Mathematical modeling of disprersive distortions of the wideband chirp signal during its propogation in the ionospheric radiochannel
Theoretical description of the chirp ionosonde operation has been made. Ionosonde registers spectrum sequence of window received-signal sampling. Mathematical modeling of dispersive distortions of spectra registered has been carried out.
On the shape of measured spectra of the ionosphere sounding by an FMCW signal under dispersion case
Abstract A theoretical description is given to the ionosonde operation configuration in the mode of sounding the decametric radio channel by the frequency-modulated continuous wave (FMCW). A preprocessing of the received signal is carried out using the frequency-compression method. It is shown that the result derived from processing an individual time sample is mathematically equivalent to a sounding of the radio channel by a complex-valued narrow-band impulse signal. The group delays of this signal determines maximum in the recorded spectrum. We give the formula for calculating the form of the recorded spectrum at ionospheric soundings by the FMCW signal subject to the dispersion. A formula for calculating the spectral width due to the dispersion is obtained. A comparison is made with experimental data.
Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations
We report on the thermospheric mass density anomaly in the vicinity of the ionospheric cusp. A systematic survey of the anomalies is presented, based on a statistical analysis of 4 years of data (2002–2005) obtained by the accelerometer onboard CHAMP. The anomalies are detected during all years and seasons in both hemispheres but with stronger signatures in the Northern Hemisphere. For the same geophysical conditions, solar flux and geomagnetic activity the anomalies in the north are larger by a factor of about 1.35. Over the course of the survey period the amplitude decreases by more than a factor of 5 while the level of solar flux reduces by a factor of 2. The anomaly strength also depends on the solar wind input. The merging electric field, E merg , is generally enhanced for about an hour before the anomaly detection. There is a quadratic response of the anomaly amplitude to E merg . For geophysical conditions of P10.7 E merg E merg , where the weight of E merg , in mV/m, is by about 50 times higher than that of the solar flux level. The solar zenith angle and the influence of particle precipitation are found to play a minor role as a controlling parameter of seasonal variation. The well-known annual variation of the thermospheric density with a minimum around June also influences the formation of the cusp anomalies. This leads to a clear hemispheric asymmetry with very weak anomalies in the south during June solstice, which is supposed to be a combined effect of the minimum in annual variation and the seasonal decrease of solar insolation in this region.