Multiple Polar Cap Arcs: Akebono &Lpar;Exos D&Rpar; Observations

Radio Science,Volume 31, Number 3, Pages645-653, May-June 1996

Multiple polar cap arcs: Akebono (Exos D) observations T. Obara, T. Mukai, H. Hayakawa, K. Tsuruda, A. Matsuoka, and A. Nishida Institute of Spaceand AstronauticalScience, Kanagawa, Japan

H. Fukunishi Department of Geophysicsand Astrophysics,Tohoku University,Sendai, Japan

Center for Atmosphericand SpaceSciences, Utah State University,Logan

Abstract. Akebono (Exos D) observationsdemonstrate that polar cap arcssometimes have a fine structure,that is, multiple (doubleor triple) arcswith spacingof a few tens of kilometers.The multiplepolar cap arcsare dominantlyobserved in the nightsidepolar cap region, suggestingthat low backgroundconductance favors the appearanceof the structuredarcs. A relationshipbetween the spacingand the averageenergy of the precipitatingelectrons is investigated.Results show that a higher energyleads to a wider spacing.Akebono observationsalso showthe existenceof a downwardcurrent region embeddedbetween upward current regions (arcs). Comparison of the observationswith resultsfrom a coupledmagnetosphere-ionosphere Sun-aligned arc model is made, which showsgood qualitativeagreement between the modelingand observationalresults on the spacing-energydependence and the effect of backgroundionospheric conductance.

Introduction dawn-to-duskcomponent of the electric field is the major contributorto the negativedivE. These results The electronsreaching low altitudes in the polar suggestthat localized electron precipitation in the cap region were categorized as "polar rain" and polar cap region shouldbe Sun-alignedand that the "polar shower"by Winninghamand Heikkila [1974]. satellitetraversed a Sun-alignedarc structure[Obara Polar rain is a relativelyuniform flux of electronsthat et al., 1996]. The polar cap arc sometimeshas an is enhancedduring southwardinterplanetary mag- internal structure,that is, multiple (double or triple) netic field (IMF) conditions[Mizera and Fennell, 1978].A polar showeris a spatiallylimited (localized) arcswith spacingof a few tens of kilometers[Zhu et and more intense electron flux [Winninghamand al., 1994]. Recently, multiple polar cap arcs have attracted much interest because of the connection Heikkila, 1974] that is enhanced during northward IMF conditions[Hardy, 1984]. Polar showerswith between multiple features and the magnetosphere- very high downward fluxes have been identified as ionosphere(M-I) couplingprocesses. Such a feature, polar cap arcs.In the past decademany observations that is, bifurcationof the polar cap arc, was theoret- focusedon the measurementsof the electrodynamical ically predicted by a time-dependentM-I coupling signaturesof the polar cap arcs [Burkeet al., 1982; model of polar cap arc [Zhu et al., 1993; Sojka et al., Carlson et al., 1988; Weberet al., 1989; Valladaresand 1994]. The model calculationproduced some inter- Carlson,1991; Obara et al., 1993]. These studieshave estingresults, including a short timescalefor the arc shownthat a large velocityshear was associatedwith formation,cross-arc plasma transport, local closure of the very intense electron precipitation. From the the arc currents, and a tendencyfor a single arc to Akebono satellite observations,Obara et al. [1996] split into multiple structures[Zhu et al., 1993]. found that (1) mostof the intenseelectron precipita- Akebono(Exos D) is a polar-orbitingsatellite that tion was seen in a dive < 0 region, and (2) the performshigh-resolution measurements of both particle precipitationand electricfields. This providesus a unique observationaltool for the study of the Copyright1996 by the AmericanGeophysical Union. electrodynamicsof multiple polar cap arcs. It also Paper number 96RS00435. complements the ongoing ground-basedobserva- 0048-6604/96/96RS-00435511.00 tional and theoreticalstudies of the multiplearcs. The

