitude band by the Odin-SMR instrument
(bottom panel) associated with the arrival of high-speed solar wind streams
(top panel) during austral winter in 2010. Middle panels show the auroral
electrojet index and the flux of 10 MeV protons (from
http://omniweb.gsfc.nasa.gov/). Vertical lines mark the times of
arrival of high-speed solar wind streams (“onset”). Observations used in
Figs. 6 and 7 are marked by open circles (pre-onset), diamonds (onset or
immediately post-onset) and asterisks (about 10 days after onset).
Observations of NO number densities by the Odin-SMR instrument
before, during and after the arrival of high-speed solar wind streams during
austral winter in 2010, plotted as a function of geomagnetic latitude. Broad,
colour-filled lines show values between zonal mean and plus and minus the
standard error of the mean. Colours indicate the different events in Table 2:
red, 2 May; green, 31 May; magenta, 29 June; cyan, 27 July.
Comparison of modelled NO production with Odin-SMR observations of
NO number density increase during HSS. Model profiles, corresponding to the
different models of incoming energetic electron fluxes listed in Table 1, are
shown in black for NO total production during 1 day (left-hand panel
labelled “ONSET”), 2 days (middle panel labelled “POST-ONSET”), and
10 days (with steadily decreasing flux, right-hand panel labelled
“+ 10 DAYS”). “Aurora” fluxes are applied for only 4 h each day, and HSS
fluxes for the whole day. Observations are the differences between pre-onset,
onset and post-onset observations for four HSS events shown in Fig. 6 (solid
lines) and for the average of all Southern Hemisphere wintertime HSS events
covered by Odin-SMR measurements during the years 2007–2014
(∗). All observations are based on
zonal averages over UT days and 5∘ bands of geomagnetic latitude.
Left and centre panels for geomagnetic latitude 60–65∘ S, and right
panel for geomagnetic latitudes 60–65∘ S (no marks along line),
70–75∘ S (circles) and 80–85∘ S (crosses)
The solar wind speeds reached in our selected events, 600–700 km s-1,
correspond to speeds between the mean and upper quartile in the study of
. The imaging riometer at Maitri registered long-lasting absorption events on the days
following the HSS arrivals – these are summarized in Table 2 in terms of
average CNA between 07:00 and 11:00 MLT, which is the time of day when CNA is expected
to be most sensitive to the HSS-related EEP. The measured values in Table 2
can be compared with our models – for winter, the mean, UQ and LQ-HSS models
give averages of 0.83, 0.45 and 0.04 dB CNA, respectively, for the 07:00–11:00 MLT
time interval. The observed values are close to the UQ-HSS model for the last
three events, but they are much higher for the first event, almost twice the level of the mean-HSS
model, so the fluxes must have been about 4 times higher than
the mean-HSS model. Thus it is reasonable to compare the observed increases in
NO density for the last three events with our UQ-HSS model, but we might expect
about 4 times more NO production than the mean-HSS model for the first event.
Figure further illustrates the NO changes
associated with the HSS, this time showing NO as a function of (geomagnetic)
latitude. Each point plotted represents an average of about 40 separate
measurements made on the same day at different longitudes and latitudes
within a geomagnetic latitude interval. During disturbed conditions the
variability from place to place at high latitudes can be expected to
increase, increasing the standard error of the mean. These uncertainties are
shown by the width of the lines on the plot (the colour fills the values
between the mean and plus/minus the standard error of the mean at each
point). The uncertainties (line widths) are indeed generally higher at
latitudes above 50∘ and are highest above 50∘ in the Southern
Hemisphere during the “POST-ONSET” phase, when they in some cases reach 1 × 1014 m-3. At other latitudes and times they are less than half this
amount. Inspection of Fig. shows a
strong increase at high latitudes above 85 km height, by much more than the
uncertainties, on the onset/post-onset days, which are within 0–2 days of
the HSS arrival. The increase is most prominent in the Southern (winter)
Hemisphere, where it is also not confined to the geomagnetic latitudes where
EEP is expected (55–70∘) but appears also at higher latitude. Below
85 km, onset and post-onset NO densities are not significantly above
pre-onset levels, when uncertainties are taken into account. By 10 days after
the HSS arrival, there is no detectable NO enhancement at northern high
latitudes at any height, and at high southern latitudes, NO densities above 95 km are also close to pre-onset values. However, below 95 km there are
persistent enhancements in the Southern Hemisphere, with a clear increase
relative to pre-HSS conditions, at both 75–85 and 85–95 km, as well as a
possible increase for some of the events at 65–75 km.
