Particle acceleration processes on auroral field lines during major geomagnetic storms

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Particle acceleration processes on auroral field lines during major geomagnetic storms

C. Cattell, P.I.

Table of Contents
1. Scientific/Technical Management Section pg. 2
2. References pg. 17
3. Summary of Personnel and Work Efforts pg. 23
4. Facilities and Equipment pg. 24

5. Budget Justification pg. 25

6. Biographical Sketches pg. 26
7. Co-I and/or Collaborator Commitments pg. 30
8. Current and Pending Support pg. 33

1. Scientific/Technical/Management Section

A. Objectives and significance to NASA’s strategic goals
Major geomagnetic storms affect the magnetosphere of the earth dramatically in a number of ways. These include intensification of the radiation belts, ionospheric heating and latitudinally extensive, extremely intense aurora, causing significant energy dissipation (Baker et al., 2001). Additionally, in the largest storms, magnetospheric activity becomes directly driven by the solar wind (e.g. Siscoe et al., 2002; Lopez et al., 2004) resulting in magnetospheric conditions that are significantly different than at other times. Understanding of the various processes occurring during such events is important for several reasons. First, these events result in acceleration of high energy particles which are a hazard to instruments and humans in space. Secondly, this alternate state of the magnetosphere is effectively an additional laboratory, complete with a good number of existing instruments, for the study of natural plasma processes in space. And third, such storms are the most extreme examples of space weather that affect near-earth space resulting in substantial energy transfer from the sun to the earth. Although most studies of major storms have focused on the ring current and radiation belts, one of the first storm phenomenon to be observed was the ‘great red aurora.’ Acceleration processes associated with auroral and cusp field lines have, therefore, also received attention. There are a number of electron features that have been identified, including stable auroral red (SAR) arcs and low latitude broadband electrons (which may produce great red aurora). Several recent studies have examined the strong outflow of ionospheric ions (Moore et al., 1999; Strangeway et al., 2000) that occurs near the polar cap boundary on the dayside. With the notable exception of studies of ionospheric sources of ring current particles (Daglis et al., 1993; McFadden et al., 2003), however, the role of particle acceleration on auroral field lines in the dynamics and evolution of major geomagnetic storms (‘superstorms’) has been largely unexplored. Recent studies of the largest storms have suggested that ionospheric particles and acceleration processes may play an important and here-to-for unrecognized role in storm dynamics (Kozyra et al., 2004b). There is evidence for strong coupling between ionospheric particles, auroral acceleration, the ring current, radiation belts and the atmosphere. Our primary science goal is to determine the mechanisms that energize electrons and ions on low latitude auroral field line during major storms and to establish their role in the dynamics and evolution of major storms, including energy deposition. The extension of FAST data acquisition to low latitudes, in conjunction with Polar and Cluster observations of particles and MHD waves on these field lines at higher altitudes and geosynchronous observations of possible source regions, provide a unique opportunity to obtain closure on these questions.

One particular phenomenon associated with these major storms is low-latitude broadband electron signatures with sufficient energy flux to produce visible aurora. Shiokawa et al. (1996; 1997) first reported such signatures using DMSP data, conjecturing that they may be the cause of red aurora observed at mid-latitude ground stations. Similar electron events have also been observed by the FAST satellite (Dombeck 2005; Dombeck et al., 2005b; Nakajima et al., 2005). The electron signatures in these events are distinct from both SAR arc events (e.g. Kozyra et al., 1993; Shiokawa et al., 2001) and diffuse auroral electrons (e.g. Chen et al. 2005b), which are also observed during storms. SAR arc electron events result from energy transfer from ring current ions to cold electrons and have an upper energy limit of ~100 eV (Kozyra et al., 1997), while diffuse auroral electrons result from plasma sheet electrons convecting to lower L-shell during a storm and have an energy spectra which is peaked around a few keV. The low-latitude broadband electron events, by contrast, generally have a non-peaked energy spectra ranging up to several keV. The acceleration mechanism for these intense, low-latitude broadband electrons observed during major storms has not yet been determined. There are, however, similarities between the signatures of these events and those of broadband electrons near the polar cap boundary which are accelerated by Alfvén waves on the plasma sheet boundary layer (PSBL) (e.g. Chaston et al., 2002; Dombeck et al., 2005a). One goal of the proposed study is to understand the acceleration mechanism for low-latitude broadband electron events observed during major storms and the differences and similarities to broadband electrons seen during other geomagnetic conditions.

