Faculty Profiles - SHIOKAWA, Kazuo

Papers - SHIOKAWA, Kazuo

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Relative Contributions of Solar Wind Dynamic Pressure and Interplanetary Electric Field on the Early Positive Part of Main Phase of Geomagnetic Storms Reviewed Open Access

Raj J.K., Balan N., Zhang Q.H., Otsuka Y., Shiokawa K., Xing Z.Y., Manu V., Nilam B., Ma Y.Z., Wang Y.

Space Weather Vol. 24 ( 3 ) 2026.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Space Weather

The important early declining part of the main phase (MP) of geomagnetic storms in SYM-H (and Dst) from positive main phase onset (MPO) to 0-level of SYM-H was somehow missed in the treatment of the storms. We included the missed part (or revised the storms) by raising the 0-level of SYM-H to MPO-level, and showed that the inclusion is important for most aspects of global space weather. However, what drives the missed part is not yet understood. We take up such a study using data and a SYM-H model. The relative contributions of the solar wind dynamic pressure P and interplanetary electric field IEFy on the missed part are studied statistically using good quality P and IEFy data available for 116 revised intense storms in 1998–2024. The results reveal that (a) the missed part in majority (62%) of storms is mainly (≥75%) due to positive IEFy; (b) in a small number of storms (12%), it is mainly (≥75%) due to decrease in P; and (c) in the remaining 26% of storms, it is due to both decrease in P and positive IEFy. The SYM-H model is developed from the existing Dst models and incorporates the delay of SYM-H response to IEFy turning positive. The model reproduces the combined effect of P and positive IEFy nearly agreeing with SYM-H data, and illustrates their relative contributions for one storm each in the three groups. In short, the missed positive part in majority of storms seems due to positive IEFy.

DOI： 10.1029/2025SW004571

Study of the Mid-Latitude Airglow Responses to Geomagnetic Storms Based on Long-Term Observations in Japan With the TIMED Satellite and Ionosondes Reviewed Open Access

Hotta Y., Shiokawa K., Otsuka Y., Nishioka M., Yue J.

Journal of Geophysical Research Space Physics Vol. 131 ( 3 ) 2026.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research Space Physics

Variations in nighttime airglow at mid-latitudes during geomagnetic disturbances reflect the propagation of atmospheric and plasma disturbances from polar heating as well as local chemical processes. However, their detailed mechanisms remain unclear. In this study, we analyzed the responses of six nighttime airglow emissions to geomagnetic storms using data from airglow temperature photometers at three locations in Japan (Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.8°N, 136.1°E), and Sata (31.0°N, 130.7°E)) over a long-term period from January 2004 to October 2023. The emissions include the OH Meinel band, O<inf>2</inf> atmospheric band, O (557.7 nm), Na (589.3 nm) from the mesopause region, and O (630.0 and 777.4 nm) from the thermosphere. The main results show an increase in the intensity of the 557.7-nm emission 2 days after geomagnetic storms in the mesopause region and an increase in the 630.0-nm emission 1 day after storms in the thermosphere. Similar responses were observed in the O<inf>2</inf> atmospheric band (mesopause region) and the 777.4-nm emission (thermosphere) to those of 557.7-nm and 630.0-nm emissions, respectively. No significant changes were found in the rotational temperatures of OH and O<inf>2</inf>. The SABER instrument onboard the TIMED satellite observed a post-storm increase in the atomic oxygen mixing ratio at altitudes of 90–100 km. We suggest that this atomic oxygen increase contributed to the enhanced 557.7-nm emission in the mesopause region. Ionosondes over Japan observed rise of ionospheric altitudes and enhancements of foF2 1 day after storms. We discuss their possible relation to the observed thermospheric 630.0-nm emission enhancements.

DOI： 10.1029/2025JA034616

Wind Induced F3 Layer in the Middle Latitude During the Mother's Day Geomagnetic Disturbances of 10–11 May 2024 Reviewed Open Access

Rai B., Kalita B.R., Bhuyan P.K., Otsuka Y., Shiokawa K., Pallamraju D.

