arXiv subjects

Hisashi Hayakawa

Publications and source records attributed to Hisashi Hayakawa.

At least 19 recordsLinked to original sources

arXiv2026

The "Other" Centuries-Long Record of Solar Magnetic Activity Cycles: Lessons From the Stars

Studies of space climate and solar magnetic activity and studies of stellar dynamos and magnetic cycles are separated by discipline and by the quality of the data we have to work with: the Sun gives us detailed knowledge of a single star, while the stars give us comparatively sparse data across a range of (sometimes poorly known) masses and ages. This review, a collaboration among scientists with expertise in stellar astronomy, space climate, and solar physics, seeks to provide a guide to what the Sun's and stars' magnetic activity records can tell us about how stellar dynamos operate on decadal, millennial, and stellar-evolutionary timescales. It reviews the historical Solar sunspot record, terrestrial cosmogenic isotope records, and decades-long chromospheric activity measurements of dozens of sun-like stars across a range of masses, ages, and metallicities near the solar value. Particular emphasis is placed on the Maunder minimum, and the hunt for similar events in the stellar record, especially the recently discovered grand minimum event ongoing in the star HD 166620. These records show that solar-like dynamos are a common feature of Sun-like stars, and magnetic grand minimum events akin to the Maunder minimum might be very rare, with only one identified in 3,000 star-years of magnetic activity records. Together, solar and stellar dynamo studies are bringing a still-fuzzy picture into focus in which stellar magnetic activity weakens as stars spin down over billions of years towards a critical Rossby number near the solar value, at which point stellar cycles begin to be frustrated. These periods of frustration manifest as magnetic grand minima, and increase in frequency until they become permanent and stars enter a period of very low and constant magnetic activity.

astro-ph.SR↗

arXiv2026

Reconstruction of pretelescopic and early telescopic solar activity cycles from auroral records

The historical record of low-latitude aurorae is essentially a poorly sampled record of the largest space weather events (SWEs). Its use for the identification of individual solar cycles is hindered by the low event rate and by the fact that the solar cycle profile of the occurrence of SWEs does not closely follow the variation of sunspot numbers. Based on recent studies of the solar cycle dependence of the occurrence rates of large SWEs, here we construct Monte-Carlo simulations of a large number of activity cycles to identify the optimal procedure to infer the characteristics of underlying solar cycles from the sparse record. We find that a reliable reconstruction of the cycle phase ($>90$% of reconstructed minima corresponding to actual minima within $\pm 2$ years) is possible whenever the long-term mean event rate (annual mean number of space weather events resulting in low-latitude auroral sightings) reaches or exceeds a value around 3. This condition is found to be satisfied during most of the the Early Modern Active Period (EMAP), a century-long period of normal solar activity between the Spörer and Maunder Minima. For the numbering of solar cycles in the EMAP we introduce the "telescopic era", where T$n$ denotes the $n$th cycle from the first telescopically observed cycle, T$0$, ongoing in 1610. Using our optimal procedure we reconstruct a series of 8 solar activity cycles from T$-5$ to T$2$ (1560-1640). Earlier cycles starting from 1540 can be reconstructed with a somewhat lower degree of reliability. Comparing our results with radionuclide-based reconstructions and sunspot observations we find a good overall correspondence, with the exception of the last cycle before the Maunder Minimum.

astro-ph.SR↗

arXiv2026

Analyses on Christoph Clavius' Reports of Total Solar Eclipses in 1560 and 1567: Key References for the Centennial Variations of the Earth's Rotation Speed and the Solar Radius

