Articles | Volume 21, issue 6
Ann. Geophys., 21, 1367–1375, 2003

Special issue: Ulysses and Beyond

Ann. Geophys., 21, 1367–1375, 2003

  30 Jun 2003

30 Jun 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 S. Dalla et al.
  • 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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