High-Lundquist Number Scaling in Three-Dimensional Simulations of Parker's Model of Coronal Heating

Parker's model is one of the most discussed mechanisms for coronal heating and has generated much debate. We have recently obtained new scaling results in a two-dimensional (2D) version of this problem suggesting that the heating rate becomes independent of resistivity in a statistical steady state [Ng and Bhattacharjee, Astrophys. J., 675, 899 (2008)]. Our numerical work has now been extended to 3D by means of large-scale numerical simulations. Random photospheric footpoint motion is applied for a time much longer than the correlation time of the motion to obtain converged average coronal heating rates. Simulations are done for different values of the Lundquist number to determine scaling. In the high-Lundquist number limit, the coronal heating rate obtained so far is consistent with a trend that is independent of the Lundquist number, as predicted by previous analysis as well as 2D simulations. In the same limit the average magnetic energy built up by the random footpoint motion tends to have a much weaker dependence on the Lundquist number than that in the 2D simulations, due to the formation of strong current layers and subsequent disruption when the equilibrium becomes unstable. We will present scaling analysis showing that when the dissipation time is comparable or larger than the correlation time of the random footpoint motion, the heating rate tends to become independent of Lundquist number, and that the magnetic energy production is also reduced significantly.Comment: Accepted for publication in Astrophysical Journa

Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

Alfv\'en waves may be generated via mode conversion from fast magneto-acoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helio-seismology. In active regions this reflection typically occurs high enough that the Alfv\'en speed $a$ greatly exceeds the sound speed $c$, well above the $a=c$ level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfv\'en conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold MHD model $c\to0$. This provides a benchmark for fast-to-Alfv\'en mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfv\'en speed profile with density scale height $h$, the Alfv\'en conversion coefficient depends on three variables only; the dimensionless transverse-to-the-stratification wavenumber $\kappa=kh$, the magnetic field inclination from the stratification direction $\theta$, and the polarization angle $\phi$ of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfv\'en waves typically occurs for small $\theta$ and large $\phi$ ($80^\circ$--$90^\circ$), though it is absent entirely when $\theta$ is exactly zero (vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.Comment: 14 pages plus supplementary tables. Astrophys J (accepted 25 May 2011). Two ancillary animations (animated gif) attache

Sharing the burden of adapting to increasing flood risk: who pays for flood insurance and flood risk management investment in the United Kingdom

Many countries are exploring alternative strategies to counter rising flood risk as there is concern at the extra burden that such increasing risk will bring. The aim of this paper is to explore the nature of these burdens, and outline responses in the United Kingdom where both the government and the private flood insurers have new policies and proposals. Our method is to collate the extensive existing authoritative data and information - from government and the insurance industry - about the risks that are being experienced and the related policy responses. The results show that these seek to concentrate somewhat more the financial burden of, respectively, flood risk management costs and insurance provisions on to those who are at risk and away from the general taxpayer and those who pay insurance premiums. Other countries may well learn from these developments. The pre-existing cross-subsidies are being reduced and, in this way, it is hoped that extra resources for risk management investment will be forthcoming (from local contributions from at-risk communities) and flood insurance will remain affordable, available and commercially viable. A key conclusion here is that it appears that any increase in flood frequency and severity in the UK appears likely to affect the financially deprived communities to a greater extent than others, not least because they are less likely to insure. Government arrangements to prioritise their contribution to risk reducing towards these financially deprived communities is a sign that this regressive effect of floods is real and serious, and those arrangements are to be welcomed

Consequences of spontaneous reconnection at a two-dimensional non-force-free current layer

