The structure of solar radio noise storms

Abstract

Context. The Nançay Radioheliograph (NRH) routinely produces snapshot images of the full sun (field of view ~3 R⊙) at 6 or 10 frequencies between 150 and 450 MHz, with typical resolution 3 arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun at 236 or 327 MHz, with the same field as the NRH, a resolution as low as 20 arcsec, and a time cadence 2 s. Aims. We seek to investigate the structure of noise storms (the most common non-thermal solar radio emission) which is yet poorly known. We focus on the relation of position and altitude of noise storms with the observing frequency and on the lower limit of their sizes. Methods. We use an improved version of a previously used method for combining NRH and GMRT visibilities to get high-resolution composite images and to investigate the fine structure of noise storms. We also use the NRH data over several consecutive days around the common observation days to derive the altitude of storms at different frequencies. Results. We present results for noise storms on four days. Noise storms consist of an extended halo and of one or several compact cores with relative intensity changing over a few seconds. We found that core sizes can be almost stable over one hour, with a minimum in the range 31–35 arcsec (less than previously reported). The heliocentric distances of noise storms are ~1.20 and 1.35 R⊙ at 432 and 150 MHz, respectively. Regions where storms originate are thus much denser than the ambient corona and their vertical extent is found to be less than expected from hydrostatic equilibrium. Conclusions. The smallest observed sizes impose upper limits on broadening effects due to scattering on density inhomogeneities in the low and medium corona and constrain the level of density turbulence in the solar corona. It is possible that scatter broadening has been overestimated in the past, and that the observed sizes cannot only be attributed to scattering. The vertical structure of the noise storms is difficult to reconcile with the classical columnar model.