Experiments consisting of the transmission and receiving of ELF ($300-3000 \mathrm{~Hz}$) whistler-mode waves were conducted on board the DSX satellite. In several events, the transmitted waves were received by the same antenna on the satellite after a time delay of several hundred milliseconds. These events suggest that the transmitted waves propagate mostly along the ambient magnetic field, reflect at some location along their path, and return to the satellite. The main questions related to these observations are: 1) What causes the guiding of the waves along the ambient magnetic field? and 2) Where in the magnetosphere are these waves reflected? We present results from analytical and numerical studies indicating that the waves are guided by the field-aligned density inhomogeneities (aka ducts), and they are reflected from the ends of the duct.

Experiments consisting of the transmission and receiving of ELF (300-3000 Hz) whistler-mode waves were conducted on board of DSX satellite. In several events, the transmitted waves were received by the same antenna on the satellite after a time delay of several hundred milliseconds. These events suggest that the transmitted waves propagate mostly along the ambient magnetic field, reflect at some location along their path, and return back to the satellite. The main questions related to these observations are: (1) What causes the guiding of the waves along the ambient magnetic field? and (2) Where in the magnetosphere are these waves reflected? The results from analytical and numerical studies presented in this paper indicate that the waves are guided by the field-aligned density inhomogeneities (aka ducts), and they are reflected from the ends of the duct.

The Air Force Research Laboratory's Demonstration and Science Experiments (DSX) spacecraft carried a high-voltage very low frequency transmitter and a sensitive broadband receiver to medium Earth orbit in 2019. During many pulsed transmission experiments, DSX detected apparent "boomerang" echoes when its emitted waves refracted in the magnetosphere and returned to the spacecraft. We simulated a series of these detected pulses using cold plasma ray tracing to characterize their likely wavelengths, indices of refraction, and initial wave normal angles. The waves were shown to remain relatively local to DSX, to be lightly damped, and to have a wide variety of wavelengths and indices of refraction, but they were all emitted with very oblique wave normal angles tightly clustered about half a degree from the Gendrin angle, which theoretical antenna models predict is preferentially excited. Our results are remarkably consistent with this prediction but are statistically biased closer to the resonance cone, possibly because of limitations in the ray tracing technique. The result is robust to perturbations of the simulation and confirms a very narrow beam of oblique radiation quite unlike the behavior of a dipole in vacuo.

The Air Force Research Laboratory's Demonstration and Science Experiments (DSX) mission investigated wave-particle interactions and the particle and space environment in Medium Earth Orbit (MEO) from June 2019 to May 2021. Its Wave-Particle Interactions Experiment conducted over 1,300 active high power very low frequency transmissions in the radiation belts providing observations of antenna performance and signal propagation from a controlled source. This included hundreds of transmissions while in magnetic conjunction with other satellites. The Loss Cone Imager and Space Weather Experiment suite observed electron and proton populations over a wide energy range, with several of these instruments providing pitch-angle resolution. The Space Environmental Effects Experiment investigated effects of the MEO environment on electronics and materials. The Adaptive Controls Experiment demonstrated technology for on-board identification and control of large structure vibrational modes. We describe the DSX instrument capabilities and on orbit performance, science planning and operations for carrying out an array of active and passive experiments, and some initial results in brief. We also describe plans for further work and data release.

The Air Force Research Laboratory (AFRL) operated the Demonstration and Science Experiments (DSX) spacecraft in a 6000 x 12,000 km, 42° orbit for nearly two years spanning 2019-2021. Along with a suite of particle sensors, DSX carried a high-power very low frequency (VLF) transmitter and a sensitive receiver, both connected to an 80m dipole antenna. The system injected a variety of VLF waveforms into the inner magnetosphere to study wave propagation effects and interactions with relativistic trapped electrons.

