Optical effects in the aurora caused by ionospheric HF heating

Ionospheric heating experiments were done by the EISCAT Heater in Tromsø on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojärvi about 100 km to the south-east of Tromsø. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O₂⁺ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.     

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.     

Whistler Growth Rates in the Magnetosphere According to Measurements of Energetic Electron Fluxes on the Van Allen Probe A Satellite

Geomagnetism and Aeronomy - Measurements of the differential fluxes of energetic electrons in the equatorial region of the magnetosphere in the energy range from units of keV to hundreds of keV...     

Observations of intensity variations and harmonics of heater induced VLF waves

Very low-frequency (VLF) radio waves have been generated in the ionosphere by the heating facility near Tromsø. VLF signals have been received at Porojärvi (69.17 ° N, 21.47 ° E, L = 6.1), in Northern Finland, about 100 km from the heating transmitter. Three heating experiments were performed under different geophysical conditions and different results were obtained. Signal intensities have been compared with magnetic variations. Correlations and anti-correlations were observed. Several harmonics of the generated signal have generally been found. In some cases when the intensity of the detected VLF signal was low the first harmonic was as intensive as the fundamental frequency. VLF response from Artificial Periodic Inhomogeneities (API) experiment was also observed.     

Imaging and EISCAT radar measurements of an auroral prebreakup event

The results of coordinated EISCAT and TV-camera observations of a prebreakup event on 15 November 1993 have been considered. The variations of the luminosity of two parallel auroral arcs, plasma depletion on the poleward edge of one of these arcs as well as electron and ion temperatures in front of a westward travelling surge were studied. It was found that a shortlived brightening of a weak zenith arc before an auroral breakup was accompanied by fading of an equatorial arc and, vice versa. A plasma depletion in the E region was detected by the EISCAT radar on the poleward edge of the zenith arc just before the auroral breakup. The plasma depletion was associated with an enhancement of ion (at the altitudes of 150–200 km) and electron (in E region) temperatures. During its occurrence, the electric field in the E-region was extremely large (150 mV/m). A significant increase in ion temperature was also observed 1 min before the arrival of a westward travelling surge (WTS) at the radar zenith. This was interpreted as the existence of an extended area of enhanced electric field ahead of the WTS.     

Solar protons in the Earth’s magnetosphere from riometric and satellite data during magnetic storms in October 2003

The fluxes and penetration boundaries of solar energetic particles on the CORONAS-F satellite during October 2003 superstorms are compared with the riometric absorption measurements on a worldwide network of riometers. The dynamics of the polar cap boundaries is investigated at various phases of magnetic storms. The dependence of absorption on time of the day and on solar proton spectrum is calculated at various phases of a solar energetic particle event.     

Auroral pulsations and accompanying VLF emissions

Results of simultaneous TV observations of pulsating auroral patches and ELF-VLF-emissions in the morning sector carried out in Sodankylä (Finland) on February 15, 1991 are presented. Auroral pulsating activity was typical having pulsating patches with characteristic periods of about 7 s. Narrow-band hiss emissions and chorus elements at intervals of 0.3–0.4 s formed the main ELF-VLF activity in the frequency range 1.0–2.5 kHz at the same time. The analysis of auroral images with time resolution of 0.04 s allowed perfectly separate analysis of spatial and temporal variations in the auroral luminosity. Mutual correspondence between the behaviour of the luminous auroral patches and the appearance of ELF noise type hiss emissions and VLF chorus trains was found in two intervals chosen for analysis. While the hiss emissions were associated with the appearance of luminosity inside a limited area close to the zenith, the structured VLF emissions were accompanied by rapid motion of luminosity inside the area. The spatial dimension of the pulsating area was about 45–50 km and luminosity propagated inside it with velocity of about 10–12 kms. We discuss a new approach to explain the 5–15 s auroral pulsation based on the theory of flowing cyclotron maser and relaxation characteristics of ionosphere.     

Spatial Structure of the Illuminated Area of the Auroral Hiss Based on Ground-Based Observations at Auroral Latitudes

Geomagnetism and Aeronomy - The article presents the results of ground-based observations of very low-frequency (VLF) auroral hiss bursts at the Lovozero (Russia) and Kannuslehto (Finland)...     

Short-period VLF emissions as solitary envelope waves in a magnetospheric plasma maser

Space and ground-based experiments have shown evidence of natural short-period VLF emissions in which separate spectral elements are repeated with a periodicity of 2–7 s. Their basic morphological properties are found on the basis of original experimental data. In our opinion, excitation of such emissions is the result of quasi-linear relaxation effects that compensate for natural spectral dispersion. The quasi-linear relaxation of the energetic electron distribution function incrementally changes wave cyclotron instability and hence the VLF emission spectral forms. Some properties of the quasi-linear interaction of whistler waves with magnetospheric radiation belt electrons are studied. It is shown that quasi-linear relaxation can increase the cyclotron instability at the leading edge of an electromagnetic pulse. This effective saturation of absorption facilitates the division of VLF hiss-like emission into separate electromagnetic pulses without spectral modification from one pulse to the next. Some features and manifestations of this effective saturation of absorption are discussed. The results are important for a better understanding of temporal and spatial structures of VLF whistler-mode emissions and energetic electron fluxes.     

First Results from VLF Observations during the Transarctica 2019 Polar Expedition

Geomagnetism and Aeronomy - The first results are presented for observations of the VLF emissions (1–15 kHz) conducted at polar latitudes during the expedition Transarctica 2019, which was...