During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including not only the stratosphere but also the thermosphere and ionosphere. The eclipse shadow has a ...supersonic motion which is theoretically expected to generate atmospheric bow waves, similar to a fast‐moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. However, previous geographically limited observations have had difficulty detecting these weak waves within the natural background atmospheric variability, and the existence of eclipse‐induced ionospheric waves and their evolution in a complex coupling system remain controversial. During the 21 August 2017 eclipse, high fidelity and wide coverage ionospheric observations provided for the first time an oversampled set of eclipse data, using a dense network of Global Navigation Satellite System receivers at ∼2,000 sites in North America. We show the first unambiguous evidence of ionospheric bow waves as electron content disturbances over central/eastern United States, with ∼1 h duration, 300–400 km wavelength and 280 m/s phase speed emanating from and tailing the totality region. We also identify large ionospheric perturbations moving at the supersonic speed of the maximum solar obscuration which are too fast to be associated with known gravity wave or large‐scale traveling ionospheric disturbance processes. This study reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere and demonstrates persistent coupling processes between different components of the Earth's atmosphere, a topic of significant community interest.
Plain Language Summary
During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including the stratosphere and both the thermosphere and ionosphere (∼100–1,000 km altitudes). Theoretical studies since the 1960s have predicted that the Moon's supersonic shadow should generate atmospheric bow waves, similar to a fast‐moving river boat. However, observations were geographically limited for these weak and complicated waves. In 2017, high fidelity and wide coverage ionospheric observations were made using a North American Global Navigation Satellite System (GNSS) ∼2,000 receiver network. Eclipse passage generated clear ionospheric bow waves in electron content disturbances emanating from totality primarily over central/eastern United States. Study of wave characteristics reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere.
Key Points
The first unambiguous evidence of eclipse‐induced ionospheric bow waves in differential TEC is shown
The bow waves have 350/270 km wavelength in zonal/meridional directions, and a 25 min period
They propagate at 280 m/s phase velocity in a direction approximately aligned with the totality path.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
On 21 August 2017, during daytime hours, a total solar eclipse with a narrow ∼160 km wide umbral shadow occurred across the continental United States. Totality was observed from the Oregon coast at ...∼9:15 local standard time (LST) (17:20 UT) to the South Carolina coast at ∼13:27 LST (18:47 UT). A dense network of Global Navigation Satellite Systems (GNSS) receivers was utilized to produce total electron content (TEC) and differential TEC. These data were analyzed for the latitudinal and longitudinal response of the TEC and for the presence of traveling ionospheric disturbances (TIDs) during eclipse passage. A significant TEC depletion, in some cases greater than 60%, was observed associated with the eclipse shadow, exceeding initial model predictions of 35%. Evidence of enhanced large‐scale TID activity was detected over the United States prior to and following the large TEC depletion observed near the time of totality. Signatures of enhanced TEC structures were observed over the Rocky Mountain chain during the main period of TEC depletion.
Plain Language Summary
On 21 August 2017, during daytime hours (16:00–20:00 UTC), a total solar eclipse was observed across the continental United States. A dense network of GPS receivers was utilized to monitor the changes in the ionosphere. GPS data were analyzed for the latitudinal and longitudinal response of the total electron content and for the presence of ionospheric perturbations during eclipse passage. A significant TEC depletion, in some cases greater than 60%, was observed associated with the eclipse shadow. Large‐scale ionospheric perturbations were detected over the United States prior to and following the major TEC depletion observed near the time of totality. Signatures of enhanced TEC structures were also observed over the Rocky Mountain chain during the main period of TEC depletion.
Key Points
GNSS observations during August 2017 eclipse show total electron content (TEC) depletions up to 60% in magnitude
Enhanced large‐scale traveling ionospheric disturbances are observed before, during, and after totality
Enhanced TEC is observed above Rocky Mountains 5‐10 min after totality
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Solar flares provide strong impulsive radiation and energy injection to the sunlit upper atmosphere. The impact on the ionosphere is immense in spatial scale, and therefore, it is not immediately ...evident if dramatically elevated neutral heating can lead to excitation of acoustic gravity waves. Using primary observations from Global Navigation Satellite System differential TEC (total electron content) over the continental United States, this paper presents postflare ionospheric observations associated with three X‐class flares on 6, 7, and 10 September 2017. Postflare ionospheric changes had two significant morphological characteristics: (1) A few minutes after the X9.3 flare peak on 6 September, clear traveling ionospheric disturbance (TID) fronts emanated near the sunrise terminator with alignment parallel to its direction—TIDs propagated predominantly eastward into the dayside with a 150 m/s phase speed and a ∼30‐min period; (2) synchronized differential TEC oscillations over continental United States with ∼60‐min periodicity and damping amplitude over time, following all three X‐class flares. Postflare ionospheric oscillation spectra exhibited significantly enhanced amplitudes and changes of periodicities (including the appearance of the 60‐min oscillations). The Millstone Hill incoherent scatter radar observed large ionospheric up‐welling occurring nearly simultaneously as detected TIDs at the X9.3 flare peak, with up to 80 m/s enhancements in vertical drift at 500 km lasting for ∼30 min. Results suggest that significant solar flare heating and associated dynamical effects may be an important factor in TID/acoustic gravity wave excitation.
