Dec 20, 1997 - R. A. Frahm, J. D. Winningham, J. R. Sharber, R. Link, and G. Crowley ...... Baker, D. N., J. B. Blake, R. W. Klebesadel, and P. R. Higbie, Highly .... Haymes, R. C., Introduction to Space Sciences, John Wiley, New York,. 1971.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 102, NO. D23, PAGES 28,203-28,214, DECEMBER
20, 1997
The diffuse aurora: A significant source of ionization in the middle atmosphere R. A. Frahm, J. D. Winningham,J. R. Sharber, R. Link, and G. Crowley Southwest Research Institute, San Antonio, Texas
E. E. Gaines
and D. L. Chenette
Researchand DevelopmentDivision,LockheedMartin Missilesand Space,Palo Alto, California
B. J. Anderson
and T. A. Potemra
Applied PhysicsLaboratory,JohnsHopkinsUniversity,Laurel, Maryland
Abstract. Energeticelectronscan penetrateinto the middle atmospherecausing excitation,dissociation,and ionizationof neutral constituents,resultingin chemical changes.In this paper, representativeelectron spectrameasuredby the Upper AtmosphereResearchSatelliteparticle environmentmonitor are usedto determinethe relative contributionsof bremsstrahlungX rays and direct electronimpact on the energy depositionand ionizationproductionrates for altitudesbetween20 and 150 km. Above 50 km most of the ionizationcomesfrom direct electronimpact. However, in the stratospherethe energycontributedbelow 50 km is mostlydue to bremsstrahlung X rays. In the diffuse aurora the ionizationfrom the bremsstrahlungcomponentexceedsthat due to the galacticcosmicray backgroundto altitudesas low as 30 km duringgeomagnetically activeperiods.This paper demonstratesthat a diffuseauroral sourcecan input as much or more energyinto the upper portion of the lower and middle atmosphereas previously reportedfor relativisticelectronevents.The effectsof the diffuseaurora (includingboth the direct electronand the bremsstrahlung contributions)on atmosphericchemistrymay be significant. 1.
REPs consistof electronswith energiesof hundredsof keV
Introduction
to low MeVs
Energetic electronscan penetrate into the middle atmospherecausingexcitation,dissociation,and ionizationof neutral constituents,resultingin chemicalchanges.Severaltypes of precipitationcarryingenergyinto the middle atmosphere have been recognized(see review by La•tovi•ka [1996]), includinggalacticcosmicrays[Thorne,1980],solarprotonevents [Vitt and Jackman, 1996], relativistic electron precipitation (REP) events[Rosenberg et al., 1972;Calliset al., 1991b],and highlyrelativisticelectron(HRE) events[Bakeret al., 1987; Goldberget al., 1994]. Galacticcosmicrays (GCRs) produce ionizationdownto the troposphere,and the GCR background is often used as a reference againstwhich to compare other ionizationsourcesin the middle and lower atmosphere[e.g., Brasseurand Solomon,1984]. The depletion of mesospheric ozoneduringsolarprotonevents(SPEs)wasfirst observedby Weekset al. [1972].Solomonet al. [1983]explained30% ozone depletionsnear 80 km duringthe July 13, 1982, SPE in terms of the productionof odd hydrogenradicals(H + OH + HO2) whichcatalyticallydestroyozonein the mesosphereand stratosphere.The productionof odd nitrogenin the middle atmospheredue to SPEsand relativisticelectronprecipitationwas comparedwith productionfrom othersourcesbyJackmanet al. [1980],while VittandJackman[1996]surveyedodd nitrogenproductionfor all significantSPE eventsin the period 1974-1993. Copyright1997 by the American GeophysicalUnion. Paper number 97JD02430. 0148-0227/97/97JD-02430509.00
at auroral
and subauroral
latitudes
in association
with magnetospheric substorms.REP energydepositiontypically peaksin the 50-80 km region. Generally,REPs are 1-2 orders of magnitude weaker than SPEs in terms of atmosphericpenetrationand energyflux [Goldberget al., 1984]; however,becausethey occur more often than SPEs, the REP events may be the dominant in situ source of mesospheric nitric oxide at subaurorallatitudesand may alsobe important in the upper stratosphere.For the REP eventsconsideredby Goldberget al. [1984], ionization rates from direct electron impact exceed those from bremsstrahlungX rays (which mainly result from softer electronsdepositednear 100 km). Gaineset al. [1995] measuredan REP event lasting 10 days usingthe particleenvironmentmonitor (PEM) instrumenton the Upper AtmosphereResearchSatellite(UARS) and con-
