P. J. Chi

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.

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.

Sudden compression of the outer magnetosphere associated with an ionospheric mass ejection

On September 24, 1998 at 2345 UT the magnetosphere was suddenly compressed as the dynamic pressure of the solar wind rapidly rose from 2 to 15 nPa. At the Polar spacecraft, at high altitudes above the center of the northern polar cap, a remarkably smooth increase in the field strength occurred while the plasma properties changed abruptly, as described in an accompanying paper. Comparisons with models and an examination of the wave amplitudes during the compression indicate that the initial change in plasma properties was most probably due to convection of pre-existing boundary layer plasma to the location of Polar rather than due to local heating by betatron acceleration and ion cyclotron waves. The smoothness of the increase in field strength is attributed to the very high velocity of compressional waves in the tail that outrun the advancing solar wind disturbance. The signatures as measured by GOES 10 at 1444 LT and at GOES 8 at 1846 LT in low latitude geosynchronous orbit are the more familiar sudden jump on the dayside, where the density is high and the compressional wave velocity low, and a weak change on the nightside, where tail current changes oppose the effects of the dayside magnetopause currents. This event is an ideal candidate for collaborative investigation of the effects of a classical sudden storm commencement on the magnetosphere.

The extreme compression of the magnetosphere on May 4, 1998, as observed by the POLAR spacecraft

Abstract 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 R E and a solar zenith angle of 19°. 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 R E and a solar zenith angle of 32° to a distance of only 8.5 R E and a solar zenith angle of 45°. This corresponds to subsolar distances of only 6.8 to 7.5 R E 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 extreme 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.

Phase skipping and Poynting flux of continuous pulsations

It is recognized that the continuous pulsations (Pc) in the magnetosphere are in general composed of many wave packets and that these wave packets are separated by phase skips. Previous observations by multiple spacecraft and ground magnetometer stations suggest that the phase skipping and packet structure of pulsation signals are most likely due to impulsive wave sources or the beating of waves. However, the magnetic field data alone have not led to an understanding of the propagation of these wave bursts. By using both the electric field and magnetic field data of the ISEE 1 spacecraft, we find that the Poynting flux of Pc3–4 pulsations in the outer magnetosphere has an impulsive nature. The direction of time-average Poynting flux changes every several wave cycles, and the phase skips in wave signals, both in the magnetic field and electric field, are often found between two adjacent Poynting flux bursts. Furthermore, the phase difference between the electric wave and the magnetic wave indicates that Pc3–4 waves are generally not in exact resonance, counter to the traditional field line resonance paradigm. These observational results provide us with strong evidence that the “continuous” pulsations in the Pc3–4 band are in fact maintained by a series of pulses, and the phase skips in the wave signals are its natural consequence. The direction of wave energy propagation and its implications are also discussed.

Observations of traveling Pc5 waves and their relation to the magnetic cloud event of January 1997

Measurements of ULF waves in the Pc5 frequency range are presented and discussed. The waves were observed during the magnetic cloud event of January 1997 by instruments on the Polar satellite and ground instrumentation. These large-amplitude waves are best interpreted as traveling shear Alfven waves rather than in the usual standing-wave scenario. Characterization of the associated complex particle environment shows that the waves were largely confined to the plasma trough. Energetic protons modulated by the wave are shown to have caused modulated proton aurora. It is argued that the waves were caused by the interaction of a magnetic hole with the magnetosphere.

