Spectral Analysis of the Electron Density Fluctuations Measured by the ICI-2 Sounding Rocket

Abstract

The Investigation of Cusp Irregularities (ICI) series of sounding rockets are aimed to study plasma instabilities and turbulence associated with cusp flow channels and F-region electron density patches. The ICI-2 sounding rocket was launched into the cusp ionosphere in December 2008 from Ny-Ålesund, Svalbard, and intersected several poleward moving auroral forms. The unprecedented high sampling rate of 5.7 kHz of absolute electron density made by the multi-needle Langmuir probes (m-NLP) allowed measurements of plasma irregularity structures down to meter scales. The rocket intersected instability regions in relation to both particle precipitation and density enhancements. In this thesis the spectral characteristics of electron density fluctuations associated with the encountered plasma structures are analyzed. We present the power laws of the irregularity spectra observed in different plasma regimes during the flight. The power spectra of the strong density fluctuations exhibit a characteristic dual-slope power law behavior with a spectral knee at frequencies in the range f=[20,55] Hz, corresponding to wavelengths l=[25,60] m. The spectral indices are p1=-1.8 below approximately 30 Hz and p2=-4 at frequencies above40 Hz. In the literature, very little evidence of this two-component spectrum exists for the high latitude F-region. Our results are discussed in the context of ionospheric turbulence and compared with the previous studies of high latitude and equatorial regions. The results strongly suggest the gradient-drift instability as being the dominant mechanism for the generation of the observed irregularities.