Dynamics of Sunspot Light Bridges as Revealed by High-Resolution Images from Hinode

Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and "canals". In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.

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“…It agrees also with the well-known observation that light bridges often decay into strings of umbral dots (e.g. Zwaan 1968;Louis et al 2008).…”

Section: Observational Connectionssupporting

confidence: 92%

Striation and convection in penumbral filaments

Spruit¹,

Scharmer

Löfdahl

2010

A&A

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“…This patch of weak flows moves along the LB as it appears at the northern end and in the central part of the LB as seen in panels 1 and 2, respectively. The horizontal speeds at the southern end are much weaker than at the northern end where the southward flow along the LB encounters northward motions from the penumbra into the umbra which is similar to the findings of Louis et al (2008). The predominantly north-to-south horizontal motion can be seen in the G-band movie which also reveals small-scale barb-like features in the LB that evolve on timescales of 10−20 min.…”

Section: Horizontal Flow In the Photospheresupporting

confidence: 76%

Small-scale chromospheric jets above a sunspot light bridge

Louis

Beck

Ichimoto

2014

A&A

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Context. The chromosphere above sunspot umbrae and penumbrae shows several different types of fast dynamic events such as running penumbral waves, umbral flashes, and penumbral microjets. Aims. The aim of this paper is to identify the physical driver responsible for the dynamic and small-scale chromospheric jets above a sunspot light bridge.Methods. High-resolution broadband filtergrams of active region NOAA 11271 in Ca ii H and G band were obtained with the Solar Optical Telescope on board Hinode. We identified the jets in the Ca ii H images using a semi-automatic routine and determined their length and orientation. We applied local correlation tracking (LCT) to the G-band images to obtain the photospheric horizontal velocity field. The magnetic field topology was derived from a Milne-Eddington inversion of a simultaneous scan with the Spectropolarimeter. Results. The chromospheric jets consist of a bright, triangular-shaped blob that lies on the light bridge, while the apex of this blob extends into a spike-like structure that is bright against the dark umbral background. Most of the jets have apparent lengths of less than 1000 km and about 30% of the jets have lengths between 1000-1600 km. The jets are oriented within ±35 • to the normal of the spine of the light bridge. Most of them are clustered near the central part of the light bridge within a 2 area. The jets are seen to move rapidly along the light bridge and many of them cannot be identified in successive images taken with a 2 min cadence. The jets are primarily located on one side of the light bridge and are directed into the umbral core. The Stokes profiles at or close to the location of the blobs on the LB exhibit both a significant net circular polarization and multiple components, including opposite-polarity lobes. The magnetic field diverges from the light bridge towards the umbral cores that it separates. The LCT reveals that in the photosphere there is a predominantly uni-directional flow with speeds of 100-150 m s −1 along the light bridge. This unidirectional flow is interrupted by a patch of weak or very small motions on the light bridge which also moves along the light bridge. Conclusions. The dynamic short-lived chromospheric jets above the LB seem to be guided by the magnetic field lines. Reconnection events are a likely trigger for such phenomenon since they occur at locations where the magnetic field changes orientation sharply and where we also observe isolated patches of opposite-polarity magnetic components. We find no clear relation between the jets and the photospheric flow pattern.

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“…AR 10953 produced several chromospheric enhancements from April 29 to the end of May 1 (Louis et al 2008;Shimizu et al 2009). Some of them were cospatial with supersonic downflows observed in the light bridge nearly 10 hr earlier than the downflows reported here (Louis et al 2009).…”

Section: Chromospheric Activitymentioning

confidence: 99%

Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots

Louis

Rubio

Mathew

et al. 2011

ApJ

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High-resolution spectropolarimetric observations of three sunspots taken with Hinode demonstrate the existence of supersonic downflows at or close to the umbra-penumbra boundary which have not been reported before. These downflows are confined to large patches, usually encompassing bright penumbral filaments, and have lifetimes of more than 14 hr. The presence of strong downflows in the center-side penumbra near the umbra rules out an association with the Evershed flow. Chromospheric filtergrams acquired close to the time of the spectropolarimetric measurements show large, strong, and long-lived brightenings in the neighborhood of the downflows. The photospheric intensity also exhibits persistent brightenings comparable to the quiet Sun. Interestingly, the orientation of the penumbral filaments at the site of the downflows is similar to that resulting from the reconnection process described by Ryutova et al. The existence of such downflows in the inner penumbra represents a challenge for numerical models of sunspots because they have to explain them in terms of physical processes likely affecting the chromosphere.

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Dynamics of Sunspot Light Bridges as Revealed by High-Resolution Images from Hinode

Cited by 45 publications

References 26 publications

Striation and convection in penumbral filaments

Striation and convection in penumbral filaments

Small-scale chromospheric jets above a sunspot light bridge

Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots

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