Faculty Profiles - SHIOKAWA, Kazuo

Papers - SHIOKAWA, Kazuo

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Special issue of SCOSTEP's 15th Quadrennial Solar-Terrestrial Physics Symposium (STP-15) Reviewed Open Access

Shiokawa K., Marsh D., Pallamraju D., Patsourakos S., Pedatella N., Ratnam M.V., Rozanov E., Srivastava N., Tulasiram S.

Journal of Atmospheric and Solar Terrestrial Physics Vol. 259 2024.6

Global Distribution of EMIC Waves and Its Association to Subauroral Proton Precipitation During the 27 May 2017 Storm: Modeling and Multipoint Observations Reviewed

P. R. Shreedevi, Yiqun Yu, Yoshizumi Miyoshi, Xingbin Tian, Minghui Zhu, Vania K. Jordanova, Satoko Nakamura, Chae‐Woo Jun, Sandeep Kumar, Kazuo Shiokawa, Martin Connors, T. Hori, Masafumi Shoji, I. Shinohara, S. Yokota, S. Kasahara, K. Keika, A. Matsuoka, Akira Kadokura, Fuminori Tsuchiya, Atsushi Kumamoto, Yoshiya Kasahara

Journal of Geophysical Research: Space Physics Vol. 129 ( 6 ) 2024.5

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

Abstract

Recent simulation studies using the RAM‐SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the magnetosphere‐ionosphere coupling. In this study, we use the BATS‐R‐US + RAM‐SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave‐particle interactions. During the 27 May 2017 storm, Arase and RBSP‐A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk‐midnight sector. Simulation results show that H‐ and He‐band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP‐A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H‐band waves in the dusk sector are stronger than He‐band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS‐R‐US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm.

DOI： 10.1029/2023ja032337

Observation and Numerical Simulation of Cold Ions Energized by EMIC Waves Reviewed Open Access

K.‐H. Kim, C.‐W. Jun, J.‐W. Kwon, J. Lee, K. Shiokawa, Y. Miyoshi, E.‐H. Kim, K. Min, J. Seough, K. Asamura, I. Shinohara, A. Matsuoka, S. Yokota, Y. Kasahara, S. Kasahara, T. Hori, K. Keika, A. Kumamoto, F. Tsuchiya

Journal of Geophysical Research: Space Physics Vol. 129 ( 5 ) 2024.5

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

Abstract

This is the first report of significant energization (up to 7,000 eV) of low‐energy He<sup>+</sup> ions, which occurred simultaneously with H‐band electromagnetic ion cyclotron (EMIC) wave activity, in a direction mostly perpendicular to the ambient magnetic field. The event was detected by the Arase satellite in the dayside plasmatrough region off the magnetic equator on 15 May 2019. The peak energy of the He<sup>+</sup> flux enhancements is mostly above 1,000 eV. At some interval, the He<sup>+</sup> ions are energized up to ∼7,000 eV. The H‐band waves are excited in a frequency band between the local crossover and helium gyrofrequencies and are close to a linear polarization state with weakly left‐handed or right‐handed polarization. The normal angle of the waves exhibits significant variation between 0° and 80°, indicating a non‐parallel propagation. We run a hybrid code with parameters estimated from the Arase observations to examine the He<sup>+</sup> energization. The simulations show that cold He<sup>+</sup> ions are energized up to more than 1,000 eV, similar to the spacecraft observations. From the analysis of the simulated wave fields and cold plasma motions, we found that the ratio of the wave frequency to He<sup>+</sup> gyrofrequency is a primary factor for transverse energization of cold He<sup>+</sup> ions. As a consequence of the numerical analysis, we suggest that the significant transverse energization of He<sup>+</sup> ions observed by Arase is attributed to H‐band EMIC waves excited near the local helium gyrofrequency.

DOI： 10.1029/2023ja032361

Westward Excursion of Pc1/EMIC Waves and Their Source Protons: Paradoxical Observations From Ground and Space Reviewed

Yin Z.F., Zhou X.Z., Hu Z.J., Yue C., Zong Q.G., Liu Z.Y., Liu J.J., Shiokawa K., Oyama S.i., Baishev D.

