Ionosphere Transfer Characteristic for Radio Astronomical Signals as Deduced from GPS Radio-sounding, Ionosphere IRI-2001, and Magnetic Field IGRF-10 Modeling

E.L. Afraimovich$^1$, C. Nomikos$^2$, Yu.Ya. Ruzhin$^3$, P.V. Tatarinov$^1$, Yu.V. Yasukevich$^1$

$^1$Institute of Solar-Terrestrial Physics Siberian Branch of the Russian Academy of Sciences, Lermontov St. 126, Irkutsk 664033, Russia
$^2$TEI of Athens, Athens, Greece
$^3$IZMIRAN, Troitsk, Moscow region, Russia

Continuous improvements of VHF radio telescopes have been made due to growing scientific and applied importance to radio astronomy. VHF signals are widely used for observations of the Sun and pulsars. Registration of solar radio emission intensity at fixed frequencies and in spectral VHF band is very important along with other methods of monitoring of coronal mass ejections. In order to extend the dimension of observable solar radio corona they endeavor to use maximum permissible low frequencies of VHF band. Nowadays huge low-frequency radio astronomical arrays (LOFAR, 30-240 MHz; MIRA, 80-300 MHz) are being constructed to record pulsar radiation at maximum possible distance. At the interpretation of the ground based radio telescopes data it is necessary take into account the possible distortions of radio astronomical signals at the Earth ionosphere. However in contrast to modern radar and navigation systems (GPS, GLONASS, GALILEO), where very accurate reconstruction of ionosphere parameters is a built-in function, in present-day radio astronomy a retrieve of ionosphere transfer characteristics has not been appropriately worked out yet. We have developed a method and software for calculation of the ionosphere measure of rotation RM, and the measure of dispersion DM for specific experimental and space weather conditions. We used the ionosphere model IRI-2001, magnetic field model IGRF-10 and values of ionosphere total electron content as deduced from GPS and satellite altimeter measurements. The obtained values of the ionosphere DM and RM were recalculated into characteristics of phase delay, Faraday amplitude modulation, spectrum distortion and polarization changes. We proposed the relevant method of ionosphere corrections permitting to reconstruct the initial parameters of radio astronomical signals (``before ionosphere''). In the paper we made calculations for different levels of geomagnetic and solar activity. On the examples of radio telescopes LOFAR and MIRA we examined dependence on different level of geomagnetic, location of radio telescope and on angular position of radio sources as well. Then we examined case of considerable time shifts of the intensity maxima of solar radio emission were registered during solar flare X38 January 17, 2005, on 6 fixed frequencies of VHF band (142-415 MHz) at 4 stations, spaced at distance not more than 150 km in Greece (Yu. Ya. Ruzhin, C. Nomicos. Geophys. Res. Abst., 2006, 8, 08151). We prove that founded phenomenon is manifested by an amplitude modulation of solar radio emission caused by a rotation of the polarization plane in the ionosphere (Faraday effect). For comparison we used the data of solar spectrograph IZMIRAN and RNST radio spectrographs in San Vito (Jtaly), Learmonth (Australia), ARTEMIS-IV (Greece), WIND WAVES, and GPS monitoring data of ionosphere for the above time period.