Secondary Energization in Compressing Plasmoids during Magnetic Reconnection

TL;DR

This paper reveals how plasmoids in relativistic magnetic reconnection can significantly energize particles internally, producing a non-thermal tail in the energy spectrum, with implications for astrophysical phenomena like blazar jets.

Contribution

It introduces a secondary energization mechanism inside compressing plasmoids during relativistic reconnection, demonstrating the formation of a high-energy tail in particle spectra.

Findings

Plasmoids grow and compress, amplifying magnetic fields.

Particles gain energy via magnetic moment conservation, forming a power-law tail.

Cutoff energy increases as the square root of time, exceeding initial magnetization energy.

Abstract

Plasmoids -- magnetized quasi-circular structures formed self-consistently in reconnecting current sheets -- were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization $\sigma_{\rm up} \gg 1$). Using two dimensional particle-in-cell simulations in pair plasmas with $\sigma_{\rm up}=10$ and $100$, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal tail $f(E)\propto E^{-3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{\rm cut}\propto\sqrt{t}$, can greatly exceed $\sigma_{\rm up} m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.