Search for photospheric footpoints of quiet Sun transition region loops

Transcription

1 Search for photospheric footpoints of quiet Sun transition region loops L. Teriaca Max Planck Institut für Sonnensystemforschung Submitted to A&A as Sánchez Almeida, Teriaca, Sütterlin, Spadaro, Schühle, and Rutten

2 The Transition Region The Transition Region (TR) is defined as the temperature region between 20 kk and 1 MK and, as such, strongly emits in the VUV spectral range. Classical, static, models depict it as a thin (10 to 100 km) thermal interface between the hot corona and the cooler photosphere.

3 The Transition Region The failure of such models in reproducing observed quantities such as the emission measure and the limb brightening curves of TR lines together with high resolution observations has changed this view. Most of the emission comes from cold (T<0.5 MK) loops Dowdy et al Filamentary TR structures Dere et al 1997.

4 The Transition Region - Corona The lower TR (T<0.7 MK) is formed by many loop-like structures 10 to 20 long located along and across network boundaries, with widths at or below the SUMER resolution of 1.5. From these data it seems that the majority of the TR emission, particularly in the quiet Sun, does not simply connect the chromosphere with the corona Feldman et al 1999.

5 The Transition Region and the Chromosphere However, there are also difficulties at connecting the TR with the chromosphere. Most strikingly, the width of the chromospheric brightening along the cell boundaries is much narrower than the widths of the bright TR structures. Feldman et al

6 What are the footpoint of the TR Structures? When comparing the TR structures with a MDI magnetogram, only some of the larger loops appear obviously connected to magnetic polarities. What about the rest? Teriaca et al

7 Where is the photospheric field? Classical (e.g., MDI) magnetograms do not have enough spatial resolution to detect the field in the interior of the supergranular cells. However, this field has been observed as weak Hanle depolarization (e.g., Trujillo Bueno et al. 2001) and Zeeman polarization (e.g., Dominguez Cerdeña et al. 2003) and small bright points (e.g., de Wijn et al. 2005). Finally, numerical simulations (e.g., Vögler & Schüssler 2007) and observations (e.g., Dominguez Cerdeña et al. 2006) indicate that a complex magnetic field pervades the non-magnetic quiet photosphere. Theoretical arguments suggest that large part of these photospheric fields reaches the TR and the corona (e.g., Jendersie & Peter 2006).

8 Observations: Photospheric field Inter-granular G-band (CH band around 430 nm) Bright Points (GBPs) have size below 250 km (0.35" possibly < 0.2"). They can be used as a proxy of (1-2) kg magnetic fields (e.g., Berger et al. 1995, 1998). Although kg fields represent only a very small fraction of the quiet Sun magnetic structures (e.g., Socas-Navarro & Sánchez Almeida 2002), these fields still trace a significant fraction of the total magnetic energy and are expected to reach and tip-over at greater altitudes (Domínguez Cerdeña et al. 2006).

9 Observations: Photospheric field Speckle reconstructed DOT images in the G-band can reach the diffraction limit of 0.2. Ca II H line core images are also taken.

10 Observations: Transition Region Structures SUMER rasters were taken in a wavelength range including the C III 97.7 nm line. This line is formed in the mid TR (T K) and is one of the brightest lines in the VUV spectrum. As such, it is an ideal tracer of the TR loops. It allows high S:N spectra to be acquired with exposure times of few seconds.

11 Observations: Transition Region Structures

12 Data Co-alignment: SUMER vs MDI Full Disk MDI magnetograms are aligned with SUMER scans by comparing the absolute magnetic flux and the scans in both the pseudo/continuum and H I Ly γ. The error in the alignment is considered to be between 1 and 2 Iso-contours of 7 G and 15 G absolute magnetic flux traced over the SUMER scan in the pseudocontinuum.

13 Data Co-alignment: DOT vs MDI DOT images are both rotated (by the angle between the geocentric North and the solar rotational North) and shifted with respect to SOHO images. After rotating (angle taken from the ephemeris), the DOT images in the Ca II H are aligned with MDI maps of the absolute magnetic flux. The error in the alignment is considered to be around 1. DOT G-band images are finally aligned to DOT Ca II H images by cross-correlation. The error in the alignment is well below 1.

14 GBPs vs TR radiances GBP are clearly related to the bright TR structures. GBPs seem to avoid the brightest cores.

15 GBPs vs TR radiances GBPs are clearly related to the bright TR structures. GBPs seem to avoid the brightest cores.

16 GBPs vs TR line shifts GBP avoids the regions of larger velocities.

17 GBPs vs TR line shifts GBP avoids the regions of larger velocities. GBPs are associated to areas that show an excess redshift of about 1.9 km/s.

18 GBPs vs TR line widths GBP avoids the regions of larger widths.

19 GBPs vs TR explosive events Although based on a very limited statistics (7 events), there never were GBPs within the SUMER pixel showing explosive events.

20 Summary & Conclusions We cannot find a clear evidence that GBPs are the foot-points of TR loops, i.e. there is not a clear relationship between the GBPs pattern and the TR structures. However, we find evidences that indicate that a link between the two phenomena may exist. In particular, the radiance (velocity) distribution of SUMER pixels associated to GBPs appear shifted towards larger radiances (downward directed velocities) than the overall distribution. The excess of red-shift in areas containing GBPs is particularly interesting as it may indicate a larger amount of vertical field, as would be expected closer to the foot-points.

21 Summary & Conclusions This may suggest loop foot-points to be connected to multiple magnetic elements in the photosphere via a complex system of field lines. A similar scenario has been suggested for moss areas, bright EUV regions at the base of hot (3 to 5 MK) AR loops. Berger et al. 1999

22 Summary & Conclusions GBPs seem to avoid explosive events. Although based on a very small statistics this may provide clues on the mechanisms responsible for such events. Further observations here are needed.

23 Future Work New SUMER rasters have been acquired during the recent SUMER campaign together with DOT. They provide a much more extended dataset to investigate the relationship between GBPs and TR structures.

24 Thank you