The impact of energetic electron precipitation on mesospheric hydroxyl during a year of solar minimum
Annet Eva Zawedde, Hilde Nesse Tyssøy, Robert Hibbins, Patrick J. Espy, Linn-Kristine Glesnes ?degaard, Marit Irene Sandanger, Johan Stadsnes
JJournal of Geophysical Research: Space Physics
The impact of energetic electron precipitation on mesospherichydroxyl during a year of solar minimum
Annet Eva Zawedde , Hilde Nesse Tyssøy , Robert Hibbins , Patrick J. Espy ,Linn-Kristine Glesnes Ødegaard , Marit Irene Sandanger , and Johan Stadsnes Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Bergen, Norway, Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway
Abstract
In 2008 a sequence of geomagnetic storms occurred triggered by high-speed solar windstreams from coronal holes. Improved estimates of precipitating fluxes of energetic electrons are derivedfrom measurements on board the NOAA/POES 18 satellite using a new analysis technique. These fluxesare used to quantify the direct impact of energetic electron precipitation (EEP) during solar minimum onmiddle atmospheric hydroxyl (OH) measured from the Aura satellite. During winter, localized longitudinaldensity enhancements in the OH are observed over northern Russia and North America at correctedgeomagnetic latitudes poleward of 55 ∘ . Although the northern Russia OH enhancement is closelyassociated with increased EEP at these longitudes, the strength and location of the North Americaenhancement appear to be unrelated to EEP. This OH density enhancement is likely due to vertical motioninduced by atmospheric wave dynamics that transports air rich in atomic oxygen and atomic hydrogendownward into the middle atmosphere, where it plays a role in the formation of OH. In the SouthernHemisphere, localized enhancements of the OH density over West Antarctica can be explained by acombination of enhanced EEP due to the local minimum in Earth’s magnetic field strength and atmosphericdynamics. Our findings suggest that even during solar minimum, there is substantial EEP-driven OHproduction. However, to quantify this e ff ect, a detailed knowledge of where and when the precipitationoccurs is required in the context of the background atmospheric dynamics.
1. Introduction
Energetic particles precipitating into the mesosphere and lower thermosphere are known to produce copi-ous amounts of odd nitrogen (NO X : N, NO, NO ) and odd hydrogen (HO X : H, HO, HO ), which can contributeto ozone (O ) destruction [e.g., Jackman et al. , 2005;
Sinnhuber et al. , 2012]. The energetic particles (electrons,protons, and heavier ions) have di ff erent solar drivers. Coronal Mass Ejections (CMEs) associated with sunspotspredominantly occur during solar maximum and are the cause of solar proton events (SPEs) which can leadto strong geomagnetic activity. The influence of the infrequent SPEs upon the middle atmosphere has beenextensively studied [see, e.g., Bates and Nicolet , 1950;
Weeks et al. , 1972;
Swider and Keneshea , 1973;
Crutzenand Solomon , 1980;
Solomon et al. , 1981;
López-Puertas et al. , 2005;
Damiani et al. , 2008, 2010;
Verronen andLehmann , 2013;
Jackman et al. , 2014;
Nesse Tyssøy and Stadsnes , 2015]. The atmospheric e ff ects of the morefrequent energetic electron precipitation (EEP) events are less known and harder to detect. During geomag-netic storms energetic electrons are injected and stored in the magnetosphere where they can be acceleratedto relativistic energies [ Foster et al. , 2014] and subsequently lost to the atmosphere [
Turner et al. , 2014]. Thepenetration depth varies with the particle energy, for example, a 30 keV electron will stop at ∼
90 km, whilea 1 MeV electron penetrates to about 60 km [
Turunen et al. , 2009]. Individually, such storms have weakergeomagnetic signatures than SPEs. It is, however, speculated that these events, because of their frequentoccurrence, will have a strong impact on the atmosphere in general [
Andersson et al. , 2014a].
