Temporal evolution of magnetic elements
aa r X i v : . [ a s t r o - ph ] D ec **FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS** Temporal evolution of magnetic elements
R. Rezaei , R. Schlichenmaier , W. Schmidt and C. Beck , Abstract.
We study the structure and evolution of the magnetic field ofthe quiet Sun by investigating weak spectro-polarimetric signals. To this end,we observed a quiet region close to the disk center with the German VTT inTenerife, July 07, 2006. We recorded 38 scans of the same area. Each scanwas eight arcsec wide and observed within about 100 seconds. We used PO-LIS to simultaneously observe Stokes profiles of the neutral iron lines at 630.15and 630.25 nm, the Stokes- I profile of the Ca ii H line at 396.8 nm, and a con-tinuum speckle channel at 500 nm. We witness two examples of magnetic fluxcancellation of small-scale opposite-polarity patches, followed by an enhancedchromospheric emission. In each case, the two opposite-polarity patches grad-ually became smaller and, within a few minutes, the smaller one completelydisappeared. The larger patch also diminished significantly. We provide evi-dence for a cancellation scenario in the photosphere which leaves minor tracesat the chromospheric level.
1. Introduction
Observations indicate that most of the magnetic flux passing through the photo-sphere is concentrated in magnetic elements , i.e., patches of high field strengththat are embedded in relatively field-free plasma (Solanki 1993). The field linesof the magnetic elements are nearly vertical to the surface because of buoyancyforces. In the hierarchy of the magnetic structures, the magnetic elements takethe position between dark and bright structures, i.e., between pores and networkbright points (Zwaan 1987, Stenflo 1994). While the magnetic elements are notvisible in continuum or line wing, they appear bright in the core of the chro-mospheric Ca ii H and K lines. The convective motion associated with the pho-tospheric granulation sweeps the magnetic flux toward the intergranular lanes.Due to the geometry of these lanes, the magnetic flux is arranged there either inchains of individual flux tubes or elongated sheets. This behavior is seen in high-resolution observations (Berger et al.2004; Rouppe van der Voort et al. 2005)as well as in numerical simulations (Steiner 2005, V¨ogler et al. 2005).Magnetic flux cancellations are common events in the solar atmosphere.There are examples in which it leads to a clear enhancement in the chromo-spheric intensity (Bellot Rubio & Beck 2005; Beck, Bellot Rubio & Nagata 2005).Magnetic reconnection at the photospheric level was also studied by Litvinenko(1999) and Takeuchi & Shibata (2001). Rezaei et al. (2007b) presented Stokes- V profiles of the Fe i
630 nm line pair, where the two lines show opposite polaritiesin a single spectrum (OP profile). They suggested that it may be understood as1
Rezaei, Schlichenmaier, Schmidt, and Beck a magnetic reconnection event at the solar photosphere with a line of argumentssimilar to Steiner (2000).In this contribution, we show the temporal evolution of physical quantitiesbefore and after this event. In addition to the polarimetric data, we investi-gate Ca ii H profiles which contain information about higher layers of the solaratmosphere.
2. Observations and data reduction
A time-series of a small quiet Sun region close to disk center (cos θ = 0.99),was observed with the VTT in Tenerife, July 07, 2006. The seeing was goodand stable during the observation. The Kiepenheuer Adaptive Optics Systemwas used for maximum spatial resolution and image stability (von der L¨uhe etal. 2003). For 64 minutes, we scanned an area eight arcsec wide with a scanningcadence of about 97 s. The scanning step size and spatial sampling along theslit were 0.5 and 0.3 arcsec, respectively. The spectrograph slit was 0.48 arcsecwide. The slit height of the blue (396.8 nm) and red (630 nm) channels was 70and 95 arcsec, respectively.Full Stokes profiles of the Fe I
630 nm line pair and the Stokes- I profile of theCa ii H line were observed strictly simultaneously with the red and blue chan-nel of POlarimetric Littrow Spectrograph (POLIS, Schmidt et al. 2003, Beck etal. 2005b). The spectral sampling of 1.92 pm for the blue channel and 1.49 pmfor the red channel leads to a velocity dispersion of 1.45 and 0.7 km s − perpixel, respectively. The spectrograph curvature was corrected using the routinedescribed in Rezaei et al. (2006). The spectro-polarimetric data of the red chan-nel were corrected for instrumental effects and telescope polarization with theprocedures described by Beck et al. (2005a,b). The rms noise level of the Stokesparameters in the continuum was σ = 6.0 × − I c . We normalized the Ca ii Hintensity profiles at the line wing at 396.490 nm to the FTS profile (Stenflo etal. 1984). Following Cram & Dam´e (1983) and Lites et al. (1993), we define aset of parameters to quantify properties of the Ca ii H line profiles (Table 1).The normalization procedure of the calcium line profiles is very similar to thatdescribed by Rezaei et al. (2007a).
Table 1. Definition of the characteristic parameters of the Ca ii H line profile(Lites et al. 1999). Wavelengths are in nm. quantity definitionH-index 396.849 ± v 396.833 ± r 396.865 ± v/H rW1 (outer wing) 396.632 ± ± ± emporal evolution of magnetic elements b ). We used thePOLIS intensity map (Figs. 1 and 2, column c ) and the reconstructed image(column b ) to align the data. Figure 1. Temporal evolution of physical parameters in a flux cancella-tion event.
