Session PM10: Mini-Conference on Reconnection: Solar

Live

Flares on the Sun, thanks to their proximity, serve as an outstanding laboratory to test our understanding of magnetic reconnection and the associated energy release and particle acceleration processes. Flare-accelerated electrons in the low solar corona emit radio waves in decimeter-centimeter wavelengths. Observations of these radio waves provide excellent means for tracing the accelerated electrons, and for probing a variety of physical processes and plasma properties in and around the magnetic reconnection site. The newly available radio spectral imaging technique from multiple recently commissioned telescope arrays opens up a new window for such studies. I will discuss recent results of this kind based on observations from the Karl G. Jansky Very Large Array and NJIT’s Expanded Owens Valley Solar Array. Examples include tracing fast electron beams from reconnection sites, mapping termination shocks driven by reconnection outflows, and measuring properties of a large-scale reconnection current sheet.   

Live

The Sun's corona is characterized by bursts of energy release that are most strikingly observed as intense X-Ray solar flares. The underlying origin for this activity is that magnetic free energy builds up and is released impulsively to the plasma in the form of heating, mass motions, and/or particle acceleration. We present high-resolution observations from NASA/ESA/JAXA space missions showing that the energy buildup process appears to be similar for flaring activity ranging across orders of magnitude in scale and energy. Furthermore, the observations demonstrate conclusively that magnetic reconnection is the energy release process. We also present very recent MHD numerical simulations of solar flares that include self-consistently both the energy buildup and explosive release. Our models show that current sheet formation leading to reconnection and energy release occurs almost continuously in the corona, but explosive energy release occurs only when there is strong feedback between the reconnection and the global ideal evolution. We discuss the mechanism for flare reconnection onset and its 3D nature. Capturing accurately the multiscale feedback inherent in flare reconnection remains as the greatest challenge to understanding and eventually predicting these critically important space weather events.   

Live

We have developed a new computational model, kglobal, to explore energetic electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of energetic electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers and does not need to resolve any kinetic scales. We use guiding center equations for macro-particle electrons that provides self-consistent feedback on the magnetic field and ion fluid through their anisotropic pressure tensor. Additionally, in order to ensure charge neutrality we include a fluid electron species as well. The result is a code with a MHD backbone that allows us to study electron energization while conserving the total amount of energy. This code has accurately simulated Alfv\'en waves, magnetohydrodynamic waves, the firehose instability, and Landau damping of electron acoustic modes. Here we present results from macroscale multi-island magnetic reconnection simulations, including evidence for a power law in the particle electrons. This model is capable of bridging the orders of magnitude gap between PIC simulations and global systems.\\ \\In collaboration with: James Drake, University of Maryland; Marc Swisdak, University of Maryland; Fan Guo, Los Alamos; Yi-Minh Huang, Princeton University; Joel Dahlin, NASA Goddard Space Flight Center\\ \\In collaboration with:Author 2 James Drake. University of Maryland; Marc Swisdak, University of Maryland; Fan Guo, Los Alamos, Yi-Minh Huang, Princeton University; Joel Dahlin, NASA Goddard Space Flight Center   

Live

Solar flare observations have suggested that magnetic reconnection in the non-relativistic low-beta regime efficiently accelerates particles and the resulting energy spectra often take a power-law. However, it has been difficult to produce a clear power-law in non-relativistic particle-in-cell (PIC) reconnection simulations. Recent progress on this has suggested that 3D physics may be a key to the power-law formation. Using a series of 3D PIC simulations with different domain sizes, we find that in the low guide field regime kink instability of reconnection flux ropes disrupts the close flux surfaces and enhance separation of magnetic field lines. This leads to efficient transport of energetic electrons out of the flux ropes and those electrons keep accessing the acceleration regions. The criterion of kink instability (safety factor at the edge of flux ropes less than 1) suggests a threshold for the dimension in the guide field direction for efficient acceleration and power-law formation.   

