We present space‐ and ground‐based multi‐instrument observations demonstrating the impact of the 2022 Tonga volcanic eruption on dayside equatorial electrodynamics. A strong counter electrojet (CEJ) ...was observed by Swarm and ground‐based magnetometers on 15 January after the Tonga eruption and during the recovery phase of a moderate geomagnetic storm. Swarm also observed an enhanced equatorial electrojet (EEJ) preceding the CEJ in the previous orbit. The observed EEJ and CEJ exhibited complex spatiotemporal variations. We combine them with the Ionospheric Connection Explorer neutral wind measurements to disentangle the potential mechanisms. Our analysis indicates that the geomagnetic storm had minimal impact; instead, a large‐scale atmospheric disturbance propagating eastward from the Tonga eruption site was the most likely driver for the observed intensification and directional reversal of the equatorial electrojet. The CEJ was associated with strong eastward zonal winds in the E‐region ionosphere, as a direct response to the lower atmosphere forcing.
Plain Language Summary
The Earth's E‐region ionosphere (∼100–150 km altitude) consists of both ionized and neutral gasses, and the two components are coupled through ion‐neutral collisions. The state of this region is closely influenced by neutral atmospheric activities from the lower atmosphere and the variability of the solar drivers. On 15 January 2022, the Tonga volcano had a massive eruption and injected an enormous amount of mass and energy into the atmosphere causing disturbances in the E‐region ionosphere or even higher. There was also a moderate geomagnetic storm that started 1 day before the eruption and ended days after. These conditions offer a unique opportunity to understand the different roles they play in controlling the ionosphere. Coordinated observations, including the atmosphere, ionosphere, and magnetosphere, were made from both space and on the ground during this event. We analyzed the magnetic field and neutral wind data and found that a large‐scale atmospheric disturbance generated by the volcano eruption was responsible for the observed directional reversal of the dayside equatorial electric field and electric current.
Key Points
Space‐ and ground‐based observations reveal dramatic equatorial electrojet variations caused by the Tonga volcanic eruption
Strong eastward turning of atmospheric zonal winds in the E‐region is responsible for the directional reversal of the equatorial electrojet
The observed complex spatiotemporal variations can be explained by a large‐scale disturbance propagating eastward from the eruption site
Observations of the nighttime thermospheric wind from two ground‐based Fabry‐Perot Interferometers are compared to the level 2.1 and 2.2 data products from the Michelson Interferometer Global ...High‐resolution Thermospheric Imaging (MIGHTI) onboard National Aeronautics and Space Administration's Ionospheric Connection Explorer to assess and validate the methodology used to generate measurements of neutral thermospheric winds observed by MIGHTI. We find generally good agreement between observations approximately coincident in space and time with mean differences less than 11 m/s in magnitude and standard deviations of about 20–35 m/s. These results indicate that the independent calculations of the zero‐wind reference used by the different instruments do not contain strong systematic or physical biases, even though the observations were acquired during solar minimum conditions when the measured airglow intensity is weak. We argue that the slight differences in the estimated wind quantities between the two instrument types can be attributed to gradients in the airglow and thermospheric wind fields and the differing viewing geometries used by the instruments.
Plain Language Summary
This study presents a validation of observations made by two different types of instruments used to measure nighttime thermospheric neutral winds. These winds represent the motion of neutral particles in the thermosphere and studying their properties is critical to gaining a complete understanding of the dynamics of the Earth's upper atmosphere. We use observations made by two ground‐based Fabry‐Perot interferometers to validate measurements from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) onboard National Aeronautics and Space Administration's recently launched Ionospheric Connection Explorer satellite. After identifying observations from the different instruments that are coincident in space and time, we show that the measurements are statistically highly correlated, thereby successfully validating the MIGHTI thermospheric wind observations.