The NO enhancements are further illustrated in Fig. 7,
which shows height profiles of the increases in NO corresponding to the HSS
events (onset, post-onset and “+ 10 day” profiles with pre-onset profiles
subtracted). Here onset and post-onset profiles are shown for the geomagnetic
latitude band 60–65∘ S (corresponding to the riometer at Maitri and
the latitude of out model calculations), together with profiles of the mean
NO increase on onset and post-onset days from all available Odin
Southern Hemisphere wintertime observations between May 2007 and September 2014 (with
error bars plotted each 7 km of height, offset in height on the different
profiles for clarity). For “+ 10 days”, profiles are included for three latitude
bands: 60–65, 70–75 and 80–85∘ S.
Our models of ionization rates due to HSS-associated energetic particle
precipitation can be used to estimate NO production rates. Here we use a
factor value of 1.25 to convert ionization rate to NO
production rate and integrate over 24 h to give the estimates shown for the
HSS “onset” day and 48 h for the “post-onset” comparison. As discussed in
Sect. 2, these can be considered upper-limit estimates for the NO
production by the modelled ionization rates. The accumulated production
amounts are shown by the black lines in Fig. 7, where
we also show the effect of adding auroral precipitation for 4 h each day.
Given the timing of the HSS arrival, and the necessity of averaging over 24 h
to derive the corresponding geomagnetic-zonal-mean NO observations, the
model-accumulated production amounts should be overestimates rather than
underestimates. Further, we have not accounted for losses. Even in the
absence of sunlight (i.e. polar winter) there will be NO losses by the
reaction N + NO - > N2+ O for example as N is produced by energetic
particle ionization. The reaction rate depends on the ionization rate and on
the partitioning of N between excited and ground states . For ionization rates below 108 m-3 s-1,
typical for our HSS models, we can expect the NO lifetime to be around 10 days. For ionization rates which are higher by an order of magnitude or more,
such as associated with the auroral precipitation, we can expect the NO
lifetime to be of the order of a day, possibly much less . Comparing the accumulated NO amounts from our models
with the observations for onset and post-onset in Fig. 7, it is clear that much more NO has appeared above 85–90 km altitude than the (overestimated) LQ, UQ and mean-HSS model
predictions – by factors of up to 3 in the case of the Odin mean profile and
factors of up to 10 for the individual HSS events during winter 2010. Although
there were indications that the first of the events (2 May) could have led
to about 4 times more NO production than the mean-HSS model, there is no
reason to believe that the other events are above that model level. However
it is also clear that the addition of the auroral source can produce very
large amounts of NO above 100 km height, even though we have assumed that it
acts for only 4 out of each 24 h. Most likely, the large increases in NO
above 85–90 km altitude are due to auroral electron precipitation.
Below 85 km there is no systematic increase in NO for the individual events
for onset/post-onset days, but the Odin mean profiles show small increases
which are similar to our HSS-mean model. Given the large variability in the
individual events, and the very low modelled NO production rates compared to
standard errors in the observed mean enhancements (error bars in the
left-hand and centre panels of Fig. 7), we can only say
that direct NO is able to contribute a significant proportion of mean (over
several events) enhancement in NO at 70–85 km in the first 2 days after HSS
arrival, but the amount is too low to be detected for individual events.
When we consider the situation about 10 days after the arrival of the HSS, we
must consider that the EEP fluxes reduce with time and also take into account
spreading of NO produced in the geomagnetic zone 55–70∘ S to other
geomagnetic latitudes. There is a large offset (about 15∘) between
the geographic and geomagnetic poles in the Southern Hemisphere, so that
geographically zonal winds can spread NO over all geomagnetic latitudes
poleward of 40∘ S. For the model estimates of accumulated NO
production in the right-hand panel of Fig. 7, we have
assumed that EEP fluxes remain the same for the first 3 days, and then reduce
by 10 % per day until the 10th day, an approximation to the statistical
results in . We make an assumption that production
is the same as our models over the whole geomagnetic zone 55–70∘ S
and that this is redistributed over the whole region poleward of 45∘ S geographic latitude. Comparing the areas of the two zones, gives a
factor of 0.52 reduction in average accumulated NO concentration when it is diluted by
spreading over the larger zone. In the observations (right-hand panel of
Fig. 7 and lowest panel of Fig. ), it is clear that the large amounts
of NO above 90 km are no longer present after 10 days, but NO has increased
substantially between 70 and 90 km, at all latitudes poleward of 60∘ S,
with the highest increase at the highest geomagnetic latitudes. In the
right-hand panel of Fig. 7, it can be seen that the
observed amounts of NO are highly variable but, on average, close to our
mean-HSS models of accumulated direct production (corrected for spatial
spread). From the CNA observed at Maitri, it seems that EEP fluxes might have
been as high as 4 times our mean-HSS model on one occasion (HSS arrival on 2
May), at least at the location of Maitri. If this applied to the whole
precipitation region, the accumulated NO production should be 4 times higher,
but the observed NO enhancements (solid red lines) are not larger than for
the other events. We have not accounted for losses in our model estimates.
