Pattern of convection at the surface of the Sun observed by the HMI instrument on board of SDO. MPI for Solar System Research /NASA

The
interior motions of the Sun are much slower than predicted. Rather than
moving at the speed of a jet plane (as previously understood) the
plasma flows at a walking pace. The result of this new study, whose lead
author is from the Max Planck Institute for Solar System Research, will
be published in an upcoming issue of Proceedings of the National
Academy of Sciences. The scientists use observations of solar
oscillations from NASA’s Solar Dynamics Observatory (SDO) to see into
the Sun’s interior. As Laurent Gizon and Aaron C. Birch from the Max
Planck Institute for Solar System Research comment in the same issue of
PNAS, these new observations demonstrate the unique capabilities of
helioseismology with SDO to probe the mysteries of the deep solar
interior.

In
its outermost third, the Sun behaves like a simmering pot of water:
heat from below causes the plasma to rise to the surface where it is
cooled and descends back down into the interior. This mechanism, named
convection, transports energy outward and controls the Sun’s structure
and evolution.

The
scientists, led by Shravan Hanasoge from Max Planck Institute for Solar
System Research, were now for the first time able to constrain the
flows deep within the convection layer from direct observations of the
Sun’s surface with the help of helioseismology. Helioseismology is
similar to Earth seismology.

“We
observe oscillations of the solar surface and use them to infer
properties, such as flows, in the solar interior,” explains Laurent
Gizon, director of the Department “Physics of the Interior of the Sun
and Sun-like Stars” at the Max Planck Institute for Solar System
Research  and Professor at the Institute for Astrophysics at the
University of Göttingen.

Plasma flows with less than one meter per second

The
team of scientists from the Max Planck Institute for Solar System
Research, Princeton University, NASA’s Goddard Flight Center and New
York University was able to determine the flow velocities at a depth of
55,000 km, which is 8% of the solar radius. Surprisingly, the flow
velocities of the plasma were found to be less than a few meters per
second. Gizon puts this into perspective saying “This is a hundred times
less than predicted by numerical models of solar convection”.

The
key to these new results was data from NASA’s space probe SDO, which
has been observing the Sun’s surface since early 2010. The scientists
analysed data from the Helioseismic and Magnetic Imager (HMI) onboard
SDO. The analysis was only possible due to the combination of high
resolution and full spatial coverage of the observations. The huge
amount of data collected by HMI  – thousands of high-resolution images
of the whole Sun per day – are archived and processed in the German Data Center for SDO hosted at the Max Planck Institute for Solar System Research, a unique facility in Europe.

Acoustic waves in the sun reveal the velocity of convection currents

HMI
measures the velocity of the Sun’s surface. When a solar acoustic wave
trapped within the Sun reaches the surface, it causes the surface to
move—and can thus be detected by HMI. In this way, the scientists were
able to measure the time it takes for solar acoustic waves to travel
from a point on the solar surface through the interior to another point
on the surface. Convective flows affect the speed of propagation of the
waves. Hence, it is possible to learn about the velocities of the
convective flows in the solar interior from measurements of wave travel
times. Modelling the interaction of solar acoustic waves with convection
is a topic of current research, undertaken within the German Research
Foundation’s Collaborative Research Center “Astrophysical flow
instabilities and Turbulence” at the Max Planck Institute for Solar
System Research  and the University of Göttingen.

Gizon
says “The unexpectedly small velocities measured using helioseismology
are the most noteworthy helioseismology result since the launch of HMI”.
Adds Birch:“There is no clear way to reconcile the observations and
theory”. Gizon then concludes: “This result not only sheds a new light
on the Sun—but also on our current inability to understand one of the
most fundamental physical processes in the Sun and stars: convection”.

Source: Max Planck Institute