S. Dalla

Properties of high heliolatitude solar energetic particle events and constraints on models of acceleration and propagation

[1]xa0We analyse 9 large solar energetic particle (SEP) events detected by the Ulysses spacecraft at high heliolatitudes during the recent solar maximum polar passes. Properties of time intensity profiles from the Ulysses/COSPIN instrument are compared with those measured by SOHO/COSTEP and Wind/3DP near Earth. We find that onset times and times to maximum at high latitude are delayed compared to in-ecliptic values. We show that the parameter which best orders these characteristics of time profiles is the difference in latitude between the associated flare and the spacecraft. We find that the presence of a shock is not necessary for the establishing of near equal intensities at Ulysses and in the ecliptic during the decay phase. The model of SEP acceleration by coronal mass ejection driven shocks does not appear to account for our observations, which would more easily be explained by particle diffusion across the interplanetary magnetic field.

Propagation of energetic particles in the high‐latitude high‐speed solar wind

[1]xa0We present observations of energetic particles in the energy range ∼1 MeV to ∼100 MeV made by the COSPIN instrument on board the Ulysses spacecraft during the recent second northern polar pass. For a short time during this pass the Ulysses spacecraft was at high heliographic latitude, above the current sheet, and immersed in high-speed solar-wind flow coming from the northern polar coronal hole. Four large solar energetic particle events were observed. For the first two or three days of each event, the spacecraft was in a homogeneous region devoid of any structures. We discuss the rise to maximum of these events. We examine the onset time and the anisotropy of the energetic particles. We find that during these events the particle angular distributions were almost isotropic, but with a net outward flow along the magnetic field lines. We conclude that particles reached these high latitudes traveling along the magnetic field lines. We do not find any evidence for cross field diffusion at Ulysses.

Further indications of a ∼140 day recurrence in energetic particle fluxes at 1 and 5 AU from the Sun

A possible ∼140 day recurrence in the fluxes and anisotropies of interplanetary MeV energy protons has previously been identified in data from the Anisotropy Telescopes (ATs) on board Ulysses. This measurement was made during 1998–1999 at a distance from the Sun of ∼5 AU. Earlier reports of a 154-day periodicity in the occurrence rate of solar flares and other solar activity indicators exist, mainly for cycle 21 data. In this paper we compare the ATs measurement, made for protons in the 1.3–2.2 MeV energy range, with fluxes of relativistic electrons (∼4–10 MeV) and protons in the range ∼30–100 MeV. We consider 5-AU data from the Ulysses Kiel Electron Telescope (KET) instrument and 1-AU measurements from the IMP 8 Cosmic Ray Nuclear Composition (CRNC) telescope. We find that at these higher energies significant fluxes at 5 AU are detected only in correspondence with a specific feature in the ATs recurrent pattern, thus supporting the hypothesis of recurrent behavior. We look for signatures of these events at 1 AU in the CRNC data and find for all of them an associated long-duration particle event, regardless of the longitudinal separation between Ulysses and IMP 8. Possible solar sources of the events are discussed.

Observation of decay phases of solar energetic particle events at 1 and 5 AU from the Sun

[1]xa0The slow decay phase of gradual solar energetic particle (SEP) events has been interpreted as an indication of particle reservoirs being established in the inner heliosphere. The same phenomenon is sometimes termed spectral invariance and explained in terms of a magnetic bottle effect, whereby a barrier impedes particle escape. In alternative to the above picture, decay-phase SEPs have been ascribed to (1) continuous acceleration at an interplanetary shock front, (2) interplanetary scattering, or (3) leakage from the solar atmosphere over several days. In this paper we investigate two large gradual SEP events characterized by comparable signatures at 1 AU from the Sun. We use measurements at 1 AU made by the ACE and IMP8 spacecraft, and at 5.2 AU by the Ulysses spacecraft. At 5.2 AU, the ∼MeV proton intensities during the decay phase of the two events are found to have strikingly different profiles, showing in one case a long-duration smooth decay and in the other a depletion in particle intensity. We discuss how the four above mentioned models on the origin of decay-phase SEPs would interpret the observations.

Recurrence in MeV proton fluxes and anisotropies at 5 AU from the Sun

We report the observation of a recurrent pattern in proton fluxes and anisotropies at 5.2 AU from the Sun, made by the Anisotropy Telescopes on board the Ulysses spacecraft between April 1998 and May 1999, during the rising phase of solar cycle 23. The time delay between recurrent features of the pattern is not constant and increases from 122 to 155 days over the three recurrences observed. We discuss a possible connection between these observations and the well documented existence of a 154-day periodicity in the rate of occurrence of solar flares.

Characterization of SEP events at high heliographic latitudes

Between February 2000 and May 2002, the Ulysses spacecraft made the first ever measurements of solar energetic particles (SEPs) at high heliographic latitudes. Nine large gradual SEP events were detected at latitudes greater than 45°, their signatures being clearest at high particle energies, i.e. protons >30 MeV and electrons >0.1 MeV. In this paper we measure the onset times of Ulysses high latitude events in several energy channels, and plot them versus inverse particle speed. We repeat the procedure for near Earth observations by Wind and SOHO. Velocity dispersion is observed in all the events near Earth and in most of them at Ulysses. The plots of onset times versus inverse speed allow to derive an experimental path length and time of release from the solar atmosphere. We find that the derived path lengths at Ulysses are longer than the length of a Parker spiral magnetic field line connecting it to the Sun, by a factor between 1.2–2.7. The time of particle release from the Sun is typically between 10...

On the energetic particle fluxes at 5 AU associated with the April/May 1998 solar activity: Ulysses observations

Located close to the ecliptic at 5.4 AU heliocentric distance, Ulysses observed energetic particle increases in April/May 1998 that were related to the second major episode of activity in the new solar cycle, the first being in November, 1997. In this paper, we report on the characteristics of the transient particle enhancements seen at Ulysses, including the compositional signatures, and suggest an interpretation in terms of the activity occurring at the Sun.