1Institute of Geophysics and Planetary Physics, University of California,Los Angeles, CA, USA
2Department of Earth and Space Sciences, University of California, Los Angeles, CA, USA
3STELAB, University of Nagoya, Japan
4Lockheed Martin, Palo Alto, CA, USA
5Space Research Laboratory, University of Michigan, USA
6Space Sciences Laboratory, University of California, Berkeley, CA, USA
Originally published in Adv. Space Res., 25(7/8), 1369-1375, 2000.
On May 4, 1998 the velocity and density of the solar wind were high and the interplanetary magnetic field was strong and southward. The POLAR spacecraft crossed the dayside magnetopause well inside geosynchronous orbit, at 5.3 RE and a solar zenith angle of 19o. After this crossing, POLAR spent most of the rest of its outbound orbit in the magnetosheath and for brief periods crossed into the solar wind at distances from 7.3 RE and a solar zenith angle of 32o to a distance of only 8.5 RE and a solar zenith angle of 45o. This corresponds to subsolar distances of only 6.8 to 7.5 RE for the shock. During this very disturbed period of time, predictions of the locations of the magnetopause by both Shue and co-workers and by Petrinec and co-workers indicate extremem distortions of the magnetopause location. Because of the importance of such events to the understanding of space weather, we recommend that this event be pursued as a special IACG 2 campaign.
Many models of the location of the magnetopause have been published but only two make a reasonable case for treating these extreme conditions (Shue et al., 1997; Petrinec and Russell, 1995). It is important that such claims be verified with extreme solar wind data that compress the magnetopause well inside synchronous orbit. Moreover, it is important to obtain data away from the equatorial plane. May 4, 1998 provides us with such an opportunity.
|Figure 1.Solar wind and interplanetary magnetic field conditions on May 4, 1998 from 0000 to 1600 UT. Left. WIND observations of the interplanetary field in GSM coordinates. Right. WIND observations of the solar wind velocity, dynamic pressure and thermal velocity.|
To anticipate what POLAR observed on this day, Figure 2 shows the predicted position of the magnetopause at the subsolar point and along the flanks according to the Shue et al. (1998) model. The magnetopause crosses synchronous orbit repeatedly, getting as close as 5 Re in the subsolar region. Above the terminators the magnetopause reaches a distance of about 8 Re.
When the disturbed solar wind reached the Earth, the POLAR spacecraft was at perigee. Figure 3 shows the projection of the POLAR orbit in the noon-midnight trajectory as expressed in solar magnetospheric coordinates. The plane of the orbit is close to this plane, being centered roughly on the 11 AM meridian at this time. We recall that the solar magnetospheric coordinate system (GSM) differs from the solar magnetic coordinate system (SM) by the tilt of the dipole axis. The GSM system is most useful for examining boundary locations that are ordered more by the direction of the solar wind flow than by the Earth's field. The SM coordinate system is most useful for examining the boundaries well within the magnetosphere where magnetic stresses dominate over solar-wind-induced stresses.
|Figure 2. Predicted size of the magnetosphere at the nose and the terminator (bottom panels) according to the southward IMF and solar-wind dynamic pressure shown in the top two panels.These panels do not account for the propogation time to the Earth from WIND||Figure 3. The projection of the POLAR orbit in the noon-midnight meridian in GSM coordinates.|
When the interplanetary magnetic field (IMF) is northward, the magnetopause location is essentially determined by the dynamic pressure scaled by the sixth root of the dynamic pressure. One way to illustrate the effect of the solar wind is to scale the location of the satellite by this factor and keep the boundary fixed. This is done in Figure 4 that shows the distance to POLAR as a function of solar zenith angle. Whenever the scaled orbit is beyond the magnetopause, POLAR is predicted to be in the magnetosheath by the solar-wind pressure alone. Figure 4 shows that this occurs during the majority of the time from approximately 0600 UT onward to 1230 UT. POLAR approaches but does not cross the bow shock according to these predictions.
|Figure 4. The POLAR orbit in GSM coordinates expressed as distance from the Earth as a function of solar zenith angle. The irregular curve is an expansion of the distance of the spacecraft from the Earth by the sixth root of the dynamic pressure. The average magnetopause and bow shock location for a dynamic pressure of 2nPa are shown.||Figure 5. The magnetic field measured at POLAR across the 0541 UT magnetopause crossing in GSM coordinates.|
Figure 5 shows the magnetic field observed by POLAR (Russell et al., 1995) on May 4, 1998 from 0540 to 0546 UT. At this time the IMF as convected to Earth should be still strongly southward but near the end of its long southward orientation period. The magnetopause crossing at 0541:22 is clearly seen and is most impressive. Prior to the crossing, the magnetic field is over 200 nT northward; it then switches to over 200 nT southward. There is a small dip in the magnitude of the field as the crossing is made but no overall change in field magnitude; thus, the discontinuity is principally a rotational discontinuity indicating strong reconnection at this point. The spacecraft returns briefly to the magnetosphere at 0544:30. At the two crossings of the magnetopause the magnetic field now decreases greatly from the magnetosphere to the magnetosheath.
|Figure 6. The magnetic field measured at POLAR at 6 sec resolution in GSM coordinates for the entire period from 0530 UT to 1530 UT.|
Figure 6 shows the POLAR magnetic field for the entire period from 0530 to 1530 UT on May 4, 1998 in GSM coordinates. On this plot the magnetopause crossing at 0541 UT, discussed above, can most readily be recognized in the Bz reversal. Later the magnetic field turns northward while still depressed in magnitude. This is probably associated with a northward turning of the IMF. At about 0615 UT the field becomes quieter and lies more in the expected direction of the magnetoshperic field. This probably signals the reentry into the magnetosphere. At 0648 UT the field rotates and the magnitude drops signaling a reentry into the magnetosheath. After this point the field is disturbed almost continually until about 1205 UT. It is difficult to rule out any magnetospheric reentry without further analysis of the field and plasma data but it is reasonable to assume that POLAR is predominantly in the magnetosheath during this interval. Finally, there is a period of disturbed and depressed magnetic field from 1336 to 1359 UT, which appears to be a final entry into the magnetosheath.