Journal of Geophysical Research Space Physics Vol. 129 ( 5 ) 2024.5

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

Pc1 geomagnetic pulsations within the frequency range of 0.2–5 Hz, the ground signatures of electromagnetic ion cyclotron (EMIC) waves, are usually observed to progress westward. A plausible explanation is that their westward-excursion speed is defined by the drift velocity of the source particles in the geomagnetic dipole field. In this study, this relationship is examined through conjugate observations from ground-based and spaceborne instruments. The ground-based observations, including Pc1 pulsations and auroral emissions, consistently demonstrate a westward-excursion speed of ∼4–5 MLT per hour, which agrees with the time delay between EMIC wave observations at two equatorial spacecraft. Observations of the wave source particles, however, indicate an inconsistency between the slow westward excursion speed and the high energy of the wave source protons. This apparent inconsistency is reconciled by the trapping motion of energetic particles within magnetic dips, which is supported by the simultaneous flux enhancements of protons across a broad energy range between ∼50 and ∼100 keV. These results improve our understanding of the westward excursion of Pc1 pulsations, and also underscore the need for caution when directly linking their westward-excursion speed to the energies of the source particles.

DOI： 10.1029/2023JA032317

Statistical Analysis of Low-Latitude Boundary of Polar-Type Medium-Scale Traveling Ionospheric Disturbances Observed by a 630-nm Airglow Imager at Nyrölä, Finland Reviewed Open Access

Masaki Sato, Kazuo Shiokawa, Shin ichiro Oyama, Yuichi Otsuka, Atsuki Shinbori, Arto Oksanen

Journal of Geophysical Research: Space Physics Vol. 129 ( 4 ) 2024.4

New observational projects in New Zealand for studying radiation belt loss processes in the deep inner magnetosphere: instrumentation, operation by solar power and initial results Reviewed Open Access

Yuki Obana, Kaori Sakaguchi, Masahito Nosé, Keisuke Hosokawa, Peter Jaquiery, Satoko Saita, Kazuo Shiokawa, Martin Connors, Akira Kadokura, Tsutomu Nagatsuma, Tanja Petersen

Earth, Planets and Space Vol. 76 ( 1 ) 2024.3

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

Abstract

This paper describes the instrumentation and the first results of an upper atmospheric observing project conducted in New Zealand. We operate an all-sky aurora camera and a 64-Hz sampling induction magnetometer at Middlemarch, as well as 1-Hz sampling fluxgate magnetometers which have been operative at three stations in New Zealand, Middlemarch, Eyrewell and Te Wharau. Green and red auroras corresponding to the 557.7 nm and 630.0 nm emissions, respectively, were observed on the night of 5 August 2019. Pc1 pulsations were observed in the frequency range of ~ 0.2–1 Hz before and after a small (minimum Dst = − 40 nT) geomagnetic storm during 4–6 October 2020. Before the geomagnetic storm, Pc1 pulsations with several center frequencies were observed regardless of local time. During the recovery phase, an IPDP (interval of pulsations of diminishing period) type of Pc1 and four subsequent intervals of Pc1s were detected. The Ionospheric Alfvén Resonator (IAR) was also identified with spectral resonance structures during this magnetic storm. Lower harmonic modes of the IAR were present throughout the local nighttime, but higher harmonic modes with frequency of 5–15 Hz seemed to disappear at the onset time of substorms. This is the first report of the IAR at such a high frequency range and this is the first IAR observation in the southern hemisphere. Examples of applying cross-phase analysis to observation data of fluxgate magnetometers are also given.

Graphical Abstract

DOI： 10.1186/s40623-024-01990-0

Other Link： https://link.springer.com/article/10.1186/s40623-024-01990-0/fulltext.html

How to identify and forecast severe space weather events Reviewed

Balan N., Zhang Q.H., Ram S.T., Shiokawa K., Manu V., Xing Z.Y.