cludedthattheNOyproduction in thelowermesosphere during this time period was a significantfraction (---1%) of the global annual sourcefrom N20 oxidation.NOx free radicals are also expectedto be producedby the REP in the stratosphere,leadingto the destructionof stratospheric ozone[Callis et al., 1991b]. Also at auroral to subauroral latitudes, HREs with electron
energies> 1 MeV depositenergyin the 40-80 km region,with peakstypicallynear 50-60 km altitudes[Bakeret al., 1987]. HREs have higher fluxesof the most energeticelectronsthan REPs and may possiblyhave their sourceat Jupiter [Bakeret al., 1993a];however,the distinctionbetweenHREs and REPs has not been properly quantifiedin the literature in terms of morphology and spectral characteristics.HREs occur most
28,203
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ET AL.: DIFFUSE
AURORA--MIDDLE
frequentlyduringthe approachto solarcycleminimumand are largely absent during solar cycle maximum. They affect the ionization, conductivity,electric field, and chemistryof the middle atmosphere,producingboth short-termand long-term changesin odd nitrogen and odd hydrogen(see review by Lagtovi&a[1996]).SinceHRE eventscanlastseveraldaysand recur over severalsolarrotations,their integratedeffect on the high-latitudemesosphere may competewith SPEs[Goldberget al., 1995a].Goldberget al. [1995a,b] measuredan HRE lasting 5 hours and predicted correspondingOH increasesof 40% between70 and 80 km, with associated03 reductionsof 25%.
Callisetal. [1991a,b] predicted 40%increases in NOynear30 km during HRE events,with associatedlarge depletionsin global 03. Most bremsstrahlung X raysare generated(highlyforward peaked)near 80-100 km by the decelerationof energeticelectrons impinging on the atmosphere.The X rays propagate down to balloon altitudes(--•30 km), graduallyisotropizing with increasingatmosphericdepth [Lazutin,1986].Becauseof photoelectricabsorptionand Comptonscattering,X rayswith initial energies--• 10 keV needbe considered.Thirty keV (isotropic)electrons,approximately the upper energy limit of electrostaticanalyzers,penetrate downto only --•95km altitude.Five MeV (isotropic)electrons penetrate into the upper stratosphere(40 km). The instrumentsdiscussed here fall roughlyinto two groups:thosewhich are primarily auroral instruments(National Oceanicand AtmosphericAdministration(NOAA) spaceenvironmentmonitor (SEM) and Defense MeteorologicalSatellite Program (DMSP) SSJ/4)andthosedesignedto detecthigh-energy (relativistic)electrons.The UARS PEM is designedto detectboth
ATMOSPHERE
IONIZATION
SOURCE
Rowelland Evans, 1987] and the SSJ/4sensors[Hardyet al., 1985] on the current DMSP Block 5D-2 satellitesare electrostatic analyzerswith upper energy limits of 20 and 30 keV, respectively.These instrumentswere designedto measureenergy inputsinto the thermosphere-ionosphere region.(SEM alsocontainsa pair of solid state detectorswhich measurethe total electron flux between 30 keV and 1 MeV, with thresholds
>30, >100, and >300 keV. These detectorsare primarilyfor detection of auroral oval boundaries and are not well suited for
atmospheric energydepositionstudiessinceit is not possibleto infer a unique electronenergyspectrumfrom three integral measurements). The NOAA SEM electrostaticanalyzerdata are (optionally)usedto specifyauroralenergyinputsinto the NationalCenterfor AtmosphericResearch(NCAR) thermosphere/ionosphere/mesosphere/electrodynamics generalcirculation model (TIME-GCM) [Roble,1995], which solvesfor winds,temperature,ion chemistry,and major and minor neutral speciesbetween30 and 400 km altitude. 2.3.
