If you've been following this SSFX or
space sound effects series about
ultra-low frequency sounds present in
space around the Earth, you might have
wondered, so what? Who cares? It's a valid question,
but there's a damn good answer to it. As
I mentioned before, these sounds play a
role in space weather. That's the
umbrella term for phenomena from space
which can affect our everyday lives,
either our health or the technology that
we so crucially depend on. Space weather
is a huge scientific field, not only
because of the potential risks but also
because there are so many different ways
things from space can affect us, with
sounds from space being just one small
part of one small part of it. ULF waves,
as with all waves in physics, are a way
of transmitting energy from one place to
another. That energy may come from the
solar wind itself or unstable processes
within Earth's magnetosphere during say
geomagnetic storms. But it's where that
energy may end up which is crucial and
one of the most dangerous places it can
go is in the outer Van Allen radiation
belt. Usually there are two radiation
belts, an inner and an outer belt, which
each form a torus or doughnut shape around the Earth. Inside these doughnuts is a
much higher level of ionizing radiation
and that's because they're filled with
trapped high-energy charged particles.
These particles are simply executing the
motion dictated by Earth's magnetic field.
As you know charged particles gyrate in
a magnetic field, meaning they trace out
helical paths along those field lines,
however as the particles travel along
the field lines towards the Earth, the
strength of that magnetic field
increases and eventually this will
reflect the particles back out into
space.
That's called a magnetic mirror.
Furthermore, because the Earth's magnetic
field is curved and isn't uniform, the
particles also drift
around the Earth and these drifts are in
opposite directions for ions and
electrons. So to summarize, there are
three different motions of particles in
the radiation belts: gyromotion,bounce
motion, and drift motion. In the outer
radiation belt, typically some 13,000 to
60,000 kilometers above the equator, it's
mostly electrons that have become
trapped. Now it turns out the periods of
the electrons' bounce and drift motions
falls within the ultra low frequency
range and that means our sounds from
space can affect them. Remember,
these sounds consist of motion of the
background plasma in space as a whole,
with the magnetic fields moving with it.
This oscillating magnetic field means
there are associated electric fields as
well and it's these electric fields that
can accelerate the radiation belt
electrons in what are known as wave
particle interactions. Now for any
electrons who's bounce or drift
frequencies match up with the
frequencies of the waves, then resonance
can occur allowing the electrons to take
huge amounts of energy from the wave. This
can accelerate these electrons up to
ultra relativistic energies where the
electrons are traveling at close to the
speed of light!
Sidenote, one of the other types of
plasma waves I talked about before,
chorus waves, also contribute to this
process because they can match the
electrons gyrofrequency. Ultra
relativistic electrons are bad news for
our satellites, they can severely damage
them or even cause them to die. That's
where they've gained the nickname "killer
electrons". The really annoying thing is
that the outer radiation belt often
overlaps with the locations of our GPS
satellites and the huge number of
satellites in geostationary orbits, which
includes satellites for our
communications, TV broadcasting and
weather forecasting. It's not all doom
and gloom though.
We increasingly use radiation hard
components in our satellites to protect
against this and satellites can be put
in a safe mode to limit damage. But this
requires us to be able to predict where
and when these killer electrons will be
present. That's currently hard for us to
do, but it's why we're working on better
understanding not only the radiation
belts but the things that can make them
deadly, like sounds from space. Thanks so much for watching this video in my SSFX
series. Do check out the other videos in
it if you haven't already and let me
know in the comments what piece of
technology you couldn't live without.
Oh and don't forget to like or
subscribe if you fancy. Thanks!
