The active plasma sheet: definition of 'events' and statistical analysis
Résumé
A statistical analysis of the plasma sheet activity is performed from CLUSTER observations (years 2001 to 2004). Different types of 'events' are defined by using the plasma flow velocity (V-events), the low frequency magnetic fluctuations (B-events), and the spectral density of higher frequency waves (HF-events). They are selected by an automatic procedure from 2 criteria: a lower threshold for the fluctuations and a minimal duration for each events. The V-events correspond to the usual 'BBF'. The three types of 'events' form an homogenous set, their number (20 to 50 for each Cluster 'tail' season, depending on the selection criteria) and their total duration (5-10% of the time spent by CLUSTER in the sheet) being comparable. 'Events' of different types are positively correlated with percentages of common detection reaching 50%. They are also organized in bunches that characterize local active states in the plasma sheet. However, these active states do not present a one-to-one relationship with substorms or auroral activations. Analysing how the number of 'events' varies with the selection criteria, it is concluded that the B-events saturate at 2-4 nT and have a rather long duration (more than 1-2 minutes) when HF-events are more likely bursty and intense since their number significantly increases for duration smaller than 1 minute. In average, B-events and HF-events begin before V-events. We cannot conclude on a cause-to-effect relationship between 'events', nevertheless, the study shows that the three types of 'events' are likely related to the same basic physical phenomena. They could be fundamental elements of the plasma sheet turbulence.
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