Sensing impacts to the Earth-ionosphere waveguide from terrestrial and space weather

Abstract

The Earth's lower ionosphere forms the lower-altitude limit of space plasma near the Earth, coexisting with the neutral mesosphere-lower thermosphere at 60-150 km altitude. In-situ measurements of this region are made difficult by the significant neutral density there, preventing long-duration satellite orbits, while at the same time being too high for balloons to reach. The ionization of this region is perturbed by dynamic solar processes. Solar flares and precipitating radiation belt particles enhance low-altitude ionization, causing communication blackouts, and generating ozone-destroying radicals in the mesosphere. In order to quantify the global impacts of solar flare ionization and energetic electron precipitation (EEP) on low-altitude ionization, measurements of the spatial extent of solar flare and EEP signatures are needed. The Earth-ionosphere waveguide, a region formed by the lower ionosphere and conducting Earth surface, allows radio waves in the very low frequency (VLF) band to propagate long distances around the world. This fact is useful for communication and other technological means, and also allows for the location of global lightning by sensing the radio waves emitted by lightning strokes, called sferics. Previous work has analyzed signal propagation in this waveguide from both natural (lightning) and artificial (transmitter) sources, in order to study the response of the waveguide to space weather drivers. However, such efforts have focused on accurately measuring ionosphere conditions either at a small number of specific locations relative to source lightning, or else along long propagation paths between a small number of source-receiver pairs. Here, we present efforts to detect and quantify the spatial signatures of both solar flare and EEP ionization in the Earth-ionosphere waveguide using lightning location data from the World Wide Lightning Location Network. First, we demonstrate a method using only changes in the stroke-to-station propagation path distribution to infer ionization associated with solar flares, and compare the timing of enhanced ionization with geostationary X-ray flux measurements for two X-class solar flares in September 2017. Then, we expand the method to include sferic waveform information, calculate the range-normalized dispersion of each WWLLN sferic, and investigate changes in range-normalized dispersion statistics associated with enhanced >300 keV electron flux detected by POES spacecraft. We find that spatiotemporal averaging of WWLLN propagation paths, and of sferic properties associated with these paths, is an effective proxy for detecting the onset and extent of enhanced ionization during strong solar flares, and may be used to determine changes in the lower ionosphere height associated with EEP. Additionally, the Earth-ionosphere waveguide, a spherical capacitor, plays host to the global electric circuit, a current system carrying charge to the ionosphere from global thunderstorms and other drivers, where it then disperses horizontally and leaks through the fair weather atmosphere to the ground. The fair-weather current density in this circuit has been thought to be spatially invariant, as supported by simultaneous measurements of the electric field and conductivity by separated stratospheric balloons. Were this invariance to be well-established, studies to determine the relative importance of various drivers to the circuit could open the door for long-term observations of global thunderstorm activity and other large-scale atmospheric electrical parameters using small numbers of instrumented stratospheric balloons. In order to test this expectation, we launched two stratospheric balloons in June 2021 that measured electric fields and conductivity in the stratosphere over several days. We found that, in disagreement with previous findings, the two balloons measured vertical return current density that differed by as much as a factor of two. We compared the balloon tracks to WWLLN and spaceborne lightning detections, measurements of cloud-top temperature related to vigorous convection, and precipitation estimates, and found no significant weather near either balloon that could account for the discrepancy in the return current density. This result suggests that fair-weather component of the global electric circuit is not as spatially invariant as previously thought, and further investigation into the causes of this invariance is needed to determine the relationships between atmospheric electrical drivers and the global circuit response.