Coronal Scattering of Radiobursts at Hectometer and Kilometer Wavelengths
Abstract
Using a Monte-Carlo ray-tracing technique, the effects of coronal scattering on hectometer and kilometer wavelengths solar bursts are computed. Between 0.2 and 3 MHz, type III space observations are available and propagation takes place between 6 and 200 solar radii (Be) altitudes. In this range of altitudes, the size 1 and r.m.s. electron density deviation A N of the coronal inhomogeneities can be deduced from the observations of radio source scintillation and angnlar broadening. The radiating point source is located higher than the critical level in the average corona in agreement with most observations; the effects of the overdense structure where type III's are most probably produced are neglected. At 1 MHz, the angular diameter of the scattered image ranges from 0.5 to 2 degrees depending upon the direction of observation. At 0.2 MHz, varies from 0 to 40 depending upon the distance of the source to the observer; for this frequency, the direction of the source has been taken 600 away from the sun-earth line to model Slysh's (1967) observations. At 0.05 MHz and 200 Be altitude, ranges from 10 to 800 when the longitude of the source varies from 25 to 1100. No computation has been carried out for altitudes larger than 1 AU because electron density inhomogeneities are poorly known in these regions. At 3 MHz, the variation of the intensity I (L) of the source with its longitude L is computed for different altitudes (plasma parameter X at the source ranging from 1/16 to 1/4). I (L) is compared to the longitude variation of the number Q (L) of type III's observed at 2.8 MHz, per unit time, on board the RAE-A satellite. It is found that for all sets of parameters (altitude, scattering power of the medium) refraction and scattering cannot reconcile the computations for an isotropic source and the only published observation. The conclusions are: 1) scattering by the known coronal inhomogeneities has to be taken into account in determining the size of the type III source at long wavelengths. 2) If the center-limb effects observed at 2.8 MHz are confirined, it is possible to reconcile the computations with the observations by assuming that the type III radiation is not isotropic when it emerges from the overdense structure where it is produced. Key words: coronal scattering - km-wave bursts - `un-wave bursts - size - center-limb effects - directivity
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 1972
- Bibcode:
- 1972A&A....18..382S