Bartels [1932] identified “ M regions” on the solar surface as the source of the sequences of recurrent geo-magnetic activity that occurred during minimum solar activity. M regions are in fact coronal holes (CHs) andare independent of sunspot activity [ Allen , 1943]. They are associated with open magnetic field lines and,high-speed, low-density flows in the solar wind [
Billings and Roberts , 1964]. CHs are the source of high-speedsolar wind streams (HSSWS) and subsequent recurrent geomagnetic activity [e.g.,
Neupert and Pizzo , 1974;
Burlaga and Lepping , 1977;
Sheeley and Harvey , 1981]. The interaction of the fast solar wind associated with
RESEARCH ARTICLE
Special Section:
Energetic Electron Loss and itsImpacts on the Atmosphere
Key Points: • There is substantial OH production byenergetic electron precipitation alsoduring solar minimum• To quantify the e ff ect, the backgroundatmospheric dynamics have to betaken into account• It also requires detail knowledgeof where and when the energeticelectron precipitation occurs Correspondence to:
H. Nesse Tyssøy,[email protected]
Citation:
Zawedde, A. E., H. Nesse Tyssøy,R. Hibbins, P. J. Espy, L.-K. G. Ødegaard,M. I. Sandanger, and J. Stadsnes (2016),The impact of energetic electronprecipitation on mesospherichydroxyl during a year of solar mini-mum,
J. Geophys. Res. , , 5914–5929,doi:10.1002/2016JA022371.Received 12 JAN 2016Accepted 29 MAY 2016Accepted article online 7 JUN 2016Published online 30 JUN 2016©2016. The Authors.This is an open access article under theterms of the Creative CommonsAttribution-NonCommercial-NoDerivsLicense, which permits use anddistribution in any medium, providedthe original work is properly cited, theuse is non-commercial and nomodifications or adaptations are made. ZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
CHs with the slow solar wind streams results in the compression of the magnetic field and plasma at theirinterfaces forming a corotating interaction region (CIR), which is the geoe ff ective structure [ Tsurutani et al. ,2006;
Gopalswamy , 2008]. However, the interplanetary magnetic field (IMF) associated with CIRs has a highlyoscillating nature, which results in only moderate intensification of the magnetospheric currents and hencemoderate geomagnetic signatures. The intensity of the resulting storm depends on the combination of solarwind speed and the direction of the B z component [ Gopalswamy , 2008].Recent studies [
Verronen et al. , 2011;
Andersson et al. , 2012, 2014b, 2014a] provide observational evidence ofradiation belt (geomagnetic latitudes 55 ∘ – 65 ∘ ) electron precipitation (100 – 300 keV) a ff ecting mesospheric(71 – 78 km) OH. Based on two case studies in the declining phase of the solar cycle, Verronen et al. [2011] foundthat 56 – 87% of the changes in OH could be explained by changes in EEP. In a follow up study,
Andersson et al. [2012] focused on a larger part of the solar cycle from solar maximum to solar minimum. They found monthsof high correlation between daily zonal mean OH mixing ratios at 70 – 78 km and the flux of 100 – 300 keVelectrons. The correlation coe ffi cients were highly dependent on season and the strength of the particleprecipitation. Andersson et al. [2014b] studied the longitudinal response of nighttime mesospheric OH to >
30 keV electron precipitation, contrasting days with daily mean count rates of >
100 c/s to days with < ff ects of EEPwere seen at magnetic latitudes 55 ∘ – 72 ∘ . In the Southern Hemisphere (SH), the OH data revealed localizedOH mixing ratio enhancements at longitudes between 150 ∘ W and 30 ∘ E, over West Antarctica, poleward ofthe South Atlantic Magnetic Anomaly (SAMA) region. In the Northern Hemisphere (NH), EEP-induced OH vari-ations were more equally distributed with longitude; however, two potential regions of enhanced OH mixingratio above Northern America and Northern Russia were found.The middle atmosphere has a strong seasonal dynamical variability, including both the background merid-ional and zonal winds, as well as the atmospheric wave activity [see, e.g.,
Shepherd , 2000;
Kleinknecht et al. ,2014]. For example,
Damiani et al. [2010] have shown that during sudden stratospheric warmings (SSWs), theOH layer may show short-term variations comparable in strength to the OH increases during SPEs.
Anders-son et al. [2014b] did not include potential seasonal or meteorological factors when considering the particleimpact upon the longitudinal distribution of OH, although there appears to be features less constrained tothe magnetic latitudes and geomagnetic activity in both hemispheres. During strong particle precipitationevents, the OH production due to background dynamics of the atmosphere might be overshadowed by theimpact of energetic particle precipitation (EPP). However, during the more frequent and modest changes, thedynamical background will be of higher importance. Moreover, for the more frequent events, the magnitudeof the direct EEP-induced HO X e ff ect on O in the mesosphere is high enough to suspect that EEP could bean important contribution to the Sun-climate connection on solar cycle time scales [ Andersson et al. , 2014a].Assessing the impact and spatial distribution of electron forcing is, therefore, important for more accuratemodeling of its atmospheric and climate e ff ects.The quantification of relativistic electron precipitation has, however, proved di ffi cult due to particle detec-tor challenges [see, e.g., Nesse Tyssøy et al. , 2016]. In addition, radiation belt electrons usually have stronganisotropic pitch angle distribution that needs to be accounted for when considering their impact upon theatmosphere [
Rodger et al. , 2013;
Nesse Tyssøy et al. , 2016]. In this study, we optimize the data from the MediumEnergy Proton and Electron Detectors (MEPED) on the Polar Orbiting Environmental Satellite (POES) NOAA-18,taking into account detector degradation, proton contamination, and combining data from both the 0 ∘ and90 ∘ telescopes to achieve a better estimate of the true loss cone fluxes. We also use electron fluxes with energy > Nesse Tyssøy et al. , 2016]. Whereas
Andersson et al. [2014b] used all available POES satellites, we onlyuse NOAA-18, which is traversing the same local time as the Aura satellite making it possible to study thelocal e ff ects of the energy deposition by relativistic electrons on OH. The data and its application are furtherexplained in the next section.Since most studies have focused on geomagnetic activity during solar maximum, it is paramount to get adeeper understanding of the contribution of EEP on HO X also during solar minimum. Therefore, we target thesolar minimum year of 2008, where a sequence of weak to moderate storms triggered by HSSWS occurred.The low intensity of the recurrent storms implies that we need to carefully consider the role of the changingbackground dynamics upon the OH distribution. In addition to OH mixing ratios, the Aura MLS provides mea-surements of temperature, water vapor (H O), and geopotential height (GPH) which reveal the backgroundZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 1.