Plain Language Summary
A solar flare injects a sudden and strong energy input to the sunlit upper atmosphere at a range of radiation wavelengths important for ionospheric photochemistry and thermospheric dynamics. Impulsive energy inputs from flares are well known to generate sudden ionospheric density enhancements with subsequent quick decay. This study addresses another type of flare‐associated ionospheric perturbation, known as traveling ionospheric disturbances (TIDs), in the form of propagating waves in space and time as well as synchronized temporal oscillations over the continental United States. Our study provides likely direct observations pointing to a previously unconfirmed TID excitation mechanism associated with solar flare impacts near the sunrise terminator. We observed TIDs with a dense network of Global Navigation Satellite System receivers, yielding detection of differential ionospheric electron content with high fidelity and excellent spatiotemporal resolution. We also used incoherent scatter radar observations at Millstone Hill to reveal ionospheric expansion/up‐welling associated with flare impact. These results address fundamental questions regarding solar flare influences on initiation of atmospheric and ionospheric waves.
Key Points
Postflare TIDs emanating near sunrise terminator propagated predominantly eastward with 150 m/s zonal phase speed and 30‐min period
Synchronized differential TEC oscillations occurred over the continental United States with 60‐min period and decreasing amplitude over time
Rapid and significant ionospheric up‐welling developed in the topside immediately after onset of X‐class flare
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This study provides new scenarios for storm time traveling ionospheric disturbance excitation and subsequent propagation at subauroral and polar latitudes. We used ground‐based total electron content ...observations from Global Navigation Satellite System receivers combined with wide field, subauroral ionospheric plasma parameters measured with the Millstone Hill Incoherent Scatter Radar during strong September 2017 geospace storms. Observations provide the first evidence of significant influences on traveling ionospheric disturbance (TID) propagation and excitation caused by the presence of large subauroral polarization stream flow channels. Simultaneous large‐ and medium‐scale TIDs evolved during the event in a broad subauroral and midlatitude area near dusk. Similar concurrent TIDs occurred near dawn sectors as well during a period of sustained southward Bz. Medium‐scale TIDs at subauroral and midlatitudes had wave fronts aligned northwest‐southeast near dusk, and northeast‐southwest near dawn. These wave fronts were highly correlated with the direction of storm time large zonal plasma drift enhancements at these latitudes. At high latitudes, unexpected, predominant, and persistent storm time TIDs were identified with 2000+ km zonal wave fronts and 15% total electron content perturbation amplitudes, moving in transpolar propagation pathways from the dayside into the nightside. This propagation direction in the polar region was opposite to the normal assumption that TIDs originated in the nightside auroral region. Results suggest that significant dayside sources, such as cusp regions, can be efficient in generating transpolar TIDs during geospace storm intervals.
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This paper reports several new findings on the traveling ionospheric disturbances (TIDs) excited during geospace storms in 7–8 September 2017. Storm time TIDs provide pathway for momentum and energy dispersion from the solar wind‐magnetosphere system to various components of the global ionosphere and thermosphere. Storm time large‐scale TIDs (LSTIDs) have been identified by many studies as being initiated generally in the auroral zone where significant heating is injected, with subsequent propagation away from the source: equatorward into lower latitudes and poleward into high latitudes. Our study indicates that, during equatorward propagation, LSTIDs can encounter strong dynamic forcing at subauroral latitudes in the zonal direction. This westward velocity forcing is provided by a SAPS (subauroral polarization stream) channel and furthermore appears to be associated with the developing of medium‐scale TIDs (MSTIDs). Thus, this paper provides the first causal link between these TIDs and SAPS flow channels. Concurrent LSTIDs and MSTIDs existed during the September storm in not only near dusk but also dawn sectors. In the polar cap region, conventionally anticipated poleward propagation away from the auroral zone was unexpectedly weak. In contrast, an opposite sense of transpolar propagation from the dayside into the nightside (i.e., equatorward at night and poleward during the day) was observed and furthermore was persistent and large in amplitude. This unexpected propagation may imply an efficient additional storm time source region of TID excitation at high latitudes on the dayside, for example, near the cusp. This study demonstrates that modern TID studies have continued potential for fresh insights into magnetosphere‐ionosphere‐thermosphere coupling processes.
