Kiyoshi Maezawa

Slow-mode shocks in the magnetotail

We have identified slow-mode shocks between the plasma sheet and lobe in the midtail to distant-tail regions by using three-dimensional magnetic field data and three-dimensional plasma data including density, velocity, temperature, and heat flux of both ions and electrons observed by the GEOTAIL satellite. Analyzing the data obtained between September 14, 1993, and February 16, 1994, we have found 303 plasma sheet-lobe boundary crossings at distances between -XGSE ∼ −30RE and XGSE ∼ −210RE. Thirty-two out of these 303 boundaries are identified as slow-mode shocks. We have found back streaming ions on the upstream side of the slow-mode shocks, which may be important in understanding the dissipation mechanism of the slow shocks in collisionless plasma. We have also found acceleration of cold ions between the upstream and the downstream of the slow-mode shocks. These cold ions are often observed in the lobe, and they are usually flowing tailward. Upon entering the plasma sheet, they are accelerated and rotate around the magnetic field and at times show ring-shaped velocity distributions. These ions may reflect the kinetic structure of slow-mode shocks. Slow shocks are at times observed also on the front side of plasmoids. These slow shocks on the front side of plasmoids have a different orientation from that of the ordinary slow shocks observed at the plasma sheet-lobe boundaries, which suggests an existence of “heart” -shaped plasmoids predicted by a numerical simulation.

Ion dynamics and resultant velocity space distributions in the course of magnetotail reconnection

We study a large-scale magnetic reconnection using 2-D hybrid simulation (ion particle, charge neutralizing massless electron fluid). After formation of fast plasma jets from the diffusion region, a quasi-steady state is achieved. The outstanding difference in field configuration between the present result and the MHD is an appearance of global cross-tail magnetic field component with a halflobe field strength. Inside the fast plasma jet, ion distribution functions show various features that can be classified into three types according to the distances from the reconnection line: (1) the modified Speiser distribution observed near the reconnection line, (2) the counterstreaming ion distribution observed around the jet front where the plasma density is compressed and the field lines are piled up due to the collision of the plasma jet with the preexisting plasma sheet, and (3) the partial-shell distribution observed in the region between the above two. The ion dynamics leading to the formation of these characteristic distributions are fairly well understood with the aid of test particle studies. The results are expected to serve as a guide to identify the reconnection dynamics on the basis of the ion distribution functions.

Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in the course of an isolated substorm event

On January 15, 1994, the ion spectrometer high energy particle - low energy particle detector (HEP-LD) on the Japanese spacecraft Geotail observed five quasi-periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid-like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP-LD observations show that Bz bipolar plasmoid-like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP-LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam-like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid-plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.

A unified model of the magnetotail convection in geomagnetically quiet and active times

Geotail observations have shown that during geomagnetically quiet times the northward magnetic field lines are convected tailward in the distant tail while they are convected earthward in the near-Earth magnetotail. These seemingly incompatible observations when the interplanetary magnetic field (IMF) Bz is northward and |By| ≥ Bz can be explained by the magnetic reconnection which occurs in the distant tail where the neutral sheet is twisted by tens of degrees. According to this interpretation, the tailward convection of the northward field lines occurs beyond the reconnection line because the reconnected field lines that cross the neutral sheet from its northern side to the southern side can have the northward polarity everywhere by virtue of the twisting of the neutral sheet. Hence the reconnection can be the generic mechanism of the plasma acceleration and heating in the magnetotail in both geomagnetically active and quiet times, namely, under both southward and northward (with |By| ≥ Bz) IMF conditions. Both Geotail and DE 2 observations suggest that the closed field lines that are formed by the reconnection return to the dayside by convecting through the inner magnetosphere in geomagnetically quiet times as well as in active times. This suggests that the dayside reconnection with the northward IMF (with |By| ≥ Bz) on the magnetopause involves the closed field lines in addition to the open field lines as considered in the classical cusp reconnection model under the northward IMF.

