A. T. Y. Lui

Current disruptions in the near-Earth neutral sheet region

Observations from the Charge Composition Explorerin 1985 and 1986 revealed fifteen current disruption events in which the magnetic field fluctuations were large and their onsets coincided well with ground onsets of substorm expansion or intensification. These events are of short durations locally (∼1–5 min). They are mostly confined to within ∼0.5 RE of the neutral sheet and 1 hour local time from the magnetic midnight. Over the disruption interval, the local magnetic field can change by as much as a factor of ∼7. In general, the stronger the current buildup and the closer to the neutral sheet, the larger the resultant field change. There is also a tendency for a larger subsequent enhancement in the AE index with a stronger current buildup prior to current disruption. For events with good pitch angle coverage and extended observation in the neutral sheet region we find that the particle pressure increases toward the disruption onset and decreases afterward. Just prior to disruption, either the total particle pressure is isotropic, or the perpendicular component (P⊥) dominates the parallel comment (P∥), the plasma beta is seen to be as high as ∼70, and the observed plasma pressure gradient at the neutral sheet is large along the tail axis. The deduced local current density associated with pressure gradient is ∼27–80 nA/m² and is ∼85–105 mA/m when integrated over the sheet thickness. We infer from these results that just prior to the onset of current disruption, (1) an extremely thin current sheet requiring P∥ > P⊥ for stress balance does not develop at these distances, (2) the thermal ion orbits are in the chaotic or Speiser regime while the thermal electrons are in the adiabatic regime and, in one case, exhibit peaked fluxes perpendicular to the magnetic field, thus implying no electron orbit chaotization to possibly initiate ion tearing instability, and (3) the neutral sheet is in the unstable regime specified by the cross-field current instability. Subsequent to the disruption onset, enhancement of magnetic noise over a broad frequency range, magnetic field aligned counterstreaming electron beams, ion energization perpendicular to the magnetic field, and current reduction in the amount similar to that of current buildup during the growth phase are observed. These features seem to be compatible with the predicted development of the cross-field current instability.

Storm‐substorm relationship: Contribution of the tail current to Dst

The Dst index has been conventionally used as a measure of the storm intensity, which ideally assumes that the associated ground magnetic disturbance is caused by the ring current. The present study examines the contribution of the tail current to Dst, focusing on the occurrence of geosynchronous dipolarization close to the Dst minimum, in other words, the start of the storm recovery phase. The Sym-H (referred to as Dst(Sym-H) hereafter) index rather than the conventional Dst index is used because of its higher time resolution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near-Earth tail when Dst(Sym-H) reached its minimum. This result indicates that the source current was located outside of geosynchronous orbit, and therefore the recovery of Dst(Sym-H) can be attributed to the reduction of the tail current rather than the decay of the ring current. A statistical study based on 59 storm events (79 GOES events) confirms the tendency for geosynchronous magnetic field to dipolarize at the Dst(Sym-H) minimum. It is therefore highly likely that the Dst(Sym-H) minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying owing to substorm-associated injection. From the magnitude of the Dst(Sym-H) recovery during the interval of geosynchronous dipolarization, the contribution of the tail current to Dst(Sym-H) at the Dst(Sym-H) minimum is estimated to be 20–25%. However, the contribution of the tail current may be even larger because the tail current may not return to preintensification levels and may continue to contribute to Dst(Sym-H) after dipolarization. The trigger of dipolarization (substorm) and the subsequent recovery of Dst(Sym-H) tend to take place in the course of the reduction of the southward interplanetary magnetic field (IMF) BZ. It is therefore suggested that the ring current (storm) recovers after the substorm since the magnetospheric convection weakens because of weaker southward IMF BZ.

Evaluation of low‐latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery

Impulsive Pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes. It is well documented that there is usually more than one Pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Although the Pi2 have become one of the most popular indicators for substorm onsets, the reliability of using the Pi2 in this fashion has not been seriously investigated. In this paper we address this question with a statistical approach by using ∼650 auroral substorm onsets identified with Polar ultraviolet images for a time interval from April 1996 to May 1997. A comparison of the low-latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with the Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as ∼10 min of quiet time preceding the onset) Pi2 bursts seen within ∼10 min from a magnetic positive bay, there were 65 events (∼55%) taking place within 3 min from breakups and 34 events (29%) indicating no sign of an auroral breakup within 10 min of the Pi2 burst. This result suggests that Pi2 may not be as a good indicator of the substorm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When compared to auroral breakups, Pi2 onsets are subject to a small delay of 1 – 3 min, with a peak around l min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast-mode wave, in a plasmapause cavity mode model, propagating in the magnetosphere.

