Article

Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere

Annales Geophysicae (Impact Factor: 1.52). 01/2007; DOI: 10.5194/angeo-25-801-2007
Source: OAI

ABSTRACT Addressing the origin of the energetic particle injections into the inner magnetosphere, we investigate the 23 February 2004 substorm using a favorable constellation of four Cluster (near perigee), LANL and Geotail spacecraft. Both an energy-dispersed and a dispersionless injection were observed by Cluster crossing the plasma sheet horn, which mapped to 9–12 RE in the equatorial plane close to the midnight meridian. Two associated narrow equatorward auroral tongues/streamers propagating from the oval poleward boundary could be discerned in the global images obtained by IMAGE/WIC. As compared to the energy-dispersed event, the dispersionless injection front has important distinctions consequently repeated at 4 spacecraft: a simultaneous increase in electron fluxes at energies ~1..300 keV, ~25 nT increase in BZ and a local increase by a factor 1.5–1.7 in plasma pressure. The injected plasma was primarily of solar wind origin. We evaluated the change in the injected flux tube configuration during the dipolarization by fitting flux increases observed by the PEACE and RAPID instruments, assuming adiabatic heating and the Liouville theorem. Mapping the locations of the injection front detected by the four spacecraft to the equatorial plane, we estimated the injection front thickness to be ~1 RE and the earthward propagation speed to be ~200–400 km/s (at 9–12 RE ). Based on observed injection properties, we suggest that it is the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp inner front propagating earthward), which accelerate and transport particles into the strong-field dipolar region.

0 Bookmarks
 · 
91 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using auroral TV data and particle precipitation data from low-altitude satellites, we identify the ionospheric signature of magnetotail dipolarizations and substorm injections measured in the near-Earth near-equatorial plasma sheet by Time History of Events and Macroscale Interactions during Substorms (THEMIS). Field line mapping exploits a recently developed time-dependent adaptive model which minimizes the variance to THEMIS in situ magnetotail observations. We present strong evidence that the equatorward edge of the auroral bulge corresponds to the innermost extent of earthward propagating dipolarization fronts in the magnetosphere, whereas individual equatorward moving auroral enhancements correspond to the motion of individual injection fronts reaching at times distances as close to Earth as 5.5 RE. The region of tail dipolarization corresponds to the auroral bulge, a broad spatial region of enhanced but structured auroral emissions, bounded on the poleward side by discrete auroral forms and on the equatorward side by a sharp drop in auroral luminosity and particle precipitation. Particle precipitation within the bulge is enhanced considerably at the energies above 30 keV. Ionospheric protons are isotropic and electrons are anisotropic but with fluctuating fluxes which are below, but on occasion comparable with, trapped levels. The equatorward edge of the bulge, herein termed the “Equatorward edge of Auroral Bulge” propagates during substorm expansion toward lower latitudes, initially fast (corresponding to 100 km/s in space at r ˜ 7 RE) but with decreasing speed after onset. Our adaptive model mapping suggests that equatorial points at near-geosynchronous altitude can map to ionospheric magnetic latitudes up to 2°-3° off of predictions using standard T96 models. The offsets can be either toward lower latitudes due to field line stretching before auroral breakup or toward higher latitudes after breakup due to the near-Earth tail dipolarization.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The strong field-aligned pitch angle distribution of electrons is observed right at the dipolarization front (DF) before the arriving of a high speed flow when the four Cluster satellites are traveling in the magnetotail around 15 R E on July 22, 2001. The increased electron fluxes only last for a short time period at the DF, corresponding to just a few bouncing periods for 1 keV electrons. The field-aligned current contributed by these electrons agrees well with that calculated by the magnetic field observations by four satellites at the front. These electron streams are found in the energy range of 200–2 keV, peak around 1 keV. It is suggested that these downward current electrons may be originated near the aurora region by some kinds of potential structure. The occurrence of these electrons implies that the formation of the dipolarization front and the associated field-aligned current play an important role in the magnetosphere-ionosphere coupling. plasma sheet, high speed flow, electron beam, dipolarization front, field-aligned current Citation: Zheng H, Fu S Y, Zong Q Q, et al. Downward current electron beams observed at the dipolarization front. Chin Sci Bull, 2012, 57: 17, doi: 10.1007/s11434-012-5478-3 Dipolarization is a significant dynamic process in the mag-netotail, in which the magnetic field topology changes from tail-like to more dipole-like in a relative short time scale. Dipolarization processes are commonly observed at the time period of substorm, plasma sheet expansion or Bursty Bulk Flows (BBFs) [1–4]. The concept of dipolarization front (DF) was first pro-posed by Nakamura et al. when they studies high speed flows [1]. Based on the analysis of the four Cluster satellites data, it is found that, on July 22, 2001, a dipolarization front moves earthward and dawnward ahead of the high-speed earthward flow. The plasma before this front is deflected, consistent with the plasma ahead of a localized plasma bub-ble centered at midnight side being pushed aside by the moving obstacle. It is believed that the evolution of the di-polarization front across the tail is directly coupled with the fast flow. Multi-point observations by five THEMIS probes situated in the near-equatorial plasma sheet at distances of (−20 R E , −10 R E) are used to carefully study the dipolariza-tion front event on February 27, 2009 [5]. The variations in magnetic field and plasma moments at the depolarization front reveal characteristic signatures of BBFs: increase in bulk velocity and magnetic pressure, and decrease in plasma density and pressure. And the quick southward variation of magnetic field observed ahead of the DF has been proved to be a spatial structure associated with the propagating of DF. Based on the observation of THEMIS on 23 February, 2008, Sergeev et al. [6] first investigated the kinetic struc-ture of the sharp dipolarization front, which was found to be a very thin current sheet along the North-South direction embedded within an earthward-propagating flow burst. Clear finite proton gyro-radius effects are observed in the center of the thin current sheet and strong E-field bursts of the lower-hybrid time scale at the injection front with a density depletion. Since enhancement of electron fluxes are commonly observed in dipolarization events, many works focus on the energization mechanism of electrons. The en-ergetic electron flux enhancements at the dipolarization fronts are found to be associated with large wave fluctua-tions extending from below the lower hybrid frequency to
    Chinese Science Bulletin 01/2012; · 1.37 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Electrons streaming along the magnetic field direction are frequently observed in the plasma sheet of Earth's geomagnetic tail. The impact of these field-aligned electrons on the dynamics of the geomagnetic tail is however not well understood. Here we report the first detection of field-aligned electrons with fluxes increasing at ∼1  keV forming a "cool" beam just prior to the dissipation of energy in the current sheet. These field-aligned beams at ∼15 R_{E} in the plasma sheet are nearly identical to those commonly observed at auroral altitudes, suggesting the beams are auroral electrons accelerated upward by electric fields parallel (E_{∥}) to the geomagnetic field. The density of the beams relative to the ambient electron density is δn_{b}/n_{e}∼5-13% and the current carried by the beams is ∼10^{-8}-10^{-7}  A m^{-2}. These beams in high β plasmas with large density and temperature gradients appear to satisfy the Bohm criteria to initiate current driven instabilities.
    Physical Review Letters 11/2012; 109(20):205001. · 7.73 Impact Factor

Full-text (2 Sources)

View
21 Downloads
Available from
May 17, 2014