Qiangwei Cai

Transverse oscillations of coronal loops not only influence coronal dynamics, but also provide a new perspective for coronal detection. To enhance their exploitation and utilization, accurately estimating the oscillation parameters is crucial. The least chi-squares fitting is a widely employed method for this estimation. However, due to the highly nonlinear nature of the fitting process, there is no established theory to provide the exact error of the fitted parameters. In this study, we systematically investigated the accuracy and precision of the fitted parameters of transverse coronal oscillations through a series of forward simulations. Our investigation indicates that the accuracy and precision of fitted oscillation parameters are influenced not only by the standard deviation of the measurement error, but also by the temporal resolution and the characteristics of the signal itself. Based on these findings, we conducted additional forward simulations to estimate the influences on the real observation examples. The results clearly show that (1) the fitted damping timescale should be overestimated in a significant number of transverse coronal loop oscillations, and (2) errors estimated by the general method may be biased. In response, we have developed a forward simulation-based schema to achieve a self-consistent error estimation for the fitted oscillation parameters. These contributions not only offer valuable insights into the error estimation of fitted transverse coronal oscillation parameters, but also establish a robust foundation for obtaining accurate and precise transverse oscillation parameters of the corona.

Context. Small-scale transient brightenings that are the consequence of magnetic reconnection play pivotal roles in the heating process of solar atmospheres. These phenomena contain key information about the dynamic evolution of the solar magnetic field. The fine-scale structures triggered by instabilities in these brightenings are intimately connected with the release of magnetic energy. Aims. To better understand the conversion and release of magnetic energy in small-scale heating events, we investigated the thermal-dynamical behaviors of a loop-like transient brightening (LTB) with plasma blobs. Methods. We used the spectroscopic and slit-jaw imaging observations taken from the Interface Region Imaging Spectrograph and the extreme-ultraviolet images taken from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to analyze the plasma properties of an LTB that occurred on February 28, 2014. The space-time maps were created to present the spatial evolution of the LTB, and the light curves were calculated to illustrate the heating process. Additionally, we employed the differential emission measure (DEM) method to compute the temperature and emission measure of the LTB. In order to investigate the plasma motion along the line-of-sight direction, a double-Gaussian function was used to fit the Si IV spectral profiles. Results. The spectrum and DEM analysis indicate that the LTB was constituted by multithermal plasma with temperatures reaching up to 5.4 × 10⁶ K. The space-time maps of the emission and the Gaussian-fitting results of the Si IV line demonstrate that the LTB not only exhibited bidirectional flows, but was also twisted. Several plasma blobs were identified in the spine of the LTB, suggesting the potential presence of a tearing-mode instability. The low-temperature bands peaked approximately one minute prior to the high-temperature bands, suggesting the occurrence of a heating process driven by magnetic reconnection. The appearance of plasma blobs closely coincided with the sudden increase in the velocity and the quick rise of light curves, providing evidence that plasma blobs facilitate the release of magnetic energy during solar activity. Conclusions. Based on these findings, we speculate that the LTB was a complex structure that occurred in the upper chromosphere-transition region. These results clearly demonstrate that plasma blobs are important for the conversion and release processes of magnetic energy.

Turbulence plays a key role in forming the complex geometry of the large-scale current sheet (CS) and fast energy release in a solar eruption. In this paper, we present full 3D high-resolution simulations for the process of a moderate coronal mass ejection (CME) and the thermodynamical evolution of the highly confined CS. Copious elongated blobs are generated owing to tearing and plasmoid instabilities, giving rise to a higher reconnection rate, and undergo the splitting, merging, and kinking processes in a more complex way in 3D. A detailed thermodynamical analysis shows that the CS is mainly heated by adiabatic and numerical viscous terms, and thermal conduction is the dominant factor that balances the energy inside the CS. Accordingly, the temperature of the CS reaches to a maximum of about 20 MK, and the range of temperatures is relatively narrow. From the face-on view in the synthetic Atmospheric Imaging Assembly 131 Å, the downflowing structures with similar morphology to supra-arcade downflows are mainly located between the post-flare loops and loop top, while moving blobs can extend spikes higher above the loop top. The downward-moving plasmoids can keep the twisted magnetic field configuration until the annihilation at the flare loop top, indicating that plasmoid reconnection dominates in the lower CS. Meanwhile, the upward-moving ones turn into turbulent structures before arriving at the bottom of the CME, implying that turbulent reconnection dominates in the upper CS. The spatial distributions of the turbulent energy and anisotropy are addressed, which show a significant variation in the spectra with height.