Very low frequency (VLF) waves (about 3–30 kHz) in the Earth’s magnetosphere interact strongly with energetic electrons and are a key element in controlling dynamics of the Van Allen radiation belts. Bistatic very low frequency (VLF) transmission experiments have recently been conducted in the magnetosphere using the high-power VLF transmitter on the Air Force Research Laboratory’s Demonstration and Science Experiments (DSX) spacecraft and an electric field receiver onboard the Japan Aerospace Exploration Agency’s Arase (ERG) spacecraft. On 4 September 2019, the spacecraft came within 410 km of each other and were in geomagnetic alignment. During this time, VLF signals were successfully transmitted from DSX to Arase, marking the first successful reception of a space-to-space VLF signal. Arase measurements were consistent with field-aligned propagation as expected from linear cold plasma theory. Details of the transmission event and comparison to VLF propagation model predictions are presented. The capability to directly inject VLF waves into near-Earth space provides a new way to study the dynamics of the radiation belts, ushering in a new era of space experimentation. Graphical Abstract

We describe the quasi‐periodic (QP) whistler‐mode emissions found in the plasmasphere as detected by electric and magnetic instrumentation onboard the Demonstration and Science Experiments (DSX) spacecraft in medium Earth orbit. Over the course of the nearly 2‐year mission, at least 45 episodes of whistler mode QP emissions were detected by the Broad Band Receiver (BBR) onboard DSX. Episodes of QP emissions were identified by discrete events having a clear unambiguous periodic nature as detected by both the electric antennae and search coil magnetic sensors in the BBR survey data at 30 s temporal resolution. Most of the QP episodes occurred in a frequency range between 1 and 4 kHz, in a band previously identified by Van Allen Probes and Cluster investigators. However, episodes were also detected by DSX at higher frequencies ‐ events in these episodes extending all the way to 15 kHz. We present our findings on these unusual high frequency events in the presentation herein. Specifically, these high frequency QP episodes tended to be observed near dawn/dusk when the spacecraft was at relatively high magnetic latitudes and on magnetic L‐shells between 3 and 5. Another unusual feature of these episodes is that individual up‐drifting events making up the episode were found to sometimes occur concurrently in time: The high frequency portion of one up‐drifting “polliwog‐shaped” event overlapped in time with the low frequency portion of the subsequent event. This behavior of the QP emissions has not been previously emphasized and we consider how this temporal concurrence relates to the source processes.

A method for tracing rays in tokamak plasmas is applied to tracing rays in the Earth's magnetosphere. The problem of computing power flux from pencil rays using classical geometric optics that occurs when constituent rays exhibit coplanarity is addressed by appealing to complex geometric optics. The construction of Gaussian beams is discussed and the quasi‐optical method of tracing such beams is given. Examples of tracing Gaussian beams from a ground transmitter, a partial beam, and from a satellite location are presented. A median method for dealing with nonviable contributions to the power flux from sections of the beam encountering caustics or otherwise experiencing coplanarity is described. The effect of beam parameter choices on the resulting power flux computation is provided for each of the examples.

This study presents analysis of very low frequency (VLF) transmitter signal measurements on the Very‐Low‐Frequency Propagation Mapper (VPM) CubeSat in low‐Earth orbit. Six months of satellite operation provided good data coverage, used to build global statistical maps of VLF power distribution. The power distribution above four powerful transmitters is used as input for ray tracing to study signal propagation to the conjugate hemisphere in two plasmaspheric density models. The ray tracing results are further compared with VPM measurements to determine which model provides better agreement with observations. As ray propagation largely depends on the background plasma density distribution, this indirect method can be used for plasmaspheric density model validation as an alternative to multipoint in situ plasma measurements that may not be readily obtainable. In addition, it can be used to investigate Landau damping and ducted versus non‐ducted propagation of VLF signals.

This study presents results from magnetic field line conjunctions between the medium‐Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low‐Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single‐axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray‐tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low‐frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid‐ and high‐frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

Space weather phenomena can threaten space technologies. A hazard among these is the population of relativistic electrons in the Van Allen radiation belts. To reduce the threat, artificial processes can be introduced by transmitting very-low-frequency (VLF) waves into the belts. The resulting wave-particle interactions may deplete these harmful electrons. However, when transmitting VLF waves in space plasma, the antenna, plasma, and waves interact in a manner that is not well-understood. We conducted a series of VLF transmission experiments in the radiation belts and measured the power and radiation impedance under various frequencies and conditions. The results demonstrate the critical role played by the plasma-antenna-wave interaction around high-voltage space antennae and open the possibility to transmit high power in space. The physical insight obtained in this study can provide guidance to future high-power space-borne VLF transmitter developments, laboratory whistler-mode wave injection experiments, and the interpretation of various astrophysical and optical phenomena.