Variations in solar radius (hereafter R_Sun) is a key reference for solar magnetic activity in time. The sunlight amount may have varied with R_Sun and had an effect on the Earth's climate in the past. Eclipse observations offer a unique opportunity to measure the absolute R_Sun value before modern direct observations. The scientific community has discussed a possible long-term R_Sun variability from 1715 onward. Prior to their coverage, Clavius' eclipse reports had been subjected to qualitative debates regarding the local eclipse visibility and a possible secular R_Sun trend. This study leverages the recent dramatic developments of lunar topography data and ephemeris data to provide an effective resolution of this debate. Clavius' eclipse reports described an explicit totality in 1560 at Coimbra and a "slender circle" around the eclipsing Moon in 1567 at Rome. Our study revised the ΔT constraints of -492 s =< ΔT =< 200 s in 1560 and 140 s =< ΔT =< 151 s in 1567 to satisfy Clavius' descriptions, considering the lunar limb profile and assuming Auwers' canonical R_Sun. This study constrains the R_Sun margin of 1567, utilising three scenarios to interpret Clavius' account. The local totality requires an upper R_Sun limit of 1567 as R_Sun =< 696200 km in absolute size (959.92" in angular size), indicating no linear secular R_Sun shrinkage but possible R_Sun oscillations on a centennial timescale. Conversely, the annularity scenario is considered unlikely because it requires an R_Sun decrease of 7.5" within 3 centuries, even beyond the capacity of extreme shrinking-Sun hypotheses.

astro-ph.SR↗

arXiv2026

Analyses on Wassenius' Report for Total Solar Eclipse in 1733: Quantifications of the Solar Radius and the Earliest Reported Prominences

Total solar eclipses (TSEs) offer a unique opportunity to observe the solar atmosphere, detect limb phenomena, and accurately measure the solar radius. Following the TSE in 1733, Wassenius first reported the existence of prominences to the scientific community. Wassenius' original manuscript is held in the Royal Academy Archives of Sweden; this study translates his report and documents the associated source materials and local eclipse visibility. The solar radius (R_Sun) during the TSE in 1733 are 696250 +/- 170 km and 959.99 +/- 0.24" in the absolute and apparent scales, respectively. This result contrasts with the modern standard (helioseismic) R_Sun of 695780 +/- 160 km and 959.34 +/- 0.22"; however, it is consistent with the solar radius recorded in 1715. The observed prominences are located at +23.5 +/- 22.5°, +66.5 +/- 22.5°, and -68.5 +/- 22.5° in the heliographic latitude. The appearance of prominences at such high latitudes contrasts with the sunspot butterfly diagram for 1725-1750, confirming 1733 as a solar minimum. These high-latitude prominences can potentially be attributed to the so-called 'polar rush' prominences that appear a few years after a solar minimum. If they are categorised as 'polar rush' prominences, the solar minimum must be re-dated to before 1733 May. Furthermore, the latitudes of at least two of the prominences reported by Wassenius enable their classification as quiescent prominences, suggesting the presence of a polarity inversion line in the polar regions in early 1733.

astro-ph.SR↗

arXiv2026

Variable Earth's Rotation Speed in the 14th to 16th Centuries: New ΔT Constraints from Chinese Eclipse Records

Total solar eclipses are not only astronomical spectacles but also great astrophysical laboratories. Their historical records are particularly helpful for assessing the past variability of the Earth's rotation speed. Chinese records played a key role for such analyses. However, Chinese eclipse records from the Míng period have not been used for ΔT reconstructions, partially because most of the contemporaneous eclipse reports are found not in official histories but in local treatises. This study examines eclipse records in the (quasi-)contemporaneous local treatises, concentrating on what explicitly mentioned eclipse totality on the day of a total solar eclipse and what were compiled during the Míng Dynasty. On their basis, our study revised the ΔT constraint in 1361 to -408 s =< ΔT =< 601 s and set new ΔT constraints of 277 s =< ΔT =< 890 s in 1514, -328 s =< ΔT =< 332 s in 1542, and -1762 s =< ΔT =< 1091 s in 1575, respectively. We also revised most of the existing ΔT constraints in the 14th to 16th centuries, using the ephemeris data of the NASA JPL DE 441. Overall, our ΔT constraints generally tighten the ΔT variations more than what M+21 fit for their ΔT spline curve, requiring downward modification and upward modifications for the ΔT reconstructions around 1361 and 1542, respectively. Our results suggest that the ΔT decrease between 1514 and 1567 was slightly steeper than previously considered.

astro-ph.EP↗

arXiv2026

The First Four Ground-Level Enhancements in the 1940s: Investigation, Digitisation, and Analysis of Forgotten Data

