Astrophysical Magnetism

Magnetic fields are ubiquitous in space and play a major role in some of the most important astrophysical phenomena.

The magnetic field along the Galactic plane
The magnetic field along the Galactic plane. The image portrays the interaction between interstellar dust in the Milky Way and the structure of our Galaxy’s magnetic field. Credit: ESA-Planck Collaboration, by Marc-Antoine Miville-Deschenes.
Unlike the familiar magnetic fields on Earth, magnetic fields in space are not the passive result of electric currents, but  are dynamically active, self-generating the currents needed to sustain them through inductive processes in the highly-conducting plasma that fills space. Without magnetic fields, stars would form in a very different way, there would be no stellar winds, no cosmic rays, no accretion disks or jets in active binary stars and  active galactic nuclei,  no pulsars and perhaps no neutron stars at all, as magnetic fields seem to be essential to the Type II supernova explosions that form them. Nearly all the complex phenomena on the surface of the Sun are driven by magnetic fields.

Although we know all this, there remains a huge amount to find out about cosmic magnetic fields. We don't know where the seed fields came from that generated the magnetic fields in the interstellar medium of spiral galaxies and the intergalactic gas in clusters of galaxies. Detailed field patterns are poorly known, especially so in our own Milky Way Galaxy, where our view from inside makes it hard to see the big picture. Even in the best-studied cases such as the Sun, the complexity of magnetic phenomena such as reconnection leaves many questions still to answer. For this reason 'The origin and Evolution of Cosmic Magnetism' is one of the Key Science projects for the Square Kilometre Array.
 
Radio astronomy provides some of the most important tracers of cosmic magnetism, including the Faraday and Zeeman effects which are the best two ways to measure the strength of magnetic fields in interstellar and intergalactic space, and so research into cosmic magnetism drives many research projects at JBCA. 
 

Research activities

  • Faraday Tomography/ Rotation Measure Synthesis
    Holding image
    Faraday rotation map of synchrotron emission from cosmic rays in our Galaxy, over 1000 square degrees from the Galactic ALFA Continuum Transit Survey (GALFACTS).

    The new generation of radio telescopes features wide-band, multi-channel receivers which are revolutionizing Faraday rotation studies by allowing us to measure the rotation across a broad range of wavelengths in a single observation. Examples include eMERLIN, the Jansky VLA, LOFAR, MeerKAT, ASKAP, and the ALFA receiver on the Arecibo radio telescope. These instruments allow us to use the Fourier technique of Rotation Measure Synthesis to measure not just the average amount of Faraday rotation but also its distribution along the line of sight, and across the sky on scales too small to resolve with the telescope.

    We are involved in large-scale polarization surveys with several of these instruments, including GALFACTS with Arecibo, POSSUM with ASKAP and SuperCLASS with eMERLIN. These surveys can be used to probe the magnetic fields in our own Galaxy, in distant galaxies and clusters of galaxies, and in deep intergalactic space.

  • Magnetic reconnection
    Two views of a model of magnetic field lines in a coronal loop
    Two views of a model of magnetic field lines in a coronal loop.
     

    Activity in the solar atmosphere is dominated by the magnetic field. We investigate phenomena such as solar flares and solar coronal heating using magnetohydrodynamics and plasma kinetic models, especially the fundamental physical process of magnetic reconnection.

    See also: Solar Plasmas research group.

  • Magnetic fields in the interstellar medium
    Polarized emission at 1.4 GHz from CGPS
    Polarized emission at 1.4 GHz from the Canadian Galactic Plane Survey (CGPS). The colour scale show a particular mode of fluctuation in the polarization intensity. The 'drapery' texture produced with 'Line Integral Convolution' technique denotes the orientation of the magnetic field.
    turbulent field from the CGPS
    This turbulent field from the CGPS has been analysed using a technique called the 'gradient of the linear polarisation vector'. The white lines trace the position of particularly sharp transition in the magnetic field as a function of the angular scale.
     

    Magnetic fields are omnipresent in the Milky Way and they play crucial roles in many physical processes in the interstellar medium. Even for parts of the Galaxy that we consider being composed of neutral gas, starlight always ionises a significant fraction of the interstellar gas. These ions link the gas to the magnetic field in the Galaxy and add an extra pressure in the medium which has important consequences for star formation processes and the turbulent motions in the interstellar medium. Therefore, it is very important to understand the close interaction between the magnetic fields and the interstellar gas, to get a better description of the physical processes occurring in our Galaxy. Since magnetic fields cannot be observed directly, we continually need to develop new techniques which allow us to interpret new sets of data revealing a broad range of unexpected features.

