Chromospheric Magnetic Chromospheric Magnetic Reconnection Reconnection

from an Observer’s Point of Viewfrom an Observer’s Point of View

Jongchul ChaeJongchul Chae

Seoul National University, KoreaSeoul National University, Korea

What is Chromospheric What is Chromospheric Reconnection ?Reconnection ?

Magnetic reconnection occurring in the chromosphere and photosphere, not in the corona

Lower Alitudes: 0 to 104 km Lower temperatures: a few 103 K to a few 105 KHigher densitiesSmall-scale low altitudeDiversities in flow speed, density and temperature strong stratification Driven reconnection

Flux emergenceSupergranular flow

Observational SignaturesObservational Signatures

Canceling Magnetic FeaturesJet-like Features seen in H and UV/FUV/EUV

UV/EUV jets, UV explosive eventsH alpha jets/ surges/ H alpha upflow events

Chromospheric Brightenings Ellerman bombsOther brightenings in UV/EUV/ H alpha

Canceling Magnetic Canceling Magnetic FeaturesFeatures

Canceling Magnetic FeatureCanceling Magnetic Feature

From Chae, Moon, Park 2003, JKAS 36, S13

Interior

CMF as a CMR eventCMF as a CMR event

cv Photosphere

Chromosphere

Corona

ivDOWNFLOW

UPFLOW

FLUX CANCELLATIONCONVERGING MOTION

Does the flux submerge in Does the flux submerge in CMFs?CMFs?

YES! The ASP observations produced the evidence for it.

From Chae, Moon, Pevtsov 2004, ApJL, 602, L65

Observables of CMFObservables of CMF

Rate of Magnetic Flux Loss

Half length of interface between two poles

Specific flux loss rate

Converging speed of each pole toward PIL

d

dt

cv

cL

From Chae, Moon, Park 2003, JKAS 36, S13

obsc

dr

L dt

Summary of CMF Summary of CMF ObservationsObservations

Chae et al. 2002 Case A

Chae et al. 2002 Case B

Chae et al. 2003

Chae et al.1998

Flux loss rate Mx/h

3.4 x 1018

2.5 x 1018

1.8x101

8

2x101

7

Contact length Mm

7.8 3.3 2.5 3

Specific flux loss rate G cm/s

1.2 x 106

1.1x106 2.0x106

2x105

Converging speed km/s

0.27 0.35 0.22

Chromospheric Jets in Active Chromospheric Jets in Active RegionsRegions

EUV Jets

-1 5 10 -3150 250 km s , 2 3 10 K, 1 10 cmev T n From Chae, J. 2003, ApJ 584, 1084

HH Jets in the same active Jets in the same active regionregion

-1 410 40 km s , 10 Kv T

Chae, J. et al. 2000, Solar Physics 195, 333

EUV/HEUV/H Jets in another AR Jets in another AR

-1 550 100 km s , 2 3 10 Kv T

-1 430 km s , 10 Kv T

From Chae et al. 1999 ApJ 513, L75

Jet-like features in the Jet-like features in the quiet Sunquiet Sun

H upflow events

-1 420 km s , 10 Kv T

From Chae et al. 1998, ApJ, 504, L123

Jet-like in the quiet SunJet-like in the quiet Sun

UV explosive events

-1 570 km s , 10 Kv T

From Chae et al. 1998, ApJ, 479, L109

Summary of Jet Summary of Jet Observations Observations

Jet-like features occur in strong association with canceling magnetic features.There is a good correlation between speed and temperature in jet-like features.Jet-like features with different temperatures often occur together at the same place.

Theoretical Considerations of Theoretical Considerations of Chromospheric ReconnectionChromospheric Reconnection

Adiabatic Current Sheet of Sweet-Parker typeInsights on Chromospheric Reconnection from Observations of Jet-like Features Insights on Chromospheric Reconnection from Observations of Canceling Magnetic Features

Adiabatic Current Sheet Model Adiabatic Current Sheet Model

of Sweet-Parker typeof Sweet-Parker type

width (inflowing length)2 : L

thickness (outflowing length)2 : l

length (in the third direction): zL

Current Sheet Model of Current Sheet Model of CMRCMR

o o z

dv B L

dt

Supposing z cL L

6 -12 10 G cm so oz

dr v B

L dt

Steady-state Current Sheet Steady-state Current Sheet ModelModel

Sweet-Parker Model

Incompressible flow

Litvinenko (1999)Compressible, isothermal flow

Chae et al. (2003)Adiabatic flowA generalized approach

i oT T

i o

i i o 1 i oT T

Steady-state EquationsSteady-state Equations

Induction equation

Mass conservation

Momentum conservation

Adiabatic energy equation

ii i o o c

Bv B v B

l

i i o ov L v l 2

21

8 2i

i i i c

Bv p p

21

2 8i o

o o o c

B B Lv p p

l

2 22 21 1

2 1 4 2 1 4i o

i i i i o o o o

B Bv p v L v p v l

SolutionsSolutions

Basic assumptions

Density compression factor

Outflow speed

Temperature Excess -outflow speed relation

2 2; ; , / 8c o i i i ip p l L v p B

4i

o Ai

i

Bv v

21

2H

o i oB

mT T T v

k

11

1o

i i

f

Insights on Chromospheric Insights on Chromospheric Reconnection from Reconnection from Observations of Observations of Jet-like FeaturesJet-like Features

Are observed jet-like features Are observed jet-like features chromospheric reconnection chromospheric reconnection

jets?jets?Temperature excess-outflow speed relation

H jets

UV Explosive events in the quiet Sun

EUV Jets

21

2H

o i oB

mT T T v

k

-1 5

5

5

70 km s , 10 K

=5/3, =0.6 0.7 10 K

=4/3, =0.6 0.4 10 K

v T

T

T

-1 5

5

200 km s , 2 3 10 K

=4/3, =0.6 3.6 10 K

v T

T

-1 4

4

4

20 km s , 10 K

=5/3, =1.0 1 10 K

=4/3, =1.0 0.6 10 K

v T

T

T

Are observed jet-like features Are observed jet-like features chromospheric reconnection chromospheric reconnection

jets? jets? Yes, very likely as seen from the temperature-speed relation.Hotter jets are better explained with a smaller value of (~ closer to isothermal process ) =4/3 fairly well explains the observed temperature-speed relations in jet-like features.

