1 Electromagnetic Ion Cyclotron Waves in the High Altitude Cusp: Polar Observations G. Le l'2 X. Blanco-Cano 3, C. T. Russell 1,X.-W. Zhou 1, F. Mozer 4, K. J. Trattner 5,S. A. Fuselier 5and B. J. Anderson 6 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 2Now at Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 3Instituto de Geofisica, UNAM, Ciudad Universitaria, Coyoacan, C.P. 04510, D.F., Mexico. 4Space Sciences Laboratory, University of California, Berkeley, California 5Lockheed-Martin Palo Alto Research Laboratory, Palo Alto, California 6The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland Abstract High-resolutiomnagneticfielddatafromthePolarMagneticFieldExperimen(tMFE)show thatnarrowbandwavesatfrequencie-s0.2to3Hzareapermanenfteatureinthevicinityofthe polarcusp.Thewaveshavebeenfoundinthemagnetosphearedjacenttothecusp(bothpoleward andequatorwarodfthecusp)andinthecuspitself. Theoccurrenceofwavesiscoincidenwt ith depressioonfmagneticfieldstrengthassociatewdithenhancepdlasmadensityi,ndicatingtheentry ofmagnetosheapthlasmaintothecuspregion.Thewavefrequencieasregenerallyscaledbythe localprotoncyclotronfrequencya,ndvarybetween0.2and1.7timeslocalprotoncyclotron frequencyT. hissuggesttshatthewavesaregeneratedin thecuspregionby theprecipitating magnetosheaptlhasmaT.hepropertieosfthewavesarehighlyvariableT. hewavesexhibitbothleft- handedandright-handedpolarizationinthespacecrafftrame.Thepropagationanglesvaryfrom nearlyparalleltonearlyperpendiculator themagneticfield.Wefindnocorrelationamongwave frequencyp,ropagationangleandpolarizationC. ombinedmagneticfieldandelectricfielddatafor thewavesindicatethattheenergyfluxofthewavesisguidedbythebackgrounmd agneticfieldand pointsdownwardtowardtheionosphere. Introduction ThePolarspacecrafitn, itshighlyellipticalorbitwithanapogee of 9 REand an inclination angle of 86° provides an excellent opportunity to study the polar cusp at high altitudes. When passing through the polar cusp region near apogee, the Polar spacecraft often enters a region with magnetosheath-like plasma, or the polar cusp, that can be readily identified by a sudden increase of the low-energy plasma density in the data from Hot Plasma Analyzer (Hydra) [Scudder et al., 1995] and Toroidal Imaging Mass-Angle Spectrograph (TIMAS) [Shelley et al., 1995] and an associated diamagnetic magnetic field depression in the magnetic field data (MFE) [Russell et al., 1995]. Zhou et al. [1999, 2000] reported the studies on the location and the solar wind control of polar cusp at high altitudes. In this paper, we report a study of the electromagnetic ion cyclotron waves inthe polar cusp region as well as in the region adjacent to the cusp observed in the Polar magnetic field data. In the high latitude region, electromagnetic ion cyclotron waves have been reported inthe data from low-altitude polar orbiting spacecraft such as Injun 5at altitudes less than 3000 km [Gumett and Frank, 1972], $3-3 between altitudes 800 to 8000 km [Temerin and Lysak, 1984], ISIS 2 between altitudes 500 to 4000 km [Saito et al., 1987], and Freja between altitudes 1200 to 1750 km [Erlandson and Zanetti, 1998]. The waves were narrow-banded with frequencies between the local proton (H÷)cyclotron frequency (fcp) and the local He+particle cyclotron frequency ranging from 20 to 400 Hz. They were found only in a narrow range of auroral latitudes inthe pre-midnight sector in association with low-energy auroral electron precipitation. At mid altitudes, the electromagnetic ion cyclotron waves were observed in the polar cusp region at - 4 REaltitude by OGO 5 [Russell et al., 1971; Scarfet al., 1972; Fredricks and Russell, 1973] and at - 2 REaltitude by Viking [Erlandson et al., 1988]. The wave frequencies at higher altitudes were observed in the range between the local H÷ 3 _e cyclotron frequency and alpha particle H_ cyclotron frequency, 4-7 Hz at OGO 5and 18to 27 Hz at Viking. The anisotropies in the electron and ion distributions were suggested as the energy sources for the waves in the cusp [Erlandson et aI., 1988]. A survey of high-resolution magnetic field data from the Polar spacecraft reveals that the electromagnetic ion cyclotron waves are a permanent feature in the polar cusp. The Polar data extended the observations of cusp electromagnetic ion cyclotron waves to high altitudes - 5 to 9 RE. In this paper, we summarize the characteristics of ion cyclotron waves in the high altitude cusp region as observed in the Polar magnetic field data. We focus on observational aspects; the generation mechanism and plasma instabilities are a topic of future study. PolarMagneticFieldData Russeleltal.[1995]describetdheMagneticFieldExperimen(tMFE)onthePolarspacecraft. ThemagneticfielddataweretransmittedtoEarthataconstanrtateof8.33Hzor 120msecper samplec,orrespondintgoaNyquistfrequencyof4.17Hz.Inthisstudy,wehavesurveyedthishigh- resolutionmagneticfielddataduringPolar'spassageosfthecuspregioninhighaltitudenorthern hemispherdeuringthetimeperiodfromApril1toDecembe3r1,1996.Therearetotally212cusp passagedsuringthistimeperiodidentifiedbyZhouetal.