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Auroras
Northern lights (aurora
Every 11 years, the Sun cycles from solar minimum to borealis) over the
solar maximum, as we discussed in a prior EarthDate. Víkurkirkja church at
And we’ve just entered another solar maximum, where Vík í Mýrdal, Iceland.
the Sun emits many more charged particles in a faster, Credit: AstroAnthony, CC BY 4.0,
stronger solar wind moving toward Earth. via Wikimedia Commons
You might experience it in disrupted GPS or cellular
signals. But you can see it in the night sky. Specifically,
in the aurora borealis, the famous northern lights.
You may remember, also from a prior episode, that
Earth’s magnetic field shields us from the charged
particles in the solar wind, allowing life on Earth.
Some particles still get through but are directed by the
magnetic field around Earth toward its magnetic poles.
These particles excite atoms in the atmosphere, which
emit light. Oxygen emits mostly green light, and nitrogen
red. They blend together in unusual ways to create many
colors including blue, purple and pink.
With stronger solar winds, more charged particles
will get through, meaning brighter auroras that will be
more visible, even at lower latitudes like the Midwest
United States.
But they’ll still be the most visible within 30 degrees of
the magnetic poles, when the pole is farthest from the
Sun, around midnight. And particularly around the
spring and autumn equinox.
So for your best shot at a spectacular nighttime light
show, head to the Arctic Circle in mid-September.
Maybe I’ll see you there.
Fact Sheet:
EarthDate.org Episode ED 384
Background: More Auroras
Synopsis: Auroras are beautiful dancing curtains of colorful light that are typically visible at high
latitudes, encircling Earth’s north and south poles. Although auroras occur in Earth’s thermosphere, they
are driven by solar activity. Charged particles in the solar wind excite atoms and molecules high in Earth’s
atmosphere, causing them to emit photons that illuminate the sky. As solar activity increases, auroras
expand southward. These eerie light shows have intrigued humans for millennia. They have now been
discovered on other planets in our solar system.
Earth’s “northern” or “southern” lights Vikings believed the lights were manifes-
appear as flowing curtains, ribbons, rays, tations of Norse gods leading the souls of
spirals or flickers of light that may cover slain earthly warriors across a pulsing bridge
part or all of the night sky. of light to Valhalla.
In Finland, myths explain that arctic foxes
Aurora borealis is the name of the northern create the aurora, running through the sky
phenomenon, while aurora australis is the so fast that their fluffy tails ignite sparks
name for the southern lights. as they brush against the mountains or
In Greek and Roman mythology, Aurora is the create whirls of snowflakes that reflect
dawn, sister of the Sun and the Moon, who the moonlight.
rides her chariot through the night skies to In North America, some indigenous people
alert her siblings to the break of day. believe the lights to be the spirits of
In Latin, borealis means northern, while departed ancestors, a way to communicate
australis means southern. with lost loved ones or torches leading
Humans have been frightened, fascinated, the spirits of those who have recently died
and inspired by auroras for centuries. across the abyss to a land of brightness.
The Meshkwahkihaki (called the Fox Indians
Dragon legends in China and Europe are
by European colonizers) have said that the
thought to have originated from auroras’
lights are the spirits of slain enemies coming
serpentine motions.
for revenge.
Auroras that made it as far south as Europe
tended to be red, driven by higher-energ y
solar storms that expanded these more
intense auroras both toward the equator and
higher in the atmosphere. Europeans saw red
auroras as omens of war, even as recently as
during the French Revolution.
Auroras result when electrons and protons
in the solar wind are deflected toward both
polar regions, where they interact with atoms
and molecules in Earth’s upper atmosphere.
When the Sun is calm, the ever-present solar
wind flows out of the Sun toward its planets
at supersonic speeds of about 900,000 mph
Aurora borealis, also known as the northern lights or (400 km/s), taking 4 to 5 days to cross the
polar light, in Tromsø, Norway, in December of 2011. 93 million mi (149 million km) distance to Earth.
