Saturday, May 11, 2024

Aurora Lights Up Skies Friday Night In The Wake of Most Energetic Solar Storm In 21 Years

 

                              Aurora seen Friday night from Brunswick, Maine

                         Aurora visible from Boulder, Colorado Friday night.
                             Aurora dances over Lambeau Field, Green Bay WI
                      Green aurora we saw from Fairbanks, AK in March 2005

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 10 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 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 Earths 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:

And:

Ensemble Modeling of Coronal Mass Ejections - A Superior Means Of Prediction? The Jury Is Still Out (brane-space.blogspot.com)

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