Abstract
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)
Highlights
The passages of the Ulysses spacecraft over the poles of the Sun in 2000 and 2001, during solar maximum conditions, provided measurements of solar energetic particles (SEPs) at high heliographic latitudes
Our analysis has shown that the release of particles to high latitudes is consistently delayed with respect to in-ecliptic release
It would seem natural to interpret this delay within the coronal mass ejection (CME) shock acceleration paradigm, as the time needed for the shock to reach the high latitude-magnetic field lines
Summary
The passages of the Ulysses spacecraft over the poles of the Sun in 2000 and 2001, during solar maximum conditions, provided measurements of solar energetic particles (SEPs) at high heliographic latitudes. In their analysis of the Bastille event, Zhang et al (2003) showed that the CME shock did not extend to high latitudes in this event They explained the large delays in particle onsets at Ulysses and the large apparent path length travelled by the particles as due to significant cross-field diffusion. Onset time analysis of Wind data has shown that the first electrons arriving at a near-Earth spacecraft have typically travelled a distance equal to the length of a Parker spiral magnetic field line connecting the spacecraft to the Sun, i.e. approximately 1.2 AU (Krucker et al, 1999) Four of the events took place while the spacecraft was immersed in fast solar wind (over the North pole of the Sun), three in slow solar wind and two in intermediate speed wind (around 600 km/s)
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