Artificial Ducts Created via High-Power HF Radio Waves at EISCAT

Abstract

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 Ne enhancements inside ducts were of 50–80% above the background Ne 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 (fH ≤ foF2 and fH > foF2), 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 Ne 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.

Comment on Blagoveshchenskaya et al. Artificial Ducts Created via High-Power HF Radio Waves at EISCAT. Remote Sens. 2023, 15, 2300

“…Two similar observations to those presented [1] were studied in detail [2] where the authors showed that the apparent electron density enhancements measured by the EISCAT UHF radar were not real because (a) in one case the natural plasma line (PL) frequency, which is a more accurate measurement than that provided by the ion line backscattered power, did not show an enhancement, and (b) in the second case the analysis of the data received in a bistatic remote receiver did not show any enhancement. In both cases, the transmitted radar beam was pointed along the magnetic field line direction.…”

“…Unfortunately, neither of these references [2,3] was mentioned in the study [1]. Examination of the natural PL frequency data for the heights where the enhanced backscatter is observed in [1], assuming that they are strong enough, will almost certainly show that there were no density enhancements of 50-80%. We now present some PL data in support of our claim.…”

“…A statistical study of similar events to those presented in [1,2] was made and a possible explanation was offered in [3]. Unfortunately, neither of these references [2,3] was mentioned in the study [1]. Examination of the natural PL frequency data for the heights where the enhanced backscatter is observed in [1], assuming that they are strong enough, will almost certainly show that there were no density enhancements of 50-80%.…”

“…The enhancements are in the power of the backscattered ion line so they are sometimes called wide altitude ion line enhancements or WAILEs for short. A statistical study of similar events to those presented in [1,2] was made and a possible explanation was offered in [3]. Unfortunately, neither of these references [2,3] was mentioned in the study [1].…”

“…In a recent paper [1] the authors present data from the EISCAT incoherent scatter radar during an ionospheric heating experiment which they claim shows evidence of artificial ducts caused by the heating experiment. The bulk of the evidence is in the apparent electron density enhancements of about 50-80% above the background density given by the standard analysis of the received radar signals.…”