We present a method for estimating incident photoelectrons’ energy spectra as a function of altitude by combining global scale far-ultraviolet (FUV) and radio-occultation (RO) measurements. This characterization provides timely insights important for accurate interpretation of ionospheric parameters inferred from the recently launched Ionospheric Connection Explorer (ICON) observations. Quantification of photoelectron impact is enabled by the fact that conjugate photoelectrons (CPEs) directly affect FUV airglow emissions but not RO measurements. We demonstrate the technique for the estimation of photoelectron fluxes and their spectra by combining coincident ICON-FUV and COSMIC2 measurements and show that a significant fraction of ICON-FUV measurements are affected by CPEs during the winter solstice. A comparison of estimated photoelectron fluxes with the SAMI2-PE model is used to gain further insights into the estimation method and reveals consistent values at low latitudes, while the results suggest that the model might be overestimating photoelectron fluxes by $\sim$10\% at mid latitudes.

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.

Using the horizontal neutral wind observations from the MIGHTI instrument onboard NASA’s ICON (Ionospheric Connection Explorer) spacecraft with continuous coverage, we determine the climatology of the mean zonal and meridional winds and the associated mean circulation at low- to middle latitudes (10S-45N) for Northern Hemisphere solstice conditions between 90 km and 200 km altitudes, specifically on 20 June 2020 solstice as well as for a one-month period from 8 June-7 July 2020. The data are averaged within appropriate altitude, longitude, latitude, solar zenith angle, and local time bins to produce mean wind distributions. The geographical distributions and local time variations of the mean horizontal circulation are evaluated. The instantaneous horizontal winds exhibit a significant degree of spatiotemporal variability often exceeding ~150 m/s. The daily averaged zonal mean winds demonstrate day-to-day variability. Eastward zonal winds and northward (winter-to-summer) meridional winds are prevalent in the lower thermosphere, which provides indirect observational evidence of the eastward momentum deposition by small-scale gravity waves. The mean neutral winds and circulation exhibit smaller scale structures in the lower thermosphere (90-120 km), while they are more homogeneous in the upper thermosphere, indicating the increasingly dissipative nature of the thermosphere. The mean wind and circulation patterns inferred from ICON/MIGHTI measurements can be used to constrain and validate general circulation models, as well as input for numerical wave models.

Simultaneous measurements of F-region neutral winds near 250 km and topside interhemispheric plasma flow near 600 km, made by the ICON satellite, allow the connection between these parameters to be observationally established for the first time. The largest variations in the topside plasma flows are seen as a function of season and are shown to depend on trans-equatorial neutral winds below the F peak in a manner that is essentially the same during the daytime and the nighttime for the solar minimum conditions that prevail in 2020. This finding is consistent with established principles of a servo model of the ionosphere for which both production and loss rates in the topside are specified by the O/N2 ratio at the F-peak height. The intermediate relationships, describing how the neutral wind influences the F-peak height and how the O+ plasma pressure gradient across the equator influences the interhemispheric plasma flow are also investigated and found to be consistent with expectations.