645 646 OBARA ET AL.' MULTIPLE POLAR CAP ARCS

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Figure 1. Akebono energy-timediagrams for electrons(top three panels)and for ions (bottom three panels)sorted by the pitch angles.Energy range is from 10 to 16 keV for both species,and the energy is logarithmicallyscaled in the vertical axis. Orbital parametersare given in the bottom of the figure. MLAT, MLT, and ALT correspondto magneticlatitude, magnetic local time, and altitude,respectively. Bifurcated electron precipitationfluxes are observedat around 0234 UT. Observationwas made on December 26, 1989. purposeof this paper is to presentAkebono observa- 10,500 km, perigee of 270 km, and initial inclination tionsof multiple polar cap arcsand to comparethem of 75.1 ø. The Exos D satellite was renamed "Ake- qualitativelywith the theoreticalpredictions of Zhu et bono," meaning daybreak,after launch. On this sat- al.'s [1993] M-I couplingmodel of polar cap arcs.As ellite, eight scientificinstruments were installed. We demonstratedby Zhu et al. [1993], the ionosphere will useparticle data [Mukai et al., 1990], electricfield playsan importantrole in the formationof polar cap data [Hayakawaet al., 1990], and magneticfield data arcs,the appearanceof multiple arcs,and the spacing [Fukunishiet al., 1990] to investigatethe fine electro- of multiple arcs. There are still many theoretical dynamicalstructure of polar cap arcs. issuesassociated with the polar cap arc phenomena Figure 1 showsan exampleof a bifurcatedelectron remaining to be answered,and theoretical modeling precipitationin the polar cap region. The top panel will be very essentialfor further understandingof the displaysthe electronspectra for pitch angleranges of physicsof polar cap arcs.In thispaper we focuson the 00-60ø (precipitation),60ø-120 ø (perpendicular),and electrodynamicalfeatures of multiple polar cap arcs 120ø-180ø (upward), and it is followed by the ion with variousspacing and comparethe in situ obser- spectra for the same ranges of pitch angles. The vationsfrom Akebono (Exos D) with model predic- energy is logarithmicallyscaled in the vertical axis, tions in a qualitativefashion. covering an energy range from 10 eV to 16 keV for both species.At around0234 UT we can seespatially Akebono Observation of the Multiple bifurcated electron precipitation.The split is very Structure of Polar Cap Arcs clear in the peakswith respectto the energy.On the The Exos D satellitewas launchedfrom Japan on contrary, we can hardly see the separation in the February22, 1989, into an orbit with initial apogeeof low-energy(-100 eV) portion,which is essentialfor OBARA ET AL.: MULTIPLE POLAR CAP ARCS 647

the definition of the bifurcation, since the single arc (A) 12h likely splits into two structures.The spacingin this caseis about 70 km with respectto the dawn-to-dusk direction.This spacinghas been determinedby map- ping the precipitation down to 120 km. The optical signatureat 630.0-nm emissionshould be seen near 18h , 6h 250 km altitude. The reasonwhy we chosean altitude of 120 km is for comparisonwith the theoreticalwork of Zhu et al. [1993]. In the model calculation [Zhu et al., 1993] the ionospherewas treated as a two- dimensionalslab with an integratedconductivity. The maximum of the altitude-dependentPedersen con- (B) 12h ductivityis most likely locatednear 120 km, sincethe E layer conductivity is greater than the modestly increasedF layer conductivity,and the field-aligned currentsare stronglyconnected to the Pedersencon- ductivityincrease at that altitude. Zhu et al. [1994] 18h ;h showed the images of polar cap arcs with spacing taken from the Qaanaaq station in Greenland. They named polar cap arcswith narrow spacingas "multiple arcs." Though we do not have simultaneous images,it is likely that Akebono traversedmultiple Oh polar cap arcs. We hereafter refer to the bifurcated (c) electronprecipitations observed by Akebono as"mul- 7 tiple polar cap arcs." 6 By usingabout 100 passesduring the period from November 1989 to April 1990 we surveyedthe mul- • 4 tiple structureof the polar cap precipitation.In total we found 19 cases.In general, we could see several • 3 polar cap arcsin one pass.The occurrenceprobability 2 of the multiple structurewas about a few percent. Figure 2 demonstratesthe distributionof spacingof the multiplepolar cap arcs,ranging from 30 to 100 km 0 20 40 60 80 100 120 140 with a center value of 70 km. Exact locations where SZA (deg.) the multiple polar cap precipitationswere observed Figure 3. (a) Distributionof multiple polar cap precipita- are plotted on the polar map (see Figure 3a). It is tion locationsshowing that most of the multiple polar cap interestingto see that all the polar cap precipitations precipitationevents were observedin the nightsideregion. (b) Coveragearea of Akebonomeasurement in the polar cap.(c) Distributionof solarzenith angle (SZA) at the locationswhere the multiplepolar cap precipitationevents were observed.

