Acceleration of runaway electrons and Joule heating in solar flares

Journal Article · Sat Jun 15 04:00:00 EDT 1985 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/163263· OSTI ID:5315572

Holman, G D

The electric-field acceleration of electrons out of a thermal plasma and the simultaneous Joule heating of the plasma are studied. Acceleration and heating time scales are derived and compared, and upper limits are obtained on the acceleration volume and the rate at which electrons can be accelerated. These upper limits, determined by the maximum magnetic-field strength observed in flaring regions, place stringent restrictions on the acceleration process. The implications of these results for the microwave and hard X-ray emission from solar flares are examined. The major conclusions are: (1) The simple electric-field acceleration of electrons is found, in agreement with Spicer, to be incapable of producing a large enough electron flux to explain the bulk of the observed hard X-ray emission from solar flares as nonthermal bremsstrahlung. For the bulk of the X-ray emission to be nonthermal, at least 10/sup 4/ oppositely directed current channels are required, or an acceleration mechanism that does not result in a net current in the acceleration region is required. (2) lf the bulk of the X-ray emission is thermal, a single current sheet can yield the required heating and acceleration time scales and the required electron energies for the microwave emission. This is accomplished with an electric field that is much smaller than the Dreicer field (E/sub D//Eroughly-equal10--50). (3) The rise time of the nonthermal emission is determined by the time needed to generate the required number of runaway electrons rather than by the time needed to accelerate the electrons to the required energies, which is generally a much shorter time scale. (4) The acceleration of enough electrons to produce a microwave flare requires the resupply of electrons to both the current sheet and the runaway region of velocity space.

• Research Organization: Laboratory for Astronomy and Solar Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland
• OSTI ID: 5315572
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 293:2; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640104* -- Astrophysics & Cosmology-- Solar Phenomena
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ACCELERATION
CURRENTS
ELECTRIC CURRENTS
ELECTRIC FIELDS
ELECTRIC HEATING
ELECTROMAGNETIC RADIATION
ELECTRONS
ELEMENTARY PARTICLES
EMISSION
FERMIONS
HARD X RADIATION
HEATING
IONIZING RADIATIONS
JOULE HEATING
LEPTONS
MAGNETIC FIELDS
MATHEMATICAL MODELS
MICROWAVE RADIATION
PLASMA
PLASMA ACCELERATION
PLASMA HEATING
RADIATIONS
RESISTANCE HEATING
RUNAWAY ELECTRONS
SOLAR ACTIVITY
SOLAR FLARES
STAR MODELS
X RADIATION