Electron energization in near-Earth space : Studies of kinetic scales using multi-spacecraft data

Sammanfattning: Plasma, a gas of charged particles exhibiting collective behavior, is everywhere in the Universe. The heating of plasma to millions of degrees and acceleration of charged particles to very high energies has been observed in many astrophysical environments. How and where the heating and acceleration occur is in many cases unclear. In most astrophysical environments, plasma consists of negative electrons and positive ions. In this thesis we focus on understanding the heating and acceleration of electrons. Several plasma processes have been proposed to explain the observed acceleration. However, the exact heating and acceleration mechanisms involved and their importance are still unclear. This thesis contributes toward a better understanding of this topic by using observations from two multi-spacecraft missions, Cluster and the Magnetospheric MultiScale (MMS), in near-Earth space.In Article I we look at magnetic nulls, regions of vanishing magnetic field B believed to be important in particle acceleration, in the Earth's nightside magnetosphere. We find that nulls are common at the nightside magnetosphere and that the characterization of the B geometry around a null can be affected by localized B fluctuations. We develop and present a method for determining the effect of the B fluctuation on the null's characterization.In Article II we look at a thin (a few km) current sheet (CS) in the turbulent magnetosheath. Observations suggest local electron heating and beam formation parallel to B inside the CS. The electron observations fits well with the theory of electron acceleration across a shock due to a potential difference. However, in our case the electron beams are formed locally inside the magnetosheath that is contrary to current belief that the beam formation only occurs at the shock.In Article III we present observations of electron energization inside a very thin (thinner than Article II) reconnecting CS located in the turbulent magnetosheath. Currently, very little is know about electron acceleration mechanisms at these small scales. MMS observe local electron heating and acceleration parallel to B when crossing the CS. We show that the energized electrons correspond to acceleration due to a quasi-static potential difference rather than electrostatic waves. This energization is similar to what has been observed inside ion diffusion regions at the magnetopause and magnetotail. Thus, despite the different plasma conditions a similar energization occurs in all these plasma regions.In Article IV we study electron acceleration by Fermi acceleration, betatron acceleration, and acceleration due to parallel electric fields inside tailward plasma jets formed due to reconnection, the so called tailward outflow region. We show that most observations are consistent with local electron heating and acceleration from a simplified two dimensional picture of Fermi acceleration and betatron acceleration in an outflow region. We find that Fermi acceleration is the dominant electron acceleration mechanism.