Abstract The development of statistical wave models that include the underlying variability of the wave distributions is an important next step toward more accurate radiation belt models. Lightning‐generated whistlers (LGW) are an important wave mode in the plasmasphere; previous time‐averaged diffusion coefficients have used assumptions about the wave distributions as a function of magnetic latitude. Shane et al. (2025, https://doi.org/10.1029/2025ja034164 ) compiled a comprehensive data set of observed event‐based LGW wave distributions using Van Allen Probes measurements. In this study, ray tracing is used to model the wave propagation to provide improved assumptions about the LGW wave variability along a magnetic field line for a source lightning discharge. The peak wave frequency increases with increasing magnetic latitude, symmetrically about the magnetic equator. The wave normal angle distribution has two components: (a) a latitudinally varying component that arises from the initial propagation of waves and (b) a component that is constant with latitude, arising from the magnetospherically reflecting waves. Bounce‐averaged diffusion coefficients are calculated using analytical wave distributions with different assumptions for the latitudinal variation of waves to quantify the sensitivity. Since the wave distributions at the equator are similar between different assumptions, the diffusion coefficients are not overly sensitive to the choice of frequency spectrum variation. However, enhanced pitch angle scattering at MeV energies occurs with the choice of the latitudinally varying wave normal angle distribution. These improved assumptions about the latitudinal wave distributions will be used in future work to calculate event‐based bounce‐averaged diffusion coefficients from the database of single point Van Allen Probes LGW observations.

The Array for VLF Imaging of the D-region ionosphere is a network of 22 transmit-receive great circle paths covering a mesoscale region of Canada. Qualitative results of amplitude data from these paths have been reported for a variety of evens in Cannon et. al., IEEE TGRS, 63 (2025), pp. 1-12. Prior work has developed an inversion technique for such an array using a Local Ensemble Transform Kalman Filter (LETKF) (F. Gasdia and R. A. Marshall, IEEE TGRS, 58.5 (2020), 3526-3543) and applied it to simulated data (F. Gasdia and R. A. Marshall, Earth and Space Sciences, 10.1 (2023), e2022EA002460). A typical Kalman filter uses a defined state and forward model to transform the state into simulated measurements which are then compared against real measurements. The model and measurements are weighted from the Kalman gain and an estimated state is solved for. When applying a Kalman filter to ionospheric measurements, a forecast model is not available so instead, the measurement update step is iterated multiple times. Using an ensemble approach where the variance of the state vector is represented by an ensemble of possible states allows for the use of a non-linear forward model, which is necessary to apply this approach to VLF measurements in the Earth-ionosphere waveguide. In this work, the state of the D-region ionosphere is defined by the Wait and Spies profile parameterized by $h^{\prime}$ and $\beta$. Fig. 1 shows an example of the Kalman filter applied to simulated data.

Very-low-frequency (VLF) radio waves are commonly used to remotely sense the D-region ionosphere. In the past few decades, a few prominent designs for VLF receivers have come to the forefront of the community, such as the AWESOME receiver developed at Stanford in the early 2000s. In the last decade, advancements in off-the-shelf electrical components and the obsoletion of others motivated a redesign of this receiver system. The result of this receiver design work is presented in this article with the measured characteristics of over ten complete systems. The newly operational array for VLF imaging of the D-region (AVID), incorporating the updated VLF receiver, is also presented. One early result from AVID is the characterization of the drift rates of the phase of two VLF transmitters. The NLK transmitter at 24.8 kHz was found to drift by -0.07886 degrees/s +/- 0.00083 degrees/s, while the NML transmitter at 25.2 kHz was found to drift by -0.07201 degrees/s +/- 0.00019 degrees/s. We also show that the transmitted power of these transmitters can change over time, by up to similar to 7% in the case of NLK. Finally, two early case studies are presented, showing AVID's ability to observe and track: 1) the effects of an M7.2 solar flare on the D-region ionosphere and 2) the onset of a -333 nT Dst geomagnetic storm and the D-region's response to it. These case studies demonstrate the tools necessary for future work incorporating spatial estimation algorithms to better characterize disturbances to the D-region ionosphere.