A synoptic study of Pc 3, 4 waves using the Air Force Geophysics Laboratory magnetometer array

We have studied the Pc 3, 4 wave amplitude recorded at the Air Force Geophysics Laboratory (AFGL) magnetometer network on August 19–20, 1978. During this period of time the wave amplitude was modulated coherently across the magnetometer network. From observations, three important factors affecting the wave amplitude measured on the ground have been determined as follows: (1) the intrinsic or magnetospheric magnitude of the wave events, (2) their local time dependence, and (3) the subsurface conductivity. We present a statistical method to calculate these three factors from the simultaneous observations of a longitudinal array of magnetometers. The intrinsic wave amplitude is found to be bursty. On short timescales the wave amplitudes do not follow our expectations based on long-term statistical studies. Amplitudes vary rapidly without concomitant changes in the solar wind. We also find that the amplification of the wave amplitude due to the subsurface conductivity is different from station to station. For the northern five AFGL stations it is found that the ratio between the largest amplification and the smallest amplification is close to 2. The local time dependence maximizes approximately at local noon. A possible reason due to the reflection of Alfven waves in the ionosphere is discussed.

Magnetopause Erosion During the 17 March 2015 Magnetic Storm: Combined Field-Aligned Currents, Auroral Oval, and Magnetopause Observations

We present multimission observations of field-aligned currents, auroral oval, and magnetopause crossings during the 17 March 2015 magnetic storm. Dayside reconnection is expected to transport magnetic flux, strengthen field-aligned currents, lead to polar cap expansion and magnetopause erosion. Our multimission observations assemble evidence for all these manifestations. After a prolonged period of strongly southward interplanetary magnetic field, Swarm and AMPERE observe significant intensification of field-aligned currents .The dayside auroral oval, as seen by DMSP, appears as a thin arc associated with ongoing dayside reconnection. Both the field-aligned currents and the auroral arc move equatorward reaching as low as approx. 60 deg. magnetic latitude. Strong magnetopause erosion is evident in the in situ measurements of the magnetopause crossings by GOES 13/15 and MMS. The coordinated Swarm, AMPERE, DMSP, MMS and GOES observations, with both global and in situ coverage of the key regions, provide a clear demonstration of the effects of dayside reconnection on the entire magnetosphere.

Pc 3 and Pc 4 activity during a long period of low interplanetary magnetic field cone angle as detected across the Institute of Geological Sciences Array

Strong Pc 3 and Pc 4 waves were recorded across the Institute of Geological Sciences ground magnetometer array for a period of about 12 hours on August 20, 1978, when the cone angle of the interplanetary magnetic field was less than 45°. The peak frequencies in the Pc 3 band varied little from station to station and were approximately the same as the frequency of the upstream waves simultaneously observed from ISEE 1 and 2. However, only certain pairs of stations saw signal waveforms having high coherence. In contrast, the waves in the Pc 4 band were seen on the surface of the Earth only at low latitudes and were not enhanced in the upstream waves. The peak frequency of Pc 4 waves on the ground was a strong function of latitude. With the exception of the ending one hour early of Pc 4 activity at a single station, both Pc 3 and Pc 4 signals ceased abruptly when the interplanetary magnetic field (IMF) cone angle increased to large angles. Although these observations exhibit the global control of occurrence of Pc 3 and Pc 4 waves by the IMF, the cross correlation analysis of the temporal variations of Pc 3 and Pc 4 power implies that the Pc 4 waves were from the nightside or the flank of the magnetosphere, while the Pc 3 waves were from the subsolar region.

Solar wind control of ultralow-frequency wave activity at L = 3

This study explores the statistical relationships between 2 years of solar wind data recorded by the IMP 8 spacecraft and data from the Mount Clemens magnetometer station (L = 3) to improve our understanding of possible energy sources of ULF wave activity. Within the four frequency bands, f = 4-8 mHz, 8-16 mHz, 16-32 mHz, and 32-64 mHz, that are studied, two distinct types of waves are found. One is at high frequencies corresponding to Pc3 pulsations, and the other is at low frequencies corresponding to Pc5 and low-frequency Pc4 pulsations. The high-frequency part clearly has an energy source in the upstream foreshock. However, our analysis shows that the magnetospheric cusps do not appear to be the conduit of energy for the wave activity in this low-latitude region. The low-frequency activity occurs most frequently and has greater wave power when the interplanetary magnetic field is southward. This correlation suggests that the major energy source of these low-frequency waves is substorm-related or is related to the reconnection on the dayside magnetopause.