Decrease In Total Electron Content Research Articles

Response of total electron content to the October 25, 2022 partial solar eclipse from high to low latitudes in the Euro-Asian region

Solar eclipses (SEs) pertain to high-energy sources. They are capable of causing significant disturbances in all geoshells and geophysical fields. Despite the almost century-long history of studying the influence of eclipses on geoshells, the task of observing and analyzing disturbances in them remains relevant. This is explained by the fact that the response of the media significantly depends on the state of atmospheric and space weather, the location and time of observation, the SE magnitude, etc. The aim of this paper is to present the results of a statistical analysis of the features of the total electron content (TEC) variations during the October 25, 2022 SE from high to low latitudes in the Euro-Asian region. Data from Global Navigation Satellite System were used for the analysis. The error in estimating TEC in the vertical column did not exceed 0.1 TECU. For the first time, the response of the ionosphere to an SE has been studied on a global scale (from the Reykjavik station to the Novosibirsk station) using methods of statistical analysis, including the time moments after sunset. The decrease in TEC reached 1.0–19.0 TECU, and the relative decrease was approximately –0.32. Changes in all TEC disturbance parameters depended in a complex manner on the relative obscuration area of the solar disk. This can be explained by non-monotonic changes in electron density during the daytime, by the intensification of the ionosphere–plasmasphere coupling, the disturbance of the ionospheric current system, vertical plasma drift in crossed electric and magnetic fields, as well as the generation of wave disturbances during the SE period. The delay time of the maximum ionospheric response (maximum |δV| value) with respect to the maximum magnitude of the eclipse was 15–25 min. The duration of the ionospheric response to the SE slightly exceeded the duration of the perturbing source influence.

2‐D Total Electron Content and 3‐D Ionospheric Electron Density Variations During the 14 October 2023 Annular Solar Eclipse

AbstractThis study investigates the ionospheric total electron content (TEC) responses in the 2‐D spatial domain and electron density variations in the 3‐D spatial domain during the annular solar eclipse on 14 October 2023, using ground‐based Global Navigation Satellite System (GNSS) observations, a novel TEC‐based ionospheric data assimilation system (TIDAS), ionosonde measurements, and satellite in situ data. The main results are summarized as follows: (a) The 2‐D TEC responses exhibited distinct latitudinal differences. The mid‐latitude ionosphere exhibited a more substantial TEC decrease of 25%–40% along with an extended recovery time of 3–4 hr. In contrast, the equatorial and low‐latitude ionosphere experienced a smaller TEC reduction of 10%–25% and a faster recovery time of 20–50 min. The minimal eclipse effect was observed near the northern equatorial ionization anomaly crest region. (b) The ionospheric electron density variations during the eclipse were effectively reconstructed by TIDAS data assimilation in the 3‐D domain, providing important altitude information with validity. (c) The ionospheric electron density variations showed a notable altitude‐dependent feature. The eclipse led to a substantial electron density reduction of 30%–50%, with the maximum depletion occurring around the ionospheric F2‐layer peak height (hmF2) of 250–350 km. The post‐eclipse recovery of electron density exhibited a relatively slower pace near the F2‐layer peak height than that at lower and higher altitudes.

Space weather impacts on the ionosphere over the southern African mid-latitude region

The ionosphere suffers major perturbations during severe space weather events such as Coronal Mass Ejections (CMEs), solar flares, high-speed streams, and Corotating Interaction Regions (CIRs). The ionosphere can experience depletions or enhancements in Total Electron Content (TEC) during severe space weather conditions. The South African National Space Agency (SANSA) near-real-time (NRT) TEC maps were used to show the ionospheric variability during the geomagnetic storm of 3–8 Nov 2021 over the southern Africa mid-latitude region. The ionosonde TEC, NRT TEC, and the quiet-time AfriTEC model were compared during the 6-day period. A negative ionospheric response was observed during the main and recovery phases of the geomagnetic storm (4–5 Nov 2021). The changes to neutral composition O/N2 was one of the physical processes attributed to the decrease in TEC over the mid-latitude region. The GPS TEC maps showed a very good agreement with ionosonde measurements and the AfriTEC model. A strong east–west TEC gradient was observed occurring between two ionosonde stations.Graphical

