An Observational Revisit of Stationary Type IV Solar Radio Bursts

Abstract

Stationary type IV solar radio bursts (IVSs) are broadband continuum emission observed at decimetric–decametric wavelength without apparent source motions. They are closely associated with solar flares and/or coronal mass ejections. Earlier studies on IVSs suffered from limited number of events, frequency coverage and available channels, and spatial resolution. Here we present an analysis on 34 IVSs using two-dimensional imaging data provided by Nançay Radioheliograh (NRH) at 10 frequencies from 150 to 445 MHz. The events are recorded from 2010 to 2014. We focus on general properties including the spatial dispersion of sources with frequency, brightness temperature ($T_{\mathrm{B}}$) and corresponding spectra, and polarization. Main findings are: (i) In the majority of events (23/34) regular and systematic source dispersion with frequency can be clearly recognized. (ii) In most (31/34) events the maximum brightness temperature ($T^{\mathrm{E}}_{\mathrm{BM}}$) exceeds $10^{8}~\text{K}$, and exceeds $10^{9}~\text{K}$ in 23 events. The histogram distribution of $T^{f}_{\mathrm{BM}}$, i.e. the maximum brightness temperature of a source at certain frequency ($f$) of a specific event (referred to as event-$f$ source, there are 247 such sources in total) exhibits a clear declining trend with increasing frequencies. The dominant type of $T_{\mathrm{B}}$ spectra is power-law like with a negative index. (iii) In most events (30/34) the sense of polarization remains unchanged and the number of events with right and left-handed polarization are comparable. In 57% of all 247 event-$f$ sources the level of polarization does not change considerably, in about 39% sources the level of polarization exhibits significant variation yet with a fixed sense, and in only 4% the sense of polarization changes. These results provide strong constraints on radiation mechanism of IVSs.