arXiv subjects

Ying D. Liu

Publications and source records attributed to Ying D. Liu.

At least 19 recordsLinked to original sources

arXiv2024

Limb Observations of Global Solar Coronal Extreme-ultraviolet Wavefronts: The Inclination, Kinematics, Coupling with the Expanding Coronal Mass Ejections, and Connection with the Coronal Mass Ejection Driven Shocks

We select and investigate six global solar extreme-ultraviolet (EUV) wave events using data from the Solar Dynamics Observatory and the Solar and Heliospheric Observatory. These eruptions are all on the limb but recorded as halo coronal mass ejections (CMEs) because the CME-driven shocks have expanded laterally to the opposite side. With the limb observations avoiding the projection effect, we have measured the inclination and speed of the EUV wavefront from 1.05 to 1.25 $R_\odot$. We also investigate the coupling and connection of the EUV wavefront with the CME boundary and the CME-driven shock, respectively. The major findings in the six events are: (1) the forward inclination of the primary and coronal-hole-transmitted EUV wavefronts is estimated, respectively, and the origins of these inclinations and their effects on the estimate of actual wavefronts speed are investigated; (2) the wavefront speed can be elevated by loop systems near the coronal base, and the average speed in the low corona has no clear correlation with the lateral expansion of the CME-driven shock in the high corona; (3) the fast magnetosonic Mach number of the wavefront is larger than unity from the coronal base; (4) the EUV wavefront is coupled with the CME driver throughout the propagation in two events; (5) after the EUV wavefront vanishes, the CME-driven shock continues traveling on the opposite side and disconnects from the EUV wavefront in four events. These results and their implications are discussed, which provide insight into the properties of global EUV waves.

astro-ph.SR↗

arXiv2024

Unveiling Key Factors in the Solar Eruptions Leading to the Solar Superstorm in 2024 May

NOAA Active Region (AR) 13664/8 produced the most intense geomagnetic effects since the ``Halloween'' event of 2003. The resulting extreme solar storm is believed to be the consequence of multiple interacting coronal mass ejections (CMEs). Notably, this AR exhibites an exceptionally rapid magnetic flux emergence. The eruptions we are focusing on all occurred along collisional polarity inversion lines (PILs) through ``collisional shearing'' during a three-day period of extraordinarily high flux emergence ($\sim$10$^{21}$ Mx hr$^{-1}$). Our key findings reveal how photospheric magnetic configurations in eruption sources influence solar superstorm formation and geomagnetic responses, and link exceptionally strong flux emergence to sequential homologous eruptions: (1) We identified the source regions of seven halo CMEs, distributed primarily along two distinct PILs, suggesting the presence of two groups of homologous CMEs. (2) The variations in magnetic flux emergence rates at the source regions correlate with CME intensities, potentially explaining the two contrasting cases of complex ejecta observed at Earth. (3) Calculations of magnetic field gradients around CME source regions show strong correlations with eruptions, providing crucial insights into solar eruption mechanisms and enhancing future prediction capabilities.

astro-ph.SR↗

arXiv2024

On the Acceleration of the Young Solar Wind from Different Source Regions

The acceleration of the young solar wind is studied using the first 17 encounters of Parker Solar Probe. We identify wind intervals from different source regions: coronal hole (CH) interiors, streamers, and low Mach number boundary layers (LMBLs), i.e. the inner boundaries of coronal holes. We present their statistical trends in the acceleration process. Most of the observations can be reproduced by a two-fluid hydrodynamic model with realistic corona temperatures. In such a model, the solar wind is accelerated by the combined thermal pressures of protons and electrons,but it is mainly the difference in the proton pressure that leads to the difference in the solar wind speed. The proton pressure is the highest in the fastest CH wind, with a high initial proton temperature that decreases slowly. It is lower in the relatively slow LMBL wind, and the lowest in the slowest streamer wind. The proton temperature is quadratically correlated with the wind speed when scaled to the same distance. In contrast, the electron temperature shows no significant differences for different wind types or wind speeds, indicating more similar contributions from the electron pressure. The model gives reasonable locations for the sonic critical point, which is on average at 3.6-7.3 solar radii and can also extend to large distances when the proton temperature is extremely low, as in the LMBL wind. In addition to the thermal pressure, we raise the possibility that Alfvén waves may contribute to the solar wind acceleration, especially for the fast CH wind.

