Kyung-Suk Cho

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2146 EUV brightenings observed with Solar Orbiter/Extreme Ultraviolet Imager and 300 EUV solar flares observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism, potentially associated with standing magnetohydrodynamic waves. Distinguishing between candidate wave modes, however, will require further observational and theoretical investigation.

Quasiperiodic disturbances in EUV emission intensity, propagating along polar plumes in the solar corona, are frequently observed. Analysis of these propagating disturbances in the 171 Å bandpass of the Solar Dynamics Observatory/Atmospheric Imaging Assembly reveals that the wave spectrum evolves with height. Near the footpoints, the dominant spectral peak corresponds to oscillation periods of 12–15 minutes. As the disturbances propagate upward, the shorter-period spectral component diminishes, while a longer-period component of 20–30 minutes becomes dominant. Theoretical modeling of coronal slow magnetoacoustic waves effectively reproduces this spectral evolution. The model incorporates field-aligned thermal conduction and the back reaction of wave-induced perturbations on local thermal equilibrium. Coronal slow waves, driven by a broadband source with a peak at a shorter period, show a decrease in shorter-period spectral power with height, while the 20–30 minute spectral power increases due to thermal overstability. Analysis of the dispersion relations reveals that, given the chosen model parameters, the most rapidly growing spectral component corresponds to a 22.5 minute period, which aligns well with the observational findings.

In near-Earth space, the interplanetary magnetic field (IMF) is generally characterized by the Parker spiral configuration with a spiral angle of approximately 45° clockwise from the radial axis of the Sun. However, spacecraft observations reveal that IMF lines can exhibit non-Parker spiral configurations with either a nearly radial (outward–inward), tangential (westward–eastward), or normal (northward–southward) direction defined in radial–tangential–normal coordinates on even hourly timescales. Using the IMF and solar wind data measured by the Wind spacecraft for the period 1995–2023, we present distinctive features in the occurrence and various solar wind parameters between these non-Parker spiral IMF configuration types. Our study reveals that (1) the occurrence of non-Parker spiral IMF configuration types has the Hale cycle (∼22 yr magnetic dynamo cycle) variations with their own phases depending on the sign of the major IMF components, (2) the solar wind in the radial/tangential IMF structure is deflected eastward/westward, which corresponds to the direction opposite to/same as the rotation direction of the Sun on the plane perpendicular to the line of sight, and (3) these IMF configuration types can be characterized by magnetic field topology, which is open (for radial IMF and tangential IMF) or closed (for normal IMF). Our findings therefore provide insights into the formation mechanisms of these non-Parker spiral IMF configurations observed in near-Earth space. Moreover, our results suggest that spacecraft missions to various areas in interplanetary space are needed to better understand spatiotemporal evolutions of the solar-originated magnetic field structure in the heliosphere.

Small-scale jets, such as chromospheric and transition region (TR) network jets, are of great interest regarding coronal heating and solar wind acceleration. Spectroscopic analysis based on multiple spectral lines with different formation temperatures is essential for understanding the physical properties and driving mechanisms of jets. Here, we conduct an investigation of the physical properties of a small-scale chromospheric jet in a quiet-Sun network region and its TR counterpart. This jet is recorded from formation to extinction using the Fast Imaging Solar Spectrograph at the Goode Solar Telescope and the Interface Region Imaging Spectrograph. The chromospheric component of the jet exhibits a high line-of-sight speed of up to 45 km s⁻¹ during its ascending phase, accompanied by spectral profiles akin to rapid blueshifted excursion and downflowing rapid redshifted excursion during the descending phase. Using a cloud model combined with a Multi-Layer Spectral Inversion, we quantify the jet’s temperature during its ascending phase, which starts at approximately 11,000 K and increases by only 1000 K over 1 minute, much smaller than a few 10⁴ K, the excess temperature expected in an ideal gas reconnection jet at an outflow speed of 45 km s⁻¹. The TR counterpart exhibits a Si iv 1394 Å line profile with a non-Gaussian shape, including a blueshifted component and a large nonthermal width. Our results suggest that if the jet is driven by magnetic reconnection in the chromosphere, the heat released by the reconnection may be mostly used to ionize the hydrogen rather than to increase the temperature so that the gas may appear almost isothermal.

