  We show that the interplay of cyclotron motion and Andreev reflection
experienced by massless-Dirac-like charge carriers in topological-insulator
surface states generates a Majorana-particle excitation. Based on an
envelope-function description of the Dirac-Andreev edge states, we discuss the
kinematic properties of the Majorana mode and find them to be possible to be
tuned by changing the superconductor's chemical potential and/or the magnitude
of the perpendicular magnetic field. Our proposal opens up new possibilities
for studying Majorana fermions in a controllable setup.