Although most storm research has focused on electrons, including those that produce SAR arcs and great red aurora, there are unusual signatures in the ions that are often seen coincident with these electron signatures. An example of these new discoveries is the banded ions with energies from 10s to 1000s of eV, which are observed during many superstorms, including the “Halloween” 2003 storms (Cattell et al., 2004b; Thomsen et al., 2004). Although energy banded ions in the auroral zone have been previously reported (Hirahara et al., 1997; Boehm et al., 1999), the observations during the Halloween storms last much longer (more than 12 hours), have more bands, the O+ and H+ bands are at the same energies, the bands are seen on the both the dayside and nightside, and band structure is more complex. They are also extensive in latitude (~50º-75º on the dayside). These bands are new phenomenon associated with all superstorms and possibly many storms and are not readily interpreted using previous models. The banded ions are associated with but occur equatorward of intense ion outflow on the dayside and equatorward of the region of discrete aurora on both the day and night sides. The distributions are peaked in the perpendicular direction (locally mirroring). At the same time, long lasting intervals of field-aligned energy dispersed ions from ~100 eV to 40 keV are seen in the LANL geosynchronous satellites, primarily on the dayside and after magnetosheath encounters (i.e. highly compressed magnetosphere). Although the geosynchronous ions have energy dispersion consistent with time-of-flight, the FAST bands only show such dispersion within an individual band and not across bands or latitude. The second goal of the proposed work is to determine the source population, acceleration mechanisms and ultimate fate of the banded ions.

At this time, we can only speculate about the role of banded ions in superstorms. They may provide an important energy source for extreme SAR arcs and great red aurora. They occur coincident with the MLAT extent of the electron temperature peak on the dawnside and thus may provide an additional heat source for plasmaspheric electrons by Coulomb collisions (Kozyra et al., 2004b). The banded ions are also often seen in the same regions as the broadband electrons. The source of the banded ions has not yet been identified. An ionospheric source may be indicated by the association with intense ion outflow on the dayside; a boundary layer or ionospheric source may be indicated by the association of the geosynchronous injections with magnetospheric compressions (Thompsen et al., 2004); or they may be related to the usual dawnside ring current source. Recent Cluster observations when Cluster was within ~30º of the equatorial plane at ~4 RE during the Halloween storms (Engebretson et al., 2005) showed Pc1 and 2 waves, which are the frequencies to interact strongly with these ions. During one interval, there was a power minimum at the O+ cyclotron frequency. The final goal of our proposed research is to understand whether there is any relationship between the electron and ion acceleration processes, the role of the banded ions in SAR and great red aurora, and the role of acceleration and loss processes on low latitude auroral field lines in storm dynamics and evolution.

One of the strategic goals of NASA and its Science Mission Directorate and part of the particular focus of the Heliophysics Research Program is to understand how solar activity affects the space environment of the earth (goal 3B). Since the most intense and direct coupling of solar activity to the near earth environment occurs during major geomagnetic storms, and particle energization is both a means by which solar wind energy is ultimately transferred to earth and constitutes a change to the environment itself, understanding these energization processes is necessary for achieving NASA’s goals.

