Title: On the estimation of the current density in space plasmas: multi versus single-point techniques,
Abstract: Thanks to multi-spacecraft mission, it has recently been possible to directly
estimate the current density in space plasmas, by using magnetic field time
series from four satellites flying in a quasi perfect tetrahedron
configuration. The technique developed, commonly called 'curlometer' permits a
good estimation of the current density when the magnetic field time series vary
linearly in space. This approximation is generally valid for small spacecraft
separation. The recent space missions Cluster and Magnetospheric Multiscale
(MMS) have provided high resolution measurements with inter-spacecraft
separation up to 100 km and 10 km, respectively. The former scale corresponds
to the proton gyroradius/ion skin depth in 'typical' solar wind conditions,
while the latter to sub-proton scale. However, some works have highlighted an
underestimation of the current density via the curlometer technique with
respect to the current computed directly from the velocity distribution
functions, measured at sub-proton scales resolution with MMS. In this paper we
explore the limit of the curlometer technique studying synthetic data sets
associated to a cluster of four artificial satellites allowed to fly in a
static turbulent field, spanning a wide range of relative separation. This
study tries to address the relative importance of measuring plasma moments at
very high resolution from a single spacecraft with respect to the
multi-spacecraft missions in the current density evaluation.