Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

Magnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of single- and multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release.

Flux Rope Formation Due to Shearing and Zipper Reconnection

Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present prior to eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope, and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope.

Shock heating in numerical simulations of kink-unstable coronal loops

An analysis of the importance of shock heating within coronal magnetic fields has hitherto been a neglected area of study. We present new results obtained from nonlinear magnetohydrodynamic simulations of straight coronal loops. This work shows how the energy released from the magnetic field, following an ideal instability, can be converted into thermal energy, thereby heating the solar corona. Fast dissipation of magnetic energy is necessary for coronal heating and this requirement is compatible with the time scales associated with ideal instabilities. Therefore, we choose an initial loop configuration that is susceptible to the fast-growing kink, an instability that is likely to be created by convectively driven vortices, occurring where the loop field intersects the photosphere (i.e. the loop footpoints). The large-scale deformation of the field caused by the kinking creates the conditions for the formation of strong current sheets and magnetic reconnection, which have previously been considered as sites of heating, under the assumption of an enhanced resistivity. However, our simulations indicate that slow mode shocks are the primary heating mechanism, since, as well as creating current sheets, magnetic reconnection also generates plasma flows that are faster than the slow magnetoacoustic wave speed.

Phase mixing of Alfvén pulses and wavetrains propagating in coronal holes

The propagation of Alfvén pulses into an inhomogeneous model of a solar coronal hole is investigated. The algebraic damping of single and bi–polar pulses remains for the leading and trailing pulses as the number of pulses is increased but the decay of the internal pulses returns to the exponential damping of an infinite wavetrain when three pulses or more are present. Thus, wavetrains with most of their energy residing in internal oscillations will be dominated by efficient exponential damping. In contrast, short wavetrains with most of their energy in the leading and trailing pulses will suffer less efficient algebraic damping. The implications of both the damping of these disturbances to the heating of coronal holes and the nonlinear wave pressure to the acceleration of the solar wind are discussed.

Learning partners: preceptor, mentor, facilitator, learner

Health care is undergoing dramatic changes that will have a significant impact on how surgical areas provide services. As perioperative nurses, we will need to perfect our skills in the area of precepting, mentoring, facilitating, and learning to position ourselves for success in the new world of health care delivery. Historically, the roles of preceptor, mentor, facilitator, and learner have been departmentalized. A new model is introduced in this article showing how nurses must move between many roles. This new model is called "Learning Partners."