Astronomers using the European Space Agency’s XMM-Newton telescope
have discovered that observing the giant planet Jupiter may actually
give them an insight in to solar activity on the far side of the Sun! In
research reported in the most recent edition of Geophysical Research
Letters, they discovered that Jupiter’s x-ray glow is due to x-rays
from the Sun being reflected back off the planet’s atmosphere.

Jupiter is an intriguing object when viewed in x-rays; it has dramatic
x-ray auroras at the poles and a variable x-ray glow from near the
equator. Researchers had theorised that these x-rays from the equatorial
regions of Jupiter, called disk x-rays, were controlled by the Sun. In
November 2003, during a period of high solar activity, they observed

“We found that Jupiter’s day-to-day disk x-rays were synchronised
with the Sun’s emissions,” says Dr Anil Bhardwaj, from NASA Marshall
Space Flight Centre and lead author on the paper. “Unfortunately, we
missed a relatively large solar flare during the 3.5-days observation
due to the perigee passage of the XMM-Newton”. “But, still we were
lucky; particularly clear was a signature of a moderate solar flare that
went off during the observing period – there was a corresponding
brightening of the Jovian disk x-rays”, says Anil Bhardwaj.

In addition to supporting the researchers’ theory, this result has
another application – in studying the Sun. The Sun is a very dynamic
environment and processes there have an impact on human activities. For
example, solar flares (the most powerful explosions in the solar system)
can damage satellites or injure astronauts in space, and on Earth they
can disrupt radio signals in the atmosphere, so it is important to
understand as much as we can about them.

There are several dedicated spacecraft watching the Sun (such as the
European Space Agency’s SOHO satellite), as well as ground-based
telescopes, but there are gaps in coverage as some areas of the Sun are
not visible by any of these means at some times.

“As Jupiter orbits the Sun, we hope to be able to learn more about
the active areas of the Sun we can’t see from Earth by watching the
Jovian x-ray emissions,” says Dr Graziella Branduardi-Raymont from the
University College London’s Mullard Space Science Laboratory. “If a
large solar flare occurs on an area of the Sun that is facing Jupiter,
we may be able to observe it in light scattered from Jupiter, even if we
cannot see that region of the Sun from around the Earth at the time.”

Jupiter’s atmosphere is not a perfect mirror of the Sunlight in
X-rays – typically one in a few thousand x-ray photons (packets of
light) is reflected back, but the more energetic the photons, the more
are reflected into space.

UK participation in this research and the UK subscription to the
European Space Agency are funded by the Particle Physics and Astronomy
Research Council (PPARC).

Notes for Editors

The paper ‘Solar Control on Jupiter’s equatorial X-ray emissions:
26-29 November 2003 XMM-Newton observation’ was selected as an AGU
Journal Highlight of Geophysical Research Letters, VOL. 32, NO. 3, 2005

What are solar flares?

A solar flare is a tremendous explosion on the Sun that happens when
energy stored in ‘twisted’ magnetic fields (usually above sunspots) is
suddenly released.

In a matter of just a few minutes they heat material to many millions
of degrees and produce a burst of radiation across the electromagnetic
spectrum, including from radio waves to x-rays and gamma rays.