New Research Enables Space Physicists To Gain Insights Into Heliosphere

For some time  space physicists and solar physicists have pondered the hypothetical properties of the heliosphere - the protective bubble created by the solar wind and depicted in the image above.  Understanding the physics at the bubble's edge- called the heliosheath -  is not easy.  This is given it's in constant flux and pushes out against the broader interstellar magnetic field that permeates our corner of the Milky Way.

For reference, the heliosheath occurs at the far edge of the heliosphere. More technically, it occurs between the "termination shock" and the "heliopause".    The heliopause  is the interface where the solar wind is stopped by the interstellar medium. One can therefore think of it as a three dimensional region or surrounding “envelope” at which the solar wind's strength is no longer sufficient to overcome the stellar winds of the external stars. In technical terms, this absence of counter-pressure signals the end of the solar system.   Thanks to data from the Voyager 1 and Voyager 2 spacecraft,  we now know the outer boundary of the heliosheath  is located roughly 18 billion kilometers from the Sun. Or 119 times the distance from the Earth to the Sun -  right where Voyager 2 found it in November, 2018.

Now,  Dialynas et al.  have combined Voyager data with observations from NASA's Cassini mission - which orbited Saturn from 2004 to 2017- to  gain much more insight.  Basically, the researchers recognized that the missions, although launched 20 years apart, had collected complementary data. Voyager 1 and 2 had instruments that measured energetic ions as the craft crossed the heliosheath and exited the solar system. Cassini, meanwhile, was able to remotely observe energetic neutral atoms  arriving in all directions from the heliosheath.


The energetic neutral atoms come from the heliosheath, where fast solar wind protons collide with neutral hydrogen atoms from interstellar space and “steal” an electron from the interlopers. The Voyager probes took in situ measurements of the parent heliosheath proton distributions as they passed through this region. Meanwhile, the protons with newly added electrons become energetic neutral atoms and shoot off in all directions.

The synergy among the spacecrafts’ observations allowed the researchers to use Voyager data from the heliosheath to ground transmissions and thereby calibrate energetic neutral atom data from Cassini, which was more sensitive to lower energetic particles than Voyager. Together, the spacecraft extended data on the intensity of both energetic neutral atoms and ions to include a broader range of energies, which gave the team a window into the physics in the heliosheath as the solar wind and interstellar medium press against each other.

The researchers found that in the energy range considered in their study (>5 kiloelectron volts), lower-energy ions with energies between about 5 and 24 kiloelectron volts played the largest role in maintaining the pressure balance inside the heliosheath. This allowed the team to calculate the strength of the magnetic field and the density of neutral hydrogen atoms in interstellar space—about 0.5 nanotesla and 0.12 per cubic centimeter, respectively.

Their finding that the lower-energy ions dominate the pressure balance in the heliosheath means that space physicists will have to rethink their assumptions about the energy distribution of such particles in the heliosheath..  See e.g. Geophysical Research Letters, https://doi.org/10.1029/2019GL083924, 2019.

Interestingly, pressure balance considerations also entered in early models and computations to do with another part of the heliosphere:  the solar corona .   In the past century an interesting question was whether the corona was static or not. In a static case its boundary would be more or less fixed, so there'd be no expansion even in times of high solar activity.  For this to occur there would need to be a consistent pressure balance, i.e. between the outward coronal  (& solar wind) pressure and inner directed pressure, from the interstellar medium.

 A static corona  superficially appeared  to be quite reasonable.  And so it was that the father of space physics,  Sydney Chapman,  first assumed a condition for hydrostatic equilibrium applied:

dp/ dr = - r {GM_s/ r²}

Where G is the usual Newtonian gravitational constant, and r defines the plasma density for the corona, with n the number density for protons, e.g. 

r = n(m_p) 

while M_s is the mass of the Sun, and r the distance from the solar center:

The coronal pressure (p) is given by:

p = 2 n T

Provided both protons and electrons are assumed to have the same temperature.

