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 +80^o 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:

ecliptic phase 2 years from Earth to Jupiter explores large heliodistances.
long south lat. scan 4 years explores high latitudes from 5 to 2 au
fast lat. scan 1 year at almost the same helioradius (2 au) from south pole to north pole.

The instruments

Magnetometer (VHM/FGM)
Solar Wind Plasma Experiment (SWOOPS)
Solar Wind Ion Composition Instrument (SWICS)
Radio and Plasma Wave Instrument (URAP)
Energetic Particle Instrument (EPAC)
Low--Energy Ion and Electron Experiment (HISCALE)
Cosmic Ray and Solar Particle Instrument (COSPIN)
Solar X--ray and Cosmic Gamma-Ray Burst Instrument (GRB)

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

Nançay Radio Heliograph (NRH)

YOHKOH Soft X--ray Telescope (SXT)

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:

scatter free events at large (4 au) distances
solar origin events at high latitudes
or inside special IM structures such as CIR or CMEs.

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:

e- beam propagation occurs inside special plasma structures called 'propagation channels' characterized by quiet behaviour of the density (top pannel) and magnetic field (second pannel) which is ordered and especially quiet.
the electron flux increase (third top pannel) has a rapid (order of hours) onset and a slow decay
the pitch angle distribution curves (flux vs cos of pitch angle) show that the beam is very well collimated (here field aligned) at event onset and decreases as the event progresses. (each PAD curve corresponds the one red part of the flux curve)
the events are associated to TIII bursts drifting down to low frequencies close to the local plasma frequencies and their corresponding LW.

'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:

the magnetic quietness of the structure is certainly responsible for SF propagation as PA-diffusion is reduced if B is steady.
the observation of thoses structures at large distances shows that they are extremely stable.

Schematisation

On this figure you can see a schematization of the 3 observations made by Ulysses during this SF event:

the plasma structure (characterised by its quiet magnetic field)
the field-aligned e- beam
the low frequency TIII burst associated to LW

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 74^oS 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:

Parker spiral footpoint longitude is 20^oE
NRH identified a flare associated to a TIII burst observed down to 20 kHz by URAP; this flare is only 30^o longitude away from Ulysses footpoint.
the direction of the anisotropy corresponds to solar injected particles.

This event differs from low latitude ones because of

the low flux amplitude (~100X less important)
a long raise time and anisotropy behaviour (supporting a long term injection process)
a delay between flare and particle injection in the IPM (explained by the opening of high latitude field lines by an expanding CME-like structure and the propagation of the particles from low lat. to high lat. newly opened field lines inside solar corona)

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 55^oS 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

2 of them: electron events above 60~keV were observed in temporal association with solar flares in the vicnity of the CME's origin site
3 others: no electrons were observed above 60~keV and no flares were reported in the vicinity of the CME origin site while Ulysses was in this structure

Conclusions:

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
The conservation of beam collimation up to large distances in such structures proves the extreme stability of the propagation channels
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
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
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''