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EI2GYB > ASTRO 18.10.25 10:05l 146 Lines 8276 Bytes #23 (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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