We started measuring geomagnetically induced current (GIC) in substations around Tokyo, Japan in 2017 to study the GIC effects on power systems. We defined a period with GIC continuously exceeding 3 A as a GIC event to analyze the long-term data. This threshold is sufficiently above the noise level of the measurement data to clearly distinguish the events. Approximately 90

AbstractSolar cycles have an asymmetrical shape with a fast rise and a slow decline, which varies from cycle to cycle. This makes it difficult to study the solar cycle phase dependence of the occurrence of intense solar-terrestrial events such as intense geomagnetic storms. In the previous works, differences in the rise and fall time lengths of each solar cycle were not fully considered. We normalized the asymmetric shape of each solar cycle using the rise and fall time lengths and showed the solar cycle phase dependence of occurrence of the intense geomagnetic storms selected using the Dst index for solar cycles 19−24 and using the aa index for solar cycles 12−24. The previous works noted that the occurrence of geomagnetic disturbances shows double peaks before and after solar maximum. Our results showed that the occurrence of the intense geomagnetic storms selected using the Dst and aa indices does not always show clear double peaks and increased more in the earlier half of the fall time than in the other time periods. The Dst and aa indices have been used to select geomagnetic storms in the previous studies. We pointed out that the solar cycle phase dependence of occurrence of the geomagnetic storms selected using the aa index is slightly different from those selected using the Dst index. We showed the geomagnetic storm on 4 August 1972 as an extreme case, which showed difference between Dst and aa. We also showed that intense geomagnetic storms associated with eruptive flares from the active sunspot groups occurred around the solar minima of cycles 17, 19, 20, and 21. Graphical Abstract

Intense magnetic storms pose a systemic threat to the electric power grid. In this study we examined the solar/interplanetary causes of such storms, their peak theoretical and observed intensities, and their occurrence frequency. Using coronal mass ejection (CME) and solar wind data, we selected the 18 intense magnetic storms from 1996 to 2021 with disturbance storm time (Dst) index of less than – 200 nT and analyzed solar events and solar wind conditions associated with them. Approximately 83% of the CMEs associated with the storms were full halo type and more than 83% of the flares associated with the storms were located within 30 degrees in longitude of solar central meridian. The integrated dawn-to-dusk electric field in the solar wind (E y ) showed a good correlation with |min. Dst| of the storms and the peak E y (E yp ) and the peak southward interplanetary magnetic field showed next good correlations with |min. Dst|. We obtained the E yp of 236 mV/m for |min. Dst| of 2500 nT of the expected upper limit of Earth’s magnetosphere using the empirical equations from the correlations between |min. Dst| and solar wind parameters and showed that this value of E y is possible according to the past observations. The E yp of 54 mV/m for the 13 March 1989 storm and that of 165/79 mV/m for the Carrington storm (|min. Dst|= 1760/850 nT) were also obtained. The analysis using the complimentary cumulative distribution function suggested the probabilities of E y of 100, 200, 250, and 340 mV/m over the next 100 years to be 0.563, 0.110, 0.060 and 0.026, respectively. Graphical Abstract

Abstract Geomagnetically induced currents (GICs) flow in power grid in response to geomagnetic field variations. Because of the potential threat of power outage, GIC is regarded as one of the important aspects of space weather. We calculated the transfer function in frequency domain that describes a linear relationship between the GICs measured at 3 substations in Japan and the geomagnetic field measured at the Kakioka observatory. The transfer function is found to show a significant time variation. For the periods at 2-10 minutes, the amplitude of the transfer function in night time is about 1 order of magnitudes larger than in day time. The day-night difference reduces for the periods >100 minutes. In addition, the amplitude of the transfer function tends to increase with local rainfall amount. These regular and irregular variations may be explained in terms of earthing resistivity depending on local weather conditions. The resistivity of the non-frozen soil decreases by about 40% when the temperature increases from 15°C to 35°C, resulting in the reduction of the earthing resistivity. Rainwater permeating into the soil also results in reduction of the earthing resistivity. Reduction of the earthing resistivity gives rise to magnification of the GICs. These results imply that a weather condition strongly affects the magnitude of GIC in Japan and challenge the conventional wisdom that the transfer function is almost steady in this time scale.

