An international research team which included staff from the University of Tasmania School of Natural Sciences has mapped the magnetic fields in space within about 150 light-years of the Sun.
Astronomers have known for decades that magnetic fields play a key role in regulating the formation of stars and planets. Still, the complicated structures formed by fields in almost empty space are impossible to observe directly. Rather, interstellar magnetism is measured indirectly, through its effect on tiny dust grains that drift through the open space between stars.
The new study, led by astronomers at the University of Turku in Finland, is based on high-precision polarisation measurements. Small dust grains, less than one micrometre (1/1000th of a millimetre) in size, can align themselves with interstellar magnetic fields in exactly the same way as a compass needle lines up with the Earth’s magnetic field. Starlight off the aligned dust grains can become polarised in turn because light is an electromagnetic wave, which preferentially oscillates in a direction that is lined up with the dust grains.
“What makes the study particularly significant is its connection with results obtained by NASA’s Interstellar Boundary EXplorer (IBEX) orbiter sent to explore the interaction between the Sun and the magnetic field in the solar neighbourhood,” explains Associate Professor Andrew Cole from the School of Natural Sciences.
As the Sun moves through space, it carries its own stream of very diffuse material along with it. This plasma is continuously blown off the Sun in a breeze known as the solar wind – responsible for the Southern Lights (aurora australis). The boundary where the solar wind ends and interstellar space begins is known as the heliopause.
IBEX receives some information about the interstellar magnetic field by observing hydrogen atoms that pass through the heliopause. The details, however, can only be accurately determined by polarisation measurements of starlight.
High-precision equipment reveals magnetic field direction
High-precision polarimetry equipment with sensitivity at the level of 0.001% or better (one part in 100,000) has been developed for these type of measurements at the Tuorla Observatory of the University of Turku, in collaboration with Leibniz-Institut fur Sonnenphysik in Freiburg, Germany. Four telescopes were used for the observations in this study: two in Hawaii (Mauna Kea and Haleakala telescopes), one in La Palma (Nordic Optical Telescope), and one in the Southern Hemisphere at the Greenhill Observatory of the University of Tasmania.
The stars observed in this study are scattered around the sky, near the edges of a vast region of hot, low-density gas. This region, known as the Local Bubble, was hollowed out by one or more exploding stars many millions years ago.
The observations have revealed interesting magnetic filament structures both ‘upstream’ of the Sun with respect to its motion through the galaxy, and in the opposite, ‘downstream’ direction. The filaments form ribbon-like arcs where dust particles and starlight polarisation have aligned with the direction of the magnetic field.
Mapping the magnetic field around the edges of the Local Bubble can help scientists learn about the very faint structures surrounding the Sun and the way in which this diffuse plasma is heated and cooled by starlight and interstellar shockwaves.
The results of the study were featured in the March edition of the European journal Astronomy & Astrophysics.
Fig. The structure of magnetic field and the distribution of dust in space is revealed by high-precision polarisation measurements. The figure shows the degree and direction of polarisation of more than 2,000 stars in galactic coordinates. The size of the dust particles responsible for polarisation at the optical wavelengths is less than one micrometre, i.e., similar to solid particles in smoke.