VOL. 78, NO. 19 JOURNAL OF GEOPHYSICAL RESEARCH JULY 1, 1973

Dependence of the Polar Cusp on the North-South Component of the Interplanetary Magnetic Field

MARGARET G. KIVELSON AND CHRISTOPHER T. RUSSELL

Institute o] Geophysics and 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 Sciences Laboratory, TR W Systems Group Redondo Beach, Cali/ornia 90273

Ogo 5 observations of the polar cusp on No.vember 1, 1968, show that the north-south com- ponent of the interplanetary field exhibits control over both the location of and the physical processes occurring 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 pola r cusp was lower and the currents were stronger than when the interplanetary field was northward. Also during these intervals of south- ward field, regions of apparently rapidly varying currents were encountered within the cusp. Associated with these regions were enhanced VLF electric field levels. When the interplane- tary 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 consistent with the existence of merging of the interplanetary magnetic field with the day- side magnetospheric field when the interplanetary field is southward and the absence of merging when it is northward. The dependence of electron temperature on field direction re- mains unexplained. •

The 'polar cusp,' defined as the regions in the dayside hemisphere in which magnetosheath plasma has access to tile magnetosphere, has been observed as a persistent feature of tile magnetosphere, having been repeatedly en- countered by satellites both at high altitudes [Frank, 1971] and at low altitudes [Heikkila and Winningham, 1971; Winningham, 1972; Frank and Ackerson, 1971, 1972]. The polar c,sp is often bounded at low latitudes by the boundary of trapped energetic electrons. At high altitudes a broad region of depressed field is centered on the polar cusp [Fairfield a•d Ness, 1972].

During a large magnetic storm on November 1, 1968, Ogo 5 passed through the dayside polar cusp. Russell et al. [1971] have sum-

Copyright (•) 1973 by the American Geophysical Union.

marized the particle and field observations in the cusp on that day, and the properties of cusp plasma waves have been described by Scarf et al. [1972].

The existence of the polar cusp is consistent with both open and closed models of the mag- netosphere. In open models, plasma can follow field lines from tile magnetosheath directly to tile deep magnetosphere, some acceleration occurring in the magnetopause current sheet [Dungey, 1962; Speiser, 1969]. in closed models, the plasma diffuses into tile polar cusp from a 'leaky magnetic bottle' in the vicinity of the dayside 'neutral point.' If, for example, the magnetosphere were to change from an open to a closed topology in response to a change in tile north-south component of tile interplanetary field, we would not expect tile polar cusp ,•o disappear. In fact, Ogo 5 ob-

3761

3762 K•vELso• r•? AL.' T•E Po•Aa Cusr

servations of the polar cusp on November 1, measured by the Jet Propulsion Laboratory 1968, show that the polar cusp existed during (JPL) plasma spectrometer, which points intervals of both northward and southward radially away from the earth, the second panel interplanetary fields [Russell et al., 1971]. shows the average energy of these electrons,

Although the existence of the polar cusp the third panel shows. the integral flux of should not depend on the orientation of the energetic electrons between 50 and 1100 key, interplanetary field, certain properties of the and the bottom panel shows the solar mag- polar cusp might. be expected to change. First, nerospheric Z component of the interplanetary the polar cusp is expected to move equator- magnetic field measured by Explorer 33. For ward as magnetic flux is added to the tail [cf. reasons discussed below, these latter data have Unti and Atkinson, 1968] as the result of been shifted by 8 min to compensate for tile erosion of the dayside magnetosphere associated average convection time from Explorer 33, with tile occurrence of a southward inter-

planetary field [Aubry et al., 1970]. Russell et al. [1971] noted that Ogo 5 encounters with 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 confirmed by Burch [1972].

We also expect to observe changes in the plasma within the polar cusp accompanying reversal of the north-south component of the interplanetary medium, if such reversal alters the topology of the magnetosphere. In partic- ular, the physical processes governing the entry and acceleration of the magnetosheath plasma may be totally different for northward and southward interplanetary fields.

In this paper we examine the polar cusp plasma during the Ogo 5 encounters with the polar cusp on November 1, 1968, •o determine the response of the plasma to changes of the orientation of the interplanetary field. The ob- servations are discussed in relation to theo-

retical descriptions of the ,olar cusp.

