arXiv subjects

George K. Parks

Publications and source records attributed to George K. Parks.

3 recordsLinked to original sources

arXiv2021

MMS direct observations of kinetic-scale shock self-reformation

Studies of shocks have long suggested that a shock can undergo cyclically self-reformation in a time scale of ion cyclotron period. This process has been proposed as a primary mechanism for energy dissipation and energetic particle acceleration at shocks. Unambiguous observational evidence, however, has remained elusive. Here, we report direct observations for the self-reformation process of a collisionless, high Mach number, quasi-perpendicular shock using MMS measurements. We find that reflected ions by the old shock ramp form a clear phase-space vortex, which gives rise to a new ramp. The new ramp observed by MMS2 has not yet developed to a mature stage during the self-reformation, and is not strong enough to reflect incident ions. Consequently, these ions are only slightly slowed down and show a flat velocity profile from the new ramp all the way to the old one. The present results provide direct evidence for shock self-reformation, and also shed light on energy dissipation and energetic particle acceleration at collisionless shocks throughout the universe.

physics.space-ph↗

arXiv2016

Global explicit particle-in-cell simulations of the nonstationary bow shock and magnetosphere

We carry out two-dimensional global particle-in-cell simulations of the interaction between the solar wind and a dipole field to study the formation of the bow shock and magnetosphere. A self-reforming bow shock ahead of a dipole field is presented by using relatively high temporal-spatial resolutions. We find that (1) the bow shock and the magnetosphere are formed and reach a quasi-stable state after several ion cyclotron periods, and (2) under the Bz southward solar wind condition the bow shock undergoes a self-reformation for low \b{eta}i and high MA. Simultaneously, a magnetic reconnection in the magnetotail is found. For high \b{eta}i and low MA, the shock becomes quasi-stationary, and the magnetotail reconnection disappears. In addition, (3) the magnetopause deflects the magnetosheath plasmas. The sheath particles injected at the quasi-perpendicular region of the bow shock can be convected to downstream of an oblique shock region. A fraction of these sheath particles can leak out from the magnetosheath at the wings of the bow shock. Hence, the downstream situation is more complicated than that for a planar shock produced in local simulations.

astro-ph.EP↗

arXiv2001

BGK Electron Solitary Waves Reexamined

This paper reexamines the physical roles of trapped and passing electrons in electron Bernstein-Greene-Kruskal (BGK) solitary waves, also called the BGK phase space electron holes (EH). It is shown that the charge density variation in the vicinity of the solitary potential is a net balance of the negative charge from trapped electrons and positive charge due to the decrease of the passing electron density. A BGK EH consists of electron density enhancements as well as a density depletion, instead of only the density depletion as previously thought. The shielding of the positive core is not a thermal screening by the ambient plasma, but achieved by trapped electrons oscillating inside the potential energy trough. The total charge of a BGK EH is therefore zero. Two separated EHs do not interact and the concept of negative mass is not needed. These features are independent of the strength of the nonlinearity. BGK EHs do not require thermal screening, and their size is thus not restricted to be greater than the Debye length $λ_D$. Our analysis predicts that BGK EHs smaller than $λ_D$ can exist. A width($δ$)-amplitude($ψ$) relation of an inequality form is obtained for BGK EHs in general. For empty-centered EHs with potential amplitude $\gg 1$, we show that the width-amplitude relation of the form $δ\propto\sqrtψ$ is common to bell-shaped potentials. For $ψ\ll 1$, the width approaches zero faster than $\sqrtψ$.

physics.plasm-ph↗