Galactic Cosmic Ray Modulation in the Global Heliosphere.

摘要

To understand the behavior of cosmic ray modulation seen by two Voyager spacecraft in the region near termination shock (TS) and heliosheath at distances of >∼ 100 AU, a realistic MagnetoHydroDynamic (MHD) global heliosphere model is incorporated into the cosmic ray transport code, so that the detailed effects of heliospheric boundaries and its plasma/magnetic geometry can be revealed. A number of simulations of cosmic ray modulation with this code reach the following conclusions. (1) Diffusive shock acceleration (DSA) by the TS can significantly affect the level of cosmic ray flux and in particular its radial gradient profile in the region near the TS and in the inner heliosheath. With the effect of acceleration, cosmic ray radial flux shows an enhancement approximately in the TS region, resulting in a difference in radial gradient of cosmic ray flux across the TS. The change of radial gradient does not occur exactly at the TS radial distance in the same direction, indicating that the acceleration effect comes from part of TS at other longitudes or latitudes. The shock acceleration effect can be easily lost if the TS is unrealistically smoothed due to a lack of spatial resolution in some previous MHD simulations. (2) The radial profile of cosmic ray flux strongly depends on longitude. There is a slight North-South asymmetry due to an asymmetric TS, but more difference of radial profile comes from the longitudinal effect. Voyager 1 and 2 are separated by ∼ 40° in longitude, simulations in these directions show large difference in the radial profile of cosmic ray flux. The apparent near zero radial gradient of cosmic ray flux derived directly out of the data from the two Voyager spacecraft does not reflect the true radial gradient in either of the directions because of the longitudinal effect. Therefore, the measured radial gradient cannot be used to extrapolate the level of cosmic ray flux in local interstellar space. Various other simulations are also performed to show how particle diffusion coefficient, cosmic ray energy, and interstellar spectrum can affect the above conclusions. In addition, the result of simulation is also compared with the cosmic ray energy spectrum obtained by the Pamela Satellite in low Earth orbit. It is shown that the cosmic ray intensity measured by Pamela is always lower than the modulation simulation result, demonstrating that the Pamela data suffer additional effect from the Earth's magnetic field.;To understand the transient modulation seen by Voyager in the heliosheath, this dissertation also studies the effect of Global Merged Interaction Region (GMIR) on cosmic ray transport. A GMIR model with intensified magnetic field and increased solar wind speed is constructed and incorporated into the cosmic ray transport code. The simulation reproduces decrease of cosmic ray flux upon the arrival of the GMIR at the spacecraft, consistent with previous simulation performed for inner region of the supersonic solar wind. However, as the simulation location is moved outside of the TS, it shows a new feature. Cosmic ray flux begins to decrease as the GMIR arrives at TS, which can be months prior to the GMIR arrival at the spacecraft. Spacecraft inside the heliosheath, such as Voyager 1, can remotely sense the time when the GMIR arrives at TS. Based on this remote sensing feature, the radial distance of the TS along the Voyager 1 direction is estimated to be about 91AU in 2006, a value agrees well with Voyager observation of an inward propagating North-South asymmetric TS.