Energetic oxygen ion bursts in the distant magnetotail as a product of intense substorms: Three case studies

On the basis of Geotail high energy particle - low energy particle detector (HEP-LD) observations, this paper reports on three energetic (144 – 4000 keV) oxygen burst sequences detected in the distant tail (Xgse = −40 to −66 RE) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam-like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component Bz assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X= −40 RE. No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a “near-Earth” neutral line that had formed beyond X=−40 RE. We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O+ ions from the polar ionosphere can be considered as “tracer ions” in the substorm dynamical process.

GEOTAIL observation of magnetospheric convection in the distant tail at 200 RE in quiet times

Magnetic field and plasma observations by GEOTAIL in the distant tail at x = −200 RE are studied for a geomagnetically quiet interval when IMF Bz is predominantly northward and |By| is larger than B on average. In the distant tail during this interval, Bz is northward and Ey is directed dusk-to-dawn on average. This combination of Bz and Ey does not seem to represent the tailward convection of the closed field lines, because Ey is much weaker in the lobe than in the plasma sheet so that the field lines would be piled up at the plasma sheet boundary if they were closed. The observations suggest instead that the plasma and field lines are convected parallel to the neutral sheet across the tail at the same time as they flow tailward. The direction of this cross-tail convection depends on the polarity of IMF By and is antisymmetric with respect to the neutral sheet, which can be twisted by tens of degrees under the influence of IMFBy. A consistent picture is obtained from observations both inside the tail and at the tail magnetopause. This convection profile agrees in topology with the cusp reconnection model, but it occurs mainly in the plasma sheet while only the lobe field lines are expected to be involved according to this model. Observations show that not only the cold-dense ions which can be linked directly with the entrant solar wind plasma but also the hot-tenuous ions in the plasma sheet take part in the convection.

Monoenergetic ion drop‐off in the inner magnetosphere

A monoenergetic drop-off of ions around 10 keV, which we term as “ion drop-off band” (IDB) in this paper, has been observed by Akebono. The IDB is identified as a sharp and deep dip at about 10 keV in ion spectra, which is usually observed at latitudes below the discrete auroral region over several degrees or more. As the ion motion of this energy in this inner part of the magnetosphere is basically described by the adiabatic theory, we have numerically traced the ion drift trajectories. From the results, it is proposed that the lower-energy boundary of the drop-off demarcates the open/closed character of the drift orbits, only below which continuous supply from the magnetotail is present. This model explains the energy, local time, and latitudinal extent of IDB as well as the formation of its poleward edge very well.

Entry process of low-energy electrons into the magnetosphere along open field lines: Polar rain electrons as field line tracers

The strahl component of solar wind electrons, which constitutes field-aligned electron heat flux running away from the Sun, is a strong candidate for the origin of the polar rain. We investigate the entry process of the strahl electrons into the distant-tail magnetosphere and discuss topologies of Earths field lines in this paper. The Geotail satellite has often observed either gradual or abrupt transitions from the magnetosheath electrons to the bidirectional lobe electrons at the magnetopause. In some cases, the strahl flux of the sheath electrons flowing tailward gradually turned near the magnetopause and finally became the earthward flux of the bidirectional lobe electrons. This transition is accompanied by the rotation of the magnetic field direction and indicates the direct entry of the strahl electrons along open field lines. On the basis of the data of 38 magnetopause crossings by Geotail, we investigate variation of density of the strahl (polar rain) electrons and that of ion density near the magnetopause. It is shown that the strahl electrons decrease upon crossing the magnetopause, and the decrease is correlated with that of ions, although, quantitatively, the strahl electrons do not decrease so much as ions. It is suggested that the strahl electrons enter the magnetosphere along open field lines more freely than ions, but their entry is under the influence of charge neutrality with ions. It remains as a problem why the Geotail data do not show the presence of electrons escaping from the magnetosphere along open field lines.