A plot showing the footprints of Aura (dark blue) and NOAA 18 (light blue) in local time and geographicallatitudes for year 2008. Midnight is at the bottom and dusk to the left of each plot. state of the atmosphere. Thus, extracting information on both the longitude and altitude distribution enablesus for the first time, to separate the OH variability caused by EEP and by atmospheric dynamics. The resultinganalysis is given in section 3, and the subsequent discussions and conclusion follows in sections 4 and 5.
2. Materials and Methods
The Microwave Limb Sounder (MLS) is one of the four instruments on board NASA’s Aura satellite [
Schoeberlet al. , 2006]. It is in a near-polar Sun-synchronous orbit at 705 km altitude, scanning the atmosphere up togeographic latitudes 82 ∘ N/S with about 14 orbits per day (period ∼
100 min). The MLS measures naturallyoccurring microwave thermal emissions from the limb of the Earth’s atmosphere to remotely sense verticalprofiles of atmospheric constituents [
Schoeberl et al. , 2006;
Waters et al. , 2006].In this study we use Aura/MLS level 2 files version 4.2x for the year 2008 screened as per
Livesey et al. [2015].Only nighttime observations with solar zenith angle (SZA ) > ∘ are considered to make sure no sunlightilluminates the sampled atmosphere below 100 km [ Pickett et al. , 2006]. At night without solar radiation, thedata should typically show low values of background OH, which makes the detection of OH enhancementsdue to EEP e ff ects easier. In the latitude range of the radiation belts, nighttime Aura measurements occur atlocal solar time (LST) 02:15-03:30 in the NH during 2008. In the SH, however, Aura measurements are from LST15:26 to 01:18. For SH nighttime observations we use LST 22:00-01:18.The temporal resolution of Aura MLS data is ∼
25 s. The vertical and horizontal resolution of OH measurementsis 2.5 km and 165 km, respectively, within mesospheric altitudes (60 – 80 km). O , H O, temperature, and GPHhave coarser and variable vertical/horizontal resolutions within mesospheric altitudes [see
Livesey et al. , 2015].The geometric height, z , can be expressed using the pressure altitude as z = − H ln ( PP s ) (1)where H is the atmospheric scale height ( ∼ Brasseur and Solomon , 2005], P s is a reference pressure(1000 hPa) and P is the pressure level given in the MLS data. In 2008, one of the five NOAA/POES satellites, NOAA 18 scanned the Earth at approximately the same localtimes as the Aura satellite (see Figure 1). This implies that the particle fluxes measured by MEPED/NOAA 18deposited their energy close in both time and space to the measurements performed by Aura. Considering theshort lifetime of OH below 80 km, the EEP impact on OH is considered to be a local e ff ect.The MEPED consists of two proton and two electron telescopes viewing almost perpendicular to each other.The electron and proton telescopes pointing radially outward are often named the 0 ∘ detectors. At highZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics Table 1.
MEPED Proton and Electron Energy Channels [
Evans and Greer , 2000]Proton Energy Channels Electron Energy ChannelsChannel Energy Range (keV) Channel Energy Range (keV)0/90 P1 30 to 80 0/90 E1 30 to 25000/90 P2 80 to 240 0/90 E2 100 to 25000/90 P3 240 to 800 0/90 E3 300 to 25000/90 P4 800 to 25000/90 P5 2500 to 69000/90 P6 > latitudes they will view particles within the loss cone and have therefore previously been used to representthe precipitating fluxes [ Verronen et al. , 2011;
Andersson et al. , 2012, 2014b, 2014a]. Electrons with energiescapable of precipitating into the middle atmosphere ( >
30 keV) have, however, often a strongly anisotropicpitch angle distribution, which decreases toward the center of the loss cone. The 0 ∘ detector looking closeto the center of the loss cone will therefore provide an underestimate of the precipitating fluxes. The otherelectron and proton telescope, called the 90 ∘ detector, view particles near the edge or outside the loss cone.Therefore, the 90 ∘ detectors will measure higher fluxes compared to the true precipitating fluxes. To overcomethis challenge, we combine data from both electron telescopes to estimate the electron fluxes over the entireloss cone. The 0 ∘ and 90 ∘ electron fluxes were fitted to the solution of the Fokker-Planck equation for pitchangle di ff usion of energetic particles [ Kennel and Petschek , 1966]. We take into account the detector sensitivitywhen the directional flux varies over the acceptance solid angle of the telescope [
Nesse Tyssøy et al. , 2016].We correct the electron data for contamination by protons [
Yando et al. , 2011]. The degradation of the protondetector is taken into account by applying the new correction factors developed by
Sandanger et al. [2015].During solar minimum, when no SPEs occur, there will be insignificant high-energy proton fluxes detectedby the MEPED proton telescope which can be confirmed by the P4 and P5 energy channels (see Table 1).Considering that the highest-energy channel of the proton detector (P6) is responsive to relativistic electrons,we utilize this contamination e ff ect to get a quantitative measure of electron fluxes larger than 1 MeV [ Yandoet al. , 2011]. The particle fluxes are sampled every 2 s. However, for purposes of comparing the particle fluxeswith composition data from Aura, we average the fluxes over 1 ∘ latitude bin equivalent to about 16 s.Combining the electron and proton channels (E1, E2, E3, and P6), we achieve a di ff erential electron spectrumcovering energies from 50 to 1000 keV. We use the electron spectra to calculate the energy deposition asa function of altitude. In these calculations, we use the cosine-dependent Isotropic over the DownwardHemisphere (IDH) model of Rees [1989]. This is a range-energy analysis based on a standard referenceatmosphere (COmmittee on SPAce Research International Reference Atmosphere 1986).