Credit: Arctic light - Frank Olsen, CC BY-SA 3.0,
via Wikimedia Commons
References: More Auroras
Aurora Tutorial | NOAA SWPC
How Auroras Form | NASA
Aurora | National Geographic
What Causes the Northern Lights? | Hurtigruten
Aurora Colors: What Causes Them and Why Do They Vary? | Space.com
EarthDate.org
Fact Sheet:
Contributors: Juli Hennings, Harry Lynch
Episode ED 384
Background: More Auroras
There are two sides to our Sun (ED-126). The photons released as particles excited by
During solar storms, explosive coronal the solar wind return to their normal states
mass ejections (CMEs) eject plasma, while give auroras their dancing colors.
wide open coronal holes spew solar wind
at as much as twice its normal speed
(1.8 million mph, 800 km/s), resulting
in chaotic disturbances to the normal
geomagnetic balance between the Sun
and Earth’s magnetic field.
Earth’s protective magnetic field is distorted
into a teardrop shape by the solar wind,
deflecting most, but not all, of the incoming
energetic particles.
Charged particles from the solar wind
that are not deflected become trapped
in Earth’s magnetic field and accelerate
along magnetic field lines toward the Charged particles from the solar wind follow Earth’s
polar regions. magnetic field lines toward the north and south
The particles strike the upper atmosphere in magnetic poles, where they dive into the upper
a donut-shaped pattern known as the auroral atmosphere and create the aurora.
oval, centered on magnetic north between The solar system’s interplanetary magnetic field
roughly 60° and 75° latitude, with a diameter (IMF in the figure) may also influence the intensity
of about 1,865 mi (3,000 km). of auroral activity, depending on its orientation.
These electrons and protons ionize and Credit: NOAA SWPC
excite the atoms and molecules in the
thermosphere, 60 to 620 mi (100–1,000 km)
above Earth’s surface.
This excitation drives the particles to
higher energy levels, and as they return
to their normal states, they release their
surplus energy as photons of light in
various wavelengths.
During periods of high solar activity,
which can last for hours or days, the auroral
oval expands toward the equator and to
higher altitudes.
Auroras may appear green, pink, red, blue,
purple and even yellow.
The color produced depends on the
altitude and, similar to neon lights, the Energetic electrons spiral down the geomagnetic
gas composition. field lines toward the polar regions, striking the
The rarefied gases in the thermosphere upper atmosphere, resulting in the display of auroral
consist of oxygen (O), nitrogen (N), and lights along the auroral oval.
helium (He). Credit: NOAA
References: More Auroras
Aurora Tutorial | NOAA SWPC
How Auroras Form | NASA
Aurora | National Geographic
What Causes the Northern Lights? | Hurtigruten
Aurora Colors: What Causes Them and Why Do They Vary? | Space.com
EarthDate.org
Fact Sheet:
Contributors: Juli Hennings, Harry Lynch
Episode ED 384
Background: More Auroras
Energized particles
from the solar
wind that are
not deflected by
Earth’s magnetic
field travel
poleward along
magnetic field
lines, striking
oxygen atoms and
nitrogen molecules
as they dive toward
Earth’s surface. Northern lights sweep across polar skies with
Energy from an a leading edge of pinkish light from nitrogen
atom or molecule molecule excitation followed by greenish light
excited by from slower oxygen atom excitation.
energized particles Credit: Chursaev13, CC BY 4.0, via Wikimedia Commons
is released as
different colors
of light. It takes nitrogen less time than oxygen to emit
Credit: NOAA these photons, so the pinkish accent often
appears at the lower leading edge of the green
curtains as they flow across the sky.
During solar storms, more-energetic solar
winds inflate the thermosphere.
This decreases particle density, reducing
During a quiet Sun, oxygen atoms release interference from nearby reactions, giving
photons at a wavelength of 557.7 nm, oxygen atoms enough time to release more-
resulting in green auroras around 60 to 185 mi energetic photons at a wavelength of 630 nm,
(100–300 km) altitude. resulting in red auroras at higher altitudes
of 185 to 380 mi (300–600 km).
The thermosphere is just dense enough that During very high intensity solar storms that
nearby reactions quench excitation reactions occur about every 11 years during the solar
before they get to higher energy states. maximum (ED-362 Solar Maximum is Heating
We can see green auroras best because Up), the entire inflated aurora may be red,
green occurs at the center of the human extending as far south as Europe and the
visible spectrum. midwestern United States.
Sometimes, glancing sunlight will turn the The thermosphere is on the edge of space,
tops of these auroral rays blue. so it has a very low density of particles.
Nitrogen releases protons at wavelengths But these particles do ebb and flow like
of 600 to 700 nm to produce red fringes at celestial tides that interact with the dynamic
the base of the green curtains just above an magnetosphere to create the ethereal
altitude of 60 mi (100 km). movement of auroras.
