Ultraviolet images of Earth's polar regions obtained by high altitude spacecraft have proved to be immensely useful for documenting numerous features of the aurora and understanding the coupling between Earth's magnetosphere and ionosphere. In this study we have examined images obtained by the far ultraviolet Spectrographic Imager camera on the IMAGE satellite during the first three years of its mission (2000–2002) for comparison with observations of large geomagnetic disturbances (GMDs) by ground‐based magnetometers in eastern Arctic Canada. To our knowledge, this is the first study to investigate the use of high‐altitude imager data to identify the global context of GMDs. We found that rapid auroral motions or localized intensifications visible in these images coincide with regions of large dB / dt as well as localized and closely spaced up/down vertical currents and increased equivalent ionospheric currents, but one of the two events presented did not appear to be related to substorm processes. These magnetic perturbations and currents can appear or disappear in a few tens of seconds, thus highlighting the importance of images with a high cadence.

Abstract First results are presented from the conjugate maneuvers performed by NASA's Ionospheric Connection Explorer (ICON) spacecraft. During each several‐minute maneuver, ICON crosses the magnetic equator, measuring the plasma drift at the ∼600‐km apex of a magnetic field line and the neutral wind profiles (∼90–300 km altitude) along both ends of that field line. The analysis utilizes 149 pairs of maneuvers separated by ∼24 hr but at nearly the same location and local time. Principal component regression reveals that 39 ± 7% and 24 ± 9% of the day‐to‐day variance in the daytime vertical and zonal drift, respectively, is attributable to conjugate neutral winds. The remaining variance is likely driven by external potentials from non‐conjugate winds and geomagnetic activity (median Kp 2−). Zonal winds at 100–113 km and >120 km altitude are the primary drivers of conjugate vertical and zonal drift variance, respectively. These observations can test vertical‐coupling mechanisms in whole‐atmosphere models.

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.

The Ion Velocity Meter (IVM) on NASA’s Ionospheric Connection Explorer (ICON) reports the in-situ ion density, ion temperature and 3-component ion drift velocity, retrieved from measurements by a retarding potential analyzer and an ion drift meter. ICON was launched during a deep solar minimum in late 2019, followed by a solar quiet (F10.7 < 80) period until September 2020. In order to quantify the uncertainties in the IVM’s drift velocity in a low plasma density environment, we compared IVM’s vertical drift velocity with eastward electric field (EEF) obtained from Swarm’s equatorial electrojet current measurements, the vertical drift from ground-based incoherent scatter radar (ISR) at Jicamarca Radio Observatory (JRO) and from Jicamarca Unattended Long-term studies of Ionosphere and Atmosphere (JULIA) coherent mode. The main results of this study show that (1) the vertical drift derived from Swarm’s EEF and ISR are in good agreement with the zonal electric field derived from JULIA’s vertical drift regardless of the F10.7 value. (2) The zonal electric field derived from IVM’s meridional drift is in good agreement with Swarm’s EEF in 2021, whereas the distribution is highly scattered in the deepest solar minimum in 2020. (3) An ad hoc IVM correction based on the 24-hour running mean of meridional drift can bring the IVM data into better agreement with Swarm and JULIA. An additional quality control based on O + fractional composition may be needed for some studies using IVM’s vertical drift. By using the same methodology presented in this work, future missions could calibrate their drift measurements to facilitate meaningful integration with ICON/IVM observations through the comparision with ground-based measurements.

