The wave 3 and wave 4 modulations of the Equatorial Ionization Anomalies are a robust feature of the low‐latitude ionosphere, when viewed at constant local time. Although initially associated, ...respectively, with DE2 and DE3, nonmigrating diurnal tides in the mesosphere and lower thermosphere region, recent results have suggested that the wave 3 and wave 4 may also have significant contributions from other tidal and stationary planetary wave (SPW) signatures. We present observations of total electron content (TEC) variations associated with tidal and SPW signatures comprising the ionospheric wave 3 and wave 4 structures from FORMOSAT‐3/COSMIC from 2007 to 2011. We find that the wave 3 (wave 4) feature is comprised predominately by DE2 (DE3) and SPW3 (SPW4) signatures in TEC throughout all 5 years, with contributions from SE1 (SE2) being less significant. The wave 3 component also has recurring contributions from DW4 during December/January. The absolute amplitudes of all the aforementioned tidal and SPW signatures are directly related to the level of solar activity and the semiannual variation in zonal mean TEC. After normalizing by the zonal mean, the relative amplitudes of the wave 4 signatures are inversely related to solar activity through 2010, which is not seen with the wave 3‐related signatures. The seasonal variation and phases of the main constituents of wave 3 and wave 4 are consistent from year to year, as evidenced by the interannual recurrence in the peak and trough locations of wave 3 and wave 4.
Key Points
Long term trends in ionosphere wave‐3 / wave‐4 examined
Consistently dominated by diurnal and stationary planetary wave signatures
SPWs suggest aliasing between nonmigrating diurnal tides and DW1 signatures.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
A moon shadow of the total solar eclipse swept through the continent of United States (CONUS) from west to east on 21 August 2017. Massive total electron content (integration of electron density from ...0 km to 20,200 km altitude) observations from 2,255 ground‐based Global Navigation Satellite System receivers show that the moon shadow ship generates a great ionospheric bow wave front which extends ~1,500 km away from the totality path covering the entire CONUS. The bow wave front consists of the acoustic shock wave due to the supersonic/near‐supersonic moon shadow ship and the significant plasma recombination due to the reduction in solar irradiation within the shadow area. The deep bow wave trough (−0.02 total electron content unit (1 TECU = 1016 el m−2) area) nearly coincides with the 100% obscuration moving along the totality path over the CONUS through the entire eclipse period. The supersonic moon shadow ship induces a bow wave crest in front of the ship (~80% obscuration). It is the first time to find the acoustic shock wave‐formed bow wave trough and crest near the totality.
Key Points
A 3,000 km wide ionospheric bow wave front is induced by the moon shadow ship of the 21 August 2017 total solar eclipse over CONUS
The TEC depression contributes to the wide bow wave front
The acoustic shock wave induces the bow wave crest and trough near the totality
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The ionospheric total electron content (TEC) derived from dense ground‐based Global Navigation Satellite System receivers over the continental United States and those from global ionosphere maps are ...utilized to find the ionosphere response to the August 21, 2017 total solar eclipse. Maximum obscurations and their associated TEC major depressions appear simultaneously at midlatitudes, while major depressions elongate toward the magnetic equator with some delays in the equatorial ionization anomaly (EIA) region. The former is due to the photochemical loss process, while the latter is caused by the plasma transport of E×B drifts and lunar gravitation forces. TECs of predawn reductions, morning enhancements, afternoon reductions, and nighttime enhancements reveal that the semidiurnal lunar tide are essential. Since a solar eclipse always occurs on a new moon day, the lunar tide results in the early EIA appearance and major depressions being underestimated/diminished before and overestimated/enhanced after about 14:00 local time.
Plain Language Summary
Observations during an eclipse offer a special opportunity for studying the Earth's ionospheric response to changes in solar ionizing radiation. Although ionospheric solar eclipse signatures in many events have been studied, we report the lunar tide effect on the signatures for the first time. A total solar eclipse swept across the continental United States (CONUS) from the west to east coast on August 21, 2017. The total electron content (TEC) along seven longitudes of the US continent derived from more than 2,200 ground‐based Global Navigation Satellite System receivers in the CONUS and extracted from global ionosphere maps (GIMs) is employed to study ionospheric solar eclipse signatures. The most prominent solar eclipse signature is major depressions (MDs) in both the CONUS and GIM TECs, when the maximum obscuration occurs. TEC extrema of predawn reductions, morning enhancements, afternoon reductions, and nighttime enhancements reveal that the semidiurnal lunar tide of about 12.42‐h period is prominent. The lunar tide causes the early appearance of equatorial ionization anomalies, which further results in the MDs being weakened in the morning and enhanced in the afternoon on the solar eclipse day. The lunar tide has to be taken into consideration for studying ionospheric solar eclipse effects.
