Traveling Atmospheric Disturbances Research Articles

Investigation of Large Scale Traveling Atmospheric/Ionospheric Disturbances Using the Coupled SAMI3 and GITM Models

AbstractWe present simulation results of the vertical structure of Large Scale Traveling Ionospheric Disturbances (LSTIDs) during synthetic geomagnetic storms. These data are produced using a one‐way coupled SAMI3/Global Ionosphere Thermosphere Model (GITM) model, where GITM provides thermospheric information to SAMI3 (SAMI3 is Another Model of the Ionosphere), producing LSTIDs. We show simulation results which demonstrate that the traveling atmospheric disturbances (TADs) generated in GITM extend to the topside ionosphere in SAMI3 as LSTIDs. The speed and wavelength (600–700 m/s and 10º–20° latitude) are consistent with LSTID observations in storms of similar magnitudes. We demonstrate the LSTIDs reach altitudes beyond the topside ionosphere with amplitudes of <5% over background which will facilitate the use of plasma measurements from the topside ionosphere to supplement measurements from Global Navigation Satellite System in the study of Traveling Ionospheric Disturbances (TIDs). Additionally, we demonstrate the dependence of the characteristics of these TADs and TIDs on longitude.

Magnetic Field Signatures of Tropospheric and Thermospheric Lamb Modes Triggered by the 15 January 2022 Tonga Volcanic Eruption

AbstractIntense eruptions of the Tonga volcano activated prominent traveling atmospheric disturbances (TADs) at 04:05UT on 15 January 2022. Himawari‐8 satellite images depict that TADs of the tropospheric Lamb wavefront propagate with a speed of 315 m/s and arrive in Taiwan at 11:30UT. Networks of 98 barometers, 28 tide gauges, an ionosonde, and 10 magnetometers are used to study the responses of magnetic fields to TADs in Taiwan. The horizontal components in magnetic field changes of the Taiwan magnetometers all point toward and away from the Tonga volcano at 11:00–12:00UT upon the tropospheric Lamb wavefront arrival and at 22:00–23:00UT when the thermospheric Lamb wavefront with speeds of 487 m/s coming, respectively. Analyses of the raytracing and beamforming techniques on the horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves efficiently activate traveling ionospheric disturbances and modify ionospheric currents of the globe.

Interhemispheric Asymmetry of the Thermospheric Neutral Density Response to the 7–9 September 2017 Geomagnetic Storms

AbstractThe thermospheric neutral density response to the 7–9 September 2017 storms is investigated based on the Swarm satellite observations and the thermosphere‐ionosphere‐electrodynamic general circulation model (TIEGCM) simulation. The Swarm data depicted a prominent interhemispheric asymmetry (IHA) in the afternoon sector during the second storm, a feature that was yet explained. Driven by realistic high‐latitude electric potential and electron precipitation patterns, the TIEGCM is able to reproduce the observed storm‐time neutral density response. The TIEGCM simulation reveals that the differences in the traveling atmospheric disturbances (TADs) is largely responsible for the observed IHA in the neutral mass density response at low and middle latitudes, whereas the difference in mean molecular mass between the two hemispheres may contribute to the IHA in neutral density at higher latitudes. The IHAs in TADs and mean molecular mass are attributed to the IHA in Joule heating dissipation on the night and dawn sides.

Study on the Hemispheric Asymmetry of Thermospheric Density Based on In-Situ Measurements from APOD Satellite

