Atmospheric Lunar Tide in the Low Latitude Thermosphere-Ionosphere.

We present simultaneous, independent measurements of the atmospheric semidiurnal lunar tide in neutral winds and plasma velocities from NASA's Ionospheric Connection Explorer, and in atomic oxygen 135.6 nm airglow measured by the Global-scale Observations of the Limb and Disk. Westward tidal winds near 115 km at the magnetic equator occur during part of the upward phase of the in-situ plasma drift. Vertical motions associated with the field-aligned plasma velocity occur away from the magnetic equator. The morphology of the lunar tide, and the phasing between the airglow and plasma velocities are consistent with E × B drift as a mechanism for linking neutral wind and plasma perturbations. This work provides the first observational quantification of global-scale E- and F-region coupling through E × B and field-aligned vertical drifts.

The Ionospheric Connection Explorer - Prime Mission Review.

The two-year prime mission of the NASA Ionospheric Connection Explorer (ICON) is complete. The baseline operational and scientific objectives have been met and exceeded, as detailed in this report. In October of 2019, ICON was launched into an orbit that provides its instruments the capability to deliver near-continuous measurements of the densest plasma in Earth's space environment. Through collection of a key set of in-situ and remote sensing measurements that are, by virtue of a detailed mission design, uniquely synergistic, ICON enables completely new investigations of the mechanisms that control the behavior of the ionosphere-thermosphere system under both geomagnetically quiet and active conditions. In a two-year period that included a deep solar minimum, ICON has elucidated a number of remarkable effects in the ionosphere attributable to energetic inputs from the lower and middle atmosphere, and shown how these are transmitted from the edge of space to the peak of plasma density above. The observatory operated in a period of low activity for 2 years and then for a year with increasing solar activity, observing the changing balance of the impacts of lower and upper atmospheric drivers on the ionosphere.

Ion Velocity and Temperature Variation Around Topside Nighttime Irregularities: Contrast Between Low- and Mid-Latitude Regions.

Studies on low-/mid-latitude ionospheric irregularities have a long history, but ion temperature and drift velocity variations within the irregularities have been rarely investigated, especially at mid-latitudes. To fill this knowledge gap, we statistically investigate ion temperature and velocity of mid-latitude ionospheric irregularities and compare them to those of low-latitude ones. This study mainly relies on in situ plasma moment data from the newly launched Ionospheric Connection Explorer (ICON) satellite and is supplemented by Republic of China Satellite 1 (ROCSAT-1) data in 2000-2004. At low latitudes (|MLAT|<10°) plasma density irregularities encountered by ICON have the following properties: lower-density regions have cooler ions and more poleward/westward/outward velocity than (|MLAT| = 10°âˆ¼30°), the opposite trends are observed: lower-density regions have hotter ions and more the ambient plasma, which agrees with previous reports on equatorial plasma bubbles. At mid-latitudes equatorward/eastward/inward drift than the ambient, which is a new finding in this study. The mid-latitude observations can be put into the context of Medium-Scale Traveling Ionospheric Disturbance (MSTID) properties reported previously, which evidences that topside irregularities at nighttime mid-latitudes generally represent MSTIDs. ROCSAT-1 data support the contrast between low- and mid-latitude ionospheric irregularities.

Isolated Peak of Oxygen Ion Fraction in the Post-Noon Equatorial F-Region: ICON and SAMI3/WACCM-X.

In the equatorial region, the fraction of oxygen ions (O+) in the topside ionosphere contains information on the source altitude of the plasma, which is controlled, in part, by the vertical plasma motion in the F-region. Previous studies on this topic are restricted by limited coverage of local time, latitude, and season, leaving a significant knowledge gap in the distribution of the topside ionospheric composition. In this study, we statistically investigate the O+ fraction measured by ICON/IVM over all the local time sectors and seasons at low/midlatitudes. For the first time, we have found that an isolated peak in the O+ fraction emerges in the post-noon equatorial region. The peak is most prominent during equinoxes, while during solstices it is connected to the O+ fraction bulges in the local summer midlatitudes. Simulations with SAMI3 coupled with thermospheric parameters from WACCM-X reproduce the peak of the O+ fraction. The post-noon equatorial peak can be explained by the net vertical motion of plasma consisting of transports either parallel or perpendicular to geomagnetic field lines.

Sensitivity of Upper Atmosphere to Different Characteristics of Flow Bursts in the Auroral Zone.

