A - Scientific rationale
In the past years a huge number of spectropolarimetric data have been provided by Hinode-SOT/SP instrument and more recently by SDO/HMI instrument. Although they use different kinds of observational techniques (slit scan is used for the former and full disk is used by the later) and different spectral lines (Fe I 6301 & 6302A and Fe I 6173A respectively), detailed comparisons of the results from both instruments would be an important tool for future coordinated observation campaigns.
Comparing vector magnetic fields and other physical quantities obtained after data reduction and inversion of Stokes profiles coming from different instruments and different inversion codes is a delicate task. On one hand, there is the influence of the instrument. On the other hand, there is the reliability of the inversion code. Usually, the inversion codes are specifically designed for an instrument, i.e. taking into account the spectral line observed and the optical features of the instrument (characterized by the PSF, spectral, spatial and temporal sampling). Of course, this information can be changed in the inversion code, and when the profiles of instrument #1 are convolved with the PSF of instrument #2, they can be used as input for the inversion code of instrument #2. However, another supplementary strategy is possible: use a third instrument
which simultaneously observes the spectral lines observed by the first and the second telescope as a reference.
Our proposal is straightly based on the work done by Berger & Lites (2003). There, the authors quantitatively compare the magnetic flux density of an active region map simultaneously observed by the SOUP instrument and the ASP instrument, and between the SoHO/MDI and ASP instruments, respectively. They found the following linear correlation between both instruments for flux density values less than 1600 Mx cm−2:
B(MDI/app) 〜 0.69 × B(ASP/app) (1)
where the magnetic flux density is Bapp = |B|fcos(φ), f being the filling factor and φ the inclination of the vector magnetic field B. Using this SoHO/MDI-ASP cross-calibration, Green et al. (2003) gave a correction flux formula for SoHO/MDI magnetograms. That formula includes the linear and non-linear response of SoHO/MDI and is:
Φcorr = 1.45(Φ + 0.3Φ（B>1200G)) (2)
B - Immediate objectives
To obtain similar relationships as given in 1 and 2 between THEMIS, Hinode-SOT/SP and SDO/HMI through the comparison of the vector magnetic field, the line-of-sight component of the velocity, magnetic flux density and others physical observables simultaneously obtained by these instruments.
C - Strategy for observing
We shall simultaneously observe the spectral lines observed by Hinode-SOT/SP and SDO/HMI instruments in THEMIS. Because spatially well-defined, well-identified structures lend themselves to more straightforward comparisons, we prefer regions containing isolated sunposts, pores and the disk-centered quiet sun. The observations will be mainly coordinated with
THEMIS, since SDO/HMI will be continuously observing the full disk. Using these instruments together we can observe the following spectral lines:
・ Photospheric lines Fe I 6173A: observed by THEMIS and SDO/HMI.
・ Photospheric lines Fe I 6301 & 6302A: observed by THEMIS and Hinode-SOT/SP.
・ Chomospheric lines Ca II 8542 & 8662A: observed by THEMIS.
We shall focus our attention on sunspots and pores because these structures can help us to align the maps with the other instruments. In addition, we are also interested in their evolution. Therefore, we propose this topic as backup program. A large area scan in the quiet sun around the center of the solar disk will be the other kind of the observations that we shall do. We have included the Ca II IR lines 8542 & 8662A because they are valuable for understanding the behavior of the penumbra and moat in the chromosphere. The presence of Ca triple IR lines does not affect the intensity of the other selected lines, that means: they are compatible with the observation of main spectral lines of our proposal.
The field of view for the sunspot observation will roughly be three or four times its size; for the pore it will be between about five to seven times its size; for the quiet sun area it will be about 40′′×40′′ around the center of the solar disk. The cadence of every target will be defined taking into account their presence and location on the solar disk. For the quiet sun area 6 scans a day is enough.
D - Strategy for data reduction and analysis
We will use the available standard software packages for the data reduction for each instrument, they are: SQUV for THEMIS-MTR and SolarSoft for Hinode-SOT/SP and SDO/HMI. The inversion of the Stokes profiles will be done with Milne-Eddington codes. For THEMIS we will use a PCA based-on inversion code developed by A. L´opez Ariste, For Hinode-SOT/SP we
shall use MERLIN code, developed by HAO. Finally, for SDO/HMI we shall use the inversions done by VFISV code Borrero et al. (2010). However, in addition to the comparison of the final products given by the instrument-specific inversions (each data inverted by its own specific inversion code), the main analysis shall include an interchange between data and inversion codes, e.g.:
・ HMI inverted data by VFISV vs. HMI-like Hinode-SOT/SP inverted data by VFISV
・ HMI inverted data by VFISV vs. HMI-like THEMIS-MTR inverted data by VFISV
・ Hinode-SOT/SP inverted data by MERLIN vs. Hinode-like THEMIS-MTR inverted data by MERLIN
・ Hinode-SOT/SP inverted data by THEMIS PCA based-on inversion code vs. THEMIS inverted data by THEMIS PCA based-on inversion code
E - Conditions necessary to reach the scientific objectives
The current solar activity is very well suited for this observation: formation of isolated sunspots and pores occurs often in this part (the rising phase) of the solar cycle. This project is better suited for moderate levels of activity, because the complex topologies of solar structures typically found during more active periods make the comparison more difficult, not only for the alignment of the maps but also for the complexity of the final observables. However, good seeing conditions are a priority. Because the three instruments have different spatial scales, and two of them are above of the Earth’s atmosphere, good seeing conditions are necessary for reliable comparisons.
Berger, T. E. & Lites, B. W. 2003, , 213, 213
Borrero, Tomczyk, Kubo, Socas-Navarro, Schou, Couvidat, & Bogart Borrero, J. M., Tomczyk, S., Kubo, M., et al. 2010, , 35
Green, D´emoulin, Mandrini, & Van Driel-Gesztelyi Green, L. M., D´emoulin, P., Mandrini, C. H., & Van Driel-Gesztelyi, L. 2003, , 215, 307