Magnetopause: Difference between revisions

Artistic rendition of the Earth's magnetopause. The magnetopause is where the pressure from the solar wind and the planet's 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 planet's 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 plasma sheet.

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 Earth's magnetic field.

$(\rho v^{2})_{sw}\approx ({\frac {4B(r)^{2}}{2\mu _{0}}})_{m}$ where $\rho$ and $v$ 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)

Since the dipole magnetic field strength varies with distance as $1/r^{3}$ the magnetic field strength can be written as $B(r)=B_{0}/r^{3}$.

$\rho v^{2}\approx {\frac {2B_{0}^{2}}{r^{6}\mu _{0}}}$.

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

$r\approx {\sqrt[{6}]{\frac {2B_{0}^{2}}{\mu _{0}\rho v^{2}}}}$

The distance from Earth to the subsolar magnetopause varies over time due to solar activity, but typical distances range from $6-15R_{\oplus }$. Emperical models using real-time solar wind data can provide a real-time estimate of the magnetopause location. A bow shock stands upstream from the magnetopause. It serves to decelerate and deflect the solar wind flow before it reaches the magnetopause

Solar System magnetopauses

Overview of the Solar System magnetopauses

Planet	Number	Magnetic moment	Magnetopause distance	Observed size of the magnetosphere	variance of magnetosphere
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.

See also

• Heliopause
• Shock wave
• Solar system
• For applications to spacecraft propulsion, see magnetic sail.