OBSERVATION 8

Fig. 1. The first three panels show the energy density and average energy of electrons from 50 to 3200 ev measured by the JPL solar wind ex- periment and the energetic electron flux from

Ogo 5 encountered the polar cusp on four 50 to 1100 key measured by the UCLA energetic separate occasions on November 1, 1968. Dur- electron spectrometer during the ego 5 outbound ing two of these encounters the interplanetary 1)ass through the magnetosphere on November 1,

1968. Data gaps that are sinmltaneous in the top field was consistently northward, (h•ring one three panels correspond to gaps in data transmis- encounter the field was consistently south- sion. Additional gaps in panels 1 and 2 cotres- ward, and during one encounter the inter- pond to interwds during which the energy den- planetary field as measured on Explorer 33 sity fell below 100 ev ('m -'•. The additional gap changed from northward to southward while in panel 3 signifies that the flux fell below 10 • cm-"" sec -• ster -•, the level of internal calibration Ogo 5 was within the polar cusp. Figure 1 sources. The bottom 1)anel shows the Z-GSM displays some of the relevant Ogo 5 measure- component of the interplanetary magnetic field ments and the north-south component of the measured by Explorer 33 at 0900 LT and 42 R r interplanetary magnetic field during this period from the earth. The Explorer 33 measurements

ß ' have been plotted with an 8-rain delay, as is dis- The top panel shows the energy density of cussed in the lext. Encounters with the polar 50- to 3200-ev electrons (assumed isotropic) cusp are indicated by horizontal bars 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 magnetosphere near noon magnetopause of the southward field, we looked local time at radial distances from 2.5 to 8 RE for evidence in the Ogo 5 data. We then made and magnetic latitudes from 35 ø to 45 ø. 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 proposed that the sequence with the occasional strong erosion of mag- of observations could most readily be in- nerosphere by the southward component of terpreted as the consequence of a polar cusp the interplanetary field caused the 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 encounter 2, 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 described in more detail by Russ.ell et al. a slightly different interpretation of the mul- [1971]. tiple encounters.) Russell et al. further argued

North-South componeni of the interplanetary that this sequence was linked to the erosion field. As there are no low-energy proton of the magnetosphere when 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 analyses to 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 exceeds a threshold of 10 '• ev 1248 UT was eonvected to the magnetopause. cm -'•. The region in which low-energy electron This would imply that, during the first part flux is accompanied by a detectable flux of of encounter 2, the interplanetary field at the 0.1-to 10-kev protons at low altitudes is more magnetopause was northward and, during the limited in extent [Winningham, 1972] but is last part of encounter 2, the interplanetary field . contained within the region that we denote was southward. as the polar cusp. Figure 1 indicates that there Table 1 lists the times of the changes of tile were four encounters with the polar cusp, north-south component of the interplanetary labeled 1-4 on the figure, between 1200 and field at, Explorer 33 to the nearest minute and 1430 UT. The first encounter occurred when the corresponding encounters of Ogo 5 with the interplanetary field was southward, and tile the polar cusp to the nearest 10 see. The time third and fourth encounters occurred when the delay between the turning of the field and the interplanetary field was northward (recall that corresponding cusp encounter is seen to average the bottom panel has been shifted by 8 man). about 13 min. This delay includes both the Between the first and third encounters the time for convection from Explorer 33, which orientation of the field at Explorer 33 fluctuated was in the solar wind at (•3.4, --27.4, --1.0) on a time scale of the order of the time re- R,• in GSM, to the magnetosphere and the quired for the field to be eonvected to the mag- time for the polar cusp boundary to move from netopa•se, a delay that depends on the solar its initial position to Ogo 5. Thus, when Ogo 5 wind velocity and the orientation of •he plane entered the polar cusp at encounter 2, the in- of the discontinuity. For reasonable solar wind terplanetary field at the magnetopause had velocities and orientations of the plane of dis- been northward for some time, and, when it contin•fity the delay time to the magnetopa•se exited from encounter 2, the interplanetary can vary from less than 1 man to tens of field had been southward for some time. If mimeres. Therefore the Explorer 33 data alone we assume, however, that the poleward ve- are no• s•fiicient to establish the time at which locity during the interval of northward field

