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Where is Coronal Plasma Heated?. James A. Klimchuk NASA Goddard Space Flight Center, USA Stephen J. Bradshaw Rice University, USA Spiros Patsourakos University of Ionnina , Greece Durgesh Tripathi Inter-University Centre for Astronomy and Astrophysics, India. Three Basic Scenarios.
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Where is Coronal Plasma Heated? James A. Klimchuk NASA Goddard Space Flight Center, USA Stephen J. Bradshaw Rice University, USA Spiros Patsourakos University of Ionnina, Greece Durgesh Tripathi Inter-University Centre for Astronomy and Astrophysics, India
Three Basic Scenarios steady heating “Steady” Coronal Heating v = 0 impulsive heating Impulsive Coronal Heating thermal cond. evaporation Impulsive Chromospheric Heating (incl. Type II Spicules) impulsive heating expansion
ChromosphericNanoflares (inc. Type II Spicules) impulsive heating expansion • Test hypothesis that all coronal plasma is heated in the chromosphere • Compare predicted and actual observations • 1D hydrodynamic approach: • Once formed, hot high-pressure plasma expands along the field • Expansion dominates; • any initial kick (e.g., spicule ejection) is relatively unimportant • Basic conclusions not altered by Lorentz forces
EUV Spectral Line Profiles(e.g., Fe XIV 274Ǻ) Line profile represents the time-averaged emission from a complete upflow-downflow cycle. Fast upflow blue wing component Slow downflow line core (small red shift) Observed wing/core intensity ratio ≤ 0.05 (Red-Blue Asymmetry) (Hara et al. 2008; De Pontieu et al. 2009; McIntosh & De Pontieu 2009; De Pontieu et al. 2011; Tian et al. 2011; Doschek 2012; Patsourakos et al. 2013; Tripathi & Klimchuk 2013) What is expected? De Pontieu et al. (2009)
Blue Wing-to-Core Intensity Ratio * if all coronal plasma comes from chomosphericnanoflares (incl. type II spicules) nc = coronal density = 3x109 (AR), 109 cm-3 (QS) hc = coronal scale height = 50,000 km A = flux tube area expansion factor = 3 l = initial length of heated plasma = 1000 km v = upflow velocity = 100 km s-1 Klimchuk (2012)
Filling Factor Thehypothesis is incorrect. Only a small fraction of the observed hot coronal plasma is created by chomosphericnanoflares (incl. type II spicules). fs < 2% (Active Regions) < 5% (Quiet Sun) < 8% (Coronal Holes) Klimchuk (2012)
1D Hydro Simulations (Work with Steve Bradshaw) HYDRAD Code: 2 fluid (electrons and ions) Nonequilibrium ionization Adaptive mesh refinement • Initial equilibrium with Tapex = 0.8 MK • Impulsively heat the upper 1000 km of the chromosphere in 10 s • Evolve for 5000 s • Average over space and time Approximate a l-o-s through an arcade with the integrated emission from a single loop of 50,000 km height
Icore IR IB The analytical results are confirmed ….also for loops of different length and heating events of different duration
Type II Spicules • Observational discrepancies if all hot plasma comes from Type II spicules: • Blue wing-to-line core intensity ratios factor 100 too big (Klimchuk 2012) • Coronal-to-LTR emission measure ratios factor 100 too big (K 2012) • Blue wing-to-line core density ratios factor 100 too big (Patsourakos, K, & Young 2013) • Good news: • Type II spicules may explain the bright emission from the LTR (T < 0.1 MK), where traditional coronal heating models fail?
Emission Measure Distribution From type II spicules? Dere & Mason (1993)
Coronal Heating Strands Type-II Spicule Strand Composite (Observed) Line Profile 100 x + = Emission Measure Distribution 100 x + =
Conclusions • Chromosphericnanoflares (incl. type II spicules) provide only a very small fraction of the hot plasma observed in the corona. • Most coronal plasma comes from chromospheric evaporation associated with coronal heating (heating that takes place above the chromosphere). • Spicules contribute substantially to the bright emission from the lower transition region, where traditional coronal heating models are inadequate. • A better understanding of the origin of spicules requires: • Detailed MHD simulations • Better observations (e.g., IRIS, Solar-C, LASSO rocket)
Brightness Decreases with Volume (Expansion) 50,000 km 1000 km EM0 0.006 xEM0 The total (spatially integrated) emission is dimmer by a factor of 157
Type II Spicules Cool (~104 K) plasma rises Most heats to ≤ 0.1 MK and falls Some at the tip heats to ~2 MK and expands to fill the flux tube Hot plasma slowly cools and drains Fe XIV (2 MK) He II (8x104 K) Ca II (104 K) v~ 100 km/s hs~ 10,000 km d ~ 200 km d ~ 10% d hs hs d
Blue Wing (Upflow) Density Expansion (type II spicules): Evaporation (coronal nanoflares): • Observed densities from the Fe XIV 264/274 ratio are: • much smaller than predicted for type II spicules • comparable to predicted for coronal nanoflares Patsourakos, Klimchuk, & Young (2013)
Coronal Nanoflare Frequency • All coronal heating is impulsive • The response of the plasma depends on the frequency of the nanoflares Low Frequency High Frequency “Impulsive” “Steady” trepeat >> tcool trepeat << tcool
Type II Spicules Hinode / SOT
Quiet Sun (De Pontieu et al. , 2007) Ca II (SOT) He II (AIA) Coronal Hole (De Pontieu et al., 2011) Fe IX (AIA)
LTR-to-Corona Emission Measure Ratio (Lower Transition Region: 4.3 < logT < 5.0) Ratio of emission measures in the LTR and corona: Predicted*: > 180 Observed: < 1 Implies a spicule filling factor fs < 1% * if all coronal plasma comes from type II spicules
Adiabatic Cooling If the hot spicule plasma cools adiabatically as it expands, the temperature will drop by a factor = 28 (Scenario A) 6 (Scenario B) For initial temperature T0 = 2 MK, the final (coronal) temperature would be Tc = 7x104 K (Scenario A) 3x105 K (Scenario B) To have Tc = 2 MK at the end of expansion requires additional coronal heating of the same magnitude that produced the hot spicule plasma in the first place!