INTERPLANETARY SHOCKS LACKING TYPE II RADIO BURSTS

Abstract

We report on the radio-emission characteristics of 222 interplanetary(IP) shocks detected by spacecraft at Sun-Earth L1 during solar cycle 23(1996 to 2006, inclusive). A surprisingly large fraction of the IPshocks (~34%) was radio quiet (RQ; i.e., the shocks lacked type II radiobursts). We examined the properties of coronal mass ejections (CMEs) andsoft X-ray flares associated with such RQ shocks and compared them withthose of the radio-loud (RL) shocks. The CMEs associated with the RQshocks were generally slow (average speed ~535 km s-1)and only ~40% of the CMEs were halos. The corresponding numbers for CMEsassociated with RL shocks were 1237 km s-1 and 72%,respectively. Thus, the CME kinetic energy seems to be the decidingfactor in the radio-emission properties of shocks. The lower kineticenergy of CMEs associated with RQ shocks is also suggested by the lowerpeak soft X-ray flux of the associated flares (C3.4 versus M4.7 for RLshocks). CMEs associated with RQ CMEs were generally accelerating withinthe coronagraph field of view (average acceleration ~+6.8 ms-2), while those associated with RL shocks weredecelerating (average acceleration ~-3.5 m s-2). Thissuggests that many of the RQ shocks formed at large distances from theSun, typically beyond 10 Rs, consistent with the absence of metric anddecameter—hectometric (DH) type II radio bursts. A small fractionof RL shocks had type II radio emission solely in the kilometric (km)wavelength domain. Interestingly, the kinematics of the CMEs associatedwith the km type II bursts is similar to those of RQ shocks, except thatthe former are slightly more energetic. Comparison of the shock Machnumbers at 1 AU shows that the RQ shocks are mostly subcritical,suggesting that they were not efficient in accelerating electrons. TheMach number values also indicate that most of these arequasi-perpendicular shocks. The radio-quietness is predominant in therise phase and decreases through the maximum and declining phases ofsolar cycle 23. About 18% of the IP shocks do not have discernibleejecta behind them. These shocks are due to CMEs moving at large anglesfrom the Sun-Earth line and hence are not blast waves. The solar sourcesof the shock-driving CMEs follow the sunspot butterfly diagram,consistent with the higher-energy requirement for driving shocks.