Electron Pitch Angle: The Loss Cone
Only electronswithin a restrictedpitch angle range, the bouncelosscone,precipitateinto the atmosphere.(The pitch angleis the anglebetweenthe electronvelocityand magnetic fieldvectors).To firstorder,electronsoutsidethe lossconeare trappedon magneticfield lines[seeHargreaves, 1992,p. 177], bouncingbetweenthe magneticallyconjugatemirror points. For relativisticelectrons,analysisof P78-1data (near 600 km altitude)indicatesthat often the energyinputsinto the atmospherecome primarily from precipitatingelectronsnear the edge of the losscone, illustratingthe inadequacyof broad angularresolutionmeasurements for precipitationstudies[Imhof and Gaines,1993]. Atmosphericionizationratesestimatedfrom electronspectra which are not measuredentirelywithin the losscone overestimate the energy deposition,as electronswithin the loss cone become depleted through atmospheric interaction [Gaineset al., 1995]. Thus a detectorflown on an orbiting spacecraftmustbe able to determineif the measuredelectrons are indeed precipitating,as the losscone orientationand angularwidth vary continuously with magneticfield line inclination and geocentricradial distance,respectively.If the magnetic mirror point is taken to be at 100 km [Haymes,1971,p. 162], then the lossconeat 100 km is 90ø,while at 600 km it is about 60ø;at geosynchronous orbit it is 3ø. Measurementsfrom geosynchronous orbit (6.6 R•r) have been used to estimate
ionization
rates of the middle
atmo-
sphereanditseffectson oddnitrogen(NOy) andozone(03) chemistry.Detectorsflown at geosynchronous orbit with fields of view >3 ømeasureboth precipitatingand trappedelectrons. As well, stochasticprocessesin the magnetospheresuch as pitch anglescatteringdue to electromagnetic wavesmay scatter particlesinto or out of the losscone,renderinganyextrapolationfrom geosynchronous orbit downinto the atmosphere problematic. 2.4.
Relativistic
Electron
Measurements
Baker et al. [1987], usingdata from 1979-053and 1982-019, suggested that precipitationinto the atmosphereis the principal lossmechanismfor relativisticelectronsat geosynchronous 2.2. Auroral Measurements orbit and discussed the implicationsfor stratospheric 03. ImBoth NOAA and DMSP operate polar-orbitingsatellites hofet al. [1991a]were ableto testthishypothesis andreported which routinely monitor the particle inputs into the auroral the first simultaneousmeasurementsof trapped relativistic zones.The SEM flownon the TIROS-NOAA satellites[Fuller- electronsat geosynchronous orbit (satellite1982-019)andpreand is described
in the next section.
FRAHM ET AL.: DIFFUSE AURORA--MIDDLE ATMOSPHERE IONIZATION SOURCE
28 205
Red: November9, 1991 (313) 1120:28UT - 1120:32 UT
Blue:January9, 1995(009)0652:37UT - 0652:41UT I
I
Ap= 119
Ap=7
•
!l"i,,
E .,-.,
0
03
-1
C•
-2
...
0
-3
Electron Spectra -7
-1
0
I
2
3
4
5
6
7
Log ElectronEnergyleVI
Plate 1. Differential numberfluxspectra precipitating nearmidnight fromthediffuse/continuous aurora. are necessary, evenat low altitudesand cipitating electrons in thelossconeat lowaltitudes (170-280 lution measurements km) fromthe S81-1satellite. Withinnarrow(1ø) latitudere- with completelatitude coverage. Calliset al. [1991a]reporteda studyof HOx andNOy progionstheprecipitating fluxwasoftenwithina factorof 10of duction in the stratosphere-lower mesosphere usingestimates thedailyaveraged trapped fluxes; however, lmhofetal. [1991a] 1977found that the daily averagedprecipitatingflux was only of electronenergydepositionfrom the geosynchronous ---0.3%of thegeosynchronous flux.Imhofetal. [1991b]studied 007, 1979-053,and 1982-019satellites,for whichtheyassumed fluxtobeathird (•)ofthetrapped flux. The highlyrelativistic electrons measured fromS81-1nearthetrap- theprecipitating particleanalyzer(CPA) [Higbieetal., 1978]andspecpingboundary in orderto quantify betterhowmanyelectrons charged areprecipitated intothe Earth'satmosphere. On oneof the trometerfor energeticelectrons(SEE) [Bakeret al., 1986] on thesesatellites hadfieldsof viewof 5.6ø and daysof strongest precipitation the totalnighttimeinputinto experiments (thelossconeatgeosynchronous orbitis•--3ø). the atmosphere from>1 MeV electrons wasstillan orderof 30ø,respectively Thus the estimates of precipitating electron flux and energy magnitude lessthantheratesestimated byBakeretal. [1987] rates,andthecorresponding effects on03 andodd from geosynchronous measurements. Herreroet al. [1991], deposition from rocket measurements of relativistic electrons at Poker
