sliding planar atmosphere

density distribution \varrho(r) = \varrho_0 \exp\left( \frac{|r - p|}{\lambda} \right), p denotes the center.

for grammage/length conversion, the approximation of a flat exponential atmosphere is used, with the axis given by \bm a \propto p - r_0, where r_0 is the starting point of the trajectory