Magnethosheath magnetic field variability

Adv. SpaceRes. Vol. 14, No. 7, pp. (7)91-(7)94, 1994 Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved. 0273-1177/94 $6.00 + 0.00

Pergamon

MAGNETOSHEATH MAGNETIC FIELD VARIABILITY D. G. Sibeck The Johns Hopkins University, Applied Physics Laboratory, Laurel MD 20723, U.&A.

ABSTRACT A case study using simultaneous IRM and CCE observations demonstrates that transient magnetospheric events correspond to pressure pulses in the magnetosheath, inward bow shock motion, and magnetopause compression. Statistical surveys indicate that the magnetosheath magnetic field orientation rarely remains constant during periods of magnetopause and bow shock motion (both characterized by periods of 1-10 min). There is no tendency for bow shock motion to occur for southward IMF orientations. INTRODUCTION Transient events, often called flux transfer events (or FTEs)/1/, are common in the outer magnetosphere. Bipolar magnetic field signatures normal to the nominal magnetopause and enhancements in the total magnetic field strength mark some events. Although FTEs were originally interpreted as evidence for magnetic merging at the dayside magnetopanse, they may also indicate wavy magnetopanse motion driven by solar wind dynamic pressure pulses/2/. It should be relatively easy to distinguish between the magnetospheric signatures of FTEs and waves. FTEs should occur almost exclusively during periods of southward interplanetary magnetic field (IMF), whereas pressure pulses should be related to solar wind features which are equally common during periods of northward and southward IMF. Simultaneous multipoint observations reveal that many transient magnetospheric events can be directly related to corresponding solar wind dynamic pressure pulses seen by a monitor appropriately located just upstream of the subsolar bow shock/2,3,4/. Thus attention must now turn to magnetospheric event occurrence patterns as a function of the IMF orientation. Under the assumption that the magnetosheath magnetic field orientation observed by a spacecraft just outside the magnetopause is similar to that when the same spacecraft is in the magnetosphere, it has been claimed that magnetospheric FTEs occur primarily during periods of southward IMF/5/. This practice has been justified on the basis that the IMF generally retains its orientation for periods of 30 min or longer/6/. In this paper, we will show that this assumption is in error and confirm that transient magnetospheric events correspond to inward magnetopanse and bow shock motion CASE STUDY Figure 1 shows CCE magnetic field observations in GSE coordinates from 1200-1500 UT on October 9,1984. The CCE was near the GSE equator, moved inward from 8.8 to 7.5 R E and from 12.4 to 13.4 LT. Several transient events may be identified on the basis of compressions in the total magnetic field strength and inward/ outward B x variations. That at 1243 UT is most prominant, but others occur at 1345, 1400, 1408,1420, 1430, and 1440 UT. JASA 14:7-6

(7)91

(7)92

D . G .

Sibeck

October9, 1984 CCE i

,

~

,

i

.

q

October 9, 1984 IRM

, 50 8 0 0

4oaz g

0

t

•

,

•

,

-

,

-

,

*

I

=

I

,

I

~

,

400

~

.

~

,

,

.

~

,

~

,

~

r

-

.

3O

0

•

~o~ -.40

10 "E" 0

300

t50

~100

lo g0

Bx

30 O

-30 150

-90

m

50

,

1200

i

,

i

,

i

1300

,

t

i

UT

,

I

1400

.

i

i

.

40

o

1500

Fig. 1, CCE magnetic field observations in GSE coordinates.

1200

,

1300

UT

1400

,

i

,

1000

Fig. 2. IRM observations in GSE coordinates. From top to bottom, the figure shows the ion velocity, density, magnetic field longitude, latitude, and strength, where • = ® = 0 ° is sunward, cI, = 90, O = 0 ° is duskward, and ® = 90 ° is northward.

Figure 2 shows simultaneous IRM plasma and magnetic field observations in GSE coordinates. The IRM was near the GSE equator, moved inward from 12.8 to 9.4 R E and from 12.8 to 13.4 LT. As shown in Figure 2, the IRM began the interval in the magnetosheath (velocity = 150-200 km s-1, ion density = 10 cm-3), briefly entered the solar wind (V = 750 km s-1, n = 4 cm "3) from 1242-1243 UT, entered the magnetosphere (steady northward magnetic field, V < 100 km s "~, n < 5 cm "3) at 1401 UT, made several exits back into the magnetosheath (1405-1407, 1420-1425, 1429-1433, and 1440-1442 UT), and ended the interval in the magnetosphere. Although the magnetosheath magnetic field was northward just prior to the outermost magnetopause crossing (at 1401 UT) and during the next magnetosheath interval (from 1405-1407 UT), it was southward at 1347 UT and from 1420-1425 UT, This suggests that the magnetosheath magnetic field orientation varies greatly in the vicinity of magnetopause crossings. The greatest densities during the 3 hr interval occurred during the brief returns to the magnetosheath following the 1401 UT magnetopause crossing. Perhaps brief, but strong, enhancements in the magnetosheath density and pressure caused the subsequent reentries into the magnetosheath. The magnetoshcath interval from 1210-1240 UT, i.e. just prior to the brief excursion into the solar wind, was highly disturbed with several fluctuations in the magnetic field orientation, as well as the density and velocity. The dashed lines in Figure 3 indicate that IRM density pulses correpond to CCE magnetic field strength increases, with lag times decreasing from -2-3 rain at the start of the interval to - 0 min at the end. The initial lag is consistent with an antisunward flow of 100 km s -1 from the IRM at 12.8 R Eto the magnetopause at 10.8 R E. The brief IRM excursions into the dense rnagnetosheath plasma from 1400-1500 UT indicate magnetopause crossings, so we may also conclude that transient events at the CCE correspond to magnetopause motion at greater radial distances. The brief inward bow shock motion observed by the IRM at 1242-1243 UT corresponded to the strong compression of the magnetospheric magnetic field observed by the CCE. Note that these correspondences imply that the IRM density features are not standing waves, but rather features

Magnetosheath

Magnetic

Field Variability

(7)93

Odo~er g, 1984 30 IIIM

I

I

b0

130 I

~110

~

I

I

~

'.

