Origin of low proton-to-electron temperature ratio in the Earth's plasma sheet

We study the proton-to-electron temperature ratio (T-p/T-e) in the plasma sheet (PS) of the Earth's magnetotail using 5 years of Cluster observations (2001-2005). The PS intervals are searched within a region defined with -19 < X <= -7 R-E and vertical bar Y vertical bar < 15 R-E (GSM) under the condition vertical bar B-X vertical bar <= 10 nT. One hundred sixty PS crossings are identified. We find an average value of < T-p/T-e> similar to 6.0. However, in many PS intervals T-p/T-e varies over a wide range from a few units to several tens of units. In 86 PS intervals the T-p/T-e decreases below 3.5. Generally, the decreases of T-p/T-e are due to some increase of T-e while T-p either decreases or remains unchanged. In the majority of these intervals the T-p/T-e drops are observed during magnetotail dipolarizations. A superposed epoch analysis applied to these events shows that the minimum value of T-p/T-e is observed after the dipolarization onset during the "turbulent phase" of dipolarization, when a number of transient B-Z pulses are reduced, but the value of B-Z is still large and an intensification of wave activity is observed. The T-p/T-e drops, and associated increases of Te often coincide either with bursts of broadband electrostatic emissions, which may include electron cyclotron harmonics, or with broadband electromagnetic emission in a frequency range from proton plasma frequency (f(pp)) up to the electron gyrofrequency (f(ce)). These findings show that the wave activity developing in the current sheet after dipolarization onset may play a role in the additional electron heating and the associated T-p/T-e decrease.