Whistler mode chorus waves in acceleration of electrons to relativistic energies in Earth's outer radiation belt
Whistler mode chorus and electron acceleration study via Combined Release and Radiation Effects Satellite data

4C00 Project Summary
Michelle Cain, mc@mssl.ucl.ac.uk (now at m.l.cain@rdg.ac.uk)
Supervisor: Dr. Nigel Meredith, npm@mssl.ucl.ac.uk


Further information: Mullard Space Science Laboratory | Project Outline | Abstract | Background | Procedure | Discussion | Conclusion
See also: Meredith, N. P., M. Cain, R. B. Horne, R. M. Thorne, D. Summers, and R. R. Anderson, Evidence for chorus-driven electron acceleration to relativistic energies from a survey of geomagnetically-disturbed periods, J. Geophys. Res., 108(A6), 1248, 10.1029/2002JA009764, 2003.
Introduction
The Combined Release and Radiation Effects Satellite (CRRES) was launched on July 25, 1990. It took readings of the plasma wave and particle environment in the Earth's radiation belts over a period of almost 15 months. This data from CRRES was used in this project to determine the mechanism that causes electrons in the outer radiation belt to be accelerated to relativistic velocities, as observations have shown.

The hypothesis put forward is that whistler mode chorus waves interact resonantly with electrons in the Earth's outer radiation belt, and so accelerate electrons of a few hundred keV up to MeV energies. The aim of this project was to study the CRRES data, and see whether the observations support this hypothesis.

This topic is of interest to scientists, as the theories describing the radiation belts do not at present adequately explain dynamic behaviour during geomagnetic storms. In addition, there is a new commercial interest in this subject. In recent years, a significant amount of money has been lost due to damage to spacecraft in orbit. The radiation belts can be an extremely hazardous environment for orbiting satellites and astronauts. For this reason, space insurers are particularly interested in evaluating all possible risks in space. A reliable model of the dynamic behaviour of the radiation belts (which includes the behaviour of relativistic electrons) would help in risk assessment.

Definitions
Dst Index Disturbance storm time index. This is a measure of the magnetospheric ring current. An increase in the ring current gives a negative excursion of the Dst.
Geomagnetic storm A geomagnetic storm is a period of intense geomagnetic activity. The storm can be best described by the Dst index (as seen in figure 1). A storm will have a main phase, in which the Dst will decrease rapidly to a minimum. A weak storm will have -30 nT > Dst min > -50 nT ; A moderate storm will have -50 nT > Dst min > -100 nT ; A strong storm will have -100 nT > Dst min > -300 nT. After the main phase is the recovery period, in which the Dst gradually recovers back to its prestorm level of ~0 nT.
Substorm activity A substorm occurs when there is an increased level of energy transfer from the solar wind to the magnetosphere. The substorm cycle typically lasts for one to three hours, causes a magnetic disturbance of 200 nT to 2000 nT, and can be repeated several times a day. Auroral electroject (AE) index is a good measure of substorm acivity (the black line on figure 1).
Whistler mode chorus waves Whistler mode waves are electromagnetic waves that travel in a direction parallel to the magnetic field. They are right handed waves, i.e. the electric field rotates in the same direction as an electron gyrates, and the right hand rule can be applied with the thumb pointing in the direction of the wave vector, to find the direction of the electric field. Amplitude enhancements are seen when there is enhanced substorm activity.

Theory
The aim of this project is to determine whether the hypothesis that whistler mode chorus waves accelerate a seed population of sub-relativistic electrons in the outer radiation belt to relativistic energies is supported by the data.

The strength of the interaction between the whistler waves and the electrons depends on the wave amplitude squared, so in order for electrons to be accelerated to relativistic speeds, the waves must be enhanced. Enhancements occur both during geomagnetic storms, and during periods of substorm activity. For this reason, this project examines data during active events that occur in the CRRES data.

There are four kinds of event, as defined in this project:

Type 1 Main phase of storm has a moderate to strong negative Dst excursion, with extended substorm activity during the recovery period. e.g. day 242-244 on figure 1
Type 2 Main phase of storm has a moderate to strong negative Dst excursion, with little substorm activity during the recovery period. e.g. day 244-246 on figure 1
Type 3 There is no geomagnetic storm (no negative excursion of Dst) but there is a period of enhanced substorm activity. e.g. day 247-250 on figure 1
Type 4 A weak geomagnetic storm (Dst has a weak negative Dst excursion) but there is a period of enhanced substorm activity. This is half way between a type 1 and a type 3. not shown on figure 1

Figure 1 illustrates the main types of storm: types 1, 2 and 3.


Figure 1: click image for larger version

Results
Software was run to produce plots (such as figure 1) of the data, and all the geomagnetic events were identified and classified. As the proposed acceleration mechanism was via whistler chorus waves, the wave amplitudes were averaged for the recovery period (the period in which the acceleration should occur) of each event. However, wave data was not available for every event, so secondary measures of wave activity were taken. As the AE index is a good measure of wave activity, the percentage of the recovery period that the AE was active (AE > 300 nT) was found, as well as the percentage of the recovery period that the AE was moderate (AE > 100 nT). These three quantities gave adequate measure of wave activity in most events.

These quantities were plotted against relativistic electron flux change at various radial distances, to show how the wave activity affected the level of relativistic electron flux (i.e. amount of acceleration). Several different processes were observed in these results, as there are many mechanisms for electron gain and loss in the outer radiaiton belt. However, there were also general trends within the data. Most obviously, there was a positive correlation between wave enhancements and relativistic electron flux increase. This can be seen in figure 2:


Figure 2: click image for larger version

Conclusion
The main conclusions were:

Overall, the results did support the hypothesis. Resonant interactions between whistler chorus waves and sub-relativistic electrons could cause acceleration to relativistic energies in this data. However, it remains for the process to be mathematicaly modelled in order to have conclusive evidence of this mechanism.


Further information: Mullard Space Science Laboratory | Project Outline | Abstract | Background | Procedure | Discussion | Conclusion

Michelle Cain, March 2002