Kinetic analysis of the energy transport of bursty bulk flows in the plasma sheet

The energy transport of bursty bulk flows (BBFs) is very important to the understanding of substorm energy transport. Previous studies all use the MHD bulk parameters to calculate the energy flux density of BBFs. In this paper, we use the kinetic approach, i.e., ion velocity distribution function, to study the energy transport of an earthward bursty bulk flow observed by Cluster C1 on 30 July 2002. The earthward energy flux density calculated using kinetic approach Q_Kx is obviously larger than that calculated using MHD bulk parameters Q_MHDx. The mean ratio Q_Kx/Q_MHDx in the flow velocity range 200-800 km/s is 2.7, implying that the previous energy transport of BBF estimated using MHD approach is much underestimated. The underestimation results from the deviation of ion velocity distribution from ideal Maxwellian distribution. The energy transport of BBF is mainly provided by ions above 10 keV although their number density N_f is much smaller than the total ion number density N. The ratio Q_Kx/Q_MHDx is basically proportional to the ratio N/N_f. The flow velocity v(E) increases with increasing energy. The ratio N_f/N is perfectly proportional to flow velocity V_x. A double ion component model is proposed to explain the above results. The increase of energy transport capability of BBF is important to understanding substorm energy transport. It is inferred that for a typical substorm, the ratio of the energy transport of BBF to the substorm energy consumption may increase from the previously estimated 5% to 34% or more.