Sky & Telescope - October 2023 - 75

AURORA FORECAST FOR BOTH HEMISPHERES: NOAA SPACE WEATHER PREDICTION
CENTER (2); JUPITER AURORA: NASA / ESA / J. NICHOLS (UNIVERSITY OF LEICESTER),
ACKNOWLEDGMENT: A. SIMON (NASA / GSFC) AND THE OPAL TEAM
FORECAST Based on the solar wind's speed
and the properties of the magnetic field it carries,
researchers can forecast how likely aurorae
are and where they'll appear. (These are for
May 8, 2023.) NOAA provides predictions for
the next hour, tonight, and tomorrow.
10%
1
Probability of Aurora
50%
2
3
4
90%
Approximate energy deposition (ergs/cm2
>4
)
and trigger a geomagnetic storm - the
space equivalent of bad weather.
But in order to break through, the
CME particles need Earth's magnetic
field to let them in. This can happen if
the CME-carried magnetic field and our
planet's field are oriented to each other
in the right way, enabling them to link
up in a process called magnetic reconnection.
The field lines will snap together
in a new arrangement, releasing energy
and opening the magnetic door.
Earth's field then shuttles the invaders
to the planet's nightside, where
they cause part of the windsock's tail
to pinch off. The pinch makes a loop
that snaps back toward Earth, sending
particles zooming toward the poles.
Large sheets of electric current speed
them up, and they smash into the upper
atmosphere.
During these storms, the auroral
oval greatly expands. Aurorae may
appear as far from the poles as Florida
and central Australia.
Painting the Sky
Aurorae appear as shimmering curtains,
arcs, bands, and swirls. A display can
last for tens of minutes or even the
entire night, sometimes rapidly changing
its appearance. The interaction of
electric currents and the magnetic field
creates these patterns. You may also see
towering rays, which trace the magnetic
field lines the particles traveled along to
reach the atmosphere.
An aurora's colors depend on what
the charged particles hit in the atmosphere
and how high up those particles
are. The characteristic green color
comes from oxygen, roughly 100 to 110
kilometers (60 to 70 miles) high. At
JUPITER'S AURORA This composite image
combines ultraviolet observations (blue) with
visible-light ones to show the aurora on the
solar system's largest planet.
lower altitudes, air is so dense that an
oxygen atom will lose its energy by colliding
with another atom before it can
emit a green photon.
Deep red also comes from oxygen,
but at much higher altitudes - we're
talking 200 or 300 km and up. Oxygen
atoms take much longer to emit this
red glow than they do the green one,
so they only have time to do so up here
where the air is super-thin.
Other reds, blues, and purples can
come from molecular nitrogen.
Pack Your Bags
The best time of year to see an aurora
is typically around the spring and fall
equinoxes, when the magnetic fields of
Earth and the solar wind align in such
a way that reconnection is more likely
to happen. (This is why we run S&T's
aurora tours in the spring and fall.)
Because more people live beneath the
northern auroral oval than the southern
one, many of us are more familiar
with the Northern Hemisphere version,
called the aurora borealis or northern
lights, than its southern counterpart,
the aurora australis. But the Southern
Hemisphere isn't entirely out of luck:
For example, on a clear night Tasmania
has on average a 1% or 2% chance of
seeing the southern lights.
Alien Aurorae?
Aurorae also appear on nearly every
planet in the solar system (poor Mercury
being the odd one out). Jupiter
has the most powerful, thanks to a
magnetic field that's some 20,000
times stronger than Earth's. Most of
the charged particles that fuel Jupiter's
aurora come from its big moons,
particularly the volcanic moon Io. The
moons actually leave auroral footprints
in Jupiter's atmosphere, glowing dots
that mark where the current from each
moon hits the planet. The moons also
have their own aurorae.
Martian aurorae look quite different
than ours, because the planet doesn't
have a global magnetic field. Diffuse
aurorae can cover the whole night sky,
as charged particles rain down into the
atmosphere without magnetic guidance.
Mars also has localized aurorae, the
brightest of which appear above places
where the planet's long-gone global
field left its imprint in the crust. Such
aurorae tend to happen in the evening
hours. They might even be visible to
future astronauts on the surface. ¢
Find the latest space-weather forecast
from the National Oceanic and Atmospheric
Administration: https://is.gd/
noaa_aurora.
sk yand tele scope .o r g * OCTOBER 2023 75
https://www.is.gd/noaa_aurora https://www.is.gd/noaa_aurora https://skyandtelescope.org/
Sky & Telescope - October 2023

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