Dear all,

 

A new paper concerning the time-dependence of active region heating based on emission measures has just been published in ApJ.

 

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

 

Bradshaw, S. J., Klimchuk, J. A., & Reep, J. W. 2012, ApJ, 758, 53

 

http://iopscience.iop.org/0004-637X/758/1/53/pdf/0004-637X_758_1_53.pdf

 

Abstract:

 

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 cm3 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.

 

 

Best wishes,

 

Steve

 

 

Dr Stephen J. Bradshaw

 

Department of Physics and Astronomy, MS-108,

Rice University,

6100 Main Street,

Houston,

TX 77005,

USA.

 

Tel: +1 713 348 4045

Email: stephen.bradshaw@rice.edu