Spatial and temporal variations in the observed intensity of solar energetic particles (SEPs) may be indirectly related to the supergranulation of the solar photosphere. Results of a numerical model reveal short-lived (~hours) intensity fall-outs during energetic-particle events associated with impulsive solar flares. These results are consistent with recent ACE/ULEIS observations (e.g. ACE news #25).
Doppler images of the solar photosphere reveal a supergranulation network consisting of individual cells with sizes on the order of tens of thousand kilometers (in the upper left is a portion of a larger image obtained from http://wwwssl.msfc.nasa.gov/ssl/pad/solar/feature1.htm, showing a photospheric convection pattern for June 15, 1996). Cells last for a day or two, and the fluid in each cell moves with a typical speed of ~0.5 km/s.
In 1969 Parker and Jokipii hypothesized that the random fluid motions associated with solar supergranulation may influence the interplanetary magnetic field. Magnetic footpoints anchored in the photosphere execute a random walk and the resulting magnetic variations are carried away by the expanding solar wind (see above right). Ulysses has observed the resulting large-scale magnetic-field fluctuations in the solar wind.
One effect of the long-wavelength magnetic turbulence on the propagation of energetic particles, such as those associated with solar eruptive phenomena, is to produce spatial variations in the observed intensity profiles. This is because the field lines, to which the particles adhere closely, are braided and meander in space on scales much longer than the particle gyroradii. The numerical simulation illustrated in the lower left shows the positions of >0.5 MeV/nuc oxygen ions 4 hours (black dots) and 14 hours (red dots) after their impulsive injection. The ULEIS instrument on ACE has observed similar time structure in the intensity of small SEP events.
J. Giacalone and J. R. Jokipii of the Univ. of Arizona, and J. Mazur of The Aerospace Corp.
Last modified June 28 1999,
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