Exploring Galactic plasma with pulsars in the SKA era

TL;DR

The paper surveys how pulsars act as natural probes of ionised plasma in the Galaxy, the Solar Wind, and the ionosphere, highlighting dispersion, scattering, scintillation, and HI absorption as key observables. It synthesises current techniques and findings, and then articulates concrete SKA-era predictions across SKA-Low and SKA-Mid configurations, including unprecedented DM precision ($10^{-8}$ to $10^{-6}$ pc cm$^{-3}$), ubiquitous scintillation arcs, expanded samples for AU-scale HI studies, and refined models of the Galactic electron density and IISM turbulence. The work emphasises the practical impact on Pulsar Timing Arrays, fast radio burst science, and solar-terrestrial physics, while outlining challenges such as ionospheric Faraday rotation removal, multi-screen scattering geometries, and inverse problems in 3D solar-wind reconstruction. Overall, the SKA is positioned as a game-changer for plasma astrophysics, enabling precise, wide-band, and multi-probe measurements that will transform our understanding of Galactic plasmas and their influence on high-precision timing experiments.

Abstract

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures between the neutron star and the observer, as well as aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy, as well as the impact on other experiments involving pulsars, such as Pulsar Timing Arrays. In this article, we review the state-of-the-art in plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements enabled by the SKA.

Figures (6)

• Figure 1: Left: Secondary spectra for PSR J0437$-$4715 from long ($>10$ hour) observations with Murriyang, the 64-m Parkes radiotelescope Reardon+20. Right: Secondary spectra for PSR J0437$-$4715 from long ($>10$ hour) observations with MeerKAT radiotelescope Reardon+25.
• Figure 2: Distribution of dispersion measures for a simulated pulsar population observed with SKA-Mid configurations AA* (blue histogram) and AA4 (orange histogram). Vertical lines show the estimated maximum DM for which scintillation is resolved at frequencies corresponding to the centre frequencies of Band 2 (1355 MHz; red) and Band 5a (6550 MHz; blue). We have assumed three possible observing modes, with 1024 frequency channels (dotted), 4096 channels (dot dash), and 16384 channels (dashed) across the bands.
• Figure 3: Distribution of known pulsars projected onto the Galactic plane, in galactocentric Cartesian coordinates. Left: Positions of all known radio pulsars, based on YMW16 distance estimates, with discoveries from four representative, major pulsar surveys highlighted: the Parkes multi-beam survey (teal), the Arecibo PALFA survey (light blue), the FAST Galactic Plane Pulsar survey (GPPS; dark blue), and the Green Bank North Celestial Cap survey (GBNCC; orange). Right: Pulsars with precise ($<25\%$ fractional uncertainty) distance measurements, based either on parallax or globular cluster associations. In both panels the Earth and Galactic centre are indicated by the open circle and cross, respectively; spiral arm models from NE2001 and rmb19 are shown by the black and light blue shaded curves. Sky regions inaccessible to SKA are shown in grey.
• Figure 4: Comparison between observed DM and scattering distributions (teal points) and Galactic electron density model predictions (black curves) vs. Galactic longitude and for all available measurements at Galactic latitudes $|b|<10^\circ$. Left: DM vs. $l$ for known radio pulsars in the ATNF catalogue (teal), compared to the maximum DM predicted by NE2001 and YMW16 for sightlines integrated through the entire Galaxy at $b = 0^\circ$. Right: Scattering time ($\tau$) vs. $l$, based on the scattering time database compiled by coc22, compared to the maximum $\tau$ for NE2001 and YMW16 at $b=0^\circ$. Scattering times are scaled from 1 GHz to 200 MHz assuming $\tau \propto \nu^{-4}$.
• Figure 5: Effect of inter-channel depolarisation. The effect of depolarisation is the most severe when there is less than one observing point per eighth of the oscillation period of Stokes Q/U. The plot shows this critical depolarisation frequency as a function of RM. Different lines demonstrate the magnitude of the effect for three channel widths. The horizontal puncture line shows the lowest frequency edge of the SKA Low.