Journal of Atmospheric and Solar Terrestrial Physics Vol. 256 2024.3

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

This paper reviews some of the work performed by the authors over the past decade regarding the identification and forecasting of severe space weather. Severe space weather (SvSW) is defined as those causing damage of ground installations such as transformers resulting in power outage and/or tele-communication networks resulting in communication failure; normal space weather (NSW) does not cause such severe effects. Severe space weather (ScSW) events are caused by very energetic coronal mass ejections (CMEs) from the Sun, in particular by the sharp changes at their leading edge or front. It is shown that (1) it is the ICME (interplanetary CME) front of large velocity jump ΔV by over 275 km s<sup>−1</sup> and sufficiently large southward interplanetary magnetic field (IMF Bz) at and beyond the velocity jump that causes SvSW. (2) A derived parameter of the geomagnetic storms called impulsive strength (IpsDst) giving the average value of Dst during the storm main phase (MP) can distinguish between SvSW and NSW. (3) The product of the observed velocity jump and associated IMF Bz southward (ΔVBz) ≤ −15000 km s<sup>−1</sup> nT or −15 mV/m) can be used for forecasting SvSW. (4) If one can estimate ΔVBz at the Sun-Earth L1 point based on observations such as from the Advanced Composition Explorer (ACE) satellite, it is possible to forecast an SvSW event with an advance warning time of about half an hour.

DOI： 10.1016/j.jastp.2024.106183

Thermospheric Wind Response to March 2023 Storm: Largest Wind Ever Observed With a Fabry-Perot Interferometer in Tromsø, Norway Since 2009 Reviewed Open Access

S. Oyama, H. Vanhamäki, L. Cai, A. Shinbori, K. Hosokawa, T. Sakanoi, K. Shiokawa, A. Aikio, I. I. Virtanen, Y. Ogawa, Y. Miyoshi, S. Kurita, N. Nishitani

Space Weather Vol. 22 ( 3 ) 2024.3

Variation in the Reflection Height of VLF/LF Transmitter Signals in the D‐Region Ionosphere and the Possible Source: A 2018 Meteoroid in Hokkaido, Japan Reviewed

H. Ohya, T. Suzuki, F. Tsuchiya, H. Nakata, K. Shiokawa

Radio Science Vol. 59 ( 3 ) 2024.2

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

Abstract

Several studies have examined ionospheric variation associated with meteorites, meteoroids, or meteors based on Global Satellite Navigation System total electron content observations. However, there have been few quantitative studies of the D‐region of the ionosphere (60–90 km), which is associated with meteoroids. We investigated variation in the D‐region during the passage of a meteoroid over northeastern Hokkaido, Japan, at 11:55:55 UT on 18 October 2018, using very low‐frequency (VLF, 3–30 kHz) and low‐frequency (LF, 30–300 kHz) signals observed by three transmitters [JJY (40 kHz), JJY (60 kHz), and JJI (22.2 kHz)], at Rikubetsu, Japan. Periodic variation of 100–200 s was observed in the VLF and LF amplitudes upon arrival of the acoustic wave. The vertical seismic velocity of Hi‐net and F‐net data also showed acoustic waves. Although the main period of the acoustic wave was 0.1–0.5 s in the seismic data, a longer period component (100–200 s) remained during propagation up to the D‐region ionosphere. The estimated velocity of the acoustic waves was ∼340 m/s on the ground according to the Hi‐net seismic data. The acoustic wave originated near the endpoint (25 km altitude) of the meteoroid trajectory. Based on the observed propagation time of the acoustic waves and ray tracing results, the acoustic waves propagated obliquely from near the endpoint of the meteoroid trajectory up to a D‐region height (about ∼90 km altitude), south of the Rikubetsu receiver.

DOI： 10.1029/2023rs007801

Longitudinal Development of Cosmic Noise Absorption Based on Multipoint Observations at Subauroral Latitudes During Storm‐Time Substorms on 25–28 August 2018 Reviewed Open Access

Yuto Kato, Kazuo Shiokawa, Yoshimasa Tanaka, Mitsunori Ozaki, Akira Kadokura, Shin‐ichiro Oyama, Alexey Oinats, Martin Connors, Dmitry Baishev

Journal of Geophysical Research: Space Physics Vol. 129 ( 1 ) 2024.1

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

Abstract

Enhancements in electron density in the D‐region ionosphere attributed to the precipitation of high‐energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground‐based riometers. However, there have been few studies of CNA observations at multi‐point stations distributed in longitudes. Thus, the spatio‐temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio‐temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti‐sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high‐energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn‐to‐dusk electric fields. These results indicate that spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.