nitrogen NOy,obtained byCallisetal. [1991a, b] should be
Flat,measured precipitating fluxesa factorof 5-25 lowerthan thosemeasuredat the sametime by the geostationary GOES satellite[Herreroet al., Figure6]. Imhofet al. [1990]andImhofand Gaines[1993]reported measurements bytheenergyelectron monitor(EEM) (68keV to 1.12MeV) andpenetrating radiationmonitor(PRM) (59 keV to 1 MeV) detectors flownon the P78-1satellite.PRM
considered upperlimits[Gaineset al., 1995]. Callisetal. [1996a,b] considered theeffectsof precipitating
performed fine-resolution measurements at 3.5ø half angle. ImhofandGaines[1993]concluded thatmeasurements of relativisticelectronenhancements at geosynchronous altitudes
electrons onmesospheric NOyanddownward transport of NO usingenergetic electrondatafromthelow-energy ion compositionanalyzer(LEICA) and protonand electrontelescope
(PET),onthesun-synchronous polar-orbiting SAMPEX[Baker et al., 1993b]satellite.LEICA is a time-of-flight massspectrometerdesigned to measure solarandmagnetospheric heavy ions[Masonetal., 1993]overa 24ø x 20øfieldof view.It also measureselectronswith E > 25 keV. The PET [Cooket al., 1993]measures electrons withE > 0.4 MeV overfieldsof
maynotrevealmaximum enhancements in precipitating fluxes whichmayoccurat otherL values[Mcllwain,1961]because of view of 58ø or 90ø.Sinceboth LEICA and PET were designed particles,the combinadrift.Theyconcluded thatprecipitation mayoftenbeprimarily to measuresolarand magnetospheric tion of instrument pointing, field of view, andspacecraft attifrom electronsnear the trappingboundary,fine angularreso-
28,206
FRAHM ET AL.: DIFFUSE AURORA--MIDDLE
ATMOSPHERE
IONIZATION
SOURCE
tude restrictsthe timeswhen electronspectraare sampled the loss cone or better. In order to cover the entire 0ø-180 ø angularrange,HEPShasfourzenithandtwonadirtelescopes, Calliset al. [1996b]pointedout that PET samplesboth mountedat _+15ø, 45ø, 90ø, and _+165 ø from the local zenith, precipitatingand trappedelectrons.They deviseda strategy measuringwithin a 30ø full-width cone. MEPS has five zenith usingmeasurements withinthe SouthAtlanticanomalyto nor- and three nadir detectorsmeasuringfrom a 5ø x 20ø rectanmalize their high-latitudetrappingzone measurements by gulararea,wherethe 5øis in the directionof the pitchangle. computingdaily averagesof the integralelectronfluxes,which The MEPS detectorsare mountedat -158.7 ø, -23.7 ø, 6.3ø, are assumedto have separateexponentialdistributionsabove 21.3ø, 36.3ø, 66.3ø, 126.3ø, and 156.3ø from the local zenith. A andbelow1 MeV. The PET fluxesare thenextrapolated down high-precision vectormagnetometer (VMAG) provides referto 0.1 MeV, althoughCalliset al. [1996a]notethat this(sub- ence magnetic field measurementsto determine the electron stantially)underestimates the low-energyelectronflux and pitch angles.Further information on PEM, measurementunNOyformationin theuppermesosphere (90 km) compared to certainties,and the procedureusedto generateatmospheric our PEM measurements (E > 5 eV) for the sameperiodand energydepositionand ionizationrate profilesfrom the eleclatitudes. On thebasisof thisextrapolation, Callisetal. [1996a] tron measurements is givenby Sharberet al. [1996]. concludedthat columnNO changesare dominatedby inIn addition to HEPS, MEPS, and VMAG, PEM also concreasesabove80-85 km becauseof precipitatingelectrons tainsthe forwardlookingatmospheric X ray imagingspecwith E < 1 MeV. We note, however,that this atmospheric trometer(AXIS), whichmeasuresbackscattered bremsstrahlregioncorresponds to (isotropic)incidentelectronswith ener- ung for the remotesensingof precipitatingelectrons.These giesE < 100 keV, the lowestenergyof the extrapolated PET electrons are not the same as those measured in situ at the measurements. In fact,in Calliset al.'s [1996a]Figure4, which spacecraft, althoughin regionsof stableprecipitation with suitshows NOyproduction ratesforthealtituderegion50-120km, ableAXIS viewinggeometry,satisfactory agreementhasbeen theentireregionabove65 km (including theproduction peak) obtainedbetweenthe two [Sharber etal., 1996],validatingour is extrapolated asthispertainsto electrons with energies E < X ray production calculations. 400 keV, belowthe lowestenergyof the PET detectorusedto purely within the losscone.
