Langmuir Waves and Electron Acceleration at Heliospheric Shocks
- Author(s): Pulupa, Marc Peter
- Advisor(s): Bale, Stuart D
- et al.
Radio waves at the local plasma frequency and its harmonic are generated upstream of collisionless shocks in foreshock regions which are magnetically connected to the shock. The radio waves are created in a multi-step process which involves the acceleration of electrons at the shock front, growth of electrostatic Langmuir waves driven by the accelerated electron beam, and conversion of the Langmuir waves into radio waves.
These radio waves can be used to remotely determine properties of the shock. For example, Type II solar radio burst observations yield information about the radial speed and angular extent of the coronal mass ejection-driven shock associated with the burst. However, in order to completely understand the generation of the radio waves and interpret the remote observations, in situ spacecraft measurements of the shock-accelerated electrons and Langmuir waves are necessary.
In this thesis, a brief introduction to the heliospheric environment is followed by a survey of the basic principles of collisionless shocks, a detailed discussion of the process of generating shock-accelerated electrons and the resulting plasma waves, and a description of the relevant instrumentation on board the Wind and STEREO spacecraft. Following this review material, several results based on in situ observations are presented:
(1) High cadence electron measurements made by Wind in the foreshock region of several IP shocks allow for a determination of the spatial scales of the source regions of Type II radio bursts. The sizes of the observed foreshock source regions are comparable to the size of the terrestrial bow shock.
(2) Langmuir waves upstream of IP shocks can be used as a diagnostic signature of foreshock electrons. Using a large database of shocks observed by Wind, different shock parameters are statistically tested for their effectiveness at accelerating electrons.
(3) Using a new type of electron detector on STEREO, the limits of the Fast Fermi theory for electron acceleration at suprathermal energies are examined. Preliminary results suggest that the mechanism may hold beyond the regime where the Larmor radius of electrons is much smaller than the scale sizes of the shock.
The prospects for future work in this area are discussed in the conclusion, and a description of an experimental antenna calibration procedure known as rheometry is included as an appendix.