SOLAR FLARES AND IGNITION OF CORONAL SHOCK WAVES

B.Vrsnak

Abstract

Conditions for the flare-ignited type II burst producing shock waves are investigated.  It is assumed that an impulsive heating of the source region and consequent expolosive expansion drives a large-amplitude MHD perturbation, propagating perpendicular to the ambient magnetic field, and subsequently steepening into the shock wave. The solutions of  1-D MHD equations for the flaring region and for the external region are matched at the boundary separating them.  The obtained results show under what conditions flares can ignite shock waves that excite the metric type II bursts. Larger and more impulsive flares generate the type II bursts more easely. The heat input rate per unit volume must be sufficiently high relative to the background magnetic field energy density, indicating that initial magnetic field in the flaring region must be highly sheared.  Furthermore, advantageous are the regions of comparatively high values of plasma $\beta$ - lower $\beta$ requires a more powerful and more impulsive flare. Flare taking place in a $\beta <0.01$ environment can hardly produce a type II burst. In general, more impulsive flares enclosed by a lower Alfv\'en velocity regions, generate type II bursts of higher starting frequencies and of shorter time delays.  The results clearly demonstrate why only a small fraction of flares is associated with type II bursts and why the association rate increases with the flare importance.
 
 
 

  Organization: Hvar Observatory
     Telephone: ++385 1 4561 222 (local 318)
           Fax: ++385 1 4828 081
        e-mail: bvrsnak@geodet.geof.hr
       Address: Bojan Vrsnak
   Hvar Observatory
   Faculty of Geodesy
   Kaciceva 26
   HR-10000 Zagreb
   Croatia