Statistics – Applications
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996esasp.392..225k&link_type=abstract
Environment Modelling for Space-based Applications, Symposium Proceedings (ESA SP-392). ESTEC Noordwijk, 18-20 September 1996. E
Statistics
Applications
Scientific paper
An enhanced intensity of HF fields at frequencies higher than FoF2 - critical frequency of the F2 layer have been registered in top-side ionosphere since an advent of satellites with the on board radio receivers. The main components were identified as broadcasting and communication transmissions, thunderstorm emissions and industrial and urban noises. Strong, structureless enhancements of HF fields in the whole frequency band from lowest instrument frequency of the order of 100kHz, up to well defined broadcasting transmissions have been registered on the Intercosmos-19, ACTIVE, and APEX satellites. The geographical coverage of these enhancements resemble geographical distribution of the most intense broadcasting signals and depends on geomagnetic activity. On many occasions the local plasma frequency fp was determined from top-side sounder or impedance data thus revealing that enhancements occurred in whistler band below local gyro frequency fc, in thermal magnetoplasma band (fc, fp) and in the electromagnetic band (fp, FoF2). The frequency band (fc, FoF2) is screened by F2 layer from direct transmissions from the ground so the observed phenomenon have been originally related to the modification of the top-side ionosphere by the ground based transmitters. Such assumption have been bolstered by the observation that amplitudes of broadcasting signals were within the limits of linear response of the receivers. Recent numerical simulations of peculiarities in impedance and wave spectra suggest that strong voltages at frequencies outside the instruments measurement bands may be responsible for them due to modification of instruments input characteristics. The measurements in frequency band .1-30MHz performed on CORONAS satellite provide valuable data to evaluate the HF contribution. On the other hand from the European ionosphere overflights with high spatial resolution there are examples of structured spectra down to 2fc which point out the existence of the narrow spatial sheets of either reduced F2 layer density or enhanced HF intensity resulting in inter-modulation. The data from low rate 16 hours long telemetry sessions can be used to prepare rough but 'instantaneous' maps of selected parameters. The data from high rate 2 hours or 30 min long telemetry sessions can be used for analysis which is more detailed but unfortunately more confined in space. The aims of the paper are to compare maps of HF intensity averaged in selected frequency bands with maps of intensity measured in selected bands below 1kHz and to analyze respective data registered with high rate telemetry. Lack of control of preamplifiers characteristics in strong external fields, insufficient spatial resolution of electromagnetic spectra, thermal plasma parameters and supra thermal fluxes at various levels of geomagnetic activity call forth dedicated measurements in order to evaluate threat of ground based transmitters for satellite systems.
Kiraga A.
Klos Z.
Rothkaehl Hanna
Zbyszynski Z.
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