Spacecraft equipped with solar telescopes are planned for deployment at various vantage points in the heliosphere to conduct coordinated, multi‐view observations of the Sun and its dynamic activities. We investigate solar visibility using imaging instruments aboard spacecraft stationed at the Sun‐Earth Lagrange points L1, L4 and L5. First, the optimal arrival time for vertical periodic orbits at L4 and L5 is determined on the basis of geometric factors that maximize the visibility of the solar poles and higher latitudes. For different orbits around L1, L4 and L5, we calculate the visibility of the solar surface (i.e., observation days per year) as a function of solar latitude. Additionally, we analyze how the solar limb observed from one Lagrange point aligns with the solar surface visible from the other two points, with a focus on studying solar eruptions like flares and coronal mass ejections. This analysis aims to assess the feasibility of coordinated observations of off‐disk erupting structures and their on‐disk magnetic footpoints. Furthermore, we evaluate the improvement in continuous tracking of solar features, such as sunspots, with multiple spacecraft in various orbital configurations. This tracking helps in studying the long‐term evolution of these features, from emergence to decay. A comprehensive comparison of observations from single (L1), double (L1 and L4), and multiple spacecraft (L1, L4 and L5) is conducted, aiding the design of future space missions involving solar observatories at the Sun‐Earth Lagrange points.

This study reports comprehensive observations for the G5-level geomagnetic storm that occurred from May 10 to 12, 2024, the most intense event since the 2003 Halloween storm. The storm was triggered by a series of coronal mass ejections (CMEs) originating from the merging of two active regions 13664/13668, which formed a large and complex photospheric magnetic configuration and produced X-class flares in early May 2024. Among the events, the most significant CME, driven by an X2.2 flare on May 9, caught up with and merged with a preceding slower CME associated with an X-class flare on May 8. These combined CMEs reached 1 AU simultaneously, resulting in an extreme geomagnetic storm. Geostationary satellite observations revealed changes in Earth’s magnetosphere due to solar wind impacts, increased fluxes of high-energy particles, and periodic magnetic field fluctuations accompanied by particle injections. Extreme geomagnetic storms resulting from the interaction of the solar wind with the Earth’s magnetosphere caused significant energy influx into Earth’s upper atmosphere over the polar regions, leading to thermospheric heating and changes in the global atmospheric composition and ionosphere. As part of this global disturbance, significant disruptions were also observed in the East Asian sector, including the Korean Peninsula. Ground-based observations show strong negative storm effects in the ionosphere, which are associated with thermospheric heating and resulting in decreases in the oxygen-to-nitrogen ratio (O/N2) in high-latitude regions. Global responses of storm-time prompt penetration electric fields were also observed from magnetometers over the East-Asian longitudinal sector. We also briefly report storm-time responses of aurora and cosmic rays using all-sky cameras and neutron monitors operated by the Korea Astronomy and Space Science Institute (KASI). The extensive observations of the G5-level storm offer crucial insights into Sun-Earth interactions during extreme space weather events and may help establish better preparation for future space weather challenges.