In this article, high spatiotemporal resolution data obtained by the atmospheric density detector carried by China’s APOD satellite are used to study the hemispheric asymmetry of thermospheric density. A detailed analysis is first performed on the dual magnetic storm event that occurred near the autumnal equinox on 8 September 2017. The results show that the enhancement ratio of atmospheric density in the southern polar region (SPR) on the duskside was approximately 1.33–1.65 times that of the northern polar region (NPR), demonstrating a strong hemispheric asymmetry of thermospheric atmospheric density response during the magnetic storm. However, the asymmetry response was smaller on the dawnside, suggesting that the hemispheric density response asymmetry is related to local time (LT). The energy injection in high-latitude regions increases local atmospheric density and forms traveling atmospheric disturbances (TADs). TADs can propagate to low-latitude regions over several hours and affect the global distribution of thermospheric atmospheric density. Similarly, the geomagnetic index fitting slope of SPR relative density difference is greater than that of NPR. The SuperDARN convection pattern indicates that the plasma convection velocity of SPR is significantly greater than that of NPR, indicating that joule heating caused by neutral friction of ions in the Southern Hemisphere may be stronger. Subsequently, an analysis of annual solar activity and seasons was carried out on the thermospheric NPR, SPR atmospheric density, and their differences from December 2015 to December 2020. The results show that thermospheric atmospheric density decreases overall as the number of sunspots decreases. The differences between the NPR and SPR atmospheric densities in the thermosphere exhibits a noticeable annual periodicity. The NPR and SPR atmospheric densities appear to have different distribution characteristics in different seasons. The NPR density peak is mainly in March or April. In particular, the “double-peak” phenomenon occurred in 2017, with peaks in March and September, while the most obvious feature of SPR atmospheric density is that its minimum value occurs in the summer months of June and July. This paper reveals the annual, seasonal, and magnetic storm response characteristics of the hemispheric asymmetry of thermospheric atmospheric density, which has significant implications for the study of multilayer energy coupling of the magnetosphere–ionosphere–thermosphere.

Response of the American equatorial ionization anomaly to 2016 Arctic sudden stratospheric warming events

Apart from the rapid ionospheric response to geomagnetic forcing originating from the Sun during extreme space weather events, forcing from the lower atmosphere below still exerts a significant influence on the ionosphere during quiet-time conditions. This study examines the ionospheric response of the equatorial ionization anomaly (EIA) in the American sector to the combined influence of the cascades of sudden stratospheric warming (SSW) events and the geomagnetic storms that coexisted with them during the period of January–March 2016. We adopted a multi-instrument and multi-modeling approach with the study locations spanning ±40° geomagnetic latitudes. Our results showed a hemispheric asymmetry in the total electron content and change in total electron content (ΔTEC) distribution with higher enhancement clearly visible in the Northern Hemisphere in comparison to the Southern Hemisphere (NH). Semidiurnal signatures were observed in both ΔTEC and equatorial electrojet parameters for some days. The double-peak zonal mean zonal wind amplitude days supported the formation of the reverse fountain effects. The different SSW peak temperature days also showed either positive or negative ionospheric response. Generally, orientation of the prompt penetration electric field (PPEF) and their strengths at either daytime or nighttime played a weak role in the ionosphere response during some of the geomagnetic storms. The negative and positive ionospheric responses under geomagnetic storm conditions were ascribed to changes in the composition of the thermosphere, prompt penetration electric field (PPEF), and traveling atmospheric disturbances (TADs).

Low- and Mid-Latitude Ionospheric Response to the 2013 St. Patrick’s Day Geomagnetic Storm in the American Sector: Global Ionosphere Thermosphere Model Simulation

In this study, the low-and mid-latitude ionospheric response to the main phase of the 2013 St. Patrick’s Day geomagnetic storm in the American sector on the dayside has been investigated using the ground-based measurements and the Global Ionosphere Thermosphere Model (GITM). First, it is found that the observed ionospheric response can be well reproduced by GITM when it is driven by the electric potential and electron precipitation patterns derived from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique. The AMIE-driven GITM simulation also significantly improves the data-model comparison as compared with the simulation driven by the high-latitude empirical models. Second, it is found that the transport process associated with the neutral wind is largely responsible for the observed ionospheric response. Specifically, the traveling atmospheric disturbances (TADs) propagating from the opposite hemisphere play an important role in the formation of the negative storm phase at low and middle latitudes. Third, it is found that the asymmetric negative storm phases occurred at the nominal equatorial ionization anomaly (EIA) peak region in the afternoon sector are mainly attributed to the interaction of the TADs launched in different hemispheres with different phase speeds. Specifically, stronger Joule heating deposited in the northern hemisphere (NH) generates TADs with faster phase speeds than those launched in the southern hemisphere (SH). Consequently, the locations where the TADs originated from the different hemispheres interact are asymmetric about the geomagnetic equator, leading to the formation of asymmetric ionospheric negative storm phases. This study highlights the importance of accurately specifying high-latitude electrodynamic forcings in global I-T simulations and provides a new insight into the cause of the interhemispheric asymmetry phenomena during geomagnetic storms.