Our proposal describes a research program designed to examine particle acceleration on low latitude auroral field lines. The primary goals are to: (1) understand the acceleration (as well as transport and loss) of low latitude broadband electrons observed during major storms and the differences and similarities to broadband electrons seen during other geomagnetic conditions; (2) determine the source population, acceleration mechanisms and ultimate fate of the banded ions; and (3) understand whether there is any relationship between the electron and ion acceleration processes, the role of the banded ions in SAR arcs and great red aurora, and the role of acceleration on low latitude auroral field lines in storm dynamics and evolution. The specific science questions and approach to answering these questions will be detailed in Section C.

The University of Minnesota and the proposed project team are ideally suited to conduct the study of these important, and not yet well studied, magnetic storm processes. In addition to having successfully completed studies using similar techniques on FAST particle data (Cattell et al., 2003, 2004a, 2006), the team has done preliminary and published studies, advancing the understanding of the likely closely related process of PSBL broadband electron/Alfvén wave events and low latitude events (Dombeck 2005; Dombeck et al., 2005a, 2005b, 2006). The University of Minnesota also has the computer expertise, computing tools, and data available in house to efficiently conduct the proposed project. All of the FAST electron CDF files as well as all Polar full resolution data are stored online at the University of Minnesota. All FAST, Polar and Cluster full resolution data are also available online or on CD at the University, while necessary ACE, Wind and geomagnetic index data are readily available online through web-services for which automated access routines already exist at the University. In addition, the PI has ongoing grants of time on the supercomputers of the Minnesota Supercomputer Institute that will be used for particle tracing codes. The PI of the proposed project is a Co-I on FAST, and the Polar and Cluster electric field instruments. Note that she does not receive project funding for Polar or Cluster studies and that her FAST funds are currently for TIMED/FAST comparisons. The group has ongoing collaborations with the Polar particle teams and the Cluster magnetic field and particle teams, and software installed and running to provide analysis of full resolution Polar particle data, Cluster magnetic and particle data. Collaborator Dr. M. Thomsen has expertise in the analysis of the LANL geosynchronous data and has worked with the PI on preliminary studies of banded ions. Collaborator Dr. J. Kozyra has expertise in theory and data analysis on SAR arcs, great red aurora, superstorms and coupling processes.

B. Impact of proposed work and relevance to NASA goals

Our research addresses NASA’s strategic goal, 3B, “Understand the Sun and its effects on Earth and the solar system.” Specifically, as called out in 3B.1, we will provide “progress in understanding the fundamental physical processes of the space environment…” that will ultimately allow the development of a predictive capability. We will provide science results to address several research focus areas from the 2005 Roadmap, including F2- “ Understand the plasma processes that accelerate and transport particles” and F3 -“Understand the role of plasma and neutral interactions in nonlinear coupling of regions throughout the solar system”, as well as H2 - “Determine how changes in the Earth’s magnetosphere, ionosphere, and upper atmosphere change in order to enable specification, prediction, and mitigation of their effects.”

Our proposed work contributes directly to the goal of the Heliophysics Guest Investigator program to fully utilize the data sets from operational missions to meet NASA strategic goals. In particular, the proposed study is a data analysis and interpretation science project that uses data, from various spacecraft of the Geospace Mission, in particular FAST, with additional analysis of data from Polar and Cluster. ACE and WIND data will be used to specify the solar wind drivers.

C. Technical Approach and Methodology

While a few major storm related low-latitude broadband electron events (Shiokawa 1996, 1997; Dombeck et al., 2005a,b; Nakajima et al., 2005) and banded ion events (Cattell et al., 2004b; Thomsen et al., 2004) have been reported on a case study basis, they have not yet been studied as classes of events. Neither process is yet understood, either observationally or theoretically. FAST is particularly well suited in orbit, mission duration, and instrumentation for its data to play the primary role in a comprehensive study of these events. Its ten year mission duration to date covers nearly an entire solar cycle allowing observation of a statistically significant number of major storms, >20, and its 133 minute, highly elliptical, polar orbit and extremely fast data rate for both particles and fields provides the temporal and spatial coverage at high resolution necessary to investigate these rare events. Further, FAST data has already been utilized for preliminary results on case studies of these and similar events (Cattell et al., 2004b; Dombeck 2005; Dombeck et al., 2005a,b; Nakajima et al., 2005). Combining FAST data with data from the Polar, Cluster, and LANL geosynchronous satellites, with Wind and ACE providing solar wind monitoring, will allow for a comprehensive understanding of these phenomena and their role in sun-earth coupling during major storms.

