G. Le

THE GGS/POLAR MAGNETIC FIELDS INVESTIGATION

The magnetometer on the POLAR Spacecraft is a high precision instrument designed to measure the magnetic fields at both high and low altitudes in the polar magnetosphere in 3 ranges of 700, 5700, and 47000 nT. This instrument will be used to investigate the behavior of fieldaligned current systems and the role they play in the acceleration of particles, and it will be used to study the dynamic fields in the polar cusp, magnetosphere, and magnetosheath. It will measure the coupling between the shocked magnetosheath plasma and the near polar cusp magnetosphere where much of the solar wind magnetosphere coupling is thought to take place. Moreover, it will provide measurements critical to the interpretation of data from other instruments. The instrument design has been influenced by the needs of the other investigations for immediately useable magnetic field data and high rate (100+vectors s−1) data distributed on the spacecraft. Data to the ground includes measurements at 10 vectors per second over the entire orbit plus snapshots of 100 vectors per second data. The design provides a fully redundant instrument with enhanced measurement capabilities that can be used when available spacecraft power permits.

Solar wind control of the polar cusp at high altitude

The POLAR mission is ideally suited to study the high-altitude polar cusp. Polar magnetometer data, together with electron and ion measurements from the Hydra and Toroidal Imaging Mass-Angle Spectrograph (Timas) instruments from March 1996 to December 1997, have been used to identify 459 polar cusp crossings. These crossings are used to study the statistical behavior of the cusp location and its dependence on the solar wind conditions. We find that the invariant latitude of the center of the cusp varies from 70° to 86° as solar wind conditions change and the magnetic local time of the footprints of the cusp magnetic field lines extends from 0800 to 1600 MLT, the cusp being slightly wider for increasing solar wind dynamic pressure. The average latitude of the center of the cusp is at 80.3° invariant latitude at noon and decreases to 78.7° at 0800 and 1600 MLT. The cusp also appears to thicken slightly in invariant latitude with increasing dynamic pressure. The center of the cusp moves equatorward with increasingly southward interplanetary magnetic field (IMF) to 73° invariant latitude for a 10 nT southward IMF. The cusp moves only slightly for northward IMF. This motion is consistent with erosion of dayside magnetic flux for southward IMF but little or no erosion for northward IMF. The cusp is also somewhat wider in invariant latitude with increasingly northward IMF. Consistent with low-altitude observations, we find that there is a clear MLT shift due to the IMF By for strongly southward IMF. We interpret the motion of the local time of the cusp for southward IMF as a shift of the reconnection site away from the noon meridian when the IMF is not due southward.

ISEE observations of low‐latitude boundary layer for northward interplanetary magnetic field: Implications for cusp reconnection

We present a study of the formation of the low-latitude boundary layer for northward interplanetary magnetic field that provides evidence for the reconnection poleward of the region of the cusp. Velocity distributions as observed by the joint Los Alamos/Max-Planck-Institut (MPE) Garching Fast Plasma Experiment on the ISEE spacecraft reveal two types of low-latitude boundary layer plasma: heated magnetosheath plasma with little or no hot magnetospheric component in the outer boundary layer and a mixture of magnetosheath and magnetospheric plasmas in the inner boundary layer. The observed plasma characteristics can be explained by the process of magnetic reconnection poleward of the region of the cusp. The outer boundary layer that contains heated magnetosheath plasma and little or no hot magnetospheric component appears to be formed by reconnection between magnetosheath and lobe field lines poleward of one cusp. It is identified to be on open field lines with one end in the ionosphere and the other one in the solar wind. The magnetosheath plasma entering the outer boundary layer is heated and accelerated at the cusp reconnection site and is then transported to the low latitudes along the magnetic field. The hot plasma from the magnetosphere is severely depleted but is not necessarily completely absent since they can escape to the magnetosheath on open field lines, with a finite time being required for total depletion to occur. The inner boundary layer is identified to be on closed field lines that have become closed by reconnection of the open end of the flux tube poleward of the second cusp. The inner boundary layer contains a mixture of magnetosheath and magnetospheric plasmas since the hot magnetospheric plasma can drift onto these closed field lines, even if the hot plasma was completely absent when the field line was open. Thus, when the interplanetary field is strongly northward, two boundary layers are formed, one on open field lines and one on closed field lines, by the sequential action of reconnection beyond the polar cusp.

The effect of solar wind dynamic pressure changes on low and mid‐latitude magnetic records

Magnetic records from low and mid latitude stations have been examined to determine their response to solar wind pressure changes. The best correlation between ground level changes and the change in the square root of the solar wind dynamic pressure occurs for stations at latitudes from 15° to 30° such as Tahiti, Honolulu, San Juan and Midway. The horizontal component of the field changes on average 16.5 nT for each change of 1(nPa)½ of the square root of dynamic pressure. This is 50% greater than the vacuum model of Tsyganenko would predict for a non-conducting Earth and therefore what would be expected for a perfectly conducting interior. Thus, low and mid latitude ground level response to solar wind pressure changes is dominated by the variation of the strength and location of the magnetopause current system and the corresponding induced currents within the Earth rather than ionospheric current sources.

Statistical studies of flux transfer events

Previous studies have revealed that flux transfer events (FTEs) have a clear dependence on the interplanetary magnetic field Bz component. Herein we examine other solar wind parameters, beta, dynamic pressure, and Mach number that possibly control the formation of FTEs. None of these other parameters appear to exercise strong control of the rate of FTE occurrence. Hence we conclude that the occurrence of FTEs is probably controlled by some intrinsic property of the magnetospheric system itself rather than by these solar wind parameters. In order to examine whether the bipolar signatures observed in the magnetosheath or magnetosphere are the same phenomenon, magnetosheath FTEs and magnetospheric FTEs are studied separately. The similar results for the two types of FTEs indicate that they do belong to the same statistical population.