computetheseprofiles.
Callisetal. [1996b]hypothesized thatreporteddepletions of 30-35% in 03 between25 and30 km maybe dueto downward transport of NOyproduced byrelativistic electrons poleward of 750-80ølatitude.However,theydid not investigate the role of in situproduction of NOyfrombremsstrahlung X raysgenerated by the precipitatingrelativisticelectrons[Gaineset al.,
4.
PEM
Observations
Plate 1 depictstwo typicalprecipitating electronspectra measured byPEM in the diffuseauroraaroundmidnightmagneticlocaltime.One spectrum wastakenduringa moderately
strongmagneticstorm(Ap = 119, Kp = 7-, Pc = 5.9, and Dst = -203 nT), and the other spectrumwas taken at low 1995] nor by energeticelectronsin the diffuse/continuous au- magneticactivity(Ap = 7, Kp = 3 +, Pc = 2.7, andDst = rora (this study). -12 nT). (Midnight spectrawere chosento minimize the To summarize, the uncertainties in the SAMPEX studiesare effectsof dispersion, whereinthe precipitatingelectronsare asfollows.(1) The lackof angularresolutionprohibitsa direct freshlyinjectedandthe energyspectrum hasnotyet degraded measurement of whether measured electron fluxes are within
thelosscone(precipitating). (2) The lowestenergyof thePET detector(0.4 MeV) restrictsthe altituderegimeto below65 km. Finally,(3) bremsstrahlung X raysgenerated by electrons belowthe PET low-energylimit canbe the dominantsourceof ionizationbelow50 km (thiswork). 3.
UARS
Particle
Environment
Monitor
because of a varietyof magnetospheric processes). The most intensespectrum (Ap = 119) camefroma passoccurring on November9, 1991(day 1991313),at 1120:28UT (for more informationon thisstorm,seeAnderson etal. [1993],Chenette et al. [1993],Sharber et al. [1993],andCooper et al. [1995]). The lower-intensity spectrum in Plate1 (Ap = 7) comes from a passon January9, 1995(day1995009),at 0652:47UT duringsolarminimum.Both spectrashowsimilarcharacteristics with a downward trend in the differential number flux with
The PEM experiment[Winningham et al., 1993] on the increasing energyanda changein slopeoccurringin the 1-10 UARS measures electronsfrom 5 eV to 5 MeV at 63 logarith- keV range.In thispaperwe presentresultsfor energydepomicallyspacedenergies.This energyrangeenablesmeasure- sitionbelow150km in the atmosphere, corresponding to elecmentsto be made of electronsinfluencingthe Earth'satmo- tron energiesE > 1 keV, includingenergydepositedby spherebetween20 and 400 km altitude.The largedynamic bremsstrahlung X raysgeneratedby the precipitatingelecrangenecessary for sucha measurement is accomplished using trons.In Plate 1, errorbarsare indicatedby the sizeof thebox two detectorsystems, the medium-energy particlespectrome- surroundingeach point. These estimatederrors are a combiter (MEPS), an electrostatic analyzermeasuring from5 eV to nationof Poisson countingstatistics, spacecraft telemetrycom32 keV, and the solidstatehigh-energy particlespectrometer pressionerrors,and instrumental uncertainties. Where por(HEPS) measuringfrom 30 keV to 5 MeV, which differ in tionsof HEPS data are not availablebecauseof saturation,we sensitivity bya factorof 105.Sensitivity thresholds of theHEPS haveinterpolatedat theseenergiesbyfittinga Kdistribution to high-energytelescopeswere chosento detect 1-5 MeV elec- thehigh-energy portionof the spectrum; for example,in Plate tronswithgoodcounting statistics. Energyresolution (AE/E) 1,Ap -- 119 andE - 30 keV - 300keV. Assuming isotropy for HEPS variesfrom 50% at