Key Points
The lunar tide results in the early appearance of the equatorial ionization anomaly on solar eclipse days
Total electron contents tend to enhance before and decrease after 14:00 local time (LT) on a solar eclipse day
The lunar tide causes major depressions being weakened before and enhanced after 14:00 LT
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Latitudinal distances between the two equatorial ionization anomaly (EIA) crests in total electron content (TEC) of global ionosphere map (GIM) are used to derive the daily dynamo eastward electric ...field by comparing the eastward electric field calculated by ROCSAT‐1 vertical ion velocity in four seasons. The electric fields derived by the EIA crests of GIM TEC, ROCSAT‐1 ion velocity, and the incoherent scatter radar (ISR) observations at Jicamarca are generally in good agreement in longitudes and seasons. Meanwhile, the electric fields derived by the EIA crests agree well with those by ROCSAT‐1, FORMOSAT‐7/COSMIC‐2, and Jicamarca ISR on individual days. A long‐term study of the GIM‐TEC crest distance reveals obvious solar activity dependency of the daily dynamo electric field during 1999–2020. These agreements confirm that the EIA crest can be used to derive the daily eastward dynamo electric field and the associated vertical ion velocity which is responsible to the strength of equatorial plasma fountain and the latitudinal extend of the EIA. Moreover, the vertical ion velocity derived by the GIM‐TEC crest distance is further used to improve the SAMI2 model for space weather applications.
Plain Language Summary
Equatorial ionization anomaly (EIA) is the most prominent feature in the ionosphere. During the morning, the eastward electric field induced by the tidal wind in the off‐equator E‐region activates the E × B plasma fountain and results in the prominent EIA crests on both sides of the magnetic equator. The global ionosphere map (GIM) routinely and globally displays the EIA crest of total electron content (TEC). Here, we show that the latitudinal distance of the GIM TEC crests between northern and southern hemispheres can be used to derive the daily eastward dynamo electric field and the associated vertical ion velocity in the equatorial ionosphere. Moreover, the derived vertical ion velocity is further used to improve ionosphere models for space weather applications.
Key Points
Daily eastward dynamo electric fields in the equatorial ionosphere are derived by the distance of equatorial ionization anomaly (EIA) crests in global ionosphere maps
The derived dynamo electric fields are used as input to ionosphere models to have a better understanding on the space weather
The distance of EIA crests minimizes the trans‐equatorial wind effects in the estimation of daily dynamo electric fields
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Traveling ionospheric disturbances generated by an epicentral ground/sea surface motion, ionospheric disturbances associated with Rayleigh‐waves as well as post‐seismic 4‐minute monoperiodic ...atmospheric resonances and other‐period atmospheric oscillations have been observed in large earthquakes. In addition, a giant tsunami after the subduction earthquake produces an ionospheric hole which is widely a sudden depletion of ionospheric total electron content (TEC) in the hundred kilometer scale and lasts for a few tens of minutes over the tsunami source area. The tsunamigenic ionospheric hole detected by the TEC measurement with Global Position System (GPS) was found in the 2011 M9.0 off the Pacific coast of Tohoku, the 2010 M8.8 Chile, and the 2004 M9.1 Sumatra earthquakes. This occurs because plasma is descending at the lower thermosphere where the recombination of ions and electrons is high through the meter‐scale downwelling of sea surface at the tsunami source area, and is highly depleted due to the chemical processes.
Key Points
A large depletion of ionospheric plasma appears above tsunami source area
A tsunamigenic ionospheric hole occurs only in the large subduction earthquake
A dense GPS network will provide the whole of the initial tsunami
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Global Navigation Satellite System radio occultation signals often show extremely strong levels of scintillation when passing through the ionospheric E region. This is related to sporadic E(Es)—dense ...layers of metallic ions that can form in the E region, influencing terrestrial and satellite radio propagation. In our report on the 2007–2014 variation of E region S4 amplitude fluctuation indices measured by the FORMOSAT‐3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) satellite constellation, we find that the spatial and temporal variation of the maximum S4 index in the E region is proportionate to the occurrence rate of extreme scintillation and by extension, sporadic E. We also find that the monthly median extreme S4 amplitude fluctuation index in the E region midlatitudes shows a dependence on variation of the El Niño–Southern Oscillation (ENSO) in the troposphere that has not been previously reported. The ENSO‐related variation of the E region median extreme S4 indices varies closely with the tropopause height, with both parameters lagging the Oceanic Niño Index by roughly 1 to 2 months, while also displaying a similar spectrum of periodicities. This similarity is especially strong in the southern midlatitudes. These results indicate that ENSO signatures can be transmitted to Es formation mechanisms, potentially through modulation of vertically propagating atmospheric tides that alter lower thermospheric wind shears. The end result is the modulation of the interannual variation of extreme Es values by ENSO.
