Top The sketch above illustrates the three principal components of the auroral oval: the dayside (A) and nightside (B) discrete arc systems and the diffuse aurora.
Bottom The false-color image below shows the northern auroral oval as observed with the IMAGE FUV Spectrographic Imager 12 (SI-12). IMAGE's SI-12 detects auroral emissions from precipitating energetic protons and is the first instrument to provide global views of the proton (as opposed to the electron) aurora. The bright spot near the geographic pole is the footpoint of the magnetospheric cusp, which is located on the day side of the Earth near local noon. The cusp is a signature of the merging of Earth's magnetic field with the interplanetary magnetic field, and is a region where energetic particles from the solar wind gain direct access to Earth's upper atmosphere.