Journal of Geophysical Research Space Physics Vol. 131 ( 2 ) 2026.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research Space Physics

The formation process of the F3 layer at sunrise in the middle latitudes is newly investigated using data from the Mother's Day geomagnetic storm of 10–11 May 2024. Ionosonde, Fabry-Perot Interferometer (FPI), satellite data, and model simulations of Equatorial Electric Field for India, South-East Asia-Australia, and Japanese sectors are utilized. The F3 layer was recorded by Ionosondes at Japanese mid-latitude stations during 19 UT–21 UT, which coincided with the local Sunrise time. Over Perth (Australia), Wuhan (China), and Ahmedabad (India), formations were delayed at 22 UT–23 UT. The COSMIC vertical profiles showed enhanced F2 layer altitude after 19 UT, and confirmed the formation of additional ionospheric stratifications above the F2 layer. The thermospheric winds measured by FPI at northern mid-latitude and southern low-latitudes indicated strong equatorward winds in both hemispheres starting around 18:30 UT. The analysis revealed that dawn-time F3 layer stratifications in middle latitude were manifestations of residual nighttime F layer, which was raised to higher altitude by the equatorward winds, even as a new weak F2 layer was formed below by photo-ionization. The reduction in thermospheric O/N<inf>2</inf> caused the F3 layer at higher altitudes to survive and remain stronger than the F2 layer for a considerable time after sunrise. In contrast, the low-latitude F3 layer was formed by vertical drift induced by the electric field, when an IMF-Bz transition resulted in an eastward penetration electric field pulse in the dawn sector. At Perth in the southern hemisphere, both mechanisms were effective in the formation (wind) and sustenance (electric field) of the F3 layer.

DOI： 10.1029/2025JA034833

Power attenuation of Pc1 waves during propagation from the magnetosphere to the ionosphere observed by the Arase satellite and PWING ground stations Reviewed Open Access

Jaeyoung Kwak, Chae-Woo Jun, Yukinaga Miyashita, Jaeheung Park, Yoshizumi Miyoshi, Kazuo Shiokawa, Ayako Matsuoka, Mariko Teramoto, Kazuhiro Yamamoto, Iku Shinohara, Yoshiya Kasahara, Fuminori Tsuchiya, Atsushi Kumamoto, Atsuki Shinbori, Dmitry Baishev, Igor Poddelsky

Earth, Planets and Space 2026.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Science and Business Media LLC

Abstract

Understanding how the properties of Pc1 waves change during their propagation from the magnetospheric source regions to the middle or low-latitude ionosphere have not yet been clearly revealed by observations. In this study, we present the first quantitative comparison of Pc1 wave power attenuation both along the geomagnetic field lines and in the ionospheric wave ducts, using simultaneous observations from the Arase satellite and dynamical variation of Particles and Waves in the INner magnetosphere using Ground-based network observations (PWING) ground magnetometers. One of our key findings is that the polarization sense of the waves changed from left-handed polarization (LHP) at the satellite to right-handed polarization (RHP) on the ground, providing observational evidence of polarization transformation from space to the ionosphere. By examining polarization angles, we confirm that the Pc1 waves observed at multiple ground stations originated from the same magnetospheric source as the EMIC waves detected by the Arase. Importantly, we quantify the wave power attenuation factor along the magnetic field line to be only 0.37 dB/1000 km, which is nearly an order of magnitude smaller than that in the ionospheric wave duct (4.7–8.2 dB/1000 km). This result establishes a previously unreported minimum Pc1 wave attenuation rate in the magnetosphere, highlighting that the wave energy loss occurs more rapidly in the ionospheric duct than in space. These findings provide new insights into Pc1 wave transmission mechanisms and emphasize the importance of combined space- and ground-based observations for characterizing wave propagation processes across geospace.

Graphical Abstract

DOI： 10.1186/s40623-026-02372-4

Characteristic behavior of SAR arc, STEVE and Red-Green arc during HILDCAA events Reviewed

Nema A., Shiokawa K., Connors M., Pathak K.N., Datta A.