When green and red light mix, the result The thermosphere can nearly double in size in
may be pink or even yellow. response to solar storms.
References: More Auroras
Aurora Tutorial | NOAA SWPC
How Auroras Form | NASA
Aurora | National Geographic
What Causes the Northern Lights? | Hurtigruten
Aurora Colors: What Causes Them and Why Do They Vary? | Space.com
EarthDate.org
Fact Sheet:
Contributors: Juli Hennings, Harry Lynch
Episode ED 384
Background: More Auroras
An image from the International Space Station shows a bright green and red aurora over our planet.
Credit: NASA
While auroras happen somewhere on Earth
every day, you can optimize your aurora
viewing by applying a few scientific concepts:
the ground beneath the auroral oval is the
best place to go to see the dazzling lights,
and auroras are most active during solar
maxima, semiannual equinoxes or around
midnight each day.
If the Sun is calm, the auroral oval occurs
15° to 30° toward the equator from the
magnetic pole (not the geographic pole,
see ED-122 Triple North Pole), however you
can count on locations along Earth’s Arctic
and Antarctic Circles (at about 66.3 degrees
Different phases of the auroral oval over a typical latitude) to provide good views.
night with moderate geomagnetic activity. The most Since the actual geographic poles tend to be
active auroras occur near midnight because that located inside the donut hole, auroral viewing
region of the oval is farthest from the Sun. is diminished there.
Credit: NOAA
References: More Auroras
Aurora Tutorial | NOAA SWPC
How Auroras Form | NASA
Aurora | National Geographic
What Causes the Northern Lights? | Hurtigruten
Aurora Colors: What Causes Them and Why Do They Vary? | Space.com
EarthDate.org
Fact Sheet:
Contributors: Juli Hennings, Harry Lynch
Episode ED 384
Background: More Auroras
As solar activity increases during the 11-year
solar cycle, the light display becomes more
dynamic and colorful while the auroral oval
inflates both vertically and laterally, making
auroras visible over more of the globe.
Auroras are most frequent around the time
of the autumn and spring equinoxes. During
equinoxes, Earth’s magnetic field is favorably
aligned with the ecliptic (the plane of
Earth’s orbit around the Sun), enhancing
auroral activity.
Beyond the equinoxes, winter is the best
viewing season for auroras because the
magnetic pole is tipped away from the Sun.
Within a given day, the most active part of
the auroral oval is the part farthest from the
Sun, which you can see best around midnight.
So, for the absolute best show, plan for a
midnight viewing on a mountaintop along
the Arctic Circle for the autumnal equinox, Jupiter’s magnificent ultraviolet aurora was
September 21 to 22, 2024, during the solar photographed by NASA’s Hubble Space Telescope
maximum of Solar Cycle 25. Imaging Spectrograph as NASA’s Juno spacecraft
approached and entered into orbit around Jupiter.
Auroras can also form as the solar wind This image superimposes the aurora on a full-
impacts other planets and their satellites, color image of Jupiter separately photographed
especially those with atmospheres and by Hubble’s Outer Planet Atmospheres Legacy
strong magnetic fields. (OPAL) program.
All the gas giants in our solar system have Credit: NASA, ESA, and J. Nichols (University of Leicester),
robust magnetic fields that support auroras, public domain, via Wikimedia Commons
but the largest planet, Jupiter, with a
magnetosphere 20,000 times stronger than
Earth’s, has the most stunning aurora, fueled
in part by particles from its volcanic moon Io.
Even Mars and Venus, with their weak
atmospheres, can display dim auroras during
intense solar activity, possibly produced from
the Sun’s own magnetic field or from ancient
artifacts of their now-defunct magnetic fields.
Photomontage of Saturn with a false-color
image of ultraviolet aurora taken with NASA’s
Hubble Space Telescope Imaging Spectrograph
superimposed on a visible-light image from
Hubble’s Advanced Camera for Surveys.
Credit: NASA, ESA, J. Clarke (Boston University), and
Z. Levay (STScI), public domain, via Wikimedia Commons
References: More Auroras
Aurora Tutorial | NOAA SWPC
How Auroras Form | NASA
Aurora | National Geographic
What Causes the Northern Lights? | Hurtigruten
Aurora Colors: What Causes Them and Why Do They Vary? | Space.com
EarthDate.org
Fact Sheet:
Contributors: Juli Hennings, Harry Lynch
Episode ED 384