Ionospheric response to the 23–31 August 2018 geomagnetic storm in the Europe-African longitude sector using multi-instrument observations

This study presents ionospheric responses of the mid and low-latitude region in the Europe-African longitude sector (along 30°±10°E) to the intense geomagnetic storm of 23–31 August 2018 (SYM-Hmin = −207 nT) using the Global Ionospheric Map (GIM) and Global Positioning System (GPS) receivers data, the satellite data (SWARM, Defense Meteorological Satellite Program (DMSP), Global Ultraviolet Imager on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (GUVI/TIMED)), and Prompt Penetration Equatorial Electric Field model (PPEFM). The percentage deviation in total electron content (TEC) denoted by ΔTEC (%) was used to observe the ionospheric storm effects. The rate of change of TEC index (ROTI) derived from GPS-TEC and the rate of change of plasma density index (RODI) obtained from SWARM satellites were utilized to quantify the occurrence of ionospheric irregularities. Results obtained from GPS receivers and GIM data revealed a large increase in TEC (positive ionospheric storm effect) in the equatorial and low-latitude region of Africa, and a decrease in TEC (negative ionospheric storm effect) over the midlatitude region of Europe and Africa during the storm recovery phase. The decrease in [O]/[N2] ratio is the possible cause for the observed negative ionospheric storm effect. Hemispheric asymmetry were noticed over Europe-African longitude sector during the storm main and recovery phases. The occurrence of ionospheric irregularities over the low-latitude region of Africa in the premidnight and post-midnight was suppressed (ROTI<0.4 TECU/min). This could be related to the local time at which the minimum SYM-H occurred which corresponded to daytime over Europe-African longitude sector. This, on the other hand, may not support the development of conducive environment for the generation of ionospheric irregularities. However, significant fluctuation in the plasma density was noticed by the SWARM-C on 23, 25, 30, and 31 August 2018 over equatorial and low-latitude region of Africa during the post-midnight period.

Multi‐Instrument Observations of the Ionospheric Response to the 26 December 2019 Solar Eclipse Over Indian and Southeast Asian Longitudes

AbstractHere, the impact of the 26 December 2019 solar eclipse on the equatorial and low latitude ionosphere has been investigated using ground and space‐based observations over Indian and Southeast Asian longitudes. The high‐resolution Ionosonde observations at Tirunelveli, GPS Total Electron Content (TEC) observations from a chain of GPS receivers along and across the eclipse path, TIMED‐SABER, and Ionospheric Connection Explorer (ICON) satellites were utilized to investigate the eclipse‐induced variations in electron density and thermospheric cooling. We noticed a tremendous increase and decrease in the base height of the F‐layer, resembling the nighttime Pre‐Reversal Enhancement (PRE). Near the eclipse maximum, a strong blanketing sporadic E layer was observed at Tirunelveli with a top frequency of ∼18 MHz for 1 hr and 26 min. Satellite traces (STs) and “U” shaped ionograms were noticed for the first time over Tirunelveli during eclipse maximum and end phases. The “STs” and “U” shaped traces indicate the presence of short‐period gravity waves or TID type of wave perturbations over the Indian region. A maximum of ∼5–7 TECU (30%–40%) decrease in TEC is observed on the eclipse day for iisc, hyde, and tiru stations. Periodogram analyses of TEC data showed the presence of wavelike structures with periodicities of 18–24 min for different stations. Simultaneous observations from the ICON satellite showed an increase and decrease in hmF2 and NmF2 which matches well with the ionosonde observations from Tirunelveli. The temperature profiles from TIMED‐SABER and ICON satellites showed a reduction and enhancement in the lower and upper E regions respectively.

