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We analyze a Neutron Star Interior Composition Explorer (NICER) observationof the black hole X-ray binary MAXI J1820+070 during a transition from type-Cto type-B quasi-periodic oscillations (QPOs). We find that below ~2 keV, forthe type-B QPOs the rms amplitude is lower and the magnitude of the phase lagsis larger than for the type-C QPOs. Above that energy, the rms and phase-lagspectra of the type-B and type-C QPOs are consistent with being the same. Weperform a joint fit of the time-averaged spectra of the source, and the rms andphase-lag spectra of the QPOs with the time-dependent Comptonization modelvkompth to study the geometry of the corona during the transition. We find thatthe data can be well-fitted with a model consisting of a small and a largecorona that are physically connected. The sizes of the small and large coronaeincrease gradually during the type-C QPO phase whereas they decrease abruptlyat the transition to type-B QPO. At the same time, the inner radius of the discmoves inward at the QPO transition. Combined with simultaneous radioobservations showing that discrete jet ejections happen around the time of theQPO transition, we propose that a corona that expands horizontally during thetype-C QPO phase, from ~10^{4} km (~800 Rg) to ~10^{5} km (~8000 Rg) overlyingthe accretion disc, transforms into a vertical jet-like corona extending over~10^{4} km (~800 Rg) during the type-B QPO phase.

A variable corona during the transition from type-C to type-B quasi-periodic oscillations in the black hole X-ray binary MAXI J1820+070