Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance
An important component of ionospheric plasma irregularity studies in the Indian low latitudes involves the study of the plasma bubbles which produce intense scintillations of the transionospheric satellite signals. Many such plasma bubble induced (PBI) scintillation events were identified while...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
1997-03-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/15/345/1997/angeo-15-345-1997.pdf |
| Summary: | An important component of ionospheric plasma
irregularity studies in the Indian low latitudes involves the study of the
plasma bubbles which produce intense scintillations of the transionospheric
satellite signals. Many such plasma bubble induced (PBI) scintillation events
were identified while recording 244 MHz signal from the geostationary satellite
Fleetsat (73°E) at Delhi (28.6°N, 77.2°E) during March-April 1991. This type
of scintillations represents changes in plasma processes. These scintillations
are spectrally analyzed using an autoregressive (AR) scheme, which is equivalent
to maximum entropy method of spectrum analysis, amenable to extracting optimum
spectral content from short data lengths (20 – 40 s). Each spectrum is assigned
a level of detectability using the final prediction error (FPE) derived from the
optimum filter order required to resolve the spectrum. Lower detectability
together with a higher order filter indicate a higher level of coherence for the
plasma irregularities (discrete structures). Consistent patterns for these
scintillations emerge from the present analysis as follows: (1) the initial and
final phases of a scintillation patch display quasiperiodic oscillations. Their
corresponding spectra show dominant (Gaussian shaped) spectral features with
detectability levels of –6 dB to –12 dB and requiring a higher order (>6) AR
filter for their spectral resolution. These are most likely associated with
discrete "filament-like" or "sheet-like" plasma structures
that exist near the bubble walls. (2) Two main features of the scintillation
spectra could be positively associated with the well-developed plasma bubble
stage: (a) spectra displaying a power-law process with a single component
spectral slope between 1.6 to 3.0. Generally such spectra are resolved with a
2nd order filter and have a 1 dB to 6 dB of detectability. (b) Spectra
displaying a double slope, indicating an inner and an outer scale regime for the
power-law irregularities. These spectra are resolved with higher order filters
(>3 but <7) and possess detectability levels of –1 dB to 3 dB. These
spectra display finer spectral changes, perhaps indicative of the nature of
continuously evolving plasma irregularities. As an example, an analysis of a
single scintillation patch is presented to highlight the geophysical
significance of the present approach. Some important parameters used in the AR
scheme of spectral analysis are given in the Appendix. |
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| ISSN: | 0992-7689 1432-0576 |