Question

the plasma density on lower ionosphere has been measured during satellite reentry to be about 10^18 m at 50km altitude,10^17 at 70km, and 10^14 at 18km. what are the plasma frequencies there?

Answer 1

If we ignore thermal motion of electrons and assume that the effective mass of electrons equals to the rest mass of electron, the plasma frequency only depends on the concentration:

$$f(n)=\frac{\omega(n)}{2\pi}=\frac{e}{2\pi}\sqrt{\frac{n}{m_e\epsilon_0}}.$$

In this equation, e is the charge of electron.

Thus:

$$\begin{vmatrix} 50\text{ km} & 8.979\text{ GHz}\\ 70\text{ km} & 2.839\text{ GHz}\\ 8\text{ km}&89.79\text{ MHz} \end{vmatrix}$$