Is null-point reconnection important for solar flux emergence?

The role of null-point reconnection in a 3D numerical MHD model of solar emerging flux is investigated. The model consists of a twisted magnetic flux tube rising through a stratified convection zone and atmosphere to interact and reconnect with a horizontal overlying magnetic field in the atmosphere. Null points appear as the reconnection begins and persist throughout the rest of the emergence, where they can be found mostly in the model photosphere and transition region, forming two loose clusters on either side of the emerging flux tube. Up to 26 nulls are present at any one time, and tracking in time shows that there is a total of 305 overall, despite the initial simplicity of the magnetic field configuration. We find evidence for the reality of the nulls in terms of their methods of creation and destruction, their balance of signs, their long lifetimes, and their geometrical stability. We then show that due to the low parallel electric fields associated with the nulls, null-point reconnection is not the main type of magnetic reconnection involved in the interaction of the newly emerged flux with the overlying field. However, the large number of nulls implies that the topological structure of the magnetic field must be very complex and the importance of reconnection along separators or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc

Right Atrial-ventricular Dissociation and Entrainment while Pacing from High Right Atrium and Coronary Sinus during Circus Movement Tachycardias

Thirteen patients with circus movement tachycardias [CMT] were studied. Twelve had left‐sided, and one, right‐sided, accessory pathways. Entrainment was possible during overdrive high right atrial stimulation in 13/13 patients, and during coronary sinus pacing in 10/12 patients. The minimal pacing rates required for this to occur were 10 to 31 beats/min faster than those of the tachycardias. Short episodes of right atrial‐ventricular dissociation occurred while pacing from the high right atrium [6/13 patients), but not from the coronary sinus (0/13 patients). It is possible to explain this phenomenon by postulating the existence of two distinct atrioventricular (AV) nodal inputs (one for right‐sided and the other for left‐sided impulses); it could also have been an expression of the close distance existing between the AV node and the coronary sinus. Entrainment, by defining a range of paring rates followed by resumption of the tachycardia upon the cessation of stimulation, indicated that faster rates were needed for the CMT to be interrupted. However, regardless of the pacing rate and pacing site, tachycardia termination occurred when an anterograde impulse was blocked at the. AV node. The information obtained from this study suggests that some patients with drug‐resistant CMT may benefit from pacing modes capable of terminating the arrhythmia “through entrainment” at the slowest atrial rate at which this is possible

Western Wildfires: A Policy Change Perspective

My research objective in this article is to analyze policy change affecting wildfire suppression programs administered by the United States Forest Service and the United States Department of the Interior. Using a variant of the punctuated equilibrium approach, a content analysis of New York Times stories dealing with wildfires over the past two decades was examined in relation to both administrative and legislative policy changes. I conclude by suggesting that administrative shifts were undertaken by federal land administrators in response to crises and media attention to protect decisional autonomy as well as forest resources, while the architect of legislative change was a president taking advantage of the combined effects of increasingly intense wildfire seasons, demographic shifts involving the movement of people and structures to the high-risk wildland urban interface areas, greater media scrutiny, and pressure to act from wildfire weary constituencies. Copyright 2006 by The Policy Studies Organization.