Heating and Dynamics of the Corona:
Why We Need to Observe Very Hot & Faint Plasmas

James A. KlimchukNASA / GSFC

Key Points•

These comments apply to all forms of heating(“ordinary” coronal heating, flares, CMEs, X-ray jets, ….)•We have yet to observe directly the energy release process– e.g., we have never seen a reconnection outflow jet•Reconnection is a fundamental process, yet we do not know its basic
properties– Is it simple (Petschek) or complex (tearing islands)?

•All heating is impulsive when viewed properly•

Heated plasma is initially very hot and faint•

Hot plasma cools very quickly– Important information is lost by the time it reaches “traditional”
coronal temperatures

All Heating is Impulsive•

To understand how plasma responds to heating (and produces
observed radiation), we need to consider individual magnetic fieldlines (flux strands).

•Plasma and heat flow along the field, so strands behave likeindependent, thermally insulated flow tubes.

•The heating on individual field lines is impulsive even when theheating over a large volume is steady.

•Three examples:
1. Long duration flare
2. Secondary instability
3. Resonant wave absorption

Courtesy of Tsuneta-san(modified)(Tsuneta, ApJ, 456, 840, 1996)

Inflow
MHD fast
mode shock

Outflow

Reconnected field lines move
rapidly away from the X-point.
Heating occurs only while they
are in contact with standing
slow mode shocks (˜ 10 sec):
impulsive.

BrightX-ray loop

Reconnection
site
MHD slowmode shock
Loop top
Hard X-ray
sourceHightemperature
ridgeStandard 2-D picture of solar flares
4

Secondary Instability• Impulsive heating occurs atcurrent sheets when themagnetic field is sufficiently
sheared• e.g., at the interface between
adjacent flux strands in acoronal field that is tangled byphotospheric convection(Parker picture)

Dahlburg, Klimchuk, & Antiochos (2003, 05, 08)

Resonant Wave Absorption• Usual damping mechanisms

are not efficient for low
frequency (τ ∼ 100 s) waves.

Heating Profile at 3 Times• Resonant absorption occurs
on magnetic field lines where
the local resonancefrequency matches the global
loop oscillation frequency.
• Resonance conditions
change as the plasma
responds to the heating• Resonance layers drift
across the loop, heating fieldlines impulsively as they
pass by.

Distance from loop axis

Ofman, Klimchuk, & Davila (1998)

Consequences of Impulsive Heating•

Temperature rises rapidly to very
high values.

•Plasma cools rapidly initially.
Valuable information is lost.

•Density and emission measure aresmall. Evaporation takes time to filla strand.

•Plasma is faint due to combination ofsmall EM and short lifetime.

•High velocity evaporative upflows(only when plasma is hot and faint).

Tnv

500 s nanoflare simulation

What Do Observations Show?
•So far, we only observe plasma that has cooled appreciably•

Strong evidence that many coronal loops are bundles ofunresolved strands that are heated by “storms” of nanoflares– Hinode 2 talk in Boulder– Klimchuk (2006, Solar Phys., 234, 41)•

True of all loops? True of the diffuse corona?

•

We do not know the details of the energy release– e.g., magnitude and duration of the nanoflare•

Some clues starting to come from Hinode/XRT

Hinode / XRT (Reale et al. 2008)log EM (cm-3)

Be_med Image

Log TT map

Hinode / XRT (Schmelz et al. 2008)
Observation

Cool part not constrainedby observations

Simulation

Two component model(large and small nanoflares)
Steady heating not plausible
20 MK equilibrium loop requires:
Energy flux ~ 109 erg cm-2 s-1
Footpoint velocity ~ 100 km s-1

Hinode/EIS:Fe XII – XVIICa IV – VI
Ni XVII

Patsourakos & Klimchuk (2008)

What can we learn by observing hot plasma?
•DEM(T) reveals the distribution of nanoflare energies•Dynamics of the energy release depend on the mechanism:– Alfvenic jets (Petschek reconnection)?
– Turbulence (secondary instability)?
– Large-amplitude oscillations (resonance absorption layer)?

•Velocity signatures disappear by the time the plasma cools to“traditional” coronal temperatures•

Flows revealed by spectral line profiles or by direct imaging inlarge events (flares, CMEs)– First ever observation of reconnection outflow– Simple jet or tearing islands?