5 with the multiple structures were located in the nightsideportion. Figure 3b givesthe coveragearea a, 2 of the satellitemeasurements, showing that there was complete coverageover the entire polar cap region, 1 both dayside and nightside,but that multiple polar 0 0 20 40 60 80 100 120 140 cap arcswere only observedon the nightside.We also Spacing (km) plotted the solar zenith angle for the multiple polar Figure 2. Distribution of arc spacingobserved by Ake- cap precipitationsin Figure 3c. These resultssuggest bono satellite.Spacing is in a rangefrom 30 to 100 km with that low backgroundconductance is a necessarycon- a center value of 70 km. dition for the existence of structured arcs. 648 OBARA ET AL.' MULTIPLE POLAR CAP ARCS

1200 with the usual value associatedwith polar cap electron precipitation[Obara et al., 1996]. This is why we 1000 - had a very small number of caseswhen Akebono 800 - observedmultiple polar cap arcs.We haveplotted the relationship between the spacing and the average 600 energyin Figure 4, where the spacingincreases with increasing average energy. The tendency that the 400 higher energy leads to wide spacingis one of the ß 200 - findingsof the Akebono observations.

2O 40 60 80 100 Spacing (kin) Existence of Downward Current Region Between Double Arcs Figure 4. Dependence of the spacingon the average energyof the electronprecipitation. It is expectedfrom the model calculationby Zhu et al. [1993] that a region of downward field-aligned current is embedded between double arcs. In order to The relationshipbetween the spacingof the multi- investigate the electrodynamic signature associated ple polar cap arcs and the average energy of the with multiple polar cap arcs we have examinedthe precipitatingelectrons was examined.In general,the current structure together with the electric field and averageenergy for multiple arcswas large compared the precipitatingelectrons.

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Four minutes of data obtained on December 8, and 5b, but the slopewas fiat for the other two cases. 1989,are plottedin Figure5a. The top panelpresents The problemwith satellitemeasurement is that the the total energy flux of the precipitatingelectrons, up-downcurrent pairs may be in suchclose proximity showingthe remarkabletwo peakswith a spacingof that measurementsof dive are smeared. This may 86 km in dawn-to-duskdirection. The secondpanel addressthe possibleexplanation for the flat slopein shows the magnitude of the field-alignedcurrent the E field shownin Figures5c and 5d. density,where a positivevalue correspondsto the upwardcurrent and a negativevalue representsthe Estimation of the Conductance From the downwardcurrent. The bottompanel shows the Ey Akebono Observations (dawn-to-dusk)component of the electricfield. Since the satellitetraversed the polar cap regionfrom dusk We surveyedthe entire polar cap regionusing data to dawn in this case,a positiveslope of the electric from half a year andfound that the distributionof the field trace correspondsto dive < 0. The upward multiple polar cap precipitationwas likely to be currentregion strictly corresponds to the regionwith limited to the nightsidepolar cap region. This sug- enhancedelectron energy flux and the dive < 0. geststhat lowbackground conductance, which may be One of the findingsby the Akebonosatellite is the led by the slowdownof plasmaconvection due to the existenceof the downwardcurrent region embedded northwardIMF condition,•s a necessarycondition for betweenthe upwardcurrent regions (arcs). Through- the existence of the multiple polar cap arcs. We out these four caseswe can identify the downward estimated the height-integratedPedersen conduc- field-alignedcurrent regions. The slopeof the electric tancein the polar caparc region.For the casesshown field was positivefor the casesshown in Figures5a in Figures 5a to 5d the averageupward current 650 OBARA ET AL.: MULTIPLE POLAR CAP ARCS

