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1. Observations and Fully Coupled Simulations of Nighttime Ionospheric Irregularities in the Subauroral E Region

   https://doi.org/10.1029/2025JA034247  

   Hysell, D L; Larsen, M F (August 2025, Journal of Geophysical Research: Space Physics)

   Abstract Observations and theory of plasma density irregularities in subauroral sporadicElayers are presented. The irregularities produce so‐called quasiperiodic (QP) coherent scatter radar echoes, and imagery of the echoes reveals large‐scale structuring which normally takes the form of bands of backscatter elongated from northwest to southeast and propagating to the southwest in the northern hemisphere. We associate the large‐scale structuring with neutral dynamics in general and Ekman‐type instability in particular. This is an inflection point instability, a generalization of Kelvin‐Helmholtz instability in turning shears, the same shears that form sporadicEionization layers in the first place. A key parameter for the instability is , the asymptotic direction of the neutral winds at the top of the shear spiral relative to magnetic east. The parameter controls the alignment of the rolls produced by inflection point instability. Numerical simulations involving fully coupled neutral and ionospheric dynamics reproduce many of the characteristics of the QP echoes. We argue that only angles within the first two quadrants are consistent with sporadicElayer formation, and only angles in the second quadrant are consistent with the production of strong field‐aligned currents which may be necessary for small‐scale field‐aligned plasma density irregularity creation. This may explain the alignment of QP‐echo striations observed by imaging radar. 

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2. VHF Imaging Radar Observations and Theory of Banded Midlatitude Sporadic E Ionization Layers

   https://doi.org/https://doi.org/10.1029/2021JA029257  

   Hysell, D. L.; Larsen, M. F. (April 2021, Journal of geophysical research)

   null (Ed.)

   Observations of backscatter from field-aligned plasma density irregularities in sporadic E (Es) layers made with a 30-MHz coherent scatter radar imager in Ithaca, New York are presented and analyzed. The volume probed by the radar lies at approximately 54° geomagnetic latitude, under the midlatitude trough and at the extreme northern edge of the zone where Es layers are prevalent. Nonetheless, the irregularities exhibit many of the characteristics of quasiperiodic echoes observed commonly at lower middle latitudes. These include a tendency to occur in elongated bands stretching from the northwest to southeast in the Northern hemisphere separated by tens of kilometers and propagating to the southwest. In addition, the irregularities were found to exhibit finer-scale structures with secondary bands oriented nearly normally to the primary bands. We investigate the proposition that the primary bands are telltale of Es-layer structuring caused by neutral Kelvin Helmholtz (KH) instability in the lower thermosphere and that the secondary bands signify secondary KH instability. Results from a 3D numerical simulation of KH support this proposition. 

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3. Turbulence in Particle-laden Midplane Layers of Planet-forming Disks

   https://doi.org/10.3847/1538-4357/ac9411  

   Sengupta, Debanjan; Umurhan, Orkan M (January 2023, The Astrophysical Journal)

   Abstract We examine the settled particle layers of planet-forming disks in which the streaming instability (SI) is thought to be either weak or inactive. A suite of low-to-moderate-resolution 3D simulations in a 0.2H-sized box, whereHis the pressure scale height, are performed using PENCIL for two Stokes numbers, St = 0.04 and 0.2, at 1% disk metallicity. We find that a complex of Ekman-layer jet flows emerge subject to three co-acting linearly growing processes: (1) the Kelvin–Helmholtz instability (KHI), (2) the planet-forming disk analog of the baroclinic Symmetric Instability (SymI), and (3) a later-time weakly acting secondary transition process, possibly a manifestation of the SI, producing a radially propagating pattern state. For St = 0.2 KHI is dominant and manifests as off-midplane axisymmetric rolls, while for St = 0.04 the axisymmetric SymI mainly drives turbulence. SymI is analytically developed in a model disk flow, predicting that it becomes strongly active when the Richardson number (Ri) of the particle–gas midplane layer transitions below 1, exhibiting growth rates $\le \sqrt{2/\mathrm{Ri}-2}·\mathrm{\Omega }$ , where Ω is the local disk rotation rate. For fairly general situations absent external sources of turbulence it is conjectured that the SI, when and if initiated, emerges out of a turbulent state primarily driven and shaped by at least SymI and/or KHI. We also find that turbulence produced in 256³resolution simulations are not statistically converged and that corresponding 512³simulations may be converged for St = 0.2. Furthermore, we report that our numerical simulations significantly dissipate turbulent kinetic energy on scales less than six to eight grid points. 

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4. Radiofrequency ice dielectric measurements at Summit Station, Greenland

   https://doi.org/10.1017/jog.2023.72  

   Aguilar, Juan Antonio; Allison, Patrick; Besson, Dave; Bishop, Abby; Botner, Olga; Bouma, Sjoerd; Buitink, Stijn; Cataldo, Maddalena; Clark, Brian A; Couberly, Kenny; et al (October 2023, Journal of Glaciology)

   We recently reported on the radio-frequency attenuation length of cold polar ice at Summit Station, Greenland, based on bistatic radar measurements of radio-frequency bedrock echo strengths taken during the summer of 2021. Those data also include echoes attributed to stratified impurities or dielectric discontinuities within the ice sheet (layers), which allow studies of a) estimation of the relative contribution of coherent (discrete layers, e.g.) vs. incoherent (bulk volumetric, e.g.) scattering, b) the magnitude of internal layer reflection coefficients, c) limits on the azimuthal asymmetry of reflections (birefringence), and d) limits on signal dispersion in-ice over a bandwidth of ~100 MHz. We find that i) after averaging 10000 echo triggers, reflected signal observable over the thermal floor (to depths of approximately 1500 m) are consistent with being entirely coherent, ii) internal layer reflection coefficients are measured at approximately -60 to -70 dB, iii) birefringent effects for vertically propagating signals are smaller by an order of magnitude relative to comparable studies performed at South Pole, and iv) within our experimental limits, glacial ice is non-dispersive over the frequency band relevant for neutrino detection experiments. 

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5. Lower-thermospheric vertical wind measurements from the Arecibo radio observatory

   https://doi.org/10.3389/fspas.2025.1677150  

   Hysell, D L; Larsen, M F (September 2025, Frontiers in Astronomy and Space Sciences)

   Vertical wind profiles in the lower thermosphere computed from data from the Arecibo Radio Observatory between 2013 and 2016 are reviewed and analyzed. The estimates are derived from ion-line spectral measurements in theEandFregions made with the Arecibo dual-beam incoherent scatter radar using a number of specialized pulsing schemes. The background electric field is estimated on the basis ofF-region samples, and its effects are then removed fromE-region samples. Statistical inverse methods are used to estimate vector wind profiles in the lower thermosphereversustime. Data from the daytimeEregion and from nighttime sporadic-Elayers are examined. We find large and highly variable vertical winds in the lower thermosphere at upperE-region altitudes. Frequency and vertical wavenumber spectra computed from the vertical wind estimates exhibit power laws, and the latter exhibit some degree of universality. The spectral forms are consistent with those measured in the mesosphere elsewhere. Theoretical interpretations of the spectral measurements are briefly reviewed. 

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