High Spatial Resolution Observations of Pores and the Formation of a Rudimentary Penumbra G. Yang, Y.Xu, H.Wangm and C.Denker 2003, 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 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 ) cm telescope - Filter based magnetograph ( bandpass of Zeiss filter : 0.25 Å ) - in the line wing of Ca I line at 6103 Å - 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 Å 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 ) 1% polarization of the BBSO magnetograph corresponds to about 200G.

- Local Correlation Tracking upper limit for features that can be tracked -The Doppler shifts in H  line wing images gives rise to fluctuations of the contrast. → Cross-talk effect

4. Results ■ Evolution of NOAA July  region minor activity 2001 July rapid development  region several C-class event 2001 July several C- and M-class events 2001 July decayed to  region July 13 July 14 July 17

Ca I 6103 Å line-of sight magnetogram

■ On 2001 July 15 NOAA9539 was loacated 241” east and 328” south of the solar disk center F : pore (positive) P : sunspot (negative) LB : light bridge FL : filament RP : Rudimentary penumbra DC : dark channel BMF : bipolar magnetic feature region of interest Schematic sketch of the emerging flux region 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 Å ) ■ 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 min. ( cf. Leka and Skumanich 1998 ) 2001 July 1519:58 20:32

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 minutes. ( cf. Nindos & Zirin 1998, Qiu et al. 2000) 20:14 UT 20:25 UT 20:35 UT 20:45 UT

20:14 UT 20:25 UT 20:35 UT 20:45 UT 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 minutes. ( cf. Nindos & Zirin 1998, Qiu et al. 2000)

H  Filaments ■ The formation of the rudimentary penumbra is closely related to chromospheric activity in the form of two short-lived filaments ( FL 1 ad FL 2 ). ■ FL 2 appears north of LB at 20:10 UT, fades away at 21:30 UT and reappeared shortly after for another minutes. This evolution triggers the activation of FL 1. ■ 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. H  Å H  Å H  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 velocity divergence map

Photospheric and Chromospheric Flow Fields ■ The horizontal velocities measured from images of the H  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 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.