the lowestenergychannelof 30 over the downwardhemisphere,the total energyflux of the keV to 2% at 5 MeV. The MEPSenergyresolution (AE/E) is datafor theAp = 7 spectrum is 0.37 + 0.13ergscm-2 s-j, fixed at ---32%. andfor theAp = 119 spectrumthe estimatedenergyfluxis At theUARS orbitalaltitude(585km) thelossconeis about 30 _+5 ergscm-2 s-•. We have found that the diffuse auroral electrons can often 60ø wide. To distinguish betweenprecipitatingand trapped electrons, PEM wasdesigned with an angularresolutionof half be characterized by a K distribution [Williams et al., 1988]of
FRAHM
ET AL.: DIFFUSE
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28,207
which a Maxwellianis a limiting case(• -• oc)representinga be seen.Between 1122 and 1126 UT the region dominatedby completelythermalizedpopulation.The • energydistribution discrete aurora is crossed. At the end of this auroral zone crossing,from 1126 UT, the diffuse auroral zone is again (f) is crossed,ending somewherearound 1133 UT. It is not uncom-
m 3/2 f(E)= 2rr•Eo N F(•+1) • 1+
to observe discrete aurora in the diffuse auroral zone. (1) monPlate 2b is the energy-timespectrogram(presentedin the
wherem is the particlemass,N is the numberdensity,F is the gammafunction,• is a constant,E is the particleenergy,and E o is the characteristicenergy.A discussionof the possible magnetospheric origins of the • distributionis beyond the scopeof this presentstudy.Plate 1 showsthe • distributions whichbestfit the observedspectra(day 1991313:E o = 2565
sameformat) from January9, 1995(day 1995009),0649-0658 UT, when the spacecrafttraversedthe auroral zone and the lessintensespectrumof Plate 1 wasobserved.Here the spacecraft neverreachesa high enoughlatitude to enter the discrete auroral zone and the entire passshowsdiffuseaurora. (Note that the intenseelectronprecipitationbelow 100 eV are photoelectrons
and do not contribute
to the ionization
below 150
km). Even thoughthis data is from a magneticquiet time, the eV, N O = 2.12 cm-3, and• = 5.11;andday1995009: Eo 1178 eV, N O= 0.116 cm-3, and• = 4.43).Notethatbelow intensity of the low-energyfluxes are high at --•10 keV, and there are more higher energyfluxesobservedabove300 keV than in Plate 2a; however,the high-energyfluxes are not as organizedas those shownin Plate 2a. The two spectrashownin Plate 1 are presentedas typically high and typicallylow values.Spectraare observedwhich exceed even these cases.An exampleis shownin Plate 2c. Plate 2c is the energy-timespectrogram(presentedin the sameformat) from November6, 1993 (day 1993310),0929-0943 UT, when the spacecrafttraversedthe auroral zone (Ap = 26, (Maxwellian)[Christonet al., 1988]. Thus the flux of high- Kp = 4 +, Pc = 4.2, and Dst = -62 nT). Again, the spaceenergy electronsinto the middle atmosphereis greater for craft traveled only into the diffuse auroral region. Recorded finite • distributionsthanwouldbe calculatedusingthe infinite here are spectrawhich are equivalentto or exceedthe high • distribution(a Maxwellian)fit of low-energy(E _30 keV) electrons electron-photoncrosssection(CEPXS)/one-dimensional from HEPS are presented(Plate 1, top) with the energyflux tigroupLegendrediscrete(ONELD) multistreamdiscreteor(ergscm-2 s-•) colorcodedusingthe scaleat the right.Just dinatescodeof Lorence[1992],whichsolvesthe coupledelecbelow is the quality indicatorfor the high-energydata; data tron-photonBoltzmann transportequationsover the energy quality from
