1. Evidence of magnetic reconnection along coronal hole boundaries

Madjarska, Doyle & van Driel-Gesztelyi (2004) ApJ, 603, L57

Zashikkai, May10th, DH Brooks

Introduction

• Coronal Holes are large magnetically “open” regions, visible as dark regions in coronal emission lines.• They are the source of the high speed (>800 km/s) solar wind.• There are two types: polar & mid-latitude.• Mid-latitude CH’s can be “isolated” or “connected” to a polar CH.• The connected ones are called Equatorial Extensions to Coronal Holes (EECH).

Famous EECH: Elephant’s Trunk Coronal Hole

EIT 195A & UVCS on 1996, October 20.

• Coronal Hole boundaries (CHB) are regions where opening and closing of magnetic fields takes place.• Reconfiguration believed to happen through magnetic reconnection between open field lines of CH and closed field lines of surrounding quiet Sun, BUT no direct evidence so far…

Kahler & Hudson (2002)

Elephant’s Trunk: a good candidate?• Distinctive feature of EECHs is their near rigid rotation, contrasts with differential rotation of photosphere: magnetic field sheared at photosphere below CH.• Largest divergence from differential flow profile (i.e.largest shearing) in low latitude extension.• Strong rigid rotation found in this EECH (13 degrees per day – Zhao et al. 1999).• Line of sight favourable for flow detections.

This paper• First evidence of existence of magnetic reconnection along

Coronal Hole Boundaries from SOHO/SUMER

Observations: 1996, October 19-20SUMER:10 hours, 1” x 300” slit60s exposure95” solar-X FOVsit-and-stareN IV (0.13MK)Ne VIII (0.6MK)

EIT: Fe XII 195A2 per hour2.62” resolutionreversed intensity

SUMER

Spectral line fittingN IVsolid-QSdash-reddot-blue

Single Gaussianto derive Dopplershift map.

Bidirectional jet profiles shift todominant wing

Non-Gaussianprofiles mark locations of BD jets.

Typical QS and CH profilessolid-QS

dot-CH

blue shift in CH5 km/s

CH boundary best detected by abrupt change to blue-shift

Results: triangles are bi-directional jets

Reversed Intensity Images. 5” coalignment Large numbers of BD jets along the CH boundary

BP blue

red

Conclusions• Bidirectional jets found along the Elephant’s Trunk coronal hole boundary at low latitudes (magnetic reconnection)• Sizes are 4-5” and lifetimes 300-1000s, velocities 150 km/s.• Compared to the quiet Sun, using the same observing sequence, their number density is 4-5 times higher indicating their predominant presence in these regions. • Also found (but not so many) in the higher temperature Ne VIII line.• The strong rigid rotation of this CH (Zhao et al. 1999) creates the necessary conditions for frequent magnetic reconnection events along the boundary.• Supports view that the slow solar wind originates from inside the CH near its boundary (reconnection between open field outside CH and closed field inside – Wang et al. 1998, Kahler & Hudson 1999)

2. Traces of the dynamic current sheet during a solar flare

Ji, Wang, Goode, Jiang & Yurchyshyn (2004) ApJ, 607, L?

This paper is a follow up to Ji et al. (2004) ApJ, 605, 938introduced by Liu-san at zashikkai on April 26th.

BackgroundFlare energy release site in the corona. Energy transported to the chromosphere.

Behaviour of flare ribbons in chromosphere can be used to study energy release in the corona.

H alpha ribbon expansion a signature of magnetic reconnection of successive field lines in the corona.

Separation speed linked to rate ofreconnection.

Yokoyama & Shibata (1998)

Background• Cannot observe current sheet directly so useful to study separation motion of H alpha ribbons.• Hard X-ray (HXR) emission useful for studying energy release processes.• Relationship between H alpha and HXR emission.

• So far, the fine temporal structure of flare ribbons has not been revealed. Position typically measured as the central position of a kernel. Here use:

to reflect the changing brightness patterns and give more accurate motion information.

jjjc IIrr

Observations: 2002 Sept. 9, M2.3 flare in NOAA 10105

BBSO:H alpha –1.3A40ms cadence0.89” pixel 228 x 228” FOV

RHESSI:25-50keV band4s time cadence 2.3” pixelFull disc FOV Flare in H alpha, and –1.3A blue wing

Reminder: main previous results• H alpha kernels a1 and a2 show a correlation with HXR emissions.• Kernel b does not (remote brightening due to loop interactions?).• 3 of the 4 HXR spikes studied are nonthermal, the other is thermal (suggested by time-delays, HXR emission sources etc.) and produced by multi-temperature plasma.• First and biggest H alpha spike failed to produce the predicted emission, may be because most of the energy was used to ionize the atoms (or model too simplistic).

New results in this letter: Ha blue wing

r1 and r2 computed centroids of boxes 1 and 2.They changed the notation: k1=a1, k2=a2, k3=b, & rotated the images.k1 and k2 connected by EIT loop.

New results in this letter: Ha blue wing

RHESSI HXR emission mainly from loop top, with occasional emission from k1, k2. k1 & k2 H alpha light curves show a close correlation with HXR light curve in high energy band. Consistent with two-ribbon flare picture.

Why use only 25-50keV energy band?

HXR emission only shows spiky structure above 25 keV and the emission falls off to below the noise level above 50 keV.

Some details of the methods• Cross-correlation used to coalign blue wing images to better than 1”.• Boxes 1 & 2 chosen to enclose the flare kernels throughout the flares’ evolution.• Centroids usually close to the box centre since flare intensity is quite small (2-3 times background).

HXR light curve, time profiles of centroid distance

40ms cadence

rebinned (4s)

Results• Distance curve correlated with HXR light curve.• Centroid distance decreases in each rising period for the four spikes (periods I-IV), most obvious in period 1.This behaviour contradicts the standard picture of two-ribbon flares.• After the flare maximum, the centroid distance increases.• Correlation coefficient between HXR light curve and rebinned curve is –0.83 (anti-correlated).• Spike III: centroid distance continues to decrease after the rise phase (no explanation but this was the only thermal spike in paper I…).• Amplitude of changes 40-130km (error of method about +/-30km).

Check that computational method is reliable:these images fall within period of most rapid decrease in distance.

Dotted lines drawn as visual aid. Two kernels appear to move closer. Bulk motion of fairly stable kernels.

Comments

• HXR source at loop top decreases in height during time period I (not analysed here…).• New automatic method useful for the 24,000 images and because variation of brightness distribution can be found more accurately.

ExplanationHXR spikes due to impulsive energy release by magnetic reconnection.

CSHKP model has vertical current sheet where reconnection occurs.

Released energypropagates down to chromosphere.

H alpha kernels

Decrease in centroid distance may reflect shrinking of volume of energy release site. Thinner current sheets release more energy.

Final Comment

• Indirect observations of the dynamics of the current sheer (pinching motion).• Current sheet thickness is too thin to be directly observed, therefore, indirect evidence is very important support for the magnetic reconnection model.