  We show that the flat bands in the chiral model of magic-angle twisted
bilayer graphene remain exactly flat in the presence of a perpendicular
magnetic field. This is shown by an exact mapping between the model and the
lowest Landau level wavefunctions at an effective magnetic field, in which the
external field is either augmented or reduced by one flux quantum per unit
cell. When the external field reaches one flux quantum per unit cell, the model
exhibits a topological phase transition. These findings allow us to analyze a
Jain-series of Fractional Chern Insulators states in the exactly flat band, and
to point out an unconventional dependence of the energy gap on the magnetic
field.
