Z. Y. Pu

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.

MHD drift ballooning instability near the inner edge of the near‐Earth plasma sheet and its application to substorm onset

The MHD drift ballooning mode (DBM) instability near the inner edge of the near-Earth plasma sheet is studied by using both the one-fluid generalized progressing wave expansion method and the two-fluid approach. It is found that in the frame of reference at rest relative to the bulk plasma the DBM may become a purely growing mode in two distinct circumstances, which, for convenience, are called the DBM1 and DBM2, respectively. The β threshold for the DBM1 is identical with that derived by Ohtani and Tamao [1993] and Southwood and Kivelson [1987], while the criterion of the DBM2 covers that of Miura et al. [1989]. Comparisons of the theory with GEOS 2 data show that the DBM2 is more easily excited in the late substorm growth phase. There is considerable evidence that the DBM is generated at expansion onsets. The characteristic features of magnetic field dipolarization can be interpreted in terms of the development of the DBM. The extremely thin current sheet cases should be studied with approaches other than those used in this work.

Motion of observed structures calculated from multi-point magnetic field measurements : Application to Cluster

A new method is described which calculates the velocity of observed, quasi-stationary structures at every moment in time from multi-point magnetic field measurements. Once the magnetic gradient tensor G=del (B) over right arrow and the time variation of the magnetic field have been estimated at every moment, the velocity can then be determined, in principle, as a function of time. One striking property of this method is that we can calculate the velocity of structures for any dimensionality: for three-dimensional structures, all three components of the velocity vector can be calculated directly; for two-dimensional (or one-dimensional) structures, we can calculate the velocity along two (or one) directions. The advantage of this method is that the velocity is determined instantaneously, point by point through any structure, and so we can see the time variation of the velocity as the spacecraft traverse the structure. In this paper, the feasibility of the method is tested by calculating the motion velocity of a three-dimensional, near cusp structure and a two-dimensional magnetotail current sheet. The results for one-dimensional structures in the magnetopause and cusp boundaries are compared to calculations for the standard techniques for analyzing discontinuities.

Ballooning instability in the presence of a plasma flow: A synthesis of tail reconnection and current disruption models for the initiation of substorms

The drift ballooning mode (DBM) instability near the inner edge of the plasma sheet (IEPS) is studied further by including a nonstationary earthward flow and flow shear in the analysis. Both equatorial and off-equatorial regions are considered. It is found that the presence of a decelerated earthward flow destabilizes both the M− and M+ branches of the DBM in a large portion of the current sheet near the IEPS and substantially increases the growth rate of the instability. The flow shear in the premidnight sector causes the conventional ballooning mode to weakly subside, while it slightly enhances the growth rate for the Alfvenic ballooning mode. The combination of the earthward flow and flow shear makes both the Alfvenic ballooning mode and conventional ballooning mode grow much faster than they would without the flow, giving rise to coupled Alfvenic slow magnetosonic waves, field-aligned currents, and the formation of a current wedge. A synthesis of tail reconnection and cross-tail current disruption scenarios is proposed for the substorm global initiation process: When the fast flow produced by magnetic reconnection in the midtail abruptly decelerates at the IEPS, it compresses the plasma populations earthward of the front, transports momentum to them, and pushes them farther earthward. This creates the configuration instability in a large portion of the inner tail magnetic field lines on both the tailward side and earthward side of the braking point. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified, which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm paradigm combines the near-Earth neutral line and near-Earth current disruption scenarios for the initiation of substorms and may also synthesize dynamical processes in the rnagnetosphere-ionosphere coupling and field line resonance during the substorm onset. We intend to use this global model to explain substorm expansion onsets occurring under the southward interplanetary magnetic field condition.

Global view of dayside magnetic reconnection with the dusk-dawn IMF orientation: A statistical study for Double Star and Cluster data

Double Star/TC-1 and Cluster data show that both component reconnection and anti-parallel reconnection occur at the magnetopause when the interplanetary magnetic field ( IMF) is predominantly dawnward. The occurrence of these different features under these very similar IMF conditions are further confirmed by a statistical study of 290 fast flows measured in both the low and high latitude magnetopause boundary layers. The directions of these fast flows suggest a possible S-shaped configuration of the reconnection X-line under such a dawnward dominated IMF orientation.

