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The combination of magnetic field and plasma in the magnetosphere means that this medium can support a variety of ultra-low frequency (ULF) wave modes. Alfvén waves can be thought of as vibrations of a field-line, rather like the vibrations of a plucked guitar string. These waves are not directly observable on the ground, but their fields can drive currents in the ionosphere which are observed as magnetic field variations by magnetometers. In certain circumstances, ULF waves in the magnetosphere can modulate the process which causes particles trapped on magnetic field lines to be precipitated into the ionosphere. These precipitated particles (usually electrons) can have high energies and penetrate to low altitudes where they increase the ionisation in the ionosphere. This increased ionisation leads to increase absorption of cosmic radio noise as measured by riometers. Modern imaging riometers have good spatial resolution and so offer an opportunity to investigate in detail the structure of ULF wave activity in a way not possible with magnetometers which are limited by their wide field-of-view.

Figure 1: An example of a "PC4" pulsation in IRIS riometer data.

Figure 2: The same pulsation seen in the IMAGE magnetometer array.

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