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\begin{document}

\title{Artificial Small-scale Field-aligned Irregularities in the High Latitude
Ionospheric F-region}



\author[1]{Nataly Blagoveshchenskaya}%
\author[1]{Tatiana Borisova}%
\author[1]{Alexey Kalishin}%
\author[2]{Timothy Yeoman}%
\affil[1]{Arctic and Antarctic Research Institute}%
\affil[2]{Department of Physics and Astronomy, University of Leicester}%


\vspace{-1em}



  
  \date{June 13, 2024}


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\begin{abstract}
The ionospheric modification experiments are providing the means for
understanding mechanisms and physical processes leading to the
generation and evolution of ionospheric irregularities. We present
experimental results concentrating on the features and evolution,
generation conditions and mechanisms of small-scale field-aligned
irregularities (FAIs) in the high latitude ionosphere F region induced
by the controlled injection of the powerful HF radio waves from the
ground into the ionosphere. Experiments have been carried out at the
EISCAT HF Heating facility near Troms\selectlanguage{ngerman}ø, Norway, at heater frequencies of
4.5 -- 8.0 MHz with an effective radiated power of 200 -- 750 MW. HF
pump wave with ordinary (O-mode) or extraordinary (X-mode) polarization
was injected along the magnetic field-aligned direction. Instrument
diagnostics included the CUTLASS (Co-operative UK Twin Located Auroral
Sounding System) radar, the European Incoherent Scatter (EISCAT) UHF
radar at 931 MHz near Tromsø and the EISCAT ionosonde (dynasonde). It
was found that the features and physical driving mechanisms of FAIs with
the spatial scale across the geomagnetic field of 7.5 -- 15 m are
significantly different for O- and X-mode HF pumping, presenting
challenges for understanding the relevant processes. The main attention
is paid to the recently discovered X-mode FAIs. By a contrast to the
O-mode FAIs excited by a thermal parametric (resonance) instability at
the upper hybrid resonance altitude, the X-mode FAIs are generated via
two-step process. In the first step the generation of elongated
large-scale irregularities (with the spatial scale across the
geomagnetic field of the order of 1 -10 km depending on the background
geophysical conditions and the pump frequency) is occurred. As a second
step, we suggest that the filamentation instability can be responsible
for the generation of small-scale FAIs. As is found from EISCAT UHF
radar measurements FAIs greatly impact on the development of strong
artificial turbulence such as Langmuir and ion-acoustic plasma waves.%
\end{abstract}\selectlanguage{ngerman}%



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