The proton temperature anisotropy associated with bursty bulk flows in the magnetotail

Abstract

We study the development of the proton temperature anisotropy T ⊥/T∥ in bursty bulk flows (BBFs), as observed by THEMIS Mission. For a set of 10 selected events, during which at least three spacecraft are aligned in the same flow, we can sample the plasma parameters along the Earth's magnetotail. The temperature anisotropy in the quiescent tail is negligible. However, as soon as the BBF passes over the spacecraft a strong anisotropy is measured. We analyze T⊥/T∥ as a function of parallel plasma beta-β∥(=nkT∥/ (B2/2μ0)) for the different THEMIS satellites and compare the spread of the data points with various instability thresholds over ion scales that can reduce the temperature anisotropy: for T⊥/ T∥<1 the parallel and oblique firehose; for T ⊥/T∥>1 the proton cyclotron and mirror mode. It is shown that the anisotropy reduces whilst the BBF is moving Earthward, and the strongest fluctuations are enhanced along the instability thresholds, indicating that these instabilities reduce the proton temperature anisotropy. Key Points As the BBFs is moving Earthward the associated anisotropy reduces The fluctuations are enhanced along the instability thresholds ©2013. American Geophysical Union. All Rights Reserved.