E. T. Sarris

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

ENERGETIC PARTICLE BURSTS IN THE EARTH'S MAGNETOTAIL

During magnetospheric substorms copious numbers of particles are accelerated to energies ranging from a few keV to ≳ 1 MeV and propagate within the magnetotail, magnetosheath, and upstream solar wind. Timescales of acceleration range from < 10 seconds to several minutes, and intensities of 0.3 MeV protons up to 105(cm2sec sr MeV)−1 have been observed. During a special class of intense proton bursts, inverse velocity dispersion if observed. Some of the bursts exhibit fast onsets and slow decays with repetitive particle injections. Pronounced dawn-dusk asymmetries are seen, especially at the higher fluxes and energies, with electron burst intensities being largest on the dawn sector of the magnetotail while proton intensities are highest on the dusk sector. General correspondence is found between burst activity and gross changes in ground-based auroral zone magnetograms. Intimate association is seen between DC electric field pulses on high latitude field lines at a few earth radii, and impulsively accelerated particles. In several instances, counterstreaming between energetic particles and plasma is observed. Oppositely directed anisotropies of protons and electrons (earthward and tailward. respectively) can be used to infer fieldaligned potential drops of tens of kV. Field-aligned electron anisotropies are used to distinguish between open and closed field lines in the magnetotail, and thus infer the large scale field topology with good (~10 sec) time resolution. Simultaneous observations with two or more spacecraft show development of anisotropies which is consistent with multiple sources within the magnetotail. In one case, the inferred source size at ~ 31 Re was ~ 3000 km and source velocity earthward was ~ 30 to 80 km/sec. These observations place most stringent constraints on magnetospheric substorm models and attendant particle acceleration mechanisms. It is suggested that the data is clearly inconsistent with models postulating a single X-type neutral line across the magnetotai1. It is found that present models cannot account for all observations.

Energetic ion distributions on both sides of the Earth's magnetopause

The AMPTE/CCE spacecraft, with an apogee of ∼8.8RE and an inclination of ∼4.3°, sampled the outer dayside equatorial magnetosphere for extended time periods and often crossed into the magnetosheath whenever the solar wind pressure was sufficiently high to compress the magnetopause to 10 keV and an earthward gradient in the subsolar magnetosheath. In addition to the steady state magnetosheath population there exists a burst-type component indicative of a magnetospheric source, and most of the time this is recognized as a flux transfer event. Overall, the results about the origin of the ≥50 keV magnetosheath ions are consistent with the continuous leakage of magnetospheric particles across a tangential discontinuity magnetopause, locally distributed according to magnetospheric drift paths. Magnetic reconnection, although present, should not be a dominant source on average, because it is not continuous in time. Fermi acceleration should not be dominant because it predicts the opposite local time asymmetry, and shock drift acceleration should be a minor contributor at E≥50 keV because of upper-energy cutoff limitations. Our observations also indicate a significant magnetospheric contribution to energies as low as ∼10 keV, where the magnetosphere-magnetosheath intensity gradient reverses. However, in order to examine the relative strength and local time distribution of all possible sources at these energies, a detailed analysis is required.

Measurements of hot plasmas in the magnetosphere of Jupiter

Abstract This paper presents an overview of a number of the principal findings regarding the hot plasmas (E ≳ 50 keV) in Jupiters magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies ≳ 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos25 α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of ∼ 10−4μAm−2. The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.