From left to right: a) V tot , b) speckle reconstructed image, c) Stokes- I , d) W1, e) W2, f )
W3, g) H , h) H , i) H-index, j) V/R, and k) Fe i x = 1, y = 2). There are weak enhancements inthe corresponding chromospheric emissions, e.g., (4 − i ). The spatial extensionof each map in both axes is 4.4 arcsec. We used the procedure explained in the Appendix of Beck et al. (2007a)to remove effect of the differential refraction between the red and blue beamsof POLIS. There is a time lag between co-spatial data in the two channels;the polarimetric data was actually recorded 10 seconds later than the calciumdata for the case shown in Fig. 1. Figures 1 and 2 show two examples offlux cancellation event: one with and one without significant chromosphericbrightening. This can be seen by comparing the maps (4 − i ) in Fig. 1 and(3 − i ) in Fig. 2.We use the signed integral of the Stokes- V profile, V tot , which traces themagnetic flux (Lites et al. 1999; Rezaei et al. 2007b). It enables us to follow weakpolarimeric signals where the Stokes- V amplitude is below the 3 σ noise level.The column a of Figs. 1 and 2 shows the variation of V tot for seven time stepsaround the reconnection frame (row 4). The cancellation starts as the smaller(white in Fig. 1) patch concentrates and lasts until it has almost disappeared. Rezaei, Schlichenmaier, Schmidt, and Beck
The V/R is a measure of the asymmetry between the violet and red emissionpeaks in the calcium profile. Signatures of bright and dark structures in V/Rmaps (column j ) can be directly compared with H and H maps (columns g and h , respectively). The wing intensities show the gradual variation of theintensity, in-between the Stokes- I and the calcium core parameters. The photo-spheric velocity, column k , shows the patterns of up- and downflow structurescorresponding to granules and intergranular lanes (e.g., compare the upper leftcorner of (3 − b ) and (3 − k )). Figure 2. Same as Fig. 1 but for another example of flux cancellation in ourdata. In this case, there is a significant enhancement in the chromosphericemission (H-index, H , and H ), far stronger than the case in Fig. 1.
3. DiscussionSimilarities:
The negative polarity patch in Fig. 1 (column a ) was a networkpatch, which was present during the whole observing run. In contrast, thepositive patch gathered from diffuse flux, enhanced, and almost disappeared(the positive patch corresponds to the white color in Fig. 1, Rezaei et al. 2007b).Since the magnetic flux is continuously replaced in the network, events like thismay be common in the solar photosphere (Schrijver et al. 1997). The strongpositive patch in Fig. 2 (column a ) was not a network component. However, itwas much stronger than the negative one and persisted for the whole observingrun. Although, the larger patches in both cases survived the cancellation event,they weakened clearly. In both cases, the cancellation event happened in anintergranular vertex (see panel (4 − b ) in Figs. 1 and 2). Therefore, these twoevents have comparable configurations at the photospheric level. emporal evolution of magnetic elements Differences:
Comparison of the two flux cancellation events presented in theprevious sections demonstrates essential differences in the chromospheric reac-tion. In the first case, we have mild chromospheric brightening in the cancel-lation site, e.g., row (4) in Fig. 1. In contrast, rows (3-5) of Fig. 2 show abrightening in the H-index on the cancellation site. Note that the colorbar hassimilar scales for the H-index. In the latter case, we observe stronger violetemission than red one, either by investigating columns g and h or by inspectingthe asymmetry parameter, V/R in column j . In contrast, we have a strongerred emission peak in the first case (map (4 − j ) in Fig. 1). Figure 3 shows theevolution of calcium profiles on and near the cancellation site. During the can-cellation, the profiles display a H peak that is much stronger than H . It isin contrast to the average quiet Sun profile where it was interpreted as the sig-nature of upwards propagating acoustic waves (e.g., Beck et al. 2007b). Hence,both the integrated intensity and the asymmetry parameter indicate importantdifferences in the reaction of the chromospheric layers to the flux cancellation. Figure 3. Variation of emission peaks at three locations ([1,3] top, [0.5, 2.6]middle, [0.5, 2.2] bottom) in time-steps 3 (left), 4 (middle), and 5(right) forthe Fig. 1 example. The gray profile is the average of a few thausand profiles.The dashed lines show a distance of ±
4. Conclusions
Time-series of co-spatial and co-temporal polarimetric data of a quiet Sun re-gion revealed a variety of connections between photospheric flux cancellations
Rezaei, Schlichenmaier, Schmidt, and Beck and chromospheric enhanced emissions. We present two examples of magneticflux cancellations with different levels of the enhanced chromospheric emission.We attribute the difference to the geometrical height at which the cancellationhappened. This is in accordance with the interpretation of the opposite polarityprofile as a signature of photospheric reconnection by Rezaei et al. (2007b).
Acknowledgments.
The POLIS instrument has been developed by theKiepenheuer-Institut in cooperation with the High Altitude Observatory (Boul-der, USA). Part of this work was supported by the Deutsche Forschungsgemein-schaft (SCHM 1168/8-1).