This answer to When a coronal mass ejection (CME) hits a spacecraft, from which direction will the particles come? states:

A single CME will impact a spacecraft from only one direction, but that direction might not be directly from the sun because a CME may zigzag en route.

Looking at that link, the cause of the zig-zagging is still unclear to me

One of the first things they noticed was how CMEs trying to go "up"—out of the plane of the solar system and away from the planets—are turned back down again. Gallagher confesses that they had to "crack the books" and spend some time at the white board to fully understand the phenomenon. In the end, the explanation was simple:

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The sun's global magnetic field, which is shaped like a bar magnet, guides the wayward CMEs back toward the sun's equator. When the clouds reach low latitudes, they get caught up in the solar wind and head out toward the planets—"like a cork bobbing along a river," says Gallagher.

Once a CME is embedded in the solar wind, it can experience significant acceleration. "This is a result of aerodynamic drag," says Byrne. "If the wind is blowing fast enough, it drags the CME along with it—something we actually observed in the STEREO data."

Question: I can't find an explanation here that I can sink my teeth into. Is it possible to explain in a more scientific way? What is the restoring force that would bring a wayward CM back towards the plane exactly? Is the zig-zagging only in the out-of-plane direction?

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    $\begingroup$ Layman pondering here: Maybe "zigzag" is the misleading, non-scientific thing here? It might imply that particles move "left, right, left, right, ..." which is probably not what they've found. To my understanding, what they describe is just the fact, that the charged particles will move along the magnetic field lines of the sun. Some of them may eventually bend back towards the sun. And since the particles move well be low c and the magnetic field gets twisted with their own rotation, a particle moving along that field line may eventually move in the opposite direction. $\endgroup$ Commented Dec 14, 2019 at 18:44
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    $\begingroup$ Consider the particles moving along this magnetic field: en.wikipedia.org/wiki/File:Heliospheric-current-sheet.gif $\endgroup$ Commented Dec 14, 2019 at 18:48
  • $\begingroup$ The paper is open sauce: nature.com/articles/ncomms1077 starts reading :-) $\endgroup$
    – user31179
    Commented Dec 14, 2019 at 21:09
  • $\begingroup$ nasa.gov/mission_pages/sunearth/news/News080411-dblpunch.html has a nice animation where I fail to see any "zig-zag" of a real CME. Scanning through the original work I got the impression that it is still wave-fronts. I.a.w. @try-catch-finally's comments seem to be the answer, or at least the skeleton of an answer. $\endgroup$
    – B--rian
    Commented Mar 17, 2021 at 9:15

1 Answer 1


Why do CME's “zig-zag” on their way to Earth?

They don't. Coronal mass ejections (CMEs) do not jump back-and-forth as they propagate outward. Their general shape does get distorted due to mass loading near the ecliptic (i.e., where the solar wind tends to be slower and denser) and different magnetic field orientations and strengths (e.g., see the following studies and references therein: 10.1007/s11207-018-1247-z, 10.1007/s11207-019-1477-8, and 10.3847/0004-637X/823/1/27). There is also some discussion of CME distortions (and useful references) in a recent review paper by Wilson et al. [2021].

Is it possible to explain in a more scientific way? What is the restoring force that would bring a wayward CM back towards the plane exactly? Is the zig-zagging only in the out-of-plane direction?

CMEs care about two primary effects: hydrodynamics and electromagnetics. The hydrodynamic forces on a CME are mostly due to it overtaking the already present solar wind. On average, the solar wind is slower and more density near the ecliptic plane than at high ecliptic latitudes. Thus, the shock wave in front of the CME will tend to develop of curved surface (e.g., see Koval and Szabo [2010]).

The magnetic fields within the CME are still connected to the sun because fields do not start or stop, right? Magnetic fields also experience effects similar to what we call tension and pressure. The fields within a CME tend to be twisted but they do usually have an overall, smooth variation on large scales. However, if that magnetic obstacle that is the core of the CME runs into the tangled mess of the solar wind, the latter can exert forces on the CME in the form of magnetic pressure. Further, magnetic reconnection within the CME can also affect the tension and field geometry of the structure as it propagates.

All of these things make the phenomena rather dynamic.

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    $\begingroup$ "rather dynamic" feels like an understatement $\endgroup$
    – uhoh
    Commented Aug 31, 2021 at 23:19

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