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Aims. We present observations of the first coronal mass ejection (CME)observed at the Solar Orbiter spacecraft on April 19, 2020, and the associatedForbush decrease (FD) measured by its High Energy Telescope (HET). This CME isa multispacecraft event also seen near Earth the next day. Methods. Wehighlight the capabilities of HET for observing small short-term variations ofthe galactic cosmic ray count rate using its single detector counters. Theanalytical ForbMod model is applied to the FD measurements to reproduce theForbush decrease at both locations. Input parameters for the model are derivedfrom both in situ and remote-sensing observations of the CME. Results. The veryslow (~350 km/s) stealth CME caused a FD with an amplitude of 3 % in thelow-energy cosmic ray measurements at HET and 2 % in a comparable channel ofthe Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the LunarReconnaissance Orbiter, as well as a 1 % decrease in neutron monitormeasurements. Significant differences are observed in the expansion behavior ofthe CME at different locations, which may be related to influence of thefollowing high speed solar wind stream. Under certain assumptions, ForbMod isable to reproduce the observed FDs in low-energy cosmic ray measurements fromHET as well as CRaTER, but with the same input parameters, the results do notagree with the FD amplitudes at higher energies measured by neutron monitors onEarth. We study these discrepancies and provide possible explanations.Conclusions. This study highlights that the novel measurements of the SolarOrbiter can be coordinated with other spacecraft to improve our understandingof space weather in the inner heliosphere. Multi-spacecraft observationscombined with data-based modeling are also essential to understand thepropagation and evolution of CMEs as well as their space weather impacts.

Radial Evolution of the April 2020 Stealth Coronal Mass Ejection between 0.8 and 1 AU -- A Comparison of Forbush Decreases at Solar Orbiter and Earth