3764 KIW, LSON •,, AL.: Ta•, POLAR CUSP

TABLE 1. Timing of Events at Explorer 33 and Ogo 5

Interplanetary Field Changes

Event

Cusp Encounters

Time Event Time, UT At, min

North turning 1 South turning 1 North turning 2

1238 Entry 2 12h 52m 17s 14 1248 Exit 2 13h 02m 08s 14

1257 Entry 3 13h 08m 16s 12

and the equatorward velocity during the in- terval of southward field were equal and also that the latitudinal thickness of the cusp re- •nained constant, then the field change at the magnetopause must have occurred at a time midway between the time of entry and exit, i.e., at 1257 UT. This implies a convection time from Explorer 33 to the magnetopause of 9 min. As is discussed below, the electron temperature changed rapidly at 1256 UT, which is 1 min earlier than the midpoint of the encounter and 8 min after the field reversal at

Explorer 33. In the arguments below we assume that this was the time at which the inter-

planetary field reversed at the magnetopause. This would imply that either the equatorward motion of the polar cusp was slower than its poleward motion or the width of the cusp in- creased when the field turned southward.

Fortunately, measurements of the magnetic field made by Explorer 34, 28 RE from the earth near 0900 LT, and Explorer 35, in the magnetosheath 61 R• from the earth near 2130 LT, are also available. The times and GSM positions at which the north-south reversal of B• were observed at the three Explorer satel- lites are tabulated in Table 2. In Table 3 are

given changes observed in the magnetic field across the discontinuity from measurements of Explorer 33 and 34. If the interplanetary field reversal takes place across a surface that, on the scale of the magnetosphere, is approxi-

mated as a plane and if the solar wind velocity can be assumed constant for tens of minutes, the solar wind velocity and the orientation of the plane can be obtained from the data of the two tables. For this purpose we use the relations

(Ax -+- Vsw At)n• q- Ayn• -Jr- Azn• = 0

bB.•- = 0

where • is a unit vector normal to the plane of discontinuity, Ax, Ay, Az, and At represent the spatial and tmnporal separation of the B, reversal observed by a pair of satellites (our data provide two such pairs), and/•B.• is the change in the normal component of B across the plane of discontinuity. Values obtained for the solar wind velocity fall in the range 7-10 R• min-• (700-1100 km sec-9, the normal to the plane of discontinuity being directed antisolar and toward negative z, as shown in Table 4. The large solar wind velocity is consistent with the high mag- netosheath velocity (700-1000 km sec-9 mea- sured at Vela 4B in the subsequent hour and with the unusually small magnetopause radius (less than 7 Re) [Russell et al., 1971].

For the calculated parameters and estimated positions of the shock and of the magnetopause (Table 2) the discontinuity would arrive at the shock approximately 4 min after passing

TABLE 2. Location of Explorer Satellites and Assumed LocatioIls of Magnetopause and Shock in GSM Coordinates and Time of Observed North-South Magnetic Field Reversal

X Y Z Time, UT

Explorer 33 33.44 Explorer 34 15.56 Explorer 35 -46.21 Magnetopause 7 Shock 0.8

-- 27.42 -- 1.05 1248 q- 40 sec -- 14.73 17.46 1250 q- 20 sec

35.35 -- 19.71 1300 q- 40 sec 0 0 1256 0 0

KIVELSON ET AL.: THE POLAR Cusp 3765

TABLE3. Change of Magnetic Field Across Plane the magnetopause between 1015 and 1337 UT. of Discontinuity.

•B• •By •B,

Explorer 33* 7.5 8.0 -35.75

Explorer 34• 2.78 -5.22 -21.4

However, from 1337 to 1415 UT, Vela was back in the magnetosheath, where it measured an exceptionally large plasma flow velocity of 700-1000 km sec -•. This sequence of measure- ments implies that sometime near 1337 UT an increase in the solar wind pressure com- pressed the magnetosphere. An increase in the dynamic pressure on the magnetosphere for

* From measurements of Explorer 33 at 1254 and fixed magnetic flux in the tail causes the polar 1245 UT.