Advances in Space Research 2026

Language：English Publishing type：Research paper (scientific journal) Publisher：Advances in Space Research

Using 630.0 nm all-sky airglow imagers at Athabasca (Canada), Kapuskasing (Canada) and Nyr(Formula presented) l(Formula presented) (Finland), we present the first statistical analysis of the Stable Auroral Red (SAR) arcs, STEVE, and Red-Green (RG) arcs during High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events from 2011 to 2019. These optical phenomena are unique and relatively faint subauroral optical phenomena observed in the Earth’s upper atmosphere at subauroral latitudes. We discovered 25 subauroral optical phenomena (SAR arc, STEVE, and Red-Green arc) after analyzing all-sky images taken over the 9-years period. The arc detachments from the auroral oval typically occur in the premidnight sector, indicating ring current ion drift to the dusk sector. Out of total 12 HILDCAA events, from 2011 to 2019, more than 90 % were accompanied by at least one of these auroras. We also found that the auroras occurring during these HILDCAA events tends to happen predominantly during midnight hours. The occurrence rate of these auroras found to be much higher during HILDCAA events reaching to approximately 45 %, compared to previous statistical studies, including non–HILDCAA times.

DOI： 10.1016/j.asr.2026.01.076

COURSE: Cross-scale cOUpling pRocesses in the Solar–tErrestrial system—SCOSTEP’s new program for 2026–2030 Reviewed Open Access

Laurenza M., Shiokawa K., Molina M.G., Liu H.L., Krivova N.A., Funke B., Kusano K., Habarulema J.B., Buresova D., West M.J., Chau J., Zhang J., Nesse H., Usoskin I., Alberti T., Alfonsi L., Coddington O., Dasso S., Fung S.F., Hayakawa H., Miyoshi Y., Nakamura R., Temmer M., Stolle C., Zong Q.G., Briand C., Goplaswamy N., Safranova J.

Earth Planets and Space Vol. 77 ( 1 ) 2025.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Earth Planets and Space

The committee nominated to define the Scientific Committee on Solar–Terrestrial Physics (SCOSTEP) Next Scientific Program (NSP) has identified cross-scale coupling as the overarching theme for conducting and promoting coordinated research and outreach activities in the upcoming period 2026–2030. The program is called COURSE (Cross-scale cOUpling pRocesses in the Solar–tErrestrial system) and is organized in three main scientific focus areas: 1) sources of space weather and space climate; 2) solar wind, magnetosphere, and ionosphere coupling; and 3) external impacts and internal dynamics of the Earth atmosphere. For each Focus Area the NSP committee has identified: 1) long-standing goals, i.e., key questions persistent through SCOSTEP scientific programs and 2) objectives, i.e., precise outcomes that can be addressed over the 5-year program duration, which contribute to achieving the goals over the long term. Moreover, the committee envisions the implementation of the program through identified novel methods, including machine learning and Artificial Intelligence techniques; integrated models; new missions; the combination of multipoint in-situ data with ground observations; improved metadata; and adoption of Findable, Accessible, Interoperable, and Reusable (FAIR) principles.

DOI： 10.1186/s40623-025-02283-w

Effects of Field Line Curvature Scattering on Energetic Proton Precipitation and Isotropy in the Magnetosphere Reviewed Open Access

S. Imajo, Y. Miyoshi, S. Kasahara, S. Yokota, A. Matsuoka, K. Keika, T. Hori, I. Shinohara, K. Shiokawa, K. Yamamoto, M. Teramoto

Journal of Geophysical Research: Space Physics Vol. 130 ( 11 ) 2025.11

Language：English Publishing type：Research paper (scientific journal) Publisher：American Geophysical Union (AGU)

Abstract

While low‐altitude observations indicate that field line curvature (FLC) scattering produces isotropic precipitation of energetic protons, it remains unclear whether this mechanism also leads to a fully isotropic pitch angle distribution in the magnetosphere. In this study, we distinguished the low‐latitude boundaries of the loss cone filling and the isotropic distribution of energetic protons in an energy range of 10–180 keV using middle‐altitude (3–5 ) off‐equatorial observations made by the Arase satellite. The isotropic distribution boundary (IDB) is defined by the ratio of proton fluxes at pitch angles of 0°–45° to 45°–90° for the northern hemisphere. The latitude of the IDB exhibits energy dependence, such that higher energy protons become isotropic at lower latitudes, implying isotropization of protons by FLC. Around the IDB, the downward loss cone was filled, while the corresponding upward loss cone was empty due to atmospheric loss. The low‐latitude boundary of the loss cone filling tended to be located at 0.1°–0.4° lower latitude than the IDB. Numerical simulations using the centrifugal impulse model for FLC scattering reproduced this latitude difference in the low‐latitude boundaries of the loss cone filling and the isotropic distribution. These results suggest that effective pitch angle scattering occurs only near the loss cone on the field line with a larger equatorial curvature radius, resulting in loss cone filling, whereas it occurs more broadly across the pitch angles on the field line with a smaller equatorial curvature radius, leading to the isotropic distribution.