Magnetic neutral points, where the magnitude of the magnetic field vanishes locally, are potential locations for energy conversion in the solar corona. The fact that the magnetic field is identically zero at these points suggests that for the study of current sheet formation and of any subsequent resistive dissipation phase, a finite beta plasma should be considered, rather than neglecting the plasma pressure as has often been the case in the past. The rapid dissipation of a finite current layer in non-force-free equilibrium is investigated numerically, after the sudden onset of an anomalous resistivity. The aim of this study is to determine how the energy is redistributed during the initial diffusion phase, and what is the nature of the outward transmission of information and energy. The resistivity rapidly diffuses the current at the null point. The presence of a plasma pressure allows the vast majority of the free energy to be transferred into internal energy. Most of the converted energy is used in direct heating of the surrounding plasma, and only about 3% is converted into kinetic energy, causing a perturbation in the magnetic field and the plasma which propagates away from the null at the local fast magnetoacoustic speed. The propagating pulses show a complex structure due to the highly non-uniform initial state. It is shown that this perturbation carries no net current as it propagates away from the null. The fact that, under the assumptions taken in this paper, most of the magnetic energy released in the reconnection converts internal energy of the plasma, may be highly important for the chromospheric and coronal heating problem

Promoting adaptive flood risk management: the role and potential of flood recovery mechanisms

There is a high potential for recovery mechanisms to be used to incentivise the uptake of flood mitigation and loss reduction measures, undertake adaptation and promote community resilience. Indeed, creating a resilient response to flooding requires flood risk management approaches to be aligned and it needs to be ensured that recovery mechanisms to not provide disincentives for individuals and business to take proactive action to reduce risk. However, the degree to which it is desirable and effective for insurers and governments providing compensation to promote resilience and risk reduction depends upon how the cover or compensation is organised and the premiums which are charged. A review of international flood recovery mechanisms has been undertaken to identify firstly the types of schemes that exist and their characteristics. Analysis of existing instruments highlights that there are various potential approaches to encourage or require the uptake of flood mitigation and also discourage the construction of new development in high flood risk. However despite the presence of these instruments, those organising recovery mechanisms could be doing much more to incentivise increased resilience

Distinguishing Solar Flare Types by Differences in Reconnection Regions

Observations show that magnetic reconnection and its slow shocks occur in solar flares. The basic magnetic structures are similar for long duration event (LDE) flares and faster compact impulsive (CI) flares, but the former require less non-thermal electrons than the latter. Slow shocks can produce the required non-thermal electron spectrum for CI flares by Fermi acceleration if electrons are injected with large enough energies to resonate with scattering waves. The dissipation region may provide the injection electrons, so the overall number of non-thermal electrons reaching the footpoints would depend on the size of the dissipation region and its distance from the chromosphere. In this picture, the LDE flares have converging inflows toward a dissipation region that spans a smaller overall length fraction than for CI flares. Bright loop-top X-ray spots in some CI flares can be attributed to particle trapping at fast shocks in the downstream flow, the presence of which is determined by the angle of the inflow field and velocity to the slow shocks.Comment: 15 pages TeX and 2 .eps figures, accepted to Ap.J.Let

Gravitational collapse of spherically symmetric plasmas in Einstein-Maxwell spacetimes

We utilize a recent formulation of a spherically symmetric spacetime endowed with a general decomposition of the energy momentum tensor [Phys. Rev. D, 75, 024031 (2007)] to derive equations governing spherically symmetric distributions of electromagnetic matter. We show the system reduces to the Reissner-Nordstrom spacetime in general, spherically symmetric coordinates in the vacuum limit. Furthermore, we show reduction to the charged Vaidya spacetime in non-null coordinates when certain equations of states are chosen. A model of gravitational collapse is discussed whereby a charged fluid resides within a boundary of finite radial extent on the initial hypersurface, and is allowed to radiate charged particles. Our formalism allows for the discussion of all regions in this model without the need for complicated matching schemes at the interfaces between successive regions. As further examples we consider the collapse of a thin shell of charged matter onto a Reissner-Nordstrom black hole. Finally, we reduce the entire system of equations to the static case such that we have the equations for hydrostatic equilibrium of a charged fluid.Comment: Accepted for publication in Phys. Rev.

