The Nonrelaxation of Magnetic Field Lines in Solar Wind Magnetic Reconnection Exhausts

Abstract We present a case study using data from multiple spacecraft and statistics from the ACE spacecraft to investigate the relaxation of reconnected magnetic field lines in solar wind magnetic reconnection exhausts, which constitute one of the two outflow regions resulting from magnetic reconnection, and which are often characterized by a plasma jet, magnetic strength depletion, and density and temperature enhancements. The normal magnetic fractions (∣B n /B∣) of the reconnection exhausts are used to indicate the degrees of relaxation of the magnetic field lines. 97 out of 98 reconnection exhausts have a relatively small ∣B n /B∣ (<0.3), while only one exhaust associated with a relatively small interplanetary coronal mass ejection has a large ∣B n /B∣ of 0.85. This result demonstrates the nonrelaxed nature of reconnected magnetic field lines in solar wind, due to the open boundary condition. As a consequence, the magnetic tension of such nonrelaxed magnetic field lines can accelerate the local plasma to produce the observed jet within the reconnection exhaust. Our results support the understanding that most solar wind reconnection exhausts are probably initiated near the Sun, where the plasma beta is low, and that they are not ongoing when they are observed at 1 au, because of the solar wind expansion from near the Sun to 1 au.