How can we measure a magnetic field across the whole Galaxy?

Astronomers infer it indirectly: polarized starlight aligned by magnetic grains, the rotation of polarized radio signals passing through magnetized plasma, and synchrotron radiation from electrons spiraling in the field all reveal its strength and direction.

Where do cosmic rays come from?

Most galactic cosmic rays are believed to be accelerated in the shock waves of supernova remnants and then to wander through the Galaxy, guided by its magnetic field, for millions of years before escaping or interacting.

Galactic Magnetic Fields and Cosmic Rays

Magnetic fields thread the Galaxy and confine cosmic rays, energetic charged particles whose pressure rivals that of the interstellar gas and shapes its dynamics.

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Definition

Galactic magnetic fields are the large-scale and turbulent magnetic fields permeating the interstellar medium, and cosmic rays are the relativistic charged particles, mostly protons and nuclei, that are confined by these fields and contribute significantly to the energy budget of the Galaxy.

Scope

This topic covers the strength and large-scale structure of the galactic magnetic field, the methods used to measure it such as Faraday rotation and polarization, the origin and propagation of cosmic rays, synchrotron emission as a tracer of fields and particles, and the rough energy equipartition among gas, fields, and cosmic rays.

Core questions

How strong is the galactic magnetic field, and what is its large-scale structure?
How are interstellar magnetic fields measured?
Where do cosmic rays come from, and how do they propagate through the Galaxy?
How do fields and cosmic rays compare in energy with the interstellar gas?

Key theories

Large-scale and turbulent fields: The galactic magnetic field has an ordered component aligned with the spiral structure plus a comparable turbulent component, mapped through Faraday rotation, polarization, and synchrotron emission.
Cosmic-ray origin and propagation: Cosmic rays are thought to be accelerated mainly in supernova remnant shocks and then diffuse through the galactic magnetic field, their composition and spectrum constraining propagation models.
Energy equipartition: The energy densities of the interstellar gas, magnetic fields, and cosmic rays are comparable, so magnetic and cosmic-ray pressure are dynamically important for the structure of the medium.

Clinical relevance

Magnetic fields and cosmic rays influence the support and dynamics of the interstellar medium, regulate star formation by providing pressure, drive galactic winds, and produce the synchrotron radiation by which galaxies are studied at radio wavelengths.

History

The detection of galactic synchrotron radio emission and the polarization of starlight in the mid-twentieth century revealed interstellar magnetic fields, while Fermi proposed mechanisms for accelerating cosmic rays. Faraday rotation surveys and space-based particle detectors have since mapped the fields and measured the cosmic-ray spectrum in detail.

Key figures

Rainer Beck
Enrico Fermi
Andrew Strong
Katia Ferriere

Seminal works

beck2001
strong2007
ferriere2001

Frequently asked questions

How can we measure a magnetic field across the whole Galaxy?: Astronomers infer it indirectly: polarized starlight aligned by magnetic grains, the rotation of polarized radio signals passing through magnetized plasma, and synchrotron radiation from electrons spiraling in the field all reveal its strength and direction.
Where do cosmic rays come from?: Most galactic cosmic rays are believed to be accelerated in the shock waves of supernova remnants and then to wander through the Galaxy, guided by its magnetic field, for millions of years before escaping or interacting.