Space, from the upper ionosphere out into the solar system contains a fully ionosed gas, called plasma. Most of the ions are hydrogen with a smattering of Helium plus electrons to give an overall, neutral plasma. Around the magnetised planets (Mercury, Earth, Jupiter, Saturn, Neptune and Uranus), magnetised plasma waves are generated and propagate in the magnetospheres. There are 3 possible wave modes for frequencies much less than the ion-cyclotron frequency. These are the slow, fast and shear Alfven modes. For low energy, (10 eV or so) plasmas, only the fast and shear Alfven modes exist.
The 'fast' mode can propagate energy oblique to the ambient magnetic field, spreading wave energy throughout the magnetosphere. It is a type of magnetic pressure wave (compressional wave).The shear mode has energy guided along the ambient magnetic field. It is a transverse wave.