Twin extreme ultraviolet waves in the solar corona

Solar extreme ultraviolet (EUV) waves are spectacular propagating disturbances with EUV enhancements in annular shapes in the solar corona. These EUV waves carry critical information about the coronal magnetised plasma that can shed light on gaining insight to the elusive physical parameters (e.g. the magnetic field strength) by global solar coronal magneto-seismology (SMS). EUV waves are closely associated with a wide range of solar atmospheric eruptions, from violent flares and coronal mass ejections (CMEs) to less energetic plasma jets or mini-filament eruptions, and can play a role in accelerating particles to non-thermal energies. However, the physical nature and driving mechanism of EUV waves is still under controversy. Here, we report the unique discovery of twin EUV waves (TEWs) that were formed in a single eruption with observations from two different perspectives. In all earlier studies, a single eruption was associated at most in a single EUV wave. The newly found TEWs urge to re-visit our theoretical understanding about the underlying formation mechanism(s) of coronal EUV waves. Two distinct scenarios of TEWs were found. In the first scenario, the two waves were separately associated with a filament eruption and a precursor jet, while in another scenario the two waves were successively associated with a filament eruption. Hence, we label these distinguished scenarios as "fraternal TEWs" and "identical TEWs", respectively. Further, we also suggest that impulsive lateral expansions of two distinct groups of coronal loops are critical to the formation of TEWs in a single eruption.

PIC Simulations of Excited Waves in the Plasmoid Instability

Fragmentation of an elongated current sheet into many reconnection X-points, and therefore multiple plasmoids, occurs frequently in the solar corona. This speeds up the release of solar magnetic energy in the form of thermal and kinetic energy. Moreover, due to the presence of multiple reconnection X-points, the particle acceleration is more efficient in terms of the number of accelerated particles. This type of instability called “plasmoid instability” is accompanied with the excitation of some electrostatic/electromagnetic waves. We carried out 2D particle-in-cell simulations of this instability in the collisionless regime, with the presence of non-uniform magnetic guide field to investigate the nature of excited waves. It is shown that the nature and properties of waves excited inside and outside the current sheet are different. While the outside perturbations are transient, the inside ones are long-lived, and are directly affected by the plasmoid instability process.

The enduring mystery of the solar corona

Physicists have long known that the Sun’s magnetic fields make its corona much hotter than the surface of the star itself. But how – and why – those fields transport and deposit their energy is still a mystery, as Philip G Judge explains.

How Does Transverse MHD Wave-driven Turbulence Influence the Density Filling Factor in the Solar Corona?

It is well established that transverse MHD waves are ubiquitous in the solar corona. One of the possible mechanisms for heating both open (e.g., coronal holes) and closed (e.g., coronal loops) magnetic field regions of the solar corona is MHD wave-driven turbulence. In this work, we study the variation of the filling factor of overdense structures in the solar corona due to the generation of transverse MHD wave-driven turbulence. Using 3D MHD simulations, we estimate the density filling factor of an open magnetic structure by calculating the fraction of the volume occupied by the overdense plasma structures relative to the entire volume of the simulation domain. Next, we perform forward modeling and generate synthetic spectra of Fe xiii 10749 Å and 10800 Å density-sensitive line pairs using FoMo. Using the synthetic images, we again estimate the filling factors. The estimated filling factors obtained from both methods are in reasonable agreement. Also, our results match fairly well with the observations of filling factors in coronal holes and loops. Our results show that the generation of turbulence increases the filling factor of the solar corona.

Atomic Structure Calculations of Landé g Factors of Astrophysical Interest with Direct Applications for Solar Coronal Magnetometry

We perform a detailed theoretical study of the atomic structure of ions with ns 2 np m ground configurations and focus on departures from LS coupling, which directly affect the Landé g factors of magnetic dipole lines between levels of the ground terms. Particular emphasis is given to astrophysically abundant ions formed in the solar corona (those with n = 2,3) with M1 transitions spanning a broad range of wavelengths. Accurate Landé g factors are needed to diagnose coronal magnetic fields using measurements from new instruments operating at visible and infrared wavelengths, such as the Daniel K. Inouye Solar Telescope. We emphasize an explanation of the dynamics of atomic structure effects for nonspecialists.

Probing Upflowing Regions in the Quiet Sun and Coronal Holes

Recent observations from Parker Solar Probe have revealed that the solar wind has a highly variable structure. How this complex behaviour is formed in the solar corona is not yet known, since it requires omnipresent fluctuations, which constantly emit material to feed the wind. In this article we analyse 14 upflow regions in the solar corona to find potential sources for plasma flow. The upflow regions are derived from spectroscopic data from the EUV Imaging Spectrometer (EIS) on board Hinode determining their Doppler velocity and defining regions which have blueshifts stronger than $-6~\mbox{km}\,\mbox{s}^{-1}$ − 6 km s − 1 . To identify the sources of these blueshift data from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), on board the Solar Dynamics Observatory (SDO), and the X-ray Telescope (XRT), on board Hinode, are used. The analysis reveals that only 5 out of 14 upflows are associated with frequent transients, like obvious jets or bright points. In contrast to that, seven events are associated with small-scale features, which show a large variety of dynamics. Some resemble small bright points, while others show an eruptive nature, all of which are faint and only live for a few minutes; we cannot rule out that several of these sources may be fainter and, hence, less obvious jets. Since the complex structure of the solar wind is known, this suggests that new sources have to be considered or better methods used to analyse the known sources. This work shows that small and frequent features, which were previously neglected, can cause strong upflows in the solar corona. These results emphasise the importance of the first observations from the Extreme-Ultraviolet Imager (EUI) on board Solar Orbiter, which revealed complex small-scale coronal structures.

Below the surface

A response to Philip G Judge’s feature article “The enduring mystery of the solar corona” in which he explores the question of why the solar corona is so much hotter than the surface of the Sun.