AMPTE/CCE-SCATHA simultaneous observations of substorm-associated magnetic fluctuations

This study examines substorm-associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near-Earth tail. Three tail reconfiguration events are selected, one event on August 28, 1986, and two consecutive events on August 30, 1986. The fractal analysis was applied to magnetic field measurements of each satellite. The result indicates that (1) the amplitude of the fluctuation of the north-south magnetic component is larger, though not overwhelmingly, than the amplitudes of the other two components and (2) the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod. In the examined events the satellite separation was less than 10 times the proton gyroradius. Nevertheless, the comparison between the AMPTE/CCE and SCATHA observations indicates that (3) there was a noticeable time delay between the onsets of the magnetic fluctuations at the two satellite positions, which is too long to ascribe to the propagation of a fast magnetosonic wave, and (4) the coherence of the magnetic fluctuations was low in the August 28, 1986, event and the fluctuations had different characteristic timescales in the first event of August 30, 1986, whereas some similarities can be found for the second event of August 30, 1986. Result 1 indicates that perturbation electric currents associated with the magnetic fluctuations tend to flow parallel to the tail current sheet and are presumably related to the reduction of the tail current intensity. Results 2 and 3 suggest that the excitation of the magnetic fluctuations and therefore the trigger of the tail current disruption is a kinetic process in which ions play an important role. It is inferred from results 3 and 4 that the characteristic spatial scale of the associated instability is of the order of the proton gyroradius or even shorter, and therefore the tail current disruption is described as a system of chaotic filamentary electric currents. However, result 4 suggests that the nature of the tail current disruption can vary from event to event.

Magnetic fluctuations associated with tail current disruption: Fractal analysis

The objective of the present study is to assess the mechanism of substorm-associated tail current disruption on the basis of magnetic field observations in the near-Earth tail. We examined 15 events observed by the Charge Composition Explorer (CCE) of the Active Magnetospheric Particle Tracer Explorers (AMPTE), with an emphasis on the August 28, 1986, event. In these events the satellite observed magnetic fluctuations to start almost simultaneously with ground substorm onsets, strongly suggesting that these fluctuations are related to the trigger of substorms. In this study we applied the new method, fractal analysis, to these fluctuations. This method enables us to examine fluctuations quantitatively and to pick up characteristic timescale(s) of fluctuations, even if fluctuations are far from sinusoidal. The results are summarized as follows: (1) Whereas before the onset of tail current disruption, magnetic fluctuations are suppressed in each of the magnetic components, after the onset, the magnitude of the H (north-south) component fluctuations is about 30% larger than the magnitudes of the fluctuations of the other components. (2) The magnetic fluctuations have a characteristic timescale, which is several times the proton gyroperiod. The first result suggests that observed magnetic fluctuations are actually related to changes in the tail current intensity, that is, tail current disruption. This result also indicates that the microprocess of tail current disruption should be described in terms of turbulent perturbation electric currents, although away from the onset region the effects of tail current disruption may be approximated by those of an orderly decrease in the tail current intensity. The second result strongly suggests that tail current disruption is driven by a certain instability, which grows most rapidly around that characteristic time scale, and in which ions should play an important role.

Geotail observations of substorm onset in the inner magnetotail

On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.

Substorm onset timing: The December 31, 1995, event

The objective of the present study is to examine the timing of various onset-associated signatures and address the cause-and-effect relationship between the formation of a near-Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950-km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near-Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near-Earth reconnection process as a major role in the substorm dynamics.

Time‐frequency decomposition of signals in a current disruption event

Wavelet transform has recently been developed to the level of sophistication suitable for application to signal processing in magnetospheric research. We explore this new technique in decomposing signals in the time-frequency domain by first conducting continuous wavelet transform on a test signal to show its ability to resolve multiple-frequency components embedded within white noise of half the amplitude as the signal. We then use this tool to examine the large-amplitude magnetic fluctuations observed during a current disruption event. The results show the current disruption to be a multiscale phenomenon, encompassing low- as well as high-frequency components. The lowest-frequency component appears to behave quite independently from the higher-frequency components. The analysis shows for the first time that in current disruption the high-frequency components constitute a broadband excitation with a nonstationary nature, i.e., some oscillations appear to cascade from high to low frequency as time progresses.

Acceleration of oxygen ions of ionospheric origin in the near‐Earth magnetotail during substorms

Measurements from the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionospheric origin at energies of 9 keV to 210 keV in the near-Earth plasma sheet during the substorm expansion phase. Substorm signatures were clearly observed on the ground at 1850 UT on May 17, 1995. In the expansion phase of this substorm, Geotail stayed in the plasma sheet at X∼−10.5 RE and observed a local dipolarization signature accompanied by strong disturbances of the magnetic field. From the energetic ion flux data of EPIC/STICS, we obtained the following results: (1) energetic flux enhancement was more pronounced for O+ than for H+; (2) the flux was enhanced almost simultaneously with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before substorm onset) relative to the H+ flux ratio, was as large as 1.31; and (4) thermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 keV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 events of local dipolarization found in the near-Earth region (X∼−6 to −16 RE). We found that EO+ is larger than unity in all ranges of radial distance and that the average value of EO+ is 1.37. These results suggest that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a mechanism in which ions are accelerated in a non-adiabatic way during substorm-associated field reconfiguration.

Generalized lower-hybrid drift instabilities in current-sheet equilibrium

A class of drift instabilities in one-dimensional current-sheet configuration, i.e., classical Harris equilibrium, with frequency ranging from low ion–cyclotron to intermediate lower-hybrid frequencies, are investigated with an emphasis placed on perturbations propagating along the direction of cross-field current flow. Nonlocal two-fluid stability analysis is carried out, and a class of unstable modes with multiple eigenstates, similar to that of the familiar quantum mechanical potential-well problem, are found by numerical means. It is found that the most unstable modes correspond to quasi-electrostatic, short-wavelength perturbations in the lower-hybrid frequency range, with wave functions localized at the edge of the current sheet where the density gradient is maximum. It is also found that there exist quasi-electromagnetic modes located near the center of the current sheet where the current density is maximum, with both kink- and sausage-type polarizations. These modes are low-frequency, long-wavelen...