New research undertaken at Northumbria University shows that
the Sun’s magnetic waves behave differently than currently believed.
Their findings have been reported in the latest edition of
the prominent journal, Nature Astronomy.
After examining data gathered over a 10-year period, the
team from Northumbria’s Department of Mathematics, Physics and Electrical Engineering found that magnetic waves in the Sun’s corona – its outermost layer of atmosphere
– react to sound waves escaping from the inside of the Sun.
These
magnetic waves, known as Alfvénic waves, play a crucial role in transporting
energy around the Sun and the solar system. The waves were previously thought
to originate at the Sun’s surface, where boiling hydrogen reaches temperatures
of 6,000 degrees and churns the Sun’s magnetic field.
However, the
researchers have found evidence that the magnetic waves also react – or are
excited – higher in the atmosphere by sound waves leaking out from the inside
of the Sun.
The team
discovered that the sound waves leave a distinctive marker on the magnetic
waves. The presence of this marker means that the Sun’s entire corona is shaking
in a collective manner in response to the sound waves. This is causing it to vibrate
over a very clear range of frequencies.
This
newly-discovered marker is found throughout the corona and was consistently
present over the 10-year time-span examined. This suggests that it is a
fundamental constant of the Sun – and could potentially be a fundamental
constant of other stars.
The findings
could therefore have significant implications for our current ideas about how
magnetic energy is transferred and used in stellar atmospheres.
Dr Richard Morton, the lead author of the report and a senior lecturer at Northumbria
University, said: “The discovery of such a distinctive marker – potentially a
new constant of the Sun – is very exciting. We have previously always thought
that the magnetic waves were excited by the hydrogen at the surface, but now we
have shown that they are excited by these sound waves. This could lead to a new
way to examine and classify the behaviour of all stars under this unique
signature. Now we know the signature is there, we can go looking for it on
other stars.
“The Sun’s corona is over one hundred times hotter than its
surface and energy stemming from the Alfvénic waves is believed to be
responsible for heating the corona to a temperature of around one million
degrees. The Alfvénic waves are also responsible for heating and accelerating
powerful solar wind from the Sun which travels through the solar system. These
winds travel at speeds of around a million miles per hour. They also affect the
atmosphere of stars and planets, impacting on their own magnetic fields, and
cause phenomena such as aurora.”
Dr Morton added: “Our evidence shows that the Sun’s internal acoustic oscillations play a
significant role in exciting the magnetic Alfvénic waves. This can give the
waves different properties and suggests that they are more susceptible to an
instability, which could lead to hotter and faster solar winds.”
The research
was funded by the UK Science and Technology Facilities Council and the US Air Force Office of
Scientific Research, and
undertaken by Dr Morton and Professor James McLaughlin from
Northumbria’s Solar Physics research group, together with Dr Micah Weberg, who recently
moved from Northumbria to Washington DC’s Naval Research Laboratory.
Dr Morton and
Professor McLaughlin are currently working with NASA to analyse images of the Sun which were taken by NASA’s High-Resolution Coronal
Imager, Hi-C.
Their paper A basal contribution from p-modes to the Alfvenic wave flux in the Sun’s corona is
published in Nature Astronomy.
