Two-dimensional velocity of the magnetic structure observed on 11 July
2017 by the Magnetospheric Multiscale spacecraft
Abstract
In order to determine particle velocities and electric field in the
frame of the magnetic structure, one first needs to determine the
velocity of the magnetic structure in the frame of the spacecraft
observations. Here, we show how to determine a two dimensional magnetic
structure velocity for the magnetic reconnection event observed in the
magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on 11 July
2017. We use two different multi-spacecraft methods, Spatio-Temporal
Difference (STD) and the recently developed polynomial reconstruction
method. Both of these methods use the magnetic field measurements, and
the reconstruction technique also uses the current density measured by
the particle instrument. We find rough agreement between the results of
our methods and with other velocity determinations previously published.
We also explain a number of features of STD and show that the polynomial
reconstruction technique is only likely to be valid within a distance of
two spacecraft spacings from the centroid of the MMS spacecraft. Both of
these methods are susceptible to contamination by magnetometer
calibration errors.