06Measurement of Solar Magnetic Field
The solar magnetic field causes structures appearing on the Sun's surface such as sunspots and various activities occurring in the solar atmosphere. How do you think we knew of the existence of the solar magnetic field? Researchers applied "the Zeeman effect", the splitting of a spectral line into several components in a magnetic field. When we observe a solar spectrum, we find that a single spectral line outside a sunspot splits into three components inside the sunspot. The separation between the components along the wavelength is proportional to the strength of the magnetic field. Thus, by the means of the Zeeman effect, astronomers measure the strength of the solar magnetic field and find it to be as strong as 3000 gauss. The magnetic field of the Sun is far stronger than geomagnetic field which is less than 1 gauss.
The Solar Optical Telescope on board Hinode (Solar-B) aims to measure the magnetic field with high accuracy. In addition to the strength, the direction (the vector) of the magnetic field is also important. To know the direction, the researchers perform spectro-polarimetry which measures weak polarization using the Zeeman effect. Examining spectral lines closely, they derive the distribution of solar magnetic field vectors.
: Twisting of the Sunspots and Solar Flares
The distribution of the magnetic vectors on the solar surface tells us how much energy is accumulated in the solar atmosphere. The accumulated magnetic energy will eventually be released and result in gigantic solar flares. Figure 2 shows a sunspot observed in December 2006. In this sunspot, umbrae with opposite polarities exist side-by-side in a penumbra. The two umbrae are connected by twisted magnetic field lines, which resembles a typhoon. This feature is a signature of accumulation of enormous magnetic energy. In fact, one day after the observation, we detected a huge solar flare.