2026-04-01
Lauren T. Ashworth, Edwin J. Mierkiewicz, Susan M. Nossal, Liying Qian, Joseph M. McInerney, L. Matthew Haffner, R. Carey Woodward, Brandon J. Myers
From 2000 to 2001, ground-based geocoronal hydrogen Balmer-α (Hα) observations were obtained from Pine Bluff Observatory (PBO) in Wisconsin (43.07°N, 270.33°E) using a high spectral resolution (R ≈ 80,000) Fabry-Perot interferometer (FPI). In previous work, the PBO March 2000 observations have been compared to forward modeled MSISE-90 Hα intensities using the radiative transport code, lyao_rt (Bishop, Journal of Qunatitative Spectroscopy and Radiative Transfer, 1999, 61, 473–491). Results indicate that hydrogen column abundances exceed those predicted by MSISE-90 (Bishop et al., Journal of Geophysical Research, 2004, 109, 473–491). Furthermore, the PBO 2000–2001 observations have been compared to NRLMSISE-00 (MSIS-00) Hα intensities generated by lyao_rt. It was found that the observed dusk-to-dawn intensity variation and MSIS-00 show good agreement near the equinoxes and summer solstice, however, MSIS-00 underestimates the dusk-to-dawn asymmetry near the winter solstice (Gallant et al., Journal of Geophysical Research: Space Physics, 2019, 124, 4525–4538). More recent work has focused on forward modeling Whole Atmosphere Community Climate Model-eXtended (WACCM-X) simulations for three separate observatory locations, including PBO. Here, we investigate variations in upper thermospheric hydrogen by comparing WACCM-X and MSIS-00 forward modeled Hα intensities for the March equinox with the PBO March 2000 evening observations used in Bishop et al. (Journal of Geophysical Research, 2004, 109, 473–491). WACCM-X underestimates the March 2000 observations by less than a factor of 2.0 rayleighs whereas MSIS-00 underestimates the observations by 2.6 rayleighs. A further analysis suggests that WACCM-X is better at simulating the underlying hydrogen density distribution than MSIS-00. Model-model-data comparisons between the 2000–2001 PBO observations, MSIS-00, and WACCM-X simulations that are representative of the observational conditions are currently underway.
DOI: 10.3389/fspas.2026.16435052026-03-31
Megha Pandya, Philemon J. Kuo, Guan Le, Mei-Ching Fok, Sang-Yun Lee, Denny M. Oliveira, Geoffrey D. Reeves
We present simultaneous multi-point observation of local-time asymmetry in keV ion and electron fluxes during the interplanetary (IP) shock impact on 19 April 2014 under geomagnetically quiet conditions. During this event, Van Allen Probe-A and Probe-B were simultaneously located in the afternoon and morning sectors, respectively, for ∼20 min (18:38-18:58 UT), while sampling nearly identical radial distances (L* = 3.7 ± 0.2). This unique conjunction provides an exceptional opportunity to investigate the local-time–dependent response of inner magnetospheric particle populations to shock-driven compression. Distinct asymmetries in the ion and electron flux responses are identified during the IP shock interval. In the afternoon sector, H+, He+, and O+ ions exhibit pronounced multiple nose structures at energies above 5 keV that extend deeper in radial distance. In contrast, the morning sector shows fewer nose structures—particularly for O+ ions and the corresponding features for all ion species are considerably less deep compared to those observed in the afternoon sector. In contrast, keV electrons show an enhancement of an order of magnitude or more in the morning sector following the IP shock, whereas no significant electron response is observed in the afternoon sector during the same interval. Our results indicate that the observed dayside local-time asymmetry of keV ions is primarily governed by shock-induced asymmetric magnetospheric compression combined with the temporal variability of the convection electric field. These findings demonstrate that even weak IP shocks under quiet geomagnetic conditions can produce significant and highly asymmetric particle responses in the inner magnetosphere.