The Observatory for Heteroscale Magnetosphere-Ionosphere Coupling (OHMIC) is an innovative moderate-scale four-spacecraft (S/C) mission that combines high-time-resolution plasma and fields measurements in the auroral acceleration region (AAR) with high-resolution local and global auroral imaging to address one of the most vexing problems in heliophysics: Determining the physical processes by which electromagnetic (EM) energy is converted into the kinetic energy of charged particles in a collisionless plasma.By (1) using two S/C to determine temporal variations and spatial structures along the magnetic field in the auroral magnetosphere and (2) using two multi-spectral imaging spacecraft to perform concurrent global and local auroral imaging with temporal and spatial resolution comparable to that of the in-situ measurements, OHMIC overcomes the limitations of previous single-S/C auroral missions to resolve fundamental outstanding questions in auroral physics and magnetosphere-ionosphere (MI) coupling.Science Background.Charged-particle acceleration is a fundamental process throughout the universe.It underlies the development of cosmic rays, galactic and stellar jets, solar eruptions and flares, and the spectacular auroral displays seen on every magnetized planet with an atmosphere.Acceleration can occur directly via electric fields (E-fields) directed along a magnetic field, stochastically through turbulence, or resonantly through coherent wave-particle interactions.In general terms, the physics of particle acceleration involves the conversion of EM energy into particle energy.This conversion and the resulting strong coupling between Earth's magnetosphere and ionosphere in the auroral region is the focus of the OHMIC mission.The auroral region displays remarkably rich and complex acceleration processes including parallel Efields and double layers, stochastic acceleration of ions leading to ion outflow, resonant and nonresonant acceleration by kinetic Alfvén waves.In spite of previous successful auroral missions, the underlying physical processes of charged-particle acceleration, in particular the conversion of EM energy into particle energy, and specifically the evolution of energy conversion and particle acceleration along auroral field lines, are yet to be discovered The time-dependent and spatially inhomogeneous nature of auroral particle acceleration has been a major obstacle to understanding its basic processes.Concurrent, high-time resolution imaging, plasma, and field measurements are needed to overcome this obstacle and advance our understanding of MI coupling.OHMIC achieves the needed measurements by (1) deploying two S/C with the hightime resolution, in-situ measurement capabilities of the FAST mission plus high-cadence ion composition and (2) employing an advanced imaging system with high-spatial and time resolution.In addition, OHMIC provides global auroral imaging in order to relate the fine-scale measurements to the disturbance state of the magnetosphere and the large-scale effects of MI coupling.OHMIC employs high-heritage particle, fields and imaging instruments and is poised for fundamental breakthroughs in the physics of charged-particle acceleration, auroral dynamics and MI coupling.Science Strategy.OHMIC explores critical regions of energy conversion in the MI interaction with novel orbital alignments, including high-accuracy magnetic conjunctions that position two S/C across important domains of spatial and temporal transition.The mission achieves >500

Intense sunward (westward) plasma flows, named Subauroral Polarization Stream (SAPS), have been known to occur equatorward of the electron auroras for decades, yet their effect on the upper thermosphere has not been well understood. On the one hand, the large velocity of SAPS results in large momentum exchange upon each ion‐neutral collision. On the other hand, the low plasma density associated with SAPS implies a low ion‐neutral collision frequency. We investigate the SAPS effect during non‐storm time by utilizing a Scanning Doppler Imager (SDI) for monitoring the upper thermosphere, SuperDARN radars for SAPS, all‐sky imagers and DMSP Spectrographic Imager for the auroral oval, and GPS receivers for the total electron content. Our observations suggest that SAPS at times drives substantial (>50 m/s) westward winds at subauroral latitudes in the dusk‐midnight sector, but not always. The occurrence of the westward winds varies with AE index, plasma content in the trough, and local time. The latitudinally averaged wind speed varies from 60 to 160 m/s, and is statistically 21% of the plasma. These westward winds also shift to lower latitude with increasing AE and increasing MLT. We do not observe SAPS driving poleward wind surges, neutral temperature enhancements, or acoustic‐gravity waves, likely due to the somewhat weak forcing of SAPS during the non‐storm time.

We provide the first comparison of the ICON-EUV O+ density profile with radio wave datasets coming from GNSS radio-occultation, ionosondes and incoherent scatter radar. The peak density and height deduced from those different observation techniques are compared. It is found that the EUV-deduced peak density is smaller than that from other techniques by 50 to 60%, while the altitude of the peak is retrieved with a slight bias of 10 to 20 km on average. These average values are found to vary between November 2019 and March 2021. Magnetic latitude and local time are not factors significantly influencing this variability. In contrast, the EUV density is closer to that deduced from radio-wave techniques in the mid latitude region, i.e. where the ionospheric crests do not play a role. The persistent very low solar activity conditions prevailing during the studied time interval challenge the EUV O+ density profile retrieval technique. These values are consistent, both in magnitude and direction, with a systematic error on the order of 10% in the data or the forward model, or a combination of both. Ultimately, the EUV instrument on-board ICON provides the only known technique capable of precisely monitoring the ionospheric peak properties at daytime from a single space platform, on a global scale and at high cadence. This feature paves the way to transpose the technology to the study of the ionosphere surrounding other planets.

