Transport of superthermal electrons in coronal loops and U(N)-type solar radio bursts.

Details Transport of superthermal electrons in coronal loops and U(N)-type solar radio bursts. Transport of superthermal electrons in coronal loops and U(N)-type solar radio bursts.

Oct 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...314..303k&link_type=abstract

Astronomy and Astrophysics, v.314, p.303-311

Astronomy and Astrophysics

Astrophysics

5

Hydrodynamics, Sun: Flares, Radio Radiation, Particle Emission

Scientific paper

Electron beams travelling with about 1/3 of the velocity of light along closed coronal loops can manifest themselves in decimeter and meter wave solar type U or U(N) radio bursts. Using a 1-D test particle model, we study trajectories of superthermal electrons in coronal loops with the aim of understanding recently published detailed radio spectral and imaging data about type U(N) bursts. The computations are carried out in a static semi-circular loop of 1 solar radius length. For modeling transport processes Coulomb collisions, mirroring of electrons in the loop magnetic field, and scattering in zones of enhanced whistler wave turbulence are taken into account. The formation of a finite zone of enhanced whistler turbulence in the loop top is consistently explained by the properties of loss-cone instability of a weak preexisting energetic particle component. In a model run initially electrons are injected upwards along the loop axis in one leg. Scanning the trajectories of electrons through the loop and representing them in space vs time and plasma frequency vs time plots, respectively, we get synthetic radio source distributions and radio spectra. The results can be analyzed in dependence on loop and particle parameters including the strength of whistler turbulence. Thus, we are able to model the essential aspects of observed U(N) bursts. We find that in a zone of sufficiently strong whistler turbulence near the loop top the initial electron beam is splitted up into two beams propagating from the top back and forward into both loop legs. Thus two widely separated radio sources brighten during the descending branch of U burst spectra. Moreover, we find that U(N) type radio bursts can be excited not only due to mirroring but also by scattering of electrons in whistler turbulence near a leg of the loop. For demonstrating the strength of the present model a simulation of an observed U(N) burst (February 23, 1993) is given.

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