M. Y. Hofer

Characterization of SEP events at high heliographic latitudes

Between February 2000 and May 2002, the Ulysses spacecraft made the first ever measurements of solar energetic particles (SEPs) at high heliographic latitudes. Nine large gradual SEP events were detected at latitudes greater than 45°, their signatures being clearest at high particle energies, i.e. protons >30 MeV and electrons >0.1 MeV. In this paper we measure the onset times of Ulysses high latitude events in several energy channels, and plot them versus inverse particle speed. We repeat the procedure for near Earth observations by Wind and SOHO. Velocity dispersion is observed in all the events near Earth and in most of them at Ulysses. The plots of onset times versus inverse speed allow to derive an experimental path length and time of release from the solar atmosphere. We find that the derived path lengths at Ulysses are longer than the length of a Parker spiral magnetic field line connecting it to the Sun, by a factor between 1.2–2.7. The time of particle release from the Sun is typically between 10...

Composition Measurements over the Solar Poles Close to Solar Maximum ‐ Ulysses COSPIN/LET Observations

We present energetic particle composition measurements acquired on board the Ulysses spacecraft in 2000 and 2001 during its recent south and north polar passages. In an earlier study using data only from the south polar pass (Hofer et al., 2002), we found that the high‐latitude composition data reflected the generally high level of solar activity present during that period. The observed particle populations during the south and north polar passages comprised predominantly of solar energetic particles (SEP) accelerated in association with CMEs, rather than particles related to SIR or CIRs. In this work, we compare the energetic particle composition signatures in the two helio‐hemispheres, and find that the latest data from the north polar pass show the same transient‐dominated signature as in the south.

Energetic particle composition measurements at high heliographic latitudes around the solar activity maximum

From 1999 until the end of 2000 the Ulysses spacecraft was traveling back towards the southern solar polar region from about 5 AU to less than 2.5 AU. In this time period the solar activity was close to the maximum. The results of a preliminary analysis of energetic particle composition data covering the energy range 4–20 MeV/n recorded by the COSPIN/LET instrument are shown. We attempt to use the composition signatures to identify the sources of energetic particles observed at high latitudes.