More Like this

1. 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. 

   more » « less
2. 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. 

   more » « less
3. 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. 

   more » « less
4. Dynamics of Mid‐Latitude Sporadic‐E and Its Impact on HF Propagation in the North American Sector

   https://doi.org/10.1029/2023JA031455  

   Kunduri, B_S R; Erickson, P J; Baker, J_B H; Ruohoniemi, J M; Galkin, I A; Sterne, K T (September 2023, Journal of Geophysical Research: Space Physics)

   Abstract Sporadic‐E (Es) are thin layers of enhanced ionization observed in the E‐region, typically between 95 and 120 km altitude. Es plays an important role in controlling the dynamics of the upper atmosphere and it is necessary to understand the geophysical factors influencing Es from both the scientific and operational perspectives. While the wind‐shear theory is widely accepted as an important mechanism responsible for the generation of Es, there are still gaps in the current state of our knowledge. For example, we are yet to determine precisely how changes in the dynamics of horizontal winds impact the formation, altitude, and destruction of Es layers. In this study, we report results from a coordinated experimental campaign between the Millstone Hill Incoherent Scatter Radar, the SuperDARN radar at Blackstone, and the Millstone Hill Digisonde to monitor the dynamics of mid‐latitude Es layers. We report observations during a 15‐hr window between 13 UT on 3 June 2022 and 4 UT on 4 June 2022, which was marked by the presence of a strong Es layer. We find that the height of the Es layer is collocated with strong vertical shears in atmospheric tides and that the zonal wind shears play a more important role than meridional wind shears in generating Es, especially at lower altitudes. Finally, we show that in the presence of Es, SuperDARN ground backscatter moves to closer ranges, and the height and critical frequency of the Es layer have a significant impact on the location and intensity of HF ground scatter. 

   more » « less
5. Simultaneous Observations of Quasi‐Orthogonal Nighttime MSTIDs at Conjugate African‐European Midlatitude Stations on 4 October 2018

   https://doi.org/10.1029/2025JA034016  

   Katamzi‐Joseph, Zama T; Martinis, Carlos; Makela, Jonathan J; Otsuka, Yuichi; Habarulema, John Bosco; Hiyadutuje, Alicreance; Toledano, Shai; Moldwin, Mark B; Baumgardner, Jeffrey (December 2025, Journal of Geophysical Research: Space Physics)

   Abstract Two quasi‐orthogonal nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) were observed by conjugate midlatitude all‐sky imagers in Sutherland (32.4S, 20.8E; magnetic latitude: −40.9) and Asiago (45.87N, 11.53E; magnetic latitude: ) on 4 October 2018. These MSTIDs had fronts elongated quasi‐orthogonally to one another as observed from each location. The first MSTID was aligned northeast‐southwest (NE‐SW) in the Southern Hemisphere (SH) and northwest‐southeast (NW‐SE) in the Northern Hemisphere (NH) and propagated equator‐westwards. These properties are typically attributed to MSTIDs generated through the coupled Perkins and sporadic E instabilities. This is supported by observed conditions in both hemispheres indicating the presence of sporadic E layers and reasonable Perkins instability growth rates. The second MSTID was aligned NW‐SE (SH) and NE‐SW (NH) and propagated equator‐eastwards and represents the first optical observations of conjugate equator‐eastward propagating MSTIDs. A possible linkage to gravity wave‐induced polarization electric fields in the NH (and mapped to the SH) is presented, as significant gravity wave activity was observed in OH and OI greenline observations by the Asiago imager. Their equator‐eastward propagation direction was favored by background winds at the hemisphere of origin, as determined from global model observations. 

   more » « less