SOHO JOP049

THE ROLE OF MHD SHOCKS IN CORONAL PROPAGATION OF SOLAR ENERGETIC PARTICLES

Flares create solar energetic particles (SEP): these can eventually
reach SOHO only if the SEP escape from the Sun at the correct longitude
(50-60 W).  To reach this escape longitude, SEP must propagate from the
flare site through the corona. How does coronal propagation occur?
According to one model, a magnetic bottle expands through the corona,
pushing an MHD shock ahead of it. This model can be tested by combining
SOHO SEP data and photon data.

Prediction:
SEP fluxes at SOHO should be relatively low if coronal
streamers lie on the path between flare site and the escape longitude.

Reason: 
An MHD shock is attenuated if it has to pass through a magnetic
neutral sheet.

Specifics of JOP to test the shock propagation model using SOHO:

(i) identify flare site (EIT); 

(ii) measure fluxes of solar energetic particles (COSTEP, ERNE); 

(iii) use solar wind velocity (CELIAS) to estimate connection longitude
at the Sun;

(iv) use magnetometer data (MDI) to compute magnetic structure in 
corona along the propagation path between flare and the connection point;

(v) check calculations of coronal magnetic field structure using
coronagraph synoptic data;

(vi) use spectroscopic data (SUMER? CDS? UVCS?) to search for Doppler shifts 
of coronal or transition region lines as the shock front (Moreton wave?) 
sweeps past: $v_{Doppler} = v_{shock}$ sin $\theta$  

(vii) how much does the shock weaken as it propagates through
a coronal streamer (H-alpha filament)? 



More detailed justification:

	In most cases, wherever a flare occurs, some coronal
propagation of the SEP is necessary before the SEP arrive at the
longitude which connects to Earth. One of the suggestions for how fast
coronal propagation might occur is a ``magnetic bottle", an expanding
structure which sweeps through the corona, preceded by a MHD shock
front (Schatten and Mullan 1977; Mullan 1980). SEP escape from the
bottle once the bottle opens up (Mullan 1983). If the opening bottle is
in contact with the footpoint of the magnetic field line which connects
to the Earth (this ``connection point" is typically at 50-60 degrees
West longitude), then the SEP can begin their interplanetary motion to
the Earth. If the opening bottle has not reached the connection point,
then some further coronal propagation is necessary: this may include
drifts and diffusion (Mullan and Schatten 1979).

	There are competing ideas for how the fast stage of coronal
propagation occurs. One of these is the ``bird cage" model of Newkirk
and Wentzel. An essential aspect which distinguishes the ``bottle" idea
from the ``bird cage" is the presence of an MHD shock.

	If the spectroscopic instruments on SOHO could detect a coronal shock
(e.g. a Moreton wave?) propagating between flare site and connection
point, this might be regarded as a "smoking gun" in favor of the
"bottle"
.

	However, even if it is impossible to identify a shock directly, there
is a possibility for an indirect proof for MHD shock propagation
through the corona. This proof depends on the fact that an MHD shock is
attenuated if it has to pass through a streamer between the flare and
the connection point. MHD modelling of the attenuation of a shock
passing through a magnetic neutral sheet in the corona was done some
years ago (Steinolfson and Mullan 1980).

	In particular: if a coronal streamer lies between a flare site
and 50-60 degrees West, then SEP will find it hard to reach the connection 
point if the ``bottle" idea is correct. Some evidence in favor of this
idea already exists in older data (Mullan 1981).

	Prediction: if two flares of comparable amplitude occur at comparable
distances from the connection point, the flare which happens to have
one or more coronal streamers lying between its location and the
connection point should be a weaker source of SEP at SOHO than the
flare which has no intervening streamers to pass through.

	How to identify coronal streamers? Quiescent H-alpha filaments trace
the location of helmet streamers, with their overlying streamers.  And
magnetometer data from MDI can be used to calculate the (potential)
coronal magnetic field.

References:

Mullan, D. J. 1980, ``Stochastic acceleration of solar cosmic rays in an 
	expanding coronal magnetic bottle", ApJ 237, 244-254.

Mullan, D. J. 1981, ``Possible evidence for attenuation of an MHD shock
	by a magnetic neutral sheet in the solar corona", 
	Proc. ICRC (Paris) Vol. 3, p. 51-54

Mullan, D. J. 1983, ``Release of solar cosmic rays from the corona:
	Rayleigh-Taylor instability and reconnection", ApJ 269, 765-778

Mullan, D. J. and Schatten, K. H. 1979, ``Motions of solar cosmic rays in the
	coronal magnetic field", Solar Phys. 62, 153-177

Schatten, K. H. and Mullan, D. J. 1977, ``Fast azimuthal transport
	of solar cosmic rays via a coronal magnetic bottle", 
	J. Geophys. Res. 82, 5609-5620  

Steinolfson, R. S. and Mullan, D. J. 1980, `` MHD shock propagation in the
	vicinity of a magnetic neutral sheet", ApJ 241, 1186-1194