Observations of Supersonic Downflows in a Sunspot Light Bridge as Revealed by Hinode

Observations of Supersonic Downflows in a Sunspot Light Bridge as Revealed by Hinode

Rohan E. Louis    Luis R. Bellot Rubio    Shibu K. Mathew, P. Venkatakrishnan
Abstract

Recent high resolution spectropolarimetric observations from Hinode detected the presence of supersonic downflows in a sunspot light bridge (Louis et al. 2009). These downflows occurred in localized patches, close to regions where the field azimuth changed by a large value. This apparent discontinuity in the field azimuth was seen along a thin ridge running along the western edge of the light bridge. Some, but not all, of these downflowing patches were co-spatial with chromospheric brightness enhancements seen in Ca ii H filtergrams. The presence of magnetic inhomogeneities at scales of 0.3 could facilitate the reconnection of field lines in the lower chromosphere whose signatures might be the supersonic downflows and the brightness enhancements that have been observed.

Udaipur Solar Observatory, Physical Research Laboratory Dewali, Badi Road, Udaipur, Rajasthan - 313004, India

Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain

Udaipur Solar Observatory, Physical Research Laboratory Dewali, Badi Road, Udaipur, Rajasthan - 313004, India

1. Introduction

Sunspot Light Bridges (LBs) are conspicuous bright intrusions in the otherwise cool, dark umbra and are often seen to exhibit a granular like morphology, bearing a ubiquitous central dark lane (Hirzberger et al. 2002). LBs are known to harbour weak inclined magnetic fields (Redi et al. 1995; Leka 1997; Jurk et al. 2006; Katsukawa et al. 2007) but the nature of sub-photospheric convection that powers them is still a matter of debate (Parker 1979; Spruit & Scharmer 2006; Rimmele 2004). The presence of supersonic downflows in a sunspot LB in NOAA 10953, using high resolution spectropolarimetric observations from Hinode (Kosugi et al. 2007) on May 1, 2007, also indicate chromospheric enhancements that are co-spatial with some of the strong downflowing patches (Louis et al. 2009). In this paper, we show that reconnection in the lower chromosphere could be a possible mechanism that could explain the downflows and the chromospheric activity.

2. Results

2.1. Supersonic Downflows

The strong downflows were observed as localized patches with velocities of 4 km.s (See Figure 1 of Louis et al. (2009)), that were retrieved from the SIR inversion code (Ruiz Cobo & del Toro Iniesta 1992). The Stokes profiles associated with the strong downflows comprise of two peaks in the red lobe which cannot be reproduced by a simple model atmosphere where all parameters are constant with height. In order to synthesize such anomalous profiles, a two-component model atmosphere is used as shown in Figure 1, where the components are stacked one on top of another. The amplitude and location of the discontinuity in the stratification are free parameters in the inversion code (SIRJUMP; Bellot Rubio in preparation). The anomalous profiles are well reproduced by this model atmosphere and as seen from the Figure, there exist supersonic downflows in the upper half of the photosphere. Louis et al. (2009) had shown that supersonic velocities are also retrieved by other model atmospheres with two-components.

Figure 1.: First and second columns: Observed (solid) and best-fit (dashed) Stokes profiles using a two-component model in which one of the components has a discontinuous stratification. Third and fourth columns: The constant background atmosphere is represented by the dashed lines, whereas the solid lines correspond to the discontinuous atmosphere. The supersonic downflows are seen in the upper half of the photosphere.

Figure 2.: Left: Ca ii H filtergram of the LB taken at 12:04:37 UT when the spectrograph slit was above the LB. The arrows indicate the transverse component of the field in the local reference frame for every alternate pixel. White contours represent fields weaker than 750 G and black contours outline bright continuum structures. The white dashed lines mark thin chromospheric threads whose ends are located in or near the downflowing patches. Right: Chromospheric event map constructed during the polarimetric scan of the LB and its neighbourhood. DF1, DF2 and DF3 indicate the downflowing regions. Black Contours have been drawn for 2.5 and 4 km.s respectively in both the panels.

2.2. Chromospheric Activity

Figure 2 shows that some, but not all, of the strong downflowing patches coincide with chromospheric Ca ii H brightness enhancements. During the early part of May 1, Louis et al. (2008) had observed that the chromosphere above the LB exhibited transient flashes across it, as well a strong brightening along a loop-like structure that was seen in the 3 hr mean image. It was also shown that the transient events were extremely fast and had lifetimes of 1-2 min. These enhancements were also observed on April 29 and 30 (Shimizu et al. 2009). In order to study the association of the downflows to the chromospheric enhancements, events maps were constructed in the following manner. All Ca ii H filtergrams in the 44 min. sequence were normalized by the Quiet Sun intensity. Using an intensity threshold of 0.9, a binary image was created for each individual image in the time sequence, setting all pixels greater than the threshold to one and the rest to zero. These binary maps were then added in time, yielding at the end, a map with pixels having values indicating the number of chromospheric events. The right panel of Figure 2 illustrates the event map constructed from the manner described above. The strongest downflowing patch DF1 is seen, lying within the sausage shaped region where a large number of events occurred. The two patches DF2 and DF3 lie 2 and 3 away from the nearest chromospheric brightening.

3. Discussion

The supersonic downflows in the LB are sometimes associated with transient chromospheric brightenings. This is illustrated from the chromospheric event map. The question that naturally arises from the above observations is the source/mechanism of the supersonic downflows and the brightenings? The strong downflows are similar to the Evershed Flow which can be supersonic in the mid and outer penumbra as well as beyond the sunspot boundary (del Toro Iniesta et al. 2001; Bellot Rubio et al. 2004). Although, the flow mechanisms responsible for Evershed Flow (Montesinos & Thomas 1997; Schlichenmaier et al. 1998) cannot be ruled out as one possibility for the supersonic downflows observed in the LB, it seems unlikely that it would account/explain the associated chromospheric phenomena as well. In the LB, of the three downflowing patches, two of them on the upper half of the LB are associated with azimuth discontinuities (Louis et al. 2009), which leads us to believe that a slingshot reconnection mechanism, as formulated by Ryutova et al. (2008), could produce the downflows as well as explain the chromospheric brightenings. The question is where does the reconnection occur? One useful hint lies in the inclination of the magnetic field. The top most downflowing patch (DF3) on the LB is 2 or 1450 km, from the nearest chromospheric brightening. The inclination of the magnetic field is 120 to the vertical. If a reconnection geometry as suggested by Isobe et al. (2007) is assumed, then the height at which the reconnection occurs would be 1450 km or 840 km which is well within the formation height of the Ca ii H line. By the same argument, it can be shown that if reconnection occurs at this height, then for fields which have an inclination of 165, the distance between the chromospheric brightening and the downflows would be 225 km or 0.3, which is 2 SP pixels. This is precisely the case for the strongest downflowing patch (DF1) that is co-spatial with the chromospheric brightenings. The chromosphere can facilitate reconnection as plasma is less than unity at these heights. Thus the photospheric downflows could be the result of downward propagating shocks resulting from the reconnection.

Acknowledgments.

We sincerely thank the Hinode team for providing the high resolution data. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway).

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