The complete methodology for our study is detailed below, but a brief overview is as follows: The first stage of the study will be to study a small statistical sample of both broadband electron and banded ion events (~10) for detailed initial analysis for use in providing preliminary results, determining the viability of the various acceleration mechanisms, and developing automated identification algorithms. These algorithms will then be used to compile a database of events from the entire FAST mission, which will include the relevant characteristics of each event from the FAST data, plus the relevant conditions of the solar wind and magnetosphere from WIND, ACE, Polar and Cluster data and global indices such as Dst, AE and Kp. This database will be used to determine characteristics and correlations for events and conditions, for comparison to the various theoretical generation mechanism and to provide constraints/input parameters to related simulations. Finally, detailed analysis of both the FAST data and related Polar, Cluster and LANL data will be used to refine and further test understanding of these processes and their role in storm dynamics and evolution.

Low-latitude downgoing broadband electrons during major storms were first reported by Shiokawa et al. (1996; 1997) using DMSP data. These broadband electron signatures were significantly different than those associated with diffuse, inverted-V and SAR arc aurora, having a very broad energy range, from ~30 eV to 30 keV and sharply defined latitudinal extents. P
Figure 1. Sample downward electron energy spectra from FAST passes through the auroral zone showing a variety of examples of low latitude broadband electron events during major geomagnetic storms. Note that the bottom three passes are equator to pole while the others are pole to equator.
reliminary FAST results, shown in Figure 1 which plots the downward energy spectra for a variety of cases, indicate that events that can be described as low-latitude broadband electrons during major storms are observed at many different local times and have a variety of signatures. These include intense, latitudinally limited events, events that extend throughout much of the auroral zone, events that have maximum energies below 1 keV, ones with energies up to 10 keV, isolated broadband populations and ones that are concurrent with inverted-V, diffuse or SAR arc auroral populations. Further, clear cut differences in properties to classify these various signatures into “types” are not readily apparent. Therefore, these events may be the result of a single mechanism operating under different conditions or a variety of different mechanisms, possibly occurring simultaneously, that result in similar electron signatures. Preliminary FAST analysis does provide some insight in that the basic properties of many of the low-latitude broadband electron events are similar to broadband electron events more commonly observed on field lines mapping to the PSBL. Also, low-latitude broadband electron events have been observed on similar field lines and in storms that also contained banded ions, although a complete correlative study of these two phenomena has not yet been done. Therefore these low-latitude broadband electron events may be related to a process similar to the PSBL events, i.e. Alfvén waves at higher altitude, a process related to the banded ions or to some other process. The proposed study will investigate these possibilities. The low-latitude broadband electrons are also sometimes observed concurrently and on the same field lines as SAR arc electron populations. The broadband signatures are distinct from the SAR arc ones, having higher energy components and sharp latitudinal extents. These population distinctions and the fact that broadband events are observed at times without SAR arc populations indicate that these two processes are likely unrelated. This conjecture will be specifically tested as a part of the proposed study, with the assistance of Collaborator Dr. J. Kozyra.

