1. 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

2. Three Basic Scenarios steady heating impulsive heating impulsive heating v = 0 evaporation expansion thermal cond. “ Steady” Coronal Heating Impulsive Coronal Heating Impulsive Chromospheric Heating (incl. Type II Spicules)

3. 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 Chromospheric Nanoflares (inc. Type II Spicules)

4. 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)

5. Blue Wing-to-Core Intensity Ratio Predicted*Observed Active Reg> 3.4≤ 0.05 Quiet Sun> 1.1≤ 0.05 Coronal Hole> 0.7≤ 0.05 n c = coronal density = 3x10 9 (AR), 10 9 cm -3 (QS) h c = 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) * if all coronal plasma comes from chomospheric nanoflares (incl. type II spicules)

6. Filling Factor f s < 2% (Active Regions) < 5% (Quiet Sun) < 8% (Coronal Holes) The hypothesis is incorrect. Only a small fraction of the observed hot coronal plasma is created by chomospheric nanoflares (incl. type II spicules). Klimchuk (2012)

7. 1D Hydro Simulations (Work with Steve Bradshaw) HYDRAD Code: 2 fluid (electrons and ions) Nonequilibrium ionization Adaptive mesh refinement Initial equilibrium with T apex = 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

8. IBIB IRIR I core The analytical results are confirmed ….also for loops of different length and heating events of different duration

9. Type II Spicules Observational discrepancies if all hot plasma comes from Type II spicules: 1.Blue wing-to-line core intensity ratios factor 100 too big (Klimchuk 2012) 2.Coronal-to-LTR emission measure ratios factor 100 too big (K 2012) 3.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?

10. Emission Measure Distribution Dere & Mason (1993) From type II spicules?

11. Line Profile Emission Measure Distribution Coronal Heating Strands Type-II Spicule Strand 100 x + + = = Composite (Observed)

12. Conclusions Chromospheric nanoflares (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)

13. Backup Slides

14. Brightness Decreases with Volume (Expansion) 1000 km 50,000 km EM 0 0.006 x EM 0 The total (spatially integrated) emission is dimmer by a factor of 157

15. Type II Spicules Fe XIV (2 MK) He II (8x10 4 K) Ca II (10 4 K) 1.Cool (~10 4 K) plasma rises 2.Most heats to ≤ 0.1 MK and falls 3.Some at the tip heats to ~2 MK and expands to fill the flux tube 4.Hot plasma slowly cools and drains v ~ 100 km/s h s ~ 10,000 km d ~ 200 km  ~ 10%  h s d hshs

16. 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)

17. Coronal Nanoflare Frequency  repeat <<  cool  repeat >>  cool Low FrequencyHigh Frequency All coronal heating is impulsive The response of the plasma depends on the frequency of the nanoflares “Steady” “Impulsive”

18. Type II Spicules Hinode / SOT

19. Quiet Sun (De Pontieu et al., 2007) Coronal Hole (De Pontieu et al., 2011) Ca II (SOT) He II (AIA) Fe IX (AIA)

20. LTR-to-Corona Emission Measure Ratio (Lower Transition Region: 4.3 < log T < 5.0) Ratio of emission measures in the LTR and corona: Predicted * : > 180 Observed: < 1 * if all coronal plasma comes from type II spicules Implies a spicule filling factor f s < 1%

21. 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 T 0 = 2 MK, the final (coronal) temperature would be T c = 7x10 4 K (Scenario A) 3x10 5 K (Scenario B) To have T c = 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!