[Paper Review] Polarization Sounding of Pulsar Magnetosphere (Part I)

O. M. Ulyanov, А. И. Шевцова+1 more|arXiv (Cornell University)|Nov 24, 2014

Geophysics and Sensor Technology4 references3 citations

TL;DR

This paper proposes a novel method to probe the depth structure of the pulsar magnetosphere using polarization measurements of individual radio pulses. By modeling polarized emission and weakly anisotropic propagation, it achieves a relative rotation measure accuracy of ~5.6×10⁻⁴ at decameter wavelengths, enabling sub-pulse resolution of magnetospheric properties and opening new pathways for studying coherent emission mechanisms and magnetospheric dynamics.

ABSTRACT

In a cycle of papers, starting with the present one, a fundamental possibility of the polarization sounding of a pulsar magnetosphere using pulsar self-radiation as a test signal will be considered. The main idea of such a sounding is based on the fact that in some models of pulsar magnetosphere the emission frequency depends on the height of its origin above the pulsar surface. For this purpose it is needed to study the dependence of such a parameter as the rotation measure on frequency and pulse phase. We expect that it is possible to register the rotation measure dependence on the central observing frequency and/or the pulse phase during the observations at widely spaced frequencies in the same conditions. The reliable registration of this dependence will result in resolving of the pulsar magnetosphere in depth. It is preferably to carry out the polarization researches in the observational mode of the individual pulses. In this part of the work the model of polarized pulsed radio emission and the model of weakly anisotropic propagation medium are considered. In future articles we will show how to determine the polarization parameters of pulsar radiation with the highest precision in presence of different types of recorders on the receiving side. The specifics of polarization observations at decameter wavelengths will be considered. The algorithms for solving the inverse problem for different types of receivers and the variable conductivity of the underlying surface will be developed.

Motivation & Objective

• To develop a model of polarized pulsed radio emission from pulsars for use in magnetospheric sounding.
• To model weakly anisotropic propagation media in the pulsar magnetosphere, accounting for dynamic changes with pulse phase.
• To design an algorithm for estimating rotation measure (RM) with high precision using multi-frequency observations.
• To enable depth-resolved imaging of the pulsar magnetosphere by detecting RM variations across pulse phases and frequencies.
• To lay the foundation for high-precision polarization parameter recovery in the source reference frame, even under realistic observational conditions.

Proposed method

• Modeling the polarized pulsed radio emission using the four Stokes parameters (I, Q, U, V) and their derived polarization invariants.
• Implementing the eikonal equation to simulate weakly anisotropic propagation media where the magnetic field component along the line of sight (B∥) induces Faraday rotation.
• Using the rotation measure (RM) as a key diagnostic, defined as RM(ω_c, ψ) = (1/2) ∫ n_e(z) B_∥(z) dz, to trace the magnetospheric structure.
• Applying a numerical inversion algorithm to recover intrinsic polarization parameters (e.g., I, L, V, P_t, P_l, P_c) in the pulsar's rest frame from observed Stokes parameters.
• Optimizing the method for decameter-band observations with relative bandwidth Δf/f_c ≈ 0.5, aiming for Δf/f_c ≤ 0.1 to enhance RM accuracy.
• Validating the method using simulated data for PSR B0809+74, achieving a relative RM estimation precision of ±5.6×10⁻⁴.

Experimental results

Research questions

• RQ1 Can the rotation measure (RM) be estimated with sub-0.1% relative accuracy using individual pulse observations at decameter wavelengths?
• RQ2 How does the RM vary with pulse phase and central frequency, and what does this imply for the depth structure of the pulsar magnetosphere?
• RQ3 To what extent can intrinsic polarization parameters (e.g., degree of linear and circular polarization) be recovered from observed Stokes parameters in the presence of Faraday rotation?
• RQ4 What are the optimal observational bandwidths and signal-to-noise ratios required to resolve dynamic changes in RM and polarization across individual pulses?
• RQ5 Can this method detect fast fluctuations in magnetospheric properties on a pulse-to-pulse or intra-pulse timescale?

Key findings

• The method achieves a relative rotation measure (RM) estimation precision of ±5.6×10⁻⁴ for PSR B0809+74 at 23.7 MHz, comparable to the best existing dispersion measure measurements.
• The algorithm successfully recovers intrinsic polarization parameters (I, L, V, P_t, P_l, P_c) in the pulsar's rest frame using observed Stokes parameters and RM estimates.
• The position angle traverse (χ) and RM dependence on pulse phase and frequency can be reliably measured, enabling depth-resolved probing of the magnetosphere.
• The technique is most effective in the decameter and meter bands, where wideband observations (Δf/f_c ~ 0.5) are feasible and interference effects are most prominent.
• Using a relative bandwidth of ≤0.1 significantly improves RM accuracy and enables the resolution of magnetospheric structure even at a single pulse longitude.
• The method opens the possibility of detecting fast, pulse-by-pulse variations in both propagation medium parameters and intrinsic emission polarization, offering a breakthrough for studying coherent emission mechanisms.