Particle and field characteristics of broadband electrons observed by the FAST satellite during a geomagnetic storm

Abstract

[1] Broadband electrons (BBEs) are remarkable flux enhancements (>10 13 eV cm À2 s À1 ) of precipitating electrons over a broad energy range (0.03-30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. We show characteristics of particles (energy spectra and pitch angle distribution) and fields (electric field, magnetic field, and wave spectra) during a BBE event observed by the Fast Auroral SnapshoT (FAST) satellite. The BBEs were observed at an altitude of $2000 km at 59°-61°i nvariant latitudes (ILATs) and 21 h magnetic local time (MLT). The event was observed at $7 min after the onset of a substorm during the main phase of the Bastille Day geomagnetic storm (minimum Dst = À301 nT) on 15 July 2000. The precipitation region of the BBEs corresponded to a localized intensification of auroral emission, lasting $14 min, observed by the Polar UVI images at 50°-60°geomagnetic latitudes (MLATs) and 20-21 MLTs. These results suggest that rapid particle acceleration was occurring in the inner magnetosphere associated with a storm-time substorm. The pitch angle distribution of BBEs was isotropic except for a loss cone feature around the field-aligned upward direction at a higher energy range above $1 keV, while field-aligned electron fluxes were larger than the perpendicular fluxes below $1 keV. These results imply that a higher energy part of the BBEs originated from higher altitudes in the inner magnetosphere and that a lower energy part was accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields were observed during this BBE event. From these results, we discuss possible acceleration of the lower energy part of BBEs through wave-particle interaction.

Characteristics of monoenergetic and broadband auroral electron precipitation as observed by the Science and Technology Satellite‐I

“…They generally occur (1) in short‐lived bursts (1–2 s in time, or equivalently, about 14 km in space when observed by a moving platform) and (2) with the pitch angle highly field aligned [ Johnstone and Winningham , ]. However, broadband precipitation may persist longer than 1 min with a spatial scale of about 10 km [ Semeter et al , ] or for about 14 min with a spatial scale of about 200 km [ Nakajima et al , ]. The occurrence rate does not depend significantly on geomagnetic activity, at least between 2000 MLT and 0300 MLT [ Nagatsuma et al , ].…”

Simultaneous FAST and Double Star TC1 observations of broadband electrons during a storm time substorm

“…These facts indicate that a substorm began at ∼0500–0600 MLTs at 1347–1349 UT. The BBEs occurred in association with the storm time substorm, which has been reported in previous studies of BBEs using the magnetic field data obtained in the geosynchronous orbit and on the ground [ Shiokawa et al , 1996; Nakajima et al , 2007, 2008]. In this study, we examined the plasma dynamics in the inner magnetosphere associated with the storm time substorm by using data obtained by DS TC1.…”

“…It is reasonable to speculate that the enhanced electron precipitation over the entire region results from the same mechanisms. In an event study of BBEs observed by FAST, Nakajima et al [2007] reported that simultaneous auroral images from the Polar satellite showed that the precipitating region of BBEs covered ∼10° in latitude and ∼1 h in local time, which included the ionospheric footprint of FAST.…”

“…BBEs have drastic electron flux enhancements over a broad energy range (0.03–30 keV) at the lower latitude side of the auroral oval during storms, which has been reported by Shiokawa et al [1996] based on particle observations by Defense Meteorological Satellite Program (DMSP) satellites. The pitch angle distribution of electrons obtained by the Fast Auroral Snapshot (FAST) satellite suggests that BBEs consist of two energy components (higher and lower energies) due to the acceleration of electrons at different altitudes [ Nakajima et al , 2007, 2008]. In these previous studies of BBEs, data from low‐altitude satellites (DMSP: ∼840 km and FAST: ∼350–4200 km) were used.…”

Satellite observations of energy‐banded ions during large geomagnetic storms: Event studies, statistics, and comparisons to source models

“…Major geomagnetic storms trigger a wide range of changes in the Earth's magnetosphere, inluding the transfer of a significant amount of energy, and are usually associated with coronal mass ejections, large interplanetary magnetic fields, and/or high-pressure solar wind plasma [Baker et al, 2001]. There are many unusual particle acceleration processes that can occur, including prompt energization of relativistic electrons in the radiation belts [Wygant et al, 1994], acceleration of electrons over a broad energy range in the auroral zone [Shiokawa et al, 1996;Dombeck et al, 2005;Nakajima et al, 2007], and strong outflow of ionospheric ions in the polar cap [Moore et al, 1999;Strangeway et al, 2000]. Although much of the research on storms in the auroral zone has focused on electrons, including those that produce subauroral red (SAR) arcs and great red aurora [Kozyra et al, 1993[Kozyra et al, , 1997Shiokawa et al, 1997], there are unusual signatures in the ions that are seen during large storms and which are sometimes coincident with these electron signatures.…”