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
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 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:
Figure 1 illustrates the main types of storm: types 1, 2 and 3.
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.
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
Results
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:
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.
Conclusion
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.
The main conclusions were:
Further information:
Mullard Space Science Laboratory |
Project Outline |
Abstract |
Background |
Procedure |
Discussion |
Conclusion