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 10 4 km Lower temperatures: a few 10 3 K to a few 10 5 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 H  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

Interior CMF as a CMR event Photosphere Chromosphere Corona DOWNFLOW UPFLOW FLUX CANCELLATION CONVERGING MOTION

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 Chae et al Flux loss rate Mx/h 3.4 x x x x10 17 Contact length Mm Specific flux loss rate G cm/s 1.2 x x x10 6 2x10 5 Converging speed km/s

Chromospheric Jets in Active Regions EUV Jets From Chae, J. 2003, ApJ 584, 1084

H  Jets in the same active region Chae, J. et al. 2000, Solar Physics 195, 333

EUV/H  Jets in another AR From Chae et al ApJ 513, L75

Jet-like features in the quiet Sun H  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 H  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  (~ closer to isothermal process )  =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  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 “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  =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.

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, 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 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,  T~10 3 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)?