There are three surprising events during the interval, two of which are shown
in Figure 7. These are crossings of the bow shock and entry into the
solar wind. The first pair of crossings is at 0735:45 and 0738:10 when the
spacecraft was at (6.17, -1.24, 3.74) RE in GSM coordinates. This
corresponds to a subsolar shock distance of about 6.8 RE close to
synchronous orbit and a magnetopause distance of about 5.3 RE. A
second pair of shock crossings is at 0919:30 and 0920:40. The third pair of
shock crossings occur at 0944 and 0950 UT. POLAR was at 8.5 RE at this time at a solar zenith angle of
45o. This corresponds roughly to a subsolar shock location of 7.5
In order to test if we can accurately predict the location of the
magnetopause under these extreme conditions, we have calculated POLAR's distance
from the magnetopause according to the Shue et al. (1998) and Petrinec and
Russell (1995) models. These predictions are shown in Figure 8. The first
entry into the magnetosheath at 0541 is correctly predicted by the both models. They also
predict an extended entry of the magnetosheath from 0845 to 1200 UT. However, it is
difficult to verify when POLAR precisely left the magnetosphere near 0845 UT from
the magnetic field observations alone. Neither model
predicts the period of magnetosheath entry from 1335 to 1400 UT but the Petrinec
and Russell (1995) model predicts closer approach to the boundary than the Shue
et al. (1998) model. This failure to predict a magnetosheath entry may be
due to the non-cylindrical symmetry of the magnetopause.
Figure 7. The magnetic field
measured at POLAR at 120 ms resolution in GSM coordinates for two periods of
entry into the solar wind. Left. 0735-0739 UT Right. 0940 to 0952 UT.
CORRESPONDENCE WITH PREDICTIONS
In order to test if we can accurately predict the location of the magnetopause under these extreme conditions, we have calculated POLAR's distance from the magnetopause according to the Shue et al. (1998) and Petrinec and Russell (1995) models. These predictions are shown in Figure 8. The first entry into the magnetosheath at 0541 is correctly predicted by the both models. They also predict an extended entry of the magnetosheath from 0845 to 1200 UT. However, it is difficult to verify when POLAR precisely left the magnetosphere near 0845 UT from the magnetic field observations alone. Neither model predicts the period of magnetosheath entry from 1335 to 1400 UT but the Petrinec and Russell (1995) model predicts closer approach to the boundary than the Shue et al. (1998) model. This failure to predict a magnetosheath entry may be due to the non-cylindrical symmetry of the magnetopause.
|Figure 8. The predicted distance of the POLAR spacecraft from the magnetopause according to two models of the dependence of the magnetopause location on IMF conditions. (Top) The Shue et al. (1998) model. (Bottom) The Petrinec and Russell (1995) model. This figure does take into account the time delay from WIND to the Earth.|
Petrinec (1998) has produced a movieof the predicted magnetopause (Petrinec and Russell, 1995; 1996) and shock locations (Farris et al., 1991; Farris and Russell, 1994) during the May 4 period. Figure 9 shows one frame of that movie at a very compressed epoch near 0500 UT. The nose of the bow shock here is predicted to reach synchronous orbit. The movie documents the rapid dynamic changes experienced by the magnetosphere on this day. It is clear that many spacecraft at geosynchronous orbit left the magnetosphere for a time on this day. Another type of prediction for this event regards the buildup of the ring current as measured by Dst. Figure 10 shows the solar-wind conditions measured by ACE and the predicted Dst index from these data according to the Fenrich and Luhmann (1998) model that modified the formula of Burton et al. (1975). The prediction very nicely agrees with the preliminary Dst shown in the bottom panel of Figure 10. The maximum Dst value is close to -250 nT. In short this interval is valuable not only as it illustrates the extreme size distortions under which the magnetosphere can undergo but it also provides a good case study of the development of a larger storm.
Figure 9.A snapshot of the distortion of the magnetosphere and bow shock on May 4, 1998 from a movie of the interaction. The top panels show the IMF Bz and solar wind dynamic pressure. The bottom panel shows the ecliptic plane cross sections of the magnetopause and bow shock. The dashed circle is synchronous orbit.
|Figure 10. The solar wind conditions observed by ACE on May 4, 1998 and the predicted and observed Dst indices. Top panel shows the solar wind density; the second panel the solar wind to speed; the third panel the IMF Bz; the fourth panel the predicted Dst index and the bottom panel the preliminary Dst index.|
The solar wind conditions on May 4, 1998 were extremely disturbed and led to a very large geomagnetic storm. These disturbances greatly compressed the magnetosphere and that compression was documented by the POLAR spacecraft. Some of the existing models do very well at predicting the compression of the magnetosphere and the ensuing buildup of the ring current; nevertheless, improvements in these models can be made and May 4 should be very useful in determining what adjustments need to be made in these models. We nominate this interval for study by the IACG Boundary Layer Campaign.
We are grateful to A.J. Lazarus, K.W. Ogilvie and J.T. Steinberg for providing the WIND ion data and R.P. Lepping for providing the WIND magnetic field measurements. This work was supported by the National Aeronautics and Space Administration under research grant NAG 5-3171.
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