Key Points
Measurements of nighttime thermospheric neutral winds made by Ionospheric Connection Explorer‐Michelson Interferometer Global High‐resolution Thermospheric Imaging agree with ground‐based Fabry‐Perot interferometer measurements to within 10 m/s
The comparison validates the independent zero‐wind removal and analysis processes employed by these instruments
The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument was built for launch and operation on the NASA Ionospheric Connection Explorer (ICON) mission. The ...instrument was designed to measure thermospheric horizontal wind velocity profiles and thermospheric temperature in altitude regions between 90 km and 300 km, during day and night. For the wind measurements it uses two perpendicular fields of view pointed at the Earth’s limb, observing the Doppler shift of the atomic oxygen red and green lines at 630.0 nm and 557.7 nm wavelength. The wavelength shift is measured using field-widened, temperature compensated Doppler Asymmetric Spatial Heterodyne (DASH) spectrometers, employing low order échelle gratings operating at two different orders for the different atmospheric lines. The temperature measurement is accomplished by a multichannel photometric measurement of the spectral shape of the molecular oxygen A-band around 762 nm wavelength. For each field of view, the signals of the two oxygen lines and the A-band are detected on different regions of a single, cooled, frame transfer charge coupled device (CCD) detector. On-board calibration sources are used to periodically quantify thermal drifts, simultaneously with observing the atmosphere. The MIGHTI requirements, the resulting instrument design and the calibration are described.
We compare coincident thermospheric neutral wind observations made by the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (MIGHTI) on the Ionospheric Connection Explorer ...(ICON) spacecraft, and four ground‐based specular meteor radars (SMRs). Using the green‐line MIGHTI channel, we analyze 1158 coincidences between Dec 2019 and May 2020 in the altitude range from 94 to 104 km where the observations overlap. We find that the two datasets are strongly correlated (r = 0.82) with a small mean difference (4.5 m/s). Although this agreement is good, an analysis of known error sources (e.g., shot noise, calibration errors, and analysis assumptions) can only account for about a quarter of the disagreement variance. The unexplained variance is 27.8% of the total signal variance and could be caused by unknown errors. However, based on an analysis of the spatial and temporal averaging of the two measurement modalities, we suggest that some of the disagreement is likely caused by temporal variability of the wind on scales ≲70 min. The observed magnitudes agree well during the night, but during the day, MIGHTI observes 16%–25% faster winds than the SMRs. This remains unresolved but is similar in certain ways to previous SMR‐satellite comparisons.
Plain Language Summary
Although Earth's atmosphere becomes less dense at high altitudes where it transitions to space, the wind speed grows faster, often exceeding 100 m/s (225 mph). One barrier to better predictions of conditions in the near‐Earth space environment is obtaining knowledge of the wind in the thermosphere, the uppermost layer of the atmosphere. Measurements of the thermospheric wind are difficult to make and historically sparse. ICON, a new NASA mission launched in October 2019, carries the MIGHTI instrument to measure the wind from 90 to 300 km altitude. In this study we compare the observations of MIGHTI to those of meteor radars, which measure the wind from the ground by analysis of radio waves reflected by meteor trails. The results indicate good agreement between the datasets when they measure the wind at the same time and place. Specifically, with 1158 coincidences over the first 6 months of the ICON mission, the correlation is 0.82 and the average difference is 4.5 m/s. This study is important because it validates the MIGHTI data, giving confidence for subsequent studies using its data. It also quantifies limits to the agreement between space‐based and ground‐based winds, which is useful information for future studies combining them.
Key Points
Coincident wind measurements by ICON‐MIGHTI and specular meteor radars are strongly correlated (r = 0.82)
The mean discrepancy between the datasets is 4.5 m/s, validating the MIGHTI v03 zero reference
The RMS discrepancy is 26 m/s, which is attributed to inherent data errors and variability on time scales ≲70 min
Two ∼2‐week Ultra‐Fast Kelvin Wave (UFKW) events centered on days 158(203) during 2021 are investigated using winds, temperatures, plasma drifts and electron densities (Ne) measured by the ...Ionospheric CONnections (ICON) mission. Eastward‐propagating longitudinal wave‐1 (s = −1) structures with periods 2.5–4.0d, thought to mainly reflect Ultra‐Fast Kelvin waves (UFKWs), reveal ±45 ms−1 zonal winds (U) at 100 km for both events. Height‐latitude structures of the 3.0(3.5)d‐period UFKWs are obtained for the first time for both temperature (T, 94–120 km) and U (94–280 km) between 12°S and 39°N latitude. Maximum values of 36(29) ms−1 for U and 12(15)K for T occur at 102(106) km altitude and within ±3° latitude. The U‐T peak height displacement remains unexplained. Vertical wavelengths are in the range 36–43 km for both U and T during both events. Concurrent with the E‐region dynamo winds, topside (580 km) F‐region field‐aligned (±20–40 ms−1), meridional (±5–10 ms−1) and vertical (±5–10 ms−1) drift and Ne (±20–40%) 2.5–4.0d s = −1 variations are also measured. These key elements of atmosphere‐ionosphere (A‐I) coupling, contemporaneously measured for the first time, are relevant to testing the internal consistency of A‐I models. The mean wind propagation environment of the UFKWs is also quantified, showing no appreciable effects on the UFKW structures, consistent with modeling and theory.
