arXiv subjects

Sachiko Akiyama

Publications and source records attributed to Sachiko Akiyama.

At least 19 recordsLinked to original sources

arXiv2026

A study of the kinematic and volumetric co-evolution of Earth-directed CMEs

While flare-associated CMEs generally show a strong association between flare X-ray flux and CME kinematics, their volumetric evolution and its link to both kinematics and flare activity remains less explored. In this study, we investigate the volumetric and kinematic co-evolution of ten Earth-directed, flare-associated CMEs using multi-viewpoint observations from STEREO-A, STEREO-B, and SOHO. We perform 3D reconstructions of the CME flux ropes with the Graduated Cylindrical Shell (GCS) model and derive their geometrical parameters. We find that the total CME volume follows a power-law dependence on the leading edge height, and that different structural components expand at different rates, with the ellipsoidal front expanding faster than the conical legs. Furthermore, the volumetric evolution follows a multi-phase pattern: initial overexpansion, a gradual reduction in the expansion rate, and finally saturation at a higher heliocentric distance. This is similar to the well-established three-phase evolution of the CME kinematics. Notably, the second-order derivative of volume with time shows a strong temporal correlation with both CME acceleration and the GOES soft X-ray flux of the associated flare. This is the first study to report such a correspondence between volumetric evolution and flare timing, highlighting the role of flare energy release in governing CME expansion dynamics. Our findings motivate further studies into the coupling between magnetic reconnection and CME volumetric evolution in the corona.

astro-ph.SR↗

arXiv2026

Solar Cycle Variation of Sustained Gamma Ray Emission from the Sun

We investigated the occurrence rate of the sustained gamma ray emission (SGRE) events from the Sun using data obtained by Fermi Large Area Telescope (LAT) since its launch in 2008. Only 16 SGRE events were observed during the first 61 months of solar cycle (SC) 25, likely due to the solar array drive assembly's malfunction in 2018; 27 SGRE events were observed in SC 24 over the corresponding epoch. The average sunspot number (SSN) increased from 56.9 in SC 24 to 79.0 in SC 25. Fast and wide (FW) CMEs and decameter-hectometric (DH) type II bursts increased significantly in SC 25 by 29% and 33%, respectively when normalized to SSN. Therefore, we expect a higher number of SGREs in SC 25. We estimated the number of SGREs in SC 25 using three methods. (i) If the SGRE number varies commensurate with SSN, we should have 38 SGRE events in SC 25. However, FW CMEs and DH type II bursts in SC 25 were overabundant by 29% and 33%, so the number SGRE events should be 48 or 50. (ii) In SC 24, ~18% of FW CMEs and 27% of DH type II bursts were associated with SGRE events. At this rate SC 25 should have 48 and 49 SGRE events. (iii) Since SGRE events are invariably associated with >100 keV hard X-ray (HXR) bursts, we identified DH type II bursts associated with >100 keV HXR bursts from Fermi's Gamma ray Burst Monitor (GBM) during LAT data gaps. Almost all SGRE events in SCs 24 and 25, and 27 of the 79 LAT-gap type IIs were associated with HXR bursts of duration > ~5 min. These DH type II bursts are indicative of SGRE, bringing the total number of SGRE events to 43 (16 + 27). Thus, the three methods provide similar estimates of the number of SGRE events in SC 25. We, therefore, conclude that SC 25 is stronger than SC 24 based on the estimated number SGRE events.

astro-ph.HE↗

arXiv2026

Unusual Circumstances of the 2024 June 8 GLE

Ground Level Enhancement (GLE) in large solar energetic particle (SEP) events is indicative of protons accelerated to GeV energies. Almost all GLE events are associated with sustained gamma-ray emission (SGRE) from the Sun because the latter require >300 MeV protons that are readily present during GLEs. Here we report on the 2024 June 8 GLE event, which has the distinction of not being associated with an SGRE event. All the associated phenomena typical of SGRE events were present: a fast and wide CME, a major solar flare, and an intense type II radio bursts that extend from the metric to kilometric wavelength domains. There was a data gap of ~51 min, but the SGRE is expected to last for hours. We suggest the east-west asymmetry in the flow of energetic particles from the shock is likely to be the reason for the lack of SGRE emission.

astro-ph.SR↗

arXiv2025

Role of CME clusters and CME-CME interactions in producing sustained $γ$-ray emission

