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The quiet-time solar wind electrons feature non-thermal characteristics when viewed from the perspective of their velocity distribution functions. They typically have an appearance of being composed of a denser thermal “core” population plus a tenuous energetic “halo” population. At first, such a feature was empirically fitted with the kappa velocity space distribution function, but ever since the ground-breaking work by Tsallis, the space physics community has embraced the potential implication of the kappa distribution as reflecting the non-extensive nature of the space plasma. From the viewpoint of microscopic plasma theory, the formation of the non-thermal electron velocity distribution function can be interpreted in terms of the plasma being in a state of turbulent quasi-equilibrium. Such a finding brings forth the possible existence of a profound inter-relationship between the non-extensive statistical state and the turbulent quasi-equilibrium state. The present paper further develops the idea of solar wind electrons being in the turbulent equilibrium, but, unlike the previous model, which involves the electrostatic turbulence near the plasma oscillation frequency (i.e., Langmuir turbulence), the present paper considers the impact of transverse electromagnetic turbulence, particularly, the turbulence in the whistler-mode frequency range. It is found that the coupling of spontaneously emitted thermal fluctuations and the background turbulence leads to the formation of a non-thermal electron velocity distribution function of the type observed in the solar wind during quiet times. This demonstrates that the whistler-range turbulence represents an alternative mechanism for producing the kappa-like non-thermal distribution, especially close to the Sun and in the near-Earth space environment. 

Abstract Terrestrial Very‐Low‐Frequency (VLF) energy from both lightning discharges and radio transmitters has a role in affecting the energetic electrons in the Van Allen radiation belts, but quantification of these effects is particularly difficult, largely due to the collisional damping experienced in the highly variable electron density in the D‐ and E‐region ionosphere. The Faraday International Reference Ionosphere (FIRI) model was specifically developed by combining lower‐ionosphere chemistry modeling with in situ rocket measurements, and represents to date the most reliable source of electron density profiles for the lower ionosphere. As a full‐resolution empirical model, FIRI is not well suited to D‐ and E‐region ionosphere inversion, and its applicability in transionospheric VLF simulation and in remote sensing of the lower ionosphere is limited. Motivated by how subionospheric VLF remote sensing has been aided by the Wait and Spies (WS) profile (Wait & Spies, 1964), in this study, we parameterize the FIRI profiles and extend the WS profile to the E‐region ionosphere by introducing two new parameters: the knee altitudeh_kand the sharpness parameter for the E‐region ionosphereβ_E. Using this modified WS profile, we calculate the expected signals at different receiver locations from the NAA, NPM, and NWC transmitters under the full range of possible ionospheric conditions. We also describe and validate a method about how these results can be readily used to translate VLF measurements into estimates of the lower ionosphere electron density. Moreover, we use this method to evaluate the sensitivity of different ground receiver locations in lower‐ionosphere remote sensing.