Multiple type II solar radio bursts

We report on the detailed analysis of a set of 38 multiple type 11 radio bursts observed by Culgoora radio spectrograph from January 1997 to July 2003. These events were selected on the basis of the following criteria: (i) more than one type 11 were reported within 30min interval, (ii) both fundamental and harmonic were identified for each of them. The X-ray flares and CMEs associated with these events are identified using GOES, Yohkoh SXT, SOHO/EIT, and SOHO/LASCO data. From the analysis of these events, the following physical characteristics are obtained: (i) In many cases, two type IIs with fundamental and harmonic were reported, and the time interval between the two type IIs is within 15 min; (ii) The mean values of starting frequency, drift rate, and shock speed of the first type 11 are significantly higher than those of the second type 11; (iii) More than 90% of the events are associated with both X-ray flares and CMEs; (iv) Nearly 75% of the flares are stronger than M1 X-ray class and 50% of CMEs have their widths larger than 200 degrees or they are halo CMEs; (v) While most of the first type IIs started within the fare impulsive phase, 22 Out of 38 second type IIs started after the fare impulsive phase. Weak correlations are found between the starting and ending frequencies of these type 11 events. On the other hand, there was no correlation between two shock speeds between the first and the second type II. Since most of the events are associated with both the flares and CMEs, and there are no events which are only associated with multiple impulsive flares or multiple mass ejections, We Suggest that the flares and CMEs (front or flank) both be sources of multiple type Its. Other possibilities on the origin of multiple type IIs are also discussed.