Astronomy & Celestial Mechanics
the sky, computed
Concept

Open flux — where it appears

Magnetic flux in a planetary magnetosphere with one end on the planet and the other connected to the solar wind. It maps to the polar caps, so the area of each cap is a direct measure of how much open flux the magnetosphere holds.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

The shield that does not shield. Above: measured ion escape rates for three planets against their surface magnetic field. Venus and Mars have no dynamo at all and Earth has one, and the three rates lie within a factor of 9 — with the magnetised planet losing the most. The intuition that a magnetosphere protects an atmosphere is not a small correction away from being right; the measurement does not support it. Below: why. A dipole's field lines are not all closed. Those emerging within a polar cap reconnect with the wind's own field and lead straight to space, and the cap's area is set by how far the magnetosphere reaches — a boundary at ten planetary radii still leaves 5.1 per cent of the surface open. So a magnetosphere is both a shield and a funnel: it deflects the wind from most of the planet and collects ions from the whole ionosphere into the polar wind, which is exactly what an instrument above the poles measures leaving. Whether the net is protection depends on quantities nobody can compute from the field strength alone, and the three points above are the state of the evidence.

The shield that is also a funnel

A magnetic field is supposed to protect an atmosphere from the stellar wind. Venus and Mars have no dynamo and Earth has one, and their measured ion escape rates lie within a factor of a few — with the magnetised planet losing the most, because a dipole's polar field lines are open and lead straight to space.

exoplanets · Atmospheric escape The rate at which the solar wind opens the Earth's field. The rate at which reconnection at the dayside magnetopause turns closed field lines into open ones, as a voltage in kilovolts — webers of flux per second — against the clock angle of the interplanetary field in the plane perpendicular to the Sun–Earth line, 0° pointing north and 180° south, for a wind at 450 km/s carrying transverse fields of 5, 10, 20 nT. The law is empirical: the rate rises as the speed to the four-thirds, in proportion to the field, and as the sine of half the clock angle to the power 4.5. Due south, a 5 nT field opens flux at 57 kV; turned to within 20° of north, at 22 volts. The steepness in angle is the whole of the solar wind's leverage on the magnetosphere: the wind's pressure decides how large the cavity is, and one component of its field decides whether anything gets in.

A substorm timed by webers and volts

The solar wind opens the Earth's magnetic field at a rate that can be quoted as a voltage — tens of kilovolts, which is webers of flux per second — and the tail closes it again. Half a gigaweber loaded at fifty kilovolts takes three hours, and three hours is how often substorms recur. The aurora's position is the open flux, its surges are the unloading, and above a certain driving the ionosphere starts to limit the circuit that drives it.

sky · Magnetosphere

Named alongside it

The objects these essays reach for when they reach for this one.

MagnetosphereAtmospheric escapeAuroral ovalClock angleDungey cycleExosphereHabitable zoneHydrodynamic escapeInduced magnetosphereIon pickupMagnetic reconnectionPedersen conductance

All concepts