[1999u]singcombinedPolarmagnetifcield data,HydrakeyparametedrataandTIMASkeyparametedrata.Thesecuspentriesoccurataltitudes from4.8to8.8RE.Themagneticfieldstrengthatthesecuspcrossingsrangesfrom36to467nT. Amongthe212cusppassagews,ehavefoundevidenceoftheexistenceofwavesin 197passages, whichoccurasnarrow-banwdavesinthefrequencyrangefrom- 0.2Hzupto~4Hz.Waveswith higherfrequenciemsaystillpresentb,utcannotberesolvedbyPolarmagnetifcielddataatitsnormal 8.33Hzdatarate. WaveFrequencies Beforewepresenetxampleosfthesewavesw, efirstshowinFigure1thefrequencieosfthese wavesasafunctionoflocalmagneticfieldstrengthfromthe197cusppassagewsithwavesN. otethat thewavesobservedinthesamecuspcrossingmayoccuratmarkedlydifferentfrequenciesM.ultiple wavefrequenciemsaybeincludedfromselectewdavepacketsineachcuspcrossingt,hus,thedata pointsin Figure1 arenot independentto eachotherandonemaynot attemptto obtainthe informationofoccurrencreateofwavefrequenciefsromthem.Itisonlyintendedtoshowtherange offrequencyvarianceF.igure1clearlyshowsthatthewavefrequencieasregenerallycontrolledby thelocalmagneticfieldstrengthT.hewavefrequenciensormalizedbythelocalprotoncyclotron 5 t frequencies are in the range of 0.2 to 1.7. This observation strongly suggests that these waves are generated locally in the cusp region. Although waves in the same frequency band occur in the magnetosheath [Anderson and Fuselier, 1993; Schwartz et al., 1996], itis very unlikely that waves in the cusp have propagated from the magnetosheath and then exhibit the control by the local magnetic field strength shown in this plot. The vertical bars in Figure 1 indicate the average local magnetic field B for the 15 cusp crossings where no evidence of the existence of waves is found. It isevident that the majority of the waves tend to be observed in the cusp crossings with smaller magnetic field strength. For all 181 cusp passages with B <250 nT, we see clear evidence of the existence of the waves. The crossings without the waves tend to have large magnetic field. The local magnetic field strength is greater than - 250 nT for all the 15 crossings without the waves (15 out of 31 passages with B > 250 nT). Thus, it is possible that there are still waves present but at higher frequencies beyond the instrument bandwidth for Polar MFE. Analysis of the wave properties shows that they are variable. The propagation angles vary in the whole range, from nearly parallel to nearly perpendicular to the background magnetic field. The waves exhibit both left-handed and right-handed polarization in the spacecraft frame with a broad range ofellipticity (from nearly linear to nearly circular polarization). We do not find any correlation among wave frequency, propagation angle, and polarization. It is possible that instabilities are at work generating both left-handed and right-handed wave modes in the cusp region. Even for waves occurring at frequencies above the local proton cyclotron frequency, both left-handed and right- handed waves are observed in the spacecraft frame. In the plasma dispersion relation, the left- handed/Alfven waves cannot propagate either in the direction perpendicular to the magnetic field or 6 at frequencies above the ion cyclotron frequency. Thus the waves observed above the proton cyclotron frequency should be in the right-handed/magnetosonic mode. Why some of the waves exhibit left-handed polarization above the ion cyclotron frequency is still a puzzle. The right- handed/magnetosonic mode above the proton cyclotron frequency has phase velocity above Alfven velocity. For typical conditions in the cusp at Polar altitudes, Alfven velocity (- 1000 km/s) ismuch greater than the plasma flow velocity (- 100 km/s), thus Doppler shift cannot reverse the wave polarization from right-handed in the plasma rest frame to left-handed in the spacecraft frame. Under some extreme conditions with highly depressed magnetic field and enhanced plasma density, or with presence of high percentage of heavy ions in the cusp, Alfven velocity may become comparable to plasma flow velocity and Doppler shifting maybe involved inthe observations of left-handed waves in the spacecraft frame. ExamplesofWavesintheCusp The May 24, 1996 Cusp Crossing. Figure 2shows an overview of the high altitude polar cusp crossing on May 24, 1996 (from 0030 to 0440 UT), where the narrow-band waves are observed throughout the passage. During this crossing, the Polar orbit plane was roughly in the noon-midnight meridian plane moving towards high latitudes. In Figure 2, the top panel shows the magnetic field data and bottom panel shows Hydra electron density and average energy key parameters. The spacecraft crossed the cusp region from the dayside magnetosphere (Bx < 0) to the tail lobe (Bx > 0). The signatures for the polar cusp are characterized by a depression of the magnetic field strength and an enhancement of low energy plasma density associated with the precipitation of the magnetosheath plasma into the cusp region (- 0052 to 0140 UT). When the spacecraft was just equatorward