• From measurements of Explorer 34 using 1-min cusp to move to higher latitudes [Atki•son and averages at about 12h 50m 3s and 12h 48m 9s UT. Unti, 1969]. We therefore believe that the

solar wind dynamic pressure increased at about Explorer 33. Propagation through the mag- 1337 UT and, as a consequence, the polar cusp netosheath at a velocity of about I Rs min -• overtook Ogo 5 (encounter 4). The fact that would'lead to the suggested total time delay the magnetosheath was observed soon after of 8 min to the magnetopause. As the velocity at only 6 Rs [Russell et al., 1971] is con- decreases near the stagnation point, these num- sistent with this description. bers appear to be consistent with expectation. The polar cusp electrons. Having estab- The estimates are imprecis.•, because the slow lished the probable orientation of the north- sampling rate produces an uncertainty in the south component of the interplanetary field at time delay of I min, or half the time delay the magnetopause, we can see in Figure I a between Explorer 33 and 34. striking correlation between this orientation

Figure 2 displays schematically our modified and the characteristics of the polar cusp dec- interpretation of the polar cusp encounters trons: the electrons were more energetic when shown in Figure 1. The satellite left the outer the interplanetary field was northward. This zone and passed through the polar cusp (en- correlation can be seen both in the average counter 1) at a time when the cusp was either energy of the low-energy electrons and in the stationary or moving southward in response level of the flux of energetic electrons. The to the southward orientation of the inter-

planetary magnetic field. Responding to a change of the orientation of this field, the polar cusp moved toward the pole, overtaking tl•e satellite (encounter 2), which had been in the polar cap region. Before the polar cusp had completely passed over the satellite, the inter- planetary field again turned south, and the polar cusp motion was reversed, leaving the •2•0 1220 1250 1240 1250 1550 1510 1520 1550 1540 1550 satellite in the polar cap region for the second u,i•e,sa• •i,, time. Another field reversal was followed by Fig. 2. Schematic reprksentation of repeated

encounters with the polar cusp (labeled 1-4). The northward motion of the polar cusp, which horizontal line represents the satellite trajectory. again overtook the satellite (encounter 3); the The circled letters N and S indicate times when poleward motion slowed or stopped, and the the interplanetary field at the magnetopause satellite once again moved out of the polar turned north or south. These times are 8 rain

later than the times of interplanetary field cusp and into the polar cap region. As there changes at Explorer 33 listed in Table 1, as is were no further field reversals, the fourth en- discussed in the text. The interval marked C counter with the polar cusp must have been indicates the approximate time when the dynamic caused by a different mechanism, for which pressure on the magnetopause increased. Slanted there is evidence based on the measurements borders for the schematic polar cusp region are

used to indicate motion of the polar cusp. Cross- of the Vela 4B satellite reported by Russell ings of the polar cusp boundary that result from et al. [1971]. Vela, near 1900 LT and 19 R•, satellite motion rather than motion of the polar had entered the magnetosphere and was inside cusp are shown with vertical borders.

3766

TABLE 4.

KIVELSON ET AL.' THE POLAR CUSP

Orientation of Plane of Discontinuity, Solar Wind Velocity, Mean Velocity to Magnetopause for Two Cases of Table 2

Field Variation Vsw, Re min -1 •x •, • Vav, Re min -1

A 6.9 -0.98 -0.01 /-0.21 3.2 B 10.2 -0.84 0.49 --0.23 5.3

average energy of the low-energy electrons was much lower during encounter 1 and the last half of encounter 2 (labeled 2B in Figure 1) than during the first half of encounter 2 and during encounters 3 and 4. The energetic elec- trons were observed above threshold in the

polar cusp only when the interplanetary field was northward. High-resolution magnetic field data show that the brief spike of energetic electrons and the increase in the average energy of the low-energy electrons at 1300 UT were accompanied by a brief northward turning of the interplanetary field. Some evidence of this is seen in the 82-sec averages shown in Figure 1.

A quantitative presentation of the correlation is given in Figure 3, which displays histograms of the percent occurrence of specific values of number density, energy density, and average energy, in logarithmically spaced intervals for northward (unshaded bars) and southward fields (shaded bars). Because of the possible uncertainty in the determination of the exact time at which a field reversal reached the mag- netopause, we have not included data from 1256 to 12581 UT in analyzing the effect of interplanetary field orientation. The figure shows that the number density is not affected by the change in polarity but that the average energy is a factor of 2 lower for southward

fields than for northward fields. The energy density, as was expected, is also •qnaller when the interplanetary field is southward.

