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One of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEFY ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm. Plain Language Summary: Life and a habitable atmosphere are sustained on Earth thanks to the protective and far‐stretched magnetic shield around the Earth, known as the Magnetosphere, which protecting the humanosphere from the hazardous solar wind particles that are continuously emanated from the Sun. However, massive solar wind ejections from the Sun often disturb the Earth's magnetosphere and affect mankind and critical space‐ and ground‐based technological infrastructure. When higher amounts of the solar wind traveling with supersonic speeds impact, the Earth's magnetosphere compresses significantly, and the crucial satellites in space become directly exposed to hazardous solar wind. Further, strong and rapid disturbances in the geomagnetic field and ionosphere could significantly affect the operations of various technical systems on ground, like electrical power grids, GNSS‐based precise navigation, etc. The strongest geomagnetic storm of the past three decades has recently occurred on 10–11 May 2024. This study investigates the solar sources and the possible mechanisms responsible for the occurrence of such an intense geomagnetic storm and also details the drastic reconfiguration of Earth's magnetosphere and significant ionospheric disturbances observed during this storm. Key Points: Strong SWDP caused a highly compressed magnetosphere with magnetopause pushed below geostationary orbit (6.6 RE) continuously for 6 hrThe highly compressed magnetosphere led an intense ring current at much closer distance to the EarthA super‐dense ICME with high SWDP (>15 nPa) and IEFY (>2.5 mV/m) sustaining for 409 min ‐ a key factor responsible for this intense storm [ABSTRACT FROM AUTHOR]

Deriving the precise true height electron density profile from the measured ionosonde virtual heights is quite a challenging problem. Recently, Ankita and Tulasi Ram (2023, https://doi.org/10.1029/2023RS007808) presented a new method, Iterative Gradient Correction (IGC) method, for true height analysis that uses HF radio wave propagation path computations to reconstruct the true height profile. Through iterative corrections on electron density gradients between successive points, the IGC method minimizes errors below a specified tolerance at each point and reconstructs a complete electron density profile. The derived profiles from the IGC method are found to be accurate when compared with Incoherent Scatter Radar and Global Navigation Satellite System—Radio Occultation observations. To facilitate true height analysis by IGC method for a wider user community, a MATLAB‐based software has been developed and is outlined in this report. The software can be installed on any Windows platform and is designed with a user‐friendly interface for easy and efficient application by the users. It can analyze multiple scaled ionograms in a single run and outputs the real height profiles in ASCII format. Further, the software also captures important ionospheric parameters such as the base altitudes and peak frequencies of E‐ and F‐layers (e.g., hE, hF, foE, and foF2) etc., from the computed true height profiles and tabulates in a separate output file for the ready use. The software also provides the option for extrapolation of true height profile into top‐side ionosphere up to a user‐specified height and reconstructs the complete vertical electron density profile. Key Points: A new software with user‐friendly interface is developed for true height analysis of ionograms using Iterative Gradient Correction methodIt can compute multiple true height profiles in one run with an option to extrapolate into top‐side ionosphere up to a user‐specified heightIt also captures the important ionospheric parameters such as hE, hF, foE, foF2, etc., for ready use in research/practical applications [ABSTRACT FROM AUTHOR]

The Equatorial Electrojet (EEJ) is one of the important near‐earth space weather phenomena which exhibits significant diurnal, seasonal and solar activity variations. This paper investigates the EEJ variations at diurnal, seasonal and solar cycle time scales from the Indian sector and portrays a new empirical EEJ field model developed using the observations spanning over nearly two solar cycles. The Method of Naturally Orthogonal Components (MNOC), also known as Principal Component Analysis (PCA), was employed to extract the dominant patterns of principal diurnal, semi‐diurnal, and ter‐diurnal components contributing to the EEJ variation. The amplitudes of these diurnal, semi‐diurnal, and ter‐diurnal components in EEJ are found to vary significantly with the season and solar