Intense solar eruptions occasionally accelerate solar energetic particles (SEPs) and can trigger ground-level enhancements (GLEs). Among the 77 known GLEs, the first four GLEs, #1 -- 4 in the 1940s took place before the advent of the standard neutron monitors and were missing from the International GLE Database. This data gap challenged their quantification. To overcome this difficulty, we systematically gathered, digitised, and quantified contemporaneous cosmic-ray records pertaining to these GLEs. These data allow us to reconstruct the temporal evolution, with the 1 -- 15 min resolutions, of these GLEs, and broaden their geographical coverage to a global scale. GLEs #1 and #3 exhibited gradual increases in their rise times, measured at 45 +/- 15 and 105 +/- 15 min, respectively. In contrast, GLEs #2 and #4 both exhibited abrupt increases of 15 +/- 15 min. We also compared integral ionisation increase on the standard ionisation chambers and their local geomagnetic cutoff rigidities Pc to qualitatively compare these GLE's spectral hardness: Our result indicates that their spectra are extremely hard for GLEs #2 and #4 and mildly hard for GLEs #1 and #3. GLE #3 showed the greatest integral ionisations for polar detectors among them.

astro-ph.SR↗

arXiv2025

The long-term solar variability, as reconstructed from historical sources: Several case studies in the 17th -- 18th centuries

On a centennial timescale, solar activity was quantified based on records of instrumental sunspot observations. This article briefly discusses several aspects of the recent archival investigations of historical sunspot records in the 17th to 18th centuries. This article also reviews the recent updates for the active day fraction and positions of the reported sunspot groups of the Maunder Minimum to show their significance within the observational history. These archival investigations serve as base datasets for reconstructing solar activity.

astro-ph.SR↗

arXiv2024

The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman-Silverman storm) apparently originated from a complex active region of moderate area (\approx 500 μsh) that was favorably situated near disk center (S19° E05°). There is circumstantial evidence for an eruption from this region at 9--10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of \approx29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at \approx14:27 UT and allow a minimum Dst estimate of £ -834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19°.9 in magnetic latitude (MLAT) and 24°. 2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below |20|° for the 1872 storm (ranging from |10°. 0|--|19°. 9| MLAT) versus one each for the 1859 storm (|17°. 3| MLAT) and the 1921 storm (|16.°2| MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25°.1 \pm 0°.5 ILAT and -949 \pm 31 nT) and 1921 May (27°.1 ILAT and -907 \pm 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.

astro-ph.SR↗

arXiv2024

The Solar and Geomagnetic Storms in May 2024: A Flash Data Report

In May 2024, the scientific community observed intense solar eruptions that resulted in a great geomagnetic storm and auroral extension, highlighting the need to document and quantify these events. This study mainly focuses on their quantification. The source active region (AR 13664) evolved from 113 to 2761 millionths of the solar hemisphere between 4 May and 14 May. AR 13664's magnetic free energy surpassed 10^33 erg on 7 May, triggering 12 X-class flares on 8 -- 15 May. Multiple interplanetary coronal mass ejections (ICMEs) were produced from this AR, accelerating solar energetic particles toward Earth. According to satellite and interplanetary scintillation data, at least 4 ICMEs erupted from 13664 eventually overcoming each other and combining. The shock arrival at 17:05 UT on 10 May significantly compressed the magnetosphere down to ~ 5.04 RE, and triggered a deep Forbush Decrease. GOES satellite data and ground-based neutron monitors confirmed a ground-level enhancement from 2 UT to 10 UT on 11 May 2024. The ICMEs induced exceptional geomagnetic storms, peaking at a Dst index of -412 nT at 2 UT on 11 May, marking the sixth-largest storm since 1957. The AE and AL indices showed great auroral extensions that located the AE/AL stations into the polar cap. We gathered auroral records at that time and reconstructed the equatorward boundary of the visual auroral oval to 29.8° invariant latitude. We compared naked-eye and camera auroral visibility, providing critical caveats on their difference. We also confirmed global enhancements of storm-enhanced density of the ionosphere.

astro-ph.SR↗

arXiv2023

Digitization of Weather Records of Seungjeongwon Ilgi: A Historical Weather Dynamics Dataset of the Korean Peninsula in 1623-1910