DOI： 10.1029/2023JA031950

A Closer Cooperation between Space and Seismology Communities – a Way to Avoid Errors in Hunting for Earthquake Precursors Reviewed Open Access

Pilipenko V.A., Shiokawa K.

Russian Journal of Earth Sciences Vol. 24 ( 1 ) 2024

Language：English Publishing type：Research paper (scientific journal) Publisher：Russian Journal of Earth Sciences

The space physicists and the earthquake (EQ) prediction community exploit the same instruments – magnetometers, but for different tasks: space physicists try to comprehend the global electrodynamics of near-Earth space on various time scales, whereas the seismic community develops electromagnetic methods of short-term EQ prediction. The lack of deep collaboration between those communities may result sometimes in erroneous conclusions. In this critical review, we demonstrate some incorrect results caused by a neglect of specifics of geomagnetic field evolution during space weather activation. The considered examples comprise: Magnetic storms as a trigger of EQs; ULF waves as a global EQ precursor; Geomagnetic impulses before seismic shocks; Long-period geomagnetic disturbances generated by strong EQs; Discrimination of underground ULF sources by amplitude-phase gradients; Depression of ULF power as a short-term EQ precursor; and Detection of seimogenic emissions by satellites. To verify the reliability of the above widely disseminated results data from available arrays of fluxgate and search-coil magnetometers have been re-analyzed. In all considered events, the “anomalous” geomagnetic field behavior can be explained by global geomagnetic activity, and it is apparently not associated with seismic activity. This critical review does not claim that ULF electromagnetic field cannot be used as a sensitive indicator of the EQ preparation processes, but we suggest that both communities must cooperate their studies more tightly using data exchange, combined usage of magnetometer networks, organization of CDAW for unique events, etc.

DOI： 10.2205/2024ES000899

Pathways to global coordination in space weather: International organizations, initiatives, and space agencies Reviewed

Ishii M., Eduardo Rezende Costa J., Kuznetsova M.M., Andries J., Gopalswamy N., Belehaki A., Alfonsi L., Shiokawa K., Stanislawska I., Bingham S., Kalegaev V., Kent Tobiska W., Rees D., Glover A., Spann J.F.

Advances in Space Research 2024

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

This paper is the second part of two papers on global coordination in space weather. In this paper the activities of established and emerging international organizations and initiatives related to space weather research and operations are introduced. As shown in the accompanying paper “Global Landscape of Space Weather Observations, Research and Operations”, most of instrumentations for space environment monitoring are operated by each country. On the other hand, it is necessary to integrate the global observation to detect space weather phenomena, and it is essential to coordinate among nations for data sharing, set standard formats and protocols, fill observational gaps, etc. There are multiple international organizations and other actors in the field of space weather, and they have their own purposes and goals. In 2022, three international organizations, WMO, ISES and COSPAR reached an agreement to explore pathways to increased coordination of activities. There is also an emerging movement by various space agencies that fund space weather research missions to establish a forum to share plans and foster discussions leading to possible collaborations that advance understanding and enable progress of space weather operations and applications.