astro-ph.SR↗

arXiv2024

High-resolution Observations of Clustered Dynamic Extreme-Ultraviolet Bright Tadpoles near the Footpoints of Corona Loops

An extreme ultraviolet (EUV) close-up view of the Sun offers unprecedented detail of heating events in the solar corona. Enhanced temporal and spatial images obtained by the Solar Orbiter during its first science perihelion enabled us to identify clustered EUV bright tadpoles (CEBTs) occurring near the footpoints of coronal loops. Combining SDO/AIA observations, we determine the altitudes of six distinct CEBTs by stereoscopy, ranging from $\sim$1300 to 3300 km. We then notice a substantial presence of dark, cooler filamentary structures seemingly beneath the CEBTs, displaying periodic up-and-down motions lasting 3 to 5 minutes. This periodic behavior suggests an association of the majority of CEBTs with Type I spicules. Out of the ten selected CEBTs with fast downward velocity, six exhibit corrected velocities close to or exceeding 50 km $s^{-1}$. These velocities notably surpass the typical speeds of Type I spicules. We explore the generation of such velocities. It indicates that due to the previous limited observations of spicules in the EUV wavelengths, they may reveal novel observational features beyond our current understanding. Gaining insights into these features contributes to a better comprehension of small-scale coronal heating dynamics.

astro-ph.SR↗

arXiv2024

A Pileup of Coronal Mass Ejections Produced the Largest Geomagnetic Storm in Two Decades

The largest geomagnetic storm in two decades occurred in 2024 May with a minimum $D_{\rm st}$ of $-412$ nT. We examine its solar and interplanetary origins by combining multipoint imaging and in situ observations. The source active region, NOAA AR 13664, exhibited extraordinary activity and produced successive halo eruptions, which were responsible for two complex ejecta observed at the Earth. In situ measurements from STEREO A, which was $12.6^{\circ}$ apart, allow us to compare the ``geo-effectiveness" at the Earth and STEREO A. We obtain key findings concerning the formation of solar superstorms and how mesoscale variations of coronal mass ejections affect geo-effectiveness: (1) the 2024 May storm supports the hypothesis that solar superstorms are ``perfect storms" in nature, i.e., a combination of circumstances resulting in an event of an unusual magnitude; (2) the first complex ejecta, which caused the geomagnetic superstorm, shows considerable differences in the magnetic field and associated ``geo-effectiveness" between the Earth and STEREO A, despite a mesoscale separation; and (3) two contrasting cases of complex ejecta are found in terms of the geo-effectiveness at the Earth, which is largely due to different magnetic field configurations within the same active region.

astro-ph.SR↗

arXiv2024

Formation of a Magnetic Cloud from the Merging of Two Successive Coronal Mass Ejections

On 2022 March 28 two successive coronal mass ejections (CMEs) were observed by multiple spacecraft and resulted in a magnetic cloud (MC) at 1 AU. We investigate the propagation and interaction properties of the two CMEs correlated with the MC using coordinated multi-point remote sensing and in situ observations from Solar Orbiter, STEREO A, SOHO, and Wind. The first CME was triggered by a filament eruption with a high inclination angle. Roughly 9 hr later, the second CME originating from the same active region erupted with a smaller tilt angle and faster speed compared to the first one. The second CME overtook the preceding CME and formed a merged front at approximately 75 \rsun{}, which developed into a complex ejecta at 1 AU. The descending speed and low proton temperature inside the complex ejecta suggest that the two CMEs have fully merged before reaching 1 AU, leading them to begin expanding rather than compressing against each other. The complex ejecta appears to have the magnetic field and plasma signatures of an MC, although there is a discontinuity in the magnetic field implying previous interactions. The cross section of the complex ejecta, reconstructed from in situ data using a Grad-Shafranov technique, exhibits a right--handed flux rope structure. These results highlight that an MC--like complex ejecta lacking interaction features could arise from the complete merging of two CMEs.

