ACE News #34
July 21, 1999

A Cool Ion Beam Observed Upstream of an Interplanetary Shock

Shock waves observed at the ACE spacecraft are driven by large, explosive ejections of solar mass from the Sun, known as coronal mass ejections. The ACE solar wind electron, proton, and alpha monitor (SWEPAM) from Los Alamos National Laboratory observed about 35 shock waves in 1998. Of these, a shock wave on April 7, 1998, offered the rare opportunity to observe a relatively cool proton beam in the upstream region of the shock. These beams are hard to detect primarily because the shocks move quickly past the spacecraft. The figure above shows the measured phase space density as a function of velocity in the rest frame of the solar wind. The velocities are parallel and perpendicular to the ambient magnetic field. The main solar wind is the high density region located at low velocities and a proton beam can be identifed in the upstream region of the shock at high speeds. The beam is the enhancement in phase space density denoted as the field-aligned or gyrating ions and is most likely produced by reflection off the shock front. Also depicted in the figure are diffuse ions presumably scattered out of the field-aligned beam. This beam has a relatively low temperature (i.e., narrow distribution in v-parallel and v-perpendicular indicating that it has not been significantly disrupted by the plasma instabilities it generates. Even so, weak instabilities were observed in association with this beam by the ACE magnetometer experiment (MAG). These are the first correlated measurements of a cool ion beam in association with beam-excited magnetic waves upstream of an interplanetary shock. The observations, together with theoretical results, give strong fresh support to the theory that ion acceleration at interplanetary shocks begins with the upstream propagation of a relatively cool proton beam, probably originating from acceleration of the thermal background ions.

Contributed by Robert L. Tokar of the Los Alamos National Laboratory.

See The SWEPAM Home Page for more information on ACE SWEPAM.

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Last modified July 21 1999, Andrew Davis
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