V. A. Kornienko

Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important. INDEX TERMS: 2403 Ionosphere: Active experiments; 6929 Radio Science: Ionospheric physics (2409); 7839 Space Plasma Physics: Nonlinear phenomena; KEYWORDS: HF-heating, ionospheric modification, electron heating, EISCAT, striations, aspect angle

Ionospheric HF pump wave triggering of local auroral activation

Experimental results from ionospheric HF pumping experiments in the nightside auroral E region above Tromso are reported. We found intriguing evidence that a modification of the ionosphere-magnetosphere coupling, due to the effects of powerful HF waves beamed into an auroral sporadic E layer, can lead to a local intensification of the auroral activity. Summarizing multi-instrument observations during two consecutive nights, one can distinguish the following peculiarities related to this auroral activation: a modification of the auroral arc and its breakup above Tromso, local changes in the horizontal currents above Tromso, a burst-like increase of the electron density and temperature, a large increase in the ion temperature in a wide altitude range and in the north-south component of the electric field, distinctive features in dynamic HF radio scatter Doppler spectra, pump-induced electron precipitation, and substorm activation exactly above Tromso. We discuss the modification of the ionosphere-magnetosphere coupling in terms of the excitation of a turbulent Alfven boundary layer between the base of the ionosphere (∼100 km) and the level of sharp increase of the Alfven velocity (at heights up to one Earth radius), and the formation of a local magnetospheric current system. The results suggest that a possible triggering of local auroral activation requires specific geophysical conditions.

Phenomena observed by HF long‐distance diagnostic tools in the HF modified auroral ionosphere during magnetospheric substorm

The HF long-distance diagnostic tools located in St. Petersburg combined with multi-instrument observations at Tromso have been operated in conjunction with the ionospheric heating facility near Tromso to examine the heater-induced phenomena in the auroral ionosphere. The distinctive feature of the heating experiment was the reflection of the pump wave from auroral sporadic Es-layer. During substorm expansion, strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magnetosphere and stimulating the precipitation of the energetic electrons (10–40 keV) of ∼1-min duration due to a cyclotron resonant interaction of the natural precipitating electrons (1–10 keV) with the heater-induced whistler waves in the magnetosphere. In the recovery phase of the auroral substorm, emissions at the heater second harmonic frequency were accompanied by excitation of AFAI in the Es layer with the large decay time up to several minutes after the heater turn-off. The possible explanation for these emissions can be found from the linear conversion process when the pump wave of O mode may directly convert to electrostatic waves in the region of the HF plasma resonances. The large decay time may result from electromagnetic instability growth in the heater-modified ionosphere at the downward field-aligned currents.

Stimulated emissions around second harmonic of Tromsø heater frequency observed by long-distance diagnostic HF tools

Observations of stimulated electromagnetic emission (SEE) around the second harmonic of Tromso heater frequency carried out by long-distance diagnostic HF tools are presented. Diagnostic tools are located at a distance of 1200 km from the Tromso HF heater. Observed SEE spectra show: (1) spectral broadening up to 30 kHz; (2) frequency shift of the maximum amplitude in the SEE spectra from the heater second harmonic frequency 8080 kHz by 16–20 kHz; (3) dependence of the maximum amplitude on the heater power level. The detected features in the SEE spectra have not been observed before in ionospheric heating experiments and may be the second harmonic of the downshifted maximum. The strongest candidate is the coalescense (head-on collisions) of Langmuir waves produced in the heated volume.

Ionospheric wave processes during HF heating experiments

Effects of ionospheric modification produced by powerful high frequency radio waves are studied using the method of field-aligned scattering of diagnostic HF radio signals. Observations of scattered HF signals have been made by the Doppler spectrum method. Analysis of the experimental data shows the appearance of quasiperiodic variations in Doppler frequency shift fd, with periods 30–60 s during the heating cycles. Powerful HF waves are assumed to excite the Alfven resonator generating oscillations of the magnetic field lines in the heated region and giving rise to fd artificial variations and magnetic pulsations. In the case of continuous action of the powerful HF transmitter ionospheric waves are sometimes observed with periods 12–25 min, typical of medium-scale travelling ionospheric disturbances.

Studying the ionosphere with the incoherent scatter radar during the magnetic storm of January 10, 1997

Specific features of the variations and structure of the ionosphere above northern Europe during the known magnetospheric storm of January 10, 1997, have been analyzed using the oblique-incidence backscatter (OIB) methods. The OIB equipment installed near St. Petersburg, Gor’kovskaya (AANII), represent a BIZON small radar, the data of which were compared with the CUTLASS radar data, GPS observations of TEC, and geophysical data of the Sweden, Finnish, and Russian high-latitude observatories. A local substorm that occurred from 1451 to 1655 UT has been studied in detail, and 52 OIB ionograms have been obtained. The types of reflections from the MIT polar wall and the narrow ionization trough have been identified.

Determining the ionospheric irregularity velocity vector based on doppler measurements in the artificially modified F 2 region of the polar ionosphere

The method for estimating the behavior of the ionospheric irregularity motion vector in the artificially disturbed HF ionospheric region has been proposed, and this behavior has been analyzed based on the simultaneous Doppler observations performed on several paths using the method of bi-static backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities. The Doppler measurements were performed during the modification of the auroral ionosphere by powerful HF radiowaves emitted by the EISCAT heating facility (Tromsø, Norway). It has been obtained that the dynamics of the ionospheric irregularity directions in the F region, calculated based on the Doppler measurements of the total vector of the ionospheric irregularity velocity above the Tromsø EISCAT radar at a frequency of 931 MHz, is in satisfactory agreement with such calculations performed using the three-position method.

Ionospheric effects caused by barium releases

Abstract Results of experimental studies of the ionospheric effects produced by CRRES barium releases are considered. The experimental observations of HF spectral characteristics by Doppler method are made by a network of long distance radio paths intersecting the L-shell of releases. The time dependence of their occurrence relative to the moment of release and the character of changes of spectral parameters produces signal effects (SE) which may be classified as: the unique burst, the quasiperiodic group of bursts, the regular changes of spectral parameters and wave processes. Observed types of SE are analogous to those seen when the releases were produced at the heights from 140 to 160 km. The result of experimental observations testify that there are special geophysical phenomena produced by barium releases.

Medium-scale ionospheric wave processes during the CRRES Barium releases

Abstract Results of experimental studies and computer simulation of medium-scale ionospheric wave processes observed during the CRRES Barium release on July 13, 1991 are considered. An attempt at reconstruction of the spatial distribution of ionosphere parameters in the region 22° – 40°N, 50° – 75°W for the time period from 08.00 to 09.00 UT on July 13, 1991 is made. Wave processes in the ionospheric F-layer with main periods in the range of 47–58 min, 19–24 min and 10–13 min have been established. Some peculiarities in Doppler data on different radio paths could be explained as an effect of abrupt ionosphere changes. The hypothesis is advanced that the disturbance in the F2- layer was initiated by the release and propagated for long distances (550 – 2200 km) with supersonic velocities in the range of 1000–2700 m/s.