physics.space-ph↗

arXiv2024

Origin and Properties of the Near Subsonic Solar Wind Observed by Parker Solar Probe

We identify and examine the solar wind intervals near the sonic critical point (i.e., $M_S \sim 1$) observed by the Parker Solar Probe (PSP). The near subsonic wind intervals show similar properties: a low density, an extremely low velocity, a low proton temperature, and essentially no magnetic field deflections compared with the surrounding solar wind. The extremely low velocity is the primary contributor to the near crossing of the sonic critical point rather than the sound speed, which is roughly constant in these intervals. Source tracing with a potential field source surface (PFSS) model suggests that the near subsonic intervals all connect to the boundaries inside coronal holes. Heliospheric current sheet (HCS) and partial HCS crossings around the near subsonic intervals indicate that the near subsonic wind is a transition layer between the slow and fast wind. The above scenario is consistent with the nature of the near subsonic wind as a low Mach-number boundary layer (LMBL), which facilitates the crossing of the sonic critical point at 15-20 $R_S$. Moreover, we find a dependence of the amplitude of switchbacks on the radial sonic Mach number. Magnetic field deflections essentially disappear near the sonic critical point, which suggests that switchbacks originate from above the sonic critical point.

astro-ph.SR↗

arXiv2024

The Alpha-Proton Differential Flow in the Alfvénic Young Solar Wind: From Sub-Alfvénic to Super-Alfvénic

Data obtained from Parker Solar Probe (PSP) since 2021 April have shown the first in situ observation of the solar corona, where the solar wind is formed and accelerated. Here we investigate the alpha-proton differential flow and its characteristics across the critical Alfvén surface (CAS) using data from PSP during encounters 8-10 and 12-13. We first show the positive correlation between the alpha-proton differential velocity and the bulk solar wind speed at PSP encounter distances. Then we explore how the characteristics of the differential flow vary across the CAS and how they are affected by Alfvénic fluctuations including switchbacks. We find that the differential velocity below the CAS is generally smaller than that above the CAS, and the local Alfvén speed well limits the differential speed both above and below the CAS. The deviations from the alignment between the differential velocity and the local magnetic field vector are accompanied by large-amplitude Alfvénic fluctuations and decreases in the differential speed. Moreover, we observe that $V_{αp}$ increases from $M_A < 1$ to $M_A \simeq 2$ and then starts to decrease, which suggests that alphas may remain preferentially accelerated well above the CAS. Our results also reveal that in the sub-Alfvénic solar wind both protons and alphas show a strong correlation between their velocity fluctuations and magnetic field fluctuations, with a weaker correlation for alphas. In contrast, in the super-Alfvénic regime the correlation remains high for protons, but is reduced for alphas.

astro-ph.SR↗

arXiv2024

Direct In Situ Measurements of a Fast Coronal Mass Ejection and Associated Structures in the Corona

We report on the first direct in situ measurements of a fast coronal mass ejection (CME) and shock in the corona, which occurred on 2022 September 5. In situ measurements from the Parker Solar Probe (PSP) spacecraft near perihelion suggest two shocks with the second one decayed, which is consistent with more than one eruptions in coronagraph images. Despite a flank crossing, the measurements indicate unique features of the young ejecta: a plasma much hotter than the ambient medium suggestive of a hot solar source, and a large plasma $β$ implying a highly non-force-free state and the importance of thermal pressure gradient for CME acceleration and expansion. Reconstruction of the global coronal magnetic fields shows a long-duration change in the heliospheric current sheet (HCS), and the observed field polarity reversals agree with a more warped HCS configuration. Reconnection signatures are observed inside an HCS crossing as deep as the sonic critical point. As the reconnection occurs in the sub-Alfvénic wind, the reconnected flux sunward of the reconnection site can close back to the Sun, which helps balance magnetic flux in the heliosphere. The nature of the sub-Alfvénic wind after the HCS crossing as a low Mach-number boundary layer (LMBL) leads to in situ measurements of the near subsonic plasma at a surprisingly large distance. Specifically, an LMBL may provide favorable conditions for the crossings of the sonic critical point in addition to the Alfvén surface.

