Presentation at CESRA 96 June 04, 96 Nouan-le-Fuzelier

A review of solar particle events during Ulysses mission


Anne Buttighoffer, Monique Pick
Observatoire de Paris--Meudon, URA 2080 CNRS
and Sang Hoang
Observatoire de Paris--Meudon, URA 246 CNRS


The Heliosphere Instrument for Spectra Composition and Anisotropy at low energies (HI-SCALE) aboard Ulysses has made during the 6 years of the mission measurements of 50keV-5 MeV ions and 30-300 keV electrons. These particles are from interplanetary, jovian or solar origin. The fluxes, pitch angle distributions or spectra established from HI-SCALE data have been studied in various particle events during the mission.

In this paper, I will present a review of solar origin particles events observed by HI-SCALE throughout Ulysses mission in the 1 to 5 a.u. in solar radius and -80 to +80o in solar latitude range. The evolution of the characteristics of those events with helio-range and -latitude and various interplanetary medium structures will be presented in attempt to characterize the propagation of solar particles in the heliosphere. The propagation of coronal electron beams will be especially discussed. The radio emissions they are associated to (type~III radio bursts and local LW) could be observed for most of the cases by the URAP experiment aboard Ulysses and their characteristics can therefore be compared to those of the particles beams.


Ulysse Trajectory


The 3 phases of the Ulysses mission:

The instruments


Solar events were jointly analysed with other non--Ulysses instruments such as :

Observations


On this plot of 40-60 keV electron fluxes throughout the mission you can see the 3 phases of the mission: the ecliptic phase, the long south scan and the fast latitude scan.

Notice during the long south scan the regular fluxe increases which have a ~26 days periodicity. Those are caused by CIRs and show the coherent structuration of the heliosphere even at high latitudes or large distances. Solar origin events were observed during the first phase of the mission but quite surprisingly: were also reported. I will present the 3 more typical such cases studied (they are the events pointed out by red arrows on the figure).

Large distance SF event



Scatter-free events have long been observed and studied at small distances away from the Sun. Their main characteristics are the following ones:
'Scatter-free' events were not expected at large distances since IPM should have been homogenised by CIR after 2-3 au therefore distroying the propagation channels. But the event we see here was observed while Ulysses was at 4.3 au away from the Sun showing that SF events and propagation channels are still present at large distances. The explainations could be the following ones:

Schematisation


On this figure you can see a schematization of the 3 observations made by Ulysses during this SF event:
The corresponding high frequency TIII burst and solar flare were observed from Earth close to the computed footpoint of the 9.4 au Parker spiral connecting Ulysses to the Sun. Flare onset time and particle event onset time at Ulysses fit very well with a ~10 au transit distance of the e- beam. The estimated size of the propagation channel on the Sun is of ~6000km.

Highest latitude solar event: 25 Oct. 94


This event is the highest latitude solar event observed to date. Ulysses was situated at 74oS and 2 au away from the Sun. The fluxes of 40-60 keV e- show a slowly increasing flux (~1.5 day between maximum and onset). A small but decernable anisotropy is measured (see the PAD inclution on the figure). Solar origin for this event is attested by the folowing:
This event differs from low latitude ones because of

What is an IM-CME


A CME is by definition a white light feature observed on solar limb by coronographs. Their signature in the IPM is believed to by 'interplanetary transients'. Those transients are plasma structures better identified on density (less fluctuating than outside the CME), solar wind speed (a back current is observed) and the very quiet behaviour of the magnetic field. Occasionaly FS and RS were observed probably associated to CME's expansion in the slower solar wind. An important issue as far as IM-CMEs are concerned has been to know wether they were 'expanding flux ropes' connected to the Sun or 'magnetic clouds' disconnected fron the Sun. Low energy particles can help answer this question as they are tracers of magnetic structure.

Solar event inside CME


Ulysses is at 55oS and 3.5 au away from the Sun. A CME (top pannel of the figure) is observed between associated FS and RS. The second pannel show low energy e- fluxes which slowly increase before the CME passage: this increase is associated to the intense solar event which occured when the CME left solar corona. As Ulysses was badly connected in lat; and longitude to the injection site, the particles reach the S/C in a rather diffusive manner which explains the slow evolution of the fluxes. We also detect a second increase better dicernible at higher energies in e- as well as protons (next 3 pannels). In association to this second increase, URAP observes a TIII burst drifting soutward. The origin of this TIII has been identified thanks to NRH (see the positons reported on the SXT picture) in the region from which the CME was initiated 7 days before. This observation attests for the solar origin of the event showing that the IM-CME is still rooted to the Sun.

IM-CME and Solar events


5 CMEs were identified at low latitudes

Conclusions:


  1. Electron beam propagation seems to be made in well--defined IPM structures, essentially characterised by a quiet magnetic field behavior well isolated from the surrounding plasma
  2. The conservation of beam collimation up to large distances in such structures proves the extreme stability of the propagation channels
  3. IPM Parker type organisation and high latitude solar events means that some coronal propagation must exist as acceleration sites are known to be low--latitude active regions
  4. Some delay between flare and particle injection in the IPM was occasionally observed and could be explained by the time required by the coronal structure's expansion to force the opening of high latitude field lines to the IPM
  5. The observation of solar origin events inside IM-CMEs show that those structures can be rooted to the Sun and not only detached ``magnetic clouds''