Time variations of HF‐induced plasma waves

Abstract

Intense plasma waves are generated by an HF pump wave in an ionospheric heating experiment at the Arecibo Observatory. These plasma waves can be observed as enhancements to the ion and plasma lines of the incoherent backscatter echo. The enhancements can be 3 or 4 orders of magnitude more intense than the unenhanced lines and tend to fluctuate wildly. Both the purely growing and the decay mode parametric instabilities are present. When the pump wave is turned on abruptly, the enhancements develop in time in a repeatable manner. A rather remarkable feature on time scales of seconds is an overshoot in instability power. Overshoots occur frequently but not universally and last for 1-6 s. They can have a magnitude from 10 to hundreds of times the average instability level. The growth of field-aligned irregularities may be the cause of the overshoots. The overshoots appear to be definitely related to an unusually rapid rise in measured electron temperature that cannot be understood in terms of ohmic energy deposition. On time scales of milliseconds th.•re is a 'miniovershoot' before the growth of the instability to a large value. The spectral details also cl•ange in a striking manner. The instabilities can first be detected 2-4 ms after pump wave turn-on. The decay mode is present as well as a broad featureless 'noise bump,' which partially sharpens into a line as time progresses. These changes of the spectra in time seem to run counter to the currently accepted theories of plasma wave saturation.

The Intensities of High Frequency‐Enhanced Plasma and Ion Lines During Ionospheric Heating

“…Eliasson and Papadopoulos () presented a numerical result that after the anomalous absorption at the upper hybrid resonance altitude, the fraction of the pump energy can reach the reflection altitude of the pump where Langmuir turbulence can be excited. Indeed, the anomalous absorption at the upper hybrid resonance altitude can lead to a decrease in the intensity of the pump when arriving at the parametric resonance altitude, then the intensity of the HFIL becomes weak in the HB (Das & Fejer, ; Fejer, ; Fejer et al, ; Fejer & Kopka, ; Gurevich, ; Showen & Kim, ). In addition, the enhancement in T e HB of up to ~ 35% should play an important role in the decrease in the intensity of the HFIL, where T e HB is the mean electron temperature in the HB.…”

“…Indeed, the overshoots in the HFPL and HFIL occur frequently but not universally (Kohl et al, ; Showen & Kim, ). In general, if f HF is fixed at a frequency very close to mf ce , the anomalous absorption of the pump falls considerably (Gurevich, ; Gurevich et al, ; Stocker et al, ; Stubbe et al, ), and T e on the traveling path of the enhanced Langmuir and ion acoustic waves will be somewhat enhanced.…”

<i>Letter to the Editor</i> UHF electromagnetic emission stimulated by HF pumping of the ionosphere

“…The instabilities lead to the development of different HF electrostatic plasma waves and LF disturbances (Hagfors et al, 1983;Showen et al, 1987;Minkoff et al, 1974). In their turn, the HF plasma waves generate the HF stimulated electromagnetic emissions (HF SEE) (Thide et al, 1982;Leyser et al, 1992Leyser et al, , 1993Karashtin et al, 1986), occurring as a result of conversion of the plasma modes into electromagnetic waves (Grach, 1985;Mjolhus, 1998;Grach et al, 1998), and accelerate electrons until energies 5-30 eV (Carlson et al, 1982).…”

Summer 2000 Student/Faculty Science Campaign, July 31-August 8, 2000

“…Radar scatter studies of the so-called plasma line' (PL) recorded during HF interaction experiments performed in the F-region of the ionosphere, have long demonstrated a complex, but reproducible, temporal and spatial structuring [Muldrew andShowen, 1977: Showen andKim, 1978;and Duncan and Sheerin, 1985]. A typical record of the time history of the PL recorded at Arecibo by Duncan and Sheerin (1985), shows an early' (a few milliseconds) return enhancement in the PL followed by a much stronger peak in the PL occurring tens of milliseconds later.…”