activity. The seasonal and solar activity dependencies of these principal components are modeled using suitable bimodal distribution functions. Finally, the empirical model for EEJ field was built by combining the principal components with their corresponding modeled amplitudes. This model accurately reproduces the diurnal, seasonal and solar activity variations of EEJ. The modeled monthly mean variations of EEJ field at ground exhibit excellent correlation of 0.96 with the observations with the root mean square error <5 nT. It also successfully captures the seasonal and solar activity variations of Counter Electrojet (CEJ). Finally, this model named "Indian Equatorial Electrojet (IEEJ) Model" is made publicly available for interested scientific users (https://iigm.res.in/system/files/IEEJ_model.html). Plain Language Summary: The equatorial electrojet (EEJ) is an intense current jet flowing in the upper atmosphere around 100 km. This is a unique phenomenon that occurs at only a few kilometers in height (e.g., 105–110 km) and with a horizontal (North‐South) width of a few hundred kilometers (e.g., ∼600 km) centered on the geomagnetic equator. This important geophysical phenomenon gives more insights into the equatorial ionospheric electrodynamical processes that have various impacts on satellite orbital dynamics, Global Positioning Systems and other satellite communication links, electrical power grids, etc. This study develops a new empirical model to predict the diurnal, seasonal, and solar activity variations in the intensity of this unique current system over the Indian longitude sector. Key Points: An empirical Equatorial Electrojet (EEJ) field model was developed using nearly two solar cycles of EEJ observations from the Indian sectorThe principal diurnal, semi‐diurnal, and ter‐diurnal components in the EEJ are modeled using suitable analytical functionsThis model can very accurately predict the diurnal, seasonal, and solar activity variations of monthly mean EEJ and CEJ fields at ground [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are one of the dominant solar wind structures that can significantly impact the Geospace when impinge on the Earth's magnetosphere. IP shocks severely distort the magnetosphere and induce dramatic changes in the magnetospheric currents, often leading to large disturbances in the geomagnetic field. Sudden enhancements in the solar wind dynamic pressure (PDyn) during IP shocks cause enhanced high‐latitude convection electric fields which penetrate promptly to equatorial latitudes. In response, the equatorial electrojet (EEJ) current exhibits sharp changes of magnitudes primarily controlled by the change in PDyn and the local time. In this paper, we further investigated the influence of shock impact angle on the EEJ response to a large number (306) of IP shocks that occurred during 2001–2021. The results consistently show that the EEJ exhibits a heightened response to the shocks that head‐on impact the magnetosphere (frontal shocks) than those with inclined impact (inclined shocks). The greater EEJ response during the frontal shocks could be due to a more intensified high‐latitude convection electric field resulting from the symmetric compression of the magnetosphere. Finally, an existing empirical relation involving PDyn and local time is improved by including the effects of impact angle, which can quantitatively better predict the EEJ response to IP shocks. Plain Language Summary: Solar Wind, a continuous stream of high‐energy particles emanating from the Sun, is ubiquitous in interplanetary (IP) space. In unison, the energetic and/or transient eruptions on the Sun often release bursts of fast solar wind. When this fast solar wind interacts with the ambient solar wind in the IP space, a shock front is formed known as IP shock. These IP shocks (if Earth‐directed) can impinge on the Earth's magnetosphere and transfer tremendous amounts of energy and momentum. As a result, the Earth's magnetic field is often severely disturbed. Severe geomagnetic disturbances are known to cause a myriad of space weather effects from the loss of satellites in space to damage of electrical power grids and transmission lines on ground. Historically, geomagnetic field disturbances are known to be more cataclysmic at high latitudes and lessen at latitudes below the auroral region. However, these disturbances again dramatically enhance at equatorial latitudes due to a unique ionospheric current system, known as equatorial electrojet (EEJ). This study provides new insights into the significant role of angle of shock impacting the magnetosphere and the resultant disturbances in EEJ. This study also derives an empirical relation to accurately estimate the EEJ disturbances during IP shocks. Key