The Aurora - The Linchpin Of Space Physics

With today commencing the recognition of World Space Week (Oct 4- 10) I plan to post space-related blogs each day on different space-related topics.


In Alaska in March, 2005, I took the above photograph 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.  An even more impressive image is shown below:




While the aurora is a fantastic sight, few may realize that it also has served as a spring board to detailed studies of near Earth plasma - treated as magnetic storms. But mote on this in a bit.

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, e.g.


These magnetic field lines have the property that any charged particles (+protons, - electron or ions) that approach from the Sun, 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 – moresso 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.

In light of the above, it would be no exaggeration to say the aurora is the linchpin for the discipline of space physics. Generically, "space physics" refers to the study of plasmas that occur in the region of Earth's atmosphere. The most accessible (and visible) such plasma we know of is the aurora .

While computer simulations and numerical analysis have dominated auroral studies, such as at the Geophysical Institute of the University of Alaska, there are also space-based techniques which have provided significant data to refine the simulations as well as auroral- magnetic substorm models. At the University of Alaska - Fairbanks, for example, the Poker Flat rocket range serves as a critically important means to acquire plasma data from the plasma itself.  The image below shows the team assembled for one of the missions:

Readers interested in many more details of the Poker Flat rocket operations to do with auroral studies  can go to this link:

http://www.pfrr.alaska.edu/

Or, get  hold of the book- 'Rockets Over Alaska' -  by Neil Davis:


A less well known mode of auroral study entails injecting electrons artificially using a space -borne accelerator.  Few are aware that NASA has previously flown devices that have fired beams with energies of a few to tens of kiloelectron volts (keV).  In a new study, Marshall et al (Journal of Geophysical Research, Space Physics, 2014) have used computer simulations to explore the capabilities of a small but powerful particle accelerator positioned at an altitude of 300 km and aimed at the atmosphere.

This space-borne accelerator would feature energies three orders of magnitude greater than the actual ones already used by NASA. Thus, it would be capable of generating a beam with particles in the 0.5 - 10 MeV range via pulses lasting 0.1 microsecond and carrying 0.1 amp of current. The simulated specs for the device are based on the Compact Accelerator for Space Science (COMPASS) currently being designed by the SLAC National Accelerator Laboratory and the nonprofit research institute SRI international.

The authors note that deploying such a device would offer many scientific opportunities to study the behavior of particles in the upper atmosphere. We'd see, for example, the paths of injected electrons trace out the arcing field lines of Earth's magnetic field - illustrating how they bend and snap during intense geomagnetic activity caused by magnetic storms.

We'd also see how visible, aurora-like emissions would be generated as the particles descend into the Earth's atmosphere and strike atmospheric molecules. Make no mistake this is exciting stuff especially for those involved in space physics. If such a particle accelerator could shed more light on magnetic substorms it would be more than welcome.

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 ).

One of the most intriguing discoveries of recent decades has been the identification of auroras on Jupiter.  A major difference is that unlike Earth's auroras, those on Jupiter are nearly continuous - driven by the planet's rapid rotation and its volcanic moon (Io),  which spews out sulfur and oxygen ions as well as electrons into space, The latter speed along the planet's magnetic field lines and - if powerful enough - slam into the atmosphere causing its particles to glow.

One of the highly enhanced images of Jupiter's aurora is visible in the Hubble Space Telescope photograph below;

Only recently Tao et al (Journal of Geophysical Research -Space Physics, Oct. 2015, p. 1002), using the Japanese Hisaki Space Telescope, measured variations in the brightness of the Jovian aurorae. They reported observing two kinds of auroral pulses. In one, the aurora brightened for up to several days at a time, and the authors attributed this to the solar wind. Thus, as it 'washes over' the planet, the charged particles are buffeted and compress Jupiter's magnetic field. This is very similar to what happens on Earth.

The team also observed much more rapid variations, with pulses typically lasting less than ten hours. By comparing the Hisaki images with images taken simultaneously by the Hubble Space Telescope, Tao et al could see these variations arose from the aurora brightening at lower altitudes, at the bottom of the auroral arc and as reported by Kimura at al (op. cit.)

Using Hisaki's onboard spectrometer, the Tao team was also able to estimate how fast the electrons were traveling on the basis of how deep in the atmosphere the light originated. They found that when the aurora flares up it's not because faster, more energetic electrons are penetrating deeper into the atmosphere. It is instead due to the overall increase in the number of electrons.

All of this indicates that Jupiter's most intense auroras occur when plasma is suddenly injected into its magnetic field, most likely from Io.

The aurora is a fantastic phenomenon for detailed plasma study that has not only aroused awe in Earthbound observers, but prompted the timely development of sophisticated space-based methods of detection.

Solar and Space Physics Students Being Left Out In The Cold

Giving a course in basic space physics ca. 1988.

The report appearing in Eos - Earth and Space Science News (15 July) was distressing to say the least. It noted the current extreme difficulty of newly minted Ph.D.s in solar and space physics getting full time academic research positions.  And despite the critical role space and solar physics plays in our daily lives - from the interference of large solar flares with airplane navigation systems to the solar wind's interaction with the magnetosphere (see image) to coronal mass ejections, solar and space physics remain relatively tiny enterprises.

