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
Last modified 2 April 1998, Rachael Kubly
Return to ACE homepage.