Historical weather records from Europe indicate that the Earth experienced substantial climate variability, which caused, for instance, the Little Ice Age and the global crisis in the period between the 14th and 19th centuries. However, it is still unclear how global this climate variability was because of the scarce meteorological data availability in other regions including East Asia, especially around the 17th century. In this context, Seungjeongwon Ilgi, a daily record of the Royal Secretariat of the Joseon Dynasty of Korea, is a precious source of historical meteorological records for the Korean Peninsula, as it covers 288 years of weather observations made during 1623-1910. We used the digital database of Seungjeongwon Ilgi to construct a machine-readable weather condition dataset. To this end, we extracted valid weather information from the original weather description text and compiled them into predefined weather categories. Additionally, we attempted to improve the usability of the dataset by converting the reported dates in the traditional calendar system to those in the Gregorian calendar. Finally, we outlined the promising implications of this dataset for meteorological and climatological studies, while describing the limitations of the dataset. Overall, future studies focusing on the climate and weather of the past could use this meteorological database for investigating long-term climate variability. Our datasets are publicly available at 10.5281/zenodo.8142701.

physics.ao-ph↗

arXiv2023

Relationship of peak fluxes of solar radio bursts and X-ray class of solar flares: Application to early great solar flares

Large solar flares occasionally trigger significant space-weather disturbances that affect the technological infrastructures of modern civilization, and therefore require further investigation. Although these solar flares have been monitored by satellite observations since the 1970s, large solar flares occur only infrequently and restrict systematic statistical research owing to data limitations. However, Toyokawa Observatory has operated solar radio observations at low frequencies (at 3.75 and 9.4 GHz) since 1951 and captured the early great flares as solar radio bursts. To estimate the magnitudes of flares that occurred before the start of solar X-ray (SXR) observations with the Geostationary Operational Environmental Satellite (GOES) satellites, we show the relationship between microwave fluxes at 3.75 and 9.4 GHz and X-ray fluxes of flares that occurred after 1988. In total, we explored 341 solar flares observed with the Nobeyama Radio Polarimeters and Toyokawa Observatory from 1988-2014 and compared them with the SXR observations recorded by the GOES satellites. The correlation coefficient was approximately 0.7. Therefore, the GOES X-ray class can be estimated from the peak flux at 3.75 and 9.4 GHz with a large variance and an error of factor of 3 (1 sigma). Thus, for the first time, we quantitatively estimated the light curves of two early solar flares observed in 1956 February by the Toyokawa solar radio observations using the relationship between SXR thermal radiation and microwave nonthermal radiation (Neupert, 1968, ApJ, 153, 59).

astro-ph.SR↗

arXiv2022

Scale Transfer in 1849 : Heinrich Schwabe to Rudolf Wolf

The focus of this study is to reveal the reason behind a scale problem detected around 1849 in the historical version of the International Sunspot Number Series, i.e. version 1 (Leussu et al, Astronomy and Astrophysics, 559, A28, 2013; Friedli, Solar Phys.291, 2505, 2016). From 1826 to 1848 Heinrich Schwabe's observations were considered primary by Rudolf Wolf, and a shift of primary observer from Schwabe to Wolf in 1849 seems to have led to an inconsistency in the Sunspot Number series. In this study we benefited from various datasets, the most important being Schwabe's raw counts from the Mittheilungen (Prof. Wolf's Journals) that have been digitised at the Royal Observatory of Belgium between 2017 and 2019. We provide a robust quantification of the detected problem by using classic algebraic calculations but also different methods such as a method inspired by Lockwood et al (Journal of Geophysical Research (Space Physics), 119(7), 5172, 2014), hence assigning a modern k-factor to Schwabe's observations before 1849. We also assess the implications of this 1849 inconsistency on the International Sunspot Number series (Versions 1 and 2) before and after 1849.

astro-ph.SR↗

arXiv2021

A Review for Japanese auroral records on the three extreme space weather events around the International Geophysical Year (1957 -- 1958)