Supporting information for "VLF Transmitters and Lightning Generated Whistlers 2: Diffusion of Radiation Belt Electrons," submitted to Journal of Geophysical Research Space Physics. Diffusion coefficients for selected values of L and energy due to Very Low Frequency (VLF) transmitters and lightning generated whistlers (LGW), as well as Da0a0 and energy drag rates |dE/dt|/E from Coulomb collisions. Also provided are precipitation lifetimes, which include Da0a0 from plasmaspheric hiss but do not account for energy drag. Calculations are presented for high and low-density plasmasphere models, for all four combinations of ducted or nonducted VLF and LGW waves.

Waves from nine major ground-based very low frequency (VLF) transmitters are modeled from their sources to 660 km altitude with a full-wave code, which reliably treats transionospheric attenuation, and then ray-and-power traced throughout the plasmasphere. Lightning-generated whistlers, previously modeled at 660 km altitude, are ray-and-power traced throughout the plasmasphere as well. The resulting profiles of electric and magnetic fields, including wave normal angles, are organized by L value. Two versions of a realistic plasmaspheric density model are used, and ducted as well as nonducted propagation are treated. Results are compared to empirical models based on near-equatorial measurements by Van Allen Probes. A companion paper will evaluate resonant interactions of these waves with radiation belt electrons.

Sampling of anomaly‐causing space environment drivers is necessary for both real‐time operations and satellite design efforts, and optimizing measurement sampling helps minimize resource demands. Relating these measurements to spacecraft anomalies requires the ability to resolve spatial and temporal variability in the energetic charged particle hazard of interest. Here we describe a method for sampling particle fluxes informed by magnetospheric phenomenology so that, along a given trajectory, the variations from both temporal dynamics and spatial structure are adequately captured while minimizing oversampling. We describe the coordinates, sampling method, and specific regions and parameters employed. We compare resulting sampling cadences with data from spacecraft spanning the regions of interest during a geomagnetically active period, showing that the algorithm retains the gross features necessary to characterize environmental impacts on space systems in diverse orbital regimes while greatly reducing the amount of sampling required. This enables sufficient environmental specification within a resource‐constrained context, such as limited telemetry bandwidth, processing requirements, and timeliness.

An algorithm has been developed for specifying > 2 MeV electron flux everywhere along geosynchronous orbit for use in operational products. The statistics of integrated electron fluxes from four GOESs for more than a solar cycle clearly indicate that the local time variation can be represented by a Gaussian distribution as a function of geomagnetic Kp index, which empirically determines the center and the half width of the Gaussian distribution. Using the most current estimated 3 h Kp value as an input, the prediction scheme requires the most recent electron flux measurements from available GOES(s) to determine the maximum and minimum for a Gaussian fit and to provide estimated electron fluxes at geosynchronous orbit with the time resolution of the instrument. In balancing between sufficient data for statistics and the change of geomagnetic configuration, the optimal length of data accumulation time for nowcasting is 6 h when one or two satellites are available. The prediction efficiency (PE) is independent of local time and solar cycle. We found that the PE values are greater than 0.5 when Kp < 5 and independent of Kp at low and moderate values; however, PE decreases dramatically with increasing Kp when Kp ≥ 5. Although the PE varies from year to year and with the choice of the test satellite, our finding resulted in a PE > 0.6 in 67.6% of the cases and PE > 0.8 more than 23.5% of the time based on our analysis from four GOESs between 1998 and 2009. Moreover, skill scores from our newly developed algorithm are ~90% of the time better than those resulting from a simpler algorithm based on a table provided by O'Brien (2009), indicating a dramatic improvement in predictive capability.