Since the average energy of the electrons is so different for northward and southward fields

in Figure 2, it seems very reasonable to identify the sudden decrease of the average energy of the electrons at 1256 UT with the arrival at

the magnetopause of the discontinuity across which the interplanetary field changed from northward to southward. We note that,, al- though on the scale of Figure 1 this change appears to be very rapid, the change actually occurred over a period of about 80 sec.

The increase in energy density of low-energy electrons concurrent with the appearance of energetic electron flux suggests that the en- ergetic electrons are simply the high-energy tail of a Maxwell distribution. This is not the

case, for, although the plasma is close to thermal equilibrium [Russell et al., 1971], the observed density of energetic electrons exceeds by many orders of magnitude the extrapolated density of electrons in the tail of their thermal distribution; furthermore, the spectrum has a secondary peak near 100 key. Representative spectra are shown in Figure 4. The spectra ob- served in the cusp resemble qualitatively spec- tra in the outer zone measured prior to the

'øø f

40

2O

4 8 8 16 16 32 32 64 I-2 2-4 4 8 8 16 I0 14 14 20 20-28 28-40 40 56 56 80

NUMBER DENSITY (CM -• ) ENERGY DENSITY (KEV CM :•) •VER•GE ENERGY (KEV)

Fig. 3. Histograms showing percent occurrence of certain values of number density, energy {!ensily, and average energy of electrons in the energy range 50-3200 ev in the polar cusp on November 1, 1968. Intcrva.ls within the polar cusp have been analyzed separately for time periods when the interplanetary magnetic field at the magnetopause was oriented northward (white bars) or southward (black bars).

KIVELSON ET AL.' THE POLAR Cusr 3767

io 9

io 4

PRIOR TO SECOND CUSP ENCOUNTER

CUSP ENCOUNTER NORTHWARD FIELD SOUTHWARD FIELD

1.0 0.1

Below Threshold

iooo

266 ev

19 cm -3 12,55 U T

i

io i

I000

1301 U T i

i , [ , i ß 1217 u T

--. •1254 U T

i

November I, 1968

Below Threshold

io 9

io 8

, Energy (key)

10 4

10 3

io 2

,

Fig. 4. Spectra of low-energy electrons (top) and of energetic electrons (bottom) in the outer zone prior to cusp encounter and in the polar cusp at times of northward and southward interplanetary fields.

cusp encounter. In both regions the pitch angle distribution was only slightly anisotropic with a maximum at 90 ø

Currents. Figure 5 illustrates another ap- parent difference between the two classes of cusp encounters. The top three panels display the three components of the field minus the earth's internal field in a fidd-aligned system. In this system the Z axis is parallel to the average magnetic field direction, and the Y axis is perpendicular to the magnetic meridian in the direction of electron gradient drift. The bottom panel shows. the low-energy electron en- ergy density. Two successive encounters are shown. During encounter 1 the interplanetary field was southward; during encounter 2A it was northward. During both encounters tile azimuthal component AB• shows features that would be expected to occur if the satellite passed through two twin sheets of oppositely directed current. The local sheet current den-

sity was about 10 -1 amp m -• during encounter 1. These currents were weaker by a factor of 5 or more when the interplanetary field was northward than when it was southward. During both encounters the current sheet on tile polar

cap side of the cusp occurred in a region in which the low-energy electron flux was at or near background. However, in both cases the depression of the magnetic field indicates that there were energetic particles present in this region, most probably protons. We note that during the second encounter the bending of the field in the magnetic meridian as measured by ABx produced field changes comparable in mag- nitude to the sheet current effects in ABe.

Armstror, g and gmuda [1970], using mag- netometer data from the satellite 1963-38C, have measured tile field-aligned currents flow- ing into the auroral zone on this day at 1505 UT, at 0900 LT, and at 66 ø invariant latitude. The current they measured at the time was the same as the cusp currents associated with south- ward interplanetary fields that Ogo 5 encoun- tered deep in the magnetosphere 2 hours earlier when these currents are corrected for the con-

vergence of the field lines from the Ogo 5 alti- tudes to 1100 km. At the time of tile Armstrong and gmuda measurements the dayside auroral oval was at low latitudes, the interplanetary field was southward, and the solar wind velocity was high [Russell et al., 1971], resembling the

3768 K•VELSO• sT AL,: T•s POLAR CUSP

,80[ OUl'

