Thermal equilibrium in planetary atmospheres occurs at altitudes where the ion, electron, and neutral temperatures are equal. Thermal equilibrium is postulated to occur in the collision-dominated lower ionosphere. This postulated altitude is above the lower boundary of all empirical models of planetary ionospheres. Physics-based model predictions of the altitude can not be validated due to a lack of adequate simultaneous observations of temperature profiles. This study presents temperature profiles from simultaneous observations on Atmosphere Explorer–C below 140 km and quiet-time neutral observations from TIMED/GUVI over Millstone Hill. These are compared with profiles from physics-based models with a discussion of their respective limitations. We conclude that there does not yet exist a quantitative understanding of the ion, electron, and neutral thermalization processes in low-altitude planetary ionospheres. Progress on this topic requires an adequate database of simultaneous ion, electron, and temperature profiles in the 110 to 140 km altitude range.

The Tonga volcano eruption at 04:14:45 UT on 2022-01-15 released enormous amounts of energy into the atmosphere, triggering very significant geophysical variations not only in the immediate proximity of the epicenter but also globally across the whole atmosphere. This study provides a global picture of ionospheric disturbances over an extended period for at least four days. We find traveling ionospheric disturbances (TIDs) radially outbound and inbound along entire Great-Circle loci at primary speeds of ~300-350 m/s (depending on the propagation direction) and 500-1000 km horizontal wavelength for front shocks, going around the globe for three times, passing six times over the continental US in 100 hours since the eruption. TIDs following the shock fronts developed for ~8 hours with 10-30 min predominant periods in near- and far- fields. TID global propagation is consistent with the effect of Lamb waves which travel at the speed of sound. Although these oscillations are often confined to the troposphere, Lamb wave energy is known to leak into the thermosphere through channels of atmospheric resonance at acoustic and gravity wave frequencies, carrying substantial wave amplitudes at high altitudes. Prevailing Lamb waves have been reported in the literature as atmospheric responses to the gigantic Krakatoa eruption in 1883 and other geohazards. This study provides substantial first evidence of their long-duration imprints up in the global ionosphere. This study was enabled by ionospheric measurements from 5,000+ world-wide Global Navigation Satellite System (GNSS) ground receivers, demonstrating the broad implication of the ionosphere measurement as a sensitive detector for atmospheric waves and geophysical disturbances.

This paper investigates the local and global ionospheric responses to the 2022 Tonga volcano eruption, using ground-based Global Navigation Satellite System (GNSS) total electron content (TEC), Swarm in-situ plasma density measurements, the Ionospheric Connection Explorer (ICON) Ion Velocity Meter (IVM) data, and ionosonde measurements. The main results are as follows: (1) A significant local ionospheric hole of more than 10 TECU depletion was observed near the epicenter ~45~min after the eruption, comprising of several cascading TEC decreases and quasi-periodic oscillations. Such a deep local plasma hole was also observed by space-borne in-situ measurements, with an estimated horizontal radius of 10-15 deg and persisted for more than 10 hours in ICON-IVM ion density profiles until local sunrise. (2) Pronounced post-volcanic evening equatorial plasma bubbles (EPBs) were continuously observed across the wide Asia-Oceania area after the arrival of volcano-induced waves; these caused a Ne decrease of 2-3 orders of magnitude at Swarm/ICON altitude between 450-575~km, covered wide longitudinal ranges of more than 140 deg and lasted around 12 hours. (3) Various acoustic-gravity wave modes due to volcano eruption were observed by accurate Beidou geostationary orbit (GEO) TEC, and the huge ionospheric hole was mainly caused by intense shock-acoustic impulses. TEC rate of change index revealed globally propagating ionospheric disturbances at a prevailing Lamb-wave mode of ~315 m/s; the large-scale EPBs could be seeded by acoustic-gravity resonance and coupling to less-damped Lamb waves, under a favorable condition of volcano-induced enhancement of dusktime plasma upward ExB drift and postsunset rise of the equatorial ionospheric F-layer.

This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated Ne/Ti/Te are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative By and negative Bz. (2) SAPS is usually associated with a midlatitude trough of 15–20\% depletion in the background density. The SAPS-related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8\% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, Te \times Ne, in the SAPS associated region is strongly dependent on 1/Ne, with Te exhibiting a negative correlation with respect to $Ne$.

We report new findings of total electron content (TEC) perturbations in the southern hemisphere at conjugate locations to the northern eclipse on 21 August 2017. We identified a persistent conjugate TEC depletion by 10-15\% during eclipse time, elongating along magnetic latitudes with $\sim$5$^\circ$ latitudinal width, moving equatorward, and becoming most pronounced at lower magnetic latitudes ($<$20$^\circ$S) when the ionosphere in the northern low latitudes were masked. This depletion was coincident with a weakening of the southern crest of the equatorial ionization anomaly (EIA), while the northern EIA crest stayed almost undisturbed or was slightly enhanced. We suggest these conjugate perturbations were associated with dramatic eclipse initiated plasma pressure reductions in the flux tubes, with a large portion of shorter tubes located at low latitudes underneath the Moon’s shadow. These small L-shell tubes transversed the F region ionosphere at low and equatorial latitudes. The plasma pressure gradient was markedly skewed northward in the flux tubes at low and equatorial latitudes, as was the neutral pressure. These effects caused a general northward motion tendency for plasma within the flux tubes, and inhibited normal southward diffusion of equatorial fountain plasma into the southern EIA region.

The mesosphere and lower thermosphere (MLT) region is dominated globally by dynamics at various scales: planetary waves, tides, gravity waves, and stratified turbulence. The latter two can co-exist and be significant at horizontal scales less than 500 km, scales that are difficult to measure. This study presents a recently deployed multi-static specular meteor radar system, SIMONe Peru, which can be used to observe these scales. The radars are positioned at and around the Jicamarca Radio Observatory, which is located at the magnetic equator. Besides presenting preliminary results of typically reported large scale features, like the dominant diurnal tide at low latitudes, we show results on selected days of spatially and temporally resolved winds obtained with two methods based on: (a) estimation of mean wind and their gradients (gradient method), and (b) an inverse theory with Tikhonov regularization (regularized wind field inversion method). The gradient method allows improved MLT vertical velocities and, for the first time, low-latitude wind field parameters such as horizontal divergence and relative vorticity. The regularized wind field inversion method allows the estimation of spatial structure within the observed area and has the potential to outperform the gradient method, in particular when more detections are available or when fine adaptive tuning of the regularization factor is done. SIMONe Peru adds important information at low latitudes to currently scarce MLT continuous observing capabilities. Results contribute to studies of the MLT dynamics at different scales inherently connected to lower atmospheric forcing and E-region dynamo related ionospheric variability.