Statistical characteristics of CMEs and flares associated with solar type II radio bursts

We have made a comprehensive statistical study of the characteristics of 290 SOHO/LASCO CMEs and their associated GOES-8 X-ray flares (>C1 class) during the period January 1997-December 2000 of solar cycle 23's rise to maximum. These events were classified into mainly three groups (Group noII - not associated with type IIs, Group II - associated with type IIs, Group DH - associated with deca/hectametric type IIs). To understand the physical characteristics associated with the origin of metric type IIs, we have examined the important parameters of CMEs and flares such as the speed and width of CMEs, and the rise time, duration and importance of flares. Major results from the present study are: (i) CMEs with type IIs are relatively bigger in size (mean width = 93degrees) with higher speed (mean = 704 km s(-1)) than those of the CMEs without type IIs (mean width = 54degrees and mean speed = 505 km s(-1)); (ii) flares with type IIs have longer rise time (mean = 16.1 min) and duration (mean = 32.7 min) than the mean values (rise time = 11.8 min; duration = 22.4 min) of flares without type IIs. Also the significant differences between Group noII and Group II have been identified from results of the Student's t-test. It may be emphasized that the impulsiveness of type II flares is found to be higher than that of no type II flares. However, there are exceptional cases: among the 229 CMEs without type IIs, nearly 50% (115/229) of CMEs in Group noII had speeds >400 km s(-1), and nearly 10% (22/229) of CMEs had widths between 100 200 deg. Similarly, in this group, there are more than 10% (30/229) flares of class > M1 and nearly 7% (17/229) of flares with duration > 45 min. These results may imply that a special condition, like 'low Alfven speed', as a favoring environment for producing MHD shocks, is necessary to generate metric type IIs. The CMEs and flares associated with both metric as well as DH type IIs are likely to be the most energetic events. The mean acceleration of flare-associated DH CMEs tends to be positive in contrast to that of all DH CMEs.