DOI： 10.1016/j.asr.2024.06.017

On the Creation, Depletion, and End of Life of Polar Cap Patches Reviewed Open Access

Eriksen, NK; Lorentzen, DA; Oksavik, K; Baddeley, L; Hosokawa, K; Shiokawa, K; Bland, E; Paxton, L; Zhang, YL; Mcwilliams, K; Yeoman, T; Themens, DR

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 128 ( 12 ) 2023.12

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

Ionospheric convection patterns from the Super Dual Auroral Radar Network are used to determine the trajectories, transit times, and decay rates of three polar cap patches from their creation in the dayside polar cap ionosphere to their end of life on the nightside. The first two polar cap patches were created within 12 min of each other and traveled through the dayside convection throat, before entering the nightside auroral oval after 104 and 92 min, respectively. When the patches approached the nightside auroral oval, an intensification in the poleward auroral boundary occurred close to their exit point, followed by a decrease in the transit velocity. The last patch (patch 3) decayed completely within the polar cap and had a lifetime of only 78 min. After a change in drift direction, patch 3 had a radar backscatter power half-life of 4.23 min, which reduced to 1.80 min after a stagnation, indicating a variable decay rate. 28 minutes after the change in direction, and 16 min after coming to a halt within the Clyde River radar field-of-view, patch 3 appeared to reach its end of life. We relate this rapid decay to increased frictional heating, which speeds up the recombination rate. Therefore, we suggest that the slowed patch motion within the polar cap convection pattern is a major factor in determining whether the patch survives as a recognizable density enhancement by the time the flux tubes comprising the initial patch cross into the nightside auroral oval.

DOI： 10.1029/2023JA031739

Geomagnetic Effect of the Atmospheric Acoustic Resonance Excited by Earthquakes and Volcano Eruptions Reviewed

Surkov, VV; Pilipenko, VA; Shiokawa, K

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 128 ( 10 ) 2023.10

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

An analytical model of the vertical acoustic resonance between the thermosphere and the earth's surface and its geomagnetic effect has been constructed. Resonance with a frequency of several mHz occurs when an atmospheric acoustic wave (AW) is excited by vibrations of the earth or ocean surface. The derived analytical relationships give a possibility to examine the dependence of the fundamental resonant frequency on the height of the reflecting atmospheric layer and horizontal AW number. Geomagnetic disturbances caused by the impact of AW on the ionosphere are calculated within the framework of a multi-layered model of the ground-atmosphere-ionosphere system. The E-layer of the ionosphere is treated in the approximation of a thin layer with an inclined geomagnetic field, while the topside ionosphere is assumed to consist of a cold collisionless plasma. The dependence of the spectral power of magnetic perturbations on the direction of the horizontal AW propagation is examined. The magnetic perturbation spectra are shown to have maxima at frequencies close to the acoustic resonance frequency. The spectral powers of magnetic perturbations and barometric variations measured during the Iwate-Miyagi Nairik earthquake are in accordance with the model predictions for realistic media and AW parameters. Field-aligned currents that arise when AWs enter the ionospheric E-layer is shown to carry measurable electromagnetic disturbances into the geomagnetically conjugated region.

DOI： 10.1029/2023JA031872

Correspondence of Pi2 pulsations, aurora luminosity, and plasma flux fluctuation near a substorm brightening aurora: Arase observations Reviewed

L. Chen, K. Shiokawa, Y. Miyoshi, S. Oyama, C‐W. Jun, Y. Ogawa, K. Hosokawa, Y. Kazama, S. Y. Wang, S. W. Y. Tam, T. F. Chang, B. J. Wang, K. Asamura, S. Kasahara, S. Yokota, T. Hori, K. Keika, Y. Kasaba, A. Kumamoto, F. Tsuchiya, M. Shoji, Y. Kasahara, A. Matsuoka, I. Shinohara, S. Nakamura

Journal of Geophysical Research: Space Physics 2023.9

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

Abstract

Although many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7‐25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase at L ∼ 5. A camera at Gakona (62.39<sup>o</sup>N, 214.78<sup>o</sup>E), Alaska, observed a substorm auroral brightening on December 28, 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere.

DOI： 10.1029/2023ja031648

A machine learning approach for total electron content (TEC) prediction over the northern anomaly crest region in Egypt Reviewed Open Access

Rukundo W., Shiokawa K., Elsaid A., AbuElezz O.A., Mahrous A.M.

