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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A09107, doi:10.1029/2007JA012414, 2007

Energetic particles during the first and third Ulysses southern high-latitude excursions: Probing global corotating interaction region structure beyond 5 AU

D. Lario

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA

E. C. Roelof

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA

Abstract

We analyze energetic particle observations during the first and third ascents of Ulysses to the southern regions of the heliosphere. The first high-latitude excursion occurred in the declining phase of solar cycle 22, while the third occurred in a later phase of solar cycle 23. In contrast to the long-lived and well-defined ∼26-day recurrent intensity enhancements observed throughout the first southern excursion, observations during the third southern excursion showed a more variable structure. Transient events of solar origin were more abundant and intense. Corotating interaction regions (CIRs) were clearly present in intermittent sequences lasting only a few solar rotations. Within these sequences the relationships between solar wind structures and energetic particle events were remarkably similar to those observed in the steady recurrences of the first southern excursion. Energetic particles associated with well-formed CIRs bounded by strong forward and reverse shock pairs were not observed until heliographic latitudes Λ 30°S (versus Λ 13°S in the first orbit). When Ulysses finally emerged into the fast (700 km s⁻¹) solar wind at Λ ≥ 39°S, particle events were variable and either associated with weak reverse shocks or without solar wind signatures indicative of local CIRs (just as in solar cycle 22). The time-intensity profiles in individual CIRs matched up well between the two southern excursions implying that essentially the same CIR structure existed in both solar cycles, albeit at a higher latitude in solar cycle 23 and only in the midst of the intermittent recurrent sequences.

Received 22 March 2007; accepted 26 June 2007; published 22 September 2007.

Keywords: corotating interaction regions; energetic particles; heliosphere.

Index Terms: 2114 Interplanetary Physics: Energetic particles (7514); 2102 Interplanetary Physics: Corotating streams; 2162 Interplanetary Physics: Solar cycle variations (7536); 2139 Interplanetary Physics: Interplanetary shocks.

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Citation: Lario, D., and E. C. Roelof (2007), Energetic particles during the first and third Ulysses southern high-latitude excursions: Probing global corotating interaction region structure beyond 5 AU, J. Geophys. Res., 112, A09107, doi:10.1029/2007JA012414.

Copyright 2007 by the American Geophysical Union.