EVIDENCE FOR GENTLY SLOPING PLASMA DENSITY PROFILES IN THE DEEP CORONA: TYPE III OBSERVATIONS

Type III radio bursts are produced near the local electron plasma frequency fp and near its harmonic 2fp by fast electrons ejected from the solar active regions and moving through the corona and solar wind. The coronal bursts have dynamic spectra with frequency rapidly falling with time, the typical duration being about 1-3 s. In the present paper, 37 well-defined coronal type III radio bursts (25-450 MHz) are analyzed. The results obtained substantiate an earlier statement that the dependence of the central frequency of the emission on time can be fitted to a power-law model, f(t) ∝ (t – t 0)–α, where α can be as low as 1. In the case of negligible plasma acceleration and conical flow, it means that the electron number density within about 1 solar radius above the photosphere will decrease as r –2, like in the solar wind. For the data set chosen, the index α varies in the range from 0.2 to 7 or bigger, with mean and median values of 1.2 and 0.5, respectively. A surprisingly large fraction of events, 84%, have α ≤ 1.2. These results provide strong evidence that in the type III source regions the electron number density scales as n(r) ∝ (r – r 0)–β, with minimum, mean, and median β = 2α of 0.4, 2.4, and 1.0, respectively. Hence, the typical density profiles are more gently sloping than those given by existing empirical coronal models. Several events are found with a wind-like dependence of burst frequency on time. Smaller power-law indices could result from the effects of non-conical geometry of the plasma flow tubes, deceleration of coronal plasma, and/or the curvature of the magnetic field lines. The last effect is shown to be too weak to explain such low power-law indices. A strong tendency is found for bursts from the same group to have similar power-law indices, thereby favoring the hypothesis that they are usually produced by the same source region.