Radial evolution of large-scale solar wind structures.

摘要

Stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs) are two types of large-scale solar wind structures. Both can cause interplanetary shocks, generate energetic particles, and affect planetary magnetospheres and ionospheres. One key element of successful space weather forecasting is being able to predict how these two structures evolve radially from the Sun.;To answer this question and eliminate the solar cycle effect as much as possible, we first compile SIR and ICME event lists using long-term solar wind observations at three different heliocentric distances: 0.72 AU using Pioneer Venus Orbiter during 1979--1988; 1 AU using Wind and Advanced Composition Explorer (ACE) during 1995--2006; 5.3 AU using Ulysses during three aphelion passes within +/-10° of solar ecliptic plane in 1992, late 1997--1998, and late 2003--2005. By analyzing the parameters of each event, we have obtained the statistics and solar cycle variations of properties of SIRs and ICMEs at each heliocentric distance, representing the space environment for Venus, Earth, and Jupiter, respectively covered in Chapters 3--5.;Through the comparison of the statistics at the three distances, we have obtained the radial evolution of SIRs and ICMEs. The SIR shock rate increases from 3% at 0.72 AU to 26% at 1 AU to 92% at 5.3 AU, and forward shocks predominate near the ecliptic plane. The ICME shock rate remains at about 60% all the way out to 5.3 AU. The SIR width increases almost linearly with heliocentric distance, changing from 0.2 to 1.2 AU from Venus to Jupiter orbit. In contrast, the ICME width increases as a power-law function of heliocentric distance with a power index of 0.82 within 1 AU and its radial velocity slows down greatly to equal that of the surrounding solar wind between 1 and 5.3 AU. The ICME expansion speed decreases by half from 1 to 5.3 AU. In addition, a third of them have interacted and become hybrid events by 5.3 AU.;Besides statistical work, we have studied some SIRs observed by multiple spacecraft, showing some merging of small SIRs into one big SIR from 1 to 5.3 AU. The CCMC/ENLIL model is run to reproduce these events. We find the arrival time of some SIRs in the model can differ 2--3 days from observed and some SIR features are missing in the model. The solar magnetogram input and solar model part are critical for ENLIL output and the whole model chain needs to be improved. Moreover, some superfast ICMEs observed by ACE, Ulysses, and Cassini, during Oct.--Nov. 2003 are also compared with the population of regular ICMEs in this dissertation. Even at a distance of 8.7 AU from the Sun, they can be many times larger and faster than regular ICMEs.