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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>Dear all,<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>A new paper concerning the time-dependence of active region heating based on emission measures has just been published in ApJ.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>Diagnosing the time-dependence of active region core heating from the emission measure. I. Low-frequency nanoflares<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>Bradshaw, S. J., Klimchuk, J. A., & Reep, J. W. 2012, ApJ, 758, 53<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><a href="http://iopscience.iop.org/0004-637X/758/1/53/pdf/0004-637X_758_1_53.pdf">http://iopscience.iop.org/0004-637X/758/1/53/pdf/0004-637X_758_1_53.pdf</a><o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>Abstract:<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt'>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 <i>low-frequency </i>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 <i>α </i>of the emission measure distribution EM(<i>T </i>) </span><span style='font-size:12.0pt;font-family:"Cambria Math","serif"'>∝</span><span style='font-size:12.0pt'> </span><i><span style='font-size:12.0pt'>T α</span></i><span style='font-size:12.0pt'>. 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</span><span style='font-size:12.0pt'>−</span><span style='font-size:12.0pt'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt'>Best wishes,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt'>Steve<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Dr Stephen J. Bradshaw<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Department of Physics and Astronomy, MS-108,<o:p></o:p></p><p class=MsoNormal>Rice University,<o:p></o:p></p><p class=MsoNormal>6100 Main Street,<o:p></o:p></p><p class=MsoNormal>Houston,<o:p></o:p></p><p class=MsoNormal>TX 77005,<o:p></o:p></p><p class=MsoNormal>USA.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Tel: +1 713 348 4045<o:p></o:p></p><p class=MsoNormal>Email: stephen.bradshaw@rice.edu<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>