Energetic solar particle dropouts detected by Ulysses at 1.63 AU: A possible encounter with the Earth's distant magnetotail

Abstract

Fast solar particles are used to trace the topology of the interplanetary magnetic field during a solar event detected by the heliosphere instrument for spectra composition and anisotropy at low energy (HISCALE) on the Ulysses spacecraft on January 2, 1991. Two sharp‐edged dropouts, lasting for 10 and 25 min, were detected in the fluxes of solar ions from ∼130 keV to >1.8 MeV and halo (heat flux) electrons from 71–461 eV, while simultaneously the flux of high‐energy, 38–315 keV solar electrons and 56–78 keV ions remained constant. The halo electrons and 130 keV to >1.8 MeV solar ions are traveling with similar speeds, ∼4 × 106 to 3 × 107 m/s, much slower than the energetic solar electrons (>108 m/s), and faster than the 56–78 keV ions, suggesting that the dropout field lines were first disconnected from the Sun and then reconnected, with the distance to the reconnection point and time of the reconnection such that >38 keV electrons had already repopulated the dropout field lines. At the time Ulysses was 0.63 AU from the Earth on its way to Jupiter, ∼2° above the ecliptic with a Sun‐Earth‐spacecraft angle of ∼172.8°, approximately where the Earth's magnetotail would be expected to be if it extended to 15,000 Earth radii. We consider the possibility that Ulysses encountered interplanetary field lines connected to the magnetotail.