In this study we use IMF and solar wind plasma parameter data for the year 2008 downloaded from the Coor-dinated Data analysis Web (http://cdaweb.gsfc.nasa.gov./istp-public/). We focus on the B z , V SW , and solar windflow pressure ( P SW ), at 1 h resolution in geocentric solar magnetospheric (GSM) coordinates. Originally, thedata are measured by either WIND or the ACE satellite and time shifted to the Earth’s bow shock nose. The Dst and AE indices also have a 1 h resolution, while the Kp index has a 3 h resolution.
3. Results
In Figure 2, daily means of Kp , Dst , AE , B z , V SW , and P SW are plotted for the whole year 2008, during which twoHSSWS associated with the 27 days solar rotation period are indicated by the vertical dashed and dotted lines.The daily mean Dst index and IMF B z clearly show decreases corresponding to the arrival of the HSSWS, indi-cating the recurrent storm activity experienced by the magnetosphere. All the storms were weak to moderatebased on the Dst index classification by
Loewe and Prölss [1997].In the period January to April, both the HSSWS were geoe ff ective. In May, the signature of the HSSWS in thegeomagnetic indices is weak. In the daily mean Dst , for example, this period shows
Dst > −
15 nT. Both Kp and AE also display low values in May. From June to September, one of the HSSWS becomes geoe ff ective againZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics Figure 2.
Daily means of Kp , Dst , AE , B z , V SW , and P SW during 2008 from top to bottom, respectively. The black verticaldashed and dotted lines indicate the 27 days recurrent period of the HSSWS. The red dashed lines show the zero linein the Dst index and B z . (dashed vertical line). The signature from this HSSWS disappears in October. The other HSSWS (dotted verticalline) also becomes geoe ff ective again in September. At the end of the year, in November and December, thesolar wind and geomagnetic signatures from the two HSSWS are weakened. To investigate the seasonal longitudinal distribution of OH, we calculate the longitudinal OH running mean(5 ∘ longitude window) within a latitude band of 40 ∘ to 80 ∘ for altitudes 60 to 81 km in both hemispheres forwinter, spring, and autumn. Note that due to the SZA selection of > ∘ , the data coverage for summer stopsat about 65 ∘ N and 70 ∘ S. The months January and February are considered winter in the NH and summer inthe SH. While the months June, July, and August are summer in the NH and winter in the SH. The other seasonsfollow accordingly. However, the month of December 2008 is not included in this study. The seasonal lon-gitudinal distribution of the OH volume mixing ratio (VMR) is shown in Figure 3. All seasons show two OHmaxima located at about 67 and 73 km in both hemispheres. Summer months exhibit another OH maxi-mum located at approximately 78 km in both hemispheres. Generally, the OH VMR decreases with decreasingaltitude. For winter in the NH, there is high OH VMR within longitudes 150 ∘ W – 100 ∘ E for altitudes 81 km toabout 76 km. In the SH winter, the high OH VMR at altitudes 76 – 81 km is almost homogeneously distributedbut strongest within longitudes 180 ∘ W – 60 ∘ W and 120 ∘ E – 180 ∘ E. High OH VMR are still visible in spring withinapproximately the same longitude region. Autumn also shows high OH VMR within approximately the samelongitudes in the NH but weaker compared to winter. In the SH, there is a stronger OH concentration withinlongitudes 115 ∘ – 0 ∘ W than during winter, with signatures up to below 76 km. Generally, these kind of high OHconcentrations at 67 – 81 km occurred during January, February, March, October, November, and December inthe NH, while in the SH, they occurred during April to September ( ∼ autumn to winter in both hemispheres). ff ects on OH We present the hemispherical distribution of disturbed and quiet conditions for the energy deposition andOH during winter in the NH and autumn in the SH averaged over altitudes between 75 and 78 km in 2008.Disturbed/quiet conditions were sorted based on daily mean energy deposition by EEP at particular altitudes.Days, for which the daily mean energy deposition is greater than the annual mean energy deposition at aparticular altitude range, were considered to be disturbed time. Whereas days, for which the daily meanZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 3.
