- 4469 - NORTHERN LIGHTS - May 2024, a huge solar flare sent a wave of energetic particles from the sun surging out through space. The wave reached Earth, and people around the world enjoyed the sight of unusually vivid aurora in both hemispheres. While the aurora is normally only visible close to the poles, this was spotted as far south as Hawaii in the northern hemisphere, and as far north as Mackay in the south.
----------------------- 4469 - NORTHERN LIGHTS - May, 2024
- This spectacular spike in auroral activity
appears because the sun is approaching the peak of its 11-year sunspot cycle,
and periods of intense aurora are likely to return over the next year.
-
- What makes the glow, and the different
colors? It is all about atoms, how
they get excited and how they relax.
-
- The auroras are caused by charged subatomic
particles (mostly electrons) smashing into Earth's atmosphere. These are
emitted from the sun all the time, but there are more during times of greater
solar activity.
-
- Most of our atmosphere is protected from
the influx of charged particles by Earth's magnetic field. But near the poles,
they can sneak in and wreak havoc.
-
- Earth's atmosphere is about 20% oxygen and
80% nitrogen, with some trace amounts of other things like water, carbon
dioxide (0.04%) and argon.
-
- When high-speed electrons smash into oxygen
molecules in the upper atmosphere, they split the oxygen molecules (O₂) into
individual atoms. Ultraviolet light from the sun does this too, and the oxygen
atoms generated can react with O₂ molecules to produce ozone (O₃), the molecule
that protects us from harmful UV radiation.
-
- But, in the case of the aurora, the oxygen
atoms generated are in an excited state. This means the atoms' electrons are
arranged in an unstable way that can "relax" by giving off energy in
the form of light.
-
- As you see in 4th of July
fireworks, atoms of different elements produce different colors of light when
they are energized. Copper atoms give a
blue light, barium is green, and sodium atoms produce a yellow–orange color
that you may also have seen in older street lamps. These emissions are
"allowed" by the rules of quantum mechanics, which means they happen
very quickly.
-
- When a sodium atom is in an excited state it
only stays there for around 17 billionths of a second before firing out a
yellow–orange photon. But, in the
aurora, many of the oxygen atoms are created in excited states with no
"allowed" ways to relax by emitting light. Nature finds a way.
-
- The green light that dominates the aurora is
emitted by oxygen atoms relaxing from a state called "¹S" to a state
called "¹D." This is a relatively slow process, which on average
takes almost a whole second.
-
- In fact, this transition is so slow it won't
usually happen at the kind of air pressure we see at ground level, because the
excited atom will have lost energy by bumping into another atom before it has a
chance to send out a green photon. But in the atmosphere's upper reaches, where
there is lower air pressure and therefore fewer oxygen molecules, they have
more time before bumping into one another and therefore have a chance to
release a photon.
-
- For this reason, it took scientists a long
time to figure out that the green light of the aurora was coming from oxygen
atoms. The yellow–orange glow of sodium was known in the 1860s, but it wasn't
until the 1920s that Canadian scientists figured out the auroral green was due
to oxygen.
-
- The green light comes from a so-called
"forbidden" transition, which happens when an electron in the oxygen
atom executes an unlikely leap from one orbital pattern to another. (“Forbidden
transitions” are much less probable than allowed ones, which means they take
longer to occur.)
-
- However, even after emitting that green
photon, the oxygen atom finds itself in yet another excited state with no
allowed relaxation. The only escape is via another forbidden transition, from
the ¹D to the ³P state, which emits red light.
-
- This transition is even more forbidden, and
the ¹D state has to survive for about about two minutes before it can finally
break the rules and give off red light. Because it takes so long, the red light
only appears at high altitudes, where the collisions with other atoms and
molecules are scarce.
-
- Also, because there is such a small amount
of oxygen up there, the red light tends to appear only in intense auroras. This is why the red light appears above the
green. While they both originate in forbidden relaxations of oxygen atoms, the
red light is emitted much more slowly and has a higher chance of being
extinguished by collisions with other atoms at lower altitudes.
-
- While green is the most common color to see
in the aurora, and red the second most common, there are also other colors. In
particular, ionized nitrogen molecules (N₂⁺, which are missing one electron and
have a positive electrical charge), can emit blue and red light. This can
produce a magenta hue at low altitudes.
-
- All these colors are visible to the naked
eye if the aurora is bright enough. However, they show up with more intensity
in the camera lens.
-
- There are two reasons for this. First,
cameras have the benefit of a long exposure, which means they can spend more
time collecting light to produce an image than our eyes can. As a result, they
can make a picture in dimmer conditions.
-
- The second is that the color sensors in our
eyes don't work very well in the dark, so we tend to see in black and white in
low light conditions. Cameras don't have this limitation.
-
- When the aurora is bright enough, the colors
are clearly visible to the naked eye.
-
- When a huge solar flare sent a wave of
energetic particles from the sun surging out through space reached Earth, and
people around the world enjoyed the sight of unusually vivid aurora in both
hemispheres.
-
- This spectacular spike in auroral activity
appears to have ended, but the sun is approaching the peak of its 11-year
sunspot cycle, and periods of intense aurora are likely to return over the next
year.
-
- Cameras have the benefit of a long exposure,
which means they can spend more time collecting light to produce an image than
our eyes can. As a result, they can make a picture in dimmer conditions.
-
- We tend to see in black and white in low
light conditions. Cameras don't have this limitation. But, when the aurora is bright enough, the
colors are clearly visible to the naked eye.
-
-
May 14, 2024 NORTHERN
LIGHTS - May, 2024 4469
------------------------------------------------------------------------------------------
-------- Comments appreciated and Pass it on to
whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Wednesday, May 15, 2024
---------------------------------
No comments:
Post a Comment