The Formation and Structure of Flux Ropes in the Magnetotail

Abstract

Flux ropes are a common signature in spacecraft observations in the plasma sheet of the nightside magnetosphere. The probability of observing the signature of a helical magnetic structure appears to increase with distance down the magnetotail, suggesting that there is a gradual transition with downtail distance from ordinary bubble-like plasmoids to flux ropes with significant magnetic fields at the core. On the other hand, a few very small flux ropes have been observed relatively close to the Earth. Thus observations of the distribution of flux ropes with distance may be biased by increases of the scale size as these structures propagate down the tail. Despite their obvious importance the formation process is not fully understood. A review of the present understanding of the formation, structure and evolution of magnetospheric flux ropes in the magnetotail is presented. The review includes a summary of flux rope observations by different spacecraft. Next, possible scenarios of flux rope formation from more bubble-like plasmoids are discussed. Analytical theory of flux rope structure is used to compare to observations. The review includes an outlook and analysis of flux rope importance in different plasma systems.