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Ann. Geophys., 21, 1367-1375, 2003
www.ann-geophys.net/21/1367/2003/
© European Geosciences Union 2003
Delay in solar energetic particle onsets at high heliographic latitudes
S. Dalla1, A. Balogh1, S. Krucker2, A. Posner3, R. Müller-Mellin3, J. D. Anglin4, M. Y. Hofer5, R. G. Marsden5, T. R. Sanderson5, B. Heber6, M. Zhang7, and R. B. McKibben8
1Blackett Laboratory, Imperial College, London, UK
2Space Sciences Laboratory, University of California, Berkeley, USA
3University of Kiel, Germany
4National Research Council of Canada, Ottawa, Canada
5Research and Scientific Support Dept. of ESA, ESTEC, The Netherlands
6University of Osnabrück, Germany
7Florida Institute of Technology, USA
8University of New Hampshire, USA
Abstract. Ulysses observations have
shown that solar energetic particles (SEPs) can easily reach high heliographic
latitudes. To obtain information on the release and propagation of SEPs prior
to their arrival at Ulysses, we analyse the onsets of nine large high-latitude
particle events. We measure the onset times in several energy channels, and
plot them versus inverse particle speed. This allows us to derive an
experimental path length and time of release from the solar atmosphere. We
repeat the procedure for near-Earth observations by Wind and SOHO. We find that
the derived path lengths at Ulysses are 1.06 to 2.45 times the length of a
Parker spiral magnetic field line connecting the spacecraft to the Sun. The
time of particle release from the Sun is between 100 and 350 min later than the
release time derived from in-ecliptic measurements. We find no evidence of
correlation between the delay in release and the inverse of the speed of the
CME associated with the event, or the inverse of the speed of the corresponding
interplanetary shock. The main parameter determining the magnitude of the delay
appears to be the difference in latitude between the flare and the footpoint of
the spacecraft.
Key words. Interplanetary physics
(energetic particles) – Solar physics, astrophysics and astronomy (energetic
particles, flares and mass ejections)
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