Hypothesis of a bi-isotropic-like plasma permeating the interstellar space

TL;DR

The paper investigates EM wave propagation in a magnetized bi-isotropic-like chiral plasma as a model for the interstellar medium. It develops a cold, uniform plasma description with bi-isotropic constitutive relations, deriving modified refractive indices for the RCP and LCP waves and identifying helicon-like low-frequency modes. The work analyzes optical activity through rotatory power and dichroism, highlighting unusual features such as double rotatory-power reversals induced by the chiral parameter $\tilde{\xi}_{c}$. By linking the theory to pulsar dispersion measures (DM) and rotation measures (RM), it derives upper bounds on $\tilde{\xi}_{c}$, with RM-based limits around $10^{-22}$, demonstrating strong constraint power on the chiral coupling. The results provide a testable framework for ISM bi-isotropic chiral plasmas and inform future low-frequency polarization observations.

Abstract

In this work, we study the propagation of electromagnetic waves in a magnetized chiral plasma that pervades the interstellar space. The Maxwell equations, supplemented by bi-isotropic-like constitutive relations, are rewritten to describe a cold, uniform, and collisionless plasma model that yields new collective electromagnetic modes for distinct pairs of refractive indices associated with right- and left-handed circularly polarized waves. We have investigated the optical behavior through the rotatory power (RP) and dichroism coefficient, reporting that the finite chiral parameter induces double RP sign reversal, an exotic optical signature that takes place in chiral dielectrics and rotating plasmas. In the low-frequency regime, a modified propagating helicon with right-handed circular polarization is obtained. Next, supposing that the interstellar medium behaves as a chiral bi-isotropic-like cold plasma, we employ Astrophysical data of radio pulsars to achieve upper limits on the magnetoelectric parameters magnitude. In particular, by using dispersion measure and rotation measure data from five pulsars, we constrain the magnitude of the chiral parameter to the order of $10^{-16}$ and $10^{-22}$, respectively.

Figures (8)

• Figure 1: Frequency dependence of the index of refraction $n_{R_{+}}$ for $\omega_{p}=\omega_{c}$. The solid (dashed) red line corresponds to the real (imaginary) piece of $n_{R_{+}}$. The same pattern holds for index ${n}_{-}$ of Eq. (\ref{['nusual2']}), shown here for comparison. Here, $\tilde{\xi_{c}}=0.8$ and $\omega_{p}=1$$\mathrm{rad}$$s^{-1}$.
• Figure 2: Frequency dependence behavior of $n_{R_{-}}$ for $\omega_{p}=\omega_{c}$. The dashed red (black) line corresponds to the imaginary piece of $n_{R_{-}}$ ($-{n}_{-}$), while the solid red (black) line represents the real piece of $n_{R_{-}}$ ($-{n}_{-}$). The index ${n}_-$ is given in Eq. (\ref{['nusual2']}). Here, $\tilde{\xi}_{c}=0.8$ and $\omega_{p}=1$$\mathrm{rad}$$s^{-1}$.
• Figure 3: Index of refraction $n_{L_{+}}$ for $\omega_{p}=\omega_{c}$. The dashed red (black) line corresponds to the imaginary piece of $n_{L_{+}}$ ($\tilde{n}_{+}$), while the solid red (black) line represents the real piece of $n_{L_{+}}$ ($\tilde{n}_{+}$), where the index $\tilde{n}_+$ is given in Eq. (\ref{['nusual2']}). Here, $\tilde{\xi}_{c}=0.8$ and $\omega_{p}=1$$\mathrm{rad}$$s^{-1}$.
• Figure 4: Index of refraction $n_{L_{-}}$ for $\omega_{p}=\omega_{c}$. The dashed red (black) line corresponds to the imaginary piece of $n_{L_{-}}$ ($\tilde{n}_{+}$), while the solid red (black) line represents the real piece of $n_{L_{-}}$ ($\tilde{n}_{+}$). The index $\tilde{n}_+$ is given in Eq. (\ref{['nusual2']}). Here, $\tilde{\xi_{c}}=0.8$ and $\omega_{p}=1$$\mathrm{rad}$$s^{-1}$.
• Figure 5: The rotatory power (\ref{['RP_RL+_bi']}) for the interval $0<\omega<\omega_{c}$. It is illustrated in dot-dashed blue line ($\tilde{\xi_{c}}=0.6$) and solid red line ($\tilde{\xi_{c}}=1.0$). The standard plasma RP ($\tilde{\xi_{c}}=0$) is represented by the dashed black line. Here, we have used $\omega_{p}=1$$\mathrm{rad}$$s^{-1}$.