Drop‐off of the polar rain flux near the plasma sheet boundary

The Akebono satellite has often observed the drop-off of polar rain flux near the polar cap boundary. The energy of the cutoff frequently showed decreasing trend with decreasing latitude. In this paper, we propose a model in which the drop-off is explained by disruption of the earthward polar rain flux at the X line in the tail. This model also predicts the behavior of the polar rain in the distant tail: The polar rain flux, usually bidirectional in the tail lobe, should become unidirectional near the plasma sheet boundary, as it is directed tailward (earthward) on the Earth (tail) side of the X line. Such a unidirectional polar rain layer has been newly detected by the Geotail satellite at XGSM = −40RE to −200 RE recently. Modeling this new feature of the polar rain is the main purpose of this study. The proposed model also predicts a relation between the direction of the polar rain flux in this layer and the direction of ion bulk flow in the adjacent plasma sheet. The Geotail data show a good agreement with this prediction. Being convinced that the model is reasonable, the position of the X line relative to the satellite has been derived. It is shown by the Geotail survey over a wide range in GSM X that the neutral line formation occurs most frequently at XGSM = −50RE to −150RE.

Substorm activity on January 11, 1994: Geotail observations in the distant tail during the leading phase of a corotating interaction region

On January 11, 1994 an interplanetary corotating interaction region (CIR) passed the Earths magnetosphere (final phase of solar cycle 22). Ground-based magnetometers, geosynchronous satellites, and Geotail (GSE −91, 15.5 and −3.5 RE), in or near the plasma sheet at the dusk flank of the magnetotail, detected a series of disturbances throughout this day, which culminated in the development of an isolated substorm between 1400 and 1710 UT (event 2). Small substorm activity early in the day (event 1) produced an energetic particle population with rather normal composition (relative helium abundance 10%). The CIR-related high-speed stream started to interact with the magnetosphere at about midday, which appears to have contributed to the initiation of the major substorm at 1400 UT (event 2). A geosynchronous Los Alamos National Laboratory (LANL) satellite at 2100 LT detected a rapid flux dropout for electrons at 1400 UT followed by a transient recovery around 1500 UT. The main injection phase, which started at 1551 UT, resulted in a massive electron flux increase. Throughout the 120-min long growth phase of this substorm the energetic particle spectrometer (HEP-LD) on board Geotail measured a rather slow flux buildup for protons and helium ions in the distant plasma sheet (PS). The azimuthal angular distributions showed only very small anisotropies in this phase. An unusual feature is the high helium abundance of 26% compared to about 10% normally found in the magnetosphere. After the onset of the electron injection at 6.6 RE, HEP-LD observed the appearance of tailward beams of energetic protons and helium ions (relative helium abundance 26%) in the PS/plasma sheet boundary layer (PSBL) and oxygen ions in the central plasma sheet (CPS). The oxygen ions were delayed by 27 min relative to the P/He beam. Plasmoid-like structures in the magnetic field accompanied the streaming ions. The synoptic observations in the geostationary orbit and in the distant tail suggest a source location at X=−50 RE (near-Earth neutral line (NENL)) for this substorm. Substorm-related extraction of oxygen ions from the polar ionosphere and subsequent drift to the acceleration region (NENL) can explain the observed delay for these ions, event 3, the last disturbance of the day, started at 2000 UT and lasted until 2300 UT. The proton/helium population was again broad in angular distribution and helium-rich in composition (20.7%) but the geostationary/distant tail association is less dynamic. The appearance of accelerated helium-rich energetic plasmas in event 2 and 3, confined to the plasma sheet and absent in the lobes, is most likely a result of the arrival of solar wind plasma with an enhanced helium content, which gained access to the magnetosphere shortly before or during the growth phase of the event 2 substorm. Helium-rich solar wind compositions are frequently observed in CIR events.