Thermospheric Density Perturbations Produced by Traveling Atmospheric Disturbances During August 2005 Storm

AbstractThermospheric mass density perturbations are commonly observed during geomagnetic storms and fundamental to upper atmosphere dynamics, but the sources of these perturbations are not well understood. Large neutral density perturbations during storms greatly affect the drag experienced by low Earth orbit. We investigated the thermospheric density perturbations at all latitudes observed along the CHAMP and GRACE satellite trajectories during the August 24–25, 2005 geomagnetic storm. Observations show that large neutral density enhancements occurred not only at high latitudes, but also globally. Large density perturbations were seen in the equatorial regions away from the high‐latitude, magnetospheric energy sources. We used the high‐resolution Multiscale Atmosphere Geospace Environment (MAGE) model to simulate consecutive neutral density changes observed by satellites during the storm. The MAGE simulation, which resolved mesoscale high‐latitude convection electric fields and field‐aligned currents, and included physics‐based specification of auroral precipitation, was contrasted with a standalone ionosphere‐thermosphere simulation driven by a high‐latitude electrodynamics empirical model. The comparison demonstrates that first‐principles representations of highly dynamic and localized Joule heating events in a fully coupled whole geospace model is critical to accurately capture both generation and propagation of traveling atmospheric disturbances (TADs) that produce neutral density perturbations globally. The MAGE simulation shows that larger density peaks in the equatorial region observed by CHAMP and GRACE are the result of TADs generated at high‐latitudes in both hemispheres, and intersect at low‐latitudes. This study reveals the importance of investigating thermospheric density variations at all latitudes in a fully coupled geospace model with sufficiently high resolving power.

Thermospheric Traveling Atmospheric Disturbances in Austral Winter From GOCE and CHAMP

AbstractIn this study, we analyze the thermospheric density data provided by the Gravity Field and Steady‐State Ocean Circulation Explorer during June–August 2010–2013 at ∼260 km altitude and the Challenging Minisatellite Payload during June–August 2004–2007 at ∼370 km altitude to study high latitude traveling atmospheric disturbances (TADs) in austral winter. We extract the TADs along the satellite tracks from the density for varying Kp, and linearly extrapolate the TAD distribution to Kp = 0; we call these the geomagnetic “quiet time” results here. We find that the quiet time spatial distribution of TADs depends on the spatial scale (along‐track horizontal wavelength ) and altitude. At z ∼ 260 km, TADs with ≤ 330 km are seen mainly around and slightly downstream of the Southern Andes‐Antarctic region, while TADs with > 800 km are distributed fairly evenly around the geographic South pole at latitudes ≥60°S. At z ∼ 370 km, TADs with ≤ 330 km are relatively weak and are distributed fairly evenly over Antarctica, while TADs with > 330 km make up a bipolar distribution. For the latter, the larger size lobe is centered at ∼60°S, and is located around, downstream and somewhat upstream of the Andes/Antarctic Peninsula, while the smaller lobe is located over the Antarctic continent at 90°–150°E. We also find that the TAD morphology for Kp ≥ 2 and > 330 km depends strongly on geomagnetic activity, likely due to auroral activity, with greatly enhanced TAD amplitudes with increasing Kp.

