Search for: All records

Abstract We demonstrate a methodology for utilizing measurements from very low frequency (VLF, 3−30 kHz) transmitters and lightning emissions to produce 3D lower electron density maps, and apply it to multiple geophysical disturbances. The D‐region lower ionosphere (60−90 km) forms the upper boundary of the Earth‐ionosphere waveguide which allows VLF radio waves to propagate to global distances. Measurements of these signals have, in many prior studies, been used to infer path‐average electron density profiles within the D region. Historically, researchers have focused on either measurements of VLF transmitters or radio atmospherics (sferics) from lightning. In this work, we build on recently published methods for each and present a method to unify the two approaches via tomography. The output of the tomographic inversion produces maps of electron density over a large portion of the United States and Gulf of Mexico. To illustrate the benefits of this unified approach, daytime and nighttime maps are compared between a sferic‐only model and the new approach suggested here. We apply the model to characterize two geophysical disturbances: solar flares and lower ionospheric changes associated with thunderstorms. 

Abstract Lightning‐induced Electron Precipitation (LEP) is a known process of electron loss in the Earth's radiation belts. An LEP event progresses with Very Low Frequency (VLF) radio wave radiation from lightning, trans‐ionospheric propagation, and wave‐particle gyroresonance interaction with energetic radiation belt electrons. Pitch angle scattered electrons then precipitate onto the ionosphere, allowing detection using VLF remote sensing using high power transmitters. The relative importance of LEP events as a radiation belt electron lifetime driver has heretofore been unclear. We build off a massive database of LEP events observed within the continental US (CONUS) by a network of VLF receivers. For each observed LEP event, based on the characteristics of the ionospheric disturbance, we apply a suite of models to estimate the total number of precipitating electrons, which we can then sum up over all LEP events to quantify lightning's contribution within CONUS. We find that LEP events within CONUS appear to be capable of removing a substantial fraction (up to 0.1%–1%) of radiation belt electrons between 33 and 1,000 keV, and may have stronger contributions to radiation belt losses than earlier estimates. 

Abstract The D‐region ionosphere (6090 km) plays an important role in long‐range communication and response to solar and space weather; however, it is difficult to directly measure with currently available technology. Very low frequency (VLF) radio remote sensing is one of the more promising approaches, using the efficient reflection of VLF waves from the D‐region. A number of VLF beacons can therefore be turned into diagnostic tools. VLF remote sensing techniques are useful and can provide global coverage, but in practice have been applied to a limited area and often on only a small number of days. In this work, we expand the use of a recently introduced machine learning based approach (Gross & Cohen, 2020,https://doi.org/10.1029/2019JA027135) to observe and model the D‐region electron density using VLF transmitting beacons and receivers. We have extended the model to cover nighttime in addition to daytime, and have applied it to track D‐region waveguide parameters, h’ and, over 400 daytimes and 150 nighttimes on up to 21 transmitter‐receiver paths across the continental US. Using an exponential fit, h’ represents the height of the ionosphere andrepresents the slope of the electron density. Using this data set, we quantify diurnal, daily and seasonal variations of the D‐region ionosphere for both daytime and nighttime D‐region ionosphere. We show that our model identifies expected variations, as well as producing results in line with other previous studies. Additionally, we show that our daytime predictions exhibit a larger autocorrelation at higher time lags than our nighttime predictions, indicating a model with persistence may perform better. 

Abstract We present a new four‐parameter model of theD‐region (60–90 km) ionospheric electron density, useful in very low frequency (VLF, 3–30 kHz) remote sensing. VLF waves have a long history of use to indirectly inferD‐region conditions, as they reflect efficiently and thus are sensitive to small changes in the electron density. Most historical efforts use VLF observations along with a forward model of theD‐region and VLF propagation. The ionospheric assumptions in the forward model are altered until the output matches the observation. The most commonD‐region model, known as the Wait‐Spies ionosphere, takes the electron density as exponentially increasing with altitude and specifies a height and steepness. This model was designed to capture the VLF propagation variations evident at a single frequency. The realD‐region is likely more complex. The limited number ofD‐region rocket passes that have previously been compiled tend to show the existence of a “ledge” somewhere between 70 and 90 km. Broadband VLF signals emitted from lightning allows a more sophisticated parametrization. Using carefully averaged amplitudes and phases of VLF sferics, we formulate a more general four‐parameterD‐region model that includes a ledge discontinuity. Using lightning‐emitted VLF observations along with a theoretical model, we find that this model better describes the ionosphere during the daytime. During the ambient nighttime and during a solar flare the two‐parameter ionosphere may be sufficient, at least for the purposes of calculating broadband VLF propagation, since the ledge either weakens or moves outside the altitude range of VLF sensitivity. 

Abstract Lightning induced perturbations of the lower ionosphere are investigated with very low frequency (VLF) remote sensing on a unique overlapping propagation path geometry. The signals from two VLF transmitters (at different frequencies) are observed at a single receiver after propagation through a common channel in the Earth‐ionosphere waveguide. This measurement diversity allows for greater certainty in quantification of perturbations to the ionosphericDregion. Changes in amplitude and phase are modeled with the Long Wave Propagation Capability (LWPC) software package to quantify changes in reference height and steepness of the two parameterDregion electron density model. Since the nighttimeDregion profile prior to the perturbation is found to strongly affect the resulting quantification, and is highly variable and generally unknown at nighttime, an error minimization method for identifying the most likely ionospheric disturbance independent of the ambient profile is used. Analysis of 12 large lightning perturbations resulting from discharges with peak currents greater than 160 kA shows that the ionospheric reference height can change by 2–8 km. We investigate both early/fast events (direct ionization and heating from lightning) and lightning‐induced electron precipitation (LEP) events, induced by lightning hundreds of kilometer away. LEP events increaseDregion electron density while early/fast events can lead to a increase or decrease in electron density. Multi‐point observations along a VLF propagation path are needed to further improve ionospheric perturbation quantification with VLF remote sensing. 

Abstract The lowest region of the ionosphere, theDregion, plays an important role in magnetosphere‐ionosphere coupling but is challenging to directly observe. The group velocity of the extremely low frequency (ELF; 3–300 Hz) portion of lightning induced electromagnetic radiation can be used to diagnose theDregion electron density profile. Day‐night conditions can be assessed using ELF receivers and lightning detection networks. Analytical formulations and the Long Wave Propagation Capability software package show that ELF group velocity has particular sensitivity to the sharpness of the exponential electron density profile. Applying the technique to sudden ionospheric disturbances shows that the group velocity increases in response to incidence of solar X‐ray flux . A small number of ELF receivers can provide a large‐scale diagnostic of theDregion. ELF remote sensing using lightning is complementary to very low frequency remote sensing and can be used to assess the Earth‐ionosphere propagation channel for very low frequency transmitters.