Fast (V$_{\rm CME}$>1000${\rm \,km\,s^{-1}}$) coronal mass ejections (CMEs) capable of accelerating protons beyond 300MeV are thought to trigger hours-long sustained $γ$-ray emission (SGRE) after the impulsive flare phase. Meanwhile, CME-CME interactions can cause enhanced proton acceleration, increasing the fluxes of solar energetic particles. This study explores the role of fast CME interactions in SGRE production during CME clusters, which we define as a series of CMEs linked to >C-class flares with waiting times <$\,$1$\,$day from the same active region (AR). We focus on clusters in major CME-productive ARs (major ARs), by defining a major AR as one that produced >$\,$1 CME-associated major (>M-class) flare. The study identified 76 major ARs between 2011 and 2019, of which 12 produced all SGRE events. SGRE-producing ARs exhibit higher median values for the speed of their fastest CMEs (2013 vs. 775${\rm \,km\,s^{-1}}$) and the class of their strongest flares (X1.8 vs. M5.8), compared to SGRE-lacking ARs. They also produced relatively faster CMEs (median speed: 1418 vs. 1206.5${\rm \,km\,s^{-1}}$), with the SGRE-associated CMEs occurring during periods of higher CME rates than typical fast CME epochs. Twelve of 22 (54.5%) SGRE events and 5 of 7 (71.4%) long-duration (>$10\,$h) SGRE events occurred during CME clusters, with high chances of CME-CME interactions. A case study on very active major ARs showed that all SGRE-associated CMEs with V$_{\rm CME}\lesssim$ 2000${\rm \,km\,s^{-1}}$ underwent CME-CME interactions within 10$\,$R$_\odot$, while SGRE-associated CMEs faster than 3000${\rm \,km\,s^{-1}}$ did not undergo interactions.

astro-ph.SR↗

arXiv2025

Multispacecraft Observations of the 2024 September 9 Backside Solar Eruption that Resulted in a Sustained Gamma Ray Emission Event

We report on the 2024 September 9 sustained gamma ray emission (SGRE) event observed by the Large Area Telescope (LAT) onboard the Fermi satellite. The event was associated with a backside solar eruption observed by multiple spacecraft such as the Solar and Heliospheric Observatory (SOHO), Solar Terrestrial Relations Observatory (STEREO), Parker Solar Probe (PSP), Solar Orbiter (SolO), Solar Dynamics Observatory (SDO), Wind, and GOES, and by ground based radio telescopes. Fermi LAT observed the SGRE after the EUV wave from the backside eruption crossed the limb to the frontside of the Sun. SolO's Spectrometer Telescope for Imaging X rays (STIX) imaged an intense (X3.3) flare, which occurred about 41 deg behind the east limb, from heliographic coordinates S13E131. Forward modeling of the CME flux rope revealed that it impulsively accelerated (3.54 km/s/s) to attain a peak speed of 2162 km/s. SolO's energetic particle detectors (EPD) observed protons up to about 1 GeV from the extended shock and electrons that produced a complex type II burst and possibly type III bursts. The durations of SGRE and type II burst are consistent with the linear relation between these quantities obtained from longer duration (>3 hours) SGRE events. All these observations are consistent with an extended shock surrounding the CME flux rope, which is the likely source of high energy protons required for the SGRE event. We compare this event with six other BTL SGRE eruptions and find that they are all consistent with energetic shock driving CMEs. We also find a significant east west asymmetry (3:1) in the BTL source locations.

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arXiv2025

Relationship between prominence eruptions and coronal mass ejections during solar cycle 24

In this article, we present the relationship between prominence eruptions (PEs) and coronal mass ejections (CMEs) from May 2010 to December 2019 covering most of solar cycle 24. We used data from the Atmospheric Imaging Assembly (AIA) for PEs and the Large Angle and Spectrometric Coronagraph (LASCO) for CMEs. We identified 1225 PEs, with 67% being radial, 32% transverse, and 1% failed PEs. The radial, transverse PEs, and the combined set have average speeds of ~53, 9, and 38 km/s, respectively. The PE association with CMEs is examined by assigning a confidence level (CL) from 0 (no association) to 3 (clear association). Out of 1225 PEs, 662 (54%) are found to be associated to CMEs including CL 1, 2, and 3. Our study reveals that the spatial and temporal relationships between PEs and CMEs vary over the solar cycle. During solar minima, CMEs tend to deflect towards the equator, possibly due to a stronger polar field. Temporal offsets are larger during solar maxima and smaller during the minima. This implies that the PEs appear in LASCO C2 FOV earlier during the minima than during the maxima. Among the 662 CMEs associated with PEs, 78% show clear bright core structures. Investigation of the morphological and temporal behavior of these CMEs indicate that the prominences evolves into CME cores at higher altitudes suggesting that PEs and CME cores are the same structure. The average speeds of the PEs, CME core, and CME leading edge are 62, 390, and 525 km/s, respectively. The speed of CME cores are more than the speed of PEs because the former are observed at larger heights where they have accelerated to higher speeds.

