On the Deceleration of CMEs in the Corona and Interplanetary Medium from Radio and White-Light Observations

It is generally acknowledged that CMEs, especially the faster ones, must deceleration somewhere between the high corona and interplanetary medium. However, the detailed characteristics and spatial region over which this deceleration occurs is still largely unknown. Simultaneous radio and white-light observations can provide information on the speed profiles of the CME or CME/shock in the high corona and/or interplanetary medium. These observations are consistent with constant deceleration for most CMEs. From the theoretical or modelling point of view, it is usually assumed that the deceleration of CMEs is due to a drag term, which is usually taken to be proportional to the relative speed of the CME compared to that of the solar wind or to this relative speed squared. We point out that such a form for the drag force on the CME inevitably leads to speed profiles that are inconsistent with both the radio and white-light observations.