The eruption of the Hunga Tonga‐Hunga Ha'apai volcano on 15 January 2022 triggered atmospheric waves at all altitudes. The National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON) and European Space Agency Swarm satellites were well placed to observe its impact on the ionospheric wind dynamo. After the Lamb wave entered the dayside, Swarm A observed an eastward and then westward equatorial electrojet (EEJ) on two consecutive orbits, each with magnitudes exceeding the 99.9th percentile of typically observed values. ICON simultaneously observed the neutral wind (90–300 km altitude) at approximately the same distance from Tonga. The observed neutral winds were also extreme (>99.9th percentile at some altitudes). The covariation of EEJ and winds is consistent with recent theoretical and observational results, indicating that the westward electrojet is driven by strong westward winds in the Pedersen region (∼120–150 km). These observations imply that the dynamo is a key mechanism in the ionospheric response to the Tonga disturbance.

Since the earliest space‐based observations of Earth's atmosphere, ultraviolet (UV) airglow has proven a useful resource for remote sensing of the ionosphere and thermosphere. The NASA Ionospheric Connection Explorer (ICON) spacecraft, whose mission is to explore the connections between ionosphere and thermosphere utilizes UV airglow in the typical way: an extreme‐UV (EUV) spectrometer uses dayglow between 54 and 88 nm to measure the density of O + , and a far‐UV spectrograph uses the O 135.6 nm doublet and N 2 Lyman‐Birge‐Hopfield band dayglow to measure the column ratio of O to N 2 in the upper thermosphere. Two EUV emission features, O + 61.6 and 83.4 nm, are used for the O + retrieval; however, many other features are captured along the EUV instrument's spectral dimension. In this study, we examine the other dayglow features observed by ICON EUV and demonstrate that it measures a nitrogen feature around 87.8 nm which can be used to observe the neutral thermosphere.

The Far-UltraViolet (FUV) imager onboard the Ionospheric Connection Explorer (ICON) spacecraft provides two-dimensional limb images of oxygen airglow in the nightside low-latitude ionosphere that are used to determine the oxygen ion density. As yet, no FUV limb imager has been used for climatological analyses of Equatorial Plasma Bubbles (EPBs). To examine the potential of ICON/FUV for this purpose, we statistically investigate small-scale (~180 km) fluctuations of oxygen ion density in its limb images. The seasonal-longitudinal variations of the fluctuation level reasonably conform to the EPB statistics in existing literature. To further validate the ICON/FUV data quality, we also inspect climatology of the ambient (unfiltered) nightside oxygen ion density. The ambient density exhibits (1) the well-known zonal wavenumber-4 signatures in the Equatorial Ionization Anomaly (EIA) and (2) off-equatorial enhancement above the Caribbean, both of which agree with previous studies. Merits of ICON/FUV observations over other conventional data sets are discussed in this paper. Furthermore, we suggest possible directions of future work, e.g., synergy between ICON/FUV and the Global-scale Observations of the Limb and Disk (GOLD) mission.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Space Physics. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]First results from the retrieved column O/N2 ratio from the Ionospheric Connection Explorer (ICON): Longitudinal structures induced by nonmigrating tidesAuthorsScott LEnglandiDRobert R.MeieriDHarald U.FreyiDStephen BMendeiDAndrew W.StephaniDChristopher ScottKrieriDChihoko Y.CullensYen-Jung JoanneWuiDColin CTriplettiDMartin MSirkEricKorpelaiDBrian JHardingiDChristoph REnglertiDThomas J.ImmeliDSee all authors Scott L EnglandiDCorresponding Author• Submitting AuthorVirginia Polytechnic Institute and State UniversityiDhttps://orcid.org/0000-0001-5336-0040view email addressThe email was not providedcopy email addressRobert R. MeieriDGeorge Mason UniversityiDhttps://orcid.org/0000-0001-8497-7115view email addressThe email was not providedcopy email addressHarald U. FreyiDUniversity of California, BerkeleyiDhttps://orcid.org/0000-0001-8955-3282view email addressThe email was not providedcopy email addressStephen B MendeiDUniversity of California, BerkeleyiDhttps://orcid.org/0000-0001-5511-3668view email addressThe email was not providedcopy email addressAndrew W. StephaniDU.S. Naval Research LaboratoryiDhttps://orcid.org/0000-0002-9487-6235view email addressThe email was not providedcopy email addressChristopher Scott KrieriDVirginia Polytechnic Institute and State UniversityiDhttps://orcid.org/0000-0002-3786-4094view email addressThe email was not providedcopy email addressChihoko Y. CullensUniversity of California, Berkeleyview email addressThe email was not providedcopy email addressYen-Jung Joanne WuiDUniversity of California, BerkeleyiDhttps://orcid.org/0000-0001-5596-4403view email addressThe email was not providedcopy email addressColin C TriplettiDUniversity of California BerkeleyiDhttps://orcid.org/0000-0002-8158-5666view email addressThe email was not providedcopy email addressMartin M SirkUniversity of California Berkeleyview email addressThe email was not providedcopy email addressEric KorpelaiDUniversity of California, BerkeleyiDhttps://orcid.org/0000-0001-8078-9395view email addressThe email was not providedcopy email addressBrian J HardingiDUC BerkeleyiDhttps://orcid.org/0000-0002-1293-9379view email addressThe email was not providedcopy email addressChristoph R EnglertiDNaval Research LaboratoryiDhttps://orcid.org/0000-0002-2145-6168view email addressThe email was not providedcopy email addressThomas J. ImmeliDUniversity of California, BerkeleyiDhttps://orcid.org/0000-0003-2558-448Xview email addressThe email was not providedcopy email address