DOI： 10.1029/2025ja034307

Estimate of (Formula Presented) Altitude Profile Using Blue Auroral Resonant-Scattering 427.8 nm Emission Observed With HySCAI During Astronomical Twilight Reviewed

Ida K., Yoshinuma M., Ebihara Y., Shiokawa K.

Geophysical Research Letters Vol. 52 ( 21 ) 2025.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Geophysical Research Letters

The spectrum of high-altitude blue auroral emissions was observed with HyperSpectral camera for auroral imaging (HySCAI) during morning astronomical twilight in Kiruna. Auroral resonance scattering of an (Formula presented.) 1NG (0, 1) (427.8 nm) emission starts to increase from the east, and then the increase of this resonance scattering emission extends to the magnetic zenith. The volume emission rate is evaluated from the rise in resonance scattering emission (time derivative of emission intensity). The volume emission rate of (Formula presented.) (427.8 nm) reaches its maximum when the sunlight shadow height reaches 200 km, although the GLOW model predicted a peak altitude of (Formula presented.) (427.8 nm) of 120 km. The higher altitude of the resonance scattering emission peak observed with HySCAI supports the idea that (Formula presented.) is produced by the charge exchange between (Formula presented.) and (Formula presented.) existing at high altitudes. However, we cannot rule out another possible mechanism of upflowing (Formula presented.) ions.

DOI： 10.1029/2025GL118375

Medium-Scale Traveling Ionospheric Disturbances Observed by Nadir-Viewing 630 nm Airglow Imaging From the International Space Station Reviewed Open Access

Yuta Hozumi, Akinori Saito, Michi Nishioka, Takeshi Sakanoi, Jia Yue, Min-Yang Chou, Satoshi Andoh, Atsushi Yamazaki, Yuichi Otsuka, Kazuo Shiokawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 130 ( 10 ) 2025.10

Multiple-Event Study of Substorm Electric Field Penetration Into Middle Latitudes Based on Simultaneous Observation of 630-nm Airglow Enhancements at Three Stations Reviewed

Morita S., Shiokawa K., Otsuka Y., Nshitani N., Shinbori A., Fujimoto A., Yoshikawa A., Nishioka M., Perwitasari S., Yamamoto M., Sori T.

Journal of Geophysical Research Space Physics Vol. 130 ( 10 ) 2025.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research Space Physics

It is known that convection electric fields develop in the polar region associated with substorm onset, and a westward electric field penetrates to lower latitudes on the night side. Two-dimensional observations of 630-nm airglow enhancements associated with the westward electric field can be conducted using all-sky cameras at middle latitudes. We investigated seven simultaneous 630-nm airglow enhancement events using all-sky cameras at Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.9°N, 136.1°E), and Sata (31.0°N, 130.7°E) in Japan. A multi-event multi-point study of 630-nm airglow enhancement has been conducted for the first time. These seven events were found from 1,800 nights in 11 years, from 2002 to 2012, indicating that simultaneous 630-nm airglow enhancements over Japan were very rare. Five of these events occurred during storms, suggesting that simultaneous enhancement requires a large penetrating electric field associated with storm-time substorms. In three events, the enhancement was identified as occurring simultaneously with clear substorm onsets, while the other four events were also associated with high-latitude magnetic disturbances. In all seven events, we observed downward motion of the ionosphere associated with the airglow enhancement. These results indicate that they are due to the westward electric field in the substorm undershielding state. In two events, the time at the end of the 630-nm airglow enhancement correlated well with the increase in ionospheric altitude observed by ionosondes and an FM-CW radar. This increase in ionospheric altitude and termination of the airglow enhancement could be caused by eastward electric field penetration associated with overshielding in the magnetosphere.

DOI： 10.1029/2025JA033721

Evolution and decay of a stable and a dynamic airglow patch Reviewed

Eriksen N.K., Nishimura Y., Zettergren M., Lorentzen D.A., Oksavik K., Baddeley L.J., Hosokawa K., Shiokawa K., Lamarche L., Redden M.E., Bhatt A.