Changes in Power Output in NCAA Football Linemen During Competitive Season

Changes in Power Output in NCAA Football Linemen During Competitive Season. Posey, Q., R. Cole, and J. Priest, Tarleton State University, Stephenville, TX 76402 Introduction Measuring power is a practice currently being developed by researchers. An available tool is the TENDO Weightlifting Analyzer (TWA). Although the TWA is a common research tool, there is little published research. The purpose of this study is to analyze OL and DL power output during in-season football. Methods Experimental Approach Seventeen NCAA division II football players in the Lone Start Conference were monitored during organized in-season weight training workouts. TWA measured and recorded their last set of squat. Bio feedback provided by the TWA was used to analyze each group. Subjects Seventeen NCAA division II football players (Age 21.1 ± 4.6 yrs, Ht. 1.6 ± 0.01m, Wt. 123.1 ± 7.4 kg , BMI 35.3 ± 3.2 kg.m-2), volunteered for the study, and had previously trained at least twice per week for 12 weeks. Subjects were familiarized with the TWA and squat protocol during pre-season. Protocol All subjects were required to lift four times a week. On the third workout of every week subjects back-squatted. All subjects completed a standardized warm-up. Subjects determine their own lifting weight. Researchers monitored squats and emphasized bar speed. The TWA was attached to the outside of the bar and measured average power (AP) and peak power (PP) output. Measurements were uploaded from the TWA into TENDO Sports Machine computer program and exported to Microsoft Excel®. Results Repeated measures ANOVA revealed no change in AP (F(3,45)=0.996, p\u3e.05), change in overall PP (F(3,45)=15.3, p\u3c.001) across 4 measures of the competitive season. No group interaction for AP (F(3,45)=.488, p\u3e.05), but PP by group interaction (F(3,45)=6.07, p=.001). AP 1 (W) AP 2 (W) AP 3 (W) AP 4 (W) PP 1 (W) PP 2 (W) PP 3 (W) PP4 (W) OL 911 ± 136 910 ± 124 850 ± 89 893 ± 19 1507 ± 251 1775 ± 258 1207 ± 141 1690 ±142 DL 947 ± 214 905 ± 184 904 ± 184 880 ± 110 1605 ± 343 1728 ± 487 1637 ± 317 1801 ± 215 Table 1. Average (AP) and Peak Power (PP) of Offensive (OL) and Defensive Linesmen (DL) During Competitive Season. Discussion The competitive football season produced normal bumps, bruises, and sprains which impacted the results obtained from bi-weekly measures of AP and PP. The observed changes in PP were attributed to the changes in peak bar velocities for this instantaneous measure, whereas the stability of AP was explained by the less volatile factor of average bar velocities. Conclusion Organized in-season weight training activities are effective at maintaining power output of offensive and defensive linemen

Time-energy correlations in solar flare occurrence

The existence of time-energy correlations in flare occurrence is still an open and much debated problem. This study addresses the question whether statistically significant correlations are present between energies of successive flares as well as energies and waiting times. We analyze the GOES catalog with a statistical approach based on the comparison of the real catalog with a reshuffled one where energies are decorrelated. This analysis reduces the effect of background activity and is able to reveal the role of obscuration. We show the existence of non-trivial correlations between waiting times and energies, as well as between energies of subsequent flares. More precisely, we find that flares close in time tend to have the second event with large energy. Moreover, after large flares the flaring rate significantly increases, together with the probability of other large flares. Results suggest that correlations between energies and waiting times are a physical property and not an effect of obscuration. These findings could give important information on the mechanisms for energy storage and release in the solar corona

Deconstructing active region AR10961 using STEREO, HINODE, TRACE and SOHO

Active region 10961 was observed over a five day period (2007 July 2-6) by instrumentation on-board STEREO, Hinode, TRACE and SOHO. As it progressed from Sun centre to the solar limb a comprehensive analysis of the EUV, X-ray and magnetic field data reveals clearly observable changes in the global nature of the region. Temperature analyses undertaken using STEREO EUVI double filter ratios and XRT single and combined filter ratios demonstrate an overall cooling of the region from between 1.6 - 3.0 MK to 1.0 - 2.0 MK over the five days. Similarly, Hinode EIS density measurements show a corresponding increase in density of 27%. Moss, cool (1 MK) outer loop areas and hotter core loop regions were examined and compared with potential magnetic field extrapolations from SOHO MDI magnetogram data. In particular it was found that the potential field model was able to predict the structure of the hotter X-ray loops and that the larger cool loops seen in 171 Angstrom images appeared to follow the separatrix surfaces. The reasons behind the high density moss regions only observed on one side of the active region are examined further