The distinctive features of the ion gyro‐harmonic discrete structures in the narrowband stimulated electromagnetic emission (NSEE) spectra (within ±1 kHz of the heater frequency), excited by an extraordinary (X‐mode) polarized high frequency (HF) pump wave in the F‐region of the high latitude ionosphere, are reported. Results were obtained from three sets of experiments at the EISCAT (European incoherent Scatter) HF Heating facility at pump frequencies of 5.423, 6.77 and 7.953 MHz, which are below the fourth, fifth, and sixth electron gyro‐harmonics. High power HF radio waves were radiated in the magnetic field‐aligned direction with an effective radiated power of 238–740 MW. Discrete spectral features, ordered by the ion gyro frequency (for O⁺ ions), were recorded at a large distance (1,200 km) away from the Heating facility. Up to 10 downshifted discrete ion gyro‐harmonic structures paired with the upshifted spectral components (Stokes and anti‐Stokes modes) in the NSEE spectra have been excited in the course of these experiments. It was found that preconditioning effects have a significant impact on the temporal evolution of the discrete spectral structures. The ion gyro‐harmonic structures coexisted with field‐aligned artificial irregularities (FAIs). Comparing the NSEE and EISCAT incoherent scatter radar observational results, it was concluded that the electron accelerations along and across the geomagnetic field coexist during X‐mode heating. The magnetized stimulated Brillouin scatter and stimulated ion Bernstein scatter are discussed as relevant processes for the observed multiple down‐ and upshifted ion gyro‐harmonic structures in the NSEE spectra.

The results of long-term studies of the effects of the high-latitude ionosphere modification by powerful high-frequency (HF) radio waves generated by the E

Ducts (field-aligned plasma density enhancements) provide a link into the magnetosphere and can guide whistler waves. Inside ducts, wave-particle interactions occur efficiently; therefore, their presence contributes to the removal of energetic particles from the magnetosphere. We present experimental results concerning the characteristics, behavior, and excitation thresholds of ducts induced by extraordinary (X-mode) polarized high-power HF radio waves emitted towards the magnetic zenith (MZ) into the upper ionosphere at EISCAT (European Incoherent SCATter). The features and behavior of ducts were diagnosed by the EISCAT incoherent scatter radar (ISR) at Tromsø and the CUTLASS (SuperDARN) Finland radar at Hankasalmi. The state of the ionosphere was monitored by the Dynasonde in Tromsø. It was found that the electron density N_e enhancements inside ducts were of 50–80% above the background N_e values and their transverse size (normal to the magnetic flux tube) corresponded to about 3–4° in the north–south direction. They were generated during magnetically quiet periods and extended from ~300 to 320 km up to the upper altitude limit of the EISCAT radar measurements (600–700 km), when heater frequencies were both below and above the critical frequency of the F2 layer (f_H ≤ f_oF2 and f_H > f_oF2), regardless of whether HF-induced plasma and ion lines were generated or not. Comparing the O-/X-mode effects from the EISCAT radar observations, it was shown that the creation of the strong N_e ducts is a typical characteristic of the X-mode pulses. As a rule, electron density enhancements were not observed during O-mode pulses. A plausible mechanism for the creation of X-mode artificial ducts is discussed.

. We have examined longitudinal-temporal variations in ionospheric parameters over Eurasia by analyzing data from a chain of high-latitude ionosondes along a latitude circle ~70° N (geomagnetic latitudes 58°<Glat<65°) in the longitudinal sector 26–171° E during severe magnetic storms of solar cycle 24 in March and June 2015. To analyze the response of ionospheric ionization to geomagnetic disturbances, we have used ionosonde data on hourly average critical frequency foF2 of the ionospheric F2 layer. Strong differences were observed between common peculiarities of temporal variations in foF2 for the analyzed periods of magnetic storms, which are likely associated with the characteristic features of the seasonal and diurnal variations in the background high-latitude ionosphere of the given geographic region. During the main and early recovery phases of magnetic storms there were periods of blackouts of ionosonde radio signals. Differences in the character of the ionospheric response to geomagnetic disturbances have been noted. This is probably due to seasonal features of the probability of occurrence of the ionospheric storm positive or negative phase in different seasons of the year. The trends of increasing ionospheric ionization over the vast region of Eastern, Western Siberia and Europe after the end of the extreme magnetic storm in March 2015, according to measurements from the chain of high-latitude ionosondes, may be associated with the formation of an area of increased [O]/[N₂] ratio over this territory. Such an increase in ionospheric ionization exceeding the background level of foF2 values can be considered as a clear manifestation of the after-effect of magnetic storms.

We have analyzed spatial and temporal variations in ionospheric parameters over high and middle latitudes of Eurasia, using data from chains of high- and mid-latitude ionosondes during a severe magnetic storm in March 2015. To analyze the ionospheric response to the severe geomagnetic disturbance of solar cycle 24, we have employed ionosonde data on hourly average values of the critical frequency foF2 of the ionospheric F2 layer, the critical frequency of the sporadic layer foEs, and the minimum reflection frequency fmin. There are strong latitudinal and longitudinal differences between the features of temporal variations in the analyzed ionospheric parameters both under quiet conditions before the magnetic storm onset and during the storm. We discuss possible causes of the observed spatial variations in ionospheric parameters. The source of spatio-temporal variations in ionospheric ionization parameters may be inhomogeneities generated in the high-latitude ionosphere under conditions of increased helio-geomagnetic activity. During the magnetic storm main and recovery phases, periods of blackouts of radio signals from ionosondes were observed at both high and middle latitudes. During these periods, there was a significant increase in the absorption of radio waves used in ionosonde sounding, as well as in the frequency of occurrence of screening sporadic Es layers. The long-term effect of the negative ionospheric storm over high and middle latitudes of Europe is explained by the movement of the vast region of the reduced density ratio [O]/[N2] at thermosphere heights from the Far East and Siberia westward to Europe during the late recovery phase of the magnetic storm. Increased ionization of the ionospheric F2 layer with foF2 exceeding the level for quiet days before the onset of the magnetic disturbance over the vast region of Eastern, Western Siberia and Eastern Europe after the end of the magnetic storm in March 2015 is a manifestation of the aftereffect of magnetic storms. The increase in ionization was especially pronounced, as measured by the chain of mid-latitude ionosondes.