JOP168 Sunspot Lightbridge Flows from the Photosphere to the Corona Received: 2003 Short title: Sunspot Lightbridges Contributors: T. E. Berger, LMSAL A. Fludra, NASA S. Gregory, Stanford T. Tarbell, LMSAL K. Schrijver, LMSAL Participating instruments: SST La Palma (GBO), CDS, MDI and TRACE Scientific Objective: --------------------- The primary objective of this observing program is to study lateral flows in the sunspot photosphere, particularly in lightbridges, and to relate these flows to dynamical effects in the chromosphere, transition region, and corona. Recent advances in very high resolution visible light observations of sunspots using the Swedish Solar Telescope (SST) on La Palma reveal that sunspot light bridges can have significant flows with 400 m/sec average speeds along their length [1]. Lightbridges demarcate locations of large magnetic field-strength and inclination gradients within active regions. Thus flows in these structures are expected to influence the sunspot chromosphere and coronal dynamics significantly. In fact, a C2.0 class flare was observed to propagate along the lightbridge studied in [1], perhaps directly related to the surface flow shearing of magnetic field lines in the lightbridge. In addition, the well-known Evershed and lateral moat flows around sunspots are clearly observed in H-alpha dopplergrams and Ca II H-line filtergrams, respectively, from the SST. The goal of this JOP is to measure lateral flows in lightbridges, penumbrae, and moats with the SST and obtain simultaneous observations of dynamics in the chromosphere, transition region, and corona from SOHO and TRACE. Scientific Justification: ------------------------ Sunspots are the central features of solar active regions, yet their formation, dynamics, and dissolution mechanisms remain mysterious. Due to their relatively dark umbral interior, dynamics in and around sunspots are typically measured only in the penumbra (Evershed flow) or in the surrounding granulation and plage (moat flows). Lightbridges offer one source of structural and dynamic information within sunspot umbral regions. Lightbridges can be formed during either the emergence and assembly phase of sunspots, or they can demarcate "fractures" along which sunspots break up during the dissolution phase [2]. In the former case, they tend to be elongated, or "filamentary" [3], and separate strong umbral regions of similar polarity. The magnetic field strength in lightbridges is typically 200-500 gauss weaker than the surrounding umbral regions [4,5,6], and is also inclined more towards the horizontal [4,6]. The structure of lightbridges can be seen in the highest resolution images to be formed by "granule-like" cells which have been shown to be consistent with magnetoconvective mechanisms [7]. In the highest resolution lightbridge observations to date [1], filamentary lightbridges are seen to have a dark central lane around which the convective cells organize. This central lane is most likely a strong downflow channel that is kept coherent by the magnetic field structure of the lightbridge. As mentioned above, we have established that strong lateral flows of the convective cells along the central lane can exist within lightbridges. This flowfield presumably acts to shear the magnetic field lines frozen into the lightbridge plasma relative to the surrounding field lines in the umbral regions, with some degree of back-reaction on the plasma expected due to the high field strengths involved. The observation of a class C2.0 flare occurring directly along the lightbridge is further evidence that lightbridge flows can lead to magnetic shearing and eventual reconnection events. Interestingly, the coordinated TRACE observations for our initial study [1] did not show conclusive evidence of a lateral flow in the 1600A chromospheric images. Instead, the data imply a back-and-forth "sloshing" motion above the lightbridge that may indicate a significantly different flowfield at higher atmospheric levels above the lightbridge. This finding is puzzling and could have interesting implications for magnetic field dynamics in the upper atmosphere of sunspots, perhaps relating to the oscillations seen in coronal plumes above sunspots [ref?]. Our initial observations observations [1] lacked high resolution magnetograms, dopplergrams, and coordinated coronal observations. We now have the SOUP tunable filter installed and working at the SST in order to obtain very high resolution magnetograms and H-alpha images. We would like to obtain further coordinated observations of lightbridge dynamics involving both TRACE and SOHO instruments in order to search for definitive chromospheric, transition region, and coronal effects of lightbridge flows in sunspots. Operation: ---------- SST La Palma: Wavelengths: G-band 4305A filtergrams G-band "continuum" 4363A filtergrams Ca II 3968A H-line filtergrams SOUP tuneable filter: Fe I 6302A magnetograms H-alpha Line center, -350mA, -700mA FOV: Filtergrams: SOUP magnetograms: Resolution: G-band, G-continuum, Ca II H-line: 0.041"/pix SOUP: 0.062"/pix Cadence: All wavelengths: 15-30s, depending on seeing. contact: Tom Berger (berger@lmsal.com) ---------------------------------------------------------------- MDI: FOV: Hi-res FOV Sequences: hr_ve_fe_me 1024x500 cutout around active region. hr_m1_v2 30 sec 1024x1024 mag and 2x2 bin vel. Contact: Sarah Gregory (gregory@mdisas.nascom.nasa.gov) ------------------------------------------- CDS: Temporal sequence: 1'x1' 18 minute raster Initial Pointing: Spectrometer NIS Slit Exposure Time Number of Repeats/Rasters Total Duration Line Selection He I 584 log 4.5 O III 599 log 5.0 Ne IV 543 log 5.2 O V 629 log 5.4 Ne VI 562 log 5.6 Ca X / Ne VI log 5.8 / 5.6 (centre window at wavelength in between that of Ca X and Ne VI, i.e. ~558.180) Mg IX 368 log 6.0 Mg X 624 log 6.1 Spectral window width No binning along the slit Raster sequence: Initial Pointing: Contact: Andrzej Fludra (fludra@cdso8.nascom.nasa.gov) --------------------------------------------------------------------------- TRACE: Wavelengths: 1600A at 10s cadence. 1550/1700/171 context every two minutes. Frame: 512x512 pixels centered subfield read, no binning. Compression: Lossless. Exposure: 10s (compatible with 15s cadence) Name of program: Modification: NO wl images interleaved. Sequence: Contacts: Ted Tarbell (tarbell@lmsal.com) Karel Schrijver (schyrver@lmsal.com) --------------------------------------------------------------------------- Proposed observation dates: May 18 - 25, 2003 Target: Large sunspot active region, preferably near disk center in the MDI HR FOV. ------------------------------------------------------------------ References: [1] Berger, T. E., & S. V. Berdyugina, ApJL, 589, L117, 2003. [2] Garcia de la Rosa, J. I. Sol. Phys., 112, 49, 1987. [3] Sobotka, M., Bonet, J. A., & M. V{\'a}squez, Astrophys. J., 426, 404, 1994. [4] Beckers, J. M., & E. H. Schr{\"o}ter, Sol. Phys., 10, 384, 1969. [5] Lites, B. W., et al., ApJ, 373, 683, 1991. [6] Leka, K. D., ApJ, 484, 900, 1997. [7] Rimmele, T. R., ApJ, 490, 458, 1997. [8] Coronal loop oscillations above sunspots.