Dependence of the Polar Cusp on the North&Hyphen;South Component of the Interplanetary Magnetic Field

VOL. 78, NO. 19 JOURNAL OF GEOPHYSICAL RESEARCH JULY 1, 1973 Dependenceof the PolarCusp on the North-SouthComponent of theInterplanetary Magnetic Field MARGARETG. KIVELSON AND CHRISTOPHER T. RUSSELL Institute o] Geophysicsand Planetary Physics,University o] California Los Angeles, California 90024 MARCIA NEUGEBAUER Jet Propulsion Laboratory, ,Ca,li]ornia Institute o! Technology Pasadena, California 91103 FREDERICK L. SCARF AND ROBERT W. FREDRICKS •pace SciencesLaboratory, TR W SystemsGroup Redondo Beach, Cali/ornia 90273 Ogo 5 observationsof the polar cusp on No.vember 1, 1968, show that the north-south component of the interplanetary field exhibits control over both the location of and the physical processesoccurring in the polar cusp. When the interplanetary field turned from north to south, the polar cusp moved equatorward. During •ntervals when the interplanetary field was southward, the electron temperature in the polar cusp was lower and the currents were strongerthan when the interplanetary field was northward. Also during theseintervals of southward field, regionsof apparentlyrapidly varying currentswere encounteredwithin the cusp. Associated with these regions were enhanced VLF electric field levels. When the interplanetary field was northward, quasi-monochromatic Pc I waves close to but below the proton gyrofrequency and energetic electrons (E > 50 key) were observed.Most of these observations are consistentwith the existenceof merging of the interplanetary magnetic field with the dayside magnetospheric field when the interplanetary field is southward and the absence of mergingwhen it is northward.The dependenceof electrontemperature on field directionre- mains unexplained. • The 'polar cusp,' defined as the regionsin marized the particle and field observationsin the daysidehemisphere in which magnetosheath the cusp on that day, and the properties of plasma has accessto tile magnetosphere,has cusp plasma waves have been describedby been observed as a persistent feature of tile Scarf et al. [1972]. magnetosphere, having been repeatedly en- The existenceof the polar cusp is consistent countered by satellites both at high altitudes with both open and closedmodels of the mag- [Frank, 1971] and at low altitudes [Heikkila netosphere.In open models, plasma can follow and Winningham, 1971; Winningham, 1972; field lines from tile magnetosheath directly Frank and Ackerson, 1971, 1972]. The polar to tile deep magnetosphere,some acceleration c,sp is often boundedat low latitudes by the occurring in the magnetopausecurrent sheet boundary of trapped energetic electrons. At [Dungey, 1962; Speiser, 1969]. in closed high altitudes a broad regionof depressedfield models,the plasma diffusesinto tile polar cusp is centeredon the polar cusp [Fairfield a•d from a 'leakymagnetic bottle' in the vicinity Ness, 1972]. of the dayside 'neutral point.' If, for example, During a large magneticstorm on November the magnetospherewere to change from an 1, 1968, Ogo 5 passedthrough the dayside open to a closed topology in responseto a polar cusp. Russell et al. [1971] have sum- change in tile north-south componentof tile interplanetary field, we would not expect tile Copyright (•) 1973 by the American Geophysical Union. polar cusp ,•o disappear. In fact, Ogo 5 ob- 3761 3762 K•vELso• r•? AL.' T•E Po•Aa Cusr servationsof the polar cuspon November1, measuredby the Jet PropulsionLaboratory 1968,show that the polarcusp existed during (JPL) plasma spectrometer,which points intervalsof both northwardand southward radiallyaway from the earth,the secondpanel interplanetaryfields [Russell et al., 1971]. showsthe averageenergy of theseelectrons, Althoughthe existenceof the polar cusp the third panel shows.the integral flux of shouldnot dependon the orientationof the energeticelectrons between 50 and 1100 key, interplanetaryfield, certainproperties of the and the bottompanel showsthe solarmag- polarcusp might. be expectedto change.First, nerosphericZ componentof the interplanetary the polar cuspis expectedto moveequator- magneticfield measuredby Explorer33. For ward as magneticflux is addedto the tail [cf. reasonsdiscussed below, these latter data have Unti and Atkinson,1968] as the result of beenshifted by 8 min to compensatefor tile erosionof the daysidemagnetosphere associated average convection time from Explorer 33, with tile occurrence of a southward interplanetary field [Aubry et al., 1970]. Russell et al. [1971] noted that Ogo 5 encounterswith the polar cusp showed the expected behavior, motion of the cusp equatorward occurring in response to a southward reorientation of the interplanetary magnetic field. This motion has also been confirmedby Burch [1972]. We also expect to observe changes in the plasma within