Points: The angle of interplanetary (IP) shock impact on the magnetosphere plays a significant role in controlling the equatorial electrojet (EEJ) responseThe magnitude of EEJ change due to the impact of frontal IP shocks is significantly larger than that for the inclined shocksAn improved empirical relation including the effects of impact angle is derived which provide better estimates of EEJ responses to IP shocks [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are known to cause significant modifications in Earth's magnetospheric and ionospheric current systems. The sudden enhancement of solar wind dynamic pressure (PDyn) associated with IP shocks could induce convection electric fields at high‐latitude ionosphere which can promptly penetrate to equatorial and low‐latitude regions. Additionally, prompt penetration electric field disturbances may also be induced due to the sudden southward/northward turnings of the Interplanetary Magnetic Field (IMF Bz) (eastward/westward turnings of the interplanetary electric field, IEFy) during IP shocks. The resultant electric field disturbances can significantly alter the ionospheric electrodynamics and equatorial electrojet (EEJ). In this study, the EEJ response to a large number of IP shocks that occurred during 2001–2021 has been investigated. The magnitude of the EEJ response to IP shocks shows a clear local time dependence and varies linearly with the change in solar wind dynamic pressure. The EEJ response is also found to depend considerably on the solar activity (F10.7 solar flux) and the polarity changes in IMF Bz associated with IP shocks. For the first time, an empirical relation is derived that can quantitatively estimate the EEJ response to IP shocks using a large number (306) of events that occurred over a span of two solar cycles. The derived empirical relation is found to be very accurate in predicting the response of the EEJ and exhibits an excellent correlation with observations. Plain Language Summary: The eruptions and the energetic events on the Sun often release fast streams of charged particles (known as solar wind) at supersonic velocities into the interplanetary space. The arrival and impingement of these fast and dense solar wind structures on the Earth's magnetosphere cause highly dynamic changes in the currents flowing in Earth's Magnetosphere and Ionosphere. These changes will result in sudden changes in the Earth's magnetic (geomagnetic) field which can have potential impacts on the electric power grids, long‐distance oil/gas pipelines and transmission lines, etc. Although these effects are more prominently known to occur at high latitude regions, the large and sharp disturbances in the geomagnetic field are also known to occur at equatorial latitudes due to the presence of an ionospheric current system known as the Equatorial Electrojet (EEJ). This study provides new insights into the important factors that control the variations in EEJ and derives an empirical relation to quantitatively predict the change in EEJ during the arrival of such fast and dense solar wind shock structures. Key Points: The EEJ responds promptly to the arrival of IP shocks with sharp enhancement/decrease on the day/night timesThe magnitude of EEJ changes in response to IP shocks vary significantly with local time and depend strongly on solar wind dynamic pressureAn empirical relation is derived that can accurately predict the quantitative change in the EEJ in terms of local time and shock parameters [ABSTRACT FROM AUTHOR]

Inversion of precise true height electron density profile from the measured virtual heights by the Ionosonde is a quite challenging and ill‐posed problem. In this paper, we present a new method to compute the true height profiles from ionograms that relies on computing the propagation path of radio waves with time. This method does not use predefined polynomial functions to fit the vertical electron density distribution; hence, it is free from fitting errors. Instead, this method implements iterative corrections in the electron density gradient between the successive points and progressively reconstructs the true height profile. This Iterative Gradient Correction (IGC) method assures minimizing the error to below a tolerance limit at all sampled points on the ionogram. The true height profiles derived from this method exhibit better accuracy than those derived from the widely used POLynomial ANalysis, particularly, at cusp and F2‐peak regions. Further, the IGC method gives the best results at higher sampling resolutions of ionograms and is less sensitive to scaling errors. Plain Language Summary: The ionosphere is an ionized layer of Earth's upper atmosphere that reflects the incident HF (high frequency) radio waves back to the