The Role of the Cusp: Introduction
The magnetosphere contains energetic particles that have been processed and energized from their parent or source populations in the solar wind or the earth's ionosphere. The Dungey cycle proposed by Dungey (1961) involving magnetic reconnection is believed to be the fundamental mechanism to accomplish the transfer of mass and momentum from the solar wind into the magnetosphere and impose a global electric field on the magnetosphere to drive these yet-to-be fully determined energization processes.
An alternative to the Dungey cycle was proposed by Tverskoy (1970) in which he recognized the different behavior of magnetospheric particles that have a gradient and curvature drift period greater than and less than one hour. Since particles of ring current energies (10 keV to 100 keV) are observed to be injected into the magnetosphere at time of geomagnetic storms, Tverskoy developed a theory in which such particles injected along the boundary of the magnetosphere and the pressure that they carried would produce electric fields equivalent to that of the Dungey cycle. Tverksoy (1970) states: "We believe that the most important element … is the ring current asymmetry accompanied by drift polarization, neutralization of the resulting charge by currents along the lines of force, and closing of these currents through the ionosphere. As it was noted above, the appearance of asymmetry may be connected with plasma injection into the magnetosphere or penetration of an external electric field into the trapped radiation region. From a formal viewpoint these two mechanisms are equivalent."
This theory by Tverskoy is outlined in Figure 1 from his paper but this theory has been neglected due to the problem with finding >20 keV protons in the solar wind that would be the injected along the dawn magnetopause to produce the asymmetric injection of the ring current protons during geomagnetic storms. We believe we have discovered such a source of protons energized from a combination of solar wind and ionospheric ions in the dayside high-altitude cusp. These energized ions form a layer of such particles on the magnetopause as well as drift along contours of constant magnetic field intensity shown in Figure 3. We present observational evidence of the existence and properties of this cusp source.