the polar cusp accompanying reversal of the north-south componentof the interplanetary medium, if such reversal alters the topology of the magnetosphere.In partic- ular, the physicalprocesses governing the entry and acceleration of the magnetosheathplasma may be totally different for northward and southward interplanetary fields. In this paper we examine the polar cusp plasma during the Ogo 5 encounterswith the polar cusp on November 1, 1968, •o determine the responseof the plasma to changesof the orientation of the interplanetary field. The observations are discussed in relation to theo- retical descriptionsof the ,olar cusp. Fig. 1. The first three panels show the energy density and average energy of electrons from 50 OBSERVATION8 to 3200 ev measured by the JPL solar wind ex- periment and the energetic electron flux from Ogo 5 encounteredthe polar cusp on four 50 to 1100key measuredby the UCLA energetic separateoccasions on November1, 1968.Dur- electronspectrometer during the ego 5 outbound ingtwo of theseencounters theinterplanetary 1)ass1968. through Data gapsthe that magnetosphere are sinmltaneousonNovember in the top1, fieldwas consistently northward, (h•ring one threepanels correspond to gaps in datatransmis- encounterthe field was consistentlysouth- sion.Additional gaps in panels1 and 2 cotres- ward, and during one encounter the inter- pond to interwdsduring which the energyden- planetaryfield as measuredon Explorer33 sityfell below100 ev ('m-'•. The additional gap changedfrom northward to southward while incm-"" panelsec -• 3ster signifies -•, the thatlevel the of internalflux fell calibrationbelow 10• Ogo 5 was within the polar cusp.Figure 1 sources.The bottom1)anel shows the Z-GSM displays some of the relevant Ogo 5 measure- componentof the interplanetary magnetic field mentsand the north-southcomponent of the measuredby Explorer33 at 0900LT and42 Rr interplanetaryß magnetic field during this period ' fromhave beenthe earth. plottedThe with Explorer an 8-rain33 delay, measurements as is dis- The top panelshows the energydensity of cussedin the lext.Encounters with the polar 50- to 3200-ev electrons(assumed isotropic) cuspare indicated by horizontalbars 1-4. KIVELSON ET AL.: THE POLAR Cusp 3763 which was 42 Rs from the earth near 0900 LT, the magnetic field reversal reached the mag- to the magnetopause. neropause. These data were obtained as Ogo 5 passed To determine the time of arrival at the through the dayside magnetospherenear noon magnetopauseof the southwardfield, we looked local time at radial distancesfrom 2.5 to 8 RE for evidencein the Ogo 5 data. We then made and magnetic latitudes from 35ø to 45ø. Dur- use of data from Explorer 33, 34, and 35 to ing this period there was a complex magnetic argue that the estimated time was consistent storm in progress,and the dynamic pressure with all available information. In using the of the solar wind was apparently unusually Ogo 5 data we follow the argument of Russell high. This high dynamic pressure combined et al. [1971], who proposedthat the sequence with the occasional strong erosion of mag- of observations could most readily be in- nerosphereby the southward component of terpreted as the consequenceof a polar cusp the interplanetary field causedthe polar cusp moving equatorward during encounter 1, a to move equatorward from its quiet time post- cusp moving poleward and then equatorward tion far poleward of the Ogo 5 orbit. The in encounter2, a cusp moving poleward in en- trajectory, the magnetic and solar wind condi- counter 3, and a stationary cusp in encounter tions, and the data shown in Figure 1 have 4. (In a later part of this paper we present been describedin more detail by Russ.ellet al. a slightly different interpretation of the mul- [1971]. tiple encounters.)Russell et al. further argued North-Southcomponeni of the interplanetary that thissequence was linked to the erosion field. As there are no low-energy proton of the magnetospherewhen the interplanetary spectra available from Ogo 5 for the period of field was southward. This interpretation re- interest.,we have used the low-energy electrons quires that the reversal of the polar cusp mo- ro define the polar cusp. For the statistical tion during encounter 2 occurred at the time analysesto follow we arbitrarily define as the when the southward turning of the inter- polar cusp those regions in which the electron planetary field detected at Explorer 33 at energy density exceedsa threshold of 10'• ev 1248 UT was eonvectedto the magnetopause. cm-'•. The region in which low-energy electron This would imply that, during the first part flux is accompaniedby a detectable flux of of encounter 2, the interplanetary field at the 0.1-to 10-kev protons at low altitudes is more magnetopausewas northward and, during the

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