The evolution of a magnetic cloud (MC) from the inner heliosphere to the outer heliosphere has been investigated for decades. Although many studies have reported on the evolution of MCs, there is no relevant statistical study about the continuous parametric evolution of the flux rope model of the Gold–Hoyle solution for MCs from near the Sun to 5.4 au. Based on the velocity-modified uniform-twist force-free flux rope model, in this study we explore the evolution with heliodistance for some parameters from 139 MCs observed by the Helios, Wind, and Ulysses spacecraft. We find a negative/positive correlation between the central axial field strength/the radius of the cross section and the heliodistance. The angle between the axis of the MC and the Sun–spacecraft line (Θ), the expansion velocity (v _e ), and the poloidal velocity (v _p ) did not show any evident tendency to increase or decrease with the heliodistance. In addition, the number of turns of the magnetic field lines per unit length winding around the magnetic flux rope, τ, shows a weak decrease with heliodistance. Also, there is an evident negative correlation between τ and the radius of the flux rope, R. The axial magnetic flux (F _z ) and the magnetic helicity (H _m ) show a tendency to decrease within 1 au, after which they remain almost unchanged until 5.5 au. Furthermore, we do not find any evident difference in the parametric properties of MCs on and outside the ecliptic.

Based on observations from the Interface Region Imaging Spectrograph and Hinode, we analyze the thermodynamic evolution of the supra-arcade fan (SAF) in the 2017 September 10 flare. The SAF presents discontinuous characters during the rising process, indicating a nonuniform process of magnetic reconnection in the solar eruption. The intensity peaks of the high-temperature spectral lines (Fe xxi 1354.08 Å, Fe xxiii 263.76 Å, and Fe xxiv 255.10 Å) basically correspond to the valley of the Doppler velocity and Doppler width. The temperature and density increase spatially at the upper boundary of the SAF. These results indicate that a compressed interface may exist in the SAF, where the plasma environment shows remarkable changes in density, temperature, and turbulence. In view of the fact that the height of the SAF is close to the hard X-ray source, we conclude that the interface could be related to termination shocks (TSs), taking into account the synthetic spectral profiles obtained from numerical experiments. In turn, the variations of the spectral profiles might be useful tools for identifying TSs from EUV spectral observations.

A GOES M1.9 flare took place in active region AR 11153 on 2011 February 9. With a resolution of 200 kHz and a time cadence of 80 ms, the reverse-drifting (RS) type-III bursts, intermittent sequence of type-U bursts, drifting pulsation structure (DPS), and fine structures were observed by the Yunnan Observatories Solar Radio Spectrometer (YNSRS). Combined information revealed by the multiwavelength data indicated that after the DPS was observed by YNSRS, the generation rate of type-U bursts suddenly increased to 5 times what it had been. In this event, the generation rate of type-U bursts may depend on the magnetic-reconnection rate. Our observations are consistent with previous numerical simulation results. After the first plasmoid produced (plasma instability occurred), the magnetic-reconnection rate suddenly increased by 5 to 8 times. Furthermore, after the DPS, the frequency range of the turnover frequency of type-U bursts was obviously broadened to thrice what it was before, which indicates a fluctuation amplitude of the density in the loop top. Our observations also support numerical simulations during the flare-impulsive phase. Turbulence occurs at the top of the flare loop and the plasmoids can trap nonthermal particles, causing density fluctuation at the loop top. The observations are generally consistent with the results of numerical simulations, helping us to better understand the characteristics of the whole physical process of eruption.