I O0 L

-40

12•0 12• 1•4 I•fi •8

Cusp 1

-200 1

ø I AZlM

-40

-40

1248 1250 1252 1254 1256

Cusp 2A

Fig. 5. The top three panels show three components of the perturbation magnetic field (the earth's internal field has been subtracted) in a coordinate system for which the Z axis is aligned along the average magnetic field and Y is positive in the direction of electron drift; zX[B[ is plotted as a dashed line. The bottom panels show the energy density of 50- to 3200-ev electrons. Panels to the left show measurements made during a period (cusp 1) when the inter- planetary magnetic field was southward. Panels to the right show corresponding measurements during a period (cusp 2a) when the interplanetary magnetic field was northward.

conditions 2 t!ou.rs earlier during the Ogo 5 polar cusp encounters.

In addition to these field-aligned sheets of current which last of the order of minutes, there are strong impulsive field-alig.ned currents last- ing seconds of which three examples are plotted in Figure 6. Here .t•e Z axis is along the field, and the Y axis is •zimuthal eastward.

We note that tile I)erturbations are almost completely perpendicular to the background field. Strong perturbations, as seen in the left- hand panels, occurred only when the inter- planetary field was southward. Associated with these strong impulsive currents were strong VLF electrostatic noise [Scarf et al., 1972; Fredricks et al., 1973]. The panel on the lower right shows an example of a wave train occur- ring at about 80% of the local ion cyclotron frequency. We note that these waves are not cotnpletely transverse to the field. Such waves were seen only when the interplanetary field was northward. Tile nature of these waves and

a possible growth mechanism have been dis- cussed by Fredricks and Russell [1973].

ELF magnetic and VLF electric records were also scanned for this orbit. Scarf et al. [1972] already described how all of the VLF electric field bandpass channel measurements and the

associated broad band dynamic spectra varied on November 1, 1968. However, the plasma wave measurements discussed by Scarf et al. do not give adequate temporal detail for the pur- poses of the present study (the bandpass chan- nel scan sequence of 3.23 min is too long, and the high-resolution broad band recordings con- tain essentially no quantitative amplitude in- formation). Another wave measurement param- eter with uniform sampling characteristics and improved time resolution was therefore analyzed to search for a possible north-south variation in the wave levels. We examined the wide band

(1-22) kHz VLF electric field strength for all cusp encounters, to determine if the local wave levels systematically varied in accordance with the changing plasma distribution functions. This parameter is measured for 1.152 sec every 9.216 sec, and it therefore provides information on an adequate time scale; in this part of the orbit on November 1, 1968, the spacecraft telemetry rate was 64 kilobits/sec, and each 1.152-sec in- terval contained 64 discrete •neasurements. No

significant differences in these records were noted for northward and southward interplan- etary fields except for tile occurrences of bursts of VLF electric noise in association with stro.ng imlmlsive cqrrents such as those ill the first

KIVELSON ET AL.: THE POLAR CUSP

two panels. These strong currents, as was men- tioned before, were seen only when the inter- planetary field was southward.

Dlscusslo•

Although the observati6ns of the polar cusp made by Ogo 5 on November 1, 1968, occurred during a large geomagnetic storm, it is probable that many of the physical processes occurring in the polar cusp were not drastically altered by the storm, but oniy increased in strength. If so, the cusp must be affected by the north- south orientation of the interplanetary magnetic field at quiet times as well. For southward inter- planetary magnetic fields the electron plasma was cooler and the field-aligned currents were stronger than for northward in;rerplanetary fields, and the cusp was displaced toward the equator. For the northward field orientation, energetic electrons resembling outer-zone elec- trons were present together with cusp particles, and Pc 1 micropulsations were observed.

The observation of equatorward motion of the polar cusp during intervals of southward interplanetary field is in agreement with the observations of the erosion of the magneto- spheric field. The erosion of the magnetospheric field, of course, implies. only the presence of a viscous interaction of the magnetosphere with the solar wind and can occur for both open and closed models of the magnetosphere. Its cor-

3769

relation with the southward component can be explained by several mechanisms, one of which is merging [Piddington, 1969].

The observation of intense currents in the

cusp when the interplanetary field is southward also can be explained by an open or a closed model of the magnetosphere. In an open model a Hall conductivity enhancement. produced by soft electron precipitation in combination with a convective electric field imposed by the merg- ing at the magnetopause leads to field-aligned sheet currents at the poleward and equatorward edges of the dayside oval (C. F. Kennel and M. H. Rees, unpublished manuscript, 1971). In a closed model these field-aligned currents could represent the closure inward across the mag- netopause of the currents responsible for the drag on the magnetosphere [Piddington, 1969].

