Moreton wave and its source of disturbances in the X12/3B WLF of AR6659 in 1991 June 4

Yunnan Observatory, Chinese Academy of Sciences, Kunming, 650011, PR China National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, PR China

Received: 17 November 1998
Accepted: 14 March 2001

Abstract

The Moreton wave that accompanied the X12/3B June 4 1991 white light flare (WLF) is analyzed. The wave, with a wavelength of km, propagated along the solar limb on the chromosphere, and the wavefronts were accelerated during the propagation: their velocity increased from 2500 km s^-1 to 4000 km s^-1. The analyses show that the wave originated in the layers of the atmosphere between the photosphere and the upper chromosphere, and it is suggested that both a magneto-hydrodynamic (MHD) disturbance and a gas-dynamic disturbance are coupled to induce the wave. The MHD disturbance is caused by a rapid variation of the magnetic field, which merges and alternatively emerges. A gas-dynamic disturbance is produced by a strong downward compression of the deep chromosphere and a spray upward ejection occurring simultaneously in the disturbance source region. In the disturbance source, the increase in the magnetic pressure is responsible for the wave acceleration. The photospheric (longitudinal) magnetic field of the source emerges continually during the wavefront propagation, and the corresponding magnetic pressure also increases. The wavefronts that are carried by the plasma around the disturbance source are accelerated, since the transverse pressure compresses the plasma. The observed results do not support the hypothesis that the Moreton wave is the "sweeping skirt" of the shock wavefront, as proposed by Uchida ([CITE]).