Selected Questions-Answers From All Experts Astronomy Forum (The Nature of the Aurora)

Question: I am interested in learning more about the aurora, what causes it and also what gives rise to the red and green displays.  Any insights here would be much appreciated! - Sally, Toronto

Answer:

To understand the aurora, one can visualize the Earth as a giant spherical magnet, with magnetic  field lines extending from the north to south (magnetic) pole.


 These  magnetic field lines have the property that any charged particles (+ protons, - electron or ions), say that are expelled from the Sun,  that approach, will spiral along them.  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. (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.  The dumbbell -shaped region,  in which the incoming particles become trapped and ultimately - via acceleration -  generate the aurora, is known as the magnetosphere.

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".  The Aurora Borealis, for example, which I observed and photographed north of Fairbanks, Alaska in 2005, e.g.

manifested as part of the huge ring known as the auroral oval, located above the Earth's Geomagnetic North pole (which is not the same as the geographical north pole).   In this region, 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. Hence, we get the green aurora. 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 also 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 two 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 can attain magnitudes of  10⁶  A  (amperes)or more. To give an example, during a quiet Sun interval  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.

Two excellent books to get, to learn much more about the physical nature and dynamics of the aurora,  are:

'Exploring the Secrets of the Aurora' - by Syun Akasofu

https://www.amazon.com/Exploring-Secrets-Astrophysics-Science-Library/dp/0387450947

And: 

'Storms in Space' - By John W. Freeman

Prof. Akasofu's text is more comprehensive  and written at an academic level, but well worth the effort if you can manage it. Freeman's book is at a more popular level, but I'd recommend it before going to Akasofu's.

Are Monster Solar Flare Disruptions On the Way?

The news that a monster -sized sunspot group (see photo) is en route to the solar meridian and likely to generate enormous magnetic storms on Earth, now has most of the solar physics community's attention, as well as space weather forecasters. This attention is crucial because it enables us to not only forecast the next large sudden ionospheric disturbances (SIDs) but also auroral displays, and even possible electric power grid failures (such as occurred in Ottawa, Canada in March, 1989, when a monster flare precipitated an inductive energy discharge that melted transformer wires and wreaked havoc).

The current large spot group, located in active region AR 1339, is as big across as 17 Earth diameters, has already triggered a class X1.9 at 20:27 UTC on Nov. 3. If current forecasts hold, even more x-ray flares will be generated over Tuesday and Wednesday this week.

What indicators if any presage these eruptions?

In my early studies of the relation of sunspot morphology to powerful, geo-effective flares during Solar cycle 20, I found that a large magnetic shear was one major indicator if the photospheric neutral line (PNL, or the defined geometric line that divides magnetic polarities) was sharply skewed so the magnetic gradient attained a certain threshold value.

The magnetic gradient is defined:

grad B = [+B_n - (-B_n)] / x

where the numerator denotes the difference in the normal components of the photospheric magnetic field (between opposite polarities of the active region) as measured by vector magnetograph, and the denominator is the scale separation (x) between them. If I calculate grad B = 0.1 Gauss/km then I know a flare is 96% probable within 24 hrs. In nearly all such cases, one also finds the magnetic class of the sunspot group to be 'delta', or having the topology most likely to trigger flares.

Subsequent to my own study others have shed light on other indicators, or what might be needed to go from necessary conditions for a solar flare, to sufficient ones. (Recall here that necessary conditions are defined such that in their absence NO event can occur, while sufficient conditions are defined such that IF they apply the event MUST occur!)

For example one subsequent study investigated a relationship between solar eruptive events and the existence of the coronal zeroth points (e.g. magnetic null points) of the B-field, using a collection of over 1800 vector magnetograms. Each of them was subjected to a charge topology analysis, including determining the presence of coronal zeroth points. Note that in such analyses the foot points of magnetic loops (of which spots are the cross sections seen in our line of sight) are stipulated as “poles” that are presumed to bear a homologous relation to “magnetic charges”

For instance, in one "quadri-polar" analysis by Somov et al (1998) a series of magnetic charges were specified as:

(+B) <-> e (N)

(-C) <-> e (S)

+A <-> e (n)

-A <-> e (s)

Where the capital letters on the left denote the identified loop foot points and the other 'e' identifiers on the right specify the respective magnetic "charges or polarity centers.

It appeared from such analyses that the majority of events over cycle 20 originated in ARs (active regions) above which no zeroth point (a magnetic null) was found. However a much larger fraction of ARs, for which a coronal zeroth point was found, were the source of an eruption than ARs for which no zeroth point was found.

Clearly the presence of a coronal zeroth point is an indication that an AR is more likely to produce an eruption, as 35 % of the ARs for which such a point was found produced eruptions, compared withonly 13 % of ARs for which no zeroth point was found.

We will wait for the magnetograph data on the current large group and hopefully, soon be able to predict when its major magnetic storm effects will manifest, if not exactly where!

The Amazing Aurora

In Alaska in March, 2005, I took the above photo of an auroral display just outside Chena Hot Springs, Alaska. I was glad my wife was with me to behold the sight (her first time) since it later evolved to an exciting electronic scene with bursts of red and green interacting and changing positions.

How are these magnificent displays caused?

One can visualize the Earth as a giant spherical magnet, with magnetic
field lines extending from its north to south magnetic poles. These
magnetic field lines, have the property that any charged particles (+
protons, - electron or ions) that approach, will spiral along them.

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. (A hot, gaseous envelope that spews these particles out continuously – moreso 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 high 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 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. The aurora or northern
lights we see displays a kind of green curtain-like shimmering. 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. (Though there have been reports
of N. lights being seen as far south as northern Florida, especially
during periods of esxceptional sunspot activity or LARGE SOLAR FLARES,
massive explosions on the Sun).

A great analogy has been given – by Syun Akasofu- comparing the aurora to images 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 TV. The electron beam in the TV (remember we are talking about the old-style cathode ray jobs!) 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 10^6 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.

If intense enough, such solar storms can herald the onset of enormous induction currents such as caused parts of the Ottawa grid to melt down in 1989.

But, as solar cycle 23 slowly ramps up, the aurora - of whatever color or shape- will be eagerly anticipated.