Various studies have been dedicated to quantifying the atmospheric chemical effects of energetic electron precipitation (EEP), but the contribution from relativistic electron precipitation (REP) was largely overlooked. Based on the precipitating fluxes estimated from Polar‐orbiting Observational Environmental Satellites, we quantify the REP‐induced atmospheric chemical effects using the Whole Atmosphere Community Climate Model. Present results show that direct stratospheric ionization caused by REP can enhance the NOx concentration by a factor of ∼2.58 at ∼37 km altitude, and the HOx concentration by a factor of ∼6.41 at ∼44 km altitude. As for the annual variation, REP causes an additional ozone loss of ∼16.2%–17.1% at ∼30–35 km altitude during winter. Moreover, REP's impact is not confined to winter since the resultant NOx and HOx production occurs in situ. Therefore, neglecting REP would significantly underestimate EEP's total effects on the stratospheric ozone.

A new array of VLF receivers is now online to advance sub-ionospheric remote sensing of the D-region ionosphere. As a part of bringing this array online, the drifting phase from the NLK (24.8 kHz) and NML (25.2 kHz) VLF transmitters has been characterized with a drift rate of -0.07885 ± 0.00079 °/s and -0.07201 ± 0.00012 °/s (2σ), respectively. We show that detrending using these drift rates recovers true phase shifts along transmit- receive great circle paths and can be used to recover phase measurements when a known reference is unavailable. A case study of two solar flares is presented, characterizing the peak phase change as a function of solar zenith angle for flares originating at different radial distances from the center of the solar disk.

Earth's magnetic field has been used as an aid in navigation for thousands of years and is an integral part of navigation systems in use today. Due to the complex and dynamic nature of the magnetic field, detailed models are required for implementation into navigation systems and for greater understanding of the magnetic field. Historically, space-based measurements have been made using relatively larger satellites. As the use of smaller platforms such as CubeSats continues to grow, more compact instrumentation is needed while maintaining performance requirements. The development of a miniaturized self-calibrating magnetometer with the capability of taking both scalar and vector measurements responds to this need. The work detailed in this paper explores the design and performance of a vector-scalar magnetometer for CubeSats with the intention of collecting measurements for predictive magnetic field models including the World Magnetic Model. Our system, the vectorized rubidium magnetometer (VRuM), comprises an optically-pumped rubidium scalar magnetometer modified to add vector capabilities. We present the operating principles of both the scalar magnetometer and our vectorization approach and describe the mechanical and electronic design and development. This manuscript is the first of two companion manuscripts. This manuscript focuses on the design and development effort for VRuM and includes preliminary testing results of the proto-flight instrument, while the second manuscript will provide a detailed characterization of the flight instrument. This will include evaluation of VRuM's accuracy, precision, and temperature-dependent performance alongside calibration efforts implemented for this system. Preliminary testing indicates that the magnetometer could eventually achieve a precision better than 2.5 nT (1 r)for the vector components and a magnitude precision better than 0.1 nT (1 r) . (c) 2025 Published by Elsevier B.V. on behalf of COSPAR.

Very Low Frequency (VLF) radio receivers have been used to remotely sense the ionosphere and whistler waves traveling through it for over 60 years (R.A. Helliwell, Stanford University Press, Stanford (1965)). A common technique to remotely sense ionospheric density uses the measurement of narrowband signals from different VLF transmitters around the world. This technique relies on the VLF signals propagating efficiently within the Earth-Ionosphere waveguide and the ability of changes in measured amplitude and phase to be tied to changes in the D-region ionospheric density profile. As a path-integrated measurement, a single receiver can measure average changes to the D-region density along the transmit-receive path but cannot be used to probe small spatial scale disturbances along that path.