Plain Language Summary
Atmospheric Ultra‐Fast Kelvin waves (UFKWs) are eastward‐ and vertically propagating oscillations with periods less than 1 week that are centered on the equator and confined to low latitudes. They are part of a whole spectrum of waves forced by the spatial‐temporal variability of the heat of condensation (“latent heating”) that is released when rising moist air forms rain droplets, mainly in the tropics. At high altitudes UFKW reach large amplitudes due to the decrease in ambient density with height. Their growth is curtailed by the viscosity of the atmosphere near 102 km altitude where they reach maximum amplitudes. Here they interact with ionized particles (the ionosphere) and generate electric fields that ultimately drive ionospheric variability at higher altitudes (>200 km), thus presenting an element of “space weather” to navigation and communications systems. In this paper, UFKWs measured in connection with the ICON mission are used to reveal, for the first time, the height and latitudinal structures of temperatures and winds associated with UFKWs in this dissipative regime, and their corresponding effect on the ionosphere. These results can thus serve to assess the internal consistency of models that seek to simulate atmosphere‐ionosphere coupling and related space weather effects induced by the wave spectrum excited in the lower atmosphere.
Key Points
First coincident UFKW winds and temperature structures covering 94‐120 km,12°S‐39°N latitude are reported
First UFKW wind profiles in the 100‐280 km altitude dissipative regime are reported
First contemporaneous measurements of ionosphere response and driving UFKW E‐region dynamo winds are reported
The design and laboratory tests of the interferometers for the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument which measures thermospheric wind and ...temperature for the NASA-sponsored Ionospheric Connection (ICON) Explorer mission are described. The monolithic interferometers use the Doppler Asymmetric Spatial Heterodyne (DASH) Spectroscopy technique for wind measurements and a multi-element photometer approach to measure thermospheric temperatures. The DASH technique and overall optical design of the MIGHTI instrument are described in an overview followed by details on the design, element fabrication, assembly, laboratory tests and thermal control of the interferometers that are the heart of MIGHTI.
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.
Plain Language Summary
The plasma that composes the ionosphere can change dramatically from one day to the next, exhibiting significant changes in its height and density which are not well predicted by models. This variability can have adverse impacts on satellite‐based navigation and communication systems, limiting their performance and availability. One of the key parameters that controls daytime ionospheric conditions is the upward and downward motion of plasma above 200 km, which affects the lifetime of newly created plasma. The force that puts the plasma in motion is electromotive, generated by the motion of the atmosphere (i.e., the wind) around 100–150 km that pushes charged particles across magnetic field lines. NASA's Ionospheric Connection Explorer is the first mission to directly observe these electrical generators, one at each “footpoint” of the arched magnetic field lines that thread the ionosphere and generator region. The results show that just under half of the day‐to‐day changes in ionospheric motion can be explained by this local generator mechanism. The major controller is the east‐west winds, with north‐south winds having only a minor influence on this mechanism.