Height-integrated Perdersen Conductivity Comparison With Model Predictions for Double-Arcs 0.4 Background Conductance and Multiple 0.35 Polar Cap Arcs •" 0.3 Zhu et al. [1993] developed a time-dependent • o.2s modelof polar cap arcsin whichthe electrodynamics • 0.2 of the polar cap arcsis treated self-consistentlyin the •0.15 frame of the coupled magnetosphere-ionosphere 0.1 (M-I) systemand the activerole of the ionosphereis

0.05 specially stressed.Figure 7 shows the asymptotic

0 field-alignedcurrent distribution from the M-I cou- -9 -6 -3 0 pling model. The ionosphere is treated as a two- •dEy/Zl Y ( 10'7V/m2) dimensionalslab with an integratedconductivity. The x axis is defined along noon-midnightmeridian and Figure 6. Relationshipbetween the field-alignedcurrent (FAC) intensity and the gradient of the electric field. pointsto the dayside,and the y axisis definedalong Though the points are scattered, there is a significant dawn-duskmeridian and points to the duskside.The relationship between them showing the rather constant ionosphericbackground conductance, which is mostly conductance of about 0.5 mho. due to solar radiation, is uniform in the dawn-dusk directionand decreasesfrom 2.5 mho on the dayside to 0.5 mho on the nightside.The initial magneto- sphericinput for the simulationis a shearflow, which densitywas plotted againstthe gradientof the electric is assumedto be carriedby a downwardpropagating field, as shown in Figure 6. We can estimate the Alfven wave. The electric potential associatedwith the shear flow is assumed to be a Gaussian distribu- height-integratedPedersen conductance from the ra- tion with a minimum of -2 kV at the center. The tio of the current density to the gradient of the large-scaleionospheric background convection is a electricfield. Although the pointswere scattered,we uniform antisunwardconvection with a magnitudeof have a very significantheight-integrated conductance 20mV m-1 [Zhuet al., 1993].The dashed curves in with an amplitude of about 0.5 mho. Figure 7 denote the upward field-aligned current, 14qnninghamand Heikkila [1974] mentioned that while the solid curves denote the downward field- the electron densityover the polar cap is low, espe- aligned current. It can be seen that in the dayside cially in the winter polar cap, during periodsclose to regions,where the ionosphericbackground conduc- solar minimum.Knudsen et al. [1977] estimatedthe tanceis higher,only a singlepolar cap arc exists.This ionizationproduct by the solar UV, showingthat the can be explainedby the fact that high ionospheric lowestelectron number density isabout 10 4 cm-3 at conductanceallows the magnetosphericcurrent to be the peak height (---250 km). In situ observationsof freely closedin the ionosphere,which actsto smooth the plasma density were measured by the AE-C localizeddiscrete structures. In the nightsideregions, satellite [Brintonet al., 1978]. When the AE-C satel- where the ionosphericconductance is low, there exist lite was in the nightsidepolar cap region, the low- multiple polar cap arcs. This is because the low densityregions of the ionosphericplasma, called the ionosphericbackground conductance in the nightside "plasmahole," were identifiedfrequently. According region blocksthe free closureof the magnetospheric to our previousobservation by the Exos C (Ohzora) current in the ionosphereand forces the current to satellite,the plasmadensity in the nightsidepolar cap return back to the magnetospherelocally thereby regionis quite low (less than 10 3 cm-3in the F layer) distortingthe initial magnetosphericcurrent pattern when the magnetic condition is quiet, forming the and leading to the structuredcurrent sheetsor struc- plasmahole state [Obaraand Oya, 1989]. It is gener- tured arcs.As shownin the precedingsection, Ake- ally recognizedthat polar cap arcs appear during bono observations revealed that most of the struc- northwardIMF conditions,and this suggeststhat the tured polar cap arcs (or multiple polar cap arcs) background conductanceshould be very low. We occurredin the nightsideregions of the polar cap. speculate that the background conductanceis less This statistical result is consistent with the above than 0.5 mho. theoreticalprediction from the M-I couplingmodel. OBARA ET AL.' MULTIPLE POLAR CAP ARCS 651

