Direct in-situ Estimates of Energy and Force Balance in Near Earth
Collisionless Plasmas
Abstract
Fundamental processes in plasmas act to convert energies into different
forms, e.g., electromagnetic, kinetic and thermal. Direct derivation
from the Valsov-Maxwell equation yields sets of equations that describe
the temporal evolution of the magnetic, kinetic and internal energies in
either the monofluid or multifluid frameworks. In this work we focus on
the main terms that affect the changes in the kinetic energy. These are
pressure gradient-related terms and electromagnetic terms. The former
account for plasma acceleration or deceleration from a pressure
gradient, while the latter from an electric field. The overall balance
between these terms is fundamental to ensure the conservation of energy
and momentum. We use in-situ observations from the Magnetospheric
MultiScale (MMS) mission to study the relationship between these terms.
We perform a statistical analysis of those parameters in the context of
magnetic reconnection by focusing on small-scale Electron Diffusion
Regions and large-scale Flux Transfer Events. The analysis reveals a
correlation between the two terms in the monofluid force balance, and in
the ion force and energy balance. However, the expected relationship
cannot be verified from electron measurements. Generally, the pressure
gradient related terms are smaller than their electromagnetic
counterparts. We perform an error analysis to quantify the expected
underestimation of gradient values as a function of the spacecraft
separation compared to the gradient scale. Our findings highlight that
MMS is capable of capturing energy and force balance for the ion fluid,
but that care should be taken for energy conversion terms based on
electron pressure gradients.