Seasonal polar cap radiation zones in dayside magnetosphere

Abstract

The phenomenon of quasi-stable trapping of charged particles in the keV to MeV energy range within the polar cusp region of the Earth’s magnetosphere is explored. The remote equatorial magnetic field lines on the dayside magnetosphere are compressed by the solar wind and exhibit two local minima in the geomagnetic field strength along the field line in high latitudes. These minima, on both sides of the equator, result in stable confinement structures. Numerical modeling of charged particle orbits that pass through the regions of these local field minima has been carried out using different seasonal Earth tilt and different magnetospheric disturbance level. These orbit tracings show when and where these off-equatorial trapped radiation zones would be situated. The existence and extent of these confinement zones depend on the tilt angle. Indeed, the northern cusp confinement zone appears only at the northern summer solstice, while the southern cusp particle capture zone appears around winter solstice. The particle orbits that pass through opposite off-equatorial field minimum during solstices reveal a bound of the geomagnetic equatorial plane on the day sector. During equinox, the particle confinement zones exist in both cusps at times of disturbed magnetosphere conditions. The trapped particles drift within the trapping zones with periods of the several minutes, conserving the 1st and 2nd adiabatic invariants.