ELECTRON-PRECIPITATION CAUSED BY CHAOTIC MOTION IN THE MAGNETOSPHERE DUE TO LARGE-AMPLITUDE WHISTLER WAVES

In the magnetosphere, energetic electrons in the radiation belts are t rapped by Earth's magnetic field and undergo bounce motion about the g eomagnetic equator. When a large-amplitude whistler wave is present, t he motion of the electrons becomes perturbed. It is shown that the non linear interaction due to the spatial dependence of the field quantiti es causes the motion of some of the trapped particles to become chaoti c. Contrary to considering a gyroresonant interaction, this chaotic sc attering does not have a directional preference and may therefore offe r a plausible explanation of the simultaneous observation of the elect ron precipitation into the upper atmosphere at geomagnetically conjuga te regions due to a single lightning flash [Burgess and Inan, 1990]. A fter simplifying the dipole configuration of the geomagnetic field, a Hamiltonian formulation is used to study the dynamics of a single, tra pped electron on the L=3 shell, subjected to a large amplitude 13.7 kH z whistler wave. A canonical transformation is introduced to remove th e time dependence from the test electron's Hamiltonian. The chaotic be havior of the electron motion is investigated with surface of section and Lyapunov exponent techniques. To show that this chaotic behavior c an lead to particle precipitation, the temporal evolution of the equat orial pitch angle of the electron is computed. Considering electrons w ith an initial pitch angle of 88 degrees, the results are found to be qualitatively independent of the bounce frequency. They show that the equatorial pitch angle of a chaotic electron varies wildly and often d ips below 25 degrees, the minimum loss cone angle one would expect to find for a charged particle in the magnetosphere. Therefore the electr ons may escape the geomagnetic trap and be precipitated into the upper atmosphere.