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. Observations from the National Aeronautics and Space Administration Ionospheric Connection Explorer confirm the link between thermospheric winds and ionospheric plasma variability.

The Far Ultra Violet (FUV) ultraviolet imager onboard the NASA-ICON mission is dedicated to the observation and study of the ionosphere dynamics at mid and low latitudes. We compare O+ density profiles provided by the ICON FUV instrument during nighttime with electron density profiles measured by the COSMIC-2 constellation (C2) and ground-based ionosondes. Co-located simultaneous observations are compared, covering the period from November 2019 to July 2020, which produces several thousands of coincidences. Manual scaling of ionogram sequences ensures the reliability of the ionosonde profiles, while C2 data are carefully selected using an automatic quality control algorithm. Photoelectron contribution coming from the magnetically conjugated hemisphere is clearly visible in FUV data around solstices and has been filtered out from our analysis. We find that the FUV observations are consistent with the C2 and ionosonde measurements, with an average positive bias lower than 1 × 1011e/m3. When restricting the analysis to cases having an NmF2 value larger than 5 × 1011e/m3, FUV provides the peak electron density with a mean difference with C2 of 10%. The peak altitude, also determined from FUV observations, is found to be 15 km above that obtained from C2, and 38 km above the ionosonde value on average.

In near-Earth space, variations in thermospheric composition have important implications for thermosphere-ionosphere coupling. The ratio of O to N-2 is often measured using far-UV airglow observations. Taking such airglow observations from space, looking below the Earth's limb allows for the total column of O and N-2 in the ionosphere to be determined. While these observations have enabled many previous studies, determining the impact of nonmigrating tides on thermospheric composition has proved difficult, owing to a small contamination of the signal by recombination of ionospheric O+. New ICON observations of far-UV are presented here, and their general characteristics are shown. Using these, along with other observations and a global circulation model, we show that during the morning hours and at latitudes away from the peak of the equatorial ionospheric anomaly, the impact of nonmigrating tides on thermospheric composition can be observed. During March-April 2020, the column O/N-2 ratio was seen to vary by 3-4% of the zonal mean. By comparing the amplitude of the variation observed with that in the model, both the utility of these observations and a pathway to enable future studies is shown.