Figure 2. A sample FAST pass during a major storm showing both low latitude (A) and PSBL (B) broadband electron signatures. Panels a and b plot the electron energy and pitch angle spectra, while panels c and d plots Alfvénic Poynting flux and dE/dB ratio for two different frequency bands. Panels e and f are sample electron energy flux distributions for the broadband electron events.
roadband electron events on PSBL field lines, which are observed both during major storms and during substorms, have been more extensively studied than the low-latitude variety and are observed coincident with small scale Alfvén waves at FAST. They are consistent with being powered by Alfvén waves on the PSBL observed at 4-9 RE (Wygant et al., 2000,2002a; Chaston et al., 2002, 2003). This theory is supported by various simulations (e.g. Chaston et al., 2002; Su et al., 2004; Chen et al., 2005a) which indicate that Alfvén waves at 4-9 RE result in a broadband electron spectra at FAST altitudes, and has recently been confirmed by a study of a simultaneous Polar and FAST PSBL crossing (Dombeck et al., 2005a). Similar findings have also been reported on dayside cusp field lines (Chaston et al. 2005). Figure 2 (Dombeck et al., 2005b) shows data from a FAST pass during the recovery phase of a major storm on 21 October 1999 with both PSBL and low-latitude broadband electron events. Panel a and b are the electron energy and pitch angle spectra, while panels c and d show the earthward directed Poynting flux and dE/dB ratio in two different frequency bands, red (blue) 10 to 100 mHz (1 to 4 Hz). dE/dB ratios are only plotted for times of significant Poynting flux (>0.04 ergs cm-2s-1 when mapped to the ionosphere). Intense broadband electrons can be observed at ‘B’, on PSBL field lines, and at low latitude (~60º ILat), e.g. ‘A’, with less intense broadband signatures at intermediate latitudes. The dE/dB ratios indicate that the lower frequency Poynting flux (red) is consistent with quasi-static structures coupling to the ionosphere through Pederson currents, while the high frequency Poynting flux (blue) is Alfvénic in nature. The broadband electrons at both ‘A’ and ‘B’, along with some of those observed at mid-latitude, occur coincident with small scale (high frequency) Alfvénic activity. Panels d and e show electron energy flux distributions at ‘A’ and ‘B’ respectively. Such similarities between the low-latitude and PSBL broadband electron observations indicate that the two processes may be similar. Further support to this conjecture is provided by preliminary analysis (Dombeck 2005; Dombeck et al., 2005b) which indicates that during at least one major storm Polar likely observed intense earthward Alfvénic Poynting flux at ~6 RE concurrent with a low-latitude broadband electron event observed by the DMSP F14 satellite.

To understand these low-latitude broadband electron events observed during major storms and their role in the solar wind/magnetosphere coupling, several specific science questions will be addressed. (1) What are the characteristics of these events and are there distinct ‘types’? (2) How are these events related to/controlled by solar wind drivers and magnetospheric conditions? (3) Are these events caused by the same mechanism as the PSBL events, how do the low-latitude Alfvén waves differ from those on the PSBL and how do they relate to major storms? (4) Are additional mechanisms required to account for all of the observed events, and if so what are those mechanisms? The proposed research project will address these questions in several steps, utilizing in situ particle and fields data from FAST, Cluster and Polar.

The first step will be to choose ~10 FAST events which include ones with and without concurrent SAR arc populations and banded ions (if possible) from varying phases of major storms. These events will be used for detailed initial analysis, including particle distribution and Alfvén wave analysis, as well as relationship to solar wind and magnetospheric conditions. This study will provide detailed initial investigation of the physics of these events addressing all four science questions and will facilitate development of an automated event identification algorithm. This algorithm development will combine the expertise developed in the previous University of Minnesota database studies with newer data mining techniques (e.g. Karimabadi et al., 2006) to produce a robust, efficient algorithm for the identification and classification of these events with their widely varying signatures. Particular attention will be given in this first stage to providing preliminary answers to questions 3 and 4 by conducting detailed analysis of the event particle distributions and comparing them and Alfvén wave characteristics to those of FAST events on PSBL field lines. The methodology for Alfvén wave analysis has been described in Dombeck (2005) and Dombeck et al. (2005a, 2006).

