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

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)

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)

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

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 km

A = 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)

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)

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

IB IRIcore

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:

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?

Emission Measure Distribution

Dere & Mason (1993)

From type II spicules?

LineProfile

EmissionMeasure

Distribution

Coronal HeatingStrands

Type-II SpiculeStrand

100 x

100 x

+

+

=

=

Composite(Observed)

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)

Backup Slides

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

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 km d ~ 10%

d hs

d

hs

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

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”

Type II Spicules

Hinode / SOT

Quiet Sun(De Pontieu et al. , 2007)

Coronal Hole(De Pontieu et al., 2011)

Ca II (SOT)

He II (AIA)

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

* if all coronal plasma comes from type II spicules

Implies a spicule filling factor fs < 1%

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!