OH running mean (5 ∘ longitude window) for the altitude range of 60–81 km. Autumn, winter, and springmonths cover a latitude band of 40 ∘ to 80 ∘ N/S for year 2008. Summer months do not extend to 80 ∘ latitude due tothe SZA selection of > ∘ . (top row) The NH and (bottom row) the SH. (first column to fourth column) Winter, spring,summer, and autumn. The months January and February are considered winter in the NH and summer in the SH.Whereas the months Jun,e, July and August are considered summer in the NH and winter in the SH. The other seasonsfollow accordingly. energy deposition is less than the annual mean energy deposition at a particular altitude range, were consid-ered to be quiet time. The data are sorted according to geographical latitude and longitude in such a way thateach 5 ∘ latitude by 10 ∘ longitude bin shows the running mean (over a window of three bins) of the energydeposition or OH/temperature/H O/GPH within that bin.We present maps of an average at 75 – 78 km for the energy deposition and OH VMR. (For comparative studieswith temperature, H O and GPH, we use data at 75 km.) All maps cover geographical latitudes 40 ∘ – 80 ∘ N/S.The first results, sorting by season, show that winter (January and February) in the NH and autumn (March,April, and May) in the SH exhibit the longitudinal signatures in OH reported by
Andersson et al. [2014b] moreclearly than in the other seasons. This is consistent with the strength of the geomagnetic activity. Althoughwinter is the best season to observe EEP e ff ects in the atmospheric OH due to low background levels,EEP-related changes in OH were stronger in the SH autumn compared to winter in 2008. Consequently, wefocus on the winter and autumn months for the NH and SH, respectively, as shown in Figures 4 and 5. As themaps of NH and SH cover di ff erent months, they also cover di ff erent EEP events throughout 2008. Figure 4 shows the night-time mean energy deposited between 40 ∘ and 80 ∘ geographic latitudes duringdisturbed and quiet conditions for winter and autumn in the NH and SH, respectively. The energy data areaveraged between 75 and 78 km. The storm time NH map is fairly homogeneous with maximum valuescovering northern Russia and part of North America at longitudes (30 ∘ E – 130 ∘ W) and minimum values overScandinavia to North America (60 ∘ W – 30 ∘ E). In the SH, the energy is deposited almost homogeneously withinthe latitude range of the radiation belts. The energy deposition during storm times is approximately 1 orderof magnitude greater than during nonstorm times in both hemispheres.
Figure 5 shows the night-time mean OH maps for disturbed and quiet conditions during winter in the NH andautumn in the SH. In the NH, OH shows clear enhancements poleward of latitude 55 ∘ N CGM during storm con-ditions, with local maxima within longitudes 90 ∘ – 10 ∘ W and 70 ∘ E – 130 ∘ W which
Andersson et al. [2014b] refersto as the North America and northern Russia hot spots. The OH enhancement over North America is, however,also present during quiet times within longitudes 90 ∘ – 0 ∘ W, and it is stronger over the North Atlantic Oceanthan over North America. (There is also a region of high-OH volume mixing ratio at 60 – 90 ∘ E about 40 – 45 ∘ N( L ∼ . − ) corresponding to the inner radiation belt. As our focus is the auroral and subauroral latitudes, weconsider this feature outside the scope of the current paper.)In the SH, there appears to be a local OH enhancement over West Antarctica both during disturbed and quietconditions which Andersson et al. [2014b] refers to as the Antarctic Peninsula hot spot. OH enhancement isZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 4.
Mean nighttime energy deposition during (top row) disturbed and (bottom row) quiet conditions at altitudes75–78 km for (left column) winter (January– February) at LST 2 ≤ LST < ≤ LST ≤ ∘ latitude by 10 ∘ longitude binbetween 40 ∘ to 80 ∘ N and longitudes 180 ∘ W to 180 ∘ E. The white line shows the approximate location of 55 ∘ N/S CGMlatitude. seen at all longitudes during disturbed times and within longitudes 150 ∘ – 0 ∘ W during quiet conditions. TheWest Antarctica hot spot appears, however, to be unbound by the geomagnetic location of Earth’s radiationbelts but seems rather to be more geographically constrained.Note that the local OH enhancements in Figure 5 generally cover smaller longitude ranges than the hot spotsseen by
Andersson et al. [2014b] but are located within the same regions. Both the current presentation andthe maps presented by
Andersson et al. [2014b] show OH features (signatures) unconstrained to the geomag-netic latitude location (or footprints) of the Earth’s radiation belts, indicating potential signatures due to thebackground dynamics.
We investigate the possibility that atmospheric dynamics is responsible for some of the observed OHenhancements. Figure 6 shows the temperature, H O and GPH (top row to bottom row) for quiet timeconditions in the NH (left column) and SH (right column), respectively. In the NH, there is a temperatureenhancement between longitudes 90 ∘ W and 110 ∘ E and an H O minimum. The temperature maximizes inZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 5.
Mean nighttime OH during (top row) disturbed and (bottom row) quiet conditions at altitudes 75– 78 km for(left column) winter (January– February) at LST 2 ≤ LST < ≤ LST < ∘ latitude by 10 ∘ longitude bin between 40 ∘ to 80 ∘ Nand longitudes 180 ∘ W to 180 ∘ E. The white line shows the approximate location of 55 ∘ N/S CGM latitude. approximately the same region where the North America OH maximum is located. A depression in the GPH isseen within longitudes 180 ∘ W – 0 ∘ W, displaced by approximately 90 ∘ westward from the location of the tem-perature maximum and H O minimum. In the SH, the same features are seen in the temperature, H O, andGPH for the West Antarctica maximum, all located over approximately the same region over West Antarcticawithout a shift in the location of the GPH minimum. The North America and West Antarctica OH enhance-ments seem to follow more closely the region of intersection of the depression in the GPH with temperaturemaximum and H O minimum.During winter time, especially in the NH, planetary wave activity is known to play an essential role in the back-ground dynamics. To determine the role of planetary wave activity upon the OH composition, Figure 7 showsthe OH (a), EEP (b), and temperature anomaly (c) in a geographic latitude band 60 ∘ – 70 ∘ N for quiet (rightcolumn) and disturbed (left column) conditions. We sorted the data based on the energy deposition as insection 3.3. The temperature anomaly is the di ff erence from the mean over the period. Then we derive thequasi-stationary planetary wave numbers 1 (S1) and 2 (S2) shown as the superposition of two sinusoidalcurves fitted to the longitudinal temperature anomaly Figures 7c and 7d.ZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics Figure 6. (top row to bottom row) Mean nighttime temperature, H O, and GPH during quiet conditions at altitudes75 km for (left column) winter (January– February) at 2 ≤ LST < ≤ LST < ∘ latitude by 10 ∘ longitude bin between 40 ∘ to 80 ∘ S and longitudes 180 ∘ W to 180 ∘ E. The white line shows the approximate location of 55 ∘ N/S CGM latitude.
ZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics a)b)c)d) a)b)c)d)
Figure 7.
Longitudinal variation of OH and energy deposition averaged between altitudes 75– 78 km during winter for the latitude band of 60–70 ∘ N in 2008for disturbed and quiet time conditions. For comparison with background atmospheric dynamics, Figures 7c and 7d show the quasi-stationary planetary waveactivity for wave number 1 and 2 derived from temperature between latitudes 60 and 70 ∘ N at an altitude of 75 km during winter for year 2008. (top row tobottom row) (a) OH (blue). Running mean of 30 ∘ longitude (black). (b) Energy deposition (blue). Running mean of 30 ∘ longitude (black). (c) Temperature anomaly(blue) fitted with a sinusoidal curve for the superposition of planetary waves S1 and S2 (black). (d) Sinusoidal curve fitting for planetary wave numbers 1 (S1)and 2 (S2). The red solid line is the zero line. The red dotted lines show the standard error of the mean. During NH winter in 2008 the S1 and S2 peak around the same longitudes during both quiet and disturbedconditions. S1 has its maximum around 0 ∘ E. S2 has its maximum around 140 ∘ E and 40 ∘ W. The amplitudes areslightly higher during disturbed conditions. However, this may well be due to the random sample of days forthe storm time conditions and unrelated to the EEP. During disturbed conditions S1 and S2 have an amplitudeof about 19 K and 7 K, respectively. The superposition of the two waves, S1 + S2 is large at 75 km driving alongitudinal variability in temperature of around 23 K with a maximum around 25 ∘ W and a minimum closeto 155 ∘ W.The OH during disturbed conditions shows that in addition to changes correlated with the temperaturevariations, an additional source is present that coincides with the distribution of EEP. As Figure 4 shows,the EEP in a geographic latitude band between 60 ∘ – 70 ∘ N samples only part of the auroral oval due tothe o ff set between geomagnetic and geographical coordinates. The energy deposition shows high values( > − s − ) between longitudes 25 ∘ E and 135 ∘ W corresponding well with the increase of OH in thisregion. In the region of little EEP, the OH appears to track the temperature variations as it does during quiettimes. To better reveal the EEP impact on the OH density, we have subtracted the quiet time conditions fromthe disturbed conditions as shown in Figure 8. The longitudinal OH behavior is generally in phase with theenergy deposition. The only exception is the longitude interval 30 ∘ – 60 ∘ E which corresponds to regions wherethe auroral oval intersects a descending area of low H O mixing ratios (see Figure 6). Note that the negativeOH values are due to the fact that some of the MLS observations are noisy in nature. Ignoring such valuesZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 8.
The di ff erence between disturbed and quiet time at 75–78 km within a latitude band of 60–70 ∘ N forwinter in 2008. The red dotted lines show the standard error of the mean. (a) Longitudinal OH (b) longitudinalenergy deposition. will automatically introduce a positive bias into any averages made of the data as part of scientific analysis[
Livesey et al. , 2015].
4. Discussion
During geomagnetic storms radiation belt particles can be accelerated to high and possibly relativistic ener-gies that precipitate deep into the atmosphere, causing enhancements of OH. EPP leads to production of HO X species through ionization, dissociation, and dissociative ionization of the most abundant chemical speciesin the atmosphere (N and O ). The abundance of H O below 80 km facilitates the formation of large watercluster ions which recombine with electrons forming ∼ X per ionization [ Solomon et al. , 1981;
Sinnhuberet al. , 2012]. EEP appears to impact the geomagnetic latitudes 55 ∘ – 72 ∘ N/S CGM as illustrated by Figure 4[see also
Andersson et al. , 2014b].The region of high OH VMR over northern Russia is collocated with the region of enhanced EEP energydeposition. The region of high OH VMR over North America and the North Atlantic Ocean, prominent duringboth disturbed and quiet conditions is not evident in the energy deposition. In the SH, we find a similarsituation. The OH concentration maximizes over the West Antarctica, while the energy deposition is found tobe rather homogeneously distributed with longitude. In the following, we will discuss the extent to which thelocal OH enhancements in the two hemispheres are related to the EEP energy deposition and the role of thebackground atmosphere on the longitudinal distribution of OH.
For January 2005 to December 2009,
Andersson et al. [2014b] found two regions of high OH concentrationduring high EEP in the NH (51 days of data in total). These regions are the North America and northernRussia, which were attributed to EEP forcing. Our energy deposition maps based on the electron fluxes fromMEPED/NOAA 18 maximizes as illustrated in Figure 4 over the northern Russia region. We do not, however,find evidence based on the energy deposition for an EEP-produced OH concentration enhancement overNorth America. The OH enhancement is prominent in both disturbed and quiet times. The energy depositionpattern cannot explain the distinct pattern found in the OH concentration.ZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics
Figure 9.
Ratio of magnetic field strength in the NH to themagnetic field in the SH found by magnetic field tracing usingthe IGRF model.