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arXiv2025

The Longest Duration SGRE Event in Solar Cycle 25

Solar Cycle (SC) 24 was the weakest in the space age, yet it produced many sustained gamma ray emission (SGRE) events from the Sun. Solar cycle (SC) 25, which is a bit stronger than SC 24 observed only a handful of SGRE events over the first five years. Here we report on the 2024 September 14 SGRE event, which has the longest duration (\~11.29 hrs) as of this writing. The associated type II radio burst is also of long duration (\~16 hr). Detailed analysis of the SGRE event reveals that the event is in good agreement with the linear relation of the SGRE duration with the ending frequency and duration of the type II burst. The kinematics of the associated coronal mass ejection (CME) shows that it is one of the fastest CMEs of SC 25, capable of driving a shock that accelerated >300 MeV protons to account for the observed SGRE. By comparing with an event with similar durations in SC 24, we find that it had a lower-speed CME but resulted in a larger-sized SGRE event. We speculate that the difference may be due to the change in the heliospheric state between the two cycles.

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arXiv2025

Third Harmonic Structure in an Interplanetary Type II Radio Burst and Other Energetic Phenomena During the 2024 September 14 Solar Eruption

We report on the observation of first, second, and third harmonic components during an interplanetary (IP) type II solar radio burst observed on 2024 September 14 by the radio instruments on board Wind, the Solar Terrestrial Relations Observatory (STEREO), and the Parker Solar Probe. The eruption resulted in an ultrafast coronal mass ejection (CME) that had a sky plane speed of \~2366 km per sec, and an X4.5 flare from NOAA active region 13825 (S15E56). Also observed were a large solar energetic particle (SEP) event and a sustained gamma ray emission (SGRE) event. The IP type II burst consists of multiple features. The first, second, and third harmonic bursts are smooth and diffuse with additional patchy bursts superposed only on the fundamental component. The existence of fundamental harmonic structure including the third harmonic can be readily explained by the coherent plasma emission mechanism and works against the possibility of synchrotron mechanism.

astro-ph.SR↗

arXiv2025

The SOHO LASCO CME Catalog -- Version 2

This paper provides an update on the coronal mass ejection (CME) catalog maintained at the CDAW Data Center, NASA Goddard Space Flight Center (https://cdaw.gsfc.nasa.gov/CME_list). This is version 2 (v2) of the Catalog that has been made as the default version as of May 1, 2024. The new features of the Catalog v2 are (i) online measurement tool, (ii) combination JavaScript movies from the STEREO and Solar Dynamics Observatory (SDO) missions, and (iii) insertion of newly identified CMEs for the period 1996 to 2004. The CME identification was revisited resulting in a set of $\sim$3000 new CMEs added to the Catalog. A vast majority of these CMEs are weak and narrow. The resulting statistical properties of CMEs are not significantly different from those reported using version 1.

astro-ph.SR↗

arXiv2025

Implications of the abundance of halo coronal mass ejections for the strength of solar cycle 25

We assess the relative strength of solar cycle (SC) 25 with respect to SCs 23 and 24 based on the abundance of halo coronal mass ejections (CMEs). We make use of the halo CME database (https://cdaw.gsfc.nasa.gov/CME_list/halo/halo.html) to compare the halo CME abundance during the first four years in each of SCs 23 to 25. The main result is that in several aspects such as the abundance, occurrence rate, source locations, and halo heights, halo CMEs are similar between SCs 24 and 25 but different from SC 23. This result follows from the fact that weaker cycles have low heliospheric total pressure, whose backreaction on CMEs allows them to expand more and hence enhancing the chance of becoming a halo. The solar cycle variation of halo CME properties is consistent with the precursor-based cycle prediction methods that indicate SC 25 is similar to or only slightly stronger than SC 24.

