A low β coronal loop model

A localized force-free current is proposed as a model for the observed coronal loops.An upper limit for the growth rate of kink instabilities in this model is found by solving numerically, in cylinder symmetry, the MHD equation of motion, with the boundary condition = 0 outside the loop.For various current densities a spectrum of kinks is found. These instabilities will disrupt the loops that are long or strongly twisted, on a time scale of a few seconds.The kinks in short or barely twisted loops are too long to fit.     

Spatially coherent oscillations in microwave bursts

Solar microwave burst observations (made with the WSRT) with high time and high spatial resolution show large-scale (> 8000 km) short-period (1.5 s) modulations of the source. It is argued that an interpretation in terms of Alfvén oscillations in the microwave source is ruled out by this observation. Instead it must be the source of the fast electrons, that produce the microwaves, that is oscillating. The fluctuating acceleration region is identified with a volume where a sheared field is compressed against a flux tube by an unstable current. MHD oscillations in the overlying fluxtube are caused by the pushing force. The rapidly expanding current plays a major role in the flare theory of van Tend and Kuperus (1978).     

Radio,X-ray,and optical observations of the flare of June 13, 1980, at 6h22m UT

A subflare of importance Sf was observed on June 13, 1980 simultaneously by instruments aboard the Solar Maximum Mission (SMM) and various ground based observatories. We describe and compare different kinds of observations, with emphasis on the Hard X-Ray Imaging Spectrometer (HXIS) images and spectra, and on the one-dimensional microwave images with high time and spatial resolution, obtained with the Westerbork Synthesis Radio Telescope (WSRT). The fast electrons causing the X-ray and microwave impulsive bursts had a common acceleration source, but the burst were produced at the opposite footpoints of the loops involved, with microwaves emitted near to a sunspot penumbra. The flare (of a ‘compact’ type) was probably triggered by an emerging flux, and two possible interpretations of this process are briefly discussed.

     

A study of the parameters of individual type-I bursts

The Dwingeloo 60-channel radio spectrograph has observed at metric wavelengths for a long time. Hundreds of type-I bursts in the digitally recorded solar noise storms were studied and the results are presented in this paper. We hoped to learn more about the emission and propagation of the radiation by trying to find patterns in the dynamic spectra of individual bursts or statistical relations between some of the parameters that can be defined to describe the spectra. The bursts are reduced and represented in a standard way. Graphical representations are inspected by eye and compared qualitatively. Numerical burst parameters are studied statistically. We describe the properties of instantaneous burst spectra. The polarization properties of bursts and the relation between burst- and continuum-polarization are studied to some extent. Apart from these results we find no significant new properties, despite the high quality of the data.     

A low-β coronal loop model

The low- coronal loop model of Sillen and Kattenberg (1980) is extended to include a surrounding current-free plasma. We calculated the dispersion curves of kink modes by solving the linearized MHD-equations of motion.We found a strong stabilizing influence on the growth rates of kink instabilities due to the surrounding plasma.In loops that are thick, have small current densities and that have a high density and a low magnetic field strength the growth times for kinks become of the order of days.Presently at Caltech, Pasadena, U.S.A.Presently at the FOM-Institute for Plasma Physics, Rijnhuizen, Nieuwegein, The Netherlands.