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Figure 9-17 RIVUXG Sunspots Have Strong Magnetic Fields (a) A black line in this image of a sunspot shows where the slit of a spectrograph was aimed. (b) This is a portion of the resulting spectrum, including a dark absorption line caused by iron atoms in the photosphere. The splitting of this line by the sunspot’s magnetic field can be used to calculate the field strength. Typical sunspot magnetic fields are more than 5000 times stronger than Earth’s fields at its north and south poles.