  • Shaping of stellar outflows

    Supernova explosions are the spectacular final gasp of stars more massive than eight times the mass of the sun, whilst smaller stars produce beautiful planetary nebulae. A main sequence star is more exactly spherical than a billiard ball (relative to its size) but supernova remnants and planetary nebulae are almost always ellipsoidal, and often show a biconical outflow or even jets. A stellar companion is often responsible for spinning up the progenitor but not all stars are binary and in some cases there is no hint of a companion and stellar surface rotation is less than ~1 km/s. Polarization of IR radiation emitted or scattered by dust, and spectral line Zeeman splitting and linear polarization reveal magnetic fields around evolved stars, and the energy density in the field is sufficient to deflect the stellar wind (if not to actually drive it). Different maser species (see LINK to maser section) are found at increasing distances from the star, showing that the magnetic field is centred on the star, but hitherto, it has not been clear whether this is a dipole, a toroidal field or even a Solar-type field. Interpretation is tricky; Fig. 1 shows SiO maser polarization vectors, which can be either parallel or perpendicular to the magnetic field direction in the plane of the sky, depending on its orientation along the line of sight. Fig. 2 shows that a preferred axis can develop in a cool star wind even before the final explosion.

    SiO maser polarization vectors
    Fig. 1 SiO maser polarization vectors.
    Preferred axis developing in a cool star wind
    Fig. 2 A preferred developing in a cool star wind.
     
  • Zeeman studies of masers/HI
    magnetic field in milligauss in the W3(OH) star-forming region
    A map of the magnetic field in milligauss in the W3(OH) star-forming region, derived from Zeeman splittings of OH maser features.

    Atoms and molecules with net electronic angular momentum have large Zeeman splittings, proportional to the Bohr magneton. Examples are hydrogen atoms and the OH and CH molecules, and species of this type typically have splittings between magnetic components of their spectral lines of several kHz per mG of magnetic field.

    This type of Zeeman effect influences, among other transitions, the 21-cm line of the H-atom, and the OH maser transitions close to 1.7 and 6.0GHz.  Closed-shell species, such as H2O and CH3OH have Zeeman splittings proportional to the nuclear magneton, which is smaller than the Bohr magneton by the electron to proton mass ratio. 

    If the Zeeman splitting significantly exceeds the line width of the individual spectral components, as is often the case for OH masers in star-forming regions, the magnetic field strength in the source can be recovered directly from the observed splitting. The sense of the field (towards or away from the observer)  is revealed by observing whether magnetic components in the spectra with left-hand elliptical polarization lie at higher or lower frequency than their right-handed counterparts. Linear polarization can also be detected, but the relationship with the magnetic field direction can be complex. Numerical modelling is required to recover the full vector magnetic field.

    We cannot observe the Zeeman splitting directly when the splitting is weak (vastly smaller than the spectral line width). However, we can still use spectra in the Stokes parameters to recover the line-of-sight component of the magnetic field in this case.

     
  • Magnetic fields in clusters of galaxies
    Merging cluster from the MACS survey (Risley et al 2016). Purple is X-ray, blue is dark matter lensing reconstruction and red is radio at 325MHz from the GMRT.

    Clusters of galaxies are the largest gravitationally bound objects in the Universe. These giant potential wells are not only sensitive probes of cosmological evolution and structure formation, but can also be used as giant astrophysical plasma laboratories - allowing us to probe a regime of temperatures, densities and pressures that are not available to us in Earth-based laboratories.

    Although clusters can contain hundreds of individual galaxies, these objects only make up ~1% of their total mass. Far more important is the hot gas between the galaxies in clusters, known as the Intra-Cluster Medium (ICM), and the dark matter that fills the cluster potential well. Dark matter makes up ~90% of the mass of a cluster and cannot be detected directly, but is mapped using the gravitational lensing effect that it has on light. Modern studies of galaxy clusters also reveal a further component of the ICM, produced by electrons moving close to the speed of light. This last component is seen in the radio window of the electromagnetic spectrum and is thought to be produced when clusters of galaxies crash into each other during merger events.

    Mergers between clusters of galaxies are some of the most violent events in the Universe and cause huge amounts of energy to be dissipated within the cluster environment. One potential consequence of such events is the (re-)acceleration of electrons in the cluster gas to velocities close to the speed of light. In the presence of the magnetic fields that pervade the cluster gas, these electrons then emit synchrotron emission to produce vast radio structures known as radio haloes and radio relics. Although exactly how this (re-)acceleration happens is still a topic of debate in astrophysics.

     
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