Diagnosing the time-dependence of active region core heating from the emission measure: II. Nanoflare trains

The time-dependence of heating in solar active regions can be studied by analyzing the slope of the emission measure distribution cool-ward of the peak. In a previous study we showed that low-frequency heating can account for 0% to 77% of active region core emission measures. We now turn our attention to heating by a finite succession of impulsive events for which the timescale between events on a single magnetic strand is shorter than the cooling timescale. We refer to this scenario as a "nanoflare train" and explore a parameter space of heating and coronal loop properties with a hydrodynamic model. Our conclusions are: (1) nanoflare trains are consistent with 86% to 100% of observed active region cores when uncertainties in the atomic data are properly accounted for; (2) steeper slopes are found for larger values of the ratio of the train duration $\Delta_H$ to the post-train cooling and draining timescale $\Delta_C$, where $\Delta_H$ depends on the number of heating events, the event duration and the time interval between successive events ($\tau_C$); (3) $\tau_C$ may be diagnosed from the width of the hot component of the emission measure provided that the temperature bins are much smaller than 0.1 dex; (4) the slope of the emission measure alone is not sufficient to provide information about any timescale associated with heating - the length and density of the heated structure must be measured for $\Delta_H$ to be uniquely extracted from the ratio $\Delta_H/\Delta_C$

X-ray Source Heights in a Solar Flare: Thick-target versus Thermal Conduction Front Heating

Observations of solar flares with RHESSI have shown X-ray sources traveling along flaring loops, from the corona down to the chromosphere and back up. The 28 November 2002 C1.1 flare, first observed with RHESSI by Sui et al. 2006 and quantitatively analyzed by O'Flannagain et al. 2013, very clearly shows this behavior. By employing numerical experiments, we use these observations of X-ray source height motions as a constraint to distinguish between heating due to a non-thermal electron beam and in situ energy deposition in the corona. We find that both heating scenarios can reproduce the observed light curves, but our results favor non-thermal heating. In situ heating is inconsistent with the observed X-ray source morphology and always gives a height dispersion with photon energy opposite to what is observed.Comment: Accepted to Ap

Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares

Observational measurements of active region emission measures contain clues to the time-dependence of the underlying heating mechanism. A strongly non-linear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly non-linear (or linear) scaling of the emission measure indicates a relatively large amount of warm plasma, suggesting that the hot active region plasma is allowed to cool and so the heating is impulsive with a long repeat time. This case is called {\it low-frequency} nanoflare heating and we investigate its feasibility as an active region heating scenario here. We explore a parameter space of heating and coronal loop properties with a hydrodynamic model. For each model run, we calculate the slope $\alpha$ of the emission measure distribution $EM(T) \propto T^\alpha$. Our conclusions are: (1) low-frequency nanoflare heating is consistent with about 36% of observed active region cores when uncertainties in the atomic data are not accounted for; (2) proper consideration of uncertainties yields a range in which as many as 77% of observed active regions are consistent with low-frequency nanoflare heating and as few as zero; (3) low-frequency nanoflare heating cannot explain observed slopes greater than 3; (4) the upper limit to the volumetric energy release is in the region of 50 erg cm$^{-3}$ to avoid unphysical magnetic field strengths; (5) the heating timescale may be short for loops of total length less than 40 Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops

VISUAL CONTROL OF HIGH-VELOCITY FOOT-TARGETING TASKS IN NOVICE AND EXPERT PERFORMERS

The visual regulation of step length and duration during target-directed locomotion was examined in relation to gait mode, approach velocity, obstacle task, and practice during a series of four experiments. Visual regulation was found to decrease in novice performers but increase in expert performers when approach velocity increased. The aptitude of expert performers is partly due to their ability to visually regulate for a greater time and distance during the approach, resulting in more accurate final foot placement. The speed/accuracy trade-off may be a mechanism that protects novice performers from harm whilst negotiating obstacles in the everyday environment

A 1-D theoretical performance analysis and optimisation of a bespoke formulastudent V-Twin engine

This paper examines the engine intended to power a SAE Formula Student car for the Oxford Brookes Racing Formula Student Team. The engine is an amalgamation of multiple students’ research work over a number of years. Therefore, this projects main aim was validation of the engine setup and specifications resulting from previous work, and to find any potential theoretical optimisations that can be implemented before the engine is run for real on a Dynamometer. The work analyses the engine's power performance and component design, as well as exploring the major parameters involved in race-engine design and development. The analysis was carried out via a 1-D simulation model constructed in GT-Suite. This was combined with combustion and dynamics models constructed in Excel, in addition to component and manifold analysis in SolidWorks and CATIA. The objectives of the work were – - Construct a combustion and dynamics model in Excel to analyse the current engine setup. - Identify areas of the engine design for improvement - Validate the previous GT-Power model and construct a new GT-Power model based on the latest version of the GT-Suite software. - Validate the findings in the Excel models using the GT-Power model. - Analyse the current engine performance using all the models. - Propose new hardware for the engine if needed. The bespoke V-Twin engine development showed a 97-98 Brake Horse Power (BHP) theoretical Performance limit. However, the starting engine performance was 70 BHP and the power output was found to be unstable and unpredictable. The analyses of the internal components found that they were optimal for the power performance expected from the engine, with the exception that the current design of the intake and exhaust manifolds were found to limit performance, with the intake plenum found to be too small. The valve timing was also found to limit the power performance, with the timing requiring changing in order to optimise performance. The validation, and the optimising recommended in this project, indicate that the engine’s theoretical power performance could be improved from 70 BHP to 90 BHP. The engine with the improvements recommended in this project can now be mounted on a Dynamometer and the final stages of the engine’s development can be completed in order to approve it for racing.

THE APPROACH, VAULTING PERFORMANCE, AND JUDGE’S SCORE IN WOMEN’S ARTISTIC GYMNASTICS

The approach step, hurdle, and round-off length characteristics of women’s vaulting were examined in relation to post-flight performance and judge’s score during five trials for five gymnasts. Two reference strips with alternating 50cm black and white intervals were placed on either side of the approach area. One 50Hz panning camera filmed the approach, with two stationary 250Hz cameras filming the post-flight vaulting performance. Two qualified judges viewed each trial and provided a performance score. A significant correlation was found between velocity during visual control of the approach, post-flight time (p≤0.01) and judge’s score (p≤0.01). Specifically, increased approach velocity leads to an increase in round-off velocity (p≤0.01), resulting in a short high velocity take-off from the board (p≤0.01)

Quantum channels in nonlinear optical processes

Quantum electrodynamics furnishes a new type of representation for the characterisation of nonlinear optical processes. The treatment elicits the detailed role and interplay of specific quantum channels, information that is not afforded by other methods. Following an illustrative application to the case of Rayleigh scattering, the method is applied to second and third harmonic generation. Derivations are given of parameters that quantify the various quantum channels and their interferences; the results are illustrated graphically. With given examples, it is shown in some systems that optical nonlinearity owes its origin to an isolated channel, or a small group of channels. © 2009 World Scientific Publishing Company