This study presents results from five months of meteor observations that included simultaneous and nearby very low frequency (VLF) wave detections. We explore the plausibility of VLF emissions from meteors resulting in the documented phenomena of simultaneous optical and audio signatures with meteor events. Most previous attempts to observe VLF emissions from meteors have been limited in duration and/or in area covered during observations. With the extended duration and an observational network of three cameras and two VLF receivers across Colorado, an exhaustive approach was taken to detect meteor VLF emissions. The ∼650 events collected were closely inspected for any signs of emissions, with an emphasis on brighter meteors and fireballs. The VLF data was filtered using interference mitigation techniques to eliminate spurious signals that could obscure the VLF signals of interest. By comparing the VLF spectral content at the time of meteors and during control times, we search for very small changes in the signal that would be statistically correlated with meteor observation times. Despite these efforts, no VLF emissions have been detected that cannot be attributed to other sources. Most commonly, lightning-generated sferics coincident with the time of meteor events lead to false attributions to meteors.

AbstractThis study evaluates the coded aperture imaging method for pitch angle observations of magnetospheric energetic electrons in the solar, Earth, and planetary space environments. We present a review of key previous energetic electron instruments with pitch angle‐resolved observations across a range of electron energies. We describe the coded aperture imaging method, typically used for high angular resolution X‐ray and gamma ray observations, and evaluate design parameters in the context of energetic electron observations. We present the results of simulations of energetic electrons in Geant4 and evaluate the method's ability to resolve sources with high angular and temporal resolution. We also evaluate the impact of secondary radiation produced from electron interactions in the tungsten coded aperture, as well as the impact of artifacts from the decoding process. With these simulated results, we identify key areas in magnetospheric science that would benefit from high angular resolution observations of energetic electrons. We find that coded aperture imaging may be well‐suited for high‐resolution observations of intense localized structures, such as low energy (tens of eV to several keV) field‐aligned electron beams or the electron strahl wind.

The total mass flux due to meteoric input is not well constrained and estimates vary greatly depending on the measurement technique used. The source of this discrepancy remains an open question in the field. Previous studies investigating the discrepancy by directly comparing mass estimates made using two techniques have been limited by extremely small sample sizes. This work presents a set of 166 meteors observed simultaneously by the MAARSY radar (53.5 MHz) and two nearby optical cameras. Independent masses are estimated using observations from both systems and compared against each other. The resulting mass estimates using both methods agree to within a factor of three on average. The results show two dominant trends: better agreement as meteoroid velocity increases and underestimation of the radar mass for the largest meteoroids observed (>10 mg). These trends had not been quantified by previous studies limited by very small sample sizes, and could help to explain the historic discrepancy between mass estimates by different systems. The general agreement between the radar and photometric masses indicates that both methods perform well independently, and can reliably be applied to radar or optical observations without restriction of simultaneous observations by two systems.

We assess the prevalence of ducted and non-ducted whistler propagation using burst mode data from the Van Allen Probes Electric Field and Waves instrument (EFW). We have identified burst periods containing lightning-generated whistlers (LGWs), resulting in a data set available for future use. The entire burst data set is filtered through an analysis of the search coil magnetometer (SCM) noise, identifying signals in frequency space that exceed an adaptive noise threshold. We implement DBSCAN (Density-Based Spatial Clustering of Applications with Noise) to identify individual whistlers and clusters of whistlers. With magnetic spectral analysis, we calculate the mean wave normal angle (WNA) for each LGW group. We use ray tracing to estimate the expected WNA distributions for non-ducted LGWs to compare to the data and determine methods for identifying potentially ducted LGWs. The ray-tracing results provide a clear threshold in WNA for ducted whistlers. Using this threshold, we estimate that at least 10.1% $10.1\%$ of LGWs in this data set are ducted. We find that the majority of potentially ducted whistlers are below L=2 $L=2$ and nearly half of LGWs below L=2 $L=2$ and within 9-18 MLT may be ducted. Lightning-generated whistlers (LGWs) are very-low-frequency waves produced when lightning-generated radio pulses known as sferics excite whistler mode waves through the ionosphere and into the magnetosphere. LGW wave power contributes to pitch angle scattering and precipitation of trapped energetic electrons, affecting atmospheric chemistry and helping maintain the slot region between the radiation belts. Field-aligned plasma density irregularities, or ducts, affect the propagation of LGWs. Ducting can determine where the wave energy travels, how long it persists, and which energetic particle populations are affected. We present distributions of LGW wave properties and explore methods of determining ducting of LGWs. We present a complete Van Allen Probes Electric Field & Waves whistler data set generated through density-based clustering Most whistlers in this data set have high wave normal angles consistent with non-ducted propagation Likely ducted whistlers are mainly below L=2 $L=2$; about half of whistlers between 9 and 18 MLT and below L=2 $L=2$ are likely ducted

The Imaging Microburst Precipitation with Atmospheric X-ray emissions (IMPAX) CubeSat was recently selected by NASA’s H-FORT program, with a planned launch date of late 2027. The overarching goal of IMPAX is to quantify relativistic electron microburst precipitation as a radiation belt loss mechanism.

