Strong Langmuir turbulence observed during EISCAT dual-radar high-frequency modi cation experiments

A major goal of ionospheric high power radio wave modiication experiments is the unraveling of the physics of the plasma turbulence created by the high frequency (HF) pump wave. The signature of this turbulence can be seen in the plasma line spectrum obtained from an incoherent scatter (sometimes called Thomson scatter) very high or ultra high frequency (VHF or UHF) radar probe beam. Here we present data obtained from the rst ever dual-radar chirped incoherent scatter observations. These data were taken at the EISCAT observatory in northern Scandinavia. EISCAT is unique in that it operates incoherent scatter systems at two distinct frequencies; it is also one of only three sites worldwide possessing co-located ISR and HF ionospheric modiication facilities. The data show a pump-induced line at the pump reeection height along with enhancements along the natural Langmuir frequency proole just below the reeection altitude. Together, these characteristics provide an unmistakable signature of strong Langmuir turbulence and show for the rst time that strong Langmuir turbulence physics occurs during high latitude experiments. Ionospheric high power radio wave modiication experiments have been performed since the early 1970s Gordon et al., 1971]. During these experiments a powerful HF pump wave having a frequency below that of the peak F region plasma frequency is transmitted using a predetermined pulse scheme. On a scale of seconds or minutes, the radio transmission can cause local changes in the ionospheric plasma density and electron and ion temperatures and can stimulate the growth of density irregularities. On shorter time scales of 100 ms or less, before thermal processes have had a chance to signiicantly aaect the background plasma, growth and saturation of instabilities and turbulence are the dominant eeects. Recent improvements in theoretical models have lead to signiicant advances in numerical simulations of the nonlinear plasma processes that occur in the rst 100 ms after the turn-on of the HF pump DuBois et al., 1993]. These results indicate that that strong and weak Langmuir turbulence can co-exist during this time period, and the calculated spectra show striking agreement with experimental incoherent scatter radar measurements taken during low latitude observations at Arecibo Sulzer and Fejer, 1994]. However, there has been signiicant controversy over whether the strong Langmuir turbulence inferred at Arecibo occurs during similar high-latitude experiments at EISCAT Stubbe et al., 1992a, b; DuBois et al., 1992]. Incoherent scatter plasma line diagnostics have been intimately involved in the study …