I

,

I

.

,

I

I

_

.

70

1200

1300

UT

1400

1500

Fig. 3. A comparison of IRM density and CCE magnetic field strength.

impinging upon the magnetopause. One does not expect a spacecraft just inside the bow shock to observe such density pulses since the bow shock would instead move inward past the spacecraft. STATISTICAL SURVEYS We examined each of 27 IRM passes from Day 245 to 304 in 1984 and noted the north/south magnetosheath magnetic field orientation just outside the outermost magnetopanse crossing. We then determined the time until the nearest southward/northward turning of the magnetosheath magnetic field. Each turning was required to last at least 1 min and reach 20 ° N or S. The top panel of Figure 4 shows that the magnetosheath magnetic field seldom retained a constant orientation longer than 16 min from a magnetopause crossing and often (>50%) changed within 10 min. Since our results differ from those concerning the average IMF/6/, we suggest that the IMF is more variable during periods of magnetopause motion than on average. We examined all of the IRM bow shock crossings for which both plasma and magnetic field observations are available during the same interval in 1984. After identifyingthe north/south IMF orientation at the time of each bow shock crossing, we determined the times to the previous and subsequent crossing, and the times to the previous and subsequent southward/northward variation in the IMF direction in the manner described above. The second panel of Figure 4 indicates that north/south turnings generally occur within 8 min of bow shock crossings, but that on occasion none are observed within 30 min. The bottom panel of Figure 4 shows a histogram of the intervals between bow shock crossings which indicates that bow shock crossings often recur on time scales of 1-8 min. We examined the north-south component of the magnetic field during all of the intervals in which two bow shock crossings were observed within 5 rain. Of 66 paired crossings, the magnetic field remained northward 20 times, southward 22 times, and varied 24 times. There is no tendency for shortperiod bow shock (and by inference magnetopause) motion to occur predominantly during periods of southward IMF. Figure 5 shows the time from each bow shock crossing to the next (previous) crossing versus the time to the next (previous) north-south IMF turning. The preponderance of points in the lower left comer indicates how common rapid bow shock motion is and its tendency to correspond to rapid IMF variations. The large number of points in the upper right comer of the plot indicates that isolated bow shock crossings are common during periods of steady IMF. The paucity of points in the lower right comer indicates that isolated bow shock crossings rarely occur during periods of varying IMF. Finally, the large number of points in the upper left comer of the plots suggests that multiple bow shock crossings may occur in the absence of rapid north/south IMF fluctuations (although east-west variations are also common). Since the location of the bow shock depends on the fast mode velocity in the magnetosheath, which in turn depends upon the IMF orientation and solar wind density, the observations are consistent with a suggestion that the IMF orientation is one factor controlling the location of the bow shock/7/.

(7)94

D. G. Sibeck

>3°12811's ~20 I_1

O 60 40

~

20

=~12 2 2 2

*~

#

0

71

Time between bow s h o ~

80 1

/

0

4

8

12

16 20 T(min)

24

28

32

Fig. 4. From top to bottom: time from magneto-

pause crossing to nearest north/south magnetosheath magnetic field turning, time from bow shock crossing to previous and next north/south magnetosheath/ interplanetary magnetic field turning, time from bow shock crossing to previous and next bow shock crossing.

13

I118

1

3

1

1 2 S"

1

3

1

1

2

11 6221 -9712 I..178372242 4 29106412 11 S i t 41

0 0

o

1

16 -3

lll

1

1 1

° - -

[]

I1 211

1 i

11 2 21

3 I

4 21 11

2 11

1 2I

I

4 8 12 16 20 24 28 >30 Time to next/last bow shock crossing

Fig. 5. The time from each bow shock crossing to the previous/next bow shock crossing versus that from each bow shock crossing to the previous/next north/ south magnetosheath or interplanetary magnetic field turning.

CONCLUSIONS We presented simultaneous IRM magnetoshcath and CCE magnetospheric observations indicating that transient magnetospheric events correspond to variations in the magnetosheath density as well as bow shock and magnetopause motion. A statistical survey indicates that the interplanetary/magnetosheath magnetic field orientation seldom remains steady longer than 15 min from either magnetopause or bow shock crossings. Like magnetopause motion, periods of 1-10 min characterize bow shock motion and IMF fluctuations. Shortperiod (<5 rain) bow shock motion exhibits no dependence upon the north/south interplanetary/magnetosheath magnetic field orientation. Variations in the solar wind dynamic pressure are a plausible explanation for transient bow shock and magnetopanse motion, as well as transient events in the outer dayside magnetosphere.

Acknowledgments. This work was supported by NASA under Task 1 of Space and Naval Warfare Systems Command contract N00039-89-C-5301 to the Navy. IRM observations were provided by the NSSDC. REFERENCES 1. C. T. Russell and R. C. Elphic, Space Sci. Rev. 22, 681 (1978). 2. D. G. Sibeck et al., J. Geophys. Res. 94, 2505 (1989). 3. D. H. Fairfield et al., J. Geophys. Res. 95, 3773 (1990). 4. D. G. Sibeck, J. Geophys. Res. 97, 4009 (1992). 5. R. P. Rijnbeek et al., J. Geophys. Res. 89, 786 (1984).

6. M. Lockwood, J. Geophys. Res. 96, 5497 (1991). 7. G. K. Waiters, J. Geophys. Res. 69, 1769 (1964).