We report the simultaneous extreme ultraviolet observation of magnetic reconnection inflow and outflow in a flare on 2010 August 18 observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We found that during the rise phase of the flare, some plasma blobs appeared in the sheet structure above the hot loops. The plasma blobs were ejected bidirectionally along the sheet structure (outflow), at the same time as the threads visible in extreme ultraviolet images moved toward the sheet structure (inflow). The upward and downward ejection velocities are 220-460 km s −1 and 250-280 km s −1 , respectively. The inflow speed changed from 90 km s −1 to 12 km s −1 in 5 minutes. By using these velocities, we estimated the nondimensional reconnection rate, which we found to vary during this period from 0.20 to 0.055. We also found that the plasma blobs in the sheet structure collided or merged with each other before they were ejected from the sheet structure. We hypothesize that the sheet structure is the current sheet and that these plasma blobs are plasmoids or magnetic islands, which could be important for understanding the dynamics of the reconnection region.

Solar flare emission is detected in all EM bands and variations in flux density of solar energetic particles. Often the EM radiation generated in solar and stellar flares shows a pronounced oscillatory pattern, with characteristic periods ranging from a fraction of a second to several minutes. These oscillations are referred to as quasi-periodic pulsations (QPPs), to emphasise that they often contain apparent amplitude and period modulation. We review the current understanding of quasi-periodic pulsations in solar and stellar flares. In particular, we focus on the possible physical mechanisms, with an emphasis on the underlying physics that generates the resultant range of periodicities. These physical mechanisms include MHD oscillations, self-oscillatory mechanisms, oscillatory reconnection/reconnection reversal, wave-driven reconnection, two loop coalescence, MHD flow over-stability, the equivalent LCR-contour mechanism, and thermal-dynamical cycles. We also provide a histogram of all QPP events published in the literature at this time. The occurrence of QPPs puts additional constraints on the interpretation and understanding of the fundamental processes operating in flares, e.g. magnetic energy liberation and particle acceleration. Therefore, a full understanding of QPPs is essential in order to work towards an integrated model of solar and stellar flares.

We report a new evaluation of the accretion properties of PDS 70b obtained with VLT/MUSE. The main difference from previous studies in Haffert et al. (2019) and Aoyama & Ikoma (2019) is in the mass accretion rate. Simultaneous multiple line observations, such as Hα and Hβ, can better constrain the physical properties of an accreting planet. While we clearly detected Hα emissions from PDS 70b, no Hβ emissions were detected. We estimate the line flux of Hβ with a 3-σ upper limit to be 2.3 × 10 −16 erg s −1 cm −2 . The flux ratio F Hβ /F Hα for PDS 70b is < 0.28. Numerical investigations by Aoyama et al. (2018) suggest that F Hβ /F Hα should be close to unity if the extinction is negligible. We attribute the reduction of the flux ratio to the extinction, and estimate the extinction of Hα (A Hα ) for PDS 70b to be > 2.0 mag using the interstellar extinction value. By combining with the Hα linewidth and the dereddening line luminosity of Hα, we derive the PDS 70b mass accretion rate to be 5 × 10 −7 M Jup yr −1 . The PDS 70b mass accretion rate is an order of magnitude larger than that of PDS 70. We found that the filling factor f f (the fractional area of the planetary surface emitting Hα) is 0.01, which is similar to the typical stellar value. The small value of f f indicates that the Hα emitting areas are localized at the surface of PDS 70b.

Unlike all previous models for QPFs, our model includes essential physics for solar flares, such as magnetic reconnection, heat conduction, and chromospheric evaporation. We revealed that QPFs can be spontaneously excited by the abovethe-loop-top oscillation. It was found that this oscillation is controlled by the backflow of the reconnection outflow. The new model revealed that flare loops and the above-the-loop-top region are full of shocks and waves, which is different from the previous expectations based on a standard flare model and previous simulations. In this paper, we will show the QPF generation process based on our new picture of flare loops and will briefly discuss a possible relationship between QPFs and QPPs. Our findings will change the current view of solar flares to a new view in which they are a very dynamic phenomenon with full of shocks and waves.