of the cusp and after the spacecraft went into the lobe region poleward of the cusp, there were multiple encounters ofmagnetosheath-like cusp plasma pulses with a magnetic field depression lasting from less than one minutes to a few minutes, probably due to the motion of the polar cusp as well as the unsteady nature of magnetosheath plasma precipitations into the cusp region. The existence of the waves in the cusp region is evident in the magnetic field data in Figure 2. We find that the wave properties vary in the same cusp crossing, which is a very common phenomenon in the data surveyed. To illustrate the details, we select several short intervals for a close examination of wave properties. Figure 3 shows an interval of waves observed inside the cusp region from 0122:00 to 0123:20 UT. The narrow band waves at - 1.2 Hz with peak-to-peak amplitude <4 nT were found in the region with a 10 nT depression of the magnetic field, as shown in the top panel of Figure 3. The wave frequency weighted by spectral power is about 0.96 fcp, only slightly lower the local proton cyclotron frequency. The waves, while having similar frequency in 8 thisshortinterval,exhibitverydifferenpt olarizationandpropagatiodnirectionsI.nthelowerpanels ofFigure3,wepresenhtodogramosfwavepackeltabeleda,bandcinthetoppanelw, here_B__,SBj and_iBKcorrespontdothewavemagneticfieldinthedirectionsoftheirmaximum(I),intermediate (J)andminimum(K)variancedirectionsr,espectivelyT.hearrowslabeledBaretheprojectionofthe localbackgrounmd agneticfieldwithmagnitude- 82nT.Theplanesofoscillationhavechanged markedlyforthesewavepacketsF.orwavepackeat ,theanglebetweenthebackgrounmd agnetic fieldandthewavekvector(0Bk)is 57° andthewaveisclearlyright-handedpolarizedwithan ellipticityof0.64.Forwavepackebt,thepolarizationchangetsoleft-handedalthoughtheelfipticity (0.68)and0Bk(56°)keeproughlythesameF.orthenextwavepacketc,thebackgrounmdagnetic fieldliesroughlyintheplaneofoscillationsothatthewavepropagateastveryobliqueangle,86° from the local magnetic field. The frequency of the wave is also quite variable for the same cusp crossing. Figure 4 shows another interval of the May 24, 1996 cusp crossing with waves occurring at much lower frequency than those in Figure 3. The top panel of Figure 4 contains magnetic field data and is plotted in the same time scale as in the top panel of Figure 3. The power spectrum for the entire interval isshown in the lower panel. The power spectrum contains multiple peaks at frequencies lower than local proton cyclotron. The frequency of the peak with maximum power is 0.60 Hz, or 0.44 fcp. In our survey, the waves are most frequently observed within and adjacent to the field depression in the cusp region. The May 24, 1996 cusp crossing provides a good example, where the waves are also observed in the short pulses of field depression region both equatorward and poleward of the cusp region. Figure 5 shows the magnetic field data and their power spectra in four of the field depression regions in the lobe on May 24, 1996. The waves are seen inside the field depression region in Figure 5b-c and both inside and near the edge of the field depression region in Figure 6a with variable frequencies. The average frequencies of the spectral peaks weighted by the power in these field depression regions are (a) 1.48 Hz or 0.90 fcp, (b) 0.49 Hz or 0.32 fcp, (c) 1.36 Hz or 0.84 fcp, and (d) 1.23 Hz or 0.78 fcp, respectively, In all the cases, the association of the waves and the magnetic field depression is very clear. The depression of the magnetic field is required by maintaining the pressure balance with the precipitation ofmagnetosheath plasma. The occurrence of the waves in association with the magnetic field depression suggests that the instabilities are most likely caused by entering magnetosheath plasma. The September 10, 1996 Cusp Crossing. We now show another example of cusp crossing on September 10, 1996. Figure 6 shows the overview of the magnetic field data and Hydra plasma data for the interval of interest. Polar orbit plane was roughly in the noon-midnight meridian plane. The spacecraft moved from high latitudes to low latitudes, i.e., from the tail lobe (Bx > 0) to the cusp region, and then into the dayside magnetosphere (Bx < 0). The crossing of the polar cusp occurred from ~ 0700 to 0915 UT and there were a few short excursions of magnetosheath-like cusp plasma associated with the magnetic field depression before and after the cusp crossing. Again, the narrow band waves are observed in the cusp region and in all intervals with magnetosheath-like cusp plasma pulses and magnetic field depression. In Figure 7 we present three short wave intervals in the field depression region poleward of the polar cusp (a and b) and in the cusp itself(c). It isalso evident that the waves have variable frequencies even within a short period of time In some intervals during this cusp crossing, we have seen co-existence of multiple narrow band waves at different frequencies in the same time. This type of waves with multiple spectral I0