In addition to covering a shorter time period,some of the discrepancies between ourenergy deposition map and the map pro-vided by
Andersson et al. [2014b] might bedue to the fact that they use the 0 ∘ detectorE1 channel measuring electrons of energylarger than 30 keV. The E1 channel countsmight be dominated by the lower energiesthat will not penetrate below 80 km. As weestimate the energy deposition at the res-pective heights we use the information fromall the energy channels. We also limit theelectron flux analysis to NOAA/POES 18which is close in both time and space to theAura OH retrieval. Andersson et al. [2014b]used multiple spacecraft at di ff erent MLTregions compared to the MLT region of theOH retrieval from Aura. Barth et al. [2001] investigated the geomagnetic longitude dependence observed in NO X at 106 km producedby auroral electrons. The most characteristics feature in their result is a minimum in a region above Scandinaviaand Greenland. A possible candidate for these longitudinal variations in the NH is the asymmetries in theEarth’s magnetic field. Electrons drifting around the Earth to the weak field associated with the SAMA are lostto the atmosphere. This depletes the electrons throughout the anomaly region. Barth et al. [2001], therefore,suggested that we are seeing the “normal” electron precipitation west of the weak magnetic field region buta much weakened precipitation of electrons within the weak magnetic field region itself and eastward of it.Figure 9 shows the square root ot the ratio: B
North /B South , where B
South is found by magnetic field tracing usingthe IGRF model. Based on this theory, the low magnetic field ratio over North Asia and Alaska would imply thatthere will be more electron precipitation there than in the SH for particles bouncing over the same magneticfield lines. Therefore, the lower magnetic field ratio and the associated electron precipitation may explainthe presence of the northern Russia hot spot. On the other hand, the magnetic field ratio is relatively higherabove Scandinavia and Greenland. As the high B
North /B South ratio overlaps with the large OH VMR, it does seemunlikely that the North America/North Atlantic OH maximum is due to EEP forcing.
In the SH autumn, there is a maximum in the OH density above West Antarctica during both disturbed andquiet conditions. The West Antarctica OH enhancement and its persistence during quiet times might beexplained by the weaker magnetic field in this region that allows a steady drizzle of radiation belt electrons.Electrons that were mirroring at other longitudes could be lost here as they penetrate deeper in the atmo-sphere and interact with the denser atmosphere. The West Antarctica hot spot is also visible in winter andspring starting at approximately 70 – 73 km altitude during quiet times (not shown).According to
Horne et al. [2009], the e ff ects on atmospheric chemistry due to relativistic electron precipitation(REP) are more likely to occur in the SH poleward of the SAMA region, because > Andersson et al. [2014b].Although the West Antarctica OH enhancement can be explained by the weaker magnetic field in that region,its features seem more constrained by geographical rather than geomagnetic location. There is a possibilitythat in addition to the above mentioned causes, atmospheric dynamics may also play an active role in theformation of the West Antarctica OH maximum.
Elevated temperatures and dry air at 75 km appear to coexist with the longitudinal region of elevated OH VMRmeasured by Aura as shown in Figure 6. The high temperatures are associated with descending air motions,bringing down dry air from higher altitudes. The descent will then also bring down odd oxygen (O and O )and atomic hydrogen (H). Atomic oxygen has a large concentration gradient from the middle mesosphere tothe mesopause, which makes the atomic oxygen highly variable in the presence of vertical motion. Even aZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics small displacement can generate large changes in the mixing ratio [ Smith , 2004]. Thus, the O mixing ratio at75 and 78 km will increase if O -rich air from the secondary O maximum is brought down. Winick et al. [2009] found elevated OH Meinel emissions related to the vertical displacements associated withSSWs. The vertical displacement of the OH airglow layer from 87 km to 78 km was observed at the time ofmajor SSW from January 2009 [
Shepherd et al. , 2010]. Assuming that this transport is fast enough to maintainan O density, it could lead to an enhanced production of vibrationally excited OH (OH*) at lower than normalaltitudes [
Winick et al. , 2009]. The pressure here will be su ffi cient to allow a third body reaction creating O : O + O + M → O + M (2)Then, O will react with atomic oxygen forming OH by the exothermic reaction: H + O → OH ∗ + O (3)The variability of the ground state OH measured with the Aura MLS instrument during periods of SSWs by Damiani et al. [2010] further corroborates the interpretation by
Winick et al. [2009]. The OH ∗ is deactivatedeither by photon emissions in the Meinel band (observed in the airglow) or by collisional quenching [ Brasseurand Solomon , 2005]. The latter depends on the density and therefore becomes more important at lower alti-tudes. The enhanced level of atomic hydrogen might also contribute to the conversion of HO into OH by thereaction [ Brasseur and Solomon , 2005]: H + HO → OH + OH (4)This potential chemical scheme seems consistent with our observation. As pointed out earlier, the quiet timelongitudinal OH enhancement is only apparent above 73 km, supporting the potential e ff ect of a steep gradi-ent in the mixing ratios of odd oxygen. In January and February 2008 the polar vortex was displaced from itsclimatological position over the pole by an SSW [ Medvedeva et al. , 2012]. There could therefore be a systematiclongitudinal response associated with SSW also in our observations. However, considering the complex chem-istry of the odd hydrogen family and its dependence on temperature, pressure, and mixing ratios, modelingstudies and/or additional satellite data are needed in order to quantitatively assess the described features.Also, the H O mixing ratios might impact the OH production e ffi ciency. At altitudes above about 65 km, HO X production depends on the ionization rate, and the atomic oxygen and H O densities [
