Browsing by Author "Reep, J.W."
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Item Diagnosing the Time Dependence of Active Region Core Heating from the Emission Measure. II. Nanoflare Trains(The American Astronomical Society, 2013) Reep, J.W.; Bradshaw, S.J.; Klimchuk, J.A.The time dependence of heating in solar active regions can be studied by analyzing the slope of the emission measure distribution coolward 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 ΔH to the post-train cooling and draining timescale ΔC, where ΔH depends on the number of heating events, the event duration and the time interval between successive events (τC); (3) τC 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 ΔH to be uniquely extracted from the ratio ΔH /ΔC.Item DIAGNOSING THE TIME-DEPENDENCE OF ACTIVE REGION CORE HEATING FROM THE EMISSION MEASURE. I. LOW-FREQUENCY NANOFLARES(The American Astronomical Society, 2012) Bradshaw, S.J.; Klimchuk, J.A.; Reep, J.W.Observational measurements of active region emission measures contain clues to the time dependence of the underlying heating mechanism. A strongly nonlinear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly nonlinear (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 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 α of the emission measure distribution EM(T ) ∝ T α. 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 40Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops.