This paper describes the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the RBSP spacecraft from an instrumentation and engineering point of view. There are four magnetic spectrometers aboard each of the two spacecraft, one low-energy unit (20-240 keV), two medium-energy units (80-1200 keV), and a high-energy unit (800-4800 keV). The high unit also contains a proton telescope (55 keV-20 MeV).The magnetic spectrometers focus electrons within a selected energy pass band upon a focal plane of several silicon detectors where pulse-height analysis is used to determine if the energy of the incident electron is appropriate for the electron momentum selected by the magnet. Thus each event is a two-parameter analysis, an approach leading to a greatly reduced background.The physics of these instruments are described in detail followed by the engineering implementation. The data outputs are described, and examples of the calibration results and early flight data presented.

We have measured the 3He abundance from approximately 0.5 to 2 MeV nucleon-1 in 12 large solar energetic particle (SEP) events during the period 1997 November-1999 June. In five of the events, the 3He time-intensity profile is similar to the 4He time-intensity profile, indicating a common acceleration and transport origin for the two species. The average 3He/4He ratio during these events is &parl0;1.9+/-0.2&parr0;x10-3, a factor of approximately 5 enhancement over the solar wind value. During this same survey period, we have also measured the low-energy ion intensities during quieter periods in between the large-particle events. We find 3He and Fe remnants from impulsive events present on a majority of the days, implying that they fill a substantial volume (>50%) of the in-ecliptic interplanetary medium during our survey. We suggest that these suprathermal ions may therefore be a source population that is available for further acceleration by interplanetary shocks that accompany large SEP events, thereby leading to the 3He enhancements in a significant fraction of large SEP events. This impulsive SEP event material might also account for recent observations of large solar particle events with energetic particle ionization states that have a wide range of ionization states that encompass values expected for both gradual and impulsive solar SEP events.

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We have surveyed the 0.1-10 MeV nucleon À1 elemental abundances at 72 interplanetary (IP) shocks observed by the Ultra-Low-Energy Isotope Spectrometer on board the Advanced Composition Explorer from 1997 October through 2002 September. We find the following: (1) The C/O ratios in IP shocks were substantially depleted (by more than $40%) relative to solar wind values. (2) The IP shock abundances were poorly correlated with those measured in the slow and fast solar wind. (3) Energetic ions above $0.1 MeV nucleon À1 from impulsive and gradual solar energetic particle events (SEPs) were present upstream of all the IP shocks in our survey. (4) The $1 MeV nucleon À1 Fe/O ratio in IP shocks was positively correlated with that measured upstream of the shocks. (5) The IP shock abundances were well correlated with the upstream abundances, with a negative dependence on mass/charge. (6) The mean Fe/O ratio in IP shocks exhibited a positive correlation with the level of solar activity, as measured by the occurrence rates of X-ray flares and sunspots. The above results are inconsistent with shock acceleration of ions originating mainly from the bulk solar wind or a suprathermal tail composed predominantly of solar wind ions. Instead, it appears that for the events surveyed here, the IP shocks accelerated a seed population predominantly comprising ions that were previously accelerated in impulsive and gradual SEPs and that the shock acceleration process accelerated higher rigidity ions less efficiently than lower rigidity ions.

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