Journal of Atmospheric and Solar Terrestrial Physics Vol. 275 2025.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Atmospheric and Solar Terrestrial Physics

This study investigates the evolution of two polar cap airglow patches, AGP1 and AGP2, observed over Resolute Bay in northern Canada. AGP1 exhibited stable shape, followed the ExB drift patterns and maintained high electron density as it transited toward the auroral oval, sustaining the maximum F-region density and redline emissions by a downward vertical drift. An initial non-uniform density decay was observed in AGP1’s density profile; the topside shifted downwards, resulting in increased bottomside density. Later, the maximum and bottomside density started to decay. Overall, the downflow contributed to a faster density decay. In contrast, AGP2 displayed dynamic characteristics, halting its motion mid-polar cap, followed by a prolonged decay in both electron density and redline emission. Observations and GEMINI simulations revealed that recombination between atomic ions and neutral molecules was the dominant mechanism for density decay in the optical layer. The decay rate was affected by vertical drift, magnetic field line orientation, and local neutral atmosphere conditions, which may be influenced by prior patch activity. Simulations of AGP1 using GEMINI and GLOW highlight the importance of vertical ExB drift in sustaining electron density maximum and redline emission intensity. By using various free parameters in the model to tune our simulations (e.g. background precipitation) we are able to achieve reasonable agreement with observed patch behavior. Our findings emphasize the interplay between vertical drift, recombination rates, and neutral wind in determining polar cap patch evolution, as well as the complexity of patch dynamics and their dependence on local ionospheric and atmospheric conditions.

DOI： 10.1016/j.jastp.2025.106593

Machine Learning Recognition and Phase Velocity Estimation of Atmospheric Gravity Waves from OI 557.7 nm All-Sky Airglow Images Reviewed Open Access

Mahmoud R., Abdelwahab M., Shiokawa K., Mahrous A.

AI Switzerland Vol. 6 ( 10 ) 2025.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AI Switzerland

Atmospheric gravity waves (AGWs) are treated as density structure perturbations of the atmosphere and play an important role in atmospheric dynamics. Utilizing All-Sky Airglow Imagers (ASAIs) with OI-Filter 557.7 nm, AGW phase velocity and propagation direction were extracted using classified images by visual inspection, where airglow images were collected from the OMTI network at Shigaraki (34.85 E, 134.11 N) from October 1998 to October 2002. Nonetheless, a large dataset of airglow images are processed and classified for studying AGW seasonal variation in the middle atmosphere. In this article, a machine learning-based approach for image recognition of AGWs from ASAIs is suggested. Consequently, three convolutional neural networks (CNNs), namely AlexNet, GoogLeNet, and ResNet-50, are considered. Out of 13,201 deviated images, 1192 very weak/unclear AGW signatures were eliminated during the quality control process. All networks were trained and tested by 12,007 classified images which approximately cover the maximum solar cycle during the time-period mentioned above. In the testing phase, AlexNet achieved the highest accuracy of 98.41%. Consequently, estimation of AGW zonal and meridional phase velocities in the mesosphere region by a cascade forward neural network (CFNN) is presented. The CFNN was trained and tested based on AGW and neutral wind data. AGW data were extracted from the classified AGW images by event and spectral methods, where wind data were extracted from the Horizontal Wind Model (HWM) as well as the middle and upper atmosphere radar in Shigaraki. As a result, the estimated phase velocities were determined with correlation coefficient (R) above 0.89 in all training and testing phases. Finally, a comparison with the existing studies confirms the accuracy of our proposed approaches in addition to AGW velocity forecasting.

DOI： 10.3390/ai6100262

Strong Substorm Development From Polar-Cap Arc Laydown Along the Auroral Poleward Boundary Reviewed

Lyons L.R., Nishimura Y., Liu J., Yadav S., Zou Y., Bristow W.A., Donovan E., Nishitani N., Shiokawa K., Hosokawa K.