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Relationship Between Spacing Distance sistentwith the theoreticalprediction from M-I cou- and Average Energy pling model of polar cap arcs. It should be noted that the above consistency Figure 8 showsthe quantitativerelationship be- between the theoretical simulation and Akebono tween the Hall-to-Pedersen conductance ratio and observationis still qualitativein nature at this time. A the spacingof the multiple polar cap arcs obtained future studyalong this line couldbe a direct quanti- from the model.The spacingis definedby the spatial tative comparison between Akebono observations separationof the peak brightness(or maximumfield- and theoretical modelings.To do this, several case alignedcurrent intensity)of two arcs and falls in a studies should be performed, and the M-I model rangefrom 50 to 100 km. In Zhu e! al.'s [1993] model needsto make further calculationsof averageenergy the ionosphereis treated as a two-dimensionalslab instead of conductance ratio for each of the cases. with an integrated conductivity.In general, the E region makesmajor contributionto the height-integrated conductance,which was the reasonwhy we Summary mappedthe spacingof the arcsto 120km ionospheric level in Figure 2. On the basis of the Akebono observations we have It is commonlyknown that the Hall-tø-Pedersen studiedthe multiplestructure of polar cap precipita- conductance ratio is an indicator of the hardness of tion. The followingare our results:(1) The electro- the electronprecipitation. A higherconductance ratio dynamicsignature associated with the multiplepolar reflectsa precipitationwith higher averageenergy. cap arcs showsthat a downwardcurrent region is Therefore the relationshipbetween the spacingof embedded between the upward current regions multiplepolar cap arcsand the averageenergy of the (arcs). (2) The multiple polar cap arc regionsare associatedprecipitation observed by Akebono is con- usuallyfound in the nightsidepolar cap region. (3) 652 OBARA ET AL.' MULTIPLE POLAR CAP ARCS

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The spacingdistance of the multiple arcsis in a range hydrodynamicM-I couplingmodel [Zhu et al., 1993] from 30 to 100 km with respect to dawn-to-dusk are adequate. direction with a dependenceon the hardnessof the electron precipitation. Acknowledgments. The authors are grateful to all the We have performed the comparisonwith the M-I membersof the Akebono tracking team for their extensive coupling model calculation in a qualitative fashion effortsto obtain the satellite data and thank T. Nagatsuma and found that model resultsare quite consistentwith and T. Sakanoifor their help in the MGF (magneticfield the Akebono observations. The observational results sensor)data processing.The USU coauthorsacknowledge NASA grant NAG5-1484 and NSF grants ATM-9302165 from Akebono indicate the importance of the M-I and ATM-9308163 to Utah State University. couplingprocesses in which the ionosphereplays an active role in the formation of the multiple polar cap References arcs. This initial study has demonstrated the close agreementbetween model and observationsof Sun- Brinton, H. C., J. M. Grebowsky,and L. H. Brace, The alignedpolar cap arcs.Furthermore, it showsthat the high-latitudewinter F region at 300 km: Thermal plasma observations from Akebono are comprehensive observationfrom AE-C, J. Geophys.Res., 83, 4767, 1978. enough to be used to constrain future M-I model Burke, W. J., et al., Electric and magneticfield character- simulations. isticsof discretearcs in the polar cap,J. Geophys.Res., 87, 2431, 1982. There is, however, still another possibility that Carlson, H. C., et al., Coherent mesoscale convection multiple polar cap arcs are due to structuredplasma pattern duringnorthward interplanetary magnetic field, J. sourcesin the magnetosphere.In order to understand Geophys.Res., 93, 501, 1988. the physicsof the multiple polar cap arcs, more Fukunishi, H., et al., Magnetic field observationson the detailed studies together with all-sky images are Akebono(EXOS D) satellite,J. Geomagn.Geoelectr., 42, needed.The follow-upstudies will determinewhether 385, 1990. assumptionsused in the formulationof the magneto- Hardy, D. A., Intense fluxes of low-energy electrons at OBARA ET AL.: MULTIPLE POLAR CAP ARCS 653