Key Points
First evidence of storm time concurrent large and medium spatial scale traveling ionospheric disturbances at subauroral latitudes was reported
Casual relation between medium‐scale traveling ionospheric disturbances and the subauroral polarization stream were found
Unexpected persistent transpolar day‐to‐night propagation of traveling ionospheric disturbances occurred
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Tonga volcano eruption at 04:14:45 UT on 2022-01-15 released enormous amounts of energy into the atmosphere, triggering very significant geophysical variations not only in the immediate proximity ...of the epicenter but also globally across the whole atmosphere. This study provides a global picture of ionospheric disturbances over an extended period for at least 4 days. We find traveling ionospheric disturbances (TIDs) radially outbound and inbound along entire Great-Circle loci at primary speeds of ∼300–350 m/s (depending on the propagation direction) and 500–1,000 km horizontal wavelength for front shocks, going around the globe for three times, passing six times over the continental US in 100 h since the eruption. TIDs following the shock fronts developed for ∼8 h with 10–30 min predominant periods in near- and far- fields. TID global propagation is consistent with the effect of Lamb waves which travel at the speed of sound. Although these oscillations are often confined to the troposphere, Lamb wave energy is known to leak into the thermosphere through channels such as atmospheric resonance at acoustic and gravity wave frequencies, carrying substantial wave amplitudes at high altitudes. Prevailing Lamb waves have been reported in the literature as atmospheric responses to the gigantic Krakatoa eruption in 1883 and other geohazards. This study provides substantial first evidence of their long-duration imprints up in the global ionosphere. This study was enabled by ionospheric measurements from 5,000+ world-wide Global Navigation Satellite System (GNSS) ground receivers, demonstrating the broad implication of the ionosphere measurement as a sensitive detector for atmospheric waves and geophysical disturbances.
We provide evidence that midlatitude postsunrise traveling ionospheric disturbances (TIDs) are comprised of electrified waves with an eastward propagation component. The post‐sunrise gravity wave ...(GW) wind‐induced dynamo action effectively generated periodic meridional polarization electric fields (PEFs), facilitating TID zonal propagation in a similar fashion as GW‐driven neutral perturbations. A combination of near‐simultaneous eastward and upward observations using the Millstone Hill incoherent scatter radar along with 2‐dimensional total electron content maps allowed resolution of TID vertical and horizontal propagation as well as zonal ion drifts Veast (meridional PEFs). In multiple observations, Veast oscillated in the early morning during periods when TIDs exhibited downward phase progression, 30–60 min period, ∼140 m/s eastward speed, and 70 km vertical wavelength. Inside these TIDs, multiple flow vortexes occurred in a vertical‐zonal plane spanning the ionospheric topside and bottomside. Subsequently, PEFs weakened after a few hours as TID horizontal wavefronts rotated clockwise.
Plain Language Summary
The solar terminator (ST) provides a repeatable, regulated forcing to the upper atmosphere, exciting thermospheric and ionospheric waves. These waves have zonal propagation components due to the terminator's orientation. Nominally, traveling ionospheric disturbances (TIDs) are considered a manifestation of dynamics produced by propagating thermospheric waves such as gravity waves (GWs). However, GW zonal propagation would be expected to be greatly attenuated in the F region since ions cannot easily move zonally across the meridionally oriented magnetic field. This study provides evidence that midlatitude postsunrise TIDs are electrified waves, due to meridional polarization electric fields (PEFs) embedded in the TIDs. We also identified plasma flow vortexes in a vertical‐zonal plane. Although observed TIDs possess some general GW characteristics, their manifestation is more complex. Particularly, GW wind‐induced dynamo action can generate oscillating PEFs and facilitate TID zonal propagation. Our results imply the importance of electrodynamics in understanding the dynamics of ST‐time ionospheric waves. Observations used the Millstone Hill incoherent scatter radar to measure TIDs and their zonal and vertical propagation, as well as F‐region plasma zonal drifts (driven by PEFs) during TID/GW passage. GNSS data were used to provide 2D TID wave characteristics.