AbstractThe possible occurrence of major space weather events, such as large solar flares within one hundred years, is studied anticipating their effects on our social facilities. However, the continuous soft X-ray (SXR) observation of flares by Geostationary Operational Environmental Satellites (GOES) started in 1975, and the period of data collection is less than 50 years. On the other hand, ground-based sunspot observations have a long history. Their duration of data collection exceeds 100 years. The possibility of the occurrence of extremely large flares is estimated using the daily sunspot area data of individual sunspot groups between 1879 and 2016 using the catalogue complied by the Debrecen Heliophysical Observatory in Hungary and the catalogue updated by Mandal, Krivova, Solanki, Shinha, and Banerjee in 2020. It had become clear that large sunspot groups with the potential to produce Carrington-class flares (areas of more than 3000 MSH) have appeared on a total of 119−139 days between 1879 and 2016, and a sunspot group with the potential to produce an X100-class flare appeared between March and April 1947. According to the past major space weather events, the large sunspot groups caused a series of multiple large flares instead of just one large flare. We tried to estimate the probabilities of occurrence of a SXR flare $$\ge$$ ≥ X100 for 30-, 50-, and 100-year periods to be 0.70−0.76, 0.87−0.91, and 0.98−0.99, respectively, using the complementary cumulative distribution function (CCDF) of sunspot areas for the 138-year data. Graphical Abstract

Large-amplitude geomagnetically induced currents (GICs) are the natural consequences of the solar–terrestrial connection triggered by solar eruptions. The threat of severe damage of power grids due to the GICs is a major concern, in particular, at high latitudes, but is not well understood as for low-latitude power grids. The purpose of this study is to evaluate the lower limit of the GICs that could flow in the Japanese power grid against a Carrington-class severe magnetic storm. On the basis of the geomagnetic disturbances (GMDs) observed at Colaba, India, during the Carrington event in 1859, we calculated the geoelectric disturbances (GEDs) by a convolution theory, and calculated GICs flowing through transformers at 3 substations in the Japanese extra-high-voltage (500-kV) power grid by a linear combination of the GEDs. The estimated GEDs could reach ~ 2.5 V/km at Kakioka, and the GICs could reach, at least, 89 ± 30 A near the storm maximum. These values are several times larger than those estimated for the 13–14 March 1989 storm (in which power blackout occurred in Canada), and the 29–31 October 2003 storm (in which power blackout occurred in Sweden). The GICs estimated here are the lower limits, and there is a probability of stronger GICs at other substations. The method introduced here will be immediately applicable for benchmark evaluation of low-latitude GICs against the Carrington-class magnetic storms if one assumes electrical parameters, such as resistance of transmission lines, with sufficient accuracy.

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

Watari et al. ( Space Weather, 2009 , 7) found that the geomagnetically induced current (GIC) in Hokkaido, Japan (35.7° geomagnetic latitude (GML)), is well correlated with the y-component magnetic field ( By ) (correlation coefficients >0.8) and poorly correlated with Bx,z and dBx,y,z/dt . The linear correlation with By would help predict the GIC, if we have capabilities of reproducing the magnetosphere–ionosphere currents during space weather disturbances. To validate the linear correlation with By for any periods ( T ) of disturbances, we made correlation analyses for the geomagnetic sudden commencements and pulsations ( T = 1–10 min), quasi-periodic DP2 fluctuations (30 min), substorm positive bays (60 min), geomagnetic storms (1–20 h), and quiet-time diurnal variations (8 h). The linear correlation is found to be valid for short periods (cc > 0.8 for T < 1 h) but not for long periods (cc < 0.3 for T > 6 h). To reproduce the GIC with any periods, we constructed one-layer model with uniform conductor and calculated the electric field (IEF) induced by By using the convolution of dBy/dt and the step response of the conductor. The IEF is found to be correlated with the GIC for long periods (cc > 0.9), while the GIC- By correlation remains better for short periods. To improve the model, we constructed a two-layer model with highly conductive upper and less conductive lower layers. The IEF is shown to reproduce the GIC with cc > 0.9 for periods ranging from 1 min to 24 h. The model is applied to the GIC measured at lower latitudes in Japan (25.3° GML) with strong B y dependence. The mechanism of the strong B y dependence of the GIC remains an issue, but a possible mechanism for the daytime GIC is due to the zeroth-order transverse magnetic (TM 0 ) mode in the Earth-ionosphere waveguide, by which the ionospheric currents are transmitted from the polar to equatorial ionosphere.

We need a typical method of directly measuring geomagnetically induced current (GIC) to compare data for estimating a potential risk of power grids caused by GIC. Here, we overview GIC measurement systems that have appeared in published papers, note necessary requirements, report on our equipment, and show several examples of our measurements in substations around Tokyo, Japan. Although they are located at middle latitudes, GICs associated with various geomagnetic disturbances are observed, such as storm sudden commencements (SSCs) or sudden impulses (SIs) caused by interplanetary shocks, geomagnetic storms including a storm caused by abrupt southward turning of strong interplanetary magnetic field (IMF) associated with a magnetic cloud, bay disturbances caused by high-latitude aurora activities, and geomagnetic variation caused by a solar flare called the solar flare effect (SFE). All these results suggest that GIC at middle latitudes is sensitive to the magnetospheric current (the magnetopause current, the ring current, and the field-aligned current) and also the ionospheric current.