The dawn‐dusk plasma sheet asymmetry of energetic particles: An Interball perspective

We systematically study the energetic particle (∼25 to 850 keV) dawn-dusk plasma sheet asymmetry using both ion and electron spectra composed of 56 energy channels. These spectra, corresponding to an average duration of ∼8 hours, have been provided by the Interball tail probe and cover the extent between 15 and 28 RE away from the Earth. The events are classified in four Y coordinate categories: the “dawnside” with YGSM Y>−13 RE, the “center dusk” with 0 13 RE. The asymmetry is profound between the two extreme dawn-dusk flanks and weaker between the two central plasma sheet regions. In all the energy channels the dusk flank ion fluxes exceed those of electrons. At the dawnside the energetic electron fluxes frequently outnumber those of ions above a bottom energy threshold. During quite times (Kp<2) the ion (electron) fluxes in the duskside (dawnside) plasma sheet exceed by far those in the dawn (dusk) flank. In general, the ion and electron fluxes significantly increase as the Kp index steps upward. However, owing to the dawn-dusk asymmetry, one can observe higher electron fluxes at dawn with low Kp than at dusk with high Kp. On the basis of two case studies, it is shown that ongoing substorm processes and plasma temperature transitions temporarily affect the spatial dawn-dusk asymmetry and the associated spectra. In one case study, via successive spectra it was concluded that the betatron acceleration mechanism is probably the dominant energization process, although the observed spectra did not preserve their spectral shape. This is interpreted as due to the action of energy-dependent large-scale drifts across the tail that strongly modify the initial source spectra. The dawn-dusk asymmetry and all the spectral characteristics from inside the plasma sheet are transmitted out into the magnetosheath, and this may help us (in the future) to discriminate between merging and leakage processes taking place over the magnetopause.

Characteristics of upstream energetic (E≥50 keV) ion events during intense geomagnetic activity

In this work we examine the statistical presence of some important features of upstream energetic (≥50 keV) ion events under some special conditions in the upstream region and the magnetosphere. The 125 ion events considered in the statistic were observed by the IMP 7 and IMP 8 spacecraft, at ∼35 R E from the Earth, during nine long time intervals of a total of 153 hours. The time intervals analyzed were selected under the following restrictions: existence of high proton flux (i.e., ≥900 p cm -2 s -1 sr -1 ) and of a great number of events (an occurrence frequency of ∼10 events per 12 hours in the whole statistics) in the energy range 50-220 keV. The most striking findings are the following: (1) The upstream events were observed during times with high values of the geomagnetic activity index Kp (≥3-); (2) all of the upstream events (100%) have energy spectra extending up to energies E ≥ 290 keV; (3) 86% of these events are accompanied by relativistic (E ≥ 220 keV) electrons; and (4) the majority of the upstream ion events (82%) showed noninverse velocity dispersion during their onset phase (22% of the events showed forward velocity dispersion, and 60% showed no velocity dispersion at all when 5.5-min averaged observations were analyzed). Further statistical analysis of this sample of upstream particle events shows that the 50- to 220-keV proton flux shows a positive correlation with the following parameters: the Kp index of geomagnetic activity and the flux of the high-energy (290-500 keV) protons and (2220 keV) electrons. More specific findings are the following: (1) The spectral index y for a power law distribution of ions detected by the National Oceanic and Atmospheric Administration Energetic Particle Experiment (EPE) instrument (50 ≥ E ≥ 220 keV) and The Johns Hopkins University Applied Physics Laboratory Charged Particle Measurement Experiment (CPME) instrument (290 ≤ E≤ 500 keV) ranges between 2 and 6, with maximum probability between 4 and 5 and (2) the peak-to-background flux ratio of the 290- to 500-keV protons and ≥220-kev electrons increases with the time duration of upstream events. We infer that the vast majority of the upstream ion events considered in this study (under conditions of intense particle activity in the upstream region and enhanced geomagnetic activity within the magnetosphere) can be consistently explained in terms of particle leakage from the magnetosphere.