Journal of Geophysical Research Space Physics Vol. 130 ( 10 ) 2025.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research Space Physics

Meso-scale enhanced flow channels often extend across the polar cap from the dayside to the nightside auroral oval, where they enter the oval via localized reconnection and cause auroral oval disturbances. Examples here show such flows associated with azimuthally moving polar cap arcs that develop an equatorward portion that bends in the direction opposite to the arc's motion, becoming nearly parallel to the nightside auroral oval and possibly becoming an oval poleward boundary arc. The arcs then continue to move equatorward, and “lay down” along the auroral oval, leading to an explosive substorm onset (or expansion enhancement) that rapidly expands azimuthally with laying down of the arc, and later expands further with westward traveling surge sliding along the arc. This gives a longitudinally broad region of expansion-phase aurora, and a similarly broad magnetic depression, indicating large and broad substorm energy release. The surge sliding along the poleward boundary arc occurs at a higher latitude than the laydown portion of onset, so that sliding leads to later magnetic depressions at locations west of the initial onset and at higher latitudes. The polar cap arc appears to be pushed equatorward and westward (for By >0) by the flow of new plasma moving across the polar cap, the arc being like a weather front that demarcates an advancing boundary between plasma regimes along polar cap field lines. The onset and expansion of the activity appear to be driven by the flow of this new plasma from the polar cap to the region of expansion phase activity.

DOI： 10.1029/2025JA034161

Reduction of High Latitude Ionospheric Electron Density by the Impact of Negative Solar Wind Pressure Pulse During the Geomagnetic Storm of 23 March 2023 Reviewed Open Access

Geetashree Kakoti, Mala S. Bagiya, Geeta Vichare, Kazuo Shiokawa, P. R. Shreedevi, Nozomu Nishitani, Yuichi Otsuka, Atsuki Shinbori, Michi Nishioka, Septi Perwitasari

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 130 ( 8 ) 2025.8

Strong Westward Current Pulse at Auroral Latitudes Extending to Dawn-Side Low-Latitudes Due To Enhanced Density Within Kelvin-Helmholtz Wave Vortex in Solar Wind Reviewed Open Access

Nilam B., Tulasi Ram S., Oliveira D.M., Remya B., Shiokawa K., Rai D., Sibeck D., Dimri A.P.

Geophysical Research Letters Vol. 52 ( 15 ) 2025.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Geophysical Research Letters

A series of coronal mass ejections (CMEs) from the Sun, interacted with one another and formed a complex interplanetary-CME (ICME) that impinged Earth's magnetosphere on 10 May 2024 and caused the strongest geomagnetic storm of the past two decades. We present a unique pulse-like enhanced solar wind density structure associated with a giant Kelvin-Helmholtz (KH) wave vortex formed within the ICME due to compression of sheath region by the surrounding ejecta. When impinged on the magnetosphere, it caused enhanced eastward and westward currents, leading to strong positive and negative geomagnetic field impulses at low and auroral latitudes, respectively. The negative disturbances extended to mid-latitudes around the midnight sector. Interestingly, the westward current pulse (negative disturbance) penetrated deep into low-latitudes (up to 24.49°N and 28.92°S) in both hemispheres, but exclusively on the dawn side. The mechanisms responsible for such an intensification and low-latitude penetration of westward current pulse are discussed.

DOI： 10.1029/2025GL117032

Ionospheric Scintillation and Geomagnetic Disturbance Caused by Space Hurricanes Reviewed Open Access

Lu S., Xing Z.Y., Zhang Q.H., Zhang Y.L., Yang H.G., Oksavik K., Lyons L.R., Shiokawa K., Wang Y., Ma Y.Z., Wang X.Y., Xu T., Sun S.J., Zhang D.

Space Weather Vol. 23 ( 7 ) 2025.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Space Weather

A space hurricane is a large-scale, cyclone-shaped aurora characterized by a rotating magnetic structure and shears in plasma convection, driven by high-latitude magnetic reconnection. It typically forms near the magnetic poles in summertime, providing an efficient energy transport channel from the solar wind to the Earth's magnetosphere under northward interplanetary magnetic field conditions. Here, we present a detailed case study of a typical space hurricane that occurred on 20 August 2014 and its impact on the polar ionosphere in the Northern Hemisphere. Multi-instrument observations from the Eureka GNSS receiver, SWARM and Defense Meteorological Satellite Program (DMSP) satellites indicate that GPS signals experienced enhanced phase scintillation near a space hurricane, coinciding with regions of large plasma density gradients and flow shears. Based on these in situ measurements, we estimated growth timescales of ∼94 s for gradient-drift instability and ∼352 s for KHI, supporting the plausibility of both instabilities. These plasma instabilities likely generated small-scale irregularities responsible for the observed scintillation effects on satellite signals. Geomagnetic data from the Greenland ground stations revealed intense local geomagnetic disturbances, likely associated with enhanced Hall currents driven by the upward field-aligned currents linked to the space hurricane. In situ plasma observations from the DMSP and SWARM satellites revealed pronounced electron density enhancements and steep gradients on the morning side of the space hurricane, likely driven by ionospheric convection and soft particle precipitation. This study provides the first comprehensive insight into the space weather effects of space hurricanes.

