A proton approaches the Earth with speed $0\mathrm{.}05$c $($c is the speed of light in vacuum$)$ at an angle of $\text{'}=40$ to Earth$$s magnetic field lines and is trapped in the lower Van Allen belt without a change in its speed. If the mean strength of the field

at this altitude is B $=105$ T$,$ find the radius of curvature for the circular part of the proton$$s helical motion.