Ulysses COSPIN observations of cosmic rays and solar energetic particles from the South Pole to the North Pole of the Sun during solar maximum

Abstract

Abstract. In 2000-2001Ulysses passed from the south to the north polar regions of the Sun in the inner heliosphere, providing a snapshot of the latitudinal structure of cosmic ray modulation and solar energetic particle populations during a period near solar maximum. Observations from the COSPIN suite of energetic charged particle telescopes show that latitude variations in the cosmic ray intensity in the inner heliosphere are nearly non-existent near solar maximum, whereas small but clear latitude gradients were observed during the similar phase of Ulysses' orbit near the 1994-95 solar minimum. At proton energies above ∼10 MeV and extending up to >70 MeV, the intensities are often dominated by Solar Energetic Particles (SEPs) accelerated near the Sun in association with intense solar flares and large Coronal Mass Ejections (CMEs). At lower energies the particle intensities are almost constantly enhanced above background, most likely as a result of a mix of SEPs and particles accelerated by interplanetary shocks. Simultaneous high-latitude Ulysses and near-Earth observations show that most events that produce large flux increases near Earth also produce flux increases at Ulysses, even at the highest latitudes attained. Particle anisotropies during particle onsets at Ulysses are typically directed outwards from the Sun, suggesting either acceleration extending to high latitudes or efficient cross-field propagation somewhere inside the orbit of Ulysses. Both cosmic ray and SEP observations are consistent with highly efficient transport of energetic charged particles between the equatorial and polar regions and across the mean interplanetary magnetic fields in the inner heliosphere.

Propagation of Energetic Particles to High Heliographic Latitudes

“…The best correlation was found with difference in latitude between the flare site and the latitude of Ulysses, this correlation being surprisingly better than the correlation with the angular separation between the site and Ulysses. This implies a very effective longitudinal transport of the particles, but a very inefficient transport latitudinally, which the authors concluded meant that cross field diffusion was the fundamental mechanism in getting the particles to high latitudes, in agreement with the suggestion by Zhang et al (2003) for the Bastille Day event. However, they did not rule out the possibility that the delay was due to the time taken for the CME to reach the field lines connected to the spacecraft.…”

“…At high latitudes and in the fast solar wind (70 -80 • N) we found no evidence for any substantial net flow across the field lines, whereas at moderately high latitudes in the slow solar wind (62 • S) Zhang et al (2003) found evidence for cross-field flow.…”

“…In Figure 2 we show summary plots of the events number 2, 3 and 4 of Sanderson et al (2003b). These events have also been studied in detail by Dalla et al (2003aDalla et al ( , 2003b, Lario et al (2004) and McKibben at al. (2003).…”

“…Both the perpendicular and parallel component amplitudes continued to decrease until both were zero at a time half way through the passage of the CME. This could imply that particles were crossing field lines within the CME, as suggested by Zhang et al (2003) although this is probably unlikely, as the field is usually very quiet inside a CME, so scattering across the field lines is not to be expected. A more likely explanation is that inside the previously existing CME there was a substantial un-measurable field aligned component, as at this time the magnetic field in the spacecraft frame of reference was almost perpendicular to the scan plane.…”

Solar Energetic Particles and Space Weather: Science and Applications

“…Comparison of simultaneous SEP observations near the ecliptic plane with the Ulysses observations at high latitudes showed that most events that produce large high-energy (>20 MeV) proton and near-relativistic electron flux increases near Earth also produce flux increases at high latitudes, regardless of the longitudinal, latitudinal and radial separation between the s/c, although with somewhat lower maximum intensities and slower rise at Ulysses (McKibben et al 2003;Lario and Pick 2008;Malandraki et al 2009). Particle anisotropies during SEP events at high latitudes are typically directed outward from the Sun and aligned with the local magnetic field (McKibben et al 2003;Malandraki et al 2009). …”