In this paper, we performed magnetohydrodynamics numerical experiments to look into the dynamic behaviour of the current sheet (CS) between the coronal mass ejection (CME) and the associated solar flare, especially the CS oscillation and plasmoid motions in coronal conditions. During the evolution, the disrupting magnetic configuration becomes asymmetric first in the buffer region at the bottom of the CME bubble. The Rayleigh−Taylor instability in the buffer region and the deflected motion of the plasma driven by the termination shock at the bottom of the CME bubble cause the buffer region to oscillate around the y-axis. The local oscillation propagates downwards through the CS, prompting an overall CS oscillation. As the buffer region grows, the oscillation period becomes longer, increasing from about 30 s to about 16 min. Meanwhile, there is another separated oscillation with a period between 0.25 and 1.5 min in the cusp region of the flare generated by velocity shearing. The tearing mode instability yields formations of plasmoids inside the CS. The motions of all the plasmoids observed in the experiment accelerate, which implies that the large-scale CME/flare CS itself in the true eruptive event is filled with the diffusion region according the the standard theory of magnetic reconnection.

Observations proved that a distributed structure named a supra-arcade fan (SAF) exists above post-flare loops in solar eruptions. The locations of the SAF are spatially consistent with various emission sources. Termination shocks (TSs) that are often regarded as an efficient driver for particle acceleration possibly exist in the SAF. We performed the numerical simulations of solar flares based on the standard flare model to study the dynamical and thermal manifestations of the SAF, as well as the possibility of detecting TSs in extreme-ultraviolet (EUV) images. In the simulations, the SAF and TSs can be clearly identified. The motion history and temperature evolution of plasmas inside the SAF indicate that the mass of the SAF comes from the corona and the plasmas are heated in the current sheet. The height of the SAF decreases with the speed of about 64.6 km s⁻¹ when the rate of magnetic reconnection quickly increases, and then increases with a slightly lower velocity of about 50.5 km s⁻¹ after the peak of the rate of magnetic reconnection. The descent−ascent path of the SAF is due to the unbalance of the Lorentz force and the pressure force inside the magnetic loops. In synthetic EUV images, emission intensity variations in the area surrounding TSs are significant, indicating that, depending on the viewing angle, TSs could be identifiable in EUV observations. The results of numerical simulations are generally consistent with observations, helping us to better understand the characteristics of the SAF and the physical natures behind it.

In a high-resolution, 3D resistive magnetohydrodynamic simulation of an eruptive magnetic flux rope (MFR), we revisit the detailed 3D magnetic structure of a coronal mass ejection (CME). Our results highlight that there exists a helical current ribbon/boundary (HCB) that wraps around the CME bubble. This HCB results from the interaction between the CME bubble and the ambient magnetic field, where it represents a tangential discontinuity in the magnetic topology. Its helical shape is ultimately caused by the kinking of the MFR that resides within the CME bubble. In synthetic Solar Dynamics Observatory/Atmospheric Imaging Assembly images, processed to logarithmic scale to enhance otherwise unobservable features, we show a clear triple-layered leading edge: a bright fast shock front, followed by a bright HCB, and within it a bright MFR. These are arranged in sequence and expand outward continuously. For kink unstable eruptions, we suggest that the HCB is a possible explanation for the bright leading edges seen near CME bubbles and also for the non-wave component of global EUV disturbances.

Magnetohydrodynamic turbulence is ubiquitous in the process of solar eruptions, and it is crucial for the fast release of energy and the formation of complex thermal structures that have been found in observations. In this paper, we focus on the turbulence in two specific regions: inside the current sheet (CS) and above the flare loops, considering the standard flare model. The gravitationally stratified solar atmosphere is used in MHD simulations, which include the Lundquist number of S = 10⁶, thermal conduction, and radiative cooling. The numerical results are generally consistent with previous simulation work, especially the thermal structures and reconnection rate in flare phases. We can observe the formation of multiple termination shocks (TSs) as well as plasmoid collisions, which make the region above the loop-top more turbulent and heat plasmas to the higher temperature. The spectrum studies show that the property of the MHD turbulence inside the CS is anisotropic, while it is quasi-isotropic above the loop-top. The magnetic spectrum becomes softer when the plasmoids interact with the multiple TSs. Meanwhile, synthetic images and light curves of the Solar Dynamics Observatory/Atmospheric Imaging Assembly 94, 131, 171, 304, and 193 Å channels show intermittent radiation enhancement by turbulence above the loop-top. The spectrum study of the radiation intensity in these five wavelengths gives quite different power indices at the same time. In particular, quasiperiodic pulsations (QPPs) in the turbulent region above the loop-top are investigated, and we also confirm that the heating for plasmas via turbulence is an important contributor to the source of QPPs.