geomagneticlatitude above 85ø, J. Geophys.Res., 89, aligned arcs in the polar cap, J. Geophys.Res., 96, 1379, 3883, 1984. 1991. Hayakawa, H., et al., Electric field measurement on the Weber, E. J., et al., Rocket measurementswithin a polar Akebono (EXOS D) satellite,J. Geomagn.Geoelectr., 42, cap arc: Plasma,particle, and electric circuit parameters, 371, 1990. J. Geophys.Res., 94, 6692, 1989. Knudsen,W. C., P.M. Banks,J. D. Winningham,and D. M. Winningham, J. D., and W. J. Heikkila, Polar cap auroral Klumpar, Numerical model of the convectingF 2 iono- electron fluxes observedwith Isis-l, J. Geophys.Res., 79, sphere at high latitude, J. Geophys.Res., 82, 4784, 1977. 949, 1974. Mizera, P. F., and J. Fennell, Satellite observationof polar Zhu, L., et al., A time-dependentmodel of the polar cap magnetotail lobe and interplanetary electrons at low arcs,J. Geophys.Res., 98, 6139, 1993. energies,Rev. Geophys.,16, 147, 1978. Zhu, L., et al., A model-observationstudy of multiple polar Mukai, T., et al., Low energycharged particle observations cap arcs,paper presentedat SecondJoint Workshop for in the "auroral" magnetosphere:First results from the CEDAR HLPS/STEP GAPS, Coupling,Energ., and Dyn. Akebono(EXOS D) satellite,J. Geomagn.Geoelectr., 42, of Atmos. Reg. High Latitude Plasma Struct. and Solar 479, 1990. Terr. Energy Program Global Aspects of Plasma Struct. Obara, T., and H. Oya, Observationof polar cuspand polar Workgroups,Lyons, Colo., June 27-29, 1994. cap ionosphericirregularities and formation of the ionospheric plasma holes using topside sounder on board D. Crain, J. Sojka, and L. Zhu, Center for Atmospheric EXOS C (Ohzora) satellite,J. Geomagn.Geoelectr., 41, and Space Science, Utah State University, Logan, UT 1025, 1989. 84322-4405. Obara, T., et al., Akebono (ExosD) observationsof small- H. Fukunishi,Department of Geophysicsand Astrophys- scale electromagnetic signaturesrelating to polar cap ics, Tohoku University, Aoba, Aramaki, Aoba-Ku, Sendai precipitation,J. Geophys.Res., 98, 11,153, 1993. 980, Japan. Obara, T., et al., Signatureof electric field associatedwith H. Hayakawa, A. Matsuoka, T. Mukai, A. Nishida, T. localizedelectron precipitations in the polar cap regionm Obara, and K. Tsuruda, Institute of Space and Astronauti- Akebono (EXOS D) results,J. Geomagn.Geoelectr., 47, cal Science,3-1-1, Yoshinodai, Sagamihara,Kanagawa 229, in press,1996. Japan. Sojka, J. J., et al., Effect of high-latitude ionospheric convectionon Sun-alignedpolar caps,J. Geophys.Res., 99, 8851, 1994. Valladares, C. E., and H. C. Carlson Jr., The electrody- (ReceivedJune 7, 1995; revisedJanuary 30, 1996; namic thermal and energetic character of intense Sun- acceptedFebruary 6, 1996.)