On the significant impact of the 17 March 2015 St. Patrick’s Day geomagnetic storm on the ionosphere over Indian region

The response of the ionosphere in the Indian region to the extreme space weather events of 17–19 March 2015 is studied using Global Positioning System (GPS) Total electron Content (TEC) measurements, Thermosphere Ionosphere Electrodynamics-General Circulation Model (TIE-GCM) simulations and High Frequency (HF) Radar observations. The data from Thiruvananthapuram (8.5° N, 77° E geographic, 0.3° N magnetic latitude) and northern hemispheric midlatitude station Hanle (32.80° N, 79° E geographic, 23.9° N magnetic latitude) have been analyzed for the above event. The TEC over Thiruvananthapuram, a dip equatorial location exhibits a decrease during the post sunset hours on 17 March 2015 and is followed by the generation of F region irregularities at an altitude of 600 km, as observed by HF radar as well as TEC scintillations. The TEC over the mid latitude station of Hanle decreased during the storm recovery phase on 18March 2015 in response to negative storm effects. The extent of decrease in TEC as observed during the recovery phase could not be simulated by TIE-GCM over Hanle. This difference is attributed to the enhanced Joule heating taking place in the polar region during St. Patrick’s Day storm and the associated composition changes. An anomalous heating scenario was introduced and the simulated TIE-GCM results were found to match the observations. The present study clearly brings out the impact of 17 March 2015, St. Patrick’s Day geomagnetic storm on the ionization distribution over equatorial as well as mid latitude stations over the Indian region using observations as well as TIE-GCM simulations and the evolutionary features of ionospheric irregularities over equatorial location using TEC and HF radar observations. The role played by thermosphere ionosphere coupling in modulating the ionization variability over equatorial as well as mid latitude region during periods of extreme geomagnetic forcing is highlighted.

Significant Ionospheric Hole and Equatorial Plasma Bubbles After the 2022 Tonga Volcano Eruption

AbstractThis paper investigates the local and global ionospheric responses to the 2022 Tonga volcano eruption, using ground‐based Global Navigation Satellite System total electron content (TEC), Swarm in situ plasma density measurements, the Ionospheric Connection Explorer (ICON) Ion Velocity Meter (IVM) data, and ionosonde measurements. The main results are as follows: (a) A significant local ionospheric hole of more than 10 TECU depletion was observed near the epicenter ∼45 min after the eruption, comprising of several cascading TEC decreases and quasi‐periodic oscillations. Such a deep local plasma hole was also observed by space‐borne in situ measurements, with an estimated horizontal radius of 10–15° and persisted for more than 10 hr in ICON‐IVM ion density profiles until local sunrise. (b) Pronounced post‐volcanic evening equatorial plasma bubbles (EPBs) were continuously observed across the wide Asia‐Oceania area after the arrival of volcano‐induced waves; these caused a Ne decrease of 2–3 orders of magnitude at Swarm/ICON altitude between 450 and 575 km, covered wide longitudinal ranges of more than 140°, and lasted around 12 hr. (c) Various acoustic‐gravity wave modes due to volcano eruption were observed by accurate Beidou geostationary orbit (GEO) TEC, and the huge ionospheric hole was mainly caused by intense shock‐acoustic impulses. TEC rate of change index revealed globally propagating ionospheric disturbances at a prevailing Lamb‐wave mode of ∼315 m/s; the large‐scale EPBs could be seeded by acoustic‐gravity resonance and coupling to less‐damped Lamb waves, under a favorable condition of volcano‐induced enhancement of dusktime plasma upward E×B drift and postsunset rise of the equatorial ionospheric F‐layer.