Solar Cycle 19 was probably the greatest solar cycle over the last four centuries and significantly disrupted the solar-terrestrial environments with a number of solar eruptions and resultant geomagnetic storms. At its peak, the International Geophysical Year (IGY: 1957 -- 1958) was organised by international collaborations and benefitted scientific developments, capturing multiple unique extreme space weather events including the third and fourth greatest geomagnetic storms in the space age. In this article, we review and analyse original records of Japanese auroral observations around the IGY. These observations were organised by Masaaki Huruhata in collaboration with professional observatories and citizen contributors. We have digitised and documented these source documents, which comprise significant auroral displays in March 1957 (minimum Dst = -255 nT), September 1957 (minimum Dst = -427 nT), and February 1958 (minimum Dst = -426 nT). These records allow us to visualise temporal and spatial evolutions of these auroral displays, reconstruct their equatorward auroral boundaries down to 41.4°, 38.3°, and 33.3° in invariant latitudes, and contextualise their occurrences following contemporary geomagnetic disturbances. Our results have been compared with significant auroral displays during other extreme space weather events. These aurorae generally showed reddish colourations occasionally with yellowish rays. Their colourations are attributed to reddish oxygen emission and its mixture with greenish oxygen emission. Overall, these archival records provide the references for future discussions on the auroral activities during the uniquely intense and extreme space weather events.

astro-ph.SR↗

arXiv2021

The current state and future directions of modeling thermosphere density enhancements during extreme magnetic storms

Satellites, crewed spacecraft and stations in low-Earth orbit (LEO) are very sensitive to atmospheric drag. A satellite's lifetime and orbital tracking become increasingly inaccurate or uncertain during magnetic storms. Given the planned increase of government and private satellite presence in LEO, the need for accurate density predictions for collision avoidance and lifetime optimization, particularly during extreme events, has become an urgent matter and requires comprehensive international collaboration. Additionally, long-term solar activity models and historical data suggest that solar activity will significantly increase in the following years and decades. In this article, we briefly summarize the main achievements in the research of thermosphere response to extreme magnetic storms occurring particularly after the launching of many satellites with state-of-the-art accelerometers from which high-accuracy density can be determined. We find that the performance of an empirical model with data assimilation is higher than its performance without data assimilation during all extreme storm phases. We discuss how forecasting models can be improved by looking into two directions: first, to the past, by adapting historical extreme storm datasets for density predictions, and second, to the future, by facilitating the assimilation of large-scale thermosphere data sets that will be collected in future events. Therefore, this topic is relevant to the scientific community, government agencies that operate satellites, and the private sector with assets operating in LEO.

physics.space-ph↗

arXiv2021

Temporal Variations of the Three Geomagnetic Field Components at Colaba Observatory around the Carrington Storm in 1859

The Carrington storm in 1859 September has been arguably identified as the greatest geomagnetic storm ever recorded. However, its exact magnitude and chronology remain controversial, while their source data have been derived from the Colaba H magnetometer. Here, we have located the Colaba 1859 yearbook, containing hourly measurements and spot measurements. We have reconstructed the Colaba geomagnetic disturbances in the horizontal component (ΔH), the eastward component (ΔY), and the vertical component (ΔZ) around the time of the Carrington storm. On their basis, we have chronologically revised the ICME transit time as =< 17.1 hrs and located the ΔH peak at 06:20 -- 06:25 UT, revealing a magnitude discrepancy between the hourly and spot measurements (-1691 nT vs. -1263 nT). Furthermore, we have newly derived the time series of ΔY and ΔZ, which peaked at ΔY ~ 378 nT (05:50 UT) and 377 nT (06:25 UT), and ΔZ ~ -173 nT (06:40 UT). We have also computed the hourly averages and removed the solar quiet (Sq) field variations from each geomagnetic component to derive their hourly variations with latitudinal weighting. Our calculations have resulted in the disturbance variations (Dist) with latitudinal weighting of Dist Y ~ 328 nT and Dist Z ~ -36 nT, and three scenarios of Dist H ~ -918, -979, and -949 nT, which also approximate the minimum Dst. These data may suggest preconditioning of the geomagnetic field after the August storm (ΔH =< -570 nT), which made the September storm even more geoeffective.