The second step will be to use the identification algorithm on the entire FAST data set as was done in the University of Minnesota FAST electron beam study (Cattell et al., 2004a; 2006) to create an event database, to provide definitive answers to science questions 1 and 2 on a statistical level and a sufficient empirical context to understand the physics of questions 2, 3 and 4. The entire FAST dataset will be used rather than just concentrating on major storms since part of the proposed study is to determine how the events are related to solar wind/magnetospheric conditions and storms. For each event, the database will store the characteristics of the event, e.g. characterization of the electron distribution and moments and their variability during the event, event spatial and temporal extent and location, along with FAST field data for Alfvén wave summary analysis, as has been done for PSBL events (Dombeck et al., 2005a, 2006), and various characteristics of the solar wind and magnetospheric conditions surrounding the time of the event, e.g. solar wind velocity, dynamic pressure and composition, IMF magnitude, direction and variability, magnetospheric composition, and AE, Dst indices. These latter solar wind and magnetospheric conditions and characteristics will be obtained from ACE, WIND, Polar, Cluster data, when available, from the CDAWeb and the World Data Center for Geomagnetism, Kyoto. These data will be obtained through automated download routines similar to those used for previous University of Minnesota correlative studies (Cattell et al., 2004a; Dombeck et al., 2006). Before running the automated database routines on the entire FAST dataset, it will be tested on a few months’ worth of data when several major storms occurred to tune the automated algorithms and provide preliminary results.

The third step of the investigation of low-latitude broadband electron events will be to investigate the properties of the plasma sheet mapping to the regions of these events during the times that these events are observed to provide definitive answers to science questions 3 and 4. This region generally corresponds to the plasma sheet between 3 and 6 RE. Wygant et al (2001, 2002b) have presented a number of cases of strong Alfvénic waves deep within the plasma sheet during major storms. Further study of this region and events of this type will clarify the role of Alfvénic acceleration in these events. The analysis in this stage of the study will utilize Polar, Cluster, and to a lesser extent LANL geosynchronous satellite data, and will incorporate two methods of analysis. The first method will search for conjunctions between Polar or Cluster and FAST during FAST observations of low-latitude broadband electrons. Similar analysis has been done for the PSBL and cusp broadband electron events (Dombeck et al., 2005a; Chaston et al., 2005). LANL geosynchronous satellite conjunctions will be somewhat common but their lack of field measurements will limit the utility of such conjunctions, particularly if the energy at these higher altitudes is primarily in waves, as in the case of Alfvén waves. Without knowing the occurrence frequency of the low-latitude broadband electron events, it is difficult to estimate the likely number of Polar or Cluster and FAST conjunctions. However, a rough estimate can be determined from visual data inspection that indicates that there are generally ~10 to 15 orbits per major storm with prominent events. Polar’s ~18 hour orbit means that it should be latitudinally conjunct with FAST during one or two of those orbits per storm. Polar has been operational for the duration of the FAST mission. However, the latitudinal precession of Polar’s perigee means that is would be conjunct in the ~3 to 6 RE mapping altitude only perhaps 10% of the time, leading to an expectation of ~2 to 4 conjunction events. Cluster, having only been available for roughly half of the FAST mission duration, has a ~54 hour orbit, but passes through the ecliptic at ~4 RE each orbit of it mission. Therefore Cluster also has an expectation of ~2 to 4 conjunctions. The challenge during such Polar and Cluster orbits will be to determine accurate mapping since active storm times require topographical mapping, and the low-latitude broadband electron events map to the central plasma sheet which does not easily facilitate such mapping. However, indirect arguments can be made for mapping of such events, as has been done for the preliminary analysis of the Polar/DMSP F14 low-latitude broadband electron event (Dombeck 2005; Dombeck et al., 2005b). These conjunction studies will provide confirmation and allow investigation of the details of the electron acceleration mechanism(s) and the energy transfer process.

