High Spatial Resolution Observations of Pores and the Formation of a Rudimentary PenumbraG. Yang, Y.Xu, H.Wangm and C.Denker2003, ApJ, 597, 1190

1. Introduction Pores : small sunspots that lack a penumbra structure

The magnetic field lines in pores : almost vertical in sunspot penumbra : highly inclined

Because they are part of the early stages of sunspot evolution, they are important in understanding the mechanism of small-scale flux emergence on the surface of the Sun.

This study focuses on .the development of penumbrae as part of the intricate time-dependent structure of sunspots.

What are the essential features that distinguish a sunspot from a pore ? - The generation of a filamentary penumbra, the onset of the Evershed flow, and the change of the magnetic field topology takes place in less than 20-30 minutes. ( Leka and Skumanich 1998)

The sudden change of the magnetic field topology should certainly affect the upper atmospheric layers. - However the exact mechanisms coupling photospheric flux tube dynamics with chromospheric activity and coronal heating are still elusive.

2. Observations 2001 July 15 during good seeing conditions using the 65cm vacuum reflector and the 25cm vacuum reflector at Big Bear Solar Observatory ( BBSO )

2. Observations 2001 July 15 during good seeing conditions using the 65cm vacuum reflector and the 25cm vacuum reflector at Big Bear Solar Observatory ( BBSO ) Digital Vector Magnetograph ( DVMG ) ----- 25cm telescope - Filter based magnetograph ( bandpass of Zeiss filter : 0.25) - in the line wing of Ca I line at 6103 - The system and its observing modes is given by Wang et al. (1998) and Spirock et al. (2001) - The Stokes I/V image is the average of 256 individual observations.

3. Data reduction Dark and flat-field correction A speckle masking reconstruction technique to process the 5200 green continuum images

Calibration of the Line-of-Sight Magnetograms using a full-disk magnetogram obtained with the spectro-magnetograph at the Kitt Peak National Observatory. ( Jones et al. 1992 ) 1% polarization of the BBSO magnetograph corresponds to about 200G.

- Local Correlation Trackingupper limit for features that can be trackedThe Doppler shifts in Ha line wing images gives rise to fluctuations of the contrast. Cross-talk effect

4. Results Evolution of NOAA9539 2001 July 12 --- b region minor activity 2001 July 14 --- rapid development bg region several C-class event 2001 July 14-17 several C- and M-class events 2001 July 18 --- decayed to a region July 13

July 14

July 17

Ca I 6103 line-of sight magnetogram

On 2001 July 15 NOAA9539 was loacated 241 east and 328 south of the solar disk centerF : pore (positive) P : sunspot (negative)LB : light bridgeFL : filamentRP : Rudimentary penumbraDC : dark channel BMF : bipolar magnetic featureregion of interestSchematic sketch of the emerging fluxregion within NOAA9539

Magnetic field evolution Overall appearance of light bridge ( LB ) is stable. The size of the pores are about 5. F1 and F2 are filled with many umbral dots ( 0.17 at 5200 ) The borders of the pores were not well defined. A penumbral segment ( RP ) is forming near the southern edge of the LB around 20:30 UT. The time interval to form a well-distinguished penumbral segment was about 20-30 min. ( cf. Leka and Skumanich 1998 )2001 July 1519:5820:32

Magnetic field evolution The penumbra forms in a region of continuously emerging flux. The feature most closely related to penumbra formation is an emerging bipolar magnetic feature ( BMP).

Temporal evolution of the magnetic flux of the dominant flux - exponential fit rise time : 40 minutes

The dominant feature for formation if the negative polarity.

Ellerman Bomb The formation of a rudimentary penumbra was associated with the sudden appearance of several Ellerman bombs (EBs) in the neighborhood of the tips of the penumbral filaments.

The EBs remained extremely stationary.

The conglomerate of EBs developed near the rudimentary penumbra and in the vicinity of an emerging magnetic flux element.

The life time of the EBs is about 20-30 minutes. ( cf. Nindos & Zirin 1998, Qiu et al. 2000)

20:14 UT 20:25 UT

20:35 UT 20:45 UT

Ellerman Bomb20:14 UT 20:25 UT

20:35 UT 20:45 UT The formation of a rudimentary penumbra was associated with the sudden appearance of several Ellerman bombs (EBs) in the neighborhood of the tips of the penumbral filaments.

The EBs remained extremely stationary.

The conglomerate of EBs developed near the rudimentary penumbra and in the vicinity of an emerging magnetic flux element.

The life time of the EBs is about 20-30 minutes. ( cf. Nindos & Zirin 1998, Qiu et al. 2000)

Ha Filaments The formation of the rudimentary penumbra is closely related to chromospheric activity in the form of two short-lived filaments ( FL1 ad FL2) .

FL2 appears north of LB at 20:10 UT, fades away at 21:30 UT and reappeared shortly after for another 10-15 minutes. This evolution triggers the activation of FL1.

One of the footpoints of the FL1 is located in the vincinity of the newly formed RP. This filament showed twisting motion and faded away by 21:30 UT.Ha - 0.6 Ha + 0.6 Ha Dopplergram

Photospheric and Chromospheric Flow Fields The granular flows around the two pores are generally moving outward.inward flows near sunspots and pores ( Wang and Zirin 1992 ) In RP, the flow near the pores was moving toward the lower pore at 0.30 km s-1, while in outer parts of penumbrae the flow is away from the pore at 0.33km s-1. There are no noticeable flows inside the light bridge.average horizontal velocitydivergence map

Photospheric and Chromospheric Flow Fields The horizontal velocities measured from images of the Ha line center shows that around the leading spot the flows in the superpenumbrae are inward, while in the following spot the flows move outward.

Summary The rudimentary penumbra develops abruptly within 20-30 minutes.

The onset of the penumbra formation is triggered by an emerging bipolar magnetic flux elements.

The rapid change of the magnetic topology allowed the condensation of cool chromospheric material and the activaton of a short-lived filament.

Ellerman bombs appears in the vicinity of the tips of the penumbral filaments and the footpoints of the activated filament.

Once a penumbra appears, it will directly influence the dynamics of the photosphere and chromosphere, and possibly affect the transition region and corona as well.