The Solar Eclipse Effects on the Upper Thermosphere

AbstractTheoretical studies have shown that the solar eclipse can have significant impacts on the thermospheric mass density, which, however, has not been confirmed from the observational perspective. In this study, we present direct observations of the thermospheric density responses to three solar eclipses based on the measurements of Gravity Recovery and Climate Experiment (GRACE) and Gravity field and steady state Ocean Circulation Explorer (GOCE). It was observed that the eclipses induced about 20% and 25% density depletions at GRACE altitudes (360 and 480 km, respectively) and about 10% depletions at GOCE altitudes (∼270 km). Moreover, the eclipse could generate large‐scale traveling atmospheric disturbances (TADs), which propagate globally even from the duskside to the dawnside after the eclipse ended. Those thermospheric responses could be generally reproduced by Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). However, the simulated densities recover more rapidly than the observations at the duskside. This study provides observational evidence to validate the results from theoretical models and further bring insights into the mechanism of eclipse effects on the ionosphere‐thermosphere system.

First Comparison of Traveling Atmospheric Disturbances Observed in the Middle Thermosphere by Global‐Scale Observations of the Limb and Disk to Traveling Ionospheric Disturbances Seen in Ground‐Based Total Electron Content Observations

AbstractTraveling ionospheric disturbances (TIDs) and their neutral counterparts known as traveling atmospheric disturbances (TADs) are believed to play a role in communicating inputs to other locations in the fluid. While these two phenomena are believed to be connected, they may not have a one‐to‐one correspondence as the geomagnetic field influences the TID but has no direct impact on the TAD. The relative amplitudes of the perturbations seen in the ionosphere and atmosphere have been observed but rarely together. This study reports results from a 3‐day campaign to observe TIDs and TADs simultaneously over a broad latitudinal region over the eastern United States using a combination of Global‐scale Observations of the Limb and Disk (GOLD) and a distributed network of ground‐based Global Navigation Satellite System (GNSS) receivers. These results demonstrate that GOLD and the ground‐based total electron content (TEC) observations can see the atmospheric and ionospheric portions of a large‐scale traveling disturbance. The phase difference in the perturbations to the GOLD airglow brightness, O/N2 and thermospheric disk temperature are consistent with an atmospheric gravity wave moving through this region. The ionospheric signatures move at the same rate as those in the atmosphere, but their amplitudes do not have a simple correspondence to the amplitude of the signal seen in the atmosphere. This campaign demonstrates a proof‐of‐concept that this combination of observations is able to provide information on TIDs and TADs, including quantifying their impact on the temperature and chemical composition of the upper atmosphere.

From Bow Waves to Traveling Atmospheric Disturbances: Thermospheric Perturbations Along Solar Eclipse Trajectory

AbstractIn this study, we investigate the characteristics of thermospheric perturbations triggered by a set of four solar eclipses, using the Thermosphere Ionosphere Electrodynamics General Circulation Model. Simulations show that temperature and horizontal wind perturbations have different wave rears but similar wavefronts, due to the coupling of energetic and dynamic processes. The perturbations are closely related to the eclipse trajectory, and their phase speed is modulated by the eclipse speed. As the eclipse shadow sweeps through the upper atmosphere, traveling atmospheric disturbances (TADs) are launched in the eclipse region and they, moving along with their moon shadow source, form bow wavefronts of thermospheric disturbances. The perturbations at a particular time are superimposed by preexisting waves and newly excited waves. With the termination of the eclipse near the sunset terminator, individual sequences are not any more organized along a specific alignment to form a bow wave. Instead, they evolve into freely propagating large‐scale TADs. The large‐scale TADs remain propagating in the direction perpendicular to the wavefronts, which is determined by history of the eclipse trajectory and thermospheric conditions. These conditions include specifically season and the latitudinal location of the eclipse trajectory.

