Presets
Playback
Solar wind
Speed and proton density of the wind at the Earth. Together they set the dynamic pressure that squeezes the magnetosphere.
Interplanetary magnetic field
Southward (negative) Bz reconnects with the Earth field; By rotates the clock angle.
Layers
Model checks
The science
Magnetopause: pressure balance
The solar wind pushes with dynamic pressure P = mp n v2 (about 1.3 nPa for 5 cm-3 at 400 km/s). The Earth's dipole field B = B0(RE/r)3 with B0 = 31 µT pushes back with the magnetic pressure of the compressed field, (2B)2/2μ0. Equating them gives the Chapman-Ferraro standoff r/RE = (2B02/μ0P)1/6, about 10 Earth radii. Because of the one-sixth power the boundary moves little even when the pressure changes a lot.
Magnetopause shape
The drawn boundary uses the empirical Shue et al. (1998) form r = r0(2/(1+cosθ))α, with r0 and α depending on pressure and on Bz: a strongly southward Bz erodes the dayside and shifts the boundary inward.
Reconnection and the oval
A southward IMF reconnects with the Earth's northward field at the nose; the convection electric field is E = v Bs (v = 400 km/s and Bz = -5 nT give 2 mV/m). The Newell et al. coupling function dΦ/dt = v4/3 BT2/3 sin8/3(θc/2) includes the clock angle θc. A dipole field line from invariant latitude Λ crosses the equator at L = 1/cos2Λ, so the oval moving equatorward means reconnected flux reaching ever smaller L. Emission colour follows altitude: nitrogen blue-violet below 100 km, green atomic oxygen 100-150 km, red above about 200 km.
Honest simplifications
The model Kp and the oval latitude use the same simple formulas as the 3D page (solar wind speed v/100 as its intensity, then Kp from the southward component), not an operational forecast. The dipole is not tilted, the tail field is not stretched, the flow past the boundary is a 2D potential flow, and the cusp is drawn at a fixed 78 degrees. The extreme preset is illustrative, not a reconstruction of any real storm.
What to try
- Pick Quiet solar wind: northward Bz, a low Kp, and an oval far toward the pole.
- Pick Southward Bz storm: watch the oval latitude fall, the orange funnel fill up and the standoff shrink.
- Raise the density at fixed speed: the red pressure-balance marker and the yellow Shue nose move inward together.
- Move the emission altitude from 90 km to 120 km to 250 km and the curtain turns blue, green, then red.