Magnetics Business & Technology - May/June 2020 - 8

FEATURE ARTICLE
stream of charged particles and magnetic
field flung out from the Sun's hot atmosphere.
When this tangle of particles and magnetic
field reaches us, it interacts with the Earth's
magnetic field. This field protects us, but
when the Sun is in a particularly violent mood,
extreme solar wind events like flares and
coronal mass ejections can interrupt satellites
and power grids, affecting our way of life.

will have a better idea of the performance of the instruments and
hopefully start collecting first scientific data in mid-May."
"The ten instruments onboard our mission will be playing together like instruments in an orchestra," says ESA Solar Orbiter project scientist Daniel Mueller. "We have just started the
rehearsal, and one by one, additional instruments will join. Once
we are complete, in a few months' time, we will be listening to the
symphony of the Sun."

Initial magnetometer data outstanding

"We're used to thinking of the Sun as a
uniform ball of light, but under the surface it
is broiling with complex magnetic fields that
can release huge amounts of energy, flinging
fast-moving charged particles in our direction, explains," explains
Horbury.

"It's early days, but the data we have so far looks good, commented O'Brien in February. "We are delighted with the measurements and the performance of the instrument - it looks even
better than we hoped it would. We're really excited to be entering
the science phase and confident it can carry out its mission well."

Professor Tim
Horbury

"We spent a long time developing the instrument and it looked
good in all the tests we did on the ground, but you never can tell
until you get to space," noted Horbury. "We only have two hours
of data so far, but it looks outstanding. The instrument has been
well behaved and done all we asked of it. It looks like we will be
able to do everything we need, and the instrument will be a key
contributor to the science of the mission."

The Earth's magnetic field can be pictured like a bar magnet.
Magnetic field lines come out from one pole, encompass the
Earth, and connect back into the other pole. The Sun, however,
is writhing with much more tangled magnetic field lines looping in
and out across its surface, getting stretched and squeezed and
sometimes coming undone, releasing energy and particles. The
particles, and with them the magnetic field, sweep out into space,
changing and transforming as they go.
But it's not simply the heat of the Sun the spacecraft and its
instruments must contend with, it's the huge variations of temperature. Out in the vacuum of space, there's no atmosphere
to evenly distribute heat. So, the side of the spacecraft facing
the Sun, which is covered by a heat shield, will reach up to 500
degrees C. Just a few meters behind this, out on a boom, the
Imperial team's magnetometer instrument will operate at -140
degrees C.

The diagram above shows pristine magnetic field measurements
taken in the solar wind by the Imperial magnetometer on Solar
Orbiter, more than a million km away from Earth. The plot shows
a smooth rotation in the magnetic field, on top of which ride
smaller waves. Capturing fine-scale waves such as these, which
interact with solar wind particles, is a key target of the Solar
Orbiter mission.

Measuring the solar wind

This first data was sent back on a low-bandwidth antenna and arrived instantly. The first science data, however, had to wait until
the high-gain antenna was deployed and came down two days
later.
The magnetometer sits on a long boom at the back of the spacecraft, to keep it away from the other instruments, which could
interfere with its readings. The whole spacecraft, which was constructed by Airbus in the UK, and all the separate instruments,
had to be very magnetically 'clean' - produce no magnetic field of
their own - for the magnetometer to work at its best.
We live inside a bubble blown by the Sun, explain the Imperial College scientists. The bubble is made of the solar wind - a

Magnetics Business & Technology * May/June 2020

At its surface, the Sun's magnetic field is around twice that of
the Earth's, but in deep space
it is tens of thousands of times
smaller, so the magnetometer
must be incredibly sensitive. "Our
instrument is so sensitive, it could
measure the magnetic field of an
MRI machine from the other side
of London," notes O'Brien. "This
means, however, that we have
to work hard to isolate it from the
other instruments on the spacecraft. Metal objects and electrical
circuits create small magnetic
fields, so we have really strict
requirements on the rest of the
project, right down to the screws
and the paint."

Close-up view of magnetometer

Before launch, as one of the final stages of testing, the spacecraft was placed in a unique facility run by German company
IABG in a forest south of Munich to avoid interference with
human-generated magnetic fields. The facility consists entirely of
non-magnetic materials like wood and contains twelve 15-metre
coils, nearly as large as the building, which create a consistent
magnetic environment that cancels out the Earth's own magnetic
field, simulating outer space conditions. The magnetometer was
then tested while the spacecraft performed some operations that
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Magnetics Business & Technology - May/June 2020

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