Observed signatures of electron precipitation from the inner radiation belt are shown to be consistent with the theory of resonant scattering by whistler‐mode plasma waves, assuming the waves originate in VLF radio transmissions from the ground station NWC. The conclusion is based on a stochastic model of electron transport that includes pitch angle diffusion, radial diffusion, energy loss, and azimuthal drift. The wave scattering causes an increase in quasi‐trapped electron intensity, forming the “wisp” signature, and a corresponding decrease in stably trapped intensity at low altitude. A smaller decrease at high altitude is expected to be obscured by inward radial diffusion. If NWC were shut down, the resulting increase in stably trapped electron intensity would be minimal.

A climatology of VLF (very low frequency) wave intensity from lightning in the plasmasphere is constructed. Starting from Optical Transient Detector/Lightning Imaging Sensor (OTD/LIS) lightning data representing 1995-2005, a climatology of strikes is assembled with 1 degrees x1 degrees latitude-longitude spatial resolution, averaged into 2h bins for each month of the year. Assuming a linear relationship between optical flash rate and VLF power flux, and that the VLF amplitude drops off as one over distance, a proxy for VLF power is developed. A typical lightning spectrum is applied and the values are scaled by appropriate transionospheric absorptions for each time and place. These values are mapped along geomagnetic field lines in order to compare them to E-field spectral densities measured by the DEMETER satellite between 2005 and 2009. An overview of the DEMETER survey mode data is presented which leads to the best scaling of the lightning VLF climatology in LEO (low earth orbit). The resulting data set represents a monthly, 2-hour, solar minimum climatology of VLF wave intensity from lightning in LEO. Finally, the E-field spectral densities are converted to Poynting flux, mapped to the plasmasphere, and converted to B-field spectral densities. Good overall agreement is found with previous observations and estimates. This new climatology is expected to have a significant impact on calculations of pitch-angle diffusion for relativistic electrons in the inner radiation belt.

Summary form only given. Recent observations of the VLF waves with frequencies close to so-called lower hybrid resonance frequency have shown that amplitudes of the observed waves are 20–30 dB smaller than those obtained in VLF propagation models. Nonlinear interactions have been suggested1 to account for the missing mechanism of energy losses in the current propagation models. Our study2 of nonlinear induced scattering in electrostatic limit based on a novel 3D code which includes so-called vector nonlinearity pinned the above nonlinear mechanism as a very likely source of this discrepancy. The results virtually reproduce the Demeter satellite observations of intense broadband lower hybrid (LH) electrostatic waves generated by whistler-mode waves from the VLF transmitter NWC. Here we present the results of the extension of the numerical model to electromagnetic (whistler) limit and discuss possible ways of doing the modeling in realistic geometry, essential for obtaining the correct spatial distribution of attenuation of the pump wave emitted from spacecraft through various latitude/longitude as well as altitude regions of the ionosphere.

The electron decay timescale (τ) is well known to be associated with radiation belt loss processes. Knowledge of τ is important for understanding pitch angle and radial diffusion mechanisms. Previous studies reported decay timescales from the inner belt to geosynchronous orbits; however, relatively few statistical studies have been focused on the region beyond the traditional outer belt. In this paper, a systematic calculation of electron decay times at 5 < L ≤ 8 is performed using 11 years (1979–1989) of electron data from the Spacecraft Charging AT High Altitudes (SCATHA) satellite. The decay timescale is determined using daily median fluxes, providing resolution of τ > 1 day. τ is examined as a function of energy, pitch angle, L shell, Kp, and AE during magnetically disturbed periods when Dst ≤ −50 nT. Results show that τ increases with increasing electron energy at L < 6.6 for electron energies from 50 to 300 keV, but is independent of energy at L > 6.6. This suggests radial transport as the dominant effect at L > 6.6. Additionally, τ decreases with increasing L‐shell. This dependence has the strongest correlation and is seen in all energies and pitch angles. However,τ has no systematic dependence with pitch angle suggesting that pitch angle diffusion also plays a key role in the electron loss process. Based on our results, τ can be expressed as a function of energy and L, and coefficients are provided for a two‐variable fit. Surprisingly,τ is slightly longer for higher activity cases at L < 6.6, which is inconsistent with the current radial or pitch angle diffusion models. Global effective decay times on the timescale of days place an upper bound on the true loss timescale.