Advances in Space Research Vol. 72 ( 3 ) page： 790 - 804 2023.8

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

In this paper, we developed an empirical model for predicting Total Electron Content (TEC) on a single station receiver located within the equatorial anomaly crest region at Helwan – Egypt (29.86°N, 31.32°E & MLAT 27.02°N). The neural network algorithm was developed from several inputs representing diurnal, seasonal, and solar cycle variations, solar and geomagnetic activity, and solar wind parameters using TEC data collected by the SCINDA GPS receiver. The TEC data collected for about nine years from 2009 to 2017 comprises almost the whole solar cycle 24 allowing the developed model to be validated and tested for different solar activity conditions. The 2009 – 2016 dataset was randomly divided into 70% for training, and 30% for validation during model development while the 2017 dataset was reserved for testing the model during independent validation. The validation was done by comparing TEC from the developed model and TEC derived from the GPS scintillation and TEC monitor (GPSTEC). Knowing that TEC modeling over the equatorial anomaly crest regions is challenging since it is characterized by high temporal and spatial variations, the developed model reconstructed diurnal GPSTEC from 2014 to 2016 with a root mean square error (RMSE) of 4.0 TECU and 3.2 TECU for high and low solar activity phases respectively. The neural network (NN) modeled TEC was highly correlated with GPSTEC, that is 0.93 and 0.94 for low and high solar activity respectively. In comparison, the International Reference Ionosphere (IRI)-2016 model predicted the general trend of GPSTEC in 2017 with RMSE of 5.43 TECU better than the developed TEC model with RMSE of 5.74 TECU. However, the NN model predicted diurnal GPSTEC for selected quiet and disturbed days, and seasonal TEC better than the IRI model in 2017. Therefore, the developed model was able to capture temporal ionospheric TEC for diurnal and seasonal variations during low solar activity. It was established that the performance of the empirical model depends on the size and distribution of the data used for model development. It's worth noting that despite the improved performance of the NN algorithm in predicting diurnal and seasonal TEC variation, the NN-modeled TEC performed poorly in reconstructing the nighttime anomalies exhibited in the GPSTEC. The inclusion of the IRI-NmF2 as network input also adds to the improved performance of the network.

DOI： 10.1016/j.asr.2022.10.052

A Ground‐Based Instrument Suite for Integrated High‐Time Resolution Measurements of Pulsating Aurora With Arase Reviewed

K. Hosokawa, S.‐I. Oyama, Y. Ogawa, Y. Miyoshi, S. Kurita, M. Teramoto, S. Nozawa, T. Kawabata, Y. Kawamura, Y.‐M. Tanaka, H. Miyaoka, R. Kataoka, K. Shiokawa, U. Brändström, E. Turunen, T. Raita, M. G. Johnsen, C. Hall, D. Hampton, Y. Ebihara, Y. Kasahara, S. Matsuda, I. Shinohara, R. Fujii

Journal of Geophysical Research: Space Physics Vol. 128 ( 8 ) 2023.8

Spatio-Temporal Characteristics of IPDP-Type EMIC Waves on April 19, 2017: Implications for Loss of Relativistic Electrons in the Outer Belt Reviewed Open Access

Asuka Hirai, Fuminori Tsuchiya, Takahiro Obara, Yuto Katoh, Yoshizumi Miyoshi, Kazuo Shiokawa, Yasumasa Kasaba, Hiroaki Misawa, Chae Woo Jun, Satoshi Kurita, Martin G. Connors, Aaron T. Hendry, Atsuki Shinbori, Yuichi Otsuka, Takuya Tsugawa, Michi Nishioka, Septi Perwitasari, Jerry W. Manweiler

Journal of Geophysical Research: Space Physics Vol. 128 ( 8 ) 2023.8

Language：English Publishing type：Research paper (scientific journal)