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arXiv2025

Solar Cycle Variation of Axial Orientations and Favorable Locations of Eruptive MFRs

Using multi-viewpoint observations from STEREO and SOHO during three solar cycles from 23 to 25, we study the magnetic flux rope (MFR) structures of coronal mass ejections (CMEs) near the Sun and magnetic clouds (MCs) at 1au. The study aims to investigate two phenomena: 1) the occurrence rate of CMEs near Hale sector boundaries (HBs) and 2) solar-cycle variation of MFR axial orientations in CMEs and MCs. Our preliminary results include: 1) the axes of MFRs in cycle 25 present a systematic northward orientation, which is the same as in cycle 23 but opposite to cycle 24; 2) the majority of the MFRs occurred near HBs (within 30 degrees) and some exceptional events occurred at non-HBs; 3) the axial fields in MCs present a similar north-south orientation, which changes from cycle to cycle. We discuss the implication of solar cycle variations of MFR axial orientations for space weather forecasts.

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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↗

arXiv2024

Novel scaling laws to derive spatially resolved flare and CME parameters from sun-as-a-star observables

Coronal mass ejections (CMEs) are often associated with X-ray (SXR) flares powered by magnetic reconnection in the low-corona, while the CME shocks in the upper corona and interplanetary (IP) space accelerate electrons often producing the type-II radio bursts. The CME and the reconnection event are part of the same energy release process as highlighted by the correlation between reconnection flux ($ϕ_{rec}$) that quantifies the strength of the released magnetic free energy during SXR flare, and the CME kinetic energy that drives the IP shocks leading to type-II bursts. Unlike the sun, these physical parameters cannot be directly inferred in stellar observations. Hence, scaling laws between unresolved sun-as-a-star observables, namely SXR luminosity ($L_X$) and type-II luminosity ($L_R$), and the physical properties of the associated dynamical events are crucial. Such scaling laws also provide insights into the interconnections between the particle acceleration processes across low-corona to IP space during solar-stellar 'flare- CME- type-II' events. Using long-term solar data in SXR to radio waveband, we derive a scaling law between two novel power metrics for the flare and CME-associated processes. The metrics of 'flare power' ($P_{flare}=\sqrt{L_Xϕ_{rec}}$) and 'CME power' ($P_{CME}= \sqrt{L_R {V_{CME}}^2}$), where $V_{CME}$ is the CME speed, scale as $P_{flare}\propto P_{CME}^{0.76 \pm 0.04}$. Besides, $L_X$ and $ϕ_{rec}$ show power-law trends with $P_{CME}$ with indices of 1.12$\pm$0.05 and 0.61$\pm$0.05 respectively. These power-laws help infer the spatially resolved physical parameters, $V_{CME}$ and $ϕ_{rec}$, from disk-averaged observables, $L_X$ and $L_R$ during solar-stellar 'flare- CME- type-II' events.

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arXiv2024

A catalog of multi-vantage point observations of type-II bursts: Statistics and correlations

Coronal mass ejection (CME) often produces a soft X-ray (SXR) flare associated with the low-coronal reconnection and a type-II radio burst associated with an interplanetary (IP) CME-shock. SXR flares and type-II bursts outshine the background emission, making them sun-as-a-star observables. Though there exist SXR flare catalogs covering decades of observations, they do not provide the associated type-II luminosity. Besides, since radio burst emission could be beamed, the observed flux dynamic spectrum may vary with line of sight. Using long-term calibrated decameter-hectometric dynamic spectra from the Wind and STEREO spacecraft, we build a catalog of multi-vantage point observations of type-II bursts. Cross-matching with existing catalogs we compile the properties of the associated flare, reconnection, and the CME. Cross-correlation analysis was done between various parameters. Two novel metrics of flare and CME power show a strong correlation revealing a link between particle acceleration strengths in the low-corona and IP space.

astro-ph.SR↗

arXiv2024

Inter-planetary type-IV solar radio bursts: A comprehensive catalog and statistical results

Decameter hectometric (DH; 1-14 MHz) type-IV radio bursts are produced by flare-accelerated electrons trapped in post-flare loops or the moving magnetic structures associated with the CMEs. From a space weather perspective, it is important to systematically compile these bursts, explore their spectro-temporal characteristics, and study the associated CMEs. We present a comprehensive catalog of DH type-IV bursts observed by the Radio and Plasma Wave Investigation (WAVES) instruments onboard Wind and STEREO spacecraft, covering the period of white-light CME observations by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the SOHO mission between November 1996 and May 2023. The catalog has 139 bursts, of which 73% are associated with a fast (>900 km/s) and wide (>60$^o$) CME, with a mean CME speed of 1301 km/s. All DH type-IV bursts are white-light CME-associated, with 78% of the events associated with halo CMEs. The CME source latitudes are within $\pm$45$^o$. 77 events had multi-vantage point observations from different spacecraft, letting us explore the impact of line of sight on the dynamic spectra. For 48 of the 77 events, there was good data from at least two spacecraft. We find that, unless occulted by nearby plasma structures, a type-IV burst is best viewed when observed within $\pm$60$^o$ line of sight. Also, the bursts with a duration above 120 min, have source longitudes within $\pm$60$^o$. Our inferences confirm the inherent directivity in the type-IV emission. Additionally, the catalog forms a sun-as-a-star DH type-IV burst database.

