Where is Coronal Plasma Heated?
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Where is Coronal Plasma Heated?
James A. KlimchukNASA Goddard Space Flight Center, USA
Stephen J. BradshawRice University, USA
Spiros Patsourakos
University of Ionnina, Greece
Durgesh TripathiInter-University Centre for Astronomy and Astrophysics, India
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Three Basic Scenarios
steadyheating
impulsive heating
impulsive heating
v = 0
evaporation
expansion
thermal cond.
“Steady”Coronal Heating
ImpulsiveCoronal Heating
ImpulsiveChromospheric Heating (incl. Type II Spicules)
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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)
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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)
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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
nc = coronal density = 3x109 (AR), 109 cm-3 (QS)hc = coronal scale height = 50,000 kmA = flux tube area expansion factor = 3l = initial length of heated plasma = 1000 kmv = upflow velocity = 100 km s-1 Klimchuk (2012)
* if all coronal plasma comes from chomospheric nanoflares (incl. type II spicules)
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Filling Factor
fs < 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)
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1D Hydro Simulations(Work with Steve Bradshaw)
HYDRAD Code:2 fluid (electrons and ions)Nonequilibrium ionizationAdaptive 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
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IB IRIcore
The analytical results are confirmed
….also for loops of different length and heating events of different duration
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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?
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Emission Measure Distribution
Dere & Mason (1993)
From type II spicules?
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LineProfile
EmissionMeasure
Distribution
Coronal HeatingStrands
Type-II SpiculeStrand
100 x
100 x
+
+
=
=
Composite(Observed)
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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)
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Backup Slides
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Brightness Decreases with Volume (Expansion)
1000 km
50,000 km
EM0
0.006 x EM0
The total (spatially integrated) emission is dimmer by a factor of 157
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Type II Spicules
Fe XIV (2 MK)He II (8x104 K)Ca II (104 K)
1. Cool (~104 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/shs ~ 10,000 kmd ~ 200 kmd ~ 10%
d hs
d
hs
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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)
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Coronal Nanoflare Frequency
trepeat << tcooltrepeat >> tcool
Low Frequency High Frequency
• All coronal heating is impulsive
• The response of the plasma depends on the frequency of the nanoflares
“Steady”“Impulsive”
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Type II Spicules
Hinode / SOT
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Quiet Sun(De Pontieu et al. , 2007)
Coronal Hole(De Pontieu et al., 2011)
Ca II (SOT)
He II (AIA)
Fe IX (AIA)
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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
* if all coronal plasma comes from type II spicules
Implies a spicule filling factor fs < 1%
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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!
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