HOP 0022

abstract of observational proposal

The solar photosphere is a boundary layer where physical quantities show very sharp depth variations and where structures such as the granulation are present on small scales.The modeling of this complex layer requires 3D hydrodynamical simulations of magneto-convection. Very efficient numerical calculations are developed (Voegler et al. 2005).They allow the formation of different solar spectral lines to be simulated (see the recent paper by Orozco Suarez et al., 2007). However,comparison of the simulations with high vertical resolution observations are quite rare. One aim of this proposal is to carry out such an "in-depth" investigation of the 3D structure of the photosphere by using a Dierential Cross-correlation Technique (DCT)similar to the one described in Grec et al. (2007). A second aim of this proposal is to take profit of the DCT to measure the depths of formation of the two FeI lines at 630.1 and 630.2nm. Those two lines are extensively used in order to determine the photospheric magnetic field. However, recently Martinez et al (2006) pointed out that spurious results may be obtained from this set of lines due to the ignored fact that they have different formation depths. DCT can provide a direct measuremen tof the difference between the two line formation depths.

request to SOT

We propose to perform SOT observations, using both SP and BFI.The slit has to be oriented along the North-South axis,in order to avoid lines Doppler shifts due to the solar rotation, and successively positioned in seven differentpositions, consisting in a reference position at disc center, and three symmetrical positions for each hemisphere. This disposition covers in an optimal way the solar diameter and will get measurements with dieffrent perspective effects. We will use SP in two modes, the Normal map and Dynamics, both coupled with the BFI position ed in the red continuum. According to the different scientific objectives, we intend to perform 2types of observations:

Type1: Use of Dynamics: Scan of 7 regions of 32 arcsecx12.8arcsec each.For each region, a scan should take about 2.5 minutes.The same observation is run 4times, which amounts toa total of 10 minutes for a given region. The 7scans are positioned from center to limb on the North-South axis at the Center, North 0.25, 0.5 and 0.75, and South 0.25, 0.5 and 0.75 as shown in the figure. In parallel we would like to have BFI images in the red continuum at the rate of 1 each minute or so,if possible. The scanning rate(4 images in 10minutes)should allow us to study the effect of the five minute oscillations.Estimated total time for 7 positions: 70minutes

Type2: Normal scans: Scan of 7 regions of 164 arcsecx4.8 arcsec each.For each region, a scan should take about 2.5minutes.The same observation is run 4 times, which amounts to a total of 10 minutes for a given region. The 7scans are positioned from center to limb on the North-South axis at the Center, North 0.3, 0.6 and 0.95, and South 0.3, 0.6 and 0.95 as shown in the figure. As for the Type1 scan we would like to have BFI images in parallel. The objective is to achieve a good polarimetric accuracy for studies of the magnetic field structure.Estimated to tal time for 7 positions: 70minutes

We propose to perform 4times the two sets of observations,this to increase the SNR and to check the coherency of our results and the effects of granular variability. The over all requested observing time is of the order of 10hours.