This award is to address outstanding scientific questions in the areas of magnetospherically generated whistler-mode waves, thunderstorm activities and global lightning discharges associated with terrestrial gamma-ray flashes, and their ionospheric effects by investigating experimental data collected by the ELF/VLF radio receiver deployed at Palmer Station, Antarctica. During and in the aftermath of geomagnetic storms wave-particle interactions result in large variations of the energetic electron population that form the Earth's radiation belts. Magnetospherically generated emissions from those wave-particle interactions are regularly observed at Palmer Station and can be used to determine magnetospheric source regions of these waves and their influence on the radiation belts dynamics. Palmer data will also be used to explore the outstanding question of the source of plasmaspheric hiss emissions, which may be terrestrial lightning or magnetospheric chorus. In addition, several scientific and industrial enterprises rely on the real-time detection and geolocation of lightning strikes from Palmer. Continued wave observation at Palmer will allow for the expansion and refinement of the current geolocation algorithm. Terrestrial gamma-ray flashes (TGF) have been strongly associated with lightning discharges, but the exact mechanism involved in their generation remains unknown. The high signal-to-noise ratio of sferics recorded at Palmer can be used to determine the properties of TGF-associated lightning, including which particular types of lightning may cause TGFs, and their geographical source. In terms of broader impacts, the research program will advance discovery and understanding while promoting teaching, training, and learning through the participation of graduate and undergraduate students in the project. The program will enhance infrastructure for research and education through collaborations and partnerships with national and international institutions.

Project Report

Under this award, Stanford University operated a very low frequency (VLF) radio receiver at Palmer Station, Antarctica that measures and records electromagnetic waves in 3-30 kHz frequency range. The magnetic component of the electromagnetic waves are received via two orthogonal, 18 meter base, triangular cross-loop wire antennas, and the two channels of data are sampled at 100 kilosamples per second with approximately 96 dB of dynamic range. The system is capable of measuring signals on the order of a femtotesla. The primary purpose of the receiver is to record waves that are naturally generated in the Earth’s high altitude plasma environment, a region known as the magnetosphere. The waves are generated by a variety of plasma processes and can propagate for long distances from the magnetospheric source region to low altitude and through the ionosphere before being measured by the receiver at Palmer. Due to its remoteness from anthropogenic electromagnetic noise sources, Palmer Station is one of the most electromagnetically quiet VLF receiving sites in the world. The waves generated in the magnetosphere and observed on the ground at Palmer play an important role in the transfer of energy and momentum in the Earth’s magnetosphere. This energy transfer occurs as the result of cyclotron resonant wave-particle interactions between the waves and energetic electrons that are geomagnetic trapped in the Earth’s magnetosphere. During geomagnetic storms that are driven by solar disturbances, the distribution of energetic electrons in the magnetosphere can change dramatically, and wave-particle interactions are believed play a major role both the removal of electrons from the magnetosphere as well as in the acceleration of low energy electrons up to relativistic energies. Under this program we performed various studies to unravel wave propagation effects in order to identify the source region and estimate the source amplitude of magnetospheric emissions observed on the ground. The results obtained allow us to properly interpret of ground-based data in studies of radiation belt dynamics during major space weather disturbances. In one particular study, we used a decade long database of wave observations in order to examine how the wave occurrence, amplitude and frequency extent varied as a function of storm phase, storm size and storm type and also examine the relationship between ground-based wave observations and changes in the energetic electron distributions as observed in space. We applied statistical hypothesis testing to determine whether the observed quantities are statistically significantly different as a function of storm phase and type. Through our analysis, we concluded that waves generated in the inner magnetosphere and observed on the ground at Palmer Station are significantly enhanced during storm main phase and for more than four days into recovery. During main phase, there are larger enhancements in wave occurrence, amplitude, and frequency extent as observed at Palmer during larger storms. During recovery phase, there are larger enhancements in wave occurrence, amplitude, and frequency extent as observed at Palmer during larger storms and during storms where the average rate of electron flux increase is higher. Our results contribute significantly to the body of observational evidence of the role of wave-particle interactions in the acceleration of magnetospheric electrons in the aftermath of geomagnetic storms.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
Standard Grant (Standard)
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Vladimir O. Papitashvili
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Stanford University
Palo Alto
United States
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