Excitation of Plasma Irregularities in the F Region of the Ionosphere by Powerful HF Radio Waves of X‐Polarization

Abstract

We discuss theoretically the excitation of artificial plasma irregularities in the auroral ionosphere by high-frequency X-mode radio wave. This is done via a two-step process. As a first step we adopt the thermal self-focusing instability excited in the F region of the ionosphere under the action of a strong high-frequency (HF) radio wave. This instability causes the formation of perturbations of the electron temperature and plasma concentration across the magnetic field. In addition, the plasma becomes depleted in the regions of the electron temperature enhancements and vice versa, since the gradients of plasma concentration and the electron temperature have opposite signs. In such conditions the temperature gradient instability comes into play. As a second step we consider plasma and electron temperature inhomogeneities that appear due to this instability to be responsible for the generation of irregularities with transverse sizes smaller than the typical scales of the self-focusing instability. Alternative mechanisms such as excitation of the gradient-drift and the current-convective instabilities, which are often attributed to the generation of plasma irregularities in the F region and can contribute to the formation of artificial irregularities in the case of X-mode heating, are also discussed.

Distinctive Features of Langmuir and Ion‐Acoustic Turbulences Induced by O‐ and X‐Mode HF Pumping at EISCAT

“…In the second step large-scale filaments collapse into the small-scale sizes. An attempt at a theoretical justification of the two-step generation of the X-mode FAIs was made by Borisov et al (2018).…”

Stimulated electromagnetic emissions spectrum observed during an X-mode heating experiment at the European Incoherent Scatter Scientific Association

“…Field-aligned irregularities have been observed during X-mode heating experiments at Tromsø, Norway (Blagoveshchenskaya et al, 2011(Blagoveshchenskaya et al, , 2014(Blagoveshchenskaya et al, , 2015. Borisov et al (2018) suggested using a two-step process to interpret the formation of 10m FAIs by the X-mode heating wave at high latitudes. The first step is the thermal self-focusing instability, and the second step is the formation of electron inhomogeneities resulting from the thermal self-focusing instability or from the gradient drift and current convective instabilities.…”

Perturbing the high-latitude upper ionosphere (F region) with powerful HF radio waves: A 25-year collaboration with EISCAT

“…It was found that the physical driving mechanisms of artifi cial fi eld-aligned irregularities with a spatial scale across the geomagnetic fi eld of 7.5 m to 15 m were signifi cantly diff erent for O-mode and X-mode HF pumping, presenting challenges for understanding the relevant processes. In contrast to the O-mode artifi cial fi eld-aligned irregularities, excited by a thermal parametric (resonance) instability at the upper hybrid resonance altitude [13,14], the X-mode artifi cial fi eld-aligned irregularities were generated via a two-step process [31,[36][37][38]. In the fi rst step, the generation of elongated large-scale irregularities (with a spatial scale across the geomagnetic fi eld of the order of 1 km to 10 km) due to the self-focusing instability occurred.…”

“…The excitation and behavior of small-scale artifi cial fi eld-aligned irregularities was driven by largescale irregularities. Their possible generation mechanisms could be the temperature-gradient-drift instability [38] or the fi lamentation instability [39].…”