Abstract
We presented experimental results of strong plasma modifications induced by X-mode powerful HF radio waves injected towards the magnetic zenith into the high latitude F region of the ionosphere. The experiments were conducted in 2009–2011 using the EISCAT Heating facility, UHF incoherent scatter radar and the EISCAT ionosonde at Tromsø, Norway; and the CUTLASS SuperDARN HF coherent radar at Hankasalmi, Finland. The results showed that the X-mode HF pump wave can generate strong small-scale artificial field aligned irregularities (AFAIs) in the F region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 9–15m, were excited when the heater frequency (fH) was above the ordinary-mode critical frequency (foF2) by 0.1–1.2MHz. It was found that the X-mode AFAIs appeared between 10s and 4min after the heater is turned on. Their decay time varied over a wide range between 3min and 30min. The excitation of X-mode AFAIs was accompanied by electron temperature (Te) enhancements and an increase in the electron density (Ne) depending on the effective radiated power (ERP). Under ERPs of about 75–180MW the Te enhances up to 50% above the background level and an increase in Ne of up to 30% were observed. Dramatic changes in the Te and Ne behavior occurred at effective radiated powers of about 370–840MW, when the Ne and Te values increased up to 100% above the background ones. It was found that AFAIs, Ne and Te enhancements occurred, when the extraordinary-mode critical frequency (fxF2) lied in the frequency range fH–fce/2≤fxF2≤fH+fce/2, where fce is the electron gyrofrequency. The strong Ne enhancements were observed only in the magnetic field-aligned direction in a wide altitude range up to the upper limit of the UHF radar measurements. In addition, the maximum value of Ne is about 50km higher than the Te enhancement peak. Such electron density enhancements (artificial ducts) cannot be explained by temperature-dependent reaction rates. They can be attributed to HF-induced ionization production by accelerated electrons. The possible mechanisms for plasma modifications induced by powerful X-mode HF radio waves were discussed.
Highlights
We presented experimental results of strong plasma modifications induced by X-mode powerful HF radio waves injected towards the magnetic zenith into the high latitude F region of the ionosphere
Among the phenomena discovered from ionospheric modification experiments one of the most outstanding is the excitation of small-scale artificial field-aligned irregularities (AFAIs) or striations
Their maximum spatial scale across the magnetic field is determined by the quantity l1⁄4c/fH, where fH is the heater frequency and c is the speed of light
Summary
AFAIs have been excited through a large number of experiments at a variety of HF heating facilities located at mid and high latitudes (Minkoff et al, 1974; Hedberg et al, 1983; Djuth et al, 1985; Noble et al, 1987; Kelley et al, 1995; Hysell et al, 1996; Frolov et al, 1997; Yeoman et al, 2007; Yampolski et al, 1997; Robinson et al, 1997;1998; 2006; Eglitis et al, 1998; Honary et al, 1999; Rietveld et al, 2003; Blagoveshchenskaya et al, 1998; 2001; 2006; 2009) They occur at the upper hybrid resonance altitude when ordinary (O-mode) polarized HF pump waves reach the ionospheric reflection height.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Journal of Atmospheric and Solar-Terrestrial Physics
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