DOI: 10.3389/fspas.2026.18085842026-03-27
Joseph E. Borovsky, Charles W. Smith
Motivated by knowledge that directional discontinuities (strong current sheets) in the solar-wind magnetic time series have dominating effects on the magnetic power spectral density of the solar wind, the question is raised as to what are the effects of the magnetic fluctuations that reside in between those discontinuities. A methodology is developed to remove the effects of discontinuities in the magnetic time series and the resulting modified time series is Fourier examined. At 1 AU, an interval of fast Alfvénic coronal-hole-origin wind and an interval of slow non-Alfvénic streamer-belt-origin wind are analyzed. For both types of solar wind, the amplitude of the trace-B power spectral density in the inertial range is about an order of magnitude smaller for the time series of fluctuations between the discontinuities than it is for the raw (with discontinuities) time series, and the spectral index is shallower when the discontinuities are removed. For future global MHD computer simulations of the “upstream turbulence effect” on the solar-wind-driven Earth’s magnetosphere, we have a methodology to determine the effects of discontinuities on the Earth’s magnetosphere versus the effects of between-the-discontinuity magnetic fluctuations.
DOI: 10.3389/fspas.2026.17953912026-03-11
Sabastian Fernandes, Gareth W. Perry, Tiago Trigo, Nathaniel A. Frissell, John Gibbons, Kristina Collins
High-frequency (HF) skywave propagation relies on the ionosphere, making it susceptible to ionospheric variability. This study analyzes long-term Doppler residual measurements of a 10 MHz HF link between Fort Collins, CO, and Newark, NJ, to characterize the impact of ionospheric conditions on the link. We report that daytime measurements of Doppler variability exhibit Cauchy statistics, while nighttime measurements show a combination of exponential and log-normal statistics. These patterns correlate with solar activity and solar zenith angle. We also use PHaRLAP numerical ray tracing simulations through the IRI 2020 ionosphere to provide insights into signal ray paths and the altitudes of the ionosphere contributing to the observed Doppler shifts. By examining diurnal variations and statistical properties of Doppler residuals, this study aims to enhance our understanding of ionospheric dynamics and their influence on HF signal characteristics.
DOI: 10.3389/fspas.2026.17139682026-03-04
Elizabeth L. M. Hanson, Laura Balboa, Alayna Corona, Cynthia A. Cattell, Evan Tyler, Tai D. Phan, Stephen A. Fuselier, Roman G. Gomez
IntroductionMotivated by observed composition dependence in reconnection energy outflow and by theoretical and simulation studies predicting differences between heating of H+ and heavier ions, we investigated the ion composition dependence of heating associated with reconnection in 28 dayside magnetopause crossings in Magnetospheric Multiscale (MMS) data.MethodsWe applied Least Squares fitting to analyze the relationship of temperature change across the magnetopause exhaust to available magnetic energy. Available magnetic energy per ion-electron pair flowing into the magnetopause from the magnetosheath and magnetosphere ranged between a few tens of eV and ∼1750 eV.ResultsThe individual fits for composition-nonspecific ions and for H+ were significantly lower than the empirical scaling relationship found previously between the temperature change and the inflowing magnetic energy; the fit for He++ was higher, with marginal significance. A composite data product combining H+ and He++ agreed with the empirical scaling relationship to within 95%.DiscussionAlthough comparisons between heating of H+ and He++ are suggestive of enhanced heating of heavy ions, differences could not be identified conclusively due to high scatter and a small number of events with adequate densities of heavy ions.
DOI: 10.3389/fspas.2025.17492712026-02-27
Stephen A. Cerwin, Jesse T. McMahan, Alexandros S. Papadopoulos, Gerard N. Piccini, Nathaniel A. Frissell, Kristina V. Collins, Aidan Montare, Paul Bilberry, Samuel Blackshear, David R. Themens
This study describes a method to deduce the ionization layer virtual height and propagation path geometry responsible for communication between two HF radio stations a fixed distance apart. The method measures the Time Difference of Arrival (TDOA) between multipath propagation modes involving the active ionospheric layers and reconciles the data with a virtual height model of the ionosphere. The TDOA approach was implemented by transmitting audio signals that are sensitive to a time delay when summed together as happens in a receiver during multipath reception. The TDOA method eliminates the need for any absolute time references or extensive equipment calibration that would be required for an absolute time of flight (TOF) measurement. The audio waveforms used by the method included 1-cycle audio bursts, linear audio chirps of controlled sweep rate, and pseudorandom noise (PN) bursts.
DOI: 10.3389/fspas.2026.1723511