The lifetime of NO at these heights in polar winter, with the HSS-related
ionization rates, is expected to be of the order of 10 days , so about half would have been lost by recombination
with N. It should also be remembered that our NO production rate
estimates are upper limits. Therefore the observed enhancements likely exceed direct
production by a factor which could be up to 4. However there also is a
possible high bias in Odin-SMR NO estimates by up to 40 % . In summary, according to our approximate
calculations, it seems unlikely that enough NO was produced directly at 70–90 km altitude to explain the observed NO increases by direct production and
horizontal transport alone. It is likely that downward transport contributed,
but, since the direct production is of the same order as the total observed
enhancement, and the uncertainties in both model and observations are also of
similar magnitude, we cannot say this for sure.
Conclusions
The first result of this study is that there is excellent quantitative
consistency between the statistical characteristics of EEP fluxes determined
from the POES satellites and the average response
in the ionospheric D region as measured by CNA .
This means there is no evidence, at least as far as average HSS-conditions
are concerned, to support the suggestion by that
POES underestimates EEP fluxes by several orders of magnitude. This gives
confidence that the HSS-associated EEP fluxes from
can be used for quantitative estimates of the contribution of HSS to NO
production in the D-region.
The second result is that seasonal
variations in the HSS-related CNA response, observed by , can be quantitatively reproduced
by seasonal changes in ion chemistry, without any seasonal changes in EEP.
This confirms the suggestion by that mesospheric
chemistry might provide an explanation.
The third result is that the ion-chemistry model shows that CNA is most
sensitive to EEP during daytime, and least sensitive during night, when
negative ions form. This means that lower-energy (auroral) precipitation can
make a significant contribution to CNA at night, since ion chemistry
affecting the lower part of the D region reduces the contribution of more
energetic electrons to the electron-density profile. This may explain the
results of , who found substantial discrepancies
between POES measurements of EEP and simultaneous/co-located CNA, as the
latter study used night-time observations in the auroral zone.
The fourth result concerns the production of NO by HSS-related EEP. For a
series of HSS events in austral winter 2010, we have shown that observations
of NO enhancements in the mesosphere over Antarctica (by the Odin satellite)
show significant enhancements after 10 days at heights 70–95 km. The
enhancements are of the same order of magnitude but possibly larger than
those expected from direct production by EEP. The largest amounts of NO are
produced by lower-energy (auroral) electrons above 90 km altitude, and
downward transport of this NO likely also contributes.
Finally, we can make a quantitative comparison with the amount of HSS-related
NO production implied by the EEP fluxes in and the
amounts found in the study by , which were suggested to
have a significant climate effect. Column production rates of NOy
(calculated as 1.2 times the ion production rate), summed over 60–90 km
altitude and averaged over 24 h after HSS-onset, are
6×1017 m-2 day-1 for our mean-HSS model and
3×1017 m-2 day-1 for UQ-HSS. In the study by , intermittent peaks in NOy production rates were
estimated to be in the range of 3–16 × 1018 m-2 day-1. This is an
order of magnitude more than the HSS-related production according to our
present study.
Acknowledgements
This research has been partly funded by the Swedish Research Council (grant 621-2010-3218).
Summary plots of Maitri riometer data were supplied by Lancaster University, England. Odin
is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB),
the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes), the
Centre National d'Etudes Spatiales (CNES) in France and through the Third Party
Missions
programme of the European Space Agency (ESA). The topical editor C. Jacobi thanks M. Friedrich and the two anonymous referees for help in evaluating this paper.
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