Differences between onset times of bursty bulk flows (BBFs) of two Cluster satellites in the magnetotail

Abstract

This paper, using the dataset of BBFs (bursty bulk flows) observed by two Cluster satellites C1 and C4, studies the difference between onset times of BBFs observed by C1 and C4. It is found that the onset time differences of most of BBFs observed by C1 and C4 are smaller than 60 s. The average onset time difference of BBFs of C1 and C4 is 68.5 s. The probabilities of onset time difference of BBFs of C1 and C4 larger than 30, 60, 90 and 120 s are respectively 55%, 35%, 27% and 23%. The largest onset time difference of BBFs of C1 and C4 decreases with the increase of earthward component of maximum velocities of BBFs. The onset time difference of BBFs of C1 and C4 results from the velocity inhomogeneity inside the flow channel of BBF, which may be produced in propagation path and/or in source region of BBFs. Such a wide range of onset time difference of BBFs suggests that the velocity inhomogeneity inside the flow channel of BBF is various. These results are very important to the current study of substorm research based on THEMIS data because they indicate that it is impossible to determine the onset time of BBF with a single satellite.