Tan, J H; Cohen, M; Inan, U S; Scherrer, P H; Scherrer, D
"VLF Remote Sensing of the Lower Ionosphere:
Solar Flares, Electron Precipitation, Sudden Ionospheric Disturbances,
Sprites, Gravity Waves and Gamma-ray Flares"
Stanford University Very Low Frequency (VLF) and Extremely Low Frequency (ELF)
radio receivers have been used extensively for remote sensing of the
ionosphere and the magnetosphere.
Among the phenomena that can be uniquely measured via ELF/VLF receivers
are radio atmospherics, whistlers, electron precipitation, solar flares,
sudden ionospheric disturbances, gravity waves, sprites,
and cosmic gamma-ray flares. With the use of simple square
air-core magnetic loop antennas of a couple of meters in size,
the sensitivity of these instruments allows the measurement of magnetic
fields as low as several tens of femtoTesla per root-Hz,
in the frequency range of ~30 Hz to 50 kHz.
This sensitivity well exceeds that required to detect any event above
the ambient atmospheric noise floor, determined by the totality of
lightning activity on the planet. In recent years, as cost of production,
timing accuracy (due to low cost GPS clocks), and data handling
flexibility of the systems has improved,
it has become possible to distribute many of these instruments in the form of
arrays, to perform interferometric and holographic imaging of the
lower ionosphere. In the context of the IHY in 2007, the ELF/VLF
receiver can used extensively as part of the United Nations initiative
to place scientific instruments in developing countries.
Stanford University's past experiences setting up arrays of
ELF/VLF receivers include an interferometer in Alaska,
the Holographic Array for Ionospheric and Lightning research (HAIL)
consisting of instruments at 13 different high schools in
mid-western United States, a broader set of ELF/VLF receivers in
Alaska, and various receivers abroad, including in France,
Japan, Greece, Turkey, Ireland, and India.
A global network of ELF/VLF receivers offer possibilities for a
wide range of scientific topics, as well as serving as a
means for educational outreach. These goals will be achieved using
the newest version of the Stanford VLF receiver, known as AWESOME:
Atmospheric Weather Educational System for Observation and Modeling
of Electromagnetics. This new version is
substantially lower in cost, and easier to set-up and use.
Nevertheless, the receivers offer the same ultimate levels of
resolution in time, sensitivity and dynamic range, as well
as ease of handling of data that is used by researchers conducting
cutting edge ionospheric and magnetospheric research. In this context,
the placement of these systems at underdeveloped host countries provides
an open-ended potential for exploration, limited only by the imagination and
drive of the users. AWESOME monitors can be placed at schools,
or universities, where they will serve the dual purpose of advancing scientific
research, as well as providing a valuable tool for scientific education.
Data collected can be pooled and publicly available to all the sites,
strengthening the potential for both cooperative education and
collaboration on the science between various regions and locations.