The presence of energetic electrons resem- bling outer-zone sp(/ctra in the cusp plasma is evidence that the cusp plasma can exist on closed field lines when the interplanetary field is northward. Both a closed magnetosphere model and a model in which the magnetosphere is open but the dayside merging rate is low are consistent with this observation. Early models of the cusp that described the diffusion of magnetosheath plasma onto closed field lines [Spreiter and Summers, 1967; Willis, 1969] were developed for a closed magnetosphere. In an open model, cusp plasma can be found on

Bx 0

By 0

$ Bz 1350y _

oo

• Bx 0

By 0

Bz 1250/

Interplanetary F•eld South Interplanetary F•eld North

Cusp 1 1220 37

------. l.k

^,,• Tsor

I0 Seconds 1222 59

Cusp 2A 1252 39

ilS)'

I0 Seconds - J•

550/

530/

Fig. 6. Examples of magnetic field variatirins observed during encounters with the polar cusp for southward oriented magnetic fields (left-hand panels) and northward oriented mag- netic fields (right-hand panels). The field components are shown in a coordinate system in which the Z axis is along the field and the Y axis is azimuthal eastward.

3770 KIVELSON ET AL.' THE POLAR Cusp

closed field lines if the plasma that diffuses onto closed field lines is only slowly convected away. Dayside merging produces convection that op- poses the plasma diffusion, and so our results indicate that the dayside merging rate becomes very low when the interplanetary field is north- ward. This conclusion is consistent with the ob-

served relation between the position of the polar cusp and the orientation of the field. For a southward orientation of the interplanetary field, outer-zone electrons remain separated from magnetosheath plasma, because the open polar cusp field lines cannot trap energetic elec- trons while the convection arising from dayside merging opposes diffusion of plasma onto closed field lines.

We do not have a simple explanation for the observation of stronger currents and cooler electrons in the polar cusp when the interplan- etary field is southward. The closed models of the magnetosphere do not appear to predict any correlation of electron temperature with the north-south component of the interplanetary field. The particles in the cusp arrive, inde- pendent of interplanetary field orientation, from the direct influx of magnetosheath particles [Spreiter and Summers, 1967; Willis, 1969]. They are stopped by some unspecified mech- anism, and their kinetic energy is transformed into thermal energy. The kinetic energy of •nagnetosheath electrons is much smaller than their thermal energy, so that some energy trans- fer from the protons (for which kinetic energy is comparable with thermal energy) is required if the electrons are to become hotter than the

magnetosheath electrons, as our observations suggest. We note that the observed electron temperatures in the cusp on November 1, 1968, were a factor of about 4 greater than those measured simultaneously at Vela when the field was northward. It is unlikely that all this tem- perature difference can be attributed to the fact the distance from the stagnation point to Vela was greater than the distance from the stagna- tion point to the intersection of the magneto- pause and the polar cusp. We note that Walters [1966] has proposed that there exists a second shock front standing upstream from the polar cusp. In analogy with the earth's bow shock we would expect this shock to transfer some of the kinetic energy of the protons into thermal energy of the electrons. However, Spreiter and

Summers [1967] argue against the existence of this second shock.

If we attribute the observed correlation to a

change in topology of the polar cusp from closed to open as the interplanetary field turns from north to south, we must conclude that there is little acceleration of the magnetosheath dec- trons in the merging process. This appears to be contrary to the predictions of the model [Speiser, 1969]; however, quantitative calcula- tions of this. acceleration have not been made.

For example, the acceleration depends on the size of the low field region in the magnetopause current layer. This region can be quite small in some merging models [cf. Petschek, 1963]. Thus, although control of the characteristics of the polar cusp by the north-south component of the interplanetary magnetic field is con- sistent with the hypothesis that the location of the polar cusp moves from closed to open field lines as the interplanetary field turns from north to south, none of the three primary ef- fects observed on November 1, 1968, is con- clusive proof of this hypothesis.

