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EI2GYB > ASTRO 14.10.25 13:03z 65 Lines 8276 Bytes #28 (0) @ WW
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Subj: Simulating Complex Coronal Mass Ejections Shows A Weakness
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Simulating Complex Coronal Mass Ejections Shows A Weakness In Space Weather Forecasting
Avoiding, or at least limiting the damage from, geomagnetic storms is one of the most compelling arguments for why we should pay attention to space. Strong solar storms can have an impact on everything from air traffic to farming, and we ignore them at our own peril and cost. Despite that threat, the tools that we have applied to tracking and analyzing them have been relatively primitive. Both simulations and the physical hardware devoted to it require an upgrade if we are to accurately assess the threat a solar storm poses. As a first step, a new paper from a group led by researchers at the University of Michigan created a much more detailed simulation that shows how important it is that we also have the appropriate sensing hardware in place to detect these storms as they happen.
The paper, which was published in The Astrophysical Journal, simulates Coronal Mass Ejections (CMEs) in much greater detail. It has been known for a long time that CMEs have a massive impact on "space weather". But the CMEs themselves are massive, and simulations that try to understand them have focused on them as monoliths, with similar magnetic stresses throughout their multiple million miles of material.
That monolithic structure flies in the face of data we have collected on CMEs themselves, which show them to be much more complex than their simulations. Trying to figure out how that complexity arises has been one of the central focuses of CME research in recent years. The researchers think their simulation shows how - using another solar structure known as a Corotating Interaction Region (CIR).
CIRs form when a fast stream of solar wind hits a slower stream that had been released earlier. Many times faster streams leave the Sun through what are known as "coronal holes", where magnetic field lines allow faster streams to pass through unimpeded. The brighter parts of the Sun emit the slower streams of solar wind. Where they collide, the plasma and magnetic fields both streams are made up of are compressed into a CIR. Typically this interaction happens even farther out that Earth's orbit, but the structures of CIRs are relatively permanent, and absolutely huge. They also co-rotate with the streams that created them, which were originally co-rotating with the Sun itself - hence the name "Corotating".
CME's often his CIRs as they are on their way away from the Sun, and that is where they start to break down into more complex structures. The simulation described in the paper breaks it down into four distinct steps.
First, the CME erupts from the Sun and runs directly into a CIR ahead on its path. Since the CIR is much slower and denser than the material in the CME, the leading edge of the CME starts to form a "cusp", like water would if it was slowly running into a barrier of mud.
Fraser discusses how bad solar storms can get.
But since this is plasma rather than water, the next step is that the cusp gets compressed by the material coming up behind it, eventually being constricted to a point where it creates a "neck". The leading part of the CME is then magnetically isolated from the rest of the material coming up behind it, creating what is known as a "bifurcating current sheet", where the magnetic field of the plasma changes abrupting, and thereby storing a larger amount of electromagnetic energy.
Magnetic fields don't like to hold a lot of energy when there is a lower energy, more stable state to be had. So eventually, during the third step of the simulation, the magnetic fields in the current sheet "break" and then rearrange into a more stable configuration, releasing the energy stored up in the current sheet. At the same time they create smaller structures called "flux ropes", which is one of the primary components of the more complex CME models seen in observational data.
In the fourth step, these newly born "flux ropes" are caught between the shock waves created by the magnetic reconnection, allowing them to maintain a level of stability as they continue to travel on towards the Earth. Other complex interactions take place on this scale, but the flux ropes, which themselves can still be millions of miles long, are the main component of the more "complex" version of a CME that is more useful for space weather prediction.
The paper's main contribution to this effort is applying huge computational resources to modeling these four distinct steps. The authors used over 260 million individual cells to allow unprecedented spatial and temporal resolution of this extremely complex process. That level of spatial resolution is what allowed them to see the formation of flux ropes in simulation for the first time as well.
Ultimately, simulations are only as good as the data used to build them, and one of the central arguments of the paper is that we need better data. The orientation of the flux ropes created as part of this process can be different from the CMEs they are created from, and could be completely missed by the single-source observational satellites typically used to monitor space weather. Missing a particularly bad flux rope could cause billions of dollars in damages, so it's worth the investment to make sure we can track these more complex structures properly.
Their answer is the Space Weather Investigation Frontier (SWIFT) mission. This mission would use a set of four probes - three offset in a triangle formation in a plane around the Earth-Sun L1 point. A fourth would be located closer to the Sun along the path between the Sun and L1. Since it would require more stationkeeping than the three at the relatively stable Lagrange point, it would be equipped with a solar sail, similar to that on the Solar Cruiser mission, that would allow it to maintain position without having to burn through fuel. This configuration would ensure that we would catch all potentially hazardous flux ropes, no matter their orientation, as well as offer earlier warning from the forward-positioned solar sail satellite.
SWIFT has yet to receive the necessary funding to get it to launch, but this paper makes an even stronger case for why we need better hardware when trying to study CMEs. If we want to make sure that a potentially catastrophic space weather event doesn't happen as we continually expand even more of our infrastructure to a place where it could, then we should strongly consider doing a better job of both monitoring and simulating the worst of it.
Learn More:
University of Michigan / Phys.org - We need a solar sail probe to detect space tornadoes earlier, researchers say
M. B. Manchester IV et al. - High-resolution Simulation of Coronal Mass Ejection-Corotating Interaction Region Interactions: Mesoscale Solar Wind Structure Formation Observable by the SWIFT Constellation
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