I’m Trying to Predict the Next Galactic Current Sheet Crossing
This project started when I was thinking about what triggers Ben Davidson’s (@SunWeatherMan / Twitter) solar micronova: a Galactic Current Sheet Crossing (GCS) at the heliosphere. I want to predict when the Sun will intersect the GCS. And, I want to build the Galactic structure from astronomical observations first, then calculate where the Sun and that surface intersect.
That means we need two things:
1. A physical surface we can locate in 3D space.
2. Some idea of how that surface moves over time.
We now have the first real version of that model.
The observational data
So far, the model uses pulsars, galactic dust, the Radcliffe Wave, and several models of the free electrons in the Milky Way.
1,892 pulsars 79 Radcliffe gas clouds 125 young stellar clusters 1,500 points along the published Radcliffe Wave We’re also using a large 3D dust map of the local Galaxy and three independent electron-density models:
YMW16 NE2001 NE2025 The pulsars are nice because their dispersion measure and rotation measure give us information about both the electrons and magnetic field along the path between us and the pulsar. The basic quantities are:
DM = ∫ nₑ ds RM = 0.812 ∫ nₑ B∥ ds DM tells us about the electrons along the path, and RM also carries information about the magnetic field.
What kind of surface are we testing?
We’re testing a specific idea: a corrugated surface that waves above and below the Galactic plane. The current geometric model is:
S(R,φ,z) = z - zₛ(R,φ) The surface itself is where:
S = 0 Its shape is described by:
zₛ = z₀ + gR(R-R₀) + A₀ sin[m(φ-φ₀) + kR(R-R₀)] The current exploratory fit gives:
z₀ = -0.022 kpc A₀ = 0.103 kpc m = 1 kR = 6.30 rad/kpc So the model is finding a large scale, single wave corrugation with an amplitude of about:
0.103 kpc ≈ 103 pc That geometry is real, but we haven’t detected a magnetic reversal yet. Our model should separate magnetic regions with different polarity. We modeled that with:
B = Bsep tanh(dS/δB) + Bguide But in the current best exploratory solution:
Bsep = 0 μG while:
Bguide ≈ 0.88 μG This means the data are fitting a guide field without resolving a magnetic sign reversal. We can model a corrugated structure, but we can’t yet say that we’ve detected a Galactic Current Sheet.
What about the Parker like winding?
We also tested whether the corrugation could behave like a wound, moving structure.
The model gives:
Ωp = 0.0646 rad/Myr and:
vadv ≈ 10 km/s with the ratio:
Ωp / vadv = 6.30 rad/kpc That matches the radial phase of the static surface. The moving model fits the Radcliffe vertical motion data better than the static version, but the individual motion parameters are still only partly constrained.
Where is the Sun relative to the fitted surface?
I calculated the Sun’s present signed distance from the exploratory surface:
dSun = -0.0488 kpc or roughly:
49 pc with a current uncertainty range of about:
28–69 pc That’s interesting geometrically, but not yet a prediction of an intersection.
So where are we?
Right now, the simplest smooth Galactic model still fits the total data slightly better than the corrugated GCS model. The evidence difference is weak though, and the corrugated geometry itself is worth continuing to test. The biggest missing piece is magnetic. We need stronger independent constraints from stellar polarization, Planck polarization and extragalactic rotation measures to see whether the proposed surface really separates magnetic sectors. Once the geometry and motion are better constrained, the next step is straightforward:
S[r☉(t), t] = 0 This will tell us when the Sun’s trajectory will cross the modeled surface. And, that my goal of this project… predict when the next intersection will be.