Global Responses of the Coupled Thermosphere and Ionosphere System to the August 2017 Great American Solar Eclipse

AbstractIt is commonly believed that solar eclipses have a great impact on the ionosphere‐thermosphere (I‐T) system within the eclipse shadow, but little attention has been paid to the global response to these events. In this study, we investigate the global upper atmospheric responses to the recent Great American Solar Eclipse that occurred on 21 August 2017 using a high‐resolution coupled ionosphere‐thermosphere‐electrodynamics model. The simulation results show that the ionosphere and thermosphere response to the eclipse is not just local but global. Large‐scale traveling atmospheric disturbances (TADs), seen in the thermospheric temperature and winds, were triggered from the eclipse region and propagated in a southeast direction when the eclipse ended. A large total electron content (TEC) enhancement occurred over South America after the eclipse was over. The TEC enhancement was primarily the result of transport by the thermospheric wind perturbations associated with the eclipse‐induced TADs. The perturbations of TEC, neutral temperature, and winds exhibited asymmetric distributions with respect to the totality path during the solar eclipse. Furthermore, ionospheric electrodynamic processes also play an important role in the global responses of the I‐T system to the solar eclipse. Unlike the case of large‐scale TADs propagating from the eclipse region to other locations in the globe, the ionospheric electric fields and plasma drifts began to show significant perturbations even during the local pre‐eclipse period when local wind and temperature had not been perturbed. This is related to the instantaneous global response of the ionospheric current system to changes in the ionospheric conductivity and winds in the eclipse region.

Equatorial and low‐latitude ionospheric response to the 17–18 March 2015 great storm over Southeast Asia longitude sector

AbstractThis study mainly investigates equatorial and low‐latitude ionospheric response to a great geomagnetic storm that occurred on 17 March 2015. We found that there were some interesting ionospheric phenomena, e.g., short‐term ionospheric positive effect, daytime spread F, and morning Equatorial Ionization Anomaly (EIA) in the topside ionosphere, emerged at equatorial and low‐latitude region along the longitude of about 100°E. Ground‐based ionosondes and in situ satellite (Swarm) were utilized to study the possible mechanisms for these ionospheric phenomena. We found that vertical downward transport of plasma or neutral induced by traveling ionospheric disturbances (TIDs) or traveling atmospheric disturbances (TADs) might make a contribution to the short‐term ionospheric positive effect at the main stage of this great storm. Additionally, results suggested that the occurrence of daytime spread F at low latitudes might be due to the diffusion of equatorial ionospheric irregularities in the topside ionosphere along geomagnetic field lines. Moreover, observational evidence shows that TIDs also might be the main driver for morning EIA‐like feature recorded by Swarm B satellite in the topside ionosphere. These ionospheric phenomena mentioned above could make us gain a better understanding of ionospheric storm effects at equatorial and low‐latitude region.

A numerical study of nighttime ionospheric variations in the American sector during 28–29 October 2003

AbstractVariations of nighttime F2 peak height (hmF2) over the American sector during the 28–29 October 2003 storm period were investigated using the National Center for Atmospheric Research Thermosphere‐Ionosphere Electrodynamics Global Circulation Model. The model was generally able to reproduce the ionospheric variations on 28–29 October 2003 observed by the ionosondes. A series of controlled model simulations were subsequently undertaken to examine the effects of electric fields and neutral winds on the ionosphere. The numerical experiments suggest that the dramatic nighttime increase of hmF2 on the storm day 29 October is mainly caused by traveling atmospheric disturbances (TADs) from the high latitudes of the Northern Hemisphere. However, the electric field plays an important role in causing the elevation of hmF2 in the equatorial region. The prompt penetration electric field (PPEF) associated with the southward component of the interplanetary magnetic field (Bz) is westward on the nightside, whereas when Bz reverses and becomes northward, the PPEF is westward in the premidnight and turns to eastward in the postmidnight. These PPEFs greatly affect the low‐latitude ionosphere during storm time. On 28 October, even though the Bz disturbance was weak with a short duration of southward Bz, the TADs from the Southern Hemisphere can propagate to the Northern Hemisphere and result in the corresponding oscillations in the nightside hmF2.