GPS observations of ionospheric TEC variations over Nepal during 22 July 2009 solar eclipse

As the study of ionospheric behavior during various solar activities is an important task, various studies of ionospheric changes during eclipse events have been widely performed in the different regions of the globe. This paper investigates the ionospheric responses to the solar eclipse on 22 July 2009 over Nepal using the total electron content (TEC) measured by dual-frequency Global Positioning System (GPS) receivers. The time-averaged Vertical TEC (vTEC) of ten GPS stations from Nepal is analyzed and it is found that the value of ionospheric TEC decreases due to the reduction of ionizing radiation. In addition, the deviation in the TEC value on eclipse day from the mean vTEC value of the top five quietest days is found to lie in the range ~1–5 TECu at those regions which were associated with the partial eclipse shadow. On the other hand, the region with the total eclipse (BRN2 and RMTE) faced ~6–7 TECu on average reduction in the TEC value. Considering that the eclipse of 22 July 2009 occurred just at sunrise in the Nepalese zone, a maximum reduction of about 5 TECu is very significant. Higher deviation in TEC is therefore linked with the path of totality and the obscuration rate. This study reveals that the ionospheric TEC over Nepal was altered by wave-like energy and momentum transport, as well as obscuration of the solar disc due to the partial and total solar eclipse. Furthermore, the cross-correlation results presented similar type signatures of the eclipse-induced ionospheric modification over Nepal. This research work serves a crucial future reference for the comparative study of change of ionospheric TEC variability over the Nepal region during Eclipse events

A Study of Possible Correlations between Seismo-Ionospheric Anomalies of GNSS Total Electron Content and Earthquake Energy

In this study, we conduct a correlation analysis between the daily occurrence times of the increase and decrease anomalies in the global total electron content (TEC) in the ionosphere, and the daily earthquake energy release within 110–130°E longitude over the following three latitude regions: A: 13°S–0.5°S (22.3°S–10°S geomagnetic), B: 0.5°S–19.5°N (10°S–10°N geomagnetic), and C: 19.5°N–32.1°N (10°N–22.5°N geomagnetic). The TEC data from global ionosphere maps (GIMs) during earthquake events of M ≥ 2.5 that occurred in 2015–2018 are used in this study. The time series of daily seismic wave energy releases within the three regions and the daily occurrence times of the TEC anomalies in each GIM grid are computed. By time-shifting the time series, the correlations are calculated and compared globally, and the temporal characteristics are also examined. The disturbance storm time (Dst) index, planetary geomagnetic index Kp, and daily observed 10.7 cm solar flux (F10.7) are used to remove data associated with space weather variations. Although the seismo-ionospheric precursor is not confirmed by the statistical investigations, the greater occurrence times of TEC decrease anomalies are observed in the southeast in Region A, and the conjugate point 13 days prior to a M6.9 earthquake in Region A, which occurred on 5 August 2018, in accordance with the statistical results. Therefore, it is required to apply more parameters to understand the causes of the ionospheric TEC variations and investigate whether ionospheric variations are caused by earthquakes.

An Investigation of Response of the Tropical Cyclone Ockhi in the Equatorial Ionosphere over the Indian region

Background/Objectives: The ionospheric response to the tropical cyclone (TC) OCKHI of December 2017 in the equatorial region is presented in the study. Methods/Statistical analysis: The ionospheric response includes a change in Total Electron Content (TEC) is utilized using the stations of Tirunelveli and Bangalore. The critical frequency (foF2) from Ionosonde over Tirunelveli station also considered to study the TC effect in the F2 region of ionosphere. Findings: It has been observed that the significant changes in the equatorial ionosphere exhibits during the tropical cyclone (TC) OCKHI. They are: Ionospheric parameter of vertical total electron content (VTEC) varies during the TC (i.e. from Day 332 to Day 336 of the year 2017) as the cyclone center is near to the Tirunelveli (8.7º N, 77.8º E) and the same follows for the IGS station of Bangalore. In addition, ionosonde measurements of foF2 perceived for the local night time support the TEC decrease noted over Tirunelveli when the cyclone attains its peak on 01 December 2017. Novelty/Applications: Using the ionosonde data over the equatorial ionosphere, such behavior may not be reported earlier to the best of my knowledge. The gravity wave induced by the TC Ockhi redistributes the element of the ionosphere due to strong convection and lightning activity. Keywords: TEC; Cyclone; foF2; OCKHI; and gravity waves

