T. D. Borisova

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.

Optical and ionospheric phenomena at EISCAT under continuous X‐mode HF pumping

We present experimental results from multiinstrument observations in the high-latitude ionospheric F2 layer at the EISCAT (European Incoherent Scatter Scientific Association) heating facility. The results come from a set of experiments, when an X-polarized HF pump wave at high heater frequencies (fH > 6.0 MHz) was injected into the F region of the ionosphere toward the magnetic zenith. Experiments were carried out under quiet magnetic conditions with an effective radiated power of 458–548 MW. HF pumping was produced at different heater frequencies, away from electron gyroharmonic frequencies, and different durations of heater pulses. We show the first experimental evidence of the excitation of artificial optical emissions at red (630 nm) and green (557.7 nm) lines in the high-latitude ionospheric F2 layer induced by an X-polarized HF pump wave. Intensities at red and green lines varied in the range 110–950 R and 50–350 R, respectively, with a ratio of green to red line of 0.35–0.5. The results of optical observations are compared with behaviors of the HF-enhanced ion and plasma lines from EISCAT UHF incoherent scatter radar data and small-scale field-aligned artificial irregularities from Cooperative UK Twin Located Auroral Sounding System observations. It was found that the X-mode radio-induced optical emissions coexisted with HF-enhanced ion and plasma lines and strong artificial field-aligned irregularities throughout the whole heater pulse. It is indicative that parametric decay or oscillating two-stream instabilities were not quenched by fully established small-scale field-aligned artificial irregularities excited by an X-mode HF pump wave.

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.

Results of Russian experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the EISCAT facilities

We present the results of complex experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the European Incoherent Scatter Scientific Association (EISCAT) facilities. During the ionospheric F-region heating by powerful extraordinary (X-mode) polarized HF radiowaves under the conditions of heating near the critical fH frequency fH ≈ fxF2 of the extraordinary wave of the F2-layer, we were first to detect the excitation of intense artificial small-scale ionospheric irregularities (ASIs), accompanied by electron temperature increases by approximately 50%. The results of coordinated satellite and ground-based observations of the powerful HF radiowave impact on the high-latitude ionosphere are considered. During ionospheric F-region heating by powerful HF radiowaves of ordinary polarization (O-mode) during evening hours, the phenomenon of ion outflow accompanied by electron temperature increases and thermal plasma expansion was revealed. Concurrent DMSP-F15 satellite measurements at a height of about 850 km indicate an O+ ion density increase. The CHAMP satellite observations identified ULF emissions at the modulation frequency (3 Hz) of the powerful HF radiowave, generated during modulated emissions of the powerful HF radiowave of O-polarization and accompanied by a substantial increase in the electron temperature and ASI generation.

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.

Comparison of the effects induced by the ordinary (O-mode) and extraordinary (X-mode) polarized powerful HF radio waves in the high-latitude ionospheric F region

Using the results of coordinated experiments on the modification of the high-latitude ionosphere by powerful HF radio emission of the EISCAT/Heating facility, effects of the impact of powerful HF radio waves of the ordinary (O-mode) and extraordinary (Х-mode) polarization on the high-latitude ionospheric F region have been compared. During the experiments, a powerful HF radio wave was emitted in the magnetic zenith direction at frequencies within the 4.5–7.9 MHz range. The effective power of the emission was 150–650 MW. The behavior and characteristics of small-scale artificial ionospheric irregularities (SAIIs) during O- and X-heating at low and high frequencies are considered in detail. A principal difference has been found in the development of the Langmuir and ion–acoustic turbulence (intensified by the heating of the plasma and ion–acoustic lines in the spectrum of the EISCAT radar of incoherent scatter of radio waves) in the О- and Х-heating cycles after switching on the heating facility. It has been shown that, under the influence on the ionospheric plasma of a powerful HF radio wave of the Х-polarization, intense spectral components in the spectrum of the narrow-band artificial ionospheric radio emission (ARI) were registered at distances on the order of 1200 km from the heating facility.

Comment on “Parametric Instability Induced by X‐Mode Wave Heating at EISCAT” by Wang et al. (2016)

The data used in this comment are available through the EISCAT Madrigal database (http://www.eiscat.se/madrigal/). Publishers version/PDF may be used 6 months after publication

Recent results from HF pumping experiments of the ionosphere and mesosphere using the EISCAT facilities near Troms

Summary form only given. The EISCAT HF ionospheric heating facility has recently been upgraded in several key areas that allow more flexibility in changing parameters such a frequency, power and phase, as well as allowing an HF radar receiver operation. These new capabilities were important for several recent results some of which are presented here. The HF radar capability has been used successfully to study how mesospheric echoes at 8 MHz are affected by artificial electron heating. Attempts have also been made to search for 8 MHz magnetospheric echoes related to auroral processes, without obvious success so far. The more traditional F-region HF-pumping experiments have shown some unexpected results, especially during X-mode heating. These include the production of electron density striations, airglow, enhanced plasma and ion lines measured with the UHF incoherent scatter radar, and electron temperature increases, phenomena which are normally expected to occur only with O-mode heating. For O-mode we sometimes see a descending HF-induced ion line which is most likely due to ionization from pump-induced fluxes of suprathermal electrons. Quantitative comparisons between measurements of electron heating and models of Ohmic and resonant heating have been made, and comparisons between the observed and theoretical thresholds of the various plasma instabilities. Furthermore, there are often unexplained 933 MHz radar backscatter enhancements above the HF reflection height, seen with both polarisations but only in directions very closely aligned along the magnetic field.

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.