The second method of investigating the region of the plasma sheet mapping to the low-latitude broadband electron events that will be utilized in the proposed study will be to use the conditions during which such events occur as determined in stage 2 of the study to investigate the Polar and Cluster passes through regions that would map to such events when FAST is not in direct conjunction. When available DMSP satellite data will be used to confirm that low-latitude broadband electron events do occur during these Polar and Cluster passes. This method of study will provide enough samples of plasma sheet conditions for statistical understanding of the region during the events. Combining the results of this method of study with the direct conjunction events will provide sufficient data for confirmation of the accuracy of understanding of the electron acceleration mechanism(s) responsible for these events and their coupling to major storm processes.

These three steps of analysis will facilitate the answering of all four specific science questions in regard to the low-latitude broadband electron events during major storms. In addition to providing characterization of the events and their ‘types’, determining how these events relate to the PSBL and cusp broadband electron and Alfvén wave events, and investigating how these events are related to major storm conditions and sun-earth energy transfer, questions 1, 3 and 2 respectively, these steps will also answer whether and how these events are related to other processes, question 4, such as banded ions and SAR arcs or here-to-for unknown processes.

Banded ions have previously been reported on auroral field lines at low altitudes (FAST, DMSP and DE-2), intermediate altitudes (DE-1, Akebono) and at high altitudes (Polar). In the low altitude auroral zone, energy dispersed discrete bands, which lasted for a few hours and had equal O+ and H+ velocities, were reported during quiet times. Two interpretations for the observed structures were proposed: (1) convective drift dispersion from an ionospheric heating source (Hirahara et al., 1997); and time-of-flight dispersion from equatorial acceleration event (Boehm et al., 1999). Both models predict that O+ and H+ ions will have the same velocity, energy bands have ratios dependent on latitude (field line length) and that energy increases with latitude. The equatorial source model assumes an impulsive acceleration process that is broad in latitude, as described by Mauk (1986). The observed energy dispersion with latitude depends on the length of the field line squared and results in bands with ratios of .25, .75, 1.25, etc. In the ionospheric acceleration model, the latitude dispersion depends on the ExB drift and results in bands with ratios of 1,2,3 … or 1.5, 2.5, 3.5, etc.. Boehm et al. (1999) concluded that both their observations and those of Hirahara et al. were most consistent with the equatorial acceleration mechanism.

Using DE data (at low and mid-altitudes), Frahm et al. (1986) and Winningham et al. (1984) described energy dispersed ion bands, within the region of diffuse aurora, from a few eV to a few keV and peaked at a pitch angle of 0º. In contrast to the Boehm et al. and Hirahara et al. observations, these occurred primarily during the main phase of storms. Similar to Hirahara et al., the bands were interpreted as being the result of convective dispersion from an ionospheric, auroral source.

At higher altitudes, Polar observations of multiple energy dispersed bands (~1-100s of keV) were reported by Fennell et al. (1998) and Peterson et al. (1998). These events had O+ and H+ at the same energy, and were weakly peaked at 90º pitch angle. They extended from L~3-8, were most often seen from ~6-18 MLT, and in quiet times following substorms. Three different explanations have been proposed: (1) convection of time variable discrete ion sources in the plasma sheet (Peterson et al., 1998); (2) time-of-flight following prompt energization in an electric field pulse associated with substorm dipolarization with bands depend on ion grad B drift time (Li et al., 2000); and (3) time-varying ExB convection of a tail source population for energies >~1keV and an ionospheric source for energies <~1keV (Fennell et al., 1998). A recent particle tracing simulation (Ebihara et al., 2003) concluded that the Fennell et al. mechanism was most likely with the bands being a result of enhanced convection (during the substorm) followed by reduced convection.

Note that there are other band-like features that have been observed and modeled, including the ion ‘gaps’ (see, for example, Kovrazkhin et al., 1999), ‘wedge’-type dispersion (Ebihara et al., 2001) and velocity-dispersed ions at the plasma sheet boundary (Ashour-Abdalla et al., 1992, 2005; Bosqued et al., 1993). McFadden et al. (2001) described low energy FAST observations of ring current injections and low energy ring current dawn-dusk asymmetry. These have all, in various cases, b