Abstract
ABSTRACTIt is shown that ions can be accelerated to about 100 keV in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves. The acceleration occurs in discrete steps of duration being a small fraction of the gyroperiod and can explain observations of ion energization to 10 keV at quasi-perpendicular shocks and to hundreds keV at quasi-parallel shocks. A general expression is provided for the maximum energy of ions accelerated in shocks of arbitrary configuration. The waves involved in the acceleration are related to three cross-field current-driven instabilities: the lower hybrid drift (LHD) instability induced by the density gradients in shocks and shocklets, followed by the modified two-stream (MTS) and electron cyclotron drift (ECD) instabilities, induced by the ExB drift of electrons in the strong LHD wave electric field. The ExB wave mechanism accelerates heavy ions to energies proportional to the atomic mass number, which is consistent with satellite observations upstream of the bow shock and also with observations of post-shocks in supernovae remnants. The results are compared with other acceleration mechanisms traditionally discussed in the literature.
Highlights
It is shown that ions can be accelerated to about 100 keV in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves
The heating process is associated with the appearance of energetic particles at energies keV, which implies significant acceleration of a suprathermal population of the solar wind ions
On the other hand, when the interplanetary magnetic field is in the quasi-parallel direction to the shock normal, an extended upstream foreshock region (Greenstadt et al 1995; Eastwood et al 2005) is formed, containing ULF waves, turbulence, non-linear structures and field-aligned beams
Summary
When the solar wind plasma streaming with a speed of 400 and containing protons with kinetic energy of 1 keV and the thermal spread of 20 eV interacts with the Earth’s quasi-perpendicular bow shock, the ion temperature increases by a factor of 10 across the shock, while the plasma flow slows down during the compression of the solar wind plasma and magnetic field. A local process that does not require moving magnetic mirrors, or electrostatic field barriers, would be more suitable to explain ion acceleration at quasi-parallel shocks It has been recently shown (Stasiewicz 2020; Stasiewicz & Eliasson 2020a,b; Stasiewicz et al 2021) that particle heating and acceleration in collisionless shocks of arbitrary orientation are related to the wave electric fields of drift instabilities triggered by shock compression of the plasma. Ckr ≈ 15, while for u⊥0 & ckr , there is significant heating only for thermal velocity comparable to the wave phase velocity, or u⊥0 ∼ Ω in the normalized variables (diagonal green line) leading to a distribution function having a high energy tail of particles. While the bulk heating is done stochastically for all particles satisfying (9), the perpendicular acceleration to high velocities along the acceleration lane (10) is selective and requires some speed and phase matching
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