Evidence for wind-like regions, acceleration of shocks in the deep corona, and relevance of 1/f dynamic spectra to coronal type II bursts

Type II radio bursts are produced near the local plasma frequency f(p) and 2f(p) near by shocks moving through the corona and solar wind. In the present Letter eight well-defined coronal type II radio bursts (30 - 300 MHz) are analyzed. Three results are presented. First, it is found that the dependence of the central frequency on time can be fitted to a power-law model, f proportional to (t - t(0))(-alpha), with 0.6 <= alpha <= 1.3. Assuming a constant shock velocity, these results provide evidence that the density profile n(e)(r) in the type II source regions closely resembles the solar ( r) wind, with n(e)(r) proportional to r(-2). One possible interpretation is that the solar wind starts within a few solar radii of the photosphere, most probably within 1 solar radius. Another relies on a gasdynamic Whitham analysis and demonstrates a possibility for blast shocks to accelerate, thereby reducing apparent power-law indices to solar wind like values. Second, for the events considered it is found that radio burst emission in the form of 1/f versus t dynamic spectra closely follows straight lines. In future this will allow much more objective identification of type II bursts in solar radio data and plausibly real-time correlation with coronagraph and other solar radar. Third, it is demonstrated that 1/f versus t dynamic spectra can provide direct evidence for acceleration of the shock deep in the corona, thus complementing coronagraph studies.