Near-Earth substorm features from multiple satellite observations
This paper was published as Journal of Geophysical Research, 2008, 113, A07S26. Copyright 2008 American Geophysical Union. It is available from http://www.agu.org/pubs/crossref/2008/2007JA012738.shtml. Doi: 10.1029/2007JA012738 Metadata only entry We investigate a substorm on 3 October 2004 during which 11 satellites were located in near-Earth magnetotail (X GSM > −10 R E). Double Star 1 (TC-1), Cluster, and LANL-97 satellites were closely aligned in the dawn-dusk direction (<1 R E apart) for this conjunction. After substorm expansion onset, TC-1 observed plasma sheet thinning at X ≈ −5.5 R E and later detected signature of plasma flow shear that may be associated with an auroral arc. Analysis of the dawn-dusk magnetic perturbations from GOES-10 and Polar suggests that these could be caused by a substorm current system consisting of not only the azimuthal closure of field-aligned currents (the substorm current wedge) but also the meridional closure of field-aligned currents. The temporal sequence of substorm activity (particle injection, current disruption, and dipolarization) revealed by these satellites indicates that the substorm expansion activity was initiated close to the Earth and spread later to further downstream distances. Furthermore, TC-1 and Cluster data show that there is no close relationship between some dipolarizations and Earthward plasma flows in the near-Earth region. The overall development of substorm activity is in agreement with the near-Earth initiation model for substorms. A temporal evolution of the magnetic field reconfiguration and plasma boundary motion during this substorm is constructed from these observations.
[Show abstract] [Hide abstract] ABSTRACT: Reconnection is the key process responsible for the magnetotail dynamics. Driven reconnection in the distant tail is not sufficient to support global magnetospheric convection and the near Earth neutral line spontaneously forms to restore the balance. Mechanisms of initiation of such near-Earth magnetotail reconnection still represent one of major unresolved issues in space physics. We review the progress in this topic during the last decade. Recent theoretical advances suggest several variants of overcoming the famous tearing stability problem. Multipoint spacecraft observations reveal detailed structure of pre-onset current sheet of and reconnection zone down to ion larmor scale, supporting the importance of unstable state development through internal magnetotail reconfiguration.0Comments 0Citations
- "The location of NENL in the magnetotail, especially the place of its initial formation at substorm onset, is one of the most debated issues in substorm studies (Angelopoulos et al. 2008; Petrukovich 2008; Lui et al. 2008). Besides the importance to the substorm initiation models, the location at a specific downtail distance defines expected initial conditions (see the previous section) for the reconnection-causing instability. "
[Show abstract] [Hide abstract] ABSTRACT: We examine a near-Earth current disruption/dipolarization (CDD) event in which two THEMIS satellites were located at nearly identical equatorial projections but separated by the distance from the neutral sheet. One satellite was very close to the neutral sheet with ∣Bx∣, ∣By∣ < 1.5 nT at CDD onset that coincided with ground signatures of substorm expansion onset. The cross-tail current density between the two satellites varied with a time scale of seconds during CDD and was reduced by ˜40% eventually. If the current density profile could be represented well by the combination of a dipole field and a one-dimensional current sheet, then the current density integrated over the current sheet thickness was reduced by only ˜12%. The difference can be attributed to plasma sheet expansion in association with CDD. There were occasional breakdowns of the frozen-in-field condition during CDD as well. The particle energization associated with CDD approached the satellite from the Earthward-dawnward direction near the neutral sheet based on the ion sounding technique, constituting compelling evidence that this near-Earth CDD arose from disturbances originating in the near-Earth region (Xgsm > -8.1 RE) and was not due to magnetic flux pileup (requiring frozen-in condition) or arrival of a dipolarization front from mid-tail (Xgsm < -15 RE) disturbances.0Comments 14Citations
