April 2, 1998

ACE News #12: An Upstream Magnetospheric Event Observed by EPAM


Well over 150 magnetospheric upstream events were observed by the EPAM instrument while ACE moved from Earth to L1, and these events continue to be observed at L1 on a frequent basis. Such upstream events are observed when the spacecraft crosses field lines connected to the bow shock and are attributed to particle acceleration at the bow shock or leakage of magnetospheric particles. The first major upstream event was observed on day 243, August 31, only 10 hours after EPAM was turned on. The data above are from EPAM's Low-Energy Magnetic Spectrometer (LEMS120) oriented 120 deg from the spacecraft spin axis. A rare-earth magnet in front of this telescope sweeps out electrons with <500 keV. Ions from 0.047 to 4.75 MeV are measured in eight separate energy channels and eight sectors equally spaced in angle.

One hour's worth of ion observations from two sectors of the lowest energy channel are shown above. The rapid onset and decay make it a likely candidate for an upstream event. One interesting feature is the 2.5 minute difference in arrival time between ions of different pitch (i.e. different sectors). Timimg differences can be seen throughout the event, including a similar 2.5 minute delay at ~3:40.

The diagram illustrates how pitch angle distributions from EPAM are calculated. Each sector of each detector has a look-direction vector. The angle between the look-direction vector and the magnetic field vector is the calculated pitch angle, obtained to 22.5 deg accuracy. The last figure shows an ion pitch angle distribution for one 12-second interval, made possible by the recent merging of ACE magnetic field data and EPAM data. The Y axis is normalized ion flux while the X axis is the cosine of the pitch angle alpha. During quasi-equilibrium, we expect the pitch angle distribution at a given angle from the field to be independent of azimuth about the field. However, in this event there is a large difference in intensity at the same pitch angle but different azimuth, causing the loop seen in the figure. Non-gyrotropic loops in the pitch angle distributions usually indicate a strong gradient anisotropy.

....contributed by Dennis Haggerty and Robert Gold of JHU/APL with special thanks to Charles Smith, Bartol Research Inst./Univ. of Delaware and Tom Armstrong and Gene Holland of Kansas

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Last modified 2 April 1998, Rachael Kubly
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