An Observational Study On Magnetic Reconnection In The Interplanetary Space

Magnetic reconnection is a fundamental dissipation process which quicklyconverts magnetic energy into the kinetic energy and heat of the plasma. It oc-curs widely in laboratory, space and astrophysical plasmas. Fast reconnection isbelieved to play a critical role in some explosive phenomena. It can trigger solarflares and corona mass ejections to eject a lot of particles and energy into inter-planetary space. Through reconnection at the magnetopause and magnetotail,the solar wind plasma can enter into the inner part of the Earth's magnetosphereto produce disastrous space weather. Magnetic reconnection plays an importantpart in the whole space weather system of Earth. Therefore, it is very helpfulto investigate the reconnection process through in situ detections to understandthe causes, development and even control of disastrous space weather.As many space projects, such as Cluster, Themis, Double Stars, come intoservice, the observational study of magnetic reconnection grows rapidly in thelast decade. However, note that most of the reconnection reposts are in themagnetopause and magnetotail. Meanwhile, the discovery of magnetic cloudboundary layer and the reconnection X-line exhausts in the solar wind introducesa brand new natural laboratory and a di?erent viewpoint to study reconnection.In this dissertation, we report several reconnection events in the interplanetaryspace by examining measurements from spacecraft at 1 AU.We report two reconnection exhaust events associated with two large-scalecurrent sheets close to the centers of sub ?ux ropes within the interior of acomplex interplanetary corona mass ejection (ICME). During November 26–272000, a complex ICME, composed of four ?ux ropes, was detected by Wind andACE at 1 AU. We identify two Petschek-like exhaust events within the interiorsof the second and third ?ux ropes, respectively. In the first event, Wind andACE detected an exhaust at the same side from the reconnection site, whichwas associated with a large-scale bifurcated current sheet with a spatial widthof～10000 ion inertial lengths and the magnetic shear was 155?. In the second event, the two spacecraft observed the oppositely directed exhausts from a singlereconnection X-line. The exhausts were also related to a large-scale current sheetwith a spatial width of～3000 ion inertial lengths and a shear angle of about 135fl.The two exhaust events resulted from fast and quasi-stationary reconnection. Therelated current sheets were both flat on the scale of a few hundred Earth radii andlocated close to the centers of sub flux ropes. The decrease of radial expansionspeed of each flux rope might account for the formation of the two current sheets.Reconnections at the centers of flux ropes may change the entire topology ofthe flux ropes and may fragment them into smaller ones. Our observations ofreconnection cross the current sheets close to the centers of flux ropes supportthe kinetically distorted flux rope mode suggested by Owens et al. (2006).For the first time to date, we report an ion diflusion region which is asso-ciated with a reconnection exhaust in the solar wind on January 06 2007. Theevent occurred in the solar minimum, when the density of solar wind plasma canbe very low. The extremely low density makes the ion inertial length relativelylarge enough to detect the ion diflusion region. In situ observations show thatthe reconnection exhaust is bounded by tow slow-mode waves, the speed varianceof which is typically less than Alfven speed. Well between the two slow-modewaves is the ion diflusion region, which shows characteristics of Hall magneticfield and density depletion layers of ions together with electrons. The pitch angledistributions of low energy electrons are consistent with the Hall current feature.The spatial width of such diflusion region is 80 ion inertial lengths that is muchlarger than the average scale of ion diflusion regions detected in the magneto-sphere. However, it remains in the range of large scale reconnection predictedby open boundary condition simulations for large systems. The dimensionlessreconnection rate calculated here is 2%, indicating fast reconnection. Althoughsimulation result of Shay et al. (1999) show Petschek-like structure can be pro-duced by reconnection in collisionless plasma, no direct detections of such caseshave been ever reported in the magnetosphere and solar wind. In this disser-tation, the Hall diflusion region associated Petschek-like reconnection exhaustfirstly confirms that reconnection can indeed produce a Petschek-like structurein the collisionless plasma.