The radial evolution of earthward BBFs during substorm

Abstract

We here study the occurrence rate, probability function of velocity and duration of earthward bursty bulk flows (BBFs) in the Inner Plasma Sheet (IPS, β>0.5) using the data of Cluster in 2001 and 2002. The occurrence rate of earthward BBFs increases with distance from the Earth up to −19 RE, which is in agreement with the previous observations of the radial evolution of BBFs. About 54% of earthward BBFs in expansion phase have a velocity larger than 600 km/s, whereas only 38% of earthward BBFs in growth and recovery phases have a velocity larger than 600 km/s. The average velocity of earthward BBFs in expansion phase is 732 km/s, larger than those in growth phase (631 km/s) and recovery phase (617 km/s). The durations of earthward BBFs decrease with the decrease of downtail distance from Earth due to the braking of earthward BBFs. The duration of earthward BBFs in expansion phase is larger than that in growth and recovery phases. The average durations in growth, expansion, and recovery phases are respectively 49.3, 71.5, and 47.6 s. Therefore, the ratios of transports of energy of earthward BBFs in growth, expansion, and recovery phases can be estimated to be 0.51:1:0.47. Thus, the earthward BBFs in the expansion phase have the largest capability of the transport of energy and can produce the largest braking effects, such as inertial currents and auroral activities.