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Recent MMS observations have discovered electron‐scale super‐thin current sheets (STCSs) with a partial electron demagnetization, which distinguishes them from the ion‐scale TCSs traditionally observed by the Cluster mission. Our investigation focuses on the dynamics of STCSs and reveals new aspects influencing their stability. We use the earlier proposed 1D collisionless self‐consistent equilibrium STCS model and show that the free parameters of this model, such as the relative part of demagnetized electrons, their flow velocity and the pressure anisotropy of magnetized electron population, can contribute to the development of tearing instability. With the growth of these parameters, the STCS becomes thinner, which leads to the accumulation excess of a free energy. Stabilizing energy decreases due to the increase of a relative part of demagnetized electrons. Thus, demagnetized electrons in STCSs can provide the development of fast and short‐wavelength electron tearing modes. Plain Language Summary: Recent MMS observations in space plasma have discovered extremely thin current sheets having the transverse electron scales. These sheets are different from the thicker current sheets usually observed by the Cluster mission. The electrons in these sheets are partially demagnetized, that is, they are not affected by magnetic fields. The presence of demagnetized electrons can lead to the development of a fast tearing mode with the shorter wavelengths in comparison with corresponding modes observed in the thicker current configurations. Our investigation focuses on the understanding of the dynamics of these super‐thin current sheets and the identification of factors that influence their stability. Key Points: Demagnetized electrons supporting STCSs can initiate the fast electron tearing instability at short wavelengthsFast growth of small‐scale magnetic perturbation results in the formation of magnetic islands, that is, plasmoidsDevelopment of fast tearing mode is followed by strong inductive electric fields [ABSTRACT FROM AUTHOR]

Recent MMS observations confirmed the existence of the quasi‐stationary electron‐scale super‐thin current sheets (STCSs) with a half‐thickness of few electron gyroradii. STCSs contain the electron population consisting of magnetized and demagnetized particles. To study the role of the demagnetized electrons, we developed a self‐consistent combined model of a STCS in which the dynamics of a part of the electron population is described by the quasi‐adiabatic approximation, while the motion of the other part is described by the guiding center approximation. Demagnetized ion population can be described in a frame of quasi‐adiabatic approach. In the presented model we introduced the weight coefficient of the demagnetized electron plasma density χ depending on the magnetic field normal component Bn/B0 and compared the characteristic profiles of two STCSs differing by the presence (χ ≠ 0) and absence (χ = 0) of demagnetized electrons. It is shown that the presence of demagnetized electrons makes the current density profile of STCS significantly more intense and narrower. The thickness and intensity of STCS depend on the anisotropy coefficient of magnetized electrons and the incoming drift velocity of demagnetized electrons. The maximum current density continuously increases with the growth of the demagnetized electron population and is controlled by the ratio of the thermal to the incoming drift electron velocities. Thus intense current sheets can be formed even in the absence of strong electron anisotropy, which is confirmed by MMS observations. Key Points: Demagnetized electrons makes the current sheets more intense and narrower with sharper gradients of magnetic field in the centerAn increase in the anisotropy coefficient of magnetized electrons and the field‐aligned flow of demagnetized electrons can lead to the increase in the intensity and thin‐ning of the electron current sheetIntense super thin current sheets can be formed even in the presence of a moderate electron anisotropy (pǁe ≈ p⊥e), which is confirmed by MMS observations [ABSTRACT FROM AUTHOR]