Solar active regions (ARs) that produce strong flares and coronal mass ejections (CMEs) are known to have a relatively high non-potentiality and are characterized by δ-sunspots and sheared magnetic structures. In this study, we conduct a series of flux emergence simulations from the convection zone to the corona and model four types of active regions that have been observationally suggested to cause strong flares, namely the Spot-Spot, Spot-Satellite, Quadrupole, and Inter-AR cases. As a result, we confirm that δ-spot formation is due to the complex geometry and interaction of emerging magnetic fields, with finding that the strong-field, high-gradient, highly-sheared polarity inversion line (PIL) is created by the combined effect of the advection, stretching, and compression of magnetic fields. We show that free magnetic energy builds up in the form of a current sheet above the PIL. It is also revealed that photospheric magnetic parameters that predict flare eruptions reflect the stored free energy with high accuracy, while CME-predicting parameters indicate the magnetic relationship between flaring zones and entire ARs.

The role of slow-mode magnetohydrodynamic (MHD) shocks in magnetic reconnection is of great importance for energy conversion and transport, but in many astrophysical plasmas the plasma is not fully ionised. In this paper, we use numerical simulations to investigate the role of collisional coupling between a proton-electron, charge-neutral fluid and a neutral hydrogen fluid for the onedimensional (1D) Riemann problem initiated in a constant pressure and density background state by a discontinuity in the magnetic field. This system, in the MHD limit, is characterised by two waves. The first is a fast-mode rarefaction wave that drives a flow towards a slow-mode MHD shock wave. The system evolves through four stages: initiation, weak coupling, intermediate coupling, and a quasisteady state. The initial stages are characterised by an over-pressured neutral region that expands with characteristics of a blast wave. In the later stages, the system tends towards a self-similar solution where the main drift velocity is concentrated in the thin region of the shock front. Because of the nature of the system, the neutral fluid is overpressured by the shock when compared to a purely hydrodynamic shock, which results in the neutral fluid expanding to form the shock precursor. Once it has formed, the thickness of the shock front is proportional to ξ −1.2 i , which is a smaller exponent than would be naively expected from simple scaling arguments. One interesting result is that the shock front is a continuous transition of the physical variables of subsonic velocity upstream of the shock front (a c-shock) to a sharp jump in the physical variables followed by a relaxation to the downstream values for supersonic upstream velocity (a j-shock). The frictional heating that results from the velocity drift across the shock front can amount to ∼2 per cent of the reference magnetic energy.

We studied the acceleration mechanisms of chromospheric jets associated with emerging flux using a two dimensional magnetohydrodynamic (MHD) simulation. We found that slow mode shock waves generated by magnetic reconnection in the chromosphere and the photosphere play key roles in the acceleration mechanisms of chromospheric jets. An important parameter is the height of magnetic reconnection. When magnetic reconnection takes place near the photosphere, the reconnection outflow collides with the region where the plasma beta is much larger than unity. Then the plasma moves along a magnetic field. This motion generates a slow mode wave. The slow mode wave develops to a strong slow shock as it propagates upward. When the slow shock crosses the transition region, the transition region is lifted up. As a result, we obtain a chromospheric jet as the lifted transition region. When magnetic reconnection takes place in the upper chromosphere, the chromospheric plasma is accelerated due to the combination of the Lorentz force and the whip-like motion of magnetic field. We found that the chromospheric plasma is further accelerated through the interaction between the transition region (steep density gradient) and a slow shock emanating from the reconnection point. This is an MHD effect which has not been discussed before.

We carried out a magnetohydrodynamics simulation where a subsurface twisted kink-unstable flux tube emerges from the solar interior to the corona. Unlike the previous expectations based on the bodily emergence of a knotted tube, we found that the kinked tube can spontaneously form a complex quadrupole structure at the photosphere. Due to the development of the kink instability before the emergence, the magnetic twist at the kinked apex of the tube is greatly reduced, although the other parts of the tube is still strongly twisted. This leads to the formation of a complex quadrupole structure: a pair of the coherent, strongly twisted spots and a narrow complex bipolar pair between it. The quadrupole is formed by the submergence of a portion of emerged magnetic fields. This result is relevant for understanding of the origin of the complex multipolar δ-spot regions that have a strong magnetic shear and emerge with polarity orientations not following Hale-Nicholson and Joy Laws.