Ultimately, detailed computations showed the static corona model could not be accurate.   If the static model were accurate, the pressure at infinity, i.e.

p(¥) = p(R_o) [exp – 7k/5 * 1/ T(R_o) R_o] 

 Should be zero  (p(¥)   = 0) , not a small finite pressure that’s effectively equal to the coronal base pressure. This finding led to the further investigations that disclosed a solar “wind” had to flow outwards from the corona. 

See also:

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068607/full


And:

New Research Into The 'Slow' Solar Wind Sheds Ligh...

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.

How Changing Solar Magnetic Fields Complicate Space Weather Forecasting

Solar magnetic field lines depicted above based on a model. They are subject to further change (twisting, looping) on encountering near Earth space. This results in errors in space weather forecasts.

As noted in previous blog posts, space weather is the term which embodies all manner of phenomena that impact the Earth or its magnetosphere including magnetic substorms, sudden ionospheric disturbances (SIDs), and CMEs or coronal mass ejections. Each of these merits forecasts but the last is particularly critical in terms of priority.  Powerful CMEs of such magnitude that they merit the name "Carrington events" and originate at the solar central meridian (relative to Earth observers) are events we wish to avoid. Even a glancing blow from a CME has the potential to knock out one or more power grids such as occurred in Quebec in 1989 after a giant solar flare.

The "ultimate" CME then is that which smacks us broadside, knocking down power grids like tenpins across the side of Earth facing the Sun when it strikes. Five years ago this led to one projection from a University of Colorado astrophysicist that sheds a good deal of insight:

"It’s believed a direct CME hit would have the potential to wipe out communication networks, GPS and electrical grids to cause widespread blackout.......Just 10 minutes without electricity, Internet or communication across the globe is a scary thought, and the effects of this event could last years. It would be chaos and disaster on an epic scale."


Thus, the interest in CMEs and space weather forecasting is not some mere armchair academic obsession but has real world consequences. Even magnetic substorms which spawn SIDs can wreak their own  form of havoc including disruption of short wave and even higher radio band signals, as well as affecting navigation controls on aircraft.

My own research had focused on the origin of SIDs from a specific type of flare identifiable from its soft x-ray signature. This led me to postulate,  in early 1984,   sudden ionospheric disturbance-generating (SID) flares, with the release attendant on a change in initial free magnetic energy (E _m = B²/2m ) given by:

¶ / ¶  t  {ò_v  B²/2m  dV} = 1/m  ò_v div[(v  X B) X B] dV 

 -   ò_v  {h_an | J_ms |² }dV       

where the first term on the right side embodies (loop) footpoint motion, and the second, joule dissipation, but with J_ms the current density at marginal stability – since the marginal stability hypothesis is required for a driven process, and h _an  is the anomalous resistivity. In the same paper, it was shown how the flare distribution corresponds to a Poisson process of the form P(t) =    =   exp (- l)   l^t  / t!, where theoretically the Poisson mean rate of occurrence is: l_m =   l Dt.

Thus tying both SIDs and CMEs together in terms of the magnitude, time and location of the flare that produced each, though in an empirical-statistical context.    As I pointed out in a paper published in The Meudon Solar -Terrestrial Predictions Workshop  e.g.

this was the best one could do  - minus the necessary physical details -  for a valid theoretical model. My paper ‘Limitations of Empirical-Statistical Methods of Solar Flare Prognostication’ appeared on pp. 276-284 of the Proceedings and received much attention from the other contributors - since of course it impacted in multiple ways on their work as well.  Up to the time of the paper (and even beyond) we have been constrained to rely on empirical statistical methods to compensate for the lack of more precise physical, quantitative models.

The project itself saw the input from over two  hundred solar and space physicists covering every aspect of the problem of solar-terrestrial interactions, including: long, medium and short term solar  forecasting, geomagnetic activity and auroral (substorm) forecasts, as well as ionospheric predictions.