To understand the mechanism of the increased frequency of intervals of pulsations of diminishing periods (IPDPs), we analyzed IPDP-type electromagnetic ion cyclotron (EMIC) waves that occurred on 19 April 2017, using ground and satellite observations. Observations by low-altitude satellites and ground-based magnetometers indicate that the increased IPDP frequency is caused by an inward (i.e., Earthward) shift of the EMIC wave source region. The EMIC wave source region moves inward along the mid-latitude trough, which we used as a proxy for the plasmapause location. A statistical analysis shows that increases in the IPDP frequency showed a positive correlation with polar cap potentials. These results suggest an enhanced convection electric field causes an inward shift of the source region. The inward shift of the source region allows EMIC waves to scatter relativistic electrons over a wide range of radial distances during the IPDP event. This mechanism suggests that IPDP-type EMIC waves are more likely to scatter relativistic electrons than other EMIC waves. We also show that the decreased phase-space density of relativistic electrons in the outer radiation belt is consistent with the extent of the source region and the resonant energy of EMIC waves, implying a possible contribution of EMIC waves to outer radiation belt loss during the main phase of geomagnetic storms.

DOI： 10.1029/2023JA031479

Equatorial plasma bubble intensities across longitudinal sectors of the globe using GNSS observations Reviewed

Okoh Daniel, Shiokawa Kazuo, Otsuka Yuichi, Rabiu Babatunde, Habarulema John Bosco, Migoya-Orue Yenca, Cesaroni Claudio

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS Vol. 249 2023.8

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

We present the first investigation of Equatorial Plasma Bubble (EPB) intensities across longitudinal sectors of the globe using observations from global navigation satellite system (GNSS) receivers. GNSS data from a total of 93 receiver stations located within ±20 degrees of the geomagnetic equator across the globe were used. The data covered periods of years 2014 and 2019 which are respectively years of high and low solar activity in solar cycle 24. We define a parameter known as the Standard deviation of Residual TEC (SRT) to characterize the EPB intensities. The EPB occurrence was defined by day-night differences of the rate of change of TEC index (ROTI). We observed a high correlation (r ∼ 0.80) between the magnitudes of the SRT and ROTI during the EPB occurrence, but the correlation is low (r ∼ 0.37) during non occurrence of EPB. The EPB intensities are greater during seasons with high occurrence rates. The EPB intensities and occurrence rates are also greater during the high solar activity. We found that the post-sunset intensities are greatest in the Atlantic region, followed by the African region, then the American, Australian, Asian, and Pacific regions in that order. The post-midnight intensities are greatest in the African region, followed by the Atlantic, American, Australian, Asian, and Pacific regions in that order.

DOI： 10.1016/j.jastp.2023.106097

Statistical Analysis of the Horizontal Phase Velocity Distribution of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances in Airglow Images Over Darwin (12.4 degrees S, 131.0 degrees E) Reviewed

Tsuboi Takuma, Shiokawa Kazuo, Otsuka Yuichi, Fujinami Hatsuki, Nakamura Takuji, Neudegg David

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 128 ( 8 ) 2023.8

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

Atmospheric gravity waves (AGWs) and medium-scale traveling ionospheric disturbances (MSTIDs) in the upper atmosphere can be observed in nocturnal airglow images. Spectral analysis of airglow images provides propagation direction and intensity of these waves. However, the spectral analysis of airglow images has not been done yet for stations in the southern hemisphere except for Antarctica. In this study, we calculated the horizontal phase velocity spectra of AGWs in the mesosphere-lower-thermosphere (MLT) region near the mesopause and MSTIDs in the F-region ionosphere from airglow images at wavelengths of 557.7 and 630.0 nm, respectively, by applying the 3-dimensional Fourier spectral analysis to the airglow images obtained at Darwin (12.4°S, 131.0°E) in Australia for 13.75 years from 2001 to 2007 and from 2011 to 2019. The spectra of AGWs in the MLT region show clear characteristics that the southward power spectral density (PSD) is stronger in summer and weaker in winter. Tropospheric convection was located at north of Darwin in summer and above Darwin in winter, suggesting that the tropospheric convection is a possible source of AGWs in the mesopause region. The spectra of MSTIDs in the F-region ionosphere show that the dominant northwestward PSD is stronger in winter and during solar quiet periods. These features can be explained if the observed MSTIDs are caused by the ionospheric instabilities. A weak positive correlation was observed between PSDs of AGWs in the mesosphere and MSTIDs. We propose that the MSTIDs propagating other than northwestward may be generated by AGWs.

DOI： 10.1029/2022JA030769

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