The dynamical disconnection of sunspots from their magnetic roots

After a dynamically active emergence phase, magnetic flux at the solar surface soon ceases to show strong signs of the subsurface dynamics of its parent magnetic structure. This indicates that some kind of disconnection of the emerged flux from its roots in the deep convection zone should take place. We propose a mechanism for the dynamical disconnection of the surface flux based upon the buoyant upflow of plasma along the field lines. Such flows arise in the upper part of a rising flux loop during the final phases of its buoyant ascent towards the surface. The combination of the pressure buildup by the upflow and the cooling of the upper layers of an emerged flux tube by radiative losses at the surface lead to a progressive weakening of the magnetic field in several Mm depth. When the field strength has become sufficiently low, convective motions and the fluting instability disrupt the flux tube into thin, passively advected flux fragments, thus providing a dynamical disconnection of the emerged part from its roots. We substantiate this scenario by considering the quasi-static evolution of a sunspot model under the effects of radiative cooling, convective energy transport, and pressure buildup by a prescribed inflow at the bottom of the model. For inflow speeds in the range shown by simulations of thin flux tubes, we find that the disconnection takes place in a depth between 2 and 6 Mm for disconnection times up to 3 days.Comment: 11 pages, 5 figures, accepted by A&

Halve the Gap by 2030: Youth Disconnection in America's Cities

One in every seven Americans between the ages of 16 and 24—5.8 million young people in all—are neither working nor in school. These vulnerable teens and young adults are unmoored from institutions that provide knowledge, networks, skills, identity, and direction. Most young people never recover from long spells of disconnection. They carry scars of their lost years for the rest of their lives in the form of lower wages and marriage rates, and higher incarceration and unemployment rates. Early spells of disconnection rain serious blows on long-term health, happiness, and job satisfaction" (p.5-6). This report uses updated data to build on a previous report that ranked the country's 25 largest metropolitan areas as well as the nation's largest racial and ethnic groups in terms of youth disconnection. The report also maps the landscape of youth disconnection and presents the data disaggregated by neighborhood cluster for twenty-five US metro areas

On pinning and snapping of elastic strands

The transient network literature up to now has considered that the connection probability of a free strand does not depend on the strand extension, in contrast with the disconnection probability. We argue that, on thermodynamic grounds, both probabilities must have the same dependence on the strand extension. We propose a model for the pinning/snapping of elastic strands reversibly binding to sticky sites, giving explicit expressions for the connection and disconnection rates.Comment: 4 pages, RevTe

Analysis of the dynamic performance of self-excited induction generators employed in renewable energy generation

Incentives, such as the Feed-in-tariff are expected to lead to continuous increase in the deployment of Small Scale Embedded Generation (SSEG) in the distribution network. Self-Excited Induction Generators (SEIG) represent a significant segment of potential SSEG. The quality of SEIG output voltage magnitude and frequency is investigated in this paper to support the SEIG operation for different network operating conditions. The dynamic behaviour of the SEIG resulting from disconnection, reconnection from/to the grid and potential operation in islanding mode is studied in detail. The local load and reactive power supply are the key factors that determine the SEIG performance, as they have significant influence on the voltage and frequency change after disconnection from the grid. Hence, the aim of this work is to identify the optimum combination of the reactive power supply (essential for self excitation of the SEIG) and the active load (essential for balancing power generation and demand). This is required in order to support the SEIG operation after disconnection from the grid, during islanding and reconnection to the grid. The results show that the generator voltage and speed (frequency) can be controlled and maintained within the statuary limits. This will enable safe disconnection and reconnection of the SEIG from/to the grid and makes it easier to operate in islanding mode

Solidification of porous interfaces and disconnection

In this article we obtain uniform estimates on the absorption of Brownian motion by porous interfaces surrounding a compact set. An important ingredient is the construction of certain resonance sets, which are hard to avoid for Brownian motion starting in the compact set. As an application of our results, we substantially strengthen the results of arXiv:1412.3960, and obtain when $d \ge 3$, large deviation upper bounds on the probability that simple random walk in $Z^d$, or random interlacements in $Z^d$, when their vacant set is in a strongly percolative regime, disconnect the discrete blow-up of a regular compact set from the boundary of the discrete blow-up of a box containing the compact set in its interior. Importantly, we make no convexity assumption on the compact set. It is plausible, although open at the moment, that the upper bounds that we derive in this work match in principal order the lower bounds of Xinyi Li and the second author (see arXiv:1310.2177) in the case of random interlacements, and of Xinyi Li (see arXiv:1412.3959) for the simple random walk.Comment: 46 pages, 4 figures, appears in the Journal of the European Mathematical Societ

Initial overview of disconnection events in Halley's Comet 1986

We present an initial overview of the disconnection events (DE's) in Comet Halley in 1986. Although disconnection events are arguably the most spectacular of all dynamic comet phenomena, the mechanisms by which they occur are not fully understood. It is generally believed that the solar wind plays a major role in determining when disconnection events occur, but the details of the solar wind/cometary interactions responsible for initiating the tail disconnection are still under debate. The three most widely accepted models are: (1) high speed streams in the solar wind cause the tail to disconnect due to pressure effects; (2) decreased production of cometary ions in a high speed stream allows magnetic field to slip away from the comet; and (3) the tail disconnects after frontside reconnection of the interplanetary magnetic field (IMF) as the comet crosses a magnetic field sector boundary. We find that the front-side magnetic reconnection model is the best explanation for the DE's we have considered