Understandably, the more energetic and complex the solar flare the more difficulty in arriving at the prognostication needed.  The sheer diversity of flare morphology, combined with an insufficiency of detailed observations - owing to lack of proper observational tools - adversely affects the degree to which reliable forecasts can be made.  This will be expected to change with the launch of the Solar Probe Plus - renamed the Parker Solar Probe. The probe will  travel to within 4 million miles of the solar surface  (photosphere) and withstand temperatures of up to 2,500 F.  We expect the optical observations to approach the resolution of 0.1 arcsec, which many solar physicists believe is the limit needed to identify the energy release volume in coronal loops. 

Even with greatly enhanced resolution and  the acquisition of other critical data, moving beyond statistical models to wholly physical ones (yielding their own self-consistent aspects) will not be easy.
In the case of CMEs,a theoretical, quantitative strategy would revolve around obtaining the rate of increase of the poloidal magnetic flux   (Φp) associated with a specific flux rope (e.g. that shows kink or other instability) e.g.

dΦp(t )/dt

Then, for a predictive basis one would require the related function be adjusted for each potential CME (dependent on its current heliographic location) that best fits the total observed data. This function would normally be given in terms of the electromotive force associated with the active region so that:

E(t ) ≡ −(1/c)dΦp(t )/dt

Where the preceding would constitute a forecast from the theory for each CME trajectory.  This would be called a "theoretical forecast" say compared to an empirical forecast, i.e. based on analyzing the frequency and intensity of fluctuating microwave bursts over time (say several  Carrington rotations).   In the above case we see that rapid changes in the poloidal magnetic field, B p ,  can throw off theoretical model forecasts.  Hence, the more we can learn about the genesis and maintenance of such localized fields the more the models (and forecasts) can be improved.

Then there is the influence of the much larger scale solar magnetic field. Beyond all the above considerations, space weather forecasting requires understanding what happens when the Earth’s magnetic field meets the Sun’s in space. When their field lines make contact, for example, they can suddenly link up and explosively realign. Like a snapping rubber band, the field lines rebound, sparking geomagnetic storms and sending dangerous radiation toward Earth that can damage satellites and threaten power grids.

However, some conditions are more conducive to this process, called magnetic reconnection. Particularly important is the orientation of the Sun’s magnetic field. Although the Earth’s magnetic field is fixed about its North and South poles, the Sun’s magnetic field is warped throughout space, and the Earth may find itself in a part of the field pointing in a different direction at any given time. The best conditions for magnetic reconnection are when the Sun’s magnetic field is aligned southward, antiparallel to Earth’s.

Recent studies have shown that the direction of the Sun’s field can shift by the time it reaches Earth’s magnetic field, apparently twisting after passing those satellites. This could lead to inaccurate space weather forecasts. To determine why this happens, Turc et al. analyzed archival data for 82 solar storms caused by approaching magnetic clouds ejected by the Sun. The team compared solar wind measurements with data from closer satellites orbiting in and around Earth’s magnetic field and used a model to reconstruct the conditions in between. Their work zeroed in on two factors.

1) The bow shock that the Earth creates in the solar wind. Like a ship plowing through water, the Earth creates a shock wave in the solar wind as it flows past, which the Sun’s field lines must traverse. Turc et al's analysis showed that depending on their relative orientations, the shock could alter the direction of the field.

2) After crossing the bow shock, the solar field lines encounter the Earth’s magnetic field. They don’t simply meet it head-on, but instead  overlap  the Earth’s field, and are warped in the process.

The authors report that these two factors combine to shift the direction of the field, which could alter the probability of magnetic reconnection. In some cases, it even reversed a benign northward field into a reconnection-prone southward field, and vice versa. These reversals spanned roughly 20% of the Earth–Sun magnetic field boundary and lasted over half an hour, making them significant enough to potentially throw off forecasts of geomagnetic storms.

The authors report that their models successfully reproduced the observations roughly 80% of the time. But more work must be done to improve their performance and incorporate them into real-time forecasts.

In the case of SIDs and CMEs the space weather forecasting difficulties are even more formidable. But I am confident that the Parker Solar Probe will finally put us on the path to genuine space weather forecasts for all phenomena that affect the near Earth space environment.