astro-ph.SR↗

arXiv2021

Analyses of a Datable Solar Eclipse Record in Maya Classic Period Monumental Inscriptions

Historical records of total solar eclipses provide vital information for computing the rotation of the Earth and understanding its long-term variations by providing data from the time before the modern measurements. While eclipses recorded around Eurasia and North Africa for millennia have been subjected to consideration in this context, eclipse records in the American continents have received little attention. In this study, we analysed the solitary observational record for a solar eclipse conducted by the ancient Maya on 16 July 790 in Julian calendar, recorded on the Stela 3 of Santa Elena Poco Uinic (N16°35', W91°44'). This stela has an eclipse glyph and is associated with a total solar eclipse. Taking the up-to-date Earth rotation (ΔT) rate into account, our calculations locate this site slightly out of the totality path. The visibility of the total solar eclipse from Santa Elena Poco Uinic would require ΔT: 4074 s < ΔT < 4873 s. In comparison with the contemporary eclipse records, this yields a short-term increase in ΔT >= 800 s between 761 and 790 and a decrease in ΔT >= 300 s till to 873. Therefore, the total solar eclipse on 16 July 790 cannot be expected to have been visible from Santa Elena Poco Uinic, unlike what has been previously considered. We conclude that this stela probably records a partial solar eclipse of great magnitude (~ 0.946) visible under favourable meteorological conditions or is based on hearsay from the southern coastal area.

astro-ph.EP↗

arXiv2021

Stephan Prantner's Sunspot Observations during the Dalton Minimum

In addition to regular Schwabe cycles (~ 11 years), solar activity also shows longer periods of enhanced or reduced activity. Of these, reconstructions of the Dalton Minimum provide controversial sunspot group numbers and limited sunspot positions, partially due to limited source record accessibility. We analysed Stephan Prantner's sunspot observations from 1804--1844, the values of which had only been known through estimates despite their notable chronological coverage during the Dalton Minimum. We identified his original manuscript in Stiftsarchiv Wilten, near Innsbruck, Austria. We reviewed his biography (1782--1873) and located his observational sites at Wilten and Waidring, which housed the principal telescopes for his early and late observations: a 3.5-inch astronomical telescope and a Reichenbach 4-feet achromatic erecting telescope, respectively. We identified 215 days of datable sunspot observations, which are twice as much data as his estimated data in the existing database (= 115 days). Prantner counted up to 7--9 sunspot groups per day and measured sunspot positions, which show their distributions in both solar hemispheres. These results strikingly emphasise the difference between the Dalton Minimum and the Maunder Minimum as well as the similarity between the Dalton Minimum and the modern solar cycles.

astro-ph.SR↗

arXiv2021

Daniel Mögling's sunspot observations in 1626 - 1629: A manuscript reference for the solar activity before the Maunder Minimum

The sunspot groups have been observed since 1610 and their numbers have been used for evaluating the amplitude of solar activity. Daniel Mögling recorded his sunspot observations for more than 100 days in 1626 - 1629 and formed a significant dataset of sunspot records before the Maunder Minimum. Here, we have analysed his original manuscripts in the Universitäts- und Landesbibliothek Darmstadt (ULBD) to review Mögling's personal profile and observational instruments and derive number and positions of the sunspot groups. In his manuscript, we have identified 134 days with an exact sunspot group number and 3 days of additional descriptions. Our analyses have completely revised their observational dates and group number, added 19 days of hitherto overlooked observations, and removed 8 days of misinterpreted observations. We have also revisited sunspot observations of Schickard and Hortensius and revised their data. These results have been compared with the contemporary observations. Moreover, we have derived the sunspot positions from his sunspot drawings and located them at 2°-23° in the heliographic latitude in both solar hemispheres. Contextualised with contemporary observations, these results indicate their temporal migration to lower heliographic latitudes and emphasise its location in the declining phase of Solar Cycle -12 in the 1620s. His observations were probably conducted using a pinhole and camera obscura, which made Mögling likely underestimate the sunspot group number by >~ 33% - 52 %. This underestimation should be noted upon their comparison with the modern datasets.

astro-ph.SR↗