Quasi-thermal Noise

Quasi-thermal noise is caused by the thermally-induced motion of the ionized particles making up the plasma. The intensity and frequency spectrum of the quasi-thermal noise depends on plasma parameters such as the density and temperature, so those parameters can be deduced by measuring the quasi-thermal noise signal. These parameters are determined by fitting the measured data to a theoretical model.

The interplanetary medium is not in thermal equilibrium and thus cannot be described by a single temperature. It has been found that the electron component can often be described by a two-temperature distribution consisting of a (relatively) cold, dense core and a hot less-dense halo. The temperature of these components is also usually different from the proton component. The quasi-thermal noise can, under ideal conditions, yield values for the density and temperature of these components, and for the solar wind velocity. Under less than ideal conditions, these quantities may not be determined well, or at all. This happens especially under conditions of very low density which occurred when Ulysses was far from the Sun. For that reason, results from the quasi-thermal noise measurements are not available for extended periods.

The files containing quasi-thermal noise data are binary files. The files contain one or more records of data, each 80 bytes long. The content of each record is as follows:

Name Length Description
DATE/TIME 8 The date and time at which the data were collected. This is in UTC at the Ulysses spacecraft. The format is a double precision floating point number giving the number of days and fractions since January 1, 1982 (day zero). For example: Noon on January 2, 1982 has the value 1.5.
Fpe 4 Electron plasma frequency (kHz).
FpeSig 4 % error in Fpe.
Tc 4 Electron core temperature (K)
TcSig 4 % error in Tc.
RNhNc 4 Halo density/core density.
RNhNcSig 4 % error in RNhNc.
RThTc 4 Halo temperature/Core temperature.
RThTcSig 4 % error in RThTc.
Vsw 4 Solar wind speed (m/s).
VswSig 4 % error in Vsw
Tp 4 Proton temperature (K).
TpSig 4 % error in Tp.
SigFit 4 Overall standard deviation of fit (%).
Spares 20 These are unused.