Tuesday, July 18, 2017

How Changing Solar Magnetic Fields Complicate Space Weather Forecasting

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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 ionsopheric 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  {òv  B2/2m  dV} = 1/m  òv div[(v  X B) X B] dV 

 -   òv  {han | Jms |2 }dV       

where the first term on the right side embodies (loop) footpoint motion, and the second, joule dissipation, but with Jms 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)   lt  / t!, where theoretically the Poisson mean rate of occurrence is: lm =   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.
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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.

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