Prolonged multiple excitation of large‐scale Traveling Atmospheric Disturbances (TADs) by successive and interacting coronal mass ejections

AbstractSuccessive and interacting coronal mass ejections (CMEs) directed earthward can have significant impacts throughout geospace. While considerable progress has been made in understanding their geomagnetic consequences over the past decade, elucidation of their atmospheric consequences remains a challenge. During 17–19 January 2005, a compound stream formed due to interaction of six successive halo CMEs impacted Earth's magnetosphere. In this paper, we report one atmospheric consequence of this impact, namely, the prolonged multiple excitation of large‐scale (>∼1000 km) traveling atmospheric disturbances (TADs). The TADs were effectively excited in auroral regions by sudden injections of energy due to the intermittent southward magnetic fields within the stream. They propagated toward the equator at speeds near 800 m/s and produced long‐duration (∼2.5 days) continuous large‐scale density disturbances of order up ± 40% in the global thermosphere.

Global ionospheric and thermospheric response to the 5 April 2010 geomagnetic storm: An integrated data‐model investigation

AbstractWe present a case study of the 5 April 2010 geomagnetic storm using observations and numerical simulations. The event was driven by a fast‐moving coronal mass ejection and despite being a moderate storm with a minimum Dst near −50 nT, the event exhibited elevated thermospheric density and surges of traveling atmospheric disturbances (TADs) more typically seen during major storms. The Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIMEGCM) was used to assess how these features were generated and developed during the storm. The model simulations gave rise to TADs that were highly nonuniform with strong latitude and longitude/local time dependence. The TAD phase speeds ranged from 640 m/s to 780 m/s at 400 km and were ~5% lower at 300 km and approximately 10–15% lower at 200 km. In the lower thermosphere around 100 km, the TAD signatures were nearly unrecognizable due to much stronger influence of upward propagating atmospheric tides. The thermosphere simulation results were compared to observations available from the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE), CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites. Comparison with GOCE data shows that the TIMEGCM reproduced the cross‐track winds over the polar region very well. The model‐data comparison also revealed some differences, specifically, the simulations underestimated neutral mass density in the upper thermosphere above ~300 km and overestimated the storm recovery tome by 6 h. These discrepancies indicate that some heating or circulation dynamics and potentially cooling processes are not fully represented in the simulations, and also that updates to some parameterization schemes in the TIMEGCM are warranted.

Thermospheric density variations: Observability using precision satellite orbits and effects on orbit propagation

AbstractThis paper examines atmospheric density estimated using precision orbit ephemerides (POE) from the CHAMP and GRACE satellites during short periods of greater atmospheric density variability. The results of the calibration of CHAMP densities derived using POEs with those derived using accelerometers are examined for three different types of density perturbations, [traveling atmospheric disturbances (TADs), geomagnetic cusp phenomena, and midnight density maxima] in order to determine the temporal resolution of POE solutions. In addition, the densities are compared to High‐Accuracy Satellite Drag Model (HASDM) densities to compare temporal resolution for both types of corrections. The resolution for these models of thermospheric density was found to be inadequate to sufficiently characterize the short‐term density variations examined here. Also examined in this paper is the effect of differing density estimation schemes by propagating an initial orbit state forward in time and examining induced errors. The propagated POE‐derived densities incurred errors of a smaller magnitude than the empirical models and errors on the same scale or better than those incurred using the HASDM model.

Effect of a solar flare on a traveling atmospheric disturbance

It is known that the sudden injection of energy during geomagnetic storms can excite atmospheric gravity waves (AGWs) or traveling atmospheric disturbances (TADs). Together with large‐scale circulation, these AGWs/TADs transport energy and momentum away from their sources. In this paper, we investigate possible involvement of AGWs/TADs during solar flares. Model simulations of an X17 flare that occurred on October 28, 2003 shows that AGWs/TADS contributed to flare energy transport from the sunlit South‐Pole region to the nightside equatorial region in 3–4 h, resulting in ∼10% nightside equatorial neutral density enhancement in the upper thermosphere. These nightside AGWs/TADs have a phase speed on the order of ∼750 m/s and a horizontal wavelength on the order of 4000 km. Enhanced solar heating to the thermosphere through enhanced ionization during flares occurs on the entire dayside, with the spatial scale of the increased solar heating being too large to excite AGWs/TADs. Further analysis revealed that strong localized enhancement of Joule heating was produced during the October 28, 2003 flare. This sudden injection of the localized heating, together with preexisting AGWs/TADs excited by moderate geomagnetic activity prior to the flare, produced intensified AGWs/TADs, which propagated energy and momentum to the equatorial region. On the other hand, model simulations showed that, under assumed geomagnetically quiet conditions, strong localized enhancement of Joule heating and AGWs/TADs were not produced during the flare. This interplay between geomagnetic activity and solar flares can be a challenge to space weather monitoring, specification, and forecasting.

