Magnetosphere & Aurora

Noon–midnight cross-section of Earth's magnetic bubble. Thousands of protons and electrons gyrate, bounce between mirror points and drift around the planet; the solar wind pushes and reconnects the field, and the particles that reach the atmosphere light up the aurora.

Storm presets

Quiet Sun: 400 km/s wind, weakly northward field.

Solar wind (drag to override)

Speed 400 km/s Density 5 cm⁻³ IMF B_z +2 nT Time warp 15 min / s

Live readout

Dynamic pressure–

Magnetopause r₀–

Bow shock–

Kp index–

Dst–

Open flux / cap–

Reconnection–

Substorm phase–

Precipitation–impacts / s

Last 24 h of space weather. The dashed line is geostationary orbit (6.6 Re): when the magnetopause crosses it, satellites there sit in the raw solar wind.

View

Show

Layers Protons Electrons Field lines Dipole L-shells Reconnection Solar wind & IMF Equatorial inset Labels

Particles

Number of particles 2500 Particle time-lapse 2.0× Electron share 50 %

Click any particle in the plot to follow it (or use the buttons above). Its trail shows gyration, bouncing between mirror points and the slow drift around the Earth.

Boris pusher self-test: …

Aurora

Observer geomagnetic latitude 65°

Geodynamo

Dynamo speed 5 / s Drive the magnetosphere with this dynamo (dipole moment scales the whole simulation)

What you are seeing

Gyration, bounce and drift

A charged particle spirals around a field line (gyration). Because the dipole field strengthens towards the poles, it is reflected at a mirror point and bounces between hemispheres. The field gradient and curvature also push ions westward and electrons eastward: the drift that forms the ring current. Trapped particles that keep drifting around the Earth form the Van Allen belts; those with small pitch angles fall in the loss cone and hit the atmosphere.

Solar wind, magnetopause and bow shock

The magnetopause sits where the solar-wind dynamic pressure ρv² balances the pressure of the Earth's field (Chapman–Ferraro). With r₀ ∝ (B₀²/ρv²)^1/6 a CME that multiplies the pressure by 100 squeezes the boundary from ~10.5 Re to ~4.5 Re. Upstream the supersonic wind is slowed at the bow shock, whose stand-off follows the magnetosonic Mach number.

Reconnection and substorms

When the interplanetary field points south (Bz < 0) it is antiparallel to the field at the nose of the magnetosphere and the two reconnect. Newly opened flux is dragged over the poles into the tail, stretching it and thinning the current sheet (growth phase). When the stored energy is released the tail reconnects, a plasmoid is ejected downtail and the near-Earth field snaps back (dipolarisation), injecting hot particles that stream into the atmosphere.

Aurora

Precipitating electrons excite atmospheric oxygen and nitrogen at 100–250 km: green (557.7 nm) low down, red (630 nm) above, blue-violet at the lower border. The oval lies just equatorward of the open–closed field-line boundary, so it expands when open flux grows during a storm and brightens and bulges at substorm onset.

Geodynamo and reversals

The field is made by convection of liquid iron in the outer core. The Rikitake two-disc model is the simplest self-exciting dynamo with chaotic polarity flips: each disc's current feeds the other's magnetic field. Real reversals take thousands of years, are irregular (chrons from 10⁴ to 10⁷ yr), and the field weakens and becomes multipolar on the way. The last full reversal was ~780 000 years ago.

Model limits

Toy model, not a forecast tool. Gyro-radii are exaggerated and the electron mass ratio is 100 so that both species are visible; each particle has its own gyro time-step, so species speeds are not to scale and the drift around the planet is shown time-lapsed. Fields are simple analytic forms (dipole, uniform Chapman–Ferraro shield, Harris-like tail sheet; ∇·B is only approximately zero in the tail). Precipitation of injected particles uses a parameterised pitch-angle scattering rate that grows with activity. The dayside/tail reconnection drawing is schematic. Energies are in relative units.