The electrodynamic influence of thermospheric winds is an effect thought to dominate the development of the daytime low-latitude ionosphere, through the generation of dynamo currents and associated vertical plasma drifts. Until recently, observations of the thermospheric and ionopsheric state variables have mainly been defined and compared on climatological time scales, due to their collection from separate observatories with disparate measurement capabilities. These datasets are inadequate for investigation of the actual action of thermospheric drivers as they modify the ionospheric state, as the response clearly changes on 24-hour timescales, and shorter when viewed in the a constant-local-time frame of reference. New observatiions of thermospheric winds, uninterrupted over the 90-300 km altitude range, are now provided by the Ionospheric Connection Explorer along with simultaneous plasma velocity and density measurments. These observations are directly comparable to the wind measurements in crossings of the magnetic equator, where the winds are magnetically conjugate to the drift measurements. Investigation of the noon-sector drifts vs wind drivers is presented. We find that the local driver is clearly evident in the noon-time vertical plasma drifts under all conditions.

Earth’s equatorial ionosphere exhibits significant and unpredictable day-to-day variations in density and morphology . This presents difficulties in preparing for adverse impacts on technological systems even 24 hours in advance . This behavior is now theoretically understood as a manifestation of thermospheric weather, where conditions in the upper atmosphere respond strongly to changes in the spectrum of atmospheric waves that propagate into space from the lower and middle atmosphere, modifying the electrodynamic environment that exerts control over the creation of plasma . The NASA Ionospheric Connection Explorer (ICON) makes the first coordinated space-based observations of the wind-driven dynamo and the plasma state to understand the relation of the plasma environment to the thermospheric weather below. Here we show the first direct measurements of the effects of a wind-driven dynamo in space, where a clear relationship is found between the vertical plasma velocities measured at the magnetic equator near 600 km and the thermospheric winds much farther below, with substantial correlations found between the plasma velocity and thermospheric winds during each of several successive precession cycles of the observatory’s orbit. Prediction of thermospheric winds in the 100 – 150 km range emerges as a key to improved prediction of the Earth’s plasma environment.

Recent efforts to compare the sprite ratios with theoretical results have not been successfully resolved due to a lack of theoretical results for sprite streamers in varying altitudes. Advances in the predicted emission ratios of sprite streamers with a simple analytic equation have opened up the possibility for direct comparisons of theoretical results with sprite observations. The study analyzed the blue-to-red ratios measured by the ISUAL array photometer with the analytical expression for the sprite emission ratio derived from the modeling of downward sprite streamers. Our statistical studies compared sprite halos and carrot sprites where the sprite halos showed fair agreement with the predicted ratios from the sprite streamer simulation. But carrot sprites had lower emission ratios. Their estimated electric field has a lower bound of greater than 0.4 times the conventional breakdown electric field (E k ). It was consistent with the results of remote electromagnetic field measurements for short delayed or big/bright sprites. An unexpectedly lower ratio in carrot sprites occurred since sprite beads or glow in carrot sprites may exist and contribute additional red emission.

During magnetospheric substorms, high‐latitude ionospheric plasma convection is known to change dramatically. How upper thermospheric winds change, however, has not been well understood, and conflicting conclusions have been reported. Here, we study the effect of substorms on high‐latitude upper thermospheric winds by taking advantage of a chain of scanning Doppler imagers (SDIs), THEMIS all‐sky imagers (ASIs), and the Poker Flat incoherent scatter radar (PFISR). SDIs provide mosaics of wind dynamics in response to substorms in two dimensions in space and as a function of time, while ASIs and PFISR concurrently monitor auroral emissions and ionospheric parameters. During the substorm growth phase, the classical two‐cell global circulation of neutral winds intensifies. After substorm onset, the zonal component of these winds is strongly suppressed in the midnight sector, whereas away from the midnight sector two‐cell circulation of winds is enhanced. Both pre and postonset enhancements are ≥100 m/s above the quiet‐time value, and postonset enhancement occurs over a broader latitude and local‐time area than preonset enhancement. The meridional wind component in the midnight and postmidnight sectors is accelerated southward to subauroral latitudes. Our findings suggest that substorms significantly modify the upper‐thermospheric wind circulation by changing the wind direction and speed and therefore are important for the entire magnetosphere‐ionosphere‐thermosphere system.