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1. Typical Characteristics of Whistler Mode Waves Categorized by Their Spectral Properties Using Van Allen Probes Observations

   https://doi.org/10.1029/2019GL082161  

   Teng, S.; Tao, X.; Li, W. (April 2019, Geophysical Research Letters)

   Abstract Properties of banded, no‐gap, lower band only, and upper band only whistler mode waves (0.1–0.8f_ce) outside the plasmasphere are investigated using Van Allen Probes data. Our analysis shows that no‐gap whistler waves have higher occurrence rate at morning side and dayside, while banded and lower band only waves have higher occurrence rate between midnight and dawn. We also find that the occurrence rate of no‐gap whistler waves peaks at magnetic latitude |MLAT|∼8–10°, while banded waves have higher occurrence rate near the equator for°. The wave normal angle distributions of these four groups of waves are similar to previous results. The distinct local time and latitudinal distribution of no‐gap and banded whistler mode waves is critical to further understand the formation mechanism of the power minimum at half electron gyrofrequency. 

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2. Localized Heating of the Martian Topside Ionosphere Through the Combined Effects of Magnetic Pumping by Large‐Scale Magnetosonic Waves and Pitch Angle Diffusion by Whistler Waves

   https://doi.org/10.1029/2019GL086408  

   Fowler, C. M.; Agapitov, O. V.; Xu, S.; Mitchell, D. L.; Andersson, L.; Artemyev, A.; Espley, J.; Ergun, R. E.; Mazelle, C. (March 2020, Geophysical Research Letters)

   Abstract We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of periodic (25 s) large‐scale (hundreds of km) magnetosonic waves propagating into the Martian dayside upper ionosphere. These waves adiabatically modulate the superthermal electron distribution function, and the induced electron temperature anisotropies drive the generation of observed electromagnetic whistler waves. The localized (in altitude) minimum in the ratio_pe/_ce provides conditions favorable for the local enhancement of efficient wave‐particle interactions, so that the induced whistlers act back on the superthermal electron population to isotropize the plasma through pitch angle scattering. These wave‐particle interactions break the adiabaticity of the large‐scale magnetosonic wave compressions, leading to local heating of the superthermal electrons during compressive wave “troughs.” Further evidence of this heating is observed as the subsequent phase shift between the observed perpendicular‐to‐parallel superthermal electron temperatures and compressive wave fronts. This heating mechanism may be important at other unmagnetized bodies. 

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3. Toward an Integrated View of Ionospheric Plasma Instabilities: 4. Behavior in the Transitional Valley Region

   https://doi.org/10.1029/2019JA027303  

   Makarevich, Roman A. (November 2019, Journal of Geophysical Research: Space Physics)

   Abstract Behavior of unstable plasma waves generated by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI) is analyzed in the transitional valley region near 120 km in altitude. The analysis is based on the expression for the FBI/GDI growth rateγthat has been recently generalized to include ion inertia effects for arbitrary altitude and wavelength, within the limits imposed by the fluid and local approaches. It is found that the ion inertia leads to a different instability behavior when the convection component is between the two critical values determined by the ion acoustic speedC_sand the ratior_ibetween the ion collision and gyrofrequency. The most interesting case occurs near 120 km, just below wherer_i=1. From analysis of electron density gradientsG=∇n/nthat result in marginal instability conditionγ=0 (i.e., critical gradientsG₀), there exists a critical scale whereG₀=0 and below which all waves are unstable to FBI. Above this scale,G₀>0 and gradients need to be sufficiently strongG>G₀for the plasma to become unstable through GDI. There also exists a maximum in dependence, which refers to the least unstable scale and gradient. For convection outside of the specified range, no critical or least unstable scale exists, which is a typical situation outside of the transitional valley region. Overall, this analysis shows that the FBI convection thresholds and the GDI critical gradients are modified by the ion inertia and that the effects are most pronounced in the transitional valley region near 120 km. 

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4. Ion Heating in the Polar Cap Under Northwards IMF Bz

   https://doi.org/10.1029/2021JA029155  

   Lamarche, Leslie J.; Varney, Roger H.; Reimer, Ashton S. (November 2021, Journal of Geophysical Research: Space Physics)

   Abstract Joule heating deposits a significant amount of energy into the high‐latitude ionosphere and is an important factor in many magnetosphere‐ionosphere‐thermosphere coupling processes. We consider the relationship between localized temperature enhancements in polar cap measured with the Resolute Bay Incoherent Scatter Radar‐North (RISR‐N) and the orientation of the interplanetary magnetic field (IMF). Based on analysis of 10 years of data, RISR‐N most commonly observes ion heating in the noon sector under northwards IMF. We interpret heating events in that sector as being primarily driven by sunwards plasma convection associated with lobe reconnection. We attempt to model two of the observed temperature enhancements with a data‐driven first principles model of ionospheric plasma transport and dynamics, but fail to fully reproduce the ion temperature enhancements. However, evaluating the ion energy equation using the locally measured ion velocities reproduces the observed ion temperature enhancements. This result indicates that current techniques for estimating global plasma convection pattern are not adequately capturing mesoscale flows in the polar cap, and this can result in underestimation of the energy deposition into the ionosphere and thermosphere. 

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5. Statistics of Multi‐MeV Electron Drift‐Periodic Flux Oscillations Using Van Allen Probes Observations

   https://doi.org/10.1029/2022GL097995  

   Zhao, Hong; Sarris, Theodore E.; Li, Xinlin; Huckabee, Isabella G.; Baker, Daniel N.; Jaynes, Allison J.; Kanekal, Shrikanth G. (March 2022, Geophysical Research Letters)

   Abstract Multi‐MeV electron drift‐periodic flux oscillations observed in Earth's radiation belts indicate radial transport and energization/de‐energization of these radiation belt core populations. Using multi‐year Van Allen Probes observations, a statistical analysis is conducted to understand the characteristics of this phenomenon. The results show that most of these flux oscillations result from resonant interactions with broadband ultralow frequency (ULF) waves and are indicators of ongoing radial diffusion. The occurrence frequency of flux oscillations is higher during high solar wind speed/dynamic pressure and geomagnetically active times; however, a large number of them were still observed under mild to moderate solar wind/geomagnetic conditions. The occurrence frequency is also highest (up to ∼30%) at low L‐shells () under various geomagnetic activity, suggesting the general presence of broadband ULF waves and radial diffusion at low L‐shells even during geomagnetically quiet times and showing the critical role of the electron phase space density radial gradient in forming drift‐periodic flux oscillations. 

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