Moving dipolar features in an emerging flux region

P.N. Bernasconi et al.

2002, Sol. Phys., 209, 119

Junko Kiyohara

2003 Dec 22

Introduction

Moving magnetic features (MMFs) :・ Sheeley (1969 ) first reported.・ Harvey and Harvey (1973) described their magnetic properties : their diameter is about 1000 km and they move about 1km/s.・ They appear as either single poles or dipoles (Shine and Title, 2001).・ They appear to be an important mechanism of sunspot decay.・ Most observation are consistent with the ‘sea serpent’ cartoon drawn by Harvey and Harvey (1973).・ Nye, Thomas and Cram (1984) observed an MMF having a 400m/s downflow.

In this paper, authors report moving dipolar features (MDFs) in a sunspot formation region.

Observation 1

The Flare Genesis Experiment (FGE) :

a balloon-borne observatory

・ 80-cm solar telescope

・ a vector polarimeter

・ a tunable Fabry-Perot narrow-band filter

・ observed data

vector magnetograms

　　　 Dopplergrams

at CaI 6122.2A

Halpha filtergrams

Observation 2NOAA 8844 10-27 Jan. 2000 during the FGE’s second flight

Location : N05W30

Data : Ca I at 6122.2A ( red and blue wings )

vector magnetograms, Dopplergrams, and photospheric images

integration time : 30ms

full set of vector polarimetric measurements : about 130s

Halpha -0.8A filtergrams

integration time : 125ms

Ca I vector polarimetric set and Hapha observation : 7.5 min

Time : 15:50UT-19:16UT on 25 Jan. 2000

Field of view : 92.2” ×92.2”

Spatial resolution : 0.5” ( 0.18” per pixel )

Flux emergence overview

17:57 UT on 25 Jan. 2000Halpha-0.8ADopplergram

Ca I 6122A blue wing line-of sight magnetogram

Until 24 Jan. NOAA 8844 had only two stable sunspots.

Rapid growth started late on the 24th or early on the 25th.

The region continued growing for only about another 24h after 25 Jan.

・ The dark fibrils ( each 3000km long ) were rising through the photosphere with typical velocities of at least 1 km/s and the upward motion was visible for 5 to 10 min.・ The tops of the arches are prominent in the Halpha, and its upward velocity about 35km/s.

There were 3 principal sites of flux emergence in the supergranules (about 10000km in diameter)

The magnetic element motions and the granulationhorizontal velocities showed general agreement.

The horizontal flux ropes emerged between two sunspot. Field orientation is 20-60 degrees.

The flow speed of the magnetic elements is about 0.3km/s.Many dipolar magnetic features are seen.

Magnetic properties of moving dipolar features

・ Most MDFs appeared from the region C, migrated at nearly constant velocity between 0.3 -0.8 km/s from the supergranule centers to the edges and /or into the sunspot. However, for short periods MDFs could dash at 1 km/s.

・ Only a few MDFs were moving toward the edge of the cell opposite to the spot. In all cases the polarity of the part closest to the spot was opposite to that of the spot.

・ The lifetimes of MDFs varied from tens of minutes to hours.

5 typical MDFs

U-loop

5 MDFs had similar characteristics.

・ MDF attained a maximum B|| at around 200G and its location is 1” from the neutral line.

・ The field lines in the negative part of each MDF were more perpendicular to the local surface than the ones in the positive part.

・ |B| appeared to be greater in the positive parts than in the negative parts. This is mainly due to the proximity of the positive parts of the MDFs to the sunspot and its stronger fields.

・ By just looking at |B| maps it is very difficult to identify the MDFs. The MDFs are not single isolated dipoles emerging from below the surface but most likely dips in the canopy of horizontal field lines.

neutral line

Material flow in MDFs・ All five of the MDFs there had a net downflow with respect to the surroundings. The downflow was not confined to the parts of the MDFs with the strongest longitudunal field. Each whole MDF was sinking.

・ After merging with the spot, in the Dopplergrams the downflow signature persisted.

・ Figure below illustrates the relationships linking the dark threads of emerging flux, the upward flows, and the MDFs.

Dopplergram Ca I wing magnetogram

Summary and Conclusions

3000km

1000km

Summary of the observation : Moving dipolar features have been discovered in emerging magnetic flux. - They move horizontally toward sunspots and supergranule boundaries. - All the MDFs were oriented opposite to the main active region polarities. - The MDFs were remarkably similar in size, field strength, and orientation. - Downflows were persistent at each pole. - Vector magnetograms show that the MDFs are really magnetic U-loops stitched into the upper photosphere.

・ Inflowing MDFs moved with the surrounding material. Outflowing dipolar MMFs move faster than the surrounding granules.

・ Outflowing dipolar MMFs are convex kinks in the horizontal fields extending outside penumbrae (Ω-loop). However, the sunspot-building MDFs are U-loops. The MDFs formed when plasma on just-emerged field lines cooled, then pooled, then distorted the field lines, making a concave depression in them. The distorted segment, a U-loop, is swept by the convective flows to the edge of a sunspot or supergranule.

・ The observation showed that the horizontal field in the MDFs is far more important than the vertical component. There could be a background pattern of horizontal fields everywhere. They could reside in the convectively stable layer just above the granulation. The plasma sloshing around in those fields should often form pools and deform those fields, giving the appearance in longitudinal magnetograms of magnetic dipoles coming and going.