magnetopause

Artistic rendition of the Earth's Magnetopause. The magnetopause is where the pressure from the solar wind and the planets magnetic field are equal. The position of the Sun would be far to the left in this image

The magnetopause is the abrupt boundary between a magnetic field, (see: magnetosphere) and surrounding plasma. The magnetopause ripples, flaps, and moves inward and outward in response to varying solar wind conditions.

In terms of planetary science, the magnetopause is the outermost boundary of the region controlled by a planets magnetic field. It separates the magnetosheath and the magnetosphere. It is the location where the outward magnetic pressure of a planet's magnetic field is counterbalanced by the solar wind plasma.

Most of the solar particles are deflected to either side of the magnetopause, much like water is deflected before the bow of a ship. At Earth and all the other planets with intrinsic magnetic fields, some particles succeed in entering and becoming trapped within the magnetosphere. At Earth, these particles form the radiation belts.

Estimating the standoff distance to the magnetopause

If the pressure from particles within the magnetosphere is neglected, it is possible to estimate the distance to the part of the magnetosphere that faces the Sun. The condition governing this position is that the dynamic ram pressure from the solar wind is equal to the magnetic pressure from the Earths magnetic field

^{[eq 1]} where ρ and v is 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)

Since the dipole magnetic field strength varies with distance as 1 / r³ the magnetic field strength can be written as B(r) = B₀ / r³.

.

Solving this equation for r leads to an estimate of the distance

1. ^ The reason for the factor of 4 is because the magnetic field strength just inside the magnetopause is twice the dipole value for a planar magnetopause

The distance from Earth to the subsolar magnetopause varies over time due to solar activity, but typical distances range from . A bow shock stands upstream from the magnetopause. It serves to decelerate and deflect the solar wind flow before it reaches the magnetopause ^[1]

Solar system magnetopauses^[2]

Planet	number	Magnetic moment [nb 1]	Magnetopause distance [nb 2]	Observed size of the magnetosphere[nb 3]	variance of magnetosphere[nb 4]
Mercury	1	0.0004	1.5	1.4	0
Venus	2	0	0	0	0
Earth	3	1	10	10	2
Mars	4	0	0	0	0
Jupiter	5	20000	42	75	25
Saturn	6	600	19	19	3
Uranus	7	50	25	18	0
Neptune	8	25	24	24.5	1.5

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.

1. ^ compared to Earth's magnetic moment ()
2. ^ typical distance between magnetopause and magnetosphere in planet radii
3. ^ in planet radii
4. ^ in planet radii, the magnetosphere varies mainly in response to solar wind dynamic pressure and interplanetary magnetic field orientation

See also

• Heliopause
• Magnetosphere
• Shock wave
• Solar system
• Van Allen radiation belt
• For applications to spacecraft propulsion, see magnetic sail.

Notes

1. ^ Imke de Pater and Jack J. Lissauer: Planetary Sciences, page 261. Cambridge University Press, 2001. ISBN 0 521 48219 4
2. ^ After Kiverson and bagenal (1999)