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

This study investigates the morphology of the GPS TEC responses in the African Equatorial Ionization Anomaly (EIA) region to intense geomagnetic storms during the ascending and maximum phases of solar cycle 24 (2012–2014). Specifically, eight intense geomagnetic storms with Dst ≤ −100 nT were considered in this investigation using TEC data obtained from 13 GNSS receivers in the East African region within 36–42°E geographic longitude; 29°N–10°S geographic latitude; ± 20°N magnetic latitude. The storm-time behavior of TEC shows clear positive and negative phases relative to the non-storm (median) behavior, with amplitudes being dependent on the time of sudden commencement of the storm and location. When a storm starts in the morning period, total electron content increases for all stations while a decrease in total electron content is manifested for a storm that had its sudden commencement in the afternoon period. The TEC and the EIA crest during the main phase of the storm is significantly impacted by the geomagnetic storm, which experiences an increase in the intensity of TEC while the location and spread of the crest usually manifest a poleward expansion.

Ionospheric effects of the August 11, 2018, solar eclipse over the People’s Republic of China

The purpose of the work is to describe the ionospheric effects of the August 11, 2018 partial solar eclipse (SE) that occurred over the People’s Republic of China, as observed via GPS technology. SEs present rare phenomena of nature. In the course of 2 to 3 hours, the rearrangement of processes acting at the Earth’s surface, in the atmosphere, geospace, i.e., in the Earth — atmosphere — ionosphere — magnetosphere system (EAIMS), occurs. The response of this system depends on the solar activity, season, time of day, and on the state of atmospheric and space weather. Therefore, the study of the EAIMS response to SEs remains an urgent need. The response is accompanied by controllable dynamic processes, the study of which improves our understanding of the near-Earth environment. The study of the EAIMS response to SEs is of fundamental importance to science. Its practical applications include the following. The SE give rise to significant perturbations in the EAIMS, which affect the propagation of radio waves virtually in all frequency bands, and consequently deteriorate the operation of radar, radio astronomy, and radio navigation systems, as well as the instruments for remotely sensing the medium. The SE effects have been studied for over more than about 100 years. Thus far, the following regular effects have been quite well studied: decreases in the electron density, electron and ion temperatures, variations in ion composition, and plasma vertical movements. The irregular effects have been studied to a significantly smaller degree, and they can vary from one solar eclipse to another. The main feature of the SE over the PRC was the fact that it was observed during before local time sunset period. The maximum phase of the eclipse within the PRC area varied from 0.07 to 0.52, while the Sun’s surface area occulted by the moon was observed to be 0.02—0.42. The beginning of the eclipse over the PRC was observed to occur in the 09:54—10:05 UT period, and the end varied from 10:07 UT to 11:10 UT. The SE duration varied from a few minutes to approximately 67 min. The insignificant duration of the eclipse and the dusk terminator affected the SE effects. The state of space weather during the solar eclipse was conducive to observing the SE effects occurring in the ionosphere. To reveal the ionospheric response to the August 11, 2018, SE, the global navigation satellite system data were processed. The ionospheric time delay and, respectively, the vertical total electron content (TEC), were calculated combining the pseudo range and integrated phase data at two frequencies. Regardless of the dusk terminator influence, we have managed to confidently detect the ionospheric SE effects, which proved to be sufficiently small because of small values of the SE phase. Over the People’s Republic of China area, a funnel-shaped decrease in TEC was observed to occur approximately 1,300 km in latitude and 2,000 km in longitude. The TEC decrease was observed to be 7 %. The solar eclipse was accompanied by the generation of aperiodic TEC disturbances at a rate of 0.4—0.8 TEC unit/h and 105-min in duration. Wave disturbances caused by the SE were not observed confidently, which is due to the small value of the SE phase and insignificant disturbances in the electron density.