astro-ph.SR↗

arXiv2023

Properties of Steady Sub-Alfvénic Solar Wind in Comparison with Super-Alfvénic Wind from Measurements of Parker Solar Probe

We identify more than ten steady sub-Alfvénic solar wind intervals from the measurements of the Parker Solar Probe (PSP) from encounter 8 to encounter 14. An analysis of these sub-Alfvénic intervals reveals similar properties and similar origins. In situ measurements show that these intervals feature a decreased radial Alfvén Mach number resulting from a reduced density and a relatively low velocity, and that switchbacks are suppressed in these intervals. Magnetic source tracing indicates that these sub-Alfvénic streams generally originate from the boundaries inside coronal holes, or narrow/small regions of open magnetic fields. Such properties and origins suggest that these streams are low Mach-number boundary layers (LMBLs), which is a special component of the pristine solar wind proposed by Liu et al. We find that the LMBL wind, the fast wind from deep inside coronal holes, and the slow streamer wind constitute three typical components of the young solar wind near the Sun. In these sub-Alfvénic intervals, the Alfvén radius varies between 15 and 25 solar radii, in contrast with a typical 12 radii for the Alfvén radius of the super-Alfvénic wind. These results give a self-consistent picture interpreting the PSP measurements in the vicinity of the Sun.

astro-ph.SR↗

arXiv2023

Observations of Mini Coronal Dimmings Caused by Small-scale Eruptions in the Quiet Sun

Small-scale eruptions could play an important role in coronal heating, generation of solar energetic particles (SEPs), and mass source of the solar wind. However, they are poorly observed, and their characteristics, distributions, and origins remain unclear. Here a mini coronal dimming was captured by the recently launched Solar Orbiter spacecraft. The observations indicate that a minifilament eruption results in the dimming and takes away approximately $(1.65\pm0.54)\times10^{13}$ g of mass, which also exhibits similar features as the sources of SEP events. The released magnetic free energy is of the order of $\sim10^{27}$ erg. Our results suggest that weak constraining force makes the flux rope associated with the minifilament easily enter a torus-unstable domain. We discuss that weak magnetic constraints from low-altitude background fields may be a general condition for the quiet-Sun eruptions, which provide a possible mechanism for the transport of coronal material and energy from the lower to the middle or even higher corona.

astro-ph.SR↗

arXiv2023

On the Generation and Evolution of Switchbacks and the Morphology of Alfvénic Transition: Low Mach-number Boundary Layers

We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfvénic wind observed by Parker Solar Probe (PSP), and the morphology of the Alfvénic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfvén radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfvénic wind detected by PSP is an LMBL flow by nature. The similar origin and similar properties of the sub-Alfvénic intervals favor a wrinkled surface for the morphology of the Alfvénic transition. We find that a larger deflection angle tends to be associated with a higher Alfvén Mach number. The magnetic deflections have an origin well below the Alfvén critical point, and deflection angles larger than $90^{\circ}$ seem to occur only when $M_{\rm A} \gtrsim 2$. The velocity enhancement in units of the local Alfvén speed generally increases with the deflection angle, which is explained by a simple model. A nonlinearly evolved, saturated state is revealed for SBs, where the local Alfvén speed is roughly an upper bound for the velocity enhancement. In the context of these results, the most promising theory on the origin of SBs is the model of expanding waves and turbulence, and the patchy distribution of SBs is attributed to modulation by reductions in the Alfvén Mach number. Finally, a picture on the generation and evolution of SBs is created based on the results.

