Radio tracking of the interplanetary coronal mass ejection driven shock crossed by Ulysses on 10 May 2001

摘要

We report on the detection of type II radio emission which was observed for more than a day prior to the arrival of an interplanetary shock at Ulysses. We use local spectral emission peaks, computerized from time-averaged intensity spectra of the type II burst, to track the associated emitting shock. In the spectrogram plotted as a function of time and inverse frequency, these peaks appear as elongated clusters of data points, organized in fundamental-harmonic bands and delineating drifting emission features. Least squares linear fittings to identified clusters give straight traces with different slopes. Most of these, when extrapolated to the Sun, are found to converge nearly at the same starting time, allowing determination of the average shock speed. Shortly before the shock crossing, intense Langmuir waves were detected at the electron plasma frequency upstream of the shock. This plasma wave enhancement together with the radio emission observed predominantly at the harmonic of the plasma frequency, all point to the occurrence of the type II radio source region in the upstream electron foreshock. We have accurately substracted the thermal noise background from the observed emission intensity to deduce the type II brightness temperatures at the fundamental and harmonic near the shock crossing. The measured brightness temperature of the type II harmonic emission is found to peak at a value of 3 × 1013 K just after the shock crossing; just before, the harmonic brightness temperature is 1012 K and the fundamental brightness temperature 8 × 1011 K.