Ionospheric Responses at Low Latitudes to the Annular Solar Eclipse on 21 June 2020

AbstractIn this study, we utilized both ground‐based and space‐borne observations including total electron content (TEC) from Beidou geostationary satellites, two‐dimensional TEC maps from the worldwide dense Global Navigation Satellite System receivers, ionosondes, and in situ electron density (Ne) and electron temperature (Te) from both Swarm and China Seismo‐Electromagnetic Satellite satellites, to investigate the low‐latitude ionospheric responses to the annular solar eclipse on 21 June 2020. The decrease in TEC during the eclipse at low latitudes showed a local time dependence with the largest depletions in the noon and afternoon sectors. It was also found that the TEC depletions at different latitudes in the equatorial ionization anomaly (EIA) region over the East Asian sector cannot solely be explained by the solar flux changes associated with the obscuration rate. The differences in TEC reduction between stations can be more than a factor of 2 at latitudes with the same obscuration rate of over 90%. Compared with TEC variations in the Northern Hemisphere, the TEC also underwent a considerable decrease in the EIA region in the conjugate hemisphere without eclipse shadow. Meanwhile, the hmF2 near the magnetic equator increased around the onset of the eclipse, indicating an enhancement of the eastward equatorial electric field. Furthermore, the TEC decrease during the eclipse in the EIA region in both hemispheres lasted for a long period of more than 7 hr after the eclipse, with a TEC depletion of 2–6 TEC units. The Ne from Swarm and China Seismo‐Electromagnetic Satellite satellites showed a complicated variation after the eclipse, whereas no visible change was observed in Te. The enhanced equatorial electric field, neutral wind changes, and the associated plasma transport act together to generate the observed ionospheric effects at low latitudes during the eclipse. Our results also suggest that the eclipse‐induced perturbations of dynamic processes can continue to impact the ionosphere after the eclipse.

Evaluation of Total Electron Content Prediction Using Three Ionosphere‐Thermosphere Models

AbstractPrediction of ionospheric state is a critical space weather problem. We expand on our previous research of medium‐range ionospheric forecasts and present new results on evaluating prediction capabilities of three physics‐based ionosphere‐thermosphere models (Thermosphere Ionosphere Electrodynamics General Circulation Model, TIE‐GCM; Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics Model, CTIPe; and Global Ionosphere Thermosphere Model, GITM). The focus of our study is understanding how current modeling approaches may predict the global ionosphere for geomagnetic storms (as studied through 35 storms during 2000–2016). Prediction approach uses physics‐based modeling without any manual model adjustment, quality control, or selection of the results. Our goal is to understand to what extent current physics‐based modeling can be used in total electron content (TEC) prediction and explore uncertainties of these prediction efforts with multiday lead times. The ionosphere‐thermosphere model runs are driven by actual interplanetary conditions, whether those data come from real‐time measurements or predicted values themselves. These model runs were performed by the Community Coordinated Modeling Center (CCMC). Jet Propulsion Laboratory (JPL)‐produced global ionospheric maps (GIMs) were used to validate model TEC estimates. We utilize the True Skill Statistic (TSS) metric for the TEC prediction evaluation, noting that this is but one metric to assess predictive skill and that complete evaluations require combinations of such metrics. The meanings of contingency table elements for the prediction performance are analyzed in the context of ionosphere modeling. Prediction success is between about 0.2 and 0.5 for weak ionospheric disturbances and decreases for strong disturbances. We evaluate the prediction of TEC decreases and increases. Our results indicate that physics‐based modeling during storms shows promise in TEC prediction with multiday lead time.

Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko

We study the evolution of a cometary ionosphere, using approximately two years of plasma measurements by the Mutual Impedance Probe on board the Rosetta spacecraft monitoring comet 67P/Churyumov-Gerasimenko (67P) during August 2014–September 2016. The in situ plasma density measurements are utilized to estimate the altitude-integrated electron number density or cometary ionospheric total electron content (TEC) of 67P based on the assumption of radially expanding plasma. The TEC is shown to increase with decreasing heliocentric distance (rh) of the comet, reaching a peak value of ~(133 ± 84) × 109cm−2averaged around perihelion (rh&lt; 1.5 au). At large heliocentric distances (rh&gt; 2.5 au), the TEC decreases by ~2 orders of magnitude. For the same heliocentric distance, TEC values are found to be significantly larger during the post-perihelion periods compared to the pre-perihelion TEC values. This “ionospheric hysteresis effect” is more prominent in the southern hemisphere of the comet and at large heliocentric distances. A significant hemispheric asymmetry is observed during perihelion with approximately two times larger TEC values in the northern hemisphere compared to the southern hemisphere. The asymmetry is reversed and stronger during post-perihelion (rh&gt; 1.5 au) periods with approximately three times larger TEC values in the southern hemisphere compared to the northern hemisphere. Hemispheric asymmetry was less prominent during the pre-perihelion intervals. The correlation of the cometary TEC with the incident solar ionizing fluxes is maximum around and slightly after perihelion (1.5 au &lt;rh&lt; 2 au), while it significantly decreases at larger heliocentric distances (rh&gt; 2.5 au) where the photo-ionization contribution to the TEC variability decreases. The results are discussed based on cometary ionospheric production and loss processes.

Geometric determination of ionospheric total electron content from dual frequency radar sounding measurements

Orbital radar sounding measurements are used to characterize the ionosphere by correcting for signal dispersion effects (de-focusing and delay) experienced as the radar wave propagates through it. For Mars, this is typically achieved either by 1) autofocusing surface echoes to maximize their strength or sharpness, or 2) comparing relative time delays to the surface at either cross-over points or with simulated radargrams. Here, a novel and robust geometric approach is proposed that is primarily applicable to Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) sounding measurements as, due to the design of the REASON and the nature of Europa’s ionosphere, typical Martian radiometric approaches may not be suitable. However, the methodology can be adapted to an arbitrary set of dual-frequency radar sounding measurements. The geometric approach leverages the dual-frequency nature of REASON and the time delay between the ionosphere-delayed 9 MHz High Frequency (HF) surface echoes and the simultaneously-recorded but differentially dispersed 60 MHz Very High Frequency (VHF) surface echoes. That time delay is then used to estimate the ionospheric total electron content (TEC). The robustness of the proposed methodology is demonstrated through testing with Martian sounding measurements. The geometric results reproduce the expected decrease in TEC in the Martian ionosphere as a function of increasing solar zenith angle (SZA), exhibit a consistent agreement between the two test regions, and quantitatively correlate with published radiometric TEC estimates. Testing provides key insights into the underlying assumptions and idiosyncrasies of the methodology and how it could best be applied with REASON datasets to characterize and search for anomalies (e.g. plumes) within the ionosphere of Europa.

Ionospheric earthquake signatures on GPS TEC

The ionosphere total electron content (TEC) can be derived from a ground based receiver of the global Positioning System (GPS). The GPS consist of 24 satellites, evenly distributed in 6 orbital planes around the globe at an altitude ∼20200 km. Each satellite transmits signals on two frequencies (f1 = 1575.42 MHz and f2 = 1227.60 MHz) with two different codes, P1 (or C/A) and P2 and two different carrier phases, L1 and L2. Since the ionosphere is a dispersive medium, the speeds of propagation of the electro-magnetic waves transmitted by the GPS satellites depend on the frequency of radio waves. The carrier phase advance and group delay of GPS transmitted radio waves in the ionosphere is proportional to electron content integrated along the propagation path. From carrier phase and pseudo-range code data observed by GPS receivers located in the vicinity of Indonesia, we derived the ionospheric GPS TEC using phase levelling method. For the aim of finding the diurnal variation anomalies associated with the large earthquakes in Indonesia and surrounding areas, we used the harmonic analysis of TEC observed at several GPS stations. The results of harmonic analysis have revealed that the amplitude of diurnal variations of TEC decreases (negative anomaly) or increases (positive anomaly) several days before major earthquakes, and by using spatial analysis of the amplitude anomaly, the magnitudes of some major earthquakes can be estimated.