Observation by Ulysses of hot (∼270 keV) coronal particles at 32° south heliolatitude and 4.6 AU

An unusual event of streaming 60 keV-2 MeV ions (with energy spectrum peaked ∼270 keV) and of 42–315 keV electrons occurred during the passage of a coronal mass ejection (CME) over the Ulysses spacecraft June 9–13, 1993, located at helioradius 4.6 AU and heliolatitude 32° south. The topology of the interplanetary magnetic field (IMF) within the CME has been identified as a helical magnetic flux rope by Gosling et al. [1994]. The ion and electron pitch angle distributions (PADs) had a bidirectional component in the outer (large-pitch) regions of the flux rope, while there were strongly unidirectional (antisunward) beams in the inner (small-pitch) core of the structure, where the electron PADs also displayed a distinctive depletion of electrons moving inward (sunward). Because the core ion beam was narrow, we can associate the observed energy spectrum in the peak direction of the beam (characterized by a Maxwellian with kT = 270 keV) directly with the spectrum injected in the inner heliosphere. The well-defined spatial structure of the event and the absence of any clear signatures of local interplanetary shock acceleration during the period June 9–17 implies that the injection source could have been a long-lived hot coronal ion population. The “weak scattering” electron PAD implies that the other (antisunward) end of the core of the flux rope was magnetically connected, not back to the sun, but rather to the outer heliosphere.

Energetic proton and electron dispersion signatures in the nightside magnetosheath supporting their leakage out of the magnetopause

Energetic proton and electron dispersion structures are detected at both the dawn and dusk nightside magnetosheath near the equatorial plane using the Geotail Energetic Particles and Ion Composition instrument data. A common feature of all the exhibited examples is that the vector magnetic field progressively scans from northward to southward through all the latitude angles while remaining almost within a plane intersecting the nightside magnetopause. For such geometries, at the duskside magnetosheath, peaks in the energetic electron flux precede those in the proton, whereas the opposite occurs at the dawnside magnetosheath. The peak fluxes of energetic protons (electrons) are measured at the duskside (dawnside) magnetosheath whenever the vector magnetic field points toward the central dusk (dawn) low-latitude boundary layer (LLBL) region of the magnetosphere. Therefore we infer that the energetic electron (proton) source is essentially spatially limited to the central dawn (dusk) LLBL. We interpret the observed dispersion structures as spatial phenomena: Energetic electrons (protons) leak out from the equatorial dawn (dusk) LLBL and then are channeled along the draped magnetic field lines over the magnetopause toward spacecraft on the duskside (dawnside). The energetic electrons (protons) are detected in the duskside (dawnside) magnetosheath for a larger range of magnetic field latitude angles. In conclusion, the leakage of energetic particles through the magnetopause seems to be a process of great importance, in parallel with the merging one.

Periodic signals in Ulysses’ energetic particle events upstream and downstream from the Jovian bow shock

Abstract We present results from a statistical analysis of energetic particle events, observed by the Heliosphere instrument for spectra, composition, and anisotrophy at low energies (HI-SCALE) instrument onboard Ulysses, upstream from the bow shock of Jupiter, during the inbound and outbound trajectory of the spacecraft. A harmonic analysis on the intensity time series of 192 distinct 61– 77 keV upstream ion events, suggests a modulation with a period of ∼40 min or ∼15– 20 min in 48% of the total number of events, at the significance level of P=0.05. In some cases, the periodicity is not significant in the ion intensities, while it is significant in the ion anisotropy and/or spectral data. Our analysis shows that the periodic modulation was more frequently observed on the out-of-ecliptic trajectory (67 events), than on the ecliptic trajectory (26 events). An ∼40/15– 20 min periodicity was also found in energetic ion and electron events in the magnetosheath. The results from the statistical analysis of HI-SCALE measurements are consistent with a magnetospheric source for the majority of the upstream events. The statistical results are also consistent with preferential leakage of energetic ions from the south high-latitude Jovian magnetosphere.

Tracing the Magnetic Topology of Coronal Mass Ejection Events by Ulysses/HI-SCALE Energetic Particle Observations In and Out of the Ecliptic

In January 2000, the Ulysses spacecraft observed an ICME event at 43° S heliographic latitude and ~4.1 AU. We use electron (E e >38 keV) observations to trace the topology of the IMF embedded within the ICME. The still controversial issue of whether ICMEs have been detached from the solar corona or are still magnetically anchored to it when they arrive at the spacecraft is tackled. An in ecliptic ICME event is also presented.