Location of the radio emitting regions of interplanetary shocks

Abstract

In this study we analyze 20 interplanetary type II radio bursts to determine the location of the type II source region relative to the interplanetary shock. We also report the first determination of a density‐distance relationship (density model) appropriate for interplanetary type II source regions. To determine source location we compare densities in type II source regions, derived from observed type II emission frequencies, to ambient solar wind densities to determine whether source regions are in the ambient, upstream solar wind or in the compressed plasma behind the shock. We compute densities in the ambient solar wind upstream of each shock by using a simple model to extrapolate solar wind plasma densities measured at 1 AU back to the shock front. Densities in the type II source regions are found to be enhanced relative to densities in the ambient solar wind by a factor which is close to the average shock density compression ratio. The simplest interpretation of this result is that the source is located in the compressed plasma within or behind the shock. Although it is possible that the emission is produced in enhanced density regions of the upstream solar wind, we feel that the weight of evidence favors the compressed postshock plasma as the source site.