The Ulysses fast latitude scans: COSPIN/KET results

Abstract

Abstract. Ulysses, launched in October 1990, began its second out-of-ecliptic orbit in December 1997, and its second fast latitude scan in September 2000. In contrast to the first fast latitude scan in 1994/1995, during the second fast latitude scan solar activity was close to maximum. The solar magnetic field reversed its polarity around July 2000. While the first latitude scan mainly gave a snapshot of the spatial distribution of galactic cosmic rays, the second one is dominated by temporal variations. Solar particle increases are observed at all heliographic latitudes, including events that produce >250 MeV protons and 50 MeV electrons. Using observations from the University of Chicago's instrument on board IMP8 at Earth, we find that most solar particle events are observed at both high and low latitudes, indicating either acceleration of these particles over a broad latitude range or an efficient latitudinal transport. The latter is supported by "quiet time" variations in the MeV electron background, if interpreted as Jovian electrons. No latitudinal gradient was found for >106 MeV galactic cosmic ray protons, during the solar maximum fast latitude scan. The electron to proton ratio remains constant and has practically the same value as in the previous solar maximum. Both results indicate that drift is of minor importance. It was expected that, with the reversal of the solar magnetic field and in the declining phase of the solar cycle, this ratio should increase. This was, however, not observed, probably because the transition to the new magnetic cycle was not completely terminated within the heliosphere, as indicated by the Ulysses magnetic field and solar wind measurements. We argue that the new A<0-solar magnetic modulation epoch will establish itself once both polar coronal holes have developed.

Latitudinal and radial variation of &gt;2 GeV/n protons and alpha-particles at solar maximum: ULYSSES COSPIN/KET and neutron monitor network observations

“…Thus, drifts, which would cause negative latitudinal gradients, were not fully "operational", leading to positive latitudinal gradients. This is in agreement with the constancy of the e/p-ratio at 2.5 GV as measured by Ulysses (Heber et al, 2003). In order to analyze and interpret the observations, further measurements are needed.…”

“…(2) to the data, more data at low latitudes are needed to determine the radial gradient with better precision. Due to the reconstruction of the heliospheric magnetic field towards a well-ordered structure, the galactic cosmic ray distribution is expected to change (Heber et al, 2003), which might not be expressed by the modulation depth, so that Eq. (2) is not applicable.…”

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

“…The greater variability in the ratios observed is most likely due in part to the difficulty of removing all contamination from solar energetic particles, and in part to the more dynamic nature of the interplanetary medium, such that the cosmic ray intensity at one spacecraft may be affected by a transient disturbance which never reaches the other. Nevertheless, it is apparent that at solar maximum the latitudinal gradients in intensity are smaller than we are able to measure (see also Heber et al, 2003). …”

“…1 were dominated by the transient increases produced by energetic solar flares and CME events. At the same time, the background intensity of cosmic rays and anomalous components had decreased by factors ranging from a few percent at GeV energies to as much as a factor of 10 or more at low energies, as a result of the increased solar modulation characteristic of solar maximum (see also Heber et al, 2003).…”

“…Studies of Jovian electron propagation have been presented elsewhere (Heber et al, 2002) and studies of local acceleration of low energy particles at high latitudes have not yet progressed beyond a basic exploratory phase. Heber et al (2003) discuss specifically and in more detail the observations from the COSPIN KET during the recent fast latitude scan.…”