Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

[1] It has been known for at least 30 years that vertically narrow thermal layers form within the middle atmosphere. Two types of temperature enhancements, the low-latitude to midlatitude mesosphere inversion layer (MIL) and the highlatitude winter stratosphere temperature enhancement (STE), have both received much attention within the atmospheric science community because of their unexplained formation mechanisms and potential impacts on the middle-atmosphere global circulation. Numerous experimental, numerical, and theoretical studies have attempted to explain certain aspects of these respective thermal layers, but no one theory consistently and satisfactorily describes all the features observed. We present a review of the literature and explicitly propose a classification scheme based on the different formation mechanisms suspected to cause these events. For the MIL we demonstrate that there are two subtypes. The first one is tidally driven and tends to occur above $85 km. This MIL originates from large-amplitude tidal waves propagating into the mesosphere and their subsequent nonlinear interactions with gravity waves, which can often create the appearance of a ''double MIL'' separated by approximately one vertical tidal wavelength ($25 km). The other subtype of MIL is formed by a climatological planetary wave dissipation mechanism that occurs at a zerowind line. The dissipation of the planetary wave tends to generate a mesoscale ($1000 km) inversion layer in the range of 65 -80 km. These two formation mechanisms explain a host of observed characteristics, including the reason behind the downward progression of some MILs and not others, the different climatological nature of the two forms of MIL events, and the relative scarcity of MIL observations at high latitudes. The STE is believed to be generated by an altogether different process, namely, the nonlinear interaction between the polar vortex and planetary waves/Aleutian High. The induced temperatures typically peak around 40 km and often exceed 300 K, generating what appears to be a ''low, hot stratopause.'' When vertical temperature profiles are combined with synoptic analyses, one observes that the STE is the consequence of high-latitude vortex interactions creating a baroclinic atmosphere, i.e., a downward adiabatic compression induced by an ageostropic flow. We summarize the details of the relationship between this feature and sudden stratospheric warmings, as well as the potential for in situ gravity wave generation. We close with a review of currently unexplained MIL/STE features and offer new directions for future middle-atmosphere thermal layer research.

Medium‐scale traveling ionospheric disturbances (MSTIDs) are wave‐like ionospheric perturbations routinely observed by high‐frequency radars. We focus on a class of MSTIDs observed during the winter daytime at high latitudes and midlatitudes. The source of these MSTIDs remains uncertain, with the two primary candidates being space weather and lower atmospheric processes. We surveyed observations from four high‐latitude and six midlatitude Super Dual Auroral Radar Network radars in the North American sector from November to May 2012 to 2015. The MSTIDs observed have horizontal wavelengths between ∼150 and 650 km and horizontal velocities between ∼75 and 325 m s−1. In local fall and winter seasons the majority of MSTIDs propagated equatorward, with bearings ranging from ∼125° to 225° geographic azimuth. No clear correlation with space weather activity as parameterized by AE and SYM‐H could be identified. Rather, MSTID observations were found to have a strong correlation with polar vortex dynamics on two timescales. First, a seasonal timescale follows the annual development and decay of the polar vortex. Second, a shorter 2–4 week timescale again corresponds to synoptic polar vortex variability, including stratospheric warmings. Additionally, statistical analysis shows that MSTIDs are more likely during periods of strong polar vortex. Direct comparison of the MSTID observations with stratospheric zonal winds suggests that a wind filtering mechanism may be responsible for the strong correlation. Collectively, these observations suggest that polar atmospheric processes, rather than space weather activity, are primarily responsible for controlling the occurrence of high‐latitude and midlatitude winter daytime MSTIDs.

Horizontal velocities ranged from 50 to 250 m s −1 with a distribution maximum between 100 and 150 m s −1 . Horizontal wavelengths ranged from 100 to 500 km with a distribution peak at 250 km, and periods between 23 and 60 min, suggesting that the MSTIDs may be consistent with thermospheric gravity waves. A local time (LT) dependence was observed such that the dominant (southeastward) population decreased in number as the day progressed until a late afternoon increase. The secondary (northwestward) population appeared only in the afternoon, possibly indicative of neutral wind effects or variability of sources. LT dependence was not observed in other parameters. Possible solar-geomagnetic and tropospheric MSTID sources were considered. The auroral electrojet (AE) index showed a correlation with MSTID statistics. Reverse ray tracing with the HINDGRATS model indicates that the dominant population has source regions over the Great Lakes and near the geomagnetic cusp, while the secondary population source region is 100 km above the Atlantic Ocean east of the Carolinas. This suggests that the dominant population may come from a region favorable to either tropospheric or geomagnetic sources, while the secondary population originates from a region favorable to secondary waves generated via lower atmospheric convection.

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