Plain Language Summary
Global Navigation Satellite System signals often become extremely unstable when passing through altitudes between approximately 90 and 110 km. This is related to dense layers of metallic ions deposited by meteors burning up in the atmosphere, known as sporadic E (Es). In our observations of the fluctuation of Global Navigation Satellite System signals passing through this region measured by the FORMOSAT‐3/Constellation Observing System for Meteorology Ionosphere and Climate satellite constellation, we find that the level of signal fluctuation shows a dependence on variation of the El Niño–Southern Oscillation (ENSO) in the troposphere that has not been previously reported. A similar ENSO‐related variation is also observed in the height of the tropopause between 10 and 20 km, showing that ENSO can affect atmospheric phenomena that are capable of propagating upward to altitudes where Es occurs. This result illustrates a vertical connection in the atmosphere that has not been previously observed.
Key Points
Extreme E region scintillation observed using 2007‐2016 COSMIC S4 indices are proportionate to sporadic E occurrence
Variation of E region zonal median extreme S4 and tropopause height are correlated with ENSO from 2007 to 2014
This is a new manifestation of troposphere‐ionosphere coupling, especially in southern midlatitudes
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The devastating Hunga Tonga-Hunga Ha’apai underwater volcano erupted at ~04:15 UT on 15 January 2022. We captured the waves that erupted from the volcano propagating in the ionosphere by monitoring ...total electron content (TEC) perturbations utilizing ground-based global navigation satellite system (GNSS) receivers that receive electromagnetic signals transmitted from the geostationary satellites operated by the BeiDou Navigation Satellite System (BDS). Meanwhile, ground barometers detected unusual enhancements of air pressure traveling in the troposphere. A novel phenomenon shows that the waves can individually propagate with a speed of ~335 m/s in the ionosphere, which is faster than its’ ~305 m/s in the troposphere. We further examined multiple geophysical data at the particular site of the novel instrumental array. Analytical results show that the pressure enhancements traveling in the troposphere not only downward trigger ground vibrations mainly in the horizontal components without obvious time difference, but also upward, leading the secondary TEC perturbations with a ~12-min delay.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Ionospheric disturbances occurred as a result of the tsunami associated with the 2011 M9.0 off the Pacific Coast of the Tohoku earthquake (EQ). The ionospheric disturbances propagated radially from ...the tsunami source area, termed the traveling ionospheric disturbance. In addition to the traveling ionospheric disturbance, an ionospheric plasma depression lasting for approximately 1 h occurred above the tsunami source area, called a tsunami ionospheric hole. In this study, we compare the ionospheric disturbances caused by large inland and submarine EQs to investigate whether an ionospheric plasma depression only occurs in association with a tsunami. Note that we term an EQ with a tsunami a submarine EQ. To investigate the presence of a plasma depression, i.e., an ionospheric hole, associated with an inland EQ, data on total electron content between the global positioning system satellite and its receivers were used. Comparison of two inland and two submarine EQ events with similar magnitudes around 7 showed that ionospheric holes were observed only for the submarine EQs. This discrepancy might be attributed to the different excitation amplitudes of the atmospheric acoustic waves between the unidirectional fault displacement and the tsunami uplift/depression, corresponding to quarter and one‐period variations. From this hypothesis, we predicted that an ionospheric hole could be observed after a significantly large inland EQ with a sufficiently large vertical ground displacement. In fact, we recognized the ionospheric hole generated by the large inland EQ that recently occurred in the Nepal with the magnitude of 7.8 on 25 April 2015.
Key Points
Ionospheric holes have been observed after submarine earthquakes
Occurrence of ionospheric holes assessed for submarine and inland earthquakes
Ionospheric holes should occur after very large earthquakes on land
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The joint Taiwan‐United States FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation ...dedicated to remotely sense Earth's atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow‐on RO mission named FORMOSAT‐7/COSMIC‐2 (COSMIC‐2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC‐2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low‐latitude plasma drifts and scintillation at multiple frequencies. COSMIC‐2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC‐2 measurements of key ionospheric parameters.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This paper presents two empirical models, the low wind bagged trees (LWBT) and high wind bagged trees (HWBT) ensemble models to estimate ocean surface wind speed using spaceborne Global Navigation ...Satellite System Reflectometry (GNSS-R) data. The models are empirically trained using NASA's Cyclone GNSS (CYGNSS) mission level 1 data (version 2.1). The truth label for the LWBT model is the wind speed product derived from European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5 and Global Data Assimilation System (GDAS), while the label for the HWBT model is wind speed measurements from Stepped Frequency Microwave Radiometer (SFMR). Testing results show that the LWBT and HWBT models achieved global wind speed retrieval root-mean-square-error (RMSE) of ~1.5 m/s and ~1.4 m/s, respectively, corresponding to an improvement of 29% and 65% with respect to the CYGNSS Level 2 standard wind speed product. The maximum bias is reduced by 65% and 60% for LWBT and HWBT over the Level 2 wind speeds, respectively. Two typhoon case studies are presented to corroborate the model performances and their retrieved wind speeds are consistent with reports from World Meteorological Organization (WMO) and with the measurement provided by the Huangmao Zhou (HMZ) weather station.
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