Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes

Abstract

We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs) at mid-and high latitudes based on the "Sura" and EISCAT/ HEATING HF facilities. Observations were performed by the method of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when the ionosphere above the "Sura" facility is illuminated by the Sun, but the solar terminator travels through the magnetically conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary ionospheric turbulence maintaining the irregularities above "Sura." A drastic increase in the Doppler spectra width of the scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the terminator, but the ionosphere above the "Sura" facility is still lighted. It is assumed that the "run away" of photoelectrons from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase in their intensity. The presence of fairly intense scattered signals was detected from the "Sura" and EISCAT/HEATING experimental results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses. In the case of pulsed heating by short pulses with duration τ p < 100 ms and average radiated power P a below the threshold power P thr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation of irregularites under the cycled HF pumping with the pulse duration τ p = 384 ms for P a comparable with P thr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The residual turbulence aftereffects played a significant role in the SSAI development.

On specific features of diurnal variations in characteristics of the diagnostic stimulated electromagnetic emission and their relation to the evolution of artificial ionospheric irregularities

“…However, the measurements of characteristics of the aspect scattering of HF radio waves, performed in August 2005, showed that under the evening conditions (but before the sunset on the Earth) the second stage of relaxation of decameter artificial irregularities during the pump-wave radiation in the diagnostic emission regime, if still observed it begins only with a level no higher than − (15−20) dB with respect to their stationary intensity, whereas this stage is recorded more regularly after the sunset and begins with the level −10 dB and sometimes from the higher level. Moreover, from the measurements [22], also performed in August 2005, it can be concluded that after the sunset the second stage of relaxation of these irregularities under conditions of pulsed heating of the ionospheric plasma can last for more than 5 min. In this case, the characteristic time of the second stage of relaxation of artificial irregularities in the diagnostic emission regime is severalfold greater than for the regime of free (in the absence of pulsed heating) relaxation, whose duration usually amounts to 30−90 s [9,11].…”

“…The latter fact can determine some features of the diurnal dependence of the properties of the stimulated electromagnetic emission and diagnostic emission, which have not been taken into account properly for now and require their further study. It should also be mentioned that the recent "Sura" experiments showed that the properties of the generated artificial turbulence in the disturbed ionosphere region varied as the solar terminator passed through the ionosphere at the magnetically conjugate point [22,25]. Such an effect is also observed in the natural ionosphere (see, e.g., [26]).…”

Consistent quasi-periodic variations of the geomagnetic pulsation level and doppler frequency shift of decametric radio waves aspect-scattered by artificial field-aligned ionospheric irregularities

“…In particular, Doppler measurements give important data on specific features of the behavior of small-scale artificial irregularities, motion of the scattering irregularities in a magnetized plasma, etc. [4][5][6][7][8][9][10].…”

Observations of unusually broadened HF radar spectra from heater-induced artificial plasma irregularities

“…Modest enhancements in the spectral width of artificial backscatter, when they have occurred, have been attributed to, for example, ULF waves [ Wright et al , 2004] or associated with the heated region being in daylight while its conjugate point was in darkness [ Blagoveshchenskaya et al , 2007]. Ponomarenko et al [1999] also reported enhancements in the spectral width of artificial backscatter by a factor of 5 when the pump frequency was close to the third and fourth harmonics of the electron gyrofrequency.…”

“…The nature of HF backscatter spectra is governed by the characteristics of the ionospheric irregularities from which the backscattered signal originates. For example, vortices in the ionospheric plasma can give rise to such effects as broadening of the HF Doppler spectrum [ André et al , 2000], the occurrence of double‐peaked Doppler spectra [ Huber and Sofko , 2000], and an increase in the decay time of artificial FAI due to additional stability supplied with respect to thermal diffusion processes [ Blagoveshchenskaya et al , 2007].…”