We investigate the formation and development of a large‐scale current system in the dawn sector during intense geomagnetic storms. Four events are selected based on the historical ranking of the westward deflections of the morningside midlatitude magnetic field. For each event the polar distribution of equivalent currents indicates a significant intensification of the westward electrojet (WEJ), which initially takes place at postmidnight and then extends eastward covering the entire dawn sector. The longitudinal confinement of the enhanced WEJ suggests that it closes with downward and upward field‐aligned currents (FACs) at its eastern and western ends, respectively, and therefore, the entire system may be envisioned as a wedge current. It is noted, however, that the primary closure of FACs is meridional, and those upward and downward FACs are considered to be unbalanced parts of the R1 and R2 currents. In one event dipolarization was observed in the dawnside plasma sheet near the magnetic conjugate point of the enhanced WEJ, and in another event a major auroral expansion was observed in the entire dawn sector. It is therefore suggested that the formation of the wedge current system and the subsequent expansion toward dayside is an ionospheric projection of the tail current reduction extending toward the dawnside flank. This wedge current system is similar to the substorm wedge current system except that it is centered at dawn. The recurrent formation of this dawnside current wedge in intense storms suggests that this is another distinct constituent of the storm time current system.

On 14-15 December 2006 an intense geomagnetic storm took place with a minimum Sym-H of -211 nT, during which a large westward magnetic disturbance was observed at Memambetsu (MMB) Japan (GM Lat: 35.7; GM Lon: 212.2). Watari et al. [2009] found that the associated magnetic variations were well correlated with local GIC signatures, and the event turned out to be the largest GIC event observed during the two-year period of the GIC measurement they conducted at MMB. The event was unique as it was observed in the late morning sector; other GIC events were observed predominantly in the evening-to-midnight sector. Motivated by this original study we examined the 30-years’ worth of MMB geomagnetic data along with various data sets available for individual events. The result shows that similar magnetic disturbances were observed almost exclusively in the late morning sector but repeatedly during severe geomagnetic storms. We also found that they can be probably attributed to a large-scale wedge current system formed in the morning sector following nightside substorm activity. Therefore, as preconditions for such extreme geomagnetic disturbances, we suggest (a) major enhancement of the solar wind driver, (b) morning location of the target site, (c) preceding substorm initiation on the night side, and possibly (d) winter season (for a reason to be presented). These conditions may serve as critical elements for warning in advance GIC events such as observed at MMB in the December 2006 event.

Solar activity of cycle 24 following the deep minimum between cycle 23 and cycle 24 is the weakest one since cycle 14 (1902–1913). Geomagnetic activity is also low in cycle 24. We show that this low geomagnetic activity is caused by the weak dawn-to-dusk solar wind electric field ( E d–d ) and that the occurrence rate of E d–d > 5 mV/m decreased in the interval from 2013 to 2014. We picked up seventeen geomagnetic storms with the minimum Dst index of less than −100 nT and identified their solar sources in cycle 24 (2009–2015). It is shown that the relatively slow coronal mass ejections contributed to the geomagnetic storms in cycle 24.

Abstract We developed a flare prediction model based on the supervised machine learning of solar observation data for 2010-2015. We used vector magnetograms, lower chromospheric brightening, and soft-X-ray data taken by Solar Dynamics Observatory and Geostationary Operational Environmental Satellite. We detected active regions and extracted 60 solar features such as magnetic neutral lines, current helicity, chromospheric brightening, and flare history. We fully shuffled the database and randomly divided it into two for training and testing. To predict the maximum size of flares occurring in the following 24 hours, we used three machine-learning algorithms independently: the support vector machine, the k nearest neighbors (kNN), and the extremely randomized trees. We achieved a skill score (TSS) of greater than 0.9 for kNN. Furthermore, we compared the prediction results in a more operational setting by shuffling and dividing the database with a week unit. It was found that the prediction score depends on the way the database is prepared.

AbstractEmanating from coronals holes (CHs), high speed streams (HSSs) cause recurrent geomagnetic disturbances in the Earth’s magnetosphere. For this reason being able to predict the occurrence and timing of the high speed solar wind is one of the more important issues in space weather forecasting. Currently, it is still difficult to estimate the effect of a CH in case that it extends from high latitudes to lower ones. To monitor the global solar wind condition we have therefore developed a three-dimensional MHD simulation code, the REProduce Plasma Universe (REPPU) code, that is driven by the solar magnetic field from the solar surface to 1AU. The connectivity of magnetic field lines from CHs to Earth’s orbit via HSSs has been investigated. Simulation results are presented and the usefulness of our model is discussed.