# Homework Help: Subsolar magnetopause distance

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1. Feb 10, 2016

### Imagin_e

1. The problem statement, all variables and given/known data

Hi!
I need to calculate the subsolar magnetopause distance (r_mp) when the distance to Earth is 0.4 AU. I have all the formulas and the data, but I still don’t get a reasonable value (it’s too small!)
Here is what I know:

Solar wind density: 5 cm^-3 (rho_sw)
Solar wind speed=400 km/s (u_sw)
Magnetic field at Earth’s surface= 5*10^-6 T (B_E)
u_0= magnetic constant permeability= 4*pi*10-7 T m/A

2. Relevant equations

3. The attempt at a solution
First I start to calculate the thermal pressure:

p= rho_sw*(u_sw)^2

next, the magnetic field when the distance is 0.4 AU, where r= 1AU * 0.4:

B(0.4)=B_E*(R_E/r)

And finally I use the following equation to get the distance I am searching for in term of Earth radius R_E:

(r_mp/R_E)=[(2*B(r)^2)/( u_0*p)]^1/6

But as I mentioned, the value is not “the right one”. Can I get help from someone?
Thanks!

2. Feb 10, 2016

### Imagin_e

I solved it, so no need to help me :)