On the Velocity of Beam Generating Fast-Drift Radio Bursts in Solar Corona

Abstract

It is shown that the data on drift velocities of III type radio bursts and the velocities of II-type HB-burst components (of “herring-bone” structure) are not contradictory to the views of free electron-beam expansion in the solar corona if the lowspeed Coulomb electron scattering is taken account of. The findings of the kinetic theory suggest that the characteristic average drift velocity is largely dependent upon the height of beam initiation and, consequently, upon the plasma density at the point of its origin. This, to a large extent, results in the difference between the velocities of the beams generated by chromospheric bursts and the beams being produced at shock wave fronts in the solar corona. The present paper is dedicated to the hitherto poorly explored problem of typical velocities of solar radio bursts generated by solar corona beams. Type III radio bursts [1,2] are found (in the vicinity of local plasma or double plasma frequencies) over the frequency range varying from hundreds of megahertz (chromospheric heights, lower corona) to tens of kilohertz. They differ essentially from the kindred HB-bursts [1] that are constituents of II-type bursts (the so-called “herring-bone” structure) both in the overlapped range of a separate element on a dynamic spectrum [3] and in the frequency drift velocity [4]. Since the drift velocity is proportional to the velocity of a radiation source it is apparent that electronic beams generating these radio bursts are bound to have different typical velocities. The typical rates of fluxes / f D df d = t * Originally published in Radiophysics and Electronics, Vol. 9, Spesial Issue, 2004, pp. 158–165. ISSN 0040-2508 © 2005 Begell House, Inc. 555 1 “Herring-bones” are commonly used in English-language publications V.M. KONTOROVICH AND A.YU. NIKITIN generating III-type bursts both at decametric (DM) [5-8] and metric waves [9] are equal to 0.07 0.3 c − , although in earlier papers [2,10,11] a reference was made to large values of 0.3 0.5 c − for all ranges under observation. The velocities of electrons accelerated on shock waves and generating the “herringbone” structure have the values of 0. (see references in [4]) 02 0.17 c − . Thus, the frequency drift velocity of III-type radio bursts is approximately twice the corresponding velocities of HB-bursts. FIGURE 1. Distribution of energy release of ( ) N E , peak intensity of ( ) N P and duration of ( ) N T bursts in R (roentgen) [12]. The distributions of ( ) N E are closely approximated by power functions: ( ) ( ) 1.4 1.8 , N E E N P P − − ∞ ∞ The present paper addresses the issue on the intrinsic velocity of radiation sources (RS) in terms of the straightforward model of free beam expansion out of the burst region. The velocity distribution of bursts (as initially described by Smerde and Wilde [2]) consist in a relatively smooth power-law decrease towards high (subrelativistic) velocities and sharp kink on the side of low beam velocities. The variation in power law at high speeds correlate with the powerseries distribution of bursts in intensity, for instance, in hardness of X-rays attendant upon them (see Fig.1 [12]) and accounts for the burst properties, like a particle acceleration efficiency. Yet the representative beam velocity is, to a large extent, determined by a sharp kink on the side of low beam velocities. In earlier works [2,10,11] such a kink was observed at min 0.2 v c = , which resulted in an average speed of 0.3c ≈ . Hereafter showed a decrease which was presumably due to the enhanced capabilities of detecting weaker beams and making measurements at low frequencies [6]. Specifically, those ones that are min v 2 Note that for the indicated lower boundary the velocities approach the thermal ones and, in general, here the Landau damping may have a strong impact