Upper Atmosphere Group

Plasma Line Research

Plasma line frequency

The plasma line is a well known, but little used component of the incoherent scatter radar spectrum. It is a signal scattered from high frequency electron waves, Langmuir waves. Two plasma lines can be detected by the radar, one upshifted from the transmitted frequency, and one downshifted. These lines correspond to scattering from downgoing and upgoing Langmuir waves respectively. The frequency shift from the transmitted signal is the frequency of the scattering Langmuir wave plus the Doppler shift caused by the bulk motion of the electron gas. The wave vector of the scattering Langmuir wave is determined by wave vector matching requirements with the incident and reflected electromagnetic waves. The reflected waves are shifted in frequency, and the up and downshifted plasma lines will have slightly different wave vectors. In practice this difference in wave vectors leads to significant differences in offset frequency and if the Doppler shift is to be measured the dispersion relation of the Langmuir waves must be known with good accuracy. The frequency of the Langmuir wave is close to the plasma frequency, i.e. a few MHz for most realistic conditions. The wave number dependence of the Langmuir wave dispersion is primarily dependent on the electron temperature.

Plasma line intensity

The Langmuir waves in a thermal plasma are relatively weak, often too weak to be usable for incoherent scatter radar measurements. In a thermal plasma the wave intensity is proportional to the electron temperature, but when superthermal electrons are present they can greatly enhance the Langmuir wave (and thus plasma line) intensity. This is because the waves are driven by electrons with a velocity component along the wave vector matching the phase velocity of the wave (Cherenkov emission), and damped by Landau damping. The Landau damping is proportional to the slope of the (one-dimensional) electron distribution function at velocities matching the wave phase velocity, while the Cherenkov emission is proportional to the number of electrons, i.e. the value of the one-dimensional distribution function, so the shape of the electron distribution function will determine the strength of the plasma line. For low fluxes (e.g. at high phase velocities), and low altitudes, collisions will damp the Langmuir waves toward their thermal level.

Use of plasma lines

Plasma lines have a number of potential uses:
  • The system constant for the incoherent scatter radar can be determined with good accuracy using plasma line frequency measurements.
  • Electron temperatures can be determined independently of the ion line analysis.
  • By fully combining ion lines and plasma lines in the analysis of incoherent scatter radar data, it is also possible to resolve the temperature / composition ambiguity in the ion line autocorrelation function.
  • High time resolution electron temperature estimates can also be made using a combination of ion and plasma lines.
  • Measurements of the plasma line strength in restricted frequency intervals (i.e. filter banks) can be used to estimate the suprathermal electron flux in different energy ranges. In this case, however, Landau or collisional damping terms must be significant, as the intensity of the superthermal flux only matters for its relative importance as compared to other influences. .
  • Maybe the most promising aspect of the plasma lines is the possibility to measure the electron drift from the Doppler shift of the signal. There are, however, still some theoretical uncertainties in the interpretation of the measurements.

    Predictions for when to measure plasma lines

    There is thus much to be gained from an increased use of plasma lines especially for the possibility to measure field aligned currents, where the remaining theoretical uncertainties can only be resolved by further measurements. It is then very valuable to be able to choose those circumstances when plasma line measurements are possible. We have used state of the art model calculations of the superthermal electron distribution for both sunlit and nighttime auroral conditions, and calculated the expected plasma line intensity for different radars (i.e. different transmitter frequencies) and different conditions.

    The calculated plasma line intensities have also been used to predict the actual signal to noise ratio for different radars, to give a guide to when plasma line measurements should be made with the different radars. The predictions for different radars can be found here:

  • EISCAT UHF radar (930 MHz)

  • EICAT UHF remote sites

  • Søndre Strømfjord radar (1280 MHz)

  • EISCAT VHF radar (224 MHz)

  • EISCAT Svalbard radar (500 MHz)

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    For comparison, we also show a plot of the midday F-region peak plasma frequency from Kiruna ionograms for 30 years. The seasonal and solar cycle dependance is very clear. For the EISCAT UHF and Søndre Strømfjord incoherent scatter radars solar maximum conditions are needed for good plasma line measurements. At other times, the lower frequency radars are to be preferred.

    Publications

    Kirkwood, S., H. Nilsson, J. Lilensten, M. Galand, Strongly enhanced incoherent-scatter plasma lines in aurora, J. Geophys. Res.,100, 21343-21355, 1995

    Nilsson, H., Kirkwood, S., Bjørnå, N., Bistatic measurements of incoherent-scatter plasma lines, J. Atmos. Terr. Phys., 58, 175-187, 1996

    Nilsson, H., S. Kirkwood, J. Lilensten, M. Galand, Enhanced incoherent scatter plasma lines, Ann. Geophys, 14, 1462-1472, 1996

    Current and future work

  • Calculate the influence of suprathermal electron fluxes on the dispersion of Langmuir waves. The model fluxes previously used for predictions of the strength of plasma lines will be used. The work is planned to be completed during 1997

  • Study EISCAT VHF plasma lines in aurora. Notably, we will analyse the results from a VHF auroral plasma line campaign run in June 1996. Work to be finished late 1997 or early 1998

  • Study EISCAT UHF daytime plasma lines for further studies of the dispersion of Langmuir waves in the ionosphere. Measurements to be performed during the next solar maximum.

  • Study EISCAT UHF plasma lines in aurora. When the theory is complete, this will provide direct observation of local thermal currents in and near auroral arcs.

  • Study the strength of daytime plasma lines from the EISCAT Svalbard radar. This can among other things be used for absolute calibrations of the Svalbard system.