Changes in the middle and upper atmosphere parameters during the January 2013 sudden stratospheric warming

We present the results of complex obser-vations of various parameters of the middle and upper atmosphere over Siberia in December 2012 – January 2013, during a major sudden stratospheric warming (SSW) event. We analyze variations in ozone concentration from microwave measurements, in stratosphere and lower mesosphere temperatures from lidar and satellite measurements, in the F2-layer critical frequency (foF2), in the total electron content (TEC), as well as in the ratio of concentrations of atomic oxygen to molecular nitrogen (O/N2) in the thermosphere. To interpret the observed disturbances in the upper atmosphere, the experimental measurements are compared with the results of model calculations obtained with the Global Self-consistent Model of Thermosphere—Ionosphere—Protonosphere (GSM TIP). The response of the upper atmosphere to the SSW event is shown to be a decrease in foF2 and TEC during the evolution of the warming event and a prolonged increase in O/N2, foF2, and TEC after the SSW maximum. For the first time, we observe the relation between the increase in stratospheric ozone, thermospheric O/N2, and ionospheric electron density for a fairly long time (up to 20 days) after the SSW maximum at midlatitudes.

Изменения параметров средней и верхней атмосферы во время внезапного стратосферного потепления в январе 2013 г.

We present the results of complex obser-vations of various parameters of the middle and upper atmosphere over Siberia in December 2012 – January 2013, during a major sudden stratospheric warming (SSW) event. We analyze variations in ozone concentration from microwave measurements, in stratosphere and lower mesosphere temperatures from lidar and satellite measurements, in the F2-layer critical frequency (foF2), in the total electron content (TEC), as well as in the ratio of concentrations of atomic oxygen to molecular nitrogen (O/N2) in the thermosphere. To interpret the observed disturbances in the upper atmosphere, the experimental measurements are compared with the results of model calculations obtained with the Global Self-consistent Model of Thermosphere—Ionosphere—Protonosphere (GSM TIP). The response of the upper atmosphere to the SSW event is shown to be a decrease in foF2 and TEC during the evolution of the warming event and a prolonged increase in O/N2, foF2, and TEC after the SSW maximum. For the first time, we observe the relation between the increase in stratospheric ozone, thermospheric O/N2, and ionospheric electron density for a fairly long time (up to 20 days) after the SSW maximum at midlatitudes.

Deep Ionospheric Hole Created by Sudden Stratospheric Warming in the Nighttime Ionosphere

AbstractMultiple observational studies have demonstrated large ionospheric variations during the daytime associated with sudden stratospheric warming (SSW) events, but only limited evidence of ionospheric disturbances during the nighttime has been reported up to now. We focus on the American longitudinal sector with its extensive observational network of Global Navigation Satellite System receivers, four Digisondes located at low and middle latitudes, and the Arecibo and Millstone Hill incoherent scatter radars. The study focuses on a major SSW event of January 2013 to investigate large‐scale disturbances in the nighttime ionosphere. We report a deep decrease in total electron content that reaches a factor of 2–5 as compared to the background level and is observed between local midnight and local sunrise (6–12 UT). This decrease is observed for several consecutive days in the range of latitudes from ~55°S to ~45°N. It is accompanied by a strong downward plasma motion and a significant decrease in ion temperature, as observed by both Arecibo and Millstone Hill radars. These results demonstrate that SSW events cause changes in the nighttime ionosphere that are even larger than in the daytime ionosphere. We discuss variations in electric field and F‐region dynamics as possible drivers of this behavior and suggest that thermospheric winds play a much larger role than previously thought.