Upward Propagating Electric Fields from Thunderstorms and VLF Transmitters.
In a pioneering experiment simultaneous rocket -borne electric field observations were made in the ionosphere and atmosphere over a nighttime thunderstorm. Lightning -induced transient electric fields were measured in the ionosphere with amplitudes of tens of mV/m with rise times as fast as 0.2 ms and typical durations of ~ 10 ms. The transients had significant components parallel to the Earth's magnetic field in the ionosphere. Both the parallel and perpendicular electric fields are large by ionospheric standards. We expect that large electron drift velocities associated with these fields can drive collective instabilities. A simple analysis shows that the lightning-induced transients result from an electrostatic source. A lightning stroke moves large quantities of charge from one location to another. This sets up a different electrostatic configuration that propagates upward as an electromagnetic disturbance and drives currents in the atmosphere and ionosphere. We attribute the rise time characteristics of the transients to the rapid change in the charge distribution around the storm and the decay characteristics to the response of the atmosphere and ionosphere to the new electrostatic configuration. The data suggest a new model for whistler generation wherein the ionosphere is viewed as an aperture antenna. Other whistler observations are also presented. Also detected during the rocket flights were standing wave patterns from two VLF transmitter signals. These were generated by reflections off a sporadic-E layer. We were able to use these patterns in a unique manner to determine electron concentration. Langmuir probes are not reliable for measuring the low electron densities in the nighttime E-region, however, we were able to calibrate our probe using the transmitter data. It is possible to make this sort of electron density measurement without a standing wave pattern and we have outlined this method. The transmitters provide single frequency measurements that we have modelled using a fullwave technique. This model will act as a basis for future modelling of the broadband lightning signals.
- Pub Date:
- Physics: Atmospheric Science; Engineering: Electronics and Electrical