SIGNATURES OF MAGNETIC RECONNECTION AT BOUNDARIES OF INTERPLANETARY SMALL-SCALE MAGNETIC FLUX ROPES

Abstract

The interaction between interplanetary small-scale magnetic flux ropes and\nthe magnetic field in the ambient solar wind is an important topic to\nunderstand- ing the evolution of magnetic structures in the heliosphere.\nThrough a survey of 125 previously reported small flux ropes from 1995 to 2005,\nwe find that 44 of them reveal clear signatures of Alfvenic fluctuations, and\nthus classify them into Alfven wave trains rather than flux ropes. Signatures\nof magnetic reconnection, generally including a plasma jet of ~30 km/s within a\nmagnetic field rotational region, are clearly present at boundaries of about\n42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are\noften observed to show a local increase in the proton temperature, density and\nplasma beta. About 66% of the reconnection events at flux rope boundaries are\nassociated with a magnetic field shear angle larger than 90 deg and 73% of them\nreveal a decrease by 20% or more in the magnetic field magnitude, suggesting a\ndominance of anti-parallel reconnec- tion at flux rope boundaries. The\noccurrence rate of magnetic reconnection at flux rope boundaries through the\nyear of 1995 to 2005 is also investigated and we find that it is relatively low\naround solar maximum and much higher when ap- proaching solar minima. The\naverage magnetic field depression and shear angle for reconnection events at\nflux rope boundaries also reveal a similar trend from 1995 to 2005. Our results\ndemonstrate for the first time that boundaries of a substantial fraction of\nsmall-scale flux ropes have properties similar to those of magnetic clouds, in\nthe sense that both of them exhibit signatures of magnetic reconnection. The\nobserved reconnection signatures could be related either to the formation of\nsmall flux ropes, or to the interaction between flux ropes and the\ninterplanetary magnetic fields.\n