Small interplanetary magnetic flux ropes(SIMFRs) are often detected by space satellites in the interplanetary space near 1 AU.These ropes can be fitted by a cylindrically symmetric magnetic model. The durations of SIM...Small interplanetary magnetic flux ropes(SIMFRs) are often detected by space satellites in the interplanetary space near 1 AU.These ropes can be fitted by a cylindrically symmetric magnetic model. The durations of SIMFRs are usually <12 h, and the diameters of SIMFRs are <0.20 AU and show power law distribution. Most SIMFRs are observed in the typically slow solar wind(<500 km/s), and only several events are observed with high speed(>700 km/s). Some SIMFRs demonstrate abnormal heavy ion compositions, such as abnormally high He abundance, abnormally high average iron ionization, and enhanced O7+abundance. These SIMFRs originate from remarkably hot coronal origins. Approximately 74.5% SIMFRs exhibit counterstreaming suprathermal electron signatures. Given their flux rope configuration, SIMFRs are potentially more effective for substorms. SIMFRs and magnetic clouds have many similar observational properties but also show some different observations.These similar properties may indicate that SIMFRs are the interplanetary counterparts of small coronal mass ejections. Some direct bodies of evidence have confirmed that several SIMFRs are interplanetary counterparts of CMEs. However, their different properties may imply that some SIMFRs have interplanetary origins. Therefore, one of the main aims of future research on SIMFRs is to determine whether SIMFRs originate from two different sources, that is, some events are formed in the solar coronal atmosphere, whereas others originate from the interplanetary space. Finally, in this paper, we offer some prospects that should be addressed in the future.展开更多
Magnetic field topology frozen in ideal magnetohydrodynamics(MHD)and its breakage in near-ideal MHD are reviewed in two parts,clarifying and expanding basic concepts.The first part gives a physically complete descript...Magnetic field topology frozen in ideal magnetohydrodynamics(MHD)and its breakage in near-ideal MHD are reviewed in two parts,clarifying and expanding basic concepts.The first part gives a physically complete description of the frozen field topology derived from magnetic flux conservation as the fundamental property,treating four conceptually related topics:Eulerian and Lagrangian descriptions of three dimensional(3D)MHD,Chandrasekhar-Kendall and Euler-potential field representations,magnetic helicity,and inviscid vortex dynamics as a fluid system in physical contrast to ideal MHD.A corollary of these developments clarifies the challenge of achieving a high degree of the frozen-in condition in numerical MHD.The second part treats field-topology breakage centered around the Parker Magnetostatic Theorem on a general incompatibility of a continuous magnetic field with the dual demand of force-free equilibrium and an arbitrarily prescribed,3D field topology.Preserving field topology as a global constraint readily results in formation of tangential magnetic discontinuities,or,equivalently,electric current-sheets of zero thickness.A similar incompatibility is present in the steady force-thermal balance of a heated radiating fluid subject to an anisotropic thermal flux conducted strictly along its frozen-in magnetic field in the low-βlimit.In a weakly resistive fluid the thinning of current sheets by these general incompatibilities inevitably results in sheet dissipation,resistive heating and topological changes in the field notwithstanding the small resistivity.Strong Faraday induction drives but also macroscopically limits this mode of energy dissipation,trapping or storing free energy in self-organized ideal-MHD structures.This property of MHD turbulence captured by the Taylor hypothesis is reviewed in relation to the Sun’s corona,calling for a basic quantitative description of the breakdown of flux conservation in the low-resistivity limit.A cylindrical initial-boundary value problem provides specificity in the general MHD ideas presented.展开更多
基金the National Natural Science Foundation of China(Grant Nos.41674170 and 41804162).The authors would like to thank Dr.TIAN Hui and HUANG Jia for helpful discussion.
文摘Small interplanetary magnetic flux ropes(SIMFRs) are often detected by space satellites in the interplanetary space near 1 AU.These ropes can be fitted by a cylindrically symmetric magnetic model. The durations of SIMFRs are usually <12 h, and the diameters of SIMFRs are <0.20 AU and show power law distribution. Most SIMFRs are observed in the typically slow solar wind(<500 km/s), and only several events are observed with high speed(>700 km/s). Some SIMFRs demonstrate abnormal heavy ion compositions, such as abnormally high He abundance, abnormally high average iron ionization, and enhanced O7+abundance. These SIMFRs originate from remarkably hot coronal origins. Approximately 74.5% SIMFRs exhibit counterstreaming suprathermal electron signatures. Given their flux rope configuration, SIMFRs are potentially more effective for substorms. SIMFRs and magnetic clouds have many similar observational properties but also show some different observations.These similar properties may indicate that SIMFRs are the interplanetary counterparts of small coronal mass ejections. Some direct bodies of evidence have confirmed that several SIMFRs are interplanetary counterparts of CMEs. However, their different properties may imply that some SIMFRs have interplanetary origins. Therefore, one of the main aims of future research on SIMFRs is to determine whether SIMFRs originate from two different sources, that is, some events are formed in the solar coronal atmosphere, whereas others originate from the interplanetary space. Finally, in this paper, we offer some prospects that should be addressed in the future.
基金The National Center for Atmospheric Researchis sponsored by the US National Science Foundation
文摘Magnetic field topology frozen in ideal magnetohydrodynamics(MHD)and its breakage in near-ideal MHD are reviewed in two parts,clarifying and expanding basic concepts.The first part gives a physically complete description of the frozen field topology derived from magnetic flux conservation as the fundamental property,treating four conceptually related topics:Eulerian and Lagrangian descriptions of three dimensional(3D)MHD,Chandrasekhar-Kendall and Euler-potential field representations,magnetic helicity,and inviscid vortex dynamics as a fluid system in physical contrast to ideal MHD.A corollary of these developments clarifies the challenge of achieving a high degree of the frozen-in condition in numerical MHD.The second part treats field-topology breakage centered around the Parker Magnetostatic Theorem on a general incompatibility of a continuous magnetic field with the dual demand of force-free equilibrium and an arbitrarily prescribed,3D field topology.Preserving field topology as a global constraint readily results in formation of tangential magnetic discontinuities,or,equivalently,electric current-sheets of zero thickness.A similar incompatibility is present in the steady force-thermal balance of a heated radiating fluid subject to an anisotropic thermal flux conducted strictly along its frozen-in magnetic field in the low-βlimit.In a weakly resistive fluid the thinning of current sheets by these general incompatibilities inevitably results in sheet dissipation,resistive heating and topological changes in the field notwithstanding the small resistivity.Strong Faraday induction drives but also macroscopically limits this mode of energy dissipation,trapping or storing free energy in self-organized ideal-MHD structures.This property of MHD turbulence captured by the Taylor hypothesis is reviewed in relation to the Sun’s corona,calling for a basic quantitative description of the breakdown of flux conservation in the low-resistivity limit.A cylindrical initial-boundary value problem provides specificity in the general MHD ideas presented.