Key Points
The first data set of simultaneous plasma drift and magnetically conjugate neutral winds in both hemispheres is presented
The day‐to‐day changes in winds account for 39 ± 7% and 24 ± 9% of the conjugate vertical and zonal daytime drift variance, respectively
Vertical drift variance is mainly driven by zonal winds at 100–113 km, and zonal drift variance is mainly driven by zonal winds above 120 km
This study cross‐compares ICON/MIGHTI and Thermosphere, Ionosphere, Mesosphere Energetics & Dynamics (TIMED)/TIMED Doppler Interferometer (TIDI) MLT region neutral winds from middle Northern ...Hemisphere to low Southern Hemisphere latitudes. We utilized MIGHTI level‐2.2 (v4) and TIDI level‐3 (v11) neutral winds from January 2020 to November 2020 and found their conjunctions using a space‐time window of LST ± 15 min, latitude ± 4°, and longitude ± 4° around each TIDI wind measurement. Due to the nature of their orbital geometry, frequent conjunctions occurred between MIGHTI and TIDI. These conjunctions are spread in longitudes and they occur at approximately fixed LSTs and latitudes, which allows us to compare their observed diurnal variability. MIGHTI and TIDI wind observations agree well (except on the TIDI coldside during forward flight) and show similar large amplitude longitudinal variations that can reach more than 100 m/s. MIGHTI and TIDI zonal and meridional winds show moderate correlations of 0.60 and 0.55, respectively. The slopes of regression fits for zonal and meridional winds are 0.92 and 0.91, respectively. The root mean square differences in zonal and meridional winds are 56 and 66 m/s, respectively. We found that TIDI coldside measurements in forward flight show a systematic bias and this behavior is repetitive as the instrument pointing direction is changed by the periodic TIMED yaw maneuver. The nature of this systematic bias suggests that the TIDI zero‐wind references (at least for the coldside telescopes) need revision. This investigation can provide guidance toward improving the TIDI data analysis. In addition, the results of this study act as a validation of MIGHTI MLT winds.
Key Points
Cross‐compare ICON/MIGHTI and Thermosphere, Ionosphere, Mesosphere Energetics & Dynamics (TIMED)/TIMED Doppler Interferometer (TIDI) MLT region neutral winds
Overall, MIGHTI and TIDI neutral wind measurements are in agreement
TIDI coldside measurements in forward flight show systematic bias
We present an algorithm to retrieve thermospheric wind profiles from measurements by the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument on NASA’s ...Ionospheric Connection Explorer (ICON) mission. MIGHTI measures interferometric limb images of the green and red atomic oxygen emissions at 557.7 nm and 630.0 nm, spanning 90–300 km. The Doppler shift of these emissions represents a remote measurement of the wind at the tangent point of the line of sight. Here we describe the algorithm which uses these images to retrieve altitude profiles of the line-of-sight wind. By combining the measurements from two MIGHTI sensors with perpendicular lines of sight, both components of the vector horizontal wind are retrieved. A comprehensive truth model simulation that is based on TIME-GCM winds and various airglow models is used to determine the accuracy and precision of the MIGHTI data product. Accuracy is limited primarily by spherical asymmetry of the atmosphere over the spatial scale of the limb observation, a fundamental limitation of space-based wind measurements. For 80% of the retrieved wind samples, the accuracy is found to be better than 5.8 m/s (green) and 3.5 m/s (red). As expected, significant errors are found near the day/night boundary and occasionally near the equatorial ionization anomaly, due to significant variations of wind and emission rate along the line of sight. The precision calculation includes pointing uncertainty and shot, read, and dark noise. For average solar minimum conditions, the expected precision meets requirements, ranging from 1.2 to 4.7 m/s.
The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) is a satellite experiment scheduled to launch on NASA’s Ionospheric Connection Explorer (ICON) in 2018. MIGHTI ...is designed to measure horizontal neutral winds and neutral temperatures in the terrestrial thermosphere. Temperatures will be inferred by imaging the molecular oxygen Atmospheric band (A band) on the limb in the lower thermosphere. MIGHTI will measure the spectral shape of the A band using discrete wavelength channels to infer the ambient temperature from the rotational envelope of the band. Here we present simulated temperature retrievals based on the as-built characteristics of the instrument and the expected emission rate profile of the A band for typical daytime and nighttime conditions. We find that for a spherically symmetric atmosphere, the measurement precision is 1 K between 90–105 km during the daytime whereas during the nighttime it increases from 1 K at 90 km to 3 K at 105 km. We also find that the accuracy is 2 K to 11 K for the same altitudes. The expected MIGHTI temperature precision is within the measurement requirements for the ICON mission.
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