Meso-scale plasma convection and particle precipitation could be significant momentum and energy sources for the ionosphere-thermosphere (I-T) system. Following our previous work on the I-T response to a typical midnight flow burst, flow bursts with different characteristics (lifetime, size, and speed) have been examined systematically with Global Ionosphere-Thermosphere Model (GITM) simulations in this study. Differences between simulations with and without additional flow bursts are used to illustrate the impact of flow bursts on the I-T system. The neutral density perturbation due to a flow burst increases with the lifetime, size, and flow speed of the flow burst. It was found that the neutral density perturbation is most sensitive to the size of a flow burst, increasing from ∼0.3% to ∼1.3% when the size changes from 80 to 200 km. A westward-eastward asymmetry has been identified in neutral density, wind, and temperature perturbations, which may be due to the changing of the forcing location in geographic coordinates and the asymmetrical background state of the I-T system. In addition to midnight flow bursts, simulations with flow bursts centered at noon, dawn, and dusk have also been carried out. A flow burst centered at noon (12.0 Local Time [LT], 73°N) produces the weakest perturbation, and a flow burst centered at dusk (18.0 LT, 71°N) produces the strongest. Single-cell and two-cell flow bursts induce very similar neutral density perturbation patterns.

Atmosphere-Ionosphere (A-I) Coupling as Viewed by ICON: Day-to-Day Variability Due to Planetary Wave (PW)-Tide Interactions.

Coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and total ion densities (:=Ne, electron density) are analyzed during January 1-21, 2020 to reveal the relationship between neutral winds and ionospheric variability on a day-to-day basis. Atmosphere-ionosphere (A-I) connectivity inevitably involves a spectrum of planetary waves (PWs), tides and secondary waves due to wave-wave nonlinear interactions. To provide a definitive attribution of dynamical origins, the current study focuses on a time interval when the longitudinal wave-4 component of the E-region winds is dominated by the eastward-propagating diurnal tide with zonal wavenumber s = -3 (DE3). DE3 is identified in winds and ionospheric parameters through its characteristic dependence on local solar time and longitude as ICON's orbit precesses. Superimposed on this trend are large variations in low-latitude DE3 wave-4 zonal winds (±40 ms-1) and topside F-region equatorial vertical drifts at periods consistent with 2-days and 6-days PWs, and a ~3-day ultra-fast Kelvin wave (UFKW), coexisting during this time interval; the DE3 winds, dynamo electric fields, and drifts are modulated by these waves. Wave-4 variability in Ne is of order 25%-35%, but the origins are more complex, likely additionally reflecting transport by ~20-25 ms-1 wave-4 in-situ winds containing strong signatures of DE3 interactions with ambient diurnal Sun-synchronous winds and ion drag. These results are the first to show a direct link between day-to-day wave-4 variability in contemporaneously measured E-region neutral winds and F-region ionospheric drifts and electron densities.

Regulation of ionospheric plasma velocities by thermospheric winds.

Earth's equatorial ionosphere exhibits substantial and unpredictable day-to-day variations in density and morphology. This presents challenges in preparing for adverse impacts on geopositioning systems and radio communications even 24 hours in advance. The variability is now theoretically understood as a manifestation of thermospheric weather, where winds in the upper atmosphere respond strongly to a spectrum of atmospheric waves that propagate into space from the lower and middle atmosphere. First-principles simulations predict related, large changes in the ionosphere, primarily through modification of wind-driven electromotive forces: the wind-driven dynamo. Here we show the first direct evidence of the action of a wind dynamo in space, using the coordinated, space-based observations of winds and plasma motion made by the National Aeronautics and Space Administration Ionospheric Connection Explorer. A clear relationship is found between vertical plasma velocities measured at the magnetic equator near 600 km and the thermospheric winds much farther below. Significant correlations are found between the plasma and wind velocities during several successive precession cycles of the Ionospheric Connection Explorer's orbit. Prediction of thermospheric winds in the 100-150 km altitude range emerges as the key to improved prediction of Earth's plasma environment.

Q2DW-Tide and -Ionosphere Interactions as Observed From ICON and Ground-Based Radars.

A quasi-2-day wave (Q2DW) event during January-February, 2020, is investigated in terms of its propagation from 96 to 250 km as a function of latitude (10°S to 30°N), its nonlinear interactions with migrating tides to produce 16 and 9.6-h secondary waves (SWs), and the plasma drift and density perturbations that it produces in the topside F-region (590-607 km) between magnetic latitudes 18°S and 18°N. This is accomplished through analysis of coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and ion densities, and wind measurements from four low-latitude (±15°) specular meteor radars (SMRs). The Q2DW westward-propagating components that existed during this period consist of zonal wavenumbers s = 2 and s = 3, that is, Q2DW+2 and Q2DW+3 (e.g., He, Chau et al., 2021, https://doi.org/10.1029/93jd00380). SWs in the ICON measurements are inferred from Q2DW+2 and Q2DW+3 characteristics derived from traditional longitude-UT fits that potentially contain aliasing contributions from SWs ("apparent" Q2DWs), from fits to space-based zonal wavenumbers that each reflect the aggregate signature of either Q2DW+2 or Q2DW+3 and its SWs combined ("effective" Q2DWs), and based on information contained in published numerical simulations. The total Q2DW ionospheric responses consists of F-region field-aligned and meridional drifts of order ±25 ms-1 and ±5-7 ms-1, respectively, and total ion density perturbations of order (±10%-25%). It is shown that the SWs can sometimes make substantial contributions to the Q2DW winds, drifts, and plasma densities.