Key Points
Postsunrise midlatitude periodic traveling ionospheric disturbances occur, propagating eastward with downward phase progression
Periodic polarization electric field in meridional direction was embedded in traveling ionospheric disturbances, large in the morning
The electrified ionospheric waves are possibly due to gravity wave wind induced F‐region dynamo effects
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A software package known as MIT Automated Processing of GPS (MAPGPS) has been developed to automate the processing of GPS data into global total electron density (TEC) maps. The goal of the MAPGPS ...software is to produce reliable TEC data automatically, although not yet in real time. Observations are used from all available GPS receivers during all geomagnetic conditions where data has been successfully collected. In this paper, the architecture of the MAPGPS software is described. Particular attention is given to the algorithms used to estimate the individual receiver biases. One of the largest sources of error in estimating TEC from GPS data is the determination of these unknown receiver biases. The MAPGPS approach to solving the receiver bias problem uses three different methods: minimum scalloping, least squares, and zero-TEC. These methods are described in detail, along with their relative performance characteristics. A brief comparison of the JPL and MAPGPS receiver biases is presented, and a possible remaining error source in the receiver bias estimation is discussed. Finally, the Madrigal database, which allows Web access to the MAPGPS TEC data and maps, is described.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Fluctuations in plasma electron density may play a role in solar coronal energy transport and the dissipation of wave energy. Transcoronal spacecraft radio sounding observations reveal frequency ...fluctuations (FFs) that encode the electron number density disturbances, allowing an exploration of the coronal compressive wave and advected inhomogeneity models. Primary FF observations from MESSENGER 2009 and published FF residuals from HELIOS 1975-1976 superior conjunctions were combined to produce a composite view of equatorial region FF near solar minimum over solar offset range 1.4-25R . Methods to estimate the electron number density fluctuation variance from the observed FF were developed. We created a simple stacked, magnetically structured slab model that incorporated both propagating slow density waves and advected spatial density variations to explain the observed FF. Slow density waves accounted for most of the FF at low solar offset, while spatial density inhomogeneities advected at solar wind speed dominated above the sonic point at 6R . Corresponding spatial scales ranged 1-38 Mm, with scales above 10 Mm contributing most to FF variance. Magnetic structuring of the model introduced radial elongation anistropy at lower solar offsets, but geometric conditions for isotropy were achieved as the slab correlation scales increased further out in the corona. The model produced agreement with the FF observations up to 12R . FF analysis provides information on electron density fluctuations in the solar corona, and should take into account the background compressive slow waves and solar wind-related advection of quasi-static spatial density variations.
Following the 2022 Tonga Volcano eruption, dramatic suppression and deformation of the equatorial ionization anomaly (EIA) crests occurred in the American sector ∼14,000 km away from the epicenter. ...The EIA crests variations and associated ionosphere‐thermosphere disturbances were investigated using Global Navigation Satellite System total electron content data, Global‐scale Observations of the Limb and Disk ultraviolet images, Ionospheric Connection Explorer wind data, and ionosonde observations. The main results are as follows: (a) Following the eastward passage of expected eruption‐induced atmospheric disturbances, daytime EIA crests, especially the southern one, showed severe suppression of more than 10 TEC Unit and collapsed equatorward over 10° latitudes, forming a single band of enhanced density near the geomagnetic equator around 14–17 UT, (b) Evening EIA crests experienced a drastic deformation around 22 UT, forming a unique X‐pattern in a limited longitudinal area between 20 and 40°W. (c) Thermospheric horizontal winds, especially the zonal winds, showed long‐lasting quasi‐periodic fluctuations between ±200 m/s for 7–8 hr after the passage of volcano‐induced Lamb waves. The EIA suppression and X‐pattern merging was consistent with a westward equatorial zonal dynamo electric field induced by the strong zonal wind oscillation with a westward reversal.
Plain Language Summary
The extreme Tonga volcano eruption on 15 January 2022 triggered profound and prolonged disturbances in the equatorial ionosphere. Using ground‐based Global Navigation Satellite System total electron content and ionosonde data, low‐Earth orbiting Ionospheric Connection Explorer wind observations, and geostationary Global‐scale Observations of the Limb and Disk ultraviolet measurements of thermospheric and ionospheric airglow emission, we found volcano‐induced atmospheric Lamb waves as a plausible source of the observed pronounced deformation of equatorial ionization anomaly (EIA) in the American sector. This deformation was triggered by volcano‐induced wave passage through the region, even after these waves had traveled 12,000–16,000 km distance for over 10 hr. The typical double‐crested EIA morphology was severely deformed during both the daytime and nighttime. Daytime EIA crests exhibited a severe suppression of more than 10 TEC Unit (TECu) (1TECu = 1016 m−2) and an equatorward collapse around 14–17 UT (10–13 LT) especially for the southern crest. Nighttime EIA crests exhibited a unique X‐pattern with crests drastically merging in a narrow longitude region between 20 and 40°W around 22 UT (20 LT). These large EIA deformations were coincident, sharing spatial scales with long‐lasting oscillations of thermospheric zonal wind following the arrival of volcano‐induced waves in the American sector. This study sheds new light on the potential far‐reaching and long‐lasting atmosphere‐ionosphere‐thermosphere impacts from a catastrophic geological event.
Key Points
Daytime equatorial ionization anomaly (EIA) crests eroded severely by 10+ TEC Unit and collapsed equatorward over 10° latitude, following the arrival of volcano‐induced Lamb waves
EIA crests exhibited a unique deformation after sunset with an X‐pattern merging in a longitudinally‐confined area between 20° and 40°W
Thermospheric zonal wind showed post‐volcanic oscillations of ±200 m/s that may explain the EIA suppression and X‐pattern via dynamo effect
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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