First Observations of Recurring HF-Enhanced Topside Ion Line Spectra Near the Fourth Gyroharmonic

• Published in: Journal of Geophysical Research: Space Physics
• Main Authors: Rexer, T, Gustavsson, B, Leyser, T, Rietveld, M, Yeoman, T, Grydeland, T
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU), Wiley 2019
• Subjects: Science & Technology; Physical Sciences; Astronomy & Astrophysics; topside ion line spectra; HF enhancements; STIMULATED ELECTROMAGNETIC EMISSIONS; HIGH-LATITUDE IONOSPHERE; ASPECT ANGLE DEPENDENCE; OPTICAL-EMISSIONS; PLASMA; STRIATIONS; EISCAT; WAVE; FREQUENCIES; INSTABILITIES; Norway
• Online Access: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JA025822; http://hdl.handle.net/2381/45163; https://doi.org/10.1029/2018JA025822

The data of EISCAT UHF radar during the experiment can be obtained from EISCAT http://www.eiscat.se/schedule/schedule.cgi. The EISCAT ISR analysis tool GUISDAP is available at https://www.eiscat.se/scientist/user-documentation/guisdap-9-0/. This paper includes color specifications and designs developed by Cynthia Brewer (http://colorbrewer.org/) and colormaps by Nathaniel J. Smith and Stefan van der Walt. We present first incoherent scatter radar observations of systematically recurring, high‐frequency (HF)‐enhanced ion line spectra at the topside F‐region ionosphere, during magnetic field aligned HF pumping in an O‐mode polarization. The European Incoherent Scatter UHF radar was directed in magnetic zenith on 9–11 March 2016 while stepping the pump frequency across the double resonance of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 3‐min‐on, 3‐min‐off pump cycle. Topside and bottomside enhancements occur at the respective plasma resonance altitude and seem to be asymmetrically conditioned by the relative proximity of the pump frequency to the double resonance frequency. Further, the topside HF‐induced ion line enhancements predominantly appear while the pump frequency is just below the double resonance frequency and only simultaneous to strong bottomside enhancements. A powerful, HF radio wave in O‐mode, transmitted in the direction of magnetic zenith is reflected a few kilometers below the plasma resonance altitude, where the pump frequency is equal to the local plasma frequency, on the bottomside F‐region in the ionosphere. Transionospheric propagation of the pump wave outside the radio window can be facilitated by density striations in the plasma, and we consider, in detail, the possible mechanisms proposed for propagation outside the standard radio window. EISCAT is an international association supported by research organisations in China (CRIRP), Finland (SA), Japan (NIPR and STEL), Norway (NFR), Sweden (VR), and the United Kingdom (NERC). Peer-reviewed Publisher Version