Daytime additional F layer stratification over low‐midlatitude station of the Indian sector under geomagnetic disturbed conditions

In this paper, the observations of additional F layer stratification are presented in a region which is situated at the outer edge of equatorial ionization anomaly (EIA) zone. The digital ionosonde data observed over Delhi (28.6° N, 77.2° E, dip 42.44° N) during March–April, 2001 has been used to carry out the present work. The observational period was geomagnetically disturbed and represents the high solar activity phase of 23rd solar cycle. The additional transient cusp is frequently observed in the considered months before the noon hours; however, in this paper only five prominent cases are presented. From the analysis of ionograms, it is observed that the transient additional cusp is formed between the pre‐existing F1 and F2 layer; hence, named as the cusp of F1.5. Study reveals that the Traveling Atmospheric Disturbances (TADs) along with the vertical expansion of F layer provides the necessary condition for the existence of this transient feature. The intensification of F1 layer along with increased altitude immediately after the disappearance of additional stratification remains one of the fascinating features of the present results. The present investigation demonstrates that the daytime F1 and F2 region over low‐midlatitude station is strongly modulated by the passage of TIDs, originating at high latitudes or by atmospheric disturbances of local origin during the high solar activity period. The concurrent presence of TADs and the associated disturbance composition appears to be plausible reason behind the present observations.

Ionospheric response of equatorial and low latitude F-region during the intense geomagnetic storm on 24–25 August 2005

In this investigation, we present and discuss the response of the ionospheric F-region in the South American and East Asian sectors during an intense geomagnetic storm in August 2005. The geomagnetic storm studied reached a minimum Dst of −216 nT at 12:00 UT on 24 August. In this work ionospheric sounding data obtained of 24, 25, and 26 August 2005 at Palmas (PAL; 10.2° S, 48.2° W; dip latitude 6.6° S), São José dos Campos (SJC, 23.2° S, 45.9° W; dip latitude 17.6° S), Brazil, Ho Chi Minh City, (HCM; 10.5° N, 106.3° E; dip latitude 2.9° N), Vietnam, Okinawa (OKI; 26.3° N, 127.8° E; dip latitude 21.2° N), Japan, are presented. Also, the GPS observations obtained at different stations in the equatorial and low-latitude regions in the Brazilian sector are presented. On the night of 24–25 August 2005, the h′F variations show traveling ionospheric disturbances associated with Joule heating in the auroral zone from SJC to PAL. The foF2 variations show a positive storm phase on the night of 24–25 August at PAL and SJC during the recovery phase. Also, the GPS-VTEC observations at several stations in the Brazilian sector show a fairly similar positive storm phase on 24 August. During the fast decrease of Dst (between 10:00 and 11:00 UT) on 24 August, there is a prompt penetration of electric field of magnetospheric origin that result in abrupt increase (∼12:00 UT) in foF2 at PAL, SJC (Brazil) and OKI (Japan) and in VTEC at IMPZ, BOMJ, PARA and SMAR (Brazil). OKI showed strong oscillations of the F-region on the night 24 August resulted to the propagation of traveling atmospheric disturbances (TADs) by Joule heating in the auroral region. These effects result a strong positive observed at OKI station. During the daytime on 25 August, in the recovery phase, the foF2 observations showed positive ionospheric storm at HCM station. Some differences in the latitudinal response of the F-region is also observed in the South American and East Asian sectors.