DOI： 10.1029/2025SW004435

A Bootstrapping Convolutional Neural Network Technique for Optimizing Automated Detection of Equatorial Plasma Bubbles by Optical All-Sky Imagers Reviewed Open Access

Okoh D., Cesaroni C., Rabiu B., Shiokawa K., Otsuka Y., Ogunjo S., Akerele A., Habarulema J.B., Nava B., Migoya-Orué Y., Jamjareegulgarn P., Seun A., Adama O., Ochieng G., Ameh J., Awuor A., Baki P.

Earth and Space Science Vol. 12 ( 6 ) 2025.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Earth and Space Science

Equatorial plasma bubbles (EPBs) disrupt satellite-based communication and navigation systems, particularly in equatorial regions. Reliable detection and classification of EPBs from all-sky imager (ASI) images are essential for accurate space weather monitoring and forecasting. This study presents a novel bootstrapping convolutional neural network (CNN) approach to optimize automated EPB detection on ASI images for operational space weather monitoring applications, and overcoming challenges related to image variability and imbalanced data sets. Data used for CNN training were obtained from the optical mesosphere thermosphere imagers ASI installed at the Space Environment Research Laboratory, National Space Research and Development Agency, Abuja during the period from 2015 to 2020. Our method involved training three sub-models, and aggregating their predictions. The CNN trainings were conducted on three sub-datasets of 3,000 images each, categorized as “EPB,” “Noisy/Cloudy” or “No EPB.” Three corresponding sub-models were developed from the CNN trainings. The three sub-model classifications independently gave prediction accuracies of 98.67%, 98.33%, and 95.83% on a reserved test data set of 600 images. Ensemble models further improved the model prediction accuracies to 99.17% and 99.33% for methods based on the mean of sub-model probabilities and the mode of sub-model classifications respectively. Our results indicate that the bootstrapping CNN technique enhanced the EPB detection accuracy, providing a powerful tool for real-time space weather monitoring applications, and implications for improving operational reliability of satellite-based navigation and communication in the equatorial region.

DOI： 10.1029/2024EA004117

Simultaneous ground-satellite observations of ELF/VLF emissions generated by a strong magnetospheric compression Reviewed Open Access

Claudia Martinez-Calderon, Kazuo Shiokawa, Ondřej Santolík, Satoshi Kurita, Kunihiro Keika, Martin Connors, Ian Schofield, Miroslav Hanzelka, William S. Kurth

Earth, Planets and Space Vol. 77 ( 1 ) 2025.4

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Science and Business Media LLC

Abstract

We report, for the first time, simultaneous nightside observations of very low frequency waves associated with a strong sudden impulse and subsequent magnetospheric compression. On 23 December 2014 at 11:15 UT we observed an intense sudden impulse characterized by an increase of SYM-H amplitude (~ 63 nT), solar wind speed (300–420 km/s), and dynamic pressure (2–6 nPa) with northwards interplanetary magnetic field. Two minutes later, the sub-auroral receiver at Athabasca (ATH, 54.7ºN, 246.7ºE, L = 4.3) detected a chorus-like burst centered at ~ 2.2 kHz from 11:17 to 11:20 UT (03 MLT). At the same time, the nearby Van Allen Probes B (RBSP-B) spacecraft, also in the pre-midnight sector, detected an emission with the same temporal and frequency variations suggesting a conjugate event. One minute before this wave burst, RBSP-B also observed hiss below 1.5 kHz, which was not detected at ATH. Magnetospheric compression can lead to betatron acceleration of plasmasheet electrons and enhancement of temperature anisotropy regulating electron cyclotron instability and whistler-mode generation. However, our observations suggest the generation mechanism between hiss and the discrete emissions differed at the beginning of the compression, in addition to them having different propagation to the ground. The decrease of magnetic curvature in the equatorial plane, caused by strong magnetospheric compression, could be responsible for the hiss being observed first. The subsequent stretching of the field lines, increasing the curvature, likely contributed to the generation of the discrete emissions. This discrete emission with mostly oblique wave angles propagated unducted to the ground, while the more field-aligned hiss did not. These results indicate detailed characteristics of the temporal development of hiss/chorus-like wave generation and propagation in response to the global magnetosphere compression associated with a solar wind pressure pulse.