Are Ellerman bombs Are Ellerman bombs chromospheric reconnection chromospheric reconnection

events?events?Ellerman bombs: Brightening in the far wing of H alpha line profile Heating events in the low chromosphere T=2000 K

They may be reconnection events. If so, we have a prediction

It would be important to measure the flow associated with Ellerman bombs.Note: Shimizu et al. 2005 “Extremely red-shifted magnetic features” as high as 10 km/s

-1~ 8 km s with =1.3, =5/3 v

What determines jet What determines jet temperatures?temperatures?

Temperature excess Outflow speed Alfven speed of inflowing region magnetic field strength and density Atmospheric level

The temperature and speed of reconnection jets strongly depend on the atmospheric level where reconnection occurs.

Higher Atmospheric level Lower density Higher Alfven speed Higher outflow speed Hotter jets

What determines the degree What determines the degree of compression?of compression?

Specific heat ratio the efficiency of radiative cooling

=1 restores the Litvinenko’s (1999) result. This is an unrealistic assumption. In general , 1 (isothermal) < <5/3 (adiabatic)It is likely that gets bigger with higher levels where the medium is more transparent.

Plasma beta of inflow restores the incompressible flow assumption of the original Sweet-Parker model In practice, this assumption is hard to achieve in solar atmosphere, and hence unrealistic.

11

1o

i i

f

i

What determines the degree What determines the degree of compression?of compression?

Near the photosphere

In the upper chromosphere

5/ 3, 1 1.6i f

11

1o

i i

f

4 / 3, 0.1 3.3i f

Insights on Chromospheric Insights on Chromospheric Reconnection from Reconnection from

Observations of Canceling Observations of Canceling Magnetic FeaturesMagnetic Features

Linking observed parameters Linking observed parameters of CMF and physical of CMF and physical parameters of CMFparameters of CMF1

o oz

dr v B

L dt

1/ 3 2 / 3 1/ 3

1/ 3 1/ 3 1/ 3

( 4 / )

( 4 / )

i i c

i i c

B L r f

v L r f

( : anomalous resitivity factor)

ipi

i

c i

pL H

g

q q

, : VAL 81

: Kubat & Karlicky 1986i i

i

z p

Do CMFs result from Do CMFs result from reconnection in the reconnection in the

temperature minimum?temperature minimum?Sturrock (1999) and Litvinenko (1999) YES We have to say NOT necessarily.

The speed of reconnection using classical conductivity of the inflow region is too slow to explain the observed converging speed in canceling magnetic features. The resistivity of the current sheet should be much bigger than that of inflowing region. anomalous resistivity

The molecular resistivity of the inflowing region is no longer the most important parameter characterizing chromospheric reconnection.

6

9 3

8

-1

-1

2 10 cgs

5 10 cgs, 1.3 10 cgs 100 km

7 10 cgs

1 0.10 km s

10 0.22 km s

i i

i

i

i

r

p L

q v

q v

Observational constraints Observational constraints on on qq

Too small values of q yield too low inflow speeds and too high outflow speedsToo big values of q yield too high inflow speeds and too low inflow speeds The observed inflow speeds and outflow speeds constrains the anomalous resistivity factor q

1/ 3 1/ 3

1/ 3 1/ 3,c ci o

i i

v q v q

A reference model A reference model 6 -11.5 10 G cm s , 50, 4 3r q

Is there any preferred height Is there any preferred height for chromospheric magnetic for chromospheric magnetic

reconnection?reconnection?No!Classical resistivity is not the major factor.Reconnection may occur at any height. It may be the geometry of two interacting flux systems that determines the reconnection height.

What we have learned so farWhat we have learned so far

Canceling magnetic features, H jets, UV explosive events, EUV jets are nicely fit into the picture of chromospheric reconnection.An adiabatic current sheet with anomalous resistivity factor of about 50 and specific ratio of 4/3 may serve as a reference model for chromospheric reconection.There may be no preferred height of chromospheric reconnection, and it may be the geometry of two interacting flux systems that determines the reconnection height.

Observational Challenges of Observational Challenges of Solar-BSolar-B

Fine-scale structure of canceling magnetic features new and more reliable measurements of specific cancellation rates and converging speeds (SOT)Discovery of reconnection outflows in the low chromosphere: v~10 km/s, T~103 K (SOT)Simultaneous observations of canceling magnetic features, low chromosphere reconnection flows, H alpha flows, UV jets, EUV jets, X-ray jets that cover diverse speeds, temperatures and atmospheric levels (SOT, EIS, XRT)

Theoretical ChallengesTheoretical ChallengesImpulsive, recurrent (often bursty) occurrence (Chae et al. 1998a, b)

existence of elementary non-steady reconnection events ? formation of a number of magnetic islands via tearing instability?

Co-occurrence of hot jets and cool jets (Chae etal. 1998b, 1999)

Two step reconnection (formation of magnetic islands in the lower atmosphere followed by its destruction in the upper atmosphere, Chae 1999)?Multi-site reconnection of many thin shredded flux sheets at different atmospheric heights (in a stratified medium)?