In the "open" model of the magnetosphere, the polar cusps are narrow regions of recently "opened" or merged magnetic field lines mapping to the high-latitude ionosphere just poleward of the last closed field line on the Earth's day side. These regions are centered on local noon and extend approximately 2-3 hours in longitude and ~1 degree in latitude. The open field lines of the cusps are connected with those of the interplanetary magnetic field, which allows the shocked solar wind plasma of the magnetosheath to enter the magnetosphere and to penetrate to the ionosphere. Associated with the cusp is the "cleft ion fountain," from which plasma flows upward from the ionosphere into the magnetosphere, with the peak outflow occurring in the pre-noon sector. The injection of magnetosheath plasma into the cusp has been found to be one of the mechanisms that heat the ionospheric plasma in the cusp/cleft region and thus drive the outflow of ionospheric plasma into the magnetosphere. (The cleft is an extended band of dayside magnetosheath particle precipitation within which the cusp is located. Though closely related, the cusp and cleft are distinguished from each other by differences in their characteristic particle fluxes and energies.)

False-color image of the Earth's magnetospheric magnetic field, illustrating the depression in magnetic field strength in the northern and southern cusps. From N. A. Tsyganenko and C. T. Russell, "Magnetic signature of distant polar cusps: observations by POLAR and quantitative modeling," a paper submitted to the Journal of Geophysical Research. Abstract and selected figures are available on the web at The image was generated with the Tsyganenko 96 magnetospheric magnetic field model.