Characterization of Magnetic Flux Contents for Flux Transfer Events and
its Implications for Flux Rope Formation at the Earth's Magnetopause
Abstract
Flux transfer events (FTEs) are a type of magnetospheric phenomena that
exhibit distinctive observational signatures from the in-situ spacecraft
measurements across the Earth’s magnetopause. They are generally
believed to possess a magnetic field configuration of a magnetic flux
rope and formed through magnetic reconnection at the dayside
magnetopause, sometimes accompanied with enhanced plasma convection in
the ionosphere. We examine two FTE events under the condition of
southward interplanetary magnetic field (IMF) with a dawn-dusk component
at the magnetopause by applying the Grad-Shafranov (GS) reconstruction
method to the in-situ measurements by the Magnetospheric Multiscale
(MMS) spacecraft to derive the magnetic flux contents associated with
the FTE flux ropes. In particular, given a cylindrical magnetic flux
rope configuration derived from the GS reconstruction, the magnetic flux
content can be characterized by both the toroidal (axial) and poloidal
fluxes. We then estimate the amount of magnetic flux (i.e., the
reconnection flux) encompassed by the area “opened” in the ionosphere,
based on the ground-based Super Dual Auroral Radar Network (SuperDARN)
observations. We find that for event 1, the FTE flux rope is oriented in
the approximate dawn-dusk direction, and the amount of its poloidal
magnetic flux agrees with the corresponding reconnection flux. For event
2, the agreement among the estimates of the magnetic fluxes is
uncertain. We provide a detailed description about our interpretation
for the topological features of the FTE flux ropes, based on a formation
scenario of sequential magnetic field reconnection between adjacent
field lines, consistent with our results.