Transcript Document

Chromospheric Magnetic
Reconnection
from an Observer’s Point of View
Jongchul Chae
Seoul National University, Korea
What is Chromospheric
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 K
Higher densities
Small-scale  low altitude
Diversities in flow speed, density and temperature 
strong stratification
Driven reconnection
Flux emergence
Supergranular flow
Observational Signatures
Canceling Magnetic Features
Jet-like Features seen in Ha and
UV/FUV/EUV
UV/EUV jets, UV explosive events
H alpha jets/ surges/ H alpha upflow events
Chromospheric Brightenings
Ellerman bombs
Other brightenings in UV/EUV/ H alpha
Canceling Magnetic Features
Canceling Magnetic Feature
From Chae, Moon, Park 2003, JKAS 36, S13
CMF as a CMR event
Corona
UPFLOW
Chromosphere
DOWNFLOW
Photosphere
FLUX CANCELLATION
CONVERGING MOTION
Interior
Does the flux submerge in CMFs?
YES! The ASP
observations produced
the evidence for it.
From Chae, Moon, Pevtsov 2004, ApJL, 602, L65
Observables 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
From Chae, Moon, Park 2003, JKAS 36, S13
Summary of CMF Observations
Chae et al.
Case A
Chae et al.
Case B
Chae et al.
2003
Chae et al.
1998
Flux loss rate
Mx/h
Contact length
Mm
Specific flux
rate G cm/s
3.4 x 1018
2.5 x 1018
1.8x1018
2x1017
7.8
3.3
2.5
3
1.2 x 106
1.1x106
2.0x106
2x105
Converging
km/s
0.27
0.35
0.22
Chromospheric Jets in Active
Regions
EUV Jets
From Chae, J. 2003, ApJ 584, 1084
Ha Jets in the same active
region
Chae, J. et al. 2000, Solar Physics 195, 333
EUV/Ha Jets in another AR
From Chae et al. 1999 ApJ 513, L75
Jet-like features in the quiet Sun
Ha upflow events
From Chae et al. 1998, ApJ, 504, L123
Jet-like in the quiet Sun
UV explosive events
From Chae et al. 1998, ApJ, 479, L109
Summary of Jet 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
Chromospheric Reconnection
Adiabatic Current Sheet of Sweet-Parker
type
Insights on Chromospheric Reconnection
from Observations of Jet-like Features
Insights on Chromospheric Reconnection
from Observations of Canceling Magnetic
Features
Adiabatic Current Sheet Model
of Sweet-Parker type
Current Sheet Model of CMR
Steady-state Current Sheet
Model
Sweet-Parker Model
Incompressible flow
Litvinenko (1999)
Compressible,
isothermal flow
Chae et al. (2003)
Adiabatic flow
A generalized approach
Steady-state Equations
Induction equation
Mass conservation
Momentum conservation
Adiabatic energy equation
Solutions
Basic assumptions
Density compression
factor
Outflow speed
Temperature Excess outflow speed relation
Insights on Chromospheric
Reconnection from Observations of
Jet-like Features
Are observed jet-like features
chromospheric reconnection jets?
Temperature excess-outflow
speed relation
Ha jets
UV Explosive events in the
quiet Sun
EUV Jets
Are observed jet-like features
chromospheric reconnection jets?
Yes, very likely as seen from the
temperature-speed relation.
Hotter jets are better explained with a
smaller value of g (~ closer to isothermal
process )
g =4/3 fairly well explains the observed
temperature-speed relations in jet-like
features.
Are Ellerman bombs chromospheric
reconnection events?
Ellerman bombs: Brightening in the far wing of H
alpha line profile  Heating events in the low
chromosphere DT=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
What determines jet
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 of
compression?
Specific heat ratio  the efficiency of radiative
cooling
g =1 restores the Litvinenko’s (1999) result. This is an
unrealistic assumption.
In general , 1 (isothermal) <g <5/3 (adiabatic)
It is likely that g 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.
What determines the degree of
compression?
Near the photosphere
In the upper chromosphere
Insights on Chromospheric
Reconnection from Observations of
Canceling Magnetic Features
Linking observed parameters of
CMF and physical parameters of
CMF
Do CMFs result from reconnection
in the 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.
Observational constraints on q
Too small values of q yield too low inflow speeds
and too high outflow speeds
Too 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
A reference model
Is there any preferred height for
chromospheric magnetic
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 far
Canceling magnetic features, Ha 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 Solar-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, DT~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 Challenges
Impulsive, 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)?