astro-ph.SR↗

arXiv2022

Spectroscopic and Imaging Observations of Spatially Extended Magnetic Reconnection in the Splitting of a Solar Filament Structure

On the Sun, Doppler shifts of bidirectional outflows from the magnetic-reconnection site have been found only in confined regions through spectroscopic observations. Without spatially resolved spectroscopic observations across an extended region, the distribution of reconnection and its outflows in the solar atmosphere cannot be made clear. Magnetic reconnection is thought to cause the splitting of filament structures, but unambiguous evidence has been elusive. Here we report spectroscopic and imaging analysis of a magnetic-reconnection event on the Sun, using high-resolution data from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. Our findings reveal that the reconnection region extends to an unprecedented length of no less than 14,000 km. The reconnection splits a filament structure into two branches, and the upper branch erupts eventually. Doppler shifts indicate clear bidirectional outflows of ~100 km/s, which decelerate beyond the reconnection site. Differential-emission-measure analysis reveals that in the reconnection region the temperature reaches over 10 MK and the thermal energy is much larger than the kinetic energy. This Letter provides definite spectroscopic evidence for the splitting of a solar filament by magnetic reconnection in an extended region.

astro-ph.SR↗

arXiv2022

Relationship between Successive Flares in the Same Active Region and Space-Weather HMI Active Region Patch (SHARP) Parameters

A solar active region (AR) may produce multiple notable flares during its passage across the solar disk. We investigate successive flares from flare-eruptive active regions, and explore their relationship with solar magnetic parameters. We examine six ARs in this study, each with at least one major flare above X1.0. The Space-Weather HMI Active Region Patch (SHARP) is employed in this study to parameterize the ARs. We aim to identify the most flare-related SHARP parameters and lay foundation for future practical flare forecasts. We first evaluate the correlation coefficients between the SHARP parameters and the successive flare production. Then we adopt a Natural Gradient Boost (NGBoost) method to analyze the relationship between the SHARP parameters and the successive flare bursts. Based on the correlation analysis and the importance distribution returned from NGBoost, we select 8 most flare-related SHARP parameters. Finally, we discuss the physical meanings of the 8 selected parameters and their relationship with flare production.

astro-ph.SR↗

arXiv2022

Multispacecraft Remote Sensing and In Situ Observations of the 2020 November 29 Coronal Mass Ejection and Associated Shock: From Solar Source to Heliospheric Impacts

We investigate the source eruption, propagation and expansion characteristics, and heliospheric impacts of the 2020 November 29 coronal mass ejection (CME) and associated shock, using remote sensing and in situ observations from multiple spacecraft. A potential--field source--surface model is employed to examine the coronal magnetic fields surrounding the source region. The CME and associated shock are tracked from the early stage to the outer corona using extreme ultraviolet and white light observations. Forward models are applied to determine the structures and kinematics of the CME and the shock near the Sun. The shock shows an ellipsoidal structure, expands in all directions, and encloses the whole Sun as viewed from both SOHO and STEREO A, which results from the large expansion of the CME flux rope and its fast acceleration. The structure and potential impacts of the shock are mainly determined by its radial and lateral expansions. The CME and shock arrive at Parker Solar Probe and STEREO A. Only based on the remote sensing observations, it is difficult to predict whether and when the CME/shock would arrive at the Earth. Combining Wind in situ measurements and WSA-ENLIL simulation results, we confirm that the far flank of the CME (or the CME leg) arrives at the Earth with no shock signature. These results highlight the importance of multipoint remote sensing and in situ observations for determining the heliospheric impacts of CMEs.