Live

Magnetic reconnection is a fundamental plasma process that plays a critical role not only in energy release in the solar atmosphere, but also in fusion, astrophysical, and other space plasma environments. One of the challenges in explaining solar observations in which reconnection is thought to play a critical role is to account for the transition of the dynamics from a slow quasi-continuous phase to a fast and impulsive energetic burst of much shorter duration. Despite the theoretical progress in identifying mechanisms that might lead to rapid onset, a lack of observations of this transition has left models poorly constrained. High-resolution spectroscopic observations from NASA’s Interface Region Imaging Spectrograph (IRIS) now reveal tell-tale signatures of the abrupt transition of reconnection from a slow phase to a fast, impulsive phase during explosive events in the Sun’s atmosphere. Our observations are consistent with numerical simulations of the plasmoid instability, and provide evidence for the onset of fast reconnection mediated by plasmoids and new opportunities for remote-sensing diagnostics of reconnection mechanisms on the Sun.   

Live

The 20 years of ACE satellite measurements of B(t) at 1AU enable detailed spectral and dynamical analyses, here supplemented by radial dependencies from Ulysses and Mariner from 0.3 - 5 AU. 1) Variable-duration spectral analyses clearly show that there is no persistent magnetic "spiral" at 1AU, merely the statistical fluctuations of "random walk" dynamics. Similarly, spectral components B(f) above f$\cong $50$\mu $Hz clearly show the $\surd $N scaling of random noise. 2) Pervasive dynamical "arc" events are observed on time-scales 10\textasciicircum 3\textless $\tau $ \textless 10\textasciicircum 5 sec, presumably related to spiky "switchbacks" observed by PSP at 0.1AU. The dynamics appears as B$\theta $-Bz, Br-Bz, and Br-B$\theta $ temporal arcs, with occurrence rates differing by direction. The observed dynamics is closely modelled by finite-duration "pinched" $+$/- current filaments, representing charge non-neutrality of 10\textasciicircum -5 of the e-/p$+$ flux over distances d$\cong $10\textasciicircum 3Mm and times $\tau \cong $2000s. 3) The Br and B$\theta $ (but not Bz) spectral components at the solar rotation frequency f$_{\mathrm{rot}}$ are quite exceptional, varying between 0{\%} and 30{\%} (average 17{\%}) of the total Brms\textasciicircum 2 magnetic energy. In \textit{only} these variable components (with differing radial dependencies) is there a Br-B$\theta $ anti-correlation, which is traditionally mis-interpreted as a persistent spiral. These f$_{\mathrm{rot}}$ components probably reflect z-currents, arising from $\theta $-z-dependent electric potentials from exceedingly small differences in e-/p$+$ ejection from the rotating solar surface.   

Live

The Caltech solar loop experiment replicates solar corona loops because of similarity in geometry, beta (low), and Lundquist number (high). Understanding loop dynamics gained from this experiment provides insight regarding solar eruption events. By exploring a large experimental parameter space, we have studied the dynamics of the arched magnetically-twisted current-carrying flux rope in regimes which have not been previously accessed. Adjustment of the initial footpoint gas injection creates a controlled density perturbation along the loop and shows that heavier loop segments have less acceleration than lighter segments, leading to dip in the loop profile. A prefilled background gas enables study of the density cavity and shock created by a rapidly expanding magnetically-driven flux rope pushing against a flux-conserving background plasma. Variation of the bias magnetic field reveals the onset of MHD instability and its role as a magnetic reconnection driver. These experimental investigations with supporting numerical and analytic methods provide a comprehensive model showing how a magnetically-twisted current-carrying flux rope exhibits a density dip, induces a magnetic cavity, drives a shock, and undergoes MHD instability leading to magnetic reconnection.   

Live

In two dimensional reconnection involving neutral sheets and magnetic islands it is not difficult to recognize reconnection sites when detailed data sets or simulations are available. This is not the case in fully three dimensional reconnection where there are no obvious reconnection sites. A Quasi Seperatrix Layer (QSL) which, in essence, measures the rapid divergence of magnetic field lines indicates that reconnection is occurring within it. This serves to narrow down the range of possible positions but does not pinpoint it. Here we use a newly developed topological framework for precisely quantifying reconnective activity in complex magnetic fields. It is demonstrated that the regions with the highest reconnective activity are not always where the largest QSL signatures are, thus indicating this is a more complete methodology for quantifying reconnective activity than standard methods. This framework should serve as a model for reconnection analysis in future studies. The work was performed at the Basic Plasma Science Facility, which is funded by DOE (DE-FC02-07ER54918) and the National Science Foundation (NSF-PHY 1036140).