Magnetopause

The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma.

For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind.

As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response.

Waves (ripples and flapping motion) along the magnetopause move in the direction of the solar wind flow in response to small-scale variations in the solar wind pressure and to Kelvin–Helmholtz instabilities.

The solar wind is supersonic and passes through a bow shock where the direction of flow is changed so that most of the solar wind plasma is deflected to either side of the magnetopause, much like water is deflected before the bow of a ship.

The zone of shocked solar wind plasma is the magnetosheath.

At Earth and all the other planets with intrinsic magnetic fields, some solar wind plasma succeeds in entering and becoming trapped within the magnetosphere.

The amount of solar wind plasma and energy that enters the magnetosphere is regulated by the orientation of the interplanetary magnetic field, which is embedded in the solar wind.

Chapman and Ferraro[2][3][4][5] proposed that a plasma was emitted by the Sun in a burst as part of a flare event which disturbed the planet's magnetic field in a manner known as a geomagnetic storm.

The collision frequency of particles in the plasma in the interplanetary medium is very low and the electrical conductivity is so high that it could be approximated to an infinite conductor.

A magnetic field in a vacuum cannot penetrate a volume with infinite conductivity.

Chapman and Bartels (1940)[1] illustrated this concept by postulating a plate with infinite conductivity placed on the dayside of a planet's dipole as shown in the schematic.

At high latitudes, the magnetic field lines are pushed backwards and over the polar regions.

The boundary between the region dominated by the planet's magnetic field (i.e., the magnetosphere) and the plasma in the interplanetary medium is the magnetopause.

The configuration equivalent to a flat, infinitely conductive plate is achieved by placing an image dipole (green arrow at left of schematic) at twice the distance from the planet's dipole to the magnetopause along the planet-Sun line.

Since the solar wind is continuously flowing outward, the magnetopause above, below and to the sides of the planet are swept backward into the geomagnetic tail as shown in the artist's concept.

The region (shown in pink in the schematic) which separates field lines from the planet which are pushed inward from those which are pushed backward over the poles is an area of weak magnetic field or day-side cusp.

Solar wind particles can enter the planet's magnetosphere through the cusp region.

The joined field lines are swept back over the poles into the planetary magnetic tail.

Since the gyro-motion of electrons and ions is in opposite directions, an electric current flows along the boundary.

The condition governing this position is that the dynamic ram pressure from the solar wind is equal to the magnetic pressure from the Earth's magnetic field:[note 1]

are the density and velocity of the solar wind, and B(r) is the magnetic field strength of the planet in SI units (B in T, μ0 in H/m).

It serves to decelerate and deflect the solar wind flow before it reaches the magnetopause.

[10] Research on the magnetopause is conducted using the LMN coordinate system (which is set of axes like XYZ).

N points normal to the magnetopause outward to the magnetosheath, L lies along the projection of the dipole axis onto the magnetopause (positive northward), and M completes the triad by pointing dawnward.