Indeed, according to the report, only 50 to 70 students in these disciplines graduate per year in North America. This is literally pathetic given that we need many more of such specialists, over say more investment bankers or Wall Street brokers.

The cruel facts? Despite the National Research Council's finding that nearly all of these grads desire research positions, only about a third find them - and even those don't last for long (especially if the person fails to get tenure.)

A lot of this has to do with the rise of temporary faculty in many universities, These adjunct professors are now approaching nearly half of the total faculty population. They are only paid per credit hour, have zero financial security and other benefits. Instead they  must cobble together enough hours to forge a  decent career. Even then they must often fall back on food stamps, e.g.

http://www.salon.com/2014/09/21/professors_on_food_stamps_the_shocking_true_story_of_academia_in_2014/

This has to make any old line  or retired academician wince given the entire purpose of getting to graduate school and hopefully gaining the coveted Ph.D. was to finally ensure financial security. Not necessarily to get rich, but just be financially secure to the point you don't have to depend on government handouts.

The other aspect, of course, is grant funding for research projects. If such money isn't available, and it generally comes from the government, then the 'well' of that academic department will dry up .

The sad fact today for the aspiring Ph.D. is that even if they make it through the sundry  hurdles there are no assurances of glory...or academic jobs. As I noted in earlier posts, the newly minted Ph.D. must compete with postdocs as well as adjunct lecturers and already ensconced TAs (and RAs!). Since most universities (especially after the credit meltdown) are on tight academic dept.  budgets (most of the $$ going for jazzed up dorms and athletic centers), it makes more sense to hire more adjuncts or keep postdocs longer than to bring in full time Ph.D.s. A potential to generate research funding is the main item which may elicit a university to look more closely at a prospect, but even here those prospects suffer because of the current impetus toward congressional budget cuts - given the size of the deficit.


Still, two authors cited in the report - Moldwin and Morrow ('Space Weather', 2016) found that success was more likely for a grad who: a) published a full paper within a year of submitting their dissertation, and b) who continued to publish for at least 4 more years.  They also found that 'there was no relationship between department size and a Ph.D.'s likelihood of finding a research career".

Nonetheless the pair still found "large differences between individual institutions" This is understandable because in many schools space and solar physics will have relatively small or poorly funded departments and the subjects may be viewed as too irrelevant or obscure to attach importance.  Indeed, the authors recommend "investigating financial support models, admissions policies and qualifying exams as well as department culture"..

I was fortunate in my solar physics research as the Barbados government paid for a full four years of it, since the importance of solar flares and their geo-effective influence was grasped. Others without such funding didn't fare as well. It was that funding that enabled me to publish multiple papers even before the dissertation was completed, see e.g.

http://brane-space.blogspot.com/2016/06/establishing-physics-identity.html

One hopes that these recent graduates will eventually be able to find needed funding, grant money to allow them to achieve successful careers in their chosen fields. A necessary component of that is not attaining a position that translates into becoming food stamp dependents.

In the meantime, those of us fortunate to have had such careers - even for a decade or so - must now share our insights into solar and space physics to make the taxpaying public more aware of their importance.

A New Model for Magnetic Substorms Shows Promise

Regions of the magnetosphere of Earth. Our planet is situated at the nexus of the two blue lobes to the left.

Space physics, alas, is not something familiar to most people who may confuse it with solar physics, or shooting rockets into space. In fact, space physics primarily involves the study of near Earth plasma particularly in the magnetosphere - which is shown in the diagram above. Basically, the magnetosphere can be considered to be the region of plasma influenced by the magnetic field of Earth.  It is the region within which all -any charged particles proximate to our planet are susceptible to that B-field.

In the diagram shown, Earth is situated at the midpoint of the two blue lobes on the left side. The immediate magnetic field is indicated by the first several outer lobes - much as you would expect to see if tracing the magnetic field lines of a bar magnet.  As can be seen, the magnetosphere is short and squat on the side of Earth that faces the Sun- the "dayside" - but features a long tail extending away on the "night side".

This "magnetotail" is shaped primarily by the powerful force of the solar wind and the energetic plasma flow within it. Here on Earth, at certain times of the year and at specific places we observe the effects of this flow in the form of the aurora, such as we photographed in Chena Hot Springs, Alaska in 2005 e.g.


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 the 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- in what we call "substorms" -  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. This  is basically what gives rise to the aurora.

Now in Eos Transactions, Vol. 96, No. 1, January  15, 2015, p. 27), it is reported that  original work published by Pritchett et al  in The Journal of Geophyical Research, it's proposed that substorms may arise from Earth-sized bubbles of low density plasma that "ride" magnetic field lines through the magnetotail toward Earth.

Using a computer model simulating how charged particles behave in magnetic fields they showed these bubbles preferentially form in parts of the magnetotail where entropy increases with distance from Earth Thus the properties of the bubbles and the "auroral streamers" they send earthward change as they move through the magnetic field . Eventually this triggers a full blown substorm.

The model is useful in that it helps to explain the link between the magnetic perturbations that seem to accompany substorms (usually covered under one of two distinct energy paradigms, the B-v (driven) and E-J ("unloading") and auroral streamers. What's encouraging is that their predictions also match observations of the structure and behavior of auroral streamers made by the THEMIS satellites (THEMIS for 'Time History of Events and Macroscale Interactions During Substorms'.)