Scientists answer question of why Comets emit X-rays

The mystery of X-ray emissions from comets has been solved by a team including University of Strathclyde Professor of Physics.

The radiation of X-rays from comets has been a long-standing mystery to science, given that X-rays are normally associated with hot objects like the sun but comets are among the coldest objects in the solar system.

When comets travel through the Solar system they interact with solar radiation, wind and magnetic field. This produces a visible atmosphere, known as a coma, and the observed cometary tail, as well as, in some cases, X-rays.

The X-rays are generated on the sunward side of the comet, where the solar wind impacts the cometary atmosphere forming a bow shock.

A team of scientists from 15 institutes probed the question by carrying out experiments at the LULI laser facility in Paris, in which they replicated the interaction of the solar wind with a comet.

The paper, Electron acceleration by wave turbulence in a magnetized plasma, has been published in Nature Physics.

Professor Bob Bingham, of Strathclyde’s Department of Physics, provided the theoretical support for the project and published a theoretical paper on X-rays from comets that formed the basis of the investigation.

He said: “These experimental results are important as they provide direct laboratory evidence that objects moving through magnetized plasmas can be sites of electron acceleration.”

“This is a very general situation in astrophysics that takes place not only in comets, but also in planetary magnetospheres - such as our own Earth - or even in supernova remnants, where the ejected material moves across the interstellar gas. The experiments also confirm theoretical models developed by the team.”

Alexandra Rigby, a University of Oxford PhD student working on the project said: “The laser beams are fired onto a plastic foil, which explodes, causing a stream of electrons and ions to be expelled, forming a high speed flow of ionized gas – plasma - like the solar wind.”;

“When this plasma flow impacts onto a solid sphere, the laboratory `comet’, placed nearly a centimetre away from the plastic foil, resembles what happens when a real comet passes through the solar system. It was found that electrons are heated to about a million degrees in the up-stream plasma by plasma turbulence.”

“These hot electrons are responsible for emitting X-rays but only in the presence of a magnetic field.”

The research also shed light on a cosmic ray mystery known as the injection problem. It is widely recognised that strong shock waves are expected to accelerate particles to very high energies, however, they require a source of particles fast enough to cross the shock. This latest experiment clearly demonstrates that plasma turbulence can provide a source of fast particles which can overcome the injection problem.

The study also involved scientists at the Science and Technology Facilities Council's (STFC) Rutherford Appleton Laboratory in Oxfordshire, where Professor Bingham is also an STFC Fellow.

12 March 2018