Solomon et al. , 1981].The main process during the formation of HO X due to EPP events is the uptake of H O, forming large clusterions and subsequent recombination with electrons. However, if the H O mixing ratios are reduced by a fewppb (parts per billion), water cluster reactions may be cut o ff by dissociative recombination of the intermedi-ates (e.g., O + , O + H O) with electrons. In this case, even the natural electron concentration may be su ffi cientto reduce the e ffi ciency of the HO X production rate [ Crutzen and Solomon , 1980]. This implies that the impactof EEP might also depend directly on the dynamical background as the EEP deposited over North Americamay be less e ffi cient in producing OH due to the low H O mixing ratios compared to the EEP over northernRussia which has higher H O mixing ratios. This explanation also applies to Figure 8 for longitudes 30 ∘ – 60 ∘ E,and it is briefly mentioned in subsection 3.4.Planetary waves are more prominent during wintertime, in particular in the NH. This is evident in Figure 7which shows that the general longitudinal trend of OH VMR within a latitude band of 60 ∘ – 70 ∘ N closely followsthat of the superposition of the planetary waves S1 + S2 especially during quiet time conditions. Quasistationary planetary wave activity is of approximately the same amplitude and phase during both disturbedand quiet times (Figures 7c and 7d). Quasi-stationary planetary waves drive the background OH densitywith a peak that persists during both disturbed and quiet time conditions regardless of the strength of theenergy deposition. The North America OH maximum which is present during both disturbed and quiet timeconditions is a feature attributed to quasi-stationary planetary wave activity.Any EEP-induced OH production will be an addition onto the already existing background OH. The OHenhancement due to EPP is visible for strong energy deposition during disturbed conditions. The visibilityof the OH variability due to EEP depends on the background OH and the strength of the energy deposited.Hence, during disturbed conditions, there exist two peaks in the OH density: one due to the backgrounddynamics and another due to EPP. Figure 7 does not show an exact one to one relationship between theZAWEDDE ET AL. IMPACT OF ELECTRON PRECIPITATION ON OH ournal of Geophysical Research: Space Physics energy deposition and the OH density in regions (longitudes 25 ∘ E – 135 ∘ W) of high energy deposition wheresuch a relationship may be expected. As already discussed, the OH production e ffi ciency is a ff ected by theH O density which may be variable over the latitude band under consideration.Therefore, we believe that in our results for the winter 2008 NH, it is only the OH maximum over northernRussia that is attributed to EEP forcing, while the North America hot spot is mainly a consequence of a dynam-ical atmosphere. The West Antarctica hot spot in the SH is attributed to both EEP and atmospheric dynamics.This mechanism has not been considered by
Andersson et al. [2014b]. Based on the increased level of OH theyfind over Greenland, which does not seem to be restricted in geomagnetic latitude, a planetary wave e ff ectmight be present in their data as well. Although our analysis supports the conclusion that even small stormscan impact the mesospheric OH, a quantitative assessment needs to firmly establish the dynamically varyingbackground. This could potentially be achieved by applying a multilinear regression analysis of OH with GPH,O , and H. The production of OH due to EEP could then be included with a potential dependency on H O.
5. Summary and Conclusions
OH enhancements due to EEP were seen poleward of CGM latitudes 55 ∘ N/S with regions of local maxima: thenorthern Russia and West Antarctica maxima. We find that the West Antarctica maximum might be explainedby a combination of EEP and the weak magnetic field in this region. Even in the cases with geomagnetic quietconditions, the weaker magnetic field in this region causes a steady drizzle of radiation belt electrons, hencecausing an OH “pool” in this region. In addition, atmospheric dynamics might contribute to the formation ofthe West Antarctica maximum.The North America OH maximum cannot be explained by the weaker magnetic field, since it is located ina region with relatively high magnetic field ratio. It rather appears that the North America OH maximum in2008 is due to dynamical e ff ects. Planetary waves in the polar winter induce downwelling of thermosphericair, bringing down dry air, atomic oxygen, and atomic hydrogen. The air density at mesospheric altitudes issu ffi cient to facilitate three-body reactions between atomic oxygen and O , which results in formation of O ,which again reacts with atomic hydrogen forming OH. The same dynamical features may be related to theWest Antarctica OH maximum. The northern Russia OH maximum, however, appears to be a feature relatedto EEP forcing alone.Our findings suggest that even during solar minimum, there is substantial EEP driven OH production. To quan-tify this e ff ect, the background atmospheric dynamics have to be taken into account, along with detailedknowledge of where and when the precipitation occurs. Background atmospheric dynamics are important inexplaining the longitudinal distribution of OH. References
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Acknowledgments
This study was supported by theResearch Council of Norway undercontract 223252/F50. The authorsthank the NOAA’s NationalGeophysical Data Center (NGDS)for providing NOAA data(http://satdat.ngdc.noaa.gov/),WDC Geomagnetism, Kyoto,Japan, for AE and Dst indices(http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html), SPDF GoddardSpace Flight Center for solar windparameters (http://omniweb.gsfc.nasa.gov/), and NASA GoddardEarth Science Data and InformationServices Center (GES DISC)for providing Aura/MLS data(http://mls.jpl.nasa.gov/).
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