Low-frequency Whistler Waves Modulate Electrons and Generate Higher-frequency Whistler Waves in the Solar Wind

The role of whistler-mode waves in the solar wind and the relationship between their electromagnetic fields and charged particles is a fundamental question in space physics. Using high-temporal-resolution electromagnetic field and plasma data from the Magnetospheric MultiScale spacecraft, we report observations of low-frequency whistler waves and associated electromagnetic fields and particle behavior in the Earth’s foreshock. The frequency of these whistler waves is close to half the lower-hybrid frequency (∼2 Hz), with their wavelength close to the ion gyroradius. The electron bulk flows are strongly modulated by these waves, with a modulation amplitude comparable to the solar wind velocity. At such a spatial scale, the electron flows are forcibly separated from the ion flows by the waves, resulting in strong electric currents and anisotropic ion distributions. Furthermore, we find that the low-frequency whistler wave propagates obliquely to the background magnetic field ( B 0), and results in spatially periodic magnetic gradients in the direction parallel to B 0. Under such conditions, large pitch-angle electrons are trapped in wave magnetic valleys by the magnetic mirror force, and may provide free perpendicular electron energy to excite higher-frequency whistler waves. This study offers important clues and new insights into wave–particle interactions, wave generation, and microscale energy conversion processes in the solar wind.

Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters

Transport equations for electron thermal energy in the high-β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te /β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L c > L/β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.

Controlling the type and intensity of low-frequency waves generated by laser plasma clots in a force tube of magnetized plasma

In this work, we study the influence of the parameters of a magnetized background plasma on the intensity of whistler waves generated by periodic laser plasma bunches in a magnetic field tube. It is shown that at 0.3 < Lpi > 0.4 Alfvén waves and whistlers are generated. In the region Lpi> 0.5, intense whistlers with an amplitude of δBmax / B0 ∼ 0.24 are generated.

Interaction of rotational discontinuities with energetic ions in the precursor of the Earth’s bow shock

We combined in-situ solar wind observations by ARTEMIS and MMS missions with kinetic hybrid simulations to study the interaction of solar wind rotational discontinuities (RDs) with the foreshock of the Earth’s bow shock. We found that whistler modes excited by diffuse energetic particles were strongly coupled with RDs and lead to their temporary dissociation. At the same time, RDs trigger the steepening of whistler waves and the generation of ’shocklets’ - small localised shock-like structures, capable of trapping energetic particles and growing up by absorbing the particles energy.