We perform azimuth time tracking of multiple thunderstorm centers on the globe, which are sources of extremely low frequency (ELF) electromagnetic waves propagating in the spherical Earth-ionosphere cavity. For observations made in September 2023 we identify azimuths of numerous global emission centers using our data sampled at 3 kHz at the Hylaty station in Poland. We confirm significant and relatively regular thunderstorm azimuth variation during the solar terminator passage over the observation site. The magnitude and duration of the azimuth deviations depend on the observed azimuths but are also varying between successive days and changing detailed thunderstorm activity patterns. The measured maximum positive (preferentially at the sunrise time) and negative (preferentially at the sunset time) azimuth deviations reach even above 20 for waves propagating close to the terminator. We discovered also particular composite deviation structures, with the negative azimuth deviation directly preceding a larger positive one, as occurring during the morning terminator passage. At azimuths distant from the terminator one can observe decreasing of the regular deviation magnitude and occasionally lower magnitude deviations with opposite sign. Variations of the observed azimuth time structures between successive days are expected to result from varying thunderstorm activity on Earth as well variability of ionospheric parameters, in particular of charge gradients and nonuniformities occurring along the terminator. We postulate that the observed deviations result from a signal diffraction at the varying ionospheric gradients.

A document by Michal Ostrowski. Click on the document to view its contents.

The first demonstration of rocket exhaust driven amplification (REDA) of whistler mode waves occurred on 26 May 2020 by transferring energy from pickup ions in a rocket exhaust plume to EM waves. The source of coherent VLF waves was the Navy NML Transmitter at 25.2 kHz located in LaMoure, North Dakota. The topside ionosphere at 480 km altitude became an amplifying medium with a 60 second firing of the Cygnus BT-4 engine. The rocket engine injected exhaust as a neutral cloud moving perpendicular to field lines that connected the NML transmitter to the VLF Radio Receiver Instrument (RRI) on e-POP/SWARM-E. Charge exchange between the ambient O+ ions and the hypersonic water molecules in the exhaust produced H2O+ ions in a ring-beam velocity distribution. The 25.2 kHz VLF signal from NML was amplified by 30 dB for a period 77 seconds as observed by the RRI. Simultaneously, preexisting coherent ELF waves at 300 Hz were amplified by 50 dB during and after the Cygnus burn. Extremely strong coherent emissions and quasi-periodic bursts in the 300 to 310 Hz frequency range lasted for 200 seconds after the release. The excitation of an ELF whistler cavity may have lasted even longer, but the orbit of the SWARM-E/e-POP moved the RRI sensor away from the wave emission region. The amplified 300 Hz ELF waves may have gained even more energy by cyclotron resonance with radiation belt electrons as they were ducted between geomagnetic-conjugate hemispheres.

Observations of radio waves in the Extremely Low Frequency and Very Low Frequency band (ELF/VLF, 0.3-30 kHz) have a host of geophysical uses, including lightning detection and characterization, D-region ionosphere remote sensing, detection of solar flares and geomagnetic storms, gravity waves, gamma-ray burst detection, observations of whistlers, chorus and hiss, to infer wave-particle interactions in the magnetosphere, plasmaspheric state. It’s been looked at for earthquake forecasting and also has commercial uses like submarine communications and subterranean prospecting. For many years ELF/VLF data have been collected at various locations and by various groups around the world for a variety of scientific purposes, but most of this data is not available publicly. We introduce the World Archive of Low-frequency Data and Observations (WALDO), a repository of ELF/VLF data from recordings taken over the past two decades by Stanford University and subsequently by Georgia Tech and University of Colorado Denver. The locations of the recordings are all around the world, including Alaska, Antarctica, Australia, and many low and mid latitude stations. Some sites were more consistent than others but there’s a lot of untapped value in this dataset. Funding for these recordings came from many years of funding from NSF, NASA, DoD, and others, on various basic science projects, and we feel a responsibility to make sure the datasets are now preserved. We are in the process of transferring many 100s of TBs of data and sharing every raw bit for anyone to download and analyze. This includes both “broadband” data that includes the entire spectrum from 500 Hz – 50 kHz, and “narrowband” data corresponding to amplitudes and phases of specific transmitting beacons. We are also including automatically generated summary plots, and a host of basic analysis tools to allow anyone to download and analyze the data. We will announce and present WALDO, update its status and timeline for full deployment, and detail some of the uses of ELF/VLF data, with the goal of enabling its use by anyone interested. We will not be finished by the Fall meeting (ripping 80,000 DVDs can take a while) but whatever we finished will be public and hopefully we will be far along by then. Finally, we will have the answer to the age-old question…”Where’s WALDO?”