Observations of Langmuir Waves in the High-Latitude Winter Auroral Ionosphere

In late January and early February of 2003 the EISCAT Svalbard Radar (ESR), located on Spitsbergen in the Arctic Ocean at 78 degrees north latitude, was used to observe incoherent scatter plasma lines due to backscatter from ionospheric Langmuir waves over a frequency band covering approximately 3.3 to 4.7 MHz. The observations were performed during the early morning hours of 6 to 10 AM local time (5-9 UT). These are the first plasma line observations with the ESR, as plasma line receivers had been installed in the 32-m, fully-steerable antenna system just two weeks before. In addition, during 2002 the ESR data-taking system was upgraded to make it much easier to record unprocessed raw data, which in the case of this plasma line data provided a maximum time resolution of between 50 and 200 ms. A 550-microsecond pulse was used with four radar frequencies, transmitted in alternating pairs of pulses. During the observations the antenna was pointed along the geomagnetic field line (82-degree elevation angle). Both up- and downshifted and both bottomside and corresponding topside F-region plasma lines were observed, and simultaneous ion line data was also recorded. There were no local photoelectrons present during the observations, as it was dark (early morning, late January, 12 degrees latitude from the north pole) and thermal-level Langmuir waves are normally too weak to produce observable plasma lines. Most of the observed lines are likely due to backscatter from Langmuir waves enhanced by a background ``drizzle'' of energetic particles. The altitudes of some of the plasma line enhancements were quite irregular with time, changing by roughly 100 km in a period of a few seconds, which may reflect the irregular characteristics of the precipitating drizzle. There may also be some contributions from conjugate photoelectrons. Some of the enhancements may be associated with backscatter from nonthermally-enhanced ion-acoustic waves via the natural excitation by energetic particles of parametric and/or cavitating Langmuir turbulence. Several examples are presented which illustrate the characteristics of natural plasma lines in the high-latitude winter auroral ionosphere.