  Scanning tunneling spectroscopy is used to study the real-space local density
of states (LDOS) of a two-dimensional electron system in magnetic field, in
particular within higher Landau levels (LL). By Fourier transforming the LDOS,
we find a set of n radial minima at fixed momenta for the nth LL. The momenta
of the minima depend only on the inverse magnetic length. By comparison with
analytical theory and numerical simulations, we attribute the minima to the
nodes of the quantum cyclotron orbits, which decouple in Fourier representation
from the random guiding center motion due to the disorder. This robustness of
the nodal structure of LL wave functions should be viewed as a key property of
quantum Hall states.
