Kinetic Plasma Dynamics in the Light of Novel in situ Heliospheric Observations: Synergistic View with Theories and Simulations

The young solar wind originating from the Sun consists of electrons and multiple species of ions. Primary ions are protons and alpha particles (He2+) and minor ions are heavier ions (e.g., O6+, Fe12+). Despite their presence and fluctuating abundance in a variety of situations (i.e., following the solar cycle) as well as in solar wind structures (e.g., coronal mass ejections, stream interaction regions, etc.), their role in heating and accelerating the solar wind remains elusive. Unlike at 1 AU, the young solar wind plasmas have distinct properties. For example, alpha particles are preferentially heated and accelerated close to the Sun. Several works during the last decades proposed various kinetic mechanisms for the super heating of heavier ions such as resonant absorption of ion-cyclotron waves, low-frequency Alfven wave turbulence, stochastic heating, reconnection, and drift instabilities. Moreover, each plasma species often has complex distribution functions that are not Maxwellian. For instance, solar wind protons are often observed to consist of core and beam populations in the velocity space. The non-Maxwellian features are the source of free energy that can drive kinetic instabilities and wave-particle interactions, eventually converting electromagnetic energy into heat and accelerating the solar wind. Despite these striking features of the solar wind especially close to the Sun, the studies of the role of nonthermal populations of solar wind plasmas on energy transfer and dissipation remain limited.

Apart from Helios and other missions that sampled plasmas close to the Sun in the 80’s - 90’s, space missions capable of distinguishing fine features of the primary ion species and electrons close to the source region of the solar wind have been available only recently, thanks to NASA’s Parker Solar Probe and ESA’s Solar Orbiter missions. Parker Solar Probe and Solar Orbiter have now entered the nominal phase; they are currently offering unprecedented opportunities where the physics at work can be probed in situ near the Sun. To understand the dynamical mechanisms at play, theoretical development and numerical modeling are critical.

Our Research Topic encourages research articles and/or review papers on pioneering observations, theories, and models that contributes to the understanding of kinetic plasma dynamics responsible for the heating and acceleration of the solar wind plasmas. We welcome a wide-range of topics including plasma kinetics, turbulence characterization, wave-particle interactions, non-Maxwellian features of velocity distributions and their evolution, temperature anisotropy, magnetic reconnection, and generation and propagation of the switchbacks.

Synergistic studies using theories, models, observations in the heliospheric environments made by Parker Solar Probe, Solar Orbiter, Magnetospheric Multiscale, as well as other missions are particularly welcome.

We welcome the following article types: Brief Research Report, Original Research, Hypothesis & Theory, Methods, Data Report, Mini Review and Review, General Commentary, Opinion, and Perspective.