Ion composition variations in the inner magnetosphere: Individual and collective storm effects in 1991

Ion composition variations in the inner magnetosphere during storm times are studied by using data sets obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES). The observations were made during the second half of the CRRES mission, which was near the maximum of solar cycle 22. Four selected storms are subjects of detailed case studies; statistical results are based on a group of moderate (50 100 nT) storms observed in 1991. The case studies show that energetic particle enhancements occur at very low equatorial altitudes (L = 3∼4) during large storms with significant delays relative to the storm sudden commencement times (of about 20 hours). The average time duration of the particle enhancements is about 47 hours. By studying the time variation of energy spectrograms of H + , it is found that low-energy (E 100 keV) protons show different time profiles during the development and decay of the ring current. The low-energy part shows a dramatic intensification and a rapid decay. However, its relative contribution to the ring current defined by the density ratio N(H + L )/N during the storm maximum is almost constant. On the other hand, high-energy protons first exhibit a flux decrease followed by a delayed increase. The density ratio N(H + H )/N shows an anticorrelation with the storm intensity. It is confirmed that the ionospheric origin particles (e.g., O + ) are important constituents of the storm time ring current. The fractional number density of O + ions increases with the intensity of the storm. The statistical results demonstrate that the energy density of O + is a steep function of Dst for moderate storms. However, it seems to increase very slowly with Dst, or even to be almost independent of Dst for large storms (|Dst| > 120 nT). The ratios of solar wind origin He ++ density to the total density show no obvious difference among large storms. The same appears for He + ions.

Ring current oxygen ions escaping into the magnetosheath

Storm-related magnetospheric oxygen bursts were observed in the dayside magnetosheath during the coronal mass ejection on January 10, 1997. These singly charged oxygen ion events exhibited a clear antisunward flow. The oxygen ions are associated with a strong negative interplanetary magnetosheath field (IMF). The average Bz was almost − 50 nT, and the field projection in the x – y plane (GSE) was nearly constant in the Sun/dawn sector forming an angle of 45° relative to the Earth-Sun axis. The magnetopause was identified as a rotational discontinuity by using the principal axis analysis (PAA) method. The three-dimensional polar versus azimuthal angle distribution of the oxygen ions showed that the oxygen flow has a north to south velocity component. The observations suggest that the dayside reconnection process is generally steady. The energy dispersion can be explained with the time-of-flight (TOF) effect assuming oxygen ions are escaping from the magnetosphere along the reconnected field lines. The lack of hydrogen and helium ions during the observed oxygen bursts can be explained, as only oxygen ions are resupplied by the gradient drift in the inner magnetosphere because of their larger bounce periods with respect to hydrogen and helium ions. Therefore only oxygen ions are observed continuously in the magnetosheath. The estimated oxygen escape rate amounts to 0.61 × 1023 ions/s, about 33% of the input rate of the ring current. The observations imply that the stormtime ring current is asymmetric. A large amount of ring current oxygen ions escape from the magnetosphere into the magnetosheath.

Tailward flowing energetic oxygen ion bursts associated with multiple flux ropes in the distant magnetotail: GEOTAil observations

An event of tailward flowing energetic (144–7959 keV) oxygen ion bursts was observed in the distant magnetotail (X=−63, Y=+7, Z=−3.8 RE) on February 13, 1994. The observations were made with the HEP-LD spectrometer on board the GEOTAIL spacecraft. The event was associated with magnetic field signatures characteristic of multiple flux ropes. During the event, which lasted from 1847 to 1907 UT, strong impulsive increases in the oxygen flux were observed. From 1846 to 1900 UT the proton counting rate also exhibited an increase, followed by a decrease until the end of the oxygen event. The oxygen flux was confined to a rather narrow range in polar and azimuthal angle (only 7–10% of 4π was occupied). This implies a streaming distribution or beam-like structure. Comparison of the particle flow angles with the polar and azimuthal angles of the magnetic field indicates that the ion beam may have been embedded in flux ropes, which may be connecting the polar ionosphere and the distant magnetotail. During the observed oxygen event the ratio is significantly higher than the ratios usually found in the center of the distant magnetotail. There is some evidence that the observed oxygen ions were more efficiently accelerated in this event than hydrogen and helium ions.

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earths high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.

Spatial structures of magnetic depression in the Earth's high-altitude cusp: Cluster multipoint observations

Magnetic depression structures (magnetic holes) of short time duration from seconds to minutes have been studied using Cluster data in the high-latitude cusp. Our multispacecraft analysis revealed that the magnetic depressions are spatial structures traveling across the spacecraft, and this result was further strengthened by the calculation of the boundary normal directions and velocities using various methods. In this article, we show that multiple properties of the magnetic depressions are consistent with those of mirror structures observed in the magnetosheath or solar wind. The plasma in the cusp is rarely unstable with respect to mirror instability. However, as has been shown by previous studies, once a large magnetic hole is created by mirror instability, it becomes relatively stable and can survive for extended periods of time even if surrounding plasma conditions drop well below the mirror threshold. Although local generation of these structures cannot be completely ruled out in some cases, we propose an interpretation of the magnetic depressions observed in the cusp as mirror structures generated upstream and convected to the cusp by plasma flow. Specifically, the magnetic holes could be generated in the magnetosheath and enter the cusp due to the open geometry of the cusp magnetic field.