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arXiv2023

What do halo CMEs tell us about solar cycle 25?

It is known that the weak state of the heliosphere due to diminished solar activity in cycle 24 back-reacted on coronal mass ejections (CMEs) to make them appear wider for a given speed. One of the consequences of the weak state of the heliosphere is that more CMEs appear as halo CMEs (HCMEs), and halos are formed at shorter heliocentric distances. Current predictions for the strength of solar cycle (SC) 25 range from half to twice the strength of SC 24. We compare the HCME occurrence rate and other properties during the rise phase of cycles 23, 24, and 25 to weigh in on the strength of SC 25. We find that HCME and solar wind properties in SC 25 are intermediate between SCs 23 and 24, but closer to SC 24. The HCME occurrence rate, normalized to the sunspot number, is higher in SCs 24 and 25 than in SC 23. The solar wind total pressure in SC 25 is ~35% smaller than that in SC 23. Furthermore, the occurrence rates of high-energy solar energetic particle events and intense geomagnetic storms are well below the corresponding values in SC 23, but similar to those in SC 24. We conclude that cycle 25 is likely to be similar to or slightly stronger than cycle 24, in agreement with polar-field precursor methods for cycle 25 prediction

astro-ph.SR↗

arXiv2023

The Solar Cause of the 2022 February 3 Geomagnetic Storm that Led to the Demise of the Starlink Satellites

We report on the solar source of the 2022 February 3 geomagnetic storm of moderate strength that contributed to the loss of 39 Starlink satellites. The geomagnetic storm was caused by the 2022 January 29 halo coronal mass ejection (CME) that was of moderate speed (about 690 km/s) originating from NOAA active region 12936 located in the northeast quadrant (N18E06) of the Sun. The eruption was marked by an M1.1 flare, which started at 22:45 UT, peaked at 23:32 UT on January 29 and ended at 00:24 UT the next day. The CME ended up as a shock-driving magnetic cloud (MC) observed at Sun-Earth L1 and at STEREO-Ahead (STA) located about 34 deg behind Earth. The geomagnetic storm was caused by a strong southward component of the MC that was boosted by a high speed solar wind stream behind the MC. Even though Earth and STA were separated by only about 34 deg, the MC appeared quite different at Earth and L1. One possibility is that the MC was writhed reflecting the curved neutral line at the Sun. In-situ observations suggest that the MC was heading closer to STA than to Earth because of the earlier arrival at STA. However, the shock arrived at STA and Earth around the same time, suggesting a weaker shock at Earth due to flank passage.

astro-ph.SR↗

arXiv2022

Effect of the Heliospheric State on CME Evolution

The culmination of solar cycle 24 by the end of 2019 has created the opportunity to compare the differing properties of coronal mass ejections (CMEs) between two whole solar cycles: Solar cycle 23 (SC 23) and Solar cycle 24 (SC 24). We report on the width evolution of limb CMEs in SC 23 and 24 in order to test the suggestion by Gopalswamy et al. (2015a) that CME flux ropes attain pressure balance at larger heliocentric distances in SC 24. We measure CME width as a function of heliocentric distance for a significantly large number of limb CMEs (~1000) and determine the distances where the CMEs reach constant width in each cycle. We introduced a new parameter: the transition height (hc) of a CME defined as the critical heliocentric distance beyond which the CME width stabilizes to a quasi-constant value. Cycle and phase-to-phase comparisons are based on this new parameter. We find that the average value of hc in SC 24 is 62% higher than in SC 23. SC 24 CMEs attain their peak width at larger distances from the Sun as compared to SC 23 CMEs. The enhanced transition height in SC 24 is new observational ratification of the anomalous expansion. The anomalous expansion of SC 24 CMEs which is caused by the weak state of the heliosphere, accounts for the larger heliocentric distance where the pressure balance between CME flux rope and the ambient medium is attained.

astro-ph.SR↗