Structures of the fluctuation precursor in an active explosive ionospheric experiment
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Structures of the Fluctuation Precursor in an Active Explosive Ionospheric Experiment I. Kh. Kovaleva Institute of Geosphere Dynamics, Russian Academy of Sciences, Leninskiœ pr. 38-1, Moscow, 119334 Russia Received June 19, 2006; in final form, September 18, 2006
Abstract—Results are presented from processing the measurement data on low-frequency fluctuations from the North Star active explosive ionospheric experiment. The fluctuation precursor signal was processed by the wavelet analysis method. The structures revealed are identified as ion acoustic envelope solitons. PACS numbers: 52.35.Sb, 94.20.Tt DOI: 10.1134/S1063780X07050054
1. INTRODUCTION In studying turbulent boundary layers in a magnetic field, attempts to reveal individual wave modes have grown in recent years into investigations of the general regularities of boundary fluctuations. Data from different experiments are usually analyzed by the same approaches. The similarity of plasma boundary phenomena in various experimental devices was analyzed by Carreras et al. [1]. Although the geophysical conditions in the ionospheric plasma are rather different (this concerns primarily the magnetic field strength), the boundary phenomena occurring during the development of plasma clouds have the same features: the generation of a turbulent layer, the formation of the fluctuation spectrum, and so on. This is why, in the present paper, the methods used to study laboratory plasmas are applied to analyze fluctuations at the front of the expanding plasma cloud produced in the ionosphere during the North Star active explosive experiment [2–4]. In this experiment, as in other analogous experiments, a plasma cloud was created explosively in the ionosphere and two spatially separated detection modules, as they passed through the boundary of the expanding plasma, recorded three components of the low-frequency (LF) fluctuations. The primary interest is in the data from measurements at the boundary of the plasma cloud. The analysis of such experiments is complicated by the following factors: the orientation of the detection modules is somewhat uncertain and unstable, and the recording time is so short that only a certain instant in the development of the plasma cloud is captured and the measurements cannot be repeated or carried out over the entire plasma volume. In all high-altitude experiments on the explosive production of plasma clouds, LF fluctuations are recorded long before the cloud expands to the observation points. The closer the boundary of the expanding
plasma to the observation point, the larger the fluctuation amplitude, but the character of the fluctuations, as well as their frequency band, remains approximately unchanged up to the arrival of the plasma boundary, which forms a magnetic cavity. This conclusion can be drawn by comparing the data from measurements of the wave electric field with those from simultaneous measurements of other parameters of the medium, such as the magnetic field, the quasi-steady electric field, the energy flux, etc. The c
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