Beyond the Sun-Earth line, spacecraft equipped with various solar telescopes are intended to be deployed at several different vantage points in the heliosphere to carry out coordinated, multi-view observations of the Sun and its dynamic activities. In this context, we investigate solar visibility by imaging instruments onboard the spacecraft orbiting the Sun-Earth Lagrange points L1, L4 and L5, respectively. An optimal arrival time for vertical periodic orbits stationed at L4 and L5 is determined based on geometric considerations that ensure maximum visibility of solar poles or higher latitudes per year. For a different set of orbits around the three Lagrange points (L1, L4 and L5), we calculate the visibility of the solar surface (i.e., observation days per year) as a function of the solar latitude. We also analyze where the solar limb viewed from one of the three Sun-Earth Lagrange points under consideration is projected onto the solar surface visible to the other two. This analysis particularly aims at determining the feasibility of studying solar eruptions, such as flares and coronal mass ejections, with coordinated observations of off-limb erupting coronal structures and their on-disk magnetic footpoints. In addition, visibility analysis of a feature (such as sunspots) on the solar surface is made for multiple spacecraft in various types of orbits with different inclinations to quantify the improvement in continuous tracking of the target feature for studying its long-term evolution from emergence, growth and to decay. A comprehensive comparison of observations from single (L1), double (L1 and L4) and multi-space missions (L1, L4 and L5) is carried out through our solar visibility analysis, and this may help us to design future space missions of constructing multiple solar observatories at the Sun-Earth Lagrange points.

Alfvén-wave turbulence is known as a plasma heating mechanism associated with the acceleration of fast solar wind, found emanating from open magnetic fields adjacent to coronal holes. In this study, we expand the scope of this mechanism to investigate the thermodynamics of Alfvénic slow solar wind, a phenomenon originating from open fields near a streamer, as observed in recent inner heliospheric missions. We demonstrate a one-dimensional two-fluid model that incorporates three components: (1) low-frequency Alfvén-wave turbulence, serving as the primary dissipation mechanism, (2) a curved magnetic field that reproduces the streamer’s boundary, and (3) the kinetic instabilities to address proton temperature anisotropy. Our findings suggest that this dissipation mechanism can be applied in common to both fast and Alfvénic slow solar winds. We identify the proton-cyclotron instability near the Sun and the oblique and parallel firehose instabilities occurring close to 1 au as crucial factors governing temperature anisotropy. This study contributes to our understanding of the complex thermodynamics of solar winds and provides valuable insights for future space missions.

The Korean heliospheric community, led by the Korea Astronomy and Space Science Institute (KASI), is currently assessing the viability of deploying a spacecraft at the Sun-Earth Lagrange Point L4 in collaboration with National Aeronautics and Space Administration (NASA). The aim of this mission is to utilize a combination of remote sensing and in situ instruments for comprehensive observations, complementing the capabilities of the L1 and L5 observatories. The paper outlines long-term scientific objectives, underscoring the significance of multi-point in-situ observations to better understand critical heliospheric phenomena. These include coronal mass ejections, magnetic flux ropes, heliospheric current sheets, kinetic waves and instabilities, suprathermal electrons and solar energetic particle events, as well as remote detection of solar radiation phenomena. Furthermore, the mission’s significance in advancing space weather prediction and space radiation exposure assessment models through the integration of L4 observations is discussed. This article is concluded with an emphasis on the potential of L4 observations to propel advancements in heliospheric science.

We study properties of intensity disturbances along polar coronal rays that are associated with plumes below. For this, we draw azimuth–time images of extreme ultraviolet (EUV) emission of 171 Å band observed by the SDO/AIA and white light (WL) observed by the SOHO/LASCO C2 in 2020 July. From the azimuth–time image, we define two tracks in which the EUV intensities were recurrently enhanced during two weeks. The two EUV tracks are rooted at 78.°8 and 81.°4 latitudes, but their projected azimuth angles are changed with time as the Sun rotates. Coherent WL tracks at different altitudes are determined by scaling the azimuth angles of the EUV tracks, accounting for the effect of inclination of coronal rays. From this, we construct time–distance images of WL intensities along WL tracks, whose projected azimuth angle changes along time and altitude, but the intensities are correlated with the EUV intensities measured below. The time–distance images of WL show repeated and inclined intensity features. The propagation speeds in the altitude range 2.3–6 solar radii are calculated to be 159 ± 8 km s⁻¹ and 300 ± 24 km s⁻¹. The EUV and WL intensities are found to be coherent at 1–2 day periods. It is also found that dynamic burst events along the EUV track are responsible for the enhanced emission. We conclude that the variation of the WL intensity along the polar coronal rays is related with the evolution of the EUV intensity below.