Graphical Abstract

DOI： 10.1186/s40623-025-02170-4

Other Link： https://link.springer.com/article/10.1186/s40623-025-02170-4/fulltext.html

Determination of electron heat flux in the topside ionosphere and its impact on the vertical profile of OI 630.0 nm emission rate during nighttime SAR arcs for different solar activity conditions Reviewed

Upadhyay K., Shiokawa K., Pallamraju D., Gololobov A.

Advances in Space Research Vol. 75 ( 6 ) page： 4731 - 4739 2025.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Advances in Space Research

Electron heat flux in the ionosphere plays a crucial role in altering the electron temperature and thermal balance of the electron population, especially over subauroral- and high-latitudes. The downward flow of electron heat flux in the subauroral-latitude ionosphere through magnetosphere-ionosphere coupling causes an enhancement in the electron temperature, thereby leading to the formation of Stable Auroral Red (SAR) arcs. Understanding the relationship between heat flux and electron temperature is critical, as no technique exists to measure heat flux directly in the SAR arc region. This study primarily reports the characteristics of thermal electron heat flux of SAR arcs in the sub-auroral ionosphere and its influence on the plasma densities in the inner-magnetosphere and the SAR arc peak emission altitude. The downward electron heat flux has been characterized by using measured atomic oxygen red line emission intensity in conjunction with simultaneous enhancements of electron temperature by in-situ measurements for several nighttime SAR arc events reported in the literature for the period of 2012–2020 of solar cycle-24, and by using GLOW, a physics-based model. The present investigation reveals considerable variation in the estimated heat flux and SAR arc emission altitudes due to the influence of solar activity. This study, thus, quantifies the electron heat flux, averaging 2.89 × 10<sup>11</sup> eV-cm<sup>−2</sup>-sec<sup>-1</sup> near solar minimum and 8.05 × 10<sup>12</sup> eV-cm<sup>−2</sup>-sec<sup>-1</sup> during solar maximum, associated with ionospheric heating during observations of nighttime SAR arcs and their relevance to space weather phenomena.

DOI： 10.1016/j.asr.2024.12.046

Automatic Detection of Equatorial Plasma Bubbles in Airglow Images Using Two-Dimensional Principal Component Analysis and Explainable Artificial Intelligence Reviewed Open Access

Yacoub M., Abdelwahab M., Shiokawa K., Mahrous A.

Machine Learning and Knowledge Extraction Vol. 7 ( 1 ) 2025.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Machine Learning and Knowledge Extraction

Equatorial plasma bubbles (EPBs) are regions of depleted electron density that form in the Earth’s ionosphere due to Rayleigh–Taylor instability. These bubbles can cause signal scintillation, leading to signal loss and errors in position calculations. EPBs can be detected in images captured by All-Sky Imager (ASI) systems. This study proposes a low-cost automatic detection method for EPBs in ASI data that can be used for both real-time detection and classification purposes. This method utilizes Two-Dimensional Principal Component Analysis (2DPCA) with Recursive Feature Elimination (RFE), in conjunction with a Random Forest machine learning model, to create an Explainable Artificial Intelligence (XAI) model capable of extracting image features to automatically detect EPBs with the lowest possible dimensionality. This led to having a small-sized and extremely fast-trained model that could be used to identify EPBs within the captured ASI images. A set of 2458 images, classified into two categories—Event and Empty—were used to build the database. This database was randomly split into two subsets: a training dataset (80%) and a testing dataset (20%). The produced XAI model demonstrated slightly higher detection accuracy compared to the standard 2DPCA model while being significantly smaller in size. Furthermore, the proposed model’s performance has been evaluated and compared with other deep learning baseline models (ResNet18, Inception-V3, VGG16, and VGG19) in the same environment.

DOI： 10.3390/make7010026

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