Spatial structure of the ionosphere disturbed volume produced by powerful HF radio waves

To study spatial structure of the ionosphere disturbed volume over the Sura heating facility (Russia) we performed: (1) satellite radio tomography measurements of plasma density perturbations (in August 2002, 2005); (2) in situ plasma density and temperature measurements at altitude h = 710 km by instruments placed on the micro-satellite DEMETER (in April–September 2005, 2006).
For radio tomography measurements three receivers constructed by Polar Geophysical Institute (Russia) were installed beneath the ionosphere disturbed volume in a chain along satellite’s pass: one receiver was placed at the Sura facility, second receiver at distance of about 100 km to the south and third one at distances of about 150 km to the north from the first. The receivers registered radio signals at two coherent frequencies 150 and 400 MHz from Russian navigational satellites flying at about 1000 km. The receiving systems are capable to measure the relative phase between the VHF and UHF waves used for tomographic reconstructions in a height range from ~ 200 km to ~ 600 km and the amplitude of the VHF signal used for scintillation measurements [1].
The micro-satellite DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) was launched in June 2004 for detecting the ionospheric disturbances associated with natural geophysical phenomena such as earthquakes, volcanic eruption or tsunamis, and also to study the electromagnetic disturbances linked with human activity. It is the first in a new series of micro satellites developed by CNES (France) and equipped by different kinds of instruments allowing to measure fundamental plasma parameters (density and temperature of electrons and ions, ion velocity), the energy spectrum of electrons in the 30 keV – 2 MeV range, as well as electric and magnetic field components in different frequency ranges from DC to 3 MHz [2].
The Sura heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia, geographic coordinates, 56.13°N, 44.1°E) was used for ionosphere modification. It consists of three HF broadcast transmitters; each of them generates in the frequency range 4 – 25 MHz the maximum output power of 250 kW in the continuous mode of radiation. When all three transmitters operate in a coherent mode and the antenna array of 300ґ300 m2 is used for pump wave radiation in a frequency range 4.3 – 9.5 MHz, the Sura facility has the effective radiated power (ERP) of 80 MW at 4.3 MHz, which increases with pump wave frequency growth up to 280 MW at 9.3 MHz. The antenna beam can be steered in the geomagnetic meridian plane within a range of ± 40° off the vertical.

1. Satellite tomography measurements of HF-induced plasma density variations

Basing on the results obtained in satellite radio tomography measurements it has been stated that [1]:
1. Under night conditions HF heating considerably affects the ionosphere resulting noticeable changes in the electron density distribution practically throughout the ionosphere body up to heights of about of 600 km, which are much higher than the F layer peak altitudes ~ 300 km).
2. Artificial large-scale plasma density irregularities occupy a large region in a horizontal direction, which is of about of 300 km being much wider than the ionosphere area illuminated by the main lobe of the heating antenna.
3. The strongest depletion with a negative plasma density variation DNe/Ne0 » 20% was observed inside the illuminated area, which is related to the magnetic zenith effect [3].
4. In the nighttime ionosphere under quiet magnetic conditions the generation of quasi-periodical variations in the electron density with characteristic scales of 20 – 30 km along the satellite trajectory (in north-south direction) was revealed in a much wider region than the ionosphere illuminated area.

2. HF-induced variations of plasma parameters measured by the DEMETER satellite

Basing on DEMETER measurements, it should be noted that experiments carried out under daytime conditions have not shown any sensible variations of the studied turbulence parameters. In ionosphere modification experiments carried out during evening hours (T = 22:00 – 22:40 LT) when pump wave frequency was only slightly below of the F2-region critical frequency (f0F2), HF-induced plasma turbulence was usually well detected by satellite instruments. It should be mentioned that the fulfillment of both these conditions is necessary to maintain ionosphere turbulence spread into the topside ionosphere. Experimental data obtained by the measurements allow concluding that [4]:
1. On the average, relative fluctuations for Te, Ne, and Ni have close magnitudes of about 10%; relative fluctuations for Ti are smaller than the level of their background natural fluctuations and thus they are at least 3 to 5 times smaller than those of Te.
2. The spatial scales for electron temperature and density variations have magnitudes of about of 30 km.
3. The variations of plasma parameters are registered in a region of about of 300 – 400 km to the north from the closest point of satellite orbits to the center of HF-disturbed magnetic tube. It is apparent that the dimension of this region along satellite trajectories is much larger than the size of the center part of the ionosphere disturbed volume near a pump wave reflection level (of about 100 km), which is determined by the main lobe of the Sura antenna beam and inside of which the strongest plasma turbulence is excited.


It can be seen that as a whole there is a good agreement for variations of the electron density and their special scales in N-S direction observed in both SURA-DEMETER experiments and tomography measurements.

References
[1] Tereshchenko E.D., Khudukon B.Z., Gurevich A.V., Zybin K.P., Frolov V.L., Myasnikov E.N., Muravieva N.V., Carlson H.C. Radio tomography and scintillation studies of ionospheric electron density modification caused by a powerful HF-wave and magnetic zenith effect at mid-latitudes. // Physics Letters A, 2004. 325, pp.381-388.
[2] First results of the DEMETER micro-satellite. // Planetary and Space Science, 2006 V.54(5) (Special issue).
[3] Gurevich A.V., Zybin K.P., Carlson H.C. Magnetic zenith effect. // Radiophys. and Quant. Electron., (2005). V.48(9), pp.772-787. (Engl. Transl.).
[4] Frolov V.L., Rapoport V.O., Komrakov G.P., Parrot M., and Rauch J.L. Preliminary results of the SURA-DEMETER experiment carried out in 2005. // Advances in Space Research, 2006 (submitted for publication).