The existence of energetic electrons in the cusp plasma, for northward fields only, how- ever, may provide evidence that such a con- figurational change takes place. The loss of energetic electrons on closed lines of force from the polar cusp is principally due to drift mo- tions out of the cusp. On open lines these elec- trons will be lost, owing to their bounce motion, which is much more rapid. Thus, if the rate at which these electrons enter the cusp region remains roughly the same, we would expect much larger fluxes. when the field was closed then when it was open.

With this interpretation the energetic elec- tron data thus indicate that the polar cusp was encountered on closed field lines when the in-

terplanetary magnetic field was northward and on open lines when it was southward.

Our observations are thus consistent with an

open model of the magnetosphere in which merging of the interplanetary magnetic field with the magnetospheric field transports flux from the dayside of the magnetosphere into the tail. The rate of this flux transport is de- pendent. on the magnitude of the north-south component of the interplanetary field such that this transport essentially, ceases for northward fields. At such times, the polar cusp plasma can

KIVELSON ET AL.: THE POLAR Cusp 3771

remain on closed field lines because convection

into the magnetopause is slow, and the dayside magnetosphere closely resembles what would be expected in a closed magnetospheric model.

The similarity of electron spectra in the mag- netosheath and the polar cusp has been noted by other investigators [Frank, 1971; Heikkila and Winningham, 1971] without reference to the orientation of the interplanetary magnetic field. If merging at the dayside magnetopause played a key role in the energization of polar cusp electrons, the polar cusp electron spectra would not resemble the magnetosheath spectra in the presence of a southward interplanetary magnetic field. The evidence that the average energy of the electrons in the polar cusp dou- bles when the interplanetary field goes from south to north suggests that merging at the day- side magnetopause plays at most a secondary role in the energization of polar cusp electrons.

These observations, at the very least, em- phasize the need during the present exploratory phase of polar cusp investigations to study the properties of the polar cusp as a function of the orientation of the interplanetary magnetic field. Unless this is done, a very confused pic- ture of the polar cusp may restrlt.

CONCLUSIONS

Ogo 5 observations on November 1, 1968, show that the polar cusp moved equatorward when the interplanetary field turned southward, that the polar cusp electrons were cooler, and that more intense currents were observed when the field was southward than when it was north-

ward. Further, regions of large ULF fluctua- tions associated with enhanced VLF electric field

oscillations occurred within the cusp during in- tervals of southward fields apparently caused by fluctuations in these current systems. When the interplanetary field was northward, these fluctuations still occurred but at a much re-

duced level. Finally, energetic electrons and quasi-monochromatic waves at about 80% of the local proton gyrofrequency were observed in the polar cusp when the interplanetary field was northward but not when it was southward.

These observations are consistent with merg- ing of the interplanetary field with the mag- netospheric field when the interplanetary field is southward and the cessation of merging when the field turns northward. They also indicate

that there is little energization of the mag- netosheath electrons in the merging process. These observations emphasize the fact that the measurements of the polar cusp properties should be ordered by the north-south com- ponent of the interplanetary fidd before general conclusions are drawn, especially when compar- ing nonsimultaneous data.

Acknowledgments. Other coinvestigators for the Ogo 5 experiments discussed in this paper were C. W. Snyder (plasma spectrometer), P. J. Coleman, Jr., T. A. Farley and D. L. Judge (flux- gate magnetometer and energetic electron spec- trometer), and G. M. Crook (VLF electric field experiment). We are indebted to C. P. Sonerr and D. S. Colburn for the interplanetary data from their NASA Ames Research Center mag- netometer on Explorer 33. We thank R. K. Bur- ton and R. E. Holzer for allowing us to examine their data in advance of publication and Harry I. West, Jr., for providing data on the flux of energetic protons. Finally, we thank J. L. Burch for many helpful suggestions, particularly for pointing out the availability of Explorer 34 inter- planetary data.

The work at UCLA was supported by the National Aeronautics and Space Administration under NASA contract NAS 5-9097 (electron spec- trometer) and under NASA grant NGR 05-007- 305 (Explorer 33 correlations). The work at, TRW was supported by NASA contract NAS 5-9278. The work at JPL represents one phase of research carried out under NAS 7-100 sponsored by the National Aeronautics and Space Administration.

, ß ß

The Editor thanks J. L. Burch and B. tt. Son-

nerup for their assistance in evaluating this paper.

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(Received November 1, 1972; accepted February 6, 1973.)