Turbulent dynamics inside the cavity of hot flow anomalies

Kovács, P. and Facskó, Gábor István and Dandouras, I. (2014) Turbulent dynamics inside the cavity of hot flow anomalies. PLANETARY AND SPACE SCIENCE, 92 (1). pp. 24-33. ISSN 0032-0633

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Abstract

In this paper the turbulent dynamics of a hot flow anomaly (HFA) event is investigated. The HFAs are transient plasma disturbances generated by the interaction of the bow shock (BS) and a tangential discontinuity (TD) embedded in the solar wind. The typical changes of the plasma parameters inside HFAs (increased plasma temperature, low bulk velocity, increased magnetic fluctuations, etc.) have been thoroughly interpreted by the records of space missions (e.g. Cluster). It is shown that the level of the turbulent intermittency inside the HFA cavity can be monitored in terms of space and time by the fourth statistical moment of the temporal differences of the time-series, i.e. by their flatness. With the multi-spacecraft observations the intermittency in the plasma fluctuations can be revealed not only in temporal but also in spatial scales. However, in the analysis, it must be taken into account that the dynamics of the foreshock region and the HFAs is governed not only by turbulent fluctuations but also by regular wave phenomena. In many cases the wave activities are more energetic than the turbulent processes, therefore the periodic signal components considerably modify the power-law behaviour of the turbulent spectra and determine the probability density functions and structure functions of the magnetic records exhibiting turbulent intermittency. On the other hand, while the wave phenomena are tight to certain time-scales, the turbulent character appears in a wide range of temporal scales. For this reason, it is argued that with the use of a dynamical high-pass filtering, the wave-like and turbulent-like components of the HFA magnetic signal can be discriminated. In our work the high-pass filtering is carried out with the use of continuous wavelet transformation. It is shown that the high-frequency components of HFA magnetic fluctuations exhibit strong intermittency referring to turbulent dynamics. It is also suggested that in the low-frequency regime, the turbulent dynamics is hidden by the wave activities.

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