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ACE News #96 - April 4, 2006
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Solar energetic particle events showing huge (>104) enrichments of the rare isotope 3He were discovered in the 1970s, with later work showing that these events were associated with impulsive electron events and type III radio emission. Some were associated with x-ray events on the Sun, but many have no obvious flare association, presumably due to their small size. The ULEIS instrument has observed many such events, which often occur in series or during periods of nearly continuous presence of 3He in the interplanetary medium. Wang, Pick, and Mason (ApJ, 639, p 495, 2006) recently investigated the origin of 25 3He-rich SEP events observed with ULEIS. The figures show the typical approach used for the case of a multi-day period in Oct. 2002 when energetic 3He was observed at 1 AU (top left). SOHO EUV images (top center) revealed a flaring active region in the western hemisphere. The white light images often show jetlike ejections around the particle injection times (center, right column). Potential field source surface (PFSS) calculations (top right) showed that the source region had magnetic field lines that were connected to the ecliptic. The source region itself (right, circled) is generally an active region, with a nearby coronal hole (whitish area, right, red arrow).
The events themselves originate from regions where strong fields of opposite polarity are in close contact, as seen in the black/white Fe I λ8688 magnetograms. These regions appear to be similar to white-light jets studied by Wang & Sheeley. We interpret the jets as signatures of magnetic reconnection (footpoint exchange) between closed and open field lines. 3He enrichments would be expected whenever Earth-connected field lines undergo footpoint exchanges with nearby source regions. Because small bipoles continually emerge inside coronal holes, 3He enrichments can occur even when there is no significant flaring activity.
Submitted by Glenn Mason, JHU/Applied Physics Laboratory. Please direct questions or comments to
Last modified 4 Apr 2006, by