Does Heavy Ions Reflect At High Mach Number Collisionless Shocks? (Particle Physics)

Researchers found first direct observational evidence for magnetic reflection of heavy ions from high mach number collisionless shocks

Fast moving collisionless shocks in space and astrophysical plasmas are regions of wave-particle interaction, energy transfer, and generation of highly energetic ions. The plasma of interplanetary shocks and planetary bow shocks is dominated by protons. Alpha particles constitute the most abundant minor heavy ion species with typical densities less than five percent of the proton density. Due to their different mass per charge ratio, alpha particles do not experience the same deceleration by the cross-shock electrostatic potential as protons, which leads to a weaker deceleration of alpha particles relative to protons. Since the potential self-adjusts to process the main proton component, it is insufficient to reflect alpha particles.

But, several studies have also reported the presence of alpha particles upstream of Earth’s bow shock. However, we don’t know, how they exactly got there. This makes researchers re-think, what if alpha particles really would have been reflected? There’s one possibility that, they are reflected from high Mach number collisionless shocks. But, we haven’t got any direct observational evidence yet.

Figure 1: A schematic illustration representing the magnetic field structure and alpha particle dynamics across the shock. The stacked magnetic field profiles (black lines) represent variations in the field strength across the shock front. The amplitude of each peak indicates the relative magnetic field strength at different times and locations. The actual magnetic field vector also changes in direction. Two example trajectories of reflected (top) and transmitted (bottom) solar wind alpha particles are also over-plotted to illustrate the ion behaviors. It should be noted that the ion dynamics have a different time dependence than the illustrated magnetic field variations. © Hadi Madanian et al.

Now, Hadi Madanian and colleagues reported on the first direct observational evidence of reflection of alpha particles (or heavier-than proton ions) at collisionless shocks in proton-dominated plasmas using high resolution in-situ measurements from Magnetospheric MultiScale (MMS) spacecraft.

“The intense magnetic amplification at the shock front associated with nonstationarity modulates the trajectory of alpha particles, some of which travel back upstream as they gyrate in the enhanced magnetic field and experience further acceleration in the upstream region.”

They argued that, unlike protons, alpha particles are reflected through gyration in the dynamically intensified magnetic field at the shock front. Since reflected ions seed the diffused ion population, their findings suggested that shocks that exhibit high magnetic amplification (i.e., reforming shocks at high Mach numbers,) are more likely to produce a diverse admixture of heavy ion species and protons in the diffused energetic ion population.

“Our observations have important implications for theoretical and simulation studies of heavy ion acceleration at high Mach number astrophysical shocks, generation of heavy solar energetic particles, and heavy ion interactions with plasma structures inside the solar corona.”

— concluded authors of the study

Figure 2. Evidence of alpha particle reflection. This figure shows ion phase space density (s³/km6) distributions across the shock. Each distribution shows FPI data over a single measurement cycle. Distributions in panels (a – d) are shown in the NIF in a plane containing n’ and the upstream motional electric field e’. The third coordinate axis in this system is into the page, tangent to the shock surface, and along the projection of the upstream magnetic field. Distributions in panels (e – h) are shown in the local plasma frame in a plane perpendicular to the magnetic field. Ion populations of protons (H+) and alpha particles (He^2+) are identified by SW (solar wind), R (reflected), and RR (returning reflected) labels. Red arrows indicate populations of non-gyrotropic alpha particles. © Hadi Madanian et al.

Featured image: The solar wind pushing on Earth’s magnetic field. © NASA


Reference: Hadi Madanian, Steven J. Schwartz, Stephen A. Fuselier, David Burgess, Drew L. Turner, Li-Jen Chen, Mihir I. Desai, Michael J. Starkey, “Direct Evidence for Magnetic Reflection of Heavy Ions from High Mach Number Collisionless Shocks”, Arxiv, pp. 1-15, 2021. https://arxiv.org/abs/2106.10214


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