Magnetopause structure and the role of reconnection at the outer planets

In situ measurements have been obtained at the magnetopause boundary of all the giant outer planets. The Jovian magnetopause was probed by Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and most recently by Galileo. Saturn was visited by Pioneer 11 and Voyagers 1 and 2, and Uranus and Neptune were visited by Voyager 2. The observations at Jupiter show evidence for flux transfer event (FTE) structures and rotational discontinuities associated with magnetic reconnection with the interplanetary magnetic field (IMF), but in accord with previous studies we find no FTE signatures beyond Jupiter in the data sets we studied. At Saturn, one magnetopause encounter shows evidence for reconnection in the form of a rotational discontinuity with a finite magnetic field component through the boundary and significant plasma acceleration. At Uranus and Neptune the flank magnetopause boundary observed by Voyager 2 exhibits a complex structure (similar to that seen at Earth during high-β conditions) for which unsteady reconnection appears to occur. Closely spaced multiple magnetopause crossings at Jupiter and Saturn are consistent with a boundary surface disturbance or wave, occurring with unsteady reconnection, and coincident with a jump in upstream solar wind dynamic pressure in the Jupiter case. Because of the transient, bursty nature of the reconnection signatures and their low (∼0.1 to 0.4 mV/m) convective electric fields compared to those of corotation at Jupiter and Saturn, the role of reconnection in driving the dynamics of these magnetospheres is thought to be minimal in the general case. Nevertheless, the unsteady reconnection observed is important to the properties of the boundary layers. At Uranus and Neptune, observations are limited (Voyager 2 only) but suggest that bursty reconnection at the flank magnetopause can only remove plasma (in the antisunward direction), and infrequent reconnection on the dayside would provide little opportunity for solar wind entry to energize the ionospheric plasma and supply the magnetosphere; these factors may contribute to producing the relatively empty magnetospheres that were seen.

Plasmaspheric depletion and refilling associated with the September 25, 1998 magnetic storm observed by ground magnetometers at L = 2

The plasmaspheric mass density at L ≃ 2 was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to ≃ 1/4 of the pre-storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as L = 2, which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern.

The polar cusp location and its dependence on dipole tilt

Polar cusp crossings at high altitudes are seen in the POLAR data as decreases in the magnetic field and increases in magnetosheath-like plasma. Close to 500 polar cusp crossings identified from the magnetic field, low-energy electron and ion data observed by POLAR, are used to determine the statistical location of the polar cusp. When compared with Tsyganenkos 1989 vacuum model with an ellipsoidal magnetopause [Tsyganenko, 1989], the medians of the cusp crossings are located between the magnetic field lines with invariant latitudes of 80° and 82°. Statistically the shape of the polar cusp in this region is consistent with this model although there is much scatter around the median value. The position of the cusp is significantly dependent on the dipole tilt angle. When dipole tilts more toward the Sun, the cusp moves more poleward to higher invariant latitude from 77.2° at −30° tilt, to 80.0° at 0° tilt, to 81.8° at 30° or roughly 1° for every 14° of tilt.

Propagation of the preliminary reverse impulse of sudden commencements to low latitudes

It has been thought that the preliminary reverse impulse (PRI) of the sudden commencements (SC) phenomena occurs simultaneously on the ground at different locations. A popular explanation is that the PRI propagates through the Earth-ionosphere waveguide at the 35 ground magnetometer stations during the SC event on September 24, 1998, and found clear differences in the arrival time of PRI. We calculated the MHD wave propagation time from the location of the first compression of the magnetosphere to the low-latitude ground stations and found good agreement with the observed PRI arrival times. Our calculation also indicates that the wavefront is seriously distorted by the inhomogeneity of the magnetosphere and the small difference in PRI arrival time between high-latitude and low-latitude observations cannot be an indicator of a super-Alfvenic propagation. We also found implications that high-latitude PRIs can be induced by the vortex of ionospheric currents at nearby latitudes, and the motion of the current vortex can affect the arrival time of high-latitude PRIs.

Flux transfer events: Spontaneous or driven?

Since flux transfer events (FTEs) occur predominantly under conditions of southward interplanetary magnetic field (IMF), one might expect that FTEs could be associated with the southward turning of the IMF which could lead to a transient increase in the merging rate. Lockwood and Wild [1993] recently suggested, in fact, that FTEs are driven by the IMF variations in which the Bz becomes more southward and rather than arising spontaneously during periods of steady IMF. To test this hypothesis, we have surveyed observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE spacecraft. The upstream solar wind data from IMP-8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE to determine the time lag between them, which enables us to survey the simultaneous solar wind data during the FTEs. We find no evidence that FTEs are directly correlated with southward turning of the IMF. Rather, the FTEs more frequently occur when the IMF Bz is steady. A survey of 10-minute intervals of simultaneous IMF data preceding each FTE show that the IMF fluctuates between northward and southward for less than 20% events and few of these turnings appear to be consistent with triggering the FTE. For majority of events, the IMF stays southward with fluctuations of clock angle less than 30° over the 10-minute period. Thus we conclude that FTEs do occur spontaneously for steady southward IMF and are not simply directly driven by the observed fluctuations in the IMF Bz component.