  The upgrade of IceTop, the surface array of IceCube, by a hybrid array of
scintillation and radio detectors is motivated by a rich science case. The
scintillators will lower the threshold for the measurement of air showers to
about 100 TeV, provide a more efficient veto of air showers for neutrino
measurements, and improve the separation of the electromagnetic and muonic
shower components due to the different responses of scintillators and
ice-Cherenkov tanks. Furthermore, the scintillators will enable the calibration
and compensation of the effect of snow accumulation above IceTop. The radio
antennas will provide a calorimetric measurement of the electromagnetic shower
component and a direct sensitivity to the shower maximum. Consequently, the
combination of the existing ice-Cherenkov detectors in the ice and at the
surface with the new scintillation and radio detectors at the surface will
enable unprecedented accuracy for event-by-event mass classification in the PeV
to EeV range. This will transform IceCube into the most accurate instrument for
high-energy Galactic cosmic rays in the Southern Hemisphere. Hence, the hybrid
array will make an important contribution to the main science case of IceCube
of understanding the origin of cosmic rays. In addition to its cosmic-ray
science goals, the hybrid array provides essential R&D for IceCube-Gen2 which
will feature a larger surface array, sophisticated timing and communication
technology, and elevated surface structures. Moreover, the hybrid array will
improve the understanding of the atmospheric background to the neutrino
measurements and opens new scientific opportunities, such as the searches for
PeV gamma rays from the Galactic Center and for mass-dependent anisotropies.
