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

Abstract

Energy‐banded ions from tens to ten thousands of eV are observed in the low‐latitude auroral and subauroral zones during every large (minimum Dst < −150 nT) geomagnetic storm encountered by the FAST satellite. The banded ions persist for many FAST orbits, lasting up to 12 h, in both the northern and southern hemispheres. The energy‐banded ions often have more than six distinct bands, and the O+, He+, and H+ bands are often observed at the same energies. The bands are extensive in latitude (~50–75° on the dayside, often extending to 45°) and magnetic local time, covering all magnetic local time over the data set of storms. The distributions are peaked in the perpendicular direction at the altitudes of the FAST satellite (~350–4175 km), although in some cases the precipitating component dominates for the lowest energy bands. At the same time, for some of the events studied in detail, long‐lasting intervals of field‐aligned energy dispersed ions from ~100 eV to 40 keV are seen in Los Alamos National Laboratory geosynchronous observations, primarily on the dayside and after magnetosheath encounters (i.e., highly compressed magnetosphere). We present both case and statistical studies of the banded ions. These bands are a new phenomenon associated with all large storms, which are distinctly different from other banded populations, and are not readily interpreted using previous models for particle sources, transport, and loss. The energy‐banded ions are an energetically important component of the inner magnetosphere during the most intense magnetic storms.

Energy‐banded ions in Saturn's magnetosphere

“… On 3 May 2005 (day 123) the cool ion distribution observed by Cassini exhibited approximately three to four discrete bands in E / q . The band energies uniformly decreased with increasing radial distance, but they appeared superimposed on a background thermal population that followed the expected general increase with r. The banded ions were a globally coherent feature, lasting at least ~4 h, and covering at least the L range from 5.7 to 7.5, the local time region from pre‐midnight to ~2 LT, and ~106° of SLS2 longitude [e.g., Kurth et al ., ]. The fact that the energy flux was larger for particles coming from the equator than for particles coming from high latitudes points to a more equatorial source of the population or possibly one in the Southern Hemisphere. The energy of the bands appeared to decrease with increasing pitch angle, even across 90°. The bands that CAPS saw were dominantly H + , and we are unable to determine whether or not the heavier species (H 2 + and W + ) were similarly banded, preventing the type of species comparisons (e.g., common energy or common time of flight) that have proven valuable at Earth [e.g., Colpitts et al , ]. Projecting backward in time, the banded ions seen by Cassini would have departed the equator in the premidnight region ~0.5–1.5 h prior to their observation (the shorter travel times corresponding to the higher‐energy bands). The times between the projected departures of the various energy bands would have been ~1000–2000 s, comparable to the estimated half bounce period of Alfvén waves on these field lines.…”

“…Energy‐banded ions have been seen under a number of different circumstances in the Earth's magnetosphere [see Colpitts et al ., , for a review]. They have been reported during both quiet and geomagnetically active intervals; in the auroral zone, at subauroral latitudes, and near the dayside magnetopause, and at ion energies of tens of eV to tens of keV.…”

“…A number of possible generation mechanisms for energy‐banded ions at Earth have been proposed (also reviewed by Colpitts et al []), including convective drift dispersion or time‐of‐flight dispersion from either an ionospheric or an equatorial heating event. The different generation scenarios have different implications for the properties of the banded ions, and some observational support seems to exist for essentially all of the mechanisms.…”