Research of Solar Very Small Velocity Fields by Magneto-Optical Filter

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The magneto-optical filter (MOF or Caccianl filter) is made of a transparent cell placed in a strong magnetic field. The cell contains the vapor of material like sodium. By utilizing the magneto-optical effect near the wavelength of a spectral line, MOF provides a very narrow and stable passband, which is perticularly suitable for helio-seismological observations. We started basic experimental studies on this type of filters in 1984. In the design of most MOF's, only the sodium reservoir is heated, so that the sodium vapor attaches onto the wall when it cools there. The cell therefore becomes less transparent as it ages, and can only be used in observations for several months. Our aim was to construct a filter that can be continuously used for 10-20 years without losing the sodium vapor at the cell wall. This was achived by heating the whole cell to about 200ﾟC.We developed the method of Introducing the new material that is stable for high temperature sodium gas. As this development was necessary very long term for each affair, we chould finish only the construciton of the new MOF system, and so the data obtainment of solar velocity fields has just started.We descrive the process of the development and experimental results.