Solar cycle and latitude dependence of high-beta suprathermal plasma conditions in interplanetary space between 1.3 and 5.4 AU

Abstract

The analysis of energetic particles and magnetic field measurements from the Ulysses spacecraft has shown that in a series of events, the energy density contained in the suprathermal tail particle distribution is comparable to or larger than that of the magnetic field, creating conditions of high-beta plasma. In this work we analyze periods of high-beta suprathermal plasma occurrences (βep>1) in interplanetary space, using the ratio (βep) of the energetic particle (20keV to ∼5MeV) and magnetic field energy densities from measurements covering the entire Ulysses mission lifetime (1990–2009) in order to reveal new or to reconfirm some recently defined interesting characteristics. The main key-results of the work are summarized as follows: (i) we verify that high-beta events are detected within well identified regions corresponding mainly to the vicinity of shock surfaces and magnetic structures, and associated with energetic particle intensity enhancements due to (a) reacceleration at shock-fronts and (b) unusually large magnetic field depressions. (ii) We define three considerable features for the high-beta events, concentrated on the next points: (a) there is an appreciable solar-activity influence on the high-beta events, during the maximum and middle solar-cycle phase, (b) the annual peak magnitude and the number of occurrences of high events are well correlated with the sunspot number, (c) the high-beta suprathermal plasma events present a spatial distribution in heliographic latitudes (HL) up to ∼±80°, and a specific important concentration on the low (−25°⩽HL<−6°, 6°<HL⩽25°) and median (−45°⩽HL<−25°, 25°<HL⩽45°) latitudes. We also reconfirm by a statistical analysis the results of Marhavilas and Sarris (2011), that the high-beta suprathermal plasma (βep>1) events are characterized by a very large parameter βep (up to 1732.5), a great total duration (406days) and a large percentage of the Ulysses-mission lifetime (which is equal to 6.34% of the total duration with usable measurements, and 11.3% of the duration in presence of suprathermal particles events).