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:
- 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 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:
- Parker spiral footpoint longitude is 20oE
- NRH identified a flare associated to a TIII burst observed down to
20 kHz by URAP; this flare is only 30o 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 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
- 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''