ground. This property of the ionosphere is used to establish radio communications over far distances beyond the horizon. Monitoring the electron density in the ionosphere is essential, as the changes in the ionospheric electron density distribution can significantly impact these communication systems. Ionosonde is one such classical instrument that is widely used to measure the vertical electron density distribution of the ionosphere. However, like any radar, Ionosonde assumes the radio waves travel with the speed of light, which is not true in the case of ionosphere. This assumption gives rise to higher estimates of reflection heights by the ionosphere known as virtual heights that are far from reality. In this paper, we present a new method that computes the actual path of radio waves with respect to time in the ionospheres and computes the true height electron density distribution from the measured virtual heights by the Ionosonde. This method, besides scientific research, would have potential applications in Skywave communications, over‐the‐horizon target detection and ranging applications. Key Points: A new method to derive the true height profiles from ionograms that relies on computing the propagation path of radio waves with timeProgressively constructs the true height profile by iteratively correcting the gradient between successive points until the error is <±1 kmThe Iterative Gradient Correction method gives more accurate results at the cusp and F2‐peak regions and is less sensitive to scaling errors compared to POLynomial ANalysis [ABSTRACT FROM AUTHOR]

Here, we report a rare observation of an extremely large and rapid change of geomagnetic field (dB/dt), a proxy for the geomagnetically induced currents (GICs), at the equator caused by a sudden drop in solar wind density at the front boundary of a magnetic cloud (MC) during the great 31 March 2001 storm. The horizontal component at the Indian equatorial station, Tirunelveli, recorded a sharp decline of ∼350 nT in just 5 min with a peak dB/dt exhibiting a concerning value of 136 nT/min, a possible GIC risk to the electric power systems. The responsible physical mechanisms were examined through magneto‐hydrodynamic model simulations and found that a prompt penetration of strong westward‐overshielding electric fields and ionospheric currents at the equator play a dominant role. This study provides some new insights into the extent of extreme geomagnetic field changes that can occur at the equatorial region due to solar wind density reduction at MC, which can have potential impacts on the electric power grid systems. Plain Language Summary: The Earth, a planet with an intrinsic magnetic field bubble around it, is immersed in a hot and energetic solar wind plasma that continuously emanated from the Sun. The sudden disturbances on the Sun, such as coronal mass ejections, induce transient structures in the solar wind which, when directed earthward, can cause severe disturbances in the Earth's (Geo) magnetic field. The rapid changes in the geomagnetic field induce electric fields at the conducting surface of Earth, which can cause strong electrical currents, known as geomagnetically induced currents (GICs), that flow through the long conducting structures, like electric power transmission lines, long pipelines, etc., on the ground. The enhanced GICs are of serious threat to the electric power and pipeline grids. The elevated GICs are most popularly known to occur at high latitudes during severe geomagnetic storms. This study reports a rare observation of a large and rapid change in geomagnetic field indicating the strong GICs at the geomagnetic equator due to a magnetic cloud structure in solar wind and reveals the underlying physical processes. Key Points: Unique report of large dB/dt (geomagnetically induced current) at the geomagnetic equator due to a decrease in solar wind density at the front boundary of a magnetic cloudThe amplitude of dB/dt is maximized at the equator on the noon side due to the Equatorial ElectroJet and is compared to that at high latitudesSolar wind density takes the control of dayside reconnection and convection electric fields through the modulation of magnetosheath Bz [ABSTRACT FROM AUTHOR]

One of the most intense geomagnetic storms of recent times occurred on 10-11 May 2024. With a peak negative excursion of Sym-H below -500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow-shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEF Y  ≥ 2.5 mV/m) within a super-dense Interplanetary Coronal Mass Ejection lasted for 409 min-is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2-12 MHz frequency band due to strong D- and E-region ionization resulting from a solar flare prior to this storm., (© 2024. The Author(s).)