Investigation On The Mid-altitude Cusp: Particle Dynamics And Configuration

The two spatially narrow cusps in the dayside magnetosphere are regionswhere the magnetosheath plasma can directly enter into the magnetosphere andthe dayside ionosphere. The cusp region plays a key role in the Solar wind-Magnetosphere-Ionosphere coupling. Based on the data set of the Cluster II andDouble Star, this dissertation studies what parameters and processes control theproperties of the cusp particles and location.The cusp is identified by the presence of ions and electrons of the magne-tosheath origin, and a diamagnetic field depression. The criteria are as follows: adecrease of the magnetic field strength from the background field greater than 1nT associated with some fluctuation; a sudden increase in the proton and electrondensities (> 5cm^-3); an electron thermal energy less than 100 eV; the presence ofsignificant He⁺⁺ (> 0.5cm^-3). We first investigate the ions density as a functionof the solar wind dynamic pressure and geomagnetic index Kp respectively. Theresults show that O⁺ density is well correlated with geomagnetic index Kp, whileO⁺, He⁺ and H⁺ density are all have a significant positive correlation with solarwind dynamic pressure.A statistical study of the diamagnetic pressure is conducted. The diamag-netic pressure shows strong dependence on the solar wind dynamic pressure, theIMF B_z component and the dipole tilt angle. The diamagnetic pressure is greaterfor greater solar wind dynamic pressure. When the dipole tilts more toward tothe sun, the diamagnetic pressure is higher. When the IMF B_z component ismore negative, the diamagnetic pressure is larger.The location of the cusp thus gives information of the state of the magne-tosphere and of the solar wind control of the magnetosphere. We investigate thecorrelations of the cusp location with the dipole tilt angle, IMF B_z and B_y com-ponent. The result shows that the dipole tilt angle has a clear control of the cusplatitudinal location. The northern cusp moves 0.065°ILAT for every increase1°in the dipole tilt angle, while the southern cusp moves 0.048°ILAT for every1°increase in the dipole tilt angle. According to the observations of di°erent satellites, the slope of the cusp location dependence on the dipole tilt angle isincreases with the increasing altitude. But in the present study, the slope forthose southern cusps is smaller than that for northern cusps, while the altitudefor those southern cusps (～7Re) is higher than that for northern cusps (～5Re).The result implies an interhemispheric asymmetry of the dipole tilt angle effectson the latitude location, which may reffect the interhemispheric asymmetry ofthe magnetospheric configuration and current system between the northern andsouthern hemispheres. For southward IMF, B_z rules the latitudinal dynamics.Observations show that the more negative the IMF B_z is, the lower in latitudethe cusp finds itself. But for northward IMF, the cusp location is less sensitiveto the magnitude of IMF B_z. The large IMF B_y could also introduce a longi-tudinal shift of the polar cusp location. For the northern hemisphere, when theIMF B_y > 3nT (B_y < -3nT), the cusp will shift to duskward (dawnward), andeverything has to be inverted for the southern hemisphere. All this is a priori inagreement with anti-parallel reconnection but the cases off the statistical trendmay result from component reconnection.We also find a strong correlation between the solar wind dynamic pressureand the cusp latitudinal location. The cusp moves down in latitude with increas-ing solar wind dynamic pressure. At last we analysis the influence of the solarwind azimuthal flow on the location of cusp. For small IMF B_y, the position ofthe cusp in MLT is clearly coupled to the azimuthal flow component of the solarwind, while the B_y has no obvious effect on the cusp location. For large IMF B_y,the IMF B_y has strong relationship with the position of the cusp in MLT, whilethe azimuthal solar wind flow has no clear effect on the cusp location. The IMFcomponent is no doubt of general importance for the cusp position, but one mustalso consider the dynamical properties of the solar wind.