astro-ph.SR↗

arXiv2022

Observations of a Flare-ignited broad Quasi-periodic Fast-propagating wave train

Large-scale Extreme-ultraviolet (EUV) waves are frequently observed as an accompanying phenomenon of flares and coronal mass ejections (CMEs). Previous studies mainly focus on EUV waves with single wavefronts that are generally thought to be driven by the lateral expansion of CMEs. Using high spatio-temporal resolution multi-angle imaging observations taken by the Solar Dynamic Observatory and the Solar Terrestrial Relations Observatory, we present the observation of a broad quasi-periodic fast propagating (QFP) wave train composed of multiple wavefronts along the solar surface during the rising phase of a GOES M3.5 flare on 2011 February 24. The wave train transmitted through a lunate coronal hole (CH) with a speed of 840 +/-67 km/s, and the wavefronts showed an intriguing refraction effect when they passed through the boundaries of the CH. Due to the lunate shape of the CH, the transmitted wavefronts from the north and south arms of the CH started to approach each other and finally collided, leading to the significant intensity enhancement at the collision site. This enhancement might hint the occurrence of interference between the two transmitted wave trains. The estimated magnetosonic Mach number of the wave train is about 1.13, which indicates that the observed wave train was a weak shock. Period analysis reveals that the period of wave train was $\sim$90 seconds, in good agreement with that of the accompanying flare. Based on our analysis results, we conclude that the broad QFP wave train was a large-amplitude fast-mode magnetosonic wave or a weak shock driven by some non-linear energy release processes in the accompanying flare.

astro-ph.SR↗

arXiv2022

Analysis of the distribution, rotation and scale characteristics of solar wind switchbacks: comparison between the first and second encounters of Parker Solar Probe

The S-shaped magnetic structure in the solar wind formed by the twisting of magnetic field lines is called a switchback, whose main characteristics are the reversal of the magnetic field and the significant increase in the solar wind radial velocity. We identify 242 switchbacks during the first two encounters of Parker Solar Probe (PSP). Statistics methods are applied to analyze the distribution and the rotation angle and direction of the magnetic field rotation of the switchbacks. The diameter of switchbacks is estimated with a minimum variance analysis (MVA) method based on the assumption of a cylindrical magnetic tube. We also make a comparison between switchbacks from inside and the boundary of coronal holes. The main conclusions are as follows: (1) the rotation angles of switchbacks observed during the first encounter seem larger than those of the switchbacks observed during the second encounter in general; (2) the tangential component of the velocity inside the switchbacks tends to be more positive (westward) than in the ambient solar wind; (3) switchbacks are more likely to rotate clockwise than anticlockwise, and the number of switchbacks with clockwise rotation is 1.48 and 2.65 times of those with anticlockwise rotation during the first and second encounters, respectively; (4) the diameter of switchbacks is about 10^5 km on average and across five orders of magnitude (10^3 -- 10^7 km).

astro-ph.SR↗

arXiv2022

Buildup of the Magnetic Flux Ropes in Homologous Solar Eruptions

Homologous coronal mass ejections (CMEs) are an interesting phenomenon, and it is possible to investigate the formation of CMEs by comparing multi-CMEs under a homologous physical condition. AR 11283 had been present on the solar surface for several days when a bipole emerged on 2011 September 4. Its positive polarity collided with the pre-existing negative polarity belonging to a different bipole, producing recurrent solar activities along the polarity inversion line (PIL) between the colliding polarities, namely the so-called collisional PIL (cPIL). Our results show that a large amount of energy and helicity were built up in the form of magnetic flux ropes (MFRs), with recurrent release and accumulation processes. These MFRs were built up along the cPIL. A flux deficit method is adopted and shows that magnetic cancellation happens along the cPIL due to the collisional shearing scenario proposed by Chintzoglou et al. The total amount of canceled flux was $\sim$0.7$\times$10$^{21}$ Mx with an uncertainty of $\sim$13.2$\%$ within the confidence region of the 30$^\circ$ sun-center distance. The canceled flux amounts to 24$\%$ of the total unsigned flux of the bipolar magnetic region. The results show that the magnetic fields beside the cPIL are very sheared, and the average shear angle is above 70$^\circ$ after the collision. The fast expansion of the twist kernels of the MFRs and the continuous eruptive activities are both driven by the collisional shearing process. These results are important for better understanding the buildup process of the MFRs associated with homologous solar eruptions.

astro-ph.SR↗