In the D‐region, the ionization rate cannot be detected directly with any known measurement technique, therefore it must be estimated. Starting from space‐based measurements of precipitating particle flux, we estimate the ionization rate in the atmosphere using the Electron Precipitation Monte Carlo transport method. This ionization rate is used to calculate the expected electron density in the D‐region with the Glukhov‐Pasko‐Inan five species (GPI5) atmospheric chemistry model. We then compare the simulated electron density with that measured by the Poker Flat Incoherent Scatter Radar (PFISR). From ground‐based radar measurements of electron density enhancements due to sub‐relativistic and relativistic electron precipitation, we present a method to extract the ionization rate altitude profiles using inverse theory. We use this estimation of ionization rate to find the energy distribution of the precipitating particles. With this inverse method, we are able to link ground measurements of electron density to the precipitating flux in a time dependent manner and with uncertainty in the inverted parameters. The method was tested on synthetic data and applied to specific PFISR data sets. The method is able to retrieve the ionization rate altitude profiles that, when forward modeled, return the expected electron densities within ∼7% error as compared to the PFISR data. For the case presented here, the arbitrary energy distribution inversion results are comparable in magnitude and shape to those presented in Turunen et al. (2016, https://doi.org/10.1002/2016jd025015 ) for the inversion of a single event of pulsating aurora observed by EISCAT.

Calculating meteoroid masses from photometric observations relies on prior knowledge of the luminous efficiency, a parameter that is not well characterized; reported values vary by several orders of magnitude. We present results from an experimental campaign to determine the luminous efficiency as a function of mass, velocity, and composition. Using a linear electrostatic dust accelerator, iron and aluminum microparticles were accelerated to v > 10 km/s and ablated, and the light production measured. The luminous efficiency of each event was calculated and functional forms fit for each species. For both materials, the luminous efficiency is lowest at low velocities, rises sharply, then falls as velocity increases. However, the exact shape and magnitude of the curve is not consistent between the materials. The difference between the luminous efficiencies for iron and aluminum, particularly at high velocities, indicates that it is not sufficient to use the same luminous efficiency for all compositions and velocities.

Energetic particle precipitation (EPP) is a key loss mechanism for radiation belt particles. Quantification of the precipitation loss rate feeds into the electron lifetimes used by radiation belt models and is needed to improve understanding of radiation belt dynamics. EPP deposits most of its energy in the D ‐region ionosphere, a layer so weakly ionized that it is not observed using standard ionosphere measurement techniques. However, very low frequency (VLF) radio signals propagate great distances because of the naturally occurring waveguide formed by Earth’s surface and the D ‐region. If the ground conductivity is known along the propagation path to a receiver, then the amplitude and phase of a VLF transmitter signal can be used to infer the average conductivity of the D ‐region ionosphere. This article simulates the propagation of narrowband VLF signals through realistic ionosphere profiles enhanced by EPP. By using a distributed array of VLF receivers, the observations can be simultaneously inverted to estimate the spatial extent of a precipitation patch. These images of the ionosphere are generated using the local ensemble transform Kalman filter. We demonstrate this method with several simulated observation experiments, including four EPP events. Precipitation patches are identified in daytime, but accurate estimation of nighttime ionospheres remains a challenge.

