Electrostatic Bursts Generated by Electrons Trapped in Whistler Mode Chorus Wave Fields.
Abstract
Recent studies of wideband plasma wave data from the ISEE-1 and ISEE-2 spacecraft have revealed that whistler mode chorus emissions in the Earth's outer magnetosphere are often accompanied by high frequency bursts of electrostatic waves with a frequency below the electron plasma frequency. Investigations have shown that in some cases the electrostatic waves are found to be modulated at the chorus frequency. Further studies indicate through use of LEPEDEA data on ISEE-1 that there bursts are being produced by a "beam" of electrons trapped in Landau (longitudinal) resonance in effective potential wells of the chorus wave and thus moving at the chorus phase velocity. There seems to be a lower threshold in chorus intensity, below which the electrostatic bursts do not appear. The high frequency electrostatic waves appear to be caused by a type of two-stream instability called the resistive-medium instability, produced by these trapped electrons. The reduction in the electrostatic burst frequency to below the plasma frequency is a characteristic of the resistive-medium instability. The instability is applicable only in the regime where V(,o)/V(,T) is on the order of 1, where V(,o) is the velocity of the beam and V(,T) is the averaged thermal velocity of the plasma electrons. The derivation assumes cold ions but warm electrons in the plasma. The instability requires Landau damping to operate, and thus the beam velocity must be in the steep slope region of the plasma electron distribution function rather than the high velocity tail region. In examined cases from LEPEDEA data the electron thermal energies are on the order of a few hundred eV. The beam velocities in the observed cases were (DBLTURN)400 eV and (DBLTURN)630 eV, thus verifying that the electrostatic bursts are in the proper regime for the resistive-medium instability. Evidence is given that the resistive-medium instability is a plausible generation mechanism for the electrostatic bursts. Several cases are examined which suggest the relevance of these electrostatic bursts to features observed in space physics data for regions other than the Earth's magnetosphere.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- July 1982
- Bibcode:
- 1982PhDT........10R
- Keywords:
-
- Physics: Fluid and Plasma;
- Dawn Chorus;
- Earth Magnetosphere;
- Electrostatic Charge;
- Electrostatics;
- Plasma Waves;
- Damping;
- Electron Distribution;
- Plasma Frequencies;
- Trapped Particles;
- Geophysics