For the first time since 2003, an extreme geomagnetic storm — the most severe of its kind — smacked Earth
on Friday evening. Glorious green, purple and red aurora displays,
also known as the northern lights, were visible across Europe and very low
latitudes in the United States, as far south as Alabama and Florida.
Scientists at the National Oceanic and
Atmospheric Administration (NOAA) also warned of potential disruptions in
satellite and radio communications as well as power grid operations.
Geomagnetic storms are created when an exploding
cloud of plasma – known as a coronal mass ejection (CME) - is unleashed from the
Sun and hurtles toward Earth. The surge of particles and plasma traveling at
500 miles per second shocks Earth’s magnetosphere, resulting in the northern
lights and sometimes technology disruptions. NOAA categorizes geomagnetic
storms on a scale of G1 to G5, where G5 is the most severe.
The agency
anticipated a severe G4 storm, but it has exceeded forecasts. Around 7 p.m.
Eastern time Friday, the storm elevated to the G5 level. The last time a storm
of this extreme severity hit Earth was in October 2003, resulting in power
outages in Sweden and damaged transformers in South Africa. The storm continued
for several hours at varying strengths, ranging from moderate to severe.
The Earth itself is "bathed" in the solar wind, a stream of high speed charged particles that flows into space, originating from the Sun's corona. This stream can be vastly more energetic at the times of CME eruption coinciding with a geomagnetic storm:
A hot, gaseous envelope that spews these particles out continuously – more so when there is a violent explosion known as a Solar Flare) Around the Earth the speed of these particles can reach 400- 500 km/second. (Because of its high temperature, over a million degrees, the corona gas is ionized so must consist of charged particles, mainly (+) protons, and (-) electrons).
During high solar activity (e.g. near sunspot
maximums) a higher flux of these charged particles inundates the solar wind,
and the region around the Earth. The Earth's magnetic field traps these charged
particles, and the highest density is around the polar regions - which we refer
to as the "auroral ovals". In these regions, very large electric
currents are set up, as the charged particles start moving in unison about the
magnetic field lines. These currents can easily reach a few million
amperes.
As this discharge occurs, one or more outer
electrons is stripped from the atoms, for example from oxygen in the atmosphere
- then recombines again - to form new (e.g. oxygen) atoms.
With this recombination - there
is accompanying emission of light, for a certain part of the visible
spectrum For example, in the case of recombination of oxygen atoms
- their emitted light is in the green region of the spectrum, such as seen
in my photograph above. The remarkable red aurora is produced by emission at
the 630 nm (nanometer) line of oxygen and at relatively high altitudes (e.g.
200-600 km) compared to green - which tends to form below 100 km and the oxygen
line at 557 nm is excited.
Auroras can display as both diffuse and discrete. In the first case the shape is ill-defined and the aurora
is believed to be formed from trapped particles originally in the magnetosphere
which then propagate into the lower ionosphere via wave-particle interactions.
Thus, multiple- colored auroras can be explained
by emissions from different atoms in the upper atmosphere, mainly in the region
of the magnetic poles. This is also why, of course, they are more often seen in
the vicinity of the N, S magnetic poles.
A great analogy has been given by Prof. Syun Akasofu- former Director of the Geophysical
Institute in Fairbanks, AK - who compared the generic aurora to
an image on a TV screen. In this case the (polar) upper atmosphere corresponds to the screen and the aurora to
the image that would be projected on it, say for a cathode ray TV. The electron
beam in the TV corresponds the electron beam in the magnetosphere. In the
conventional TV motions of the image are generated by the changing impact
points of the electron beam on the screen. Similarly, with the aurora, its
motions – such as moving sheets or curtains- are produced by moving impact
points of the magnetospheric electron beams.
In gauging the power and intensity of auroras at
different times, it is useful to remember that ultimately the aurora derives
its power and potential from the Sun and specifically the charged particles of
the solar wind. This is why the most spectacular displays are usually near
sunspot maximum. Around those times the currents I noted earlier are “amped” up
– no pun intended- to 106 A or more. To give an example, during a
quiet Sun interval (like we are in now) the residual power for the
magnetospheric generator is on the order of maybe a tenth of a megawatt. If we
see a new cycle coming on and solar wind activated – we may get that power up
to a million megawatts for a few hours.
How does space physics
approach the aurora?
Given the aurora is basically regarded as a
magnetic substorm we use what are known as "substorm models".
In most current ones some dynamo action is required which sends currents to
specific regions to provide a Lorentz force: (J ⊥
X B) where J ⊥
is the vertical current density and B the magnetic induction associated with the
system.. At a particular altitude then, these J ⊥
currents can trigger a substorm. In the proper space physics (magnetospheric)
context, a “neutral
wind” arises from a force associated with the
neutral air of the Earth’s atmosphere This force can be expressed:
F = mU f
where f is the collision frequency.If one solves
for f above, and uses the magnitude of magnetic force (F = qvB) where B is the magnetic induction, and v the velocity one arrives at two horizontal
flows for electrons and ions moving in opposite directions. mU f = qvB = (-e)
vB = (e) vB
Thus,
v1 = mU f / (-e) B and v2 = mU f / (e) B
A terrific articulation of this process can be
found in a paper by Syun-Ichi Akasofu ( Auroral Substorms: Paradigm
Shifts in Research', Eos Transactions, Vol. 91, No. 31, August 3, 2010, p. 269) wherein
the author points out:
"The transfer mechanism of solar wind energy to
the magnetosphere...is now known to be a dynamo process that converts the
kinetic energy of the solar wind to electrical energy on the magnetopause -
because most auroral and geomagnetic phenomena are various manifestations of
energy dissipation processes."
This amounted to a brilliant recognition of the
energy aspects but more work needs to be done, especially in developing
self-consistent models for "loading" and "unloading"
processes. These refer to two distinct energy paradigms, the B-v (driven) and E-J . In the former observations of
the magnetic field and motions govern the models, in the latter it is the
magnitude of currents or current densities (J ).
See Also:
Syun-Ichi Akasofu Lecture Recording, 2023 (youtube.com)
And:
Where to see northern lights this weekend, and what times they will be visible
And:
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