- "There are observations advocating the onset process to be magnetic reconnection [e.g., Hones, 1979; Shiokawa et al., 1998; Nagai et al., 1998; , ballooning instability [e.g., Roux et al., 1991; Samson et al., 1992; Cheng and Lui, 1998; Pu et al., 2001; Chen et al., 2003; Saito et al., 2008; Liang et al., 2008; Liu et al., 2008; Henderson, 2009; Rae et al., 2009 Rae et al., , 2010 Zhu et al., 2009; Xing et al., 2010], cross-field current instability [e.g., Lui, 1996; Vogiatzis et al., 2005; Liang et al., 2008; Rae et al., 2009 Rae et al., , 2010 Yoon et al., 2009] , Kelvin-Helmholtz instabil- ity [Rostoker, 1996], ionospheric feedback instability [e.g., Lysak and Song, 2002; Streltsov et al., 2010], and others [e.g., Lin et al., 2009; Liu and Liang, 2009; Machida et al., 2009; Nishimura et al., 2010; Lyons et al., 2010; Haerendel, 2010].  The substorm current system consists of a substorm current wedge and a meridional current system, linking the cross-tail current to the ionospheric westward electrojet during substorms [Ahn et al., 1995; Akasofu, 2003; Lui et al., 2008] . Associated with the substorm current wedge development is the near-Earth magnetic field dipolarization that may be related to current disruption, i.e., a reduction of the cross-tail current [e.g., Lui et al., 1988] . "
[Show abstract] [Hide abstract] ABSTRACT: The high-latitude polar ionosphere is characterized by two regions, the polar cap and the auroral oval. In the polar cap, the geomagnetic field lines are open and connect to the solar wind, whereas the field lines in the auroral oval are closed and map to the plasma sheet and the plasma sheet boundary layer in the magnetosphere. The two substantially different magnetic and plasma domains are separated by a separatrix, the polar cap boundary (PCB), which is an ionospheric projection of the open-closed field line boundary (OCB) in the magnetosphere. In this thesis, a new method to determine the location of the PCB in the nightside ionosphere based on electron temperature measurements by EISCAT incoherent scatter radars is introduced. Comparisons with other PCB proxies like poleward boundary of the auroral emissions, poleward edge of the auroral electrojets and poleward boundary of energetic particle precipitation show general agreement. By applying the method to several events together with other supporting ground-based and space-borne observations, dynamic processes and phenomena in the vicinity of the PCB and inside the auroral oval are studied. The main results include the following. During substorm expansion, the PCB moves poleward in a burstlike manner with individual bursts separated by 2-10 min, indicating impulsive reconnection in the magnetotail. In one event, a possible signature of the high-altitude counterpart of the Earthward flowing field-aligned current of the Hall current system at the magnetotail reconnection site is observed. Investigation of the relation between the auroral activity and the local reconnection rate estimated from the EISCAT measurements reveals direct association between individual auroral poleward boundary intensifications (PBIs) and intensifications in the ionospheric reconnection electric field within the same MLT sector. The result confirms earlier suggestions of positive correlation between PBIs and enhanced flux closure in the magnetotail. In another event, quiet-time bursty bulk flows (BBFs) and their ionospheric signatures are studied. The BBFs are found to be consistent with the so called "bubble" model with Earthward fast flows inside the region of depleted plasma density (bubble). The tailward return flows show an interesting asymmetry in plasma density. Whereas the duskside return flows show signatures of a depleted wake, consistent with a recent suggestion, no similar feature is seen for the dawnside return flows, but rather increase in density. The BBFs are associated with auroral streamers in the conjugate ionosphere, consistently with previous findings. The related ionospheric plasma flow patterns are interpreted as ionospheric counterpart of the BBF flows, excluding the dawnside return flows which could not be identified in the ionosphere. The BBFs and streamers are found to appear during an enhanced reconnection electric field in the magnetotail.0Comments 0Citations
- "Shiokawa et al., 1998]. In the competing current disruption (CD) model [Lui , 1996; Erickson et al., 2000; Donovan et al., 2008; Lui et al., 2008; Henderson, 2009], the expansion phase is triggered by reduction of the tail current and formation of the SCW due to plasma instabilities developing in the inner magnetosphere at around 6–10 R E . The current disruption spreads downtail as a rarefaction wave, which then initiates the formation of the NENL farther in the tail. "