Particle injections observed at the morning sector as a response to IMF turning
Introduction
Dramatic changes occur both in the magnetosphere and in the high latitude ionosphere during substorms (Akasofu, 1977). The most dynamic auroras of the substorm, the westward traveling surge, is associated with strong upward field-aligned current (FAC) near the western edge of the substorm auroral bulge (McPherron et al., 1973, Chao and Heacock, 1980, Rostoker and Samson, 1981, Pashin et al., 1982, Murphy et al., 2013). The dynamical processes which occur in the region east of onset have received much less attention than that west of onset. Liang et al. (2006) have reported that after the substorm onset, a postmidnight anticlockwise convection vortex emerges at latitudes higher than the auroral brightening region, and an east-to-west flow reversal wrapping around the intensified auroras extends into the post-midnight sector.
In the near-Earth magnetosphere the injections of energetic (at energies of tens to hundreds of keV) particles and fast reconfiguration of the magnetic field from stretched tail-like to more dipole shape (so-called “dipolarization”) are among the most common features of the substorm events. The injection is dispersionless in energy within an injection region near midnignt MLT but is dispersed at MLTs away this region due to energy dependence of the drift velocity (e.g. Reeves et al., 1990, Birn et al., 1997, Birn et al., 2000, Thomsen et al., 2001, Keiling et al., 2008a; Gkioulidou et al., 2015, Gabrielse et al., 2014). Birn et al. (1997) showed five classes of dispersionless substorm injection events at geosynchronous orbit (6.6 RE) ordered by their average local times. They range from “ion-only” injections, to ion injections followed a few minutes later by electron injections (i → e), to simultaneous ion and electron injections close to midnight, to electron injections that are followed by ion injections (e → i) ∼1 h postmidnight, and finally to “electron-only” ∼2 h postmidnight.
Observations by satellites located at geosynchronous orbit have led to an idea of azimuthally wide “injection boundary”, under an enhanced cross-tail electric field (Mauk and McIlwain, 1974, Konradi et al., 1975, Mauk and Meng, 1983). Russell and McPherron (1973) have reported ‘dynamic’ injections of plasma, which inward propagating from 9 to 6.6 RE. Moore et al.(1981) proposed the ‘injection front’ model, where injections correspond to some kind of compressional wave (or, more carefully, fast magnetosonic (FMS) wave mode) front. The main assumption of the injection boundary model is that tailward of this boundary all ions and electrons are energized together at the same time, at substorm onset. Substorm related particle injections have internal fine structure (Belian et al., 1981; Lui et al., 1988, Holter et al., 1995, Maynard et al., 1996, Friedel et al., 1996, Lazutin et al., 1998, Kozelova and Kozelov, 2013). The observed fine structure is supposed an existence of more localized, near-Earth acceleration mechanism which energize the particles by local plasma instability (such as the ballooning-mode instability (Roux et al., 1991)).
Recently Gabrielse et al. (2014) demonstrated good correlation between injections and bursty bulk flows (BBFs) from ∼9 to 30 RE, indicating that particle injection occur along a large range of radial distances. These results are inconsistent with the above-mentioned classical concept of an azimuthally wide injection boundary moving earthward from ∼9–12 RE to geosynchronous orbit under an enhanced cross-tail electric field.
Thus, the source of particle injections and nature of injection process are still unclear. Are the FMS waves excited during the substorm breakup?
In this paper we try to explain the issues on the example of substorm event on January 6, 2008 in the interval 13–15 UT. We focus on the changes occur both in the magnetosphere and in the ionosphere in the postmidnight sector eastward of the initial substorm onset. We consider the IMF observations from the ACE satellite and analyze the substorm evolution from the ground-based observations (Section 2). In Section 3, we analyze the associated changes in the particle fluxes and the magnetic fields observed by four THEMIS satellites from r ∼ 11.35 RE to 5.8 RE and at the intervals from MLT = 2.9 to MLT = 5.1 as shown at Table 1. In Section 4, we discuss the role of the local diamagnetic effects and other distant current during the substorm intensification near the boundary of transition between different configurations of the magnetic field in the magnetosphere. The paper is finished by short conclusions in Section 5.
Section snippets
Substorm dynamics
Fig. 1 shows the map of ground-based stations and the projections of the THEMIS satellites on January 6, 2008 at 13:30–13:45 UT. The dashed circle on the map is the field of view of the all-sky camera at Whitehorse station (WHIT). The dashed-dotted line shows position of the 270 magnetic meridian (270 MM) on the considered date. This region we will call hereinafter as sector of near the ‘270 MM’. Another region which we refer in the text is located westward, in the East Siberia, near the 210
Observations in the magnetosphere
Fig. 5, Fig. 6, Fig. 7 present the magnetic field and the energy density of the particles observed at three THEMIS satellites (THE, THE, and THA) during the substorm. Fig. 5, Fig. 6, Fig. 7 show from top to bottom: (i) inclination angle X of the magnetic field relative to the XY plane; (ii) Pi2-like fluctuations of the magnetic field calculated as the deviations of observed magnetic field from 108s-smoothed values, the most appreciable dBx component of the magnetic field is shown here; (iii)
Discussion
In this work we investigate the injections of energetic particles and behavior of lower-energetic plasma (<25 keV) in the morning sector magnetosphere eastward the initial onset location during stimulated substorm on January 6, 2008. The following important features of this event should be noted:
- •
The substorm of interest presents the (from ground-based stations) magnetic disturbance consisted from the large-scale pulsations (4–5 min) superposed on a substorm bay. One can distinguish at least two
Conclusions
We report a detailed case study of substorm on January 6, 2008 in the interval 13–15 UT using data from four THEMIS satellites located in the morning sector magnetosphere eastward the onset location. This substorm onset was stimulated by a short-living northward burst of the IMF Bz and the consecutive northward turning IMF Bz.
The following peculiarities of this event have been demonstrated:
- (1)
The magnetic disturbance at the ground-based stations consisted from the large-scale pulsations (4–5 min)
Acknowledgments
Authors thank V. Angelopoulos, C.W. Carlson and J. McFadden at UCB, NASA, NAS5-02099 and CDAWeb for THEMIS and ACE satellites data, Professor Dr. Kiyohumi Yumoto for data of 210 MM Magnetic Observation Network.
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