•Evaporative upflows are a sensitive diagnostic of heatingDEM(T) depends on nanoflare energy distribution

Nanoflare Power Law

Two Nanoflare Components

Seems to best fit the observations.
Surprising! Important!
Uncertain. Need Solar-C!

What can we learn by observing hot plasma?
•DEM(T) reveals the distribution of nanoflare energies•

Dynamics of the energy release depend on the mechanism:– Alfvenic jets (Petschek reconnection)?
– Turbulence (secondary instability)?
– Large-amplitude oscillations (resonance absorption layer)?

•Velocity signatures disappear by the time the plasma cools to“traditional” coronal temperatures•Flows revealed by spectral line profiles or by direct imaging inlarge events (flares, CMEs)– First ever observation of reconnection outflow– Simple jet or tearing islands?

•Evaporative upflows are a sensitive diagnostic of heating

Simulated Line Profiles
Fe XVII
(254)
5.1MK

Fe XVII
(254)
5.1MK

Footpoint

Fe XVII
(254)
5.1MK

Mg X(625)
1.3MK

Patsourakos & Klimchuk (2006)

Observed Fe XVII Profile

EIS sit and stareobservations
Line is very faintProposed x100
increase inthroughput is aquantum
improvement

See also Hara et al. (2008)

Observational Priorities
1.

Sensitivity to faint hot emission (high throughput)–
T ~ 5-10 MK minimum, 10-30 MK desired–
DEM(T) ~ 1019 cm-5 K-1

2.

Temperature discrimination–
ΔT ~ 1 MK–
Ability to distinguish faint hot emission from bright cool emission
along the l-o-s

3.

Spatial resolution–
0.1 arc sec (100 km) for elemental coronal flux strands–
Short-lived hot plasma reduces l-o-s overlap problems–
Tremendous progress possible with much coarser resolution

4.

Temporal resolution–
None required for DEM(T) over large areas–
100 s for coronal loops–
1-10 s for spatially resolved reconnection outflows (flares, CMEs)

BACKUP SLIDESPredicted Hot Line Intensities
DEM(T) = 1018 cm-5 K-1
0.1 x 0.1 arcsec2 pixel
100 s exposure
Wave.

Intensity*

Ion

logT

Wave.

Intensity*

Ion

logT

15.0150
15.2620
16.0760
16.7770
17.0500
17.0970
93.9232101.5498
102.2172103.9370
108.3555
117.1543
117.4996
118.6801
119.9836
121.8448
128.7526
132.8405

5.68e-01
1.61e-01
1.13e-01
3.27e-01
4.17e-01
3.48e-01
7.29e-01
1.74e-01
1.30e-01
2.64e-01
5.56e-01
3.28e-01
1.18e-01
2.98e-01
1.49e-01
5.84e-01
7.49e-01
8.29e-01

Fe XVIIFe XVIIFe XVIIIFe XVIIFe XVIIFe XVIIFe XVIIIFe XIXFe XXIFe XVIIIFe XIXFe XXII
Fe XXIFe XX
Fe XIXFe XX
Fe XXI
Fe XX

6.6

132.9065

3.29e-01

Fe XXIII

7.2

135.7912
192.8198204.6655
254.3466
302.1902
320.5660
344.7605
350.4782
353.9203
567.8666
592.2357
721.5593
845.5715
974.8602
1118.0575
1354.0665

3.22e-01
3.57e-01
1.01e-01
1.12e-01
2.49e-01
2.85e-01
1.29e-01
1.19e-01
1.18e-01
2.08e-01
2.54e-01
3.76e-01
2.29e-01
8.46e-01
4.57e-01
9.17e-01

Fe XXIICa XVIIFe XVIIFe XVIICa XVIIINi XVIIICa XVIIIFe XVIIAr XVIFe XX
Fe XIXFe XX
Fe XXIIFe XVIIIFe XIXFe XXI

7.1
6.7

6.8

6.9
7.0

7.0

* photons cm-2 sr-1

6.7

Significant Heating Occurs in the Corona•

Coronal magnetic field becomestangled and twisted by random
footpoint motions associated withphotospheric convection.

•“Reconnection” between coronal
flux strands is necessary to preventa monotonic buildup of magnetic
stress and energy.

•Reconnection between a coronal
flux strand and a short magnetic
carpet loop increases the tangling(makes the problem worse).

•

There is no magnetic carpet in
active regions, so cannot heat there.

Hinode / SOT G-band