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.
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
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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