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The above magnetogram was recorded from a three-component Narod Ring Core Magnetometer, with the signals digitized 10 times per second. magZ is the vertical magnetic intensity, magH is the magnetic intensity toward magnetic North (which is 5 degrees East of True North at Leonard, OK,) and magD is horizontal magnetic intensity toward magnetic East. In magD, D stands for declination, which is the difference between magnetic North and True North. Near Leonard, Oklahoma, magZ is about 48175 nT (nanoTesla), magH is about 22275 nT, and magD is near zero. The magnetogram indicates only the fluctuations in the geomagnetic field, not how far from zero the components are. The fluctuation is usually within a 200 nT range on each component. Magnetic storms occur when a solar flare sends a gust if ionized hydrogen into the otherwise steady solar wind. From 24 to 48 hours after the flare, the gust in the solar wind strikes the outer edge of the earth's magnetic field, causing a sudden jump in the field measured at the surface. This jump is called the SSC, which stands for Storm Sudden Commencement. In this storm, the traces dropped a bit before their large increase, in which case the SSC label is changed to SSC*. The SSC* in this storm (as in most) largest on the horizontal intensity in the direction of magnetic North (magH). The SSC* on magH is enclosed in a red rectangle. Note that this magnetogram is slightly over 24 hours. This is a much longer time than most seismograms, except those showing Earth Tides. The red box in the first figure is enlarged below to show the SSC* in detail.
