• magnetic reconnection
• magnetopause
• Ohm's law
• space plasmas

Magnetic reconnection is a fundamental process by which two plasmas with different magnetic fields interact, changing the magnetic field topology, and transferring energy from the magnetic field to particles, resulting in electron and ion acceleration and heating.
It is a multi-scale process: the small spatial and temporal scales are strongly coupled with the large scales. Although magnetic reconnection can be individuated in a bunch of different situation in space plasmas, we are interested in reconnection processes driven by the interaction between the solar wind and the magnetosphere.
This work focuses on asymmetric reconnection - in
which the two reconnection plasmas have different features - produced at Earth's magnetopause.
In this study, we propose a study of magnetic reconnection focused on the microphysics of the process, which is still poorly understood. In particular, we investigate the relative contributions of various terms in the generalized Ohm's law by means of \textit{in situ} measurements by the Magnetospheric Multiscale (MMS) mission. MMS is a NASA four-spacecraft constellation mission launched in 2015 with the aim to conduct a definitive experiment to determine the causes of collisionless magnetic reconnection.
The evaluation of Ohm's law has always been a challenge because of the lack of high resolution data preventing the achievement of a proper computation of the electron scale terms of Ohm's law. Indeed, the computation of the pressure and the inertia term using in situ measurements has never been achieved with data from previous missions.
We consider the event of October, 3 2015 in which several consecutive magnetopause crossings occurred. Around 1500 UTC the MMS satellites were located in the afternoon side of magnetosphere and separated by a distance of about 20 km. The analysis focuses in particular on two consecutive complete crossings: during the first crossing signatures of reconnection are observed while these are absent in the second one. We report an encounter with an ion diffusion region. Then, we compute of all the terms of the generalised Ohm's law. Since the spacecraft did not cross the electron diffusion region, we expect the inertia and the pressure terms to be negligible compared to the others. This is what is found at the end of the analysis: the reconnection electric field is mainly sustained by the Hall term in presence of reconnection; ideal Ohm's law holds when reconnection stops.