Abstract of observational proposal
The purpose of this project is to study 3D structures of magnetic field and velocity field at magnetic flux cancellation sites from multi-line observations. The magnetic flux cancellation is a descriptive term to indicate a mutual flux loss due to the apparent collision of the oppositepolarity magnetic elements. The flux cancellation has the key to understanding the dissipation ofvthe magnetic flux from the photosphere. We plan joint observations with the IBIS (Cavallini 2006), Hinode and IRIS. Our proposal to the IBIS has been already accepted.
Two major possible processes have been proposed to explain the observed flux cancellation (Zwaan 1987): submergence (retract) of Ω-shaped loops or emergence of U-shaped loops across the photosphere. The approaches to understanding the physical process of magnetic flux cancellation could be categorized into two types. One is to examine the motion of the photospheric horizontal magnetic field between the canceling magnetic elements from the vector field measurements. The horizontal field with the downward motion should be observed in the submerging Ω-loop model, and the observation of the emerging horizontal field is expected in the emerging U-loop model.
Another approach is to investigate the relative timing of disappearance of the photospheric and chromospheric magnetic flux from a time series of line-of-sight magnetograms like Harvey et al. (1999). When the chromospheric magnetic flux disappears before the photospheric magnetic flux, some submerging activities are suggested at the cancellation site, and vice versa.
We expect two new results from our proposed joint observations. (1) We will improve the Harvey et al. (1999) study. The cadence of their study was about 14 minutes, which was insufficient to detect the dynamic motion of magnetic field lines at the cancellation sites. The IBIS can provide us the photospheric and chromospheric magnetic flux (Fe I 637.3 nm, Na I D 589.6 nm, and Ca II 854.2 nm) with about a 40 second cadence. We can examine the relative timing of the magnetic flux disappearance from only the longitudinal fields. (2) We will also investigate the consistency between the timing of the photospheric/chromospheric (line-of-sight) magnetic flux disappearances and the motion of the photospheric horizontal fields. For this purpose, we propose the repeat scans by the Hinode/SP. The SP is necessary for the accurate measurements of the photospheric horizontal fields between the canceling magnetic elements. In addition, IRIS can provide us the timing of brightening and Doppler motions at the cancellation sites in the chromosphere or transition region. These information are also important to identify the height of magnetic reconnection and motion of plasma related to the magnetic flux cancellation.
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