S. Ohtani

Radial expansion of the tail current disruption during substorms: A new approach to the substorm onset region

The substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint. The reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease (disruption) in tail current intensity. The north-south component (BZ) is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region, while the change in the Sun-Earth component (BX) is most sensitive to the change in tail current intensity near the spacecraft. If the current disruption starts in a localized range of radial distance and expands radially, a distinctive phase relationship between the changes in BX and BZ is expected to be observed. This phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft. Thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft. This method is applied to ISEE observations of a tail reconfiguration event that occurred on March 6, 1979. The phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event. This model prediction is consistent with the time lag of magnetic signatures observed by the two ISEE spacecraft. The expansion velocity is estimated at 2 RE/min (∼200 km/s) for this event. Furthermore, it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption. The method is used statistically for 13 events selected from the ISEE magnetometer data. It is found that the current disruption usually starts in the near-Earth magnetotail (|X| < 20RE and often within 15 RE of the Earth. The phase relationship between BX and BZ is discussed in terms of the near-Earth neutral line model, and the result is compared with previous studies on the substorm onset region.

Initial signatures of magnetic field and energetic particle fluxes at tail reconfiguration : explosive growth phase

Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetometer and Medium Energy Particle Analyzer (MEPA) data are used to examine the initial signatures of tail field reconfiguration observed in the near-Earth magnetotail (< 9 RE). Sixteen events are selected preliminarily from 9 months (January-September 1985) of magnetometer data according to two criteria, that is, an unambiguous commencement of tail field reconfiguration and a sharp recovery of the north-south (H) component. The second criterion requires that the satellite was close to the onset region of current disruption. Although these strict criteria result in the small number of events, the magnetic and particle flux signatures of the events are considered to be informative concerning the mechanism of substorm onsets. It is found that these tail reconfiguration events are classified into two types: Type I and Type II. In Type I events a current disruption starts in a flux tube that is inward (earthward/equatorward) of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type II) is characterized by a distinctive interval (explosive growth phase) just prior to the local commencement of tail reconfiguration. The duration of this interval is typically 1 min, much shorter than that of the so-called growth phase. During this interval the north-south magnetic (H) component is depressed sharply, and the flux of energetic ions increases outward (tailward/poleward) of the spacecraft, suggesting that the cross-tail current is explosively enhanced. It is also found that the radial magnetic (V) component changes with a distinctive phase relationship relative to the north-south component, which can also be explained in terms of the explosive enhancement in the cross-tail current intensity just prior to the current disruption. This enhancement is inferred to be a local process, rather than a result of a current disruption which has occurred somewhere else, although it is possible that the commencement of the H recovery observed is not exactly simultaneous with a substorm onset. The present results contribute significantly to modeling efforts regarding the triggering mechanism of substorms in the magnetotail.

Statistical analysis of Pi 2 pulsations observed by the AMPTE CCE Spacecraft in the inner magnetosphere

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii and at geomagnetic latitudes from −16° to 16° and at the Kakioka ground station located at magnetic shell of L = 1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE. An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial Bx, azimuthal By, and compressional Bz components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal H component of the Kakioka data and then examined how these parameters depend on the location of CCE. It is found that high-coherence events (coherence between CCE and Kakioka > 0.6) are observed primarily when CCE is on the nightside and at L < 4. For these events the magnetic field perturbations at CCE are dominated by the poloidal components Bx and Bz, and these components exhibit a ground-to-satellite cross phase of either ∼0 or ∼180°, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity-mode-type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are midlatitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that midlatitude (2 <L< 5) Pi 2 pulsations observed on the ground originate from a cavity-mode-type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfven velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large-scale magnetic reconfiguration associated with cross-tail current disruption.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

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.

Does the ballooning instability trigger substorms in the near‐Earth magnetotail?

The stability of the near-Earth magnetotail against ballooning (or configurational) instability is examined in the framework of the MHD approximation. It is emphasized that a change in plasma pressure induced by a meridional electric field drift δun is an important factor that determines the stability. We have to consider two ways in which plasma pressure changes, that is, a convective change −δun · ▽P0, where P0 is background plasma pressure, and plasma expansion/compression -P0▽ · δun. Since δun is perpendicular to the magnetic field and its magnitude is inversely proportional to the magnetic field strength, δun diverges/converges in usual tail magnetic field configurations. For the instability, the convective change must overwhelm the effects of the plasma expansion/compression. However, near the equator in the near-Earth tail, the latter may overcompensate for the former. We describe the ballooning instability in terms of a coupling between the Alfven and slow magnetosonic waves in an inhomogeneous plasma and derive instability conditions. The result shows that the excessive curvature stabilizes, rather than destabilizes, perturbations. It is also found that the field-aligned flow stabilizes perturbations, as well as the field-aligned current. We infer that under quiet conditions, the plasma pressure gradient in the near-Earth tail is not sharp enough to trigger the instability. The plasma sheet is expected to become more stable during the substorm growth phase because of an increase in the field line curvature associated with the plasma sheet thinning. In the region closer to the Earth, including the ring current, the plasma pressure gradient may be localized in a limited range of the radial distance during the growth phase. However, recently reported plasma and magnetic field parameters before substorm onsets do not provide very convincing evidence that the ballooning instability is the triggering mechanism of substorms.

A multisatellite study of a pseudo-substorm onset in the near-Earth magnetotail

This paper reports the multisatellite and ground observations of two pseudo-substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm-associated tail reconfiguration started in a very localized region in the near-Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (∼ 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near-Earth tail reconfiguration. Positive bay onsets at mid-latitudes also had a longitudinally wide distribution. From these features we infer that in the present event the current disruption took place filamentarily near AMPTE/CCE at ∼8.8 RE. It is also inferred that pseudo-substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scale of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.

The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.

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.