The positions, experiences, and viewpoints expressed below are those of

4. NASA Heliophysics division should fund more in situ measurements to quantify lightning/thunderstorm effects in the upper atmosphere, including sounding rockets, short-

Solar X-rays, auroral and radiation belt electrons, and other energetic particles deposit their energy in the atmosphere at altitudes corresponding to the D-region of the Earth's ionosphere.Subionospheric Very-low-frequency (VLF) remote sensing remains the most effective method for continuous observation of the D-region, thanks to the efficient reflection of probe VLF waves from transmitters and lightning at these altitudes.These VLF waves are sensitive to changes in D-region conditions, so that VLF measurements are indicative of the changing D-region state.However, it is extremely challenging to infer properties of the ionization sources (e.g.solar X-ray or radiation belt particle flux) from these VLF measurements, because the ionization process, D-region chemistry, and VLF signal propagation together form a complex and underdetermined system for inversion.Here we present a new method to estimate the D-region state using an array of overlapping VLF signal paths covering a mesoscale region of thousands of kilometers.This array of VLF receivers is notionally designed to cover radiation belt latitudes over western Canada, corresponding to L-shells between L = 3 and L = 7, where radiation belt precipitation occurs.The inversion procedure uses the amplitude and phase of VLF transmitter signals measured at these VLF receivers to infer D-region electron density profile parameters in a grid over the array.This inversion procedure uses a Local Ensemble Transform Kalman Filter (LETKF) to estimate the state of the ionosphere by minimizing both data errors and forward model errors.In this talk we describe the design of this array of VLF receivers, the LETKF-based inversion procedure, and some of the limitations of this inversion method.Finally, we describe the design of a new VLF receiver that will be deployed to make up the Array for VLF Imaging of the D-region (AVID) to monitor radiation belt precipitation and measure precipitation scale sizes.Once operational in summer 2023, this array will make continuous observations of the D-region over a large portion of Canada using continuous ground-based VLF observations of transmitter signals.

Electron lifetimes are important for understanding the dominant loss processes of radiation belt electrons to the atmosphere and for accurate radiation belt modeling. We estimate electron lifetimes from the precipitating population measured in Low Earth Orbit (LEO) from the Polar Orbiting Environmental Satellites (POES). We compare our estimates to previous estimates from the Radiation Belt Storm Probes (RBSP) by Claudepierre et al. (2020b, https://doi.org/10.1029/2019GL086053 ). We also present the first complete pitch angle resolved lifetimes in the radiation belts. Quasi‐linear theory predicts the pitch angle distribution decays uniformly, therefore if steady‐state decay is realized, POES and RBSP should measure similar lifetimes. Lifetime estimates from LEO are shown to be in good agreement with those from Van Allen Probes in the outer belt and pitch angle resolved lifetimes indicate that steady‐state decay is realized. However, a systematic slight overestimation of the POES lifetimes reveal that the POES instruments may suffer from bremsstrahlung contamination. At L = 2–3, no decay intervals were identified in the POES electron fluxes and the near loss cone RBSP electron fluxes. We show that at these L‐shells, where lifetimes are long, there is an apparent decoupling of the perpendicular and parallel flux. Large pitch angle anisotropy and processes that affect low pitch angle electrons are two explanations for this apparent decoupling. Electron lifetime models that use pitch angle independent lifetimes, derived from the equatorially mirroring electron flux, likely underestimate the amount of precipitating flux and may not capture the dynamics of low equatorial pitch angle electrons at L < 3.

George Clark, Jim Kinnison, Dan Kelly, Peter Kollmann, Wen Li, Allison Jaynes, Lauren Blum, Robert Marshall, Drew Turner, Ian Cohen, Sasha Ukhorskiy, Barry Mauk, Elias Roussos, Quentin Nénon, Sasha Drozdov, Xinlin Li, Emma Woodfield, Will Dunn, Grant Berland, Ralph Kraft, Peter Williams, Todd Smith, Kareem Sorathia , Anthony Sciola, George Hospodarsky, Xin Wu, Paul O’Brian, Mark Looper, Angelica Sicard, Andy Santo, Meagan Leary , Amanda Haapala, Fazle Siddique, Michelle Donegan, Ben Clare, Derek Emmell, Kim Slack, John Wirzburger, Daniel Sepulveda, Lew Roufberg, Jackie Perry, John Schellhase, Darrius Pergosky, Liz Able, Mike O’Neill, Cris Fernandes, Deb Chattopadhyay, Samuel Bibelhauser, Seth Kijewski, Joe Pulkowski, and Mike Furrow