Rotation Measure Substructures Induced by the Ponderomotive Force of Inertial \alfven Waves

TL;DR

This paper proposes a physical mechanism involving inertial Alfvén waves and the ponderomotive force to explain short-term rotation measure substructures observed in repeating fast radio bursts.

Contribution

It introduces a model where wave-driven density cavitation causes RM fluctuations, linking plasma dynamics to observed RM variability in FRBs.

Findings

Inertial Alfvén waves induce nonlinear density perturbations.

Density redistribution can produce RM suppression matching observations.

The model explains short-term RM variability via wave-driven plasma cavitation.

Abstract

The rotation measure (RM) and dispersion measure (DM) of fast radio bursts (FRBs) serve as critical probes of the magneto-ionic environments along the line of sight. The significant temporal evolution of RM observed in some repeating FRBs is generally attributed to the local environment of the source, since the intergalactic medium is not expected to vary on such short timescales. Recent observations of repeating FRB 20201124A and FRB 20220529 exhibit complex RM phenomenology, including large-amplitude global fluctuations and short-term substructures. Here, we attribute these short-term RM variations to the ponderomotive force exerted by inertial \alfven~waves (IAWs). We propose that IAWs, generated via magnetic reconnection or turbulent cascades in a low-$\beta$ plasma, induce nonlinear density perturbations in the source environment. We demonstrate that the resulting plasma density redistribution can produce RM suppression consistent with observed substructures. This model presents a physically motivated mechanism for the short-term RM variability observed in active repeaters. It demonstrates that such fluctuations can arise from wave-driven density cavitation within a broad, coupled parameter space involving wave amplitude, plasma density, and temperature, thereby characterizing the localized plasma dynamics required to produce the observed RM jitters.