On October 11, 2019, NASA's ICON satellite was launched into a circular orbit at about 590 km altitude, inclined by 27°. The spacecraft carries four scientific instruments dedicated to the study of the coupling between the lower atmosphere, the upper atmosphere, and the solar wind. Besides the in-situ plasma measurement performed by the Ion Velocity Meter (IVM) (Heelis et al., 2017), the remaining three instruments remotely sense the neutral and ionized atmosphere at altitudes ranging from about 90-600 km by observing airglow emissions in several wavelength ranges. In the visible domain, the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) observes the red and green oxygen airglow lines for wind speed retrieval and the 𝐴𝐴 O2 A-band in the near-infrared to measure the thermospheric temperature (Englert et al., 2017;Harding et al., 2017;Stevens et al., 2017). 𝐴𝐴 O + density profiles are retrieved at the 12-s measurement cadence by the two complementary instruments operating in the ultraviolet: the Far Ultra Violet Imaging Spectrograph (FUV) and the Extreme Ultra Violet Spectrograph (EUV). The first one simultaneously measures the 𝐴𝐴 OI -135.6 nm emission of atomic oxygen and the Lyman-Birge-Hopfield (LBH) band of 𝐴𝐴 N2 near 157 nm (Mende et al., 2017). During nighttime, the 135.6-nm channel is used alone to infer the 𝐴𝐴 O + density profile by observing the radiative recombination of oxygen ions with ambient electrons (Kamalabadi et al., 2018). On the dayside, both the 135.6 nm and LBH emissions are measured and combined to determine 𝐴𝐴 O and 𝐴𝐴 N2 altitude profiles and column 𝐴𝐴 O∕N2, used to monitor the atmospheric composition changes (Stephan et al., 2018). The EUV spectrograph records limb altitude profiles of terrestrial emissions in the extreme ultraviolet spectrum from 54 to 88 nm (Sirk et al., 2017). Specifically, the 𝐴𝐴 OII -61.7 and 83.4 nm emissions are used to retrieve daytime 𝐴𝐴 O + altitude profiles (Stephan et al., 2017).The radio-occultation space mission program COSMIC-2 (C2) currently provides up to 3,000 electron density profiles on a daily basis since October 1, 2019, using six spacecraft orbiting above low latitudes at similar altitudes as ICON. Additionally, ground-based ionosondes allow retrieving precise and accurate measurements of the electron density profile up to the peak altitude. These two data sets provide a large and robust
Simultaneous measurements of F‐region neutral winds near 250 km and topside field‐aligned interhemispheric plasma flow near 600 km, made by the ionospheric connections 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 near 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.
The Far Ultraviolet Imaging Spectrograph (FUV) onboard the NASA-ICON spacecraft has been providing nighttime O + density profiles over mid-and low-latitude since December 2019. These profiles are compared to electron density profiles provided by GNSS radiooccultations and ground-based ionosondes, mainly at the F-peak level where both density and height are compared. This work is an important update of the earlier study published by Wautelet et al. (J. Geophys. Res. Space Phys. 126(11):e2021JA029360, 2021) for two reasons: First, several methodological improvements have been implemented at the calibration and inversion levels. Second, the present work relies on an extended time range, ranging from December 2019 to August 2022, covering therefore periods of increased solar activity, which was not the case for the previous work. It is found that the peak density and height are, on average, similar to radio-based observations by about 10% in density and 7 km in height, meaning that FUV provides peak characteristics compatible with existing ionospheric datasets based on radio signals. However, comparisons of FUV and radio-occultation profiles have to be considered very carefully due to the potentially large difference in the observation geometry, which can account for large density discrepancies even between profiles being closely located and mostly simultaneous. This is particularly important around the crests of the equatorial anomaly where the largest density discrepancies have been observed. In addition, this study highlights the variability of the FUV profiles at the bottomside level, with the analysis of cases where rather large density values were observed while small density values are expected. The latter observation does nevertheless not impact the statis-The Ionospheric Connection Explorer (ICON) Mission: First Results Edited by David E. Siskind and Ruth S. Lieberman G. Wautelet
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 a technique for estimation of photoelectron fluxes and their spectra by combining coincident ICON and COSMIC2 measurements and show that a significant fraction of ICON‐FUV measurements is affected by CPEs during the winter solstice. A comparison of estimated photoelectron fluxes with measured photoelectron spectra is used to gain further insights into the estimation method and reveals consistent values within 10–60 eV.
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