Examining Flux Tube Interactions as a Cause of Sub-alfvénic Outflow

In accepted models, magnetic tension drives reconnected magnetic flux away from the reconnection site at the local Alfvén speed. Numerous observational signatures of these outflows have been identified in solar flares, notable among them being supra-arcade downflows (SADs), almost none move at the Alfvén speed as predicted by models. Well-studied examples of SADs or SAD loops found in the flare of 2017 September 10 (SOL2017-09-10T15:35:00) move at a quarter or less of the expected Alfvén speed. Among those reasons posited to explain such discrepancies is the possibility that reconnected flux experiences a drag force during its outflow. Drag has not been included in previous reconnection models. Here, we develop the first such model in order to test the possibility that drag can explain sub-alfveńic reconnection outflows. Our model uses thin flux tube dynamics, previously shown to match features of flare observations other than outflow speed, including for the 2017 September 10 flare. We supplement the dynamics with a drag force representing the tube’s interaction with surrounding plasma through the formation of a wake. The wake’s width appears as a parameter in the force. We perform simulations, varying the drag parameter and synthesizing EUV observations, to test whether a drag force can produce a reasonable fit to observed features of the September 10 flare. We find that that slower retraction increases the brightness of emission and lowers the temperature of the synthetic plasma sheet. With proper choice of parameters the drag enables the simulation to agree reasonably with the observations.

MMS observations of electron firehose fluctuations in the magnetic reconnection outflow

<p>Plasma waves and instabilities driven by temperature anisotropies are known to play a significant role in plasma dynamics, scattering the particles and affecting particle heating and energy conversion between the electromagnetic fields and the particles. Among these instabilities, the electron firehose instability is driven by electron temperature anisotropy T<sub>e,</sub> > T<sub>e,perp</sub> (with respect to the background magnetic field) and produce nonpropagating oblique modes. </p><p>Magnetic reconnection is characterized by regions of enhanced temperature anisotropy that could drive instabilities - including the electron firehose instability - affecting the particle dynamics and the energy conversion of the process. Yet, the electron firehose instability and its role in the reconnection process is still rather unexplored, especially with in situ measurements. </p><p>We report MMS observations of electron firehose fluctuations observed in the exhaust region of a reconnection site in the magnetotail. The fluctuations are observed in the Earthward outflow relatively close (less than 2 d<sub>i</sub> distance) to the electron diffusion region (EDR). While the characteristics of the fluctuations are compatible with oblique electron firehose fluctuations, the associated firehose instability threshold is not exceeded in the interval where the fluctuations are observed. However, the threshold is exceeded in the EDR. The wave analysis in the EDR suggests that the firehose instability could be active at the reconnection site. We suggest that the firehose fluctuations observed in the outflow region may have been originated at the EDR, where the electron temperature anisotropy exceeds the threshold values, and then advected in the outflow region.</p>

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between an MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented further understanding. Here, we provide comprehensive observations such as turbulence spreading into an MI and turbulence enhancement at the reconnection site, elucidating intricate effects of plasma turbulence on the nonlinear MI evolution.

The role of small-scale surface motions in the transfer of twist to a solar jet from a remote stable flux rope

Context. Jets often have a helical structure containing ejected plasma that is both hot and also cooler and denser than the corona. Various mechanisms have been proposed to explain how jets are triggered, primarily attributed to a magnetic reconnection between the emergence of magnetic flux and environment or that of twisted photospheric motions that bring the system into a state of instability. Aims. Multi-wavelength observations of a twisted jet observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory and the Interface Region Imaging Spectrograph (IRIS) were used to understand how the twist was injected into the jet, thanks to the IRIS spectrographic slit fortuitously crossing the reconnection site at that time. Methods. We followed the magnetic history of the active region based on the analysis of the Helioseismic and Magnetic Imager vector magnetic field computed with the UNNOFIT code. The nature and dynamics of the jet reconnection site are characterised by the IRIS spectra. Results. This region is the result of the collapse of two emerging magnetic fluxes (EMFs) overlaid by arch filament systems that have been well-observed with AIA, IRIS, and the New Vacuum Solar Telescope in Hα. In the magnetic field maps, we found evidence of the pattern of a long sigmoidal flux rope (FR) along the polarity inversion line between the two EMFs, which is the site of the reconnection. Before the jet, an extension of the FR was present and a part of it was detached and formed a small bipole with a bald patch (BP) region, which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time of the reconnection, the Mg II spectra exhibited a strong extension of the blue wing that is decreasing over a distance of 10 Mm (from −300 km s−1 to a few km s−1). This is the signature of the transfer of the twist to the jet. Conclusions. A comparison with numerical magnetohydrodynamics simulations confirms the existence of the long FR. We conjecture that there is a transfer of twist to the jet during the extension of the FR to the reconnection site without FR eruption. The reconnection would start in the low atmosphere in the BP reconnection region and extend at an X-point along the current sheet formed above.

How Alfvén waves induce compressive flows in the neighborhood of a 2.5D magnetic null-point

The aim of the present study is to provide insight on the induced compressive perturbations together with the modifications of the environmental parameters in the course of Alfvén wave interaction with a solar magnetic null-point. The shock-capturing Godunov-type code PLUTO is used to solve the set of ideal magnetohydrodynamic equations. The nonlinear effects connected with an initial Alfvén pulse nearing a magnetic null point induces fast and slow magnetoacoustic waves with anti phase conduct. The induced current density and flows are independent of the local plasma-$$\beta$$ β at the reconnection site. The induced inflows and outflows highly depend on the polarization. The inflows have a stronger effect compared to the outflows in both the x and y directions showing its peak in the x-direction. The dominant wave that couples to flows is the fast wave due to the in-phase harmony between perturbations of the compressive parameters and the fast wave. The induced current density possesses a steady orientation at the reconnection site which governs the diffusion or propagation of the waves. Induced perturbations by the nonlinear forces together with their back reaction on the Alfvén wave have a significant role in the current density excitation being responsible for the creation of inflows and outflows that are possible candidates for the creation of solar jets which has a significant contribution towards coronal seismology.

Reconnection site and ion scale turbulence generation

<p><span>We analyze in detail a reconnection site observed by the Magnetospheric Multiscale (MMS) mission in the magnetotail.</span><span> The interval around the X-line is identified based on the ion jet reversal, Hall electric fields and other reconnection signatures. At the reconnection site strong electric fields with amplitudes above 100mV/m are observed. In addition, the region shows strong turbulent variations on ion scales, including magnetic island-like structures. We discuss the cause of strong electric fields, their relation to ion scale structures and associated particle acceleration in this region. </span><span>Of particular interest is the relation of the reconnection site to the generation of kinetic Alfven waves.</span></p>