On the features of great Forbush effect during May 2024 extreme geomagnetic storm

• Large Forbush Effect up to 15.7% decrease in cosmic ray intensity. • Abnormally low cosmic ray anisotropy (0.9%) for such large events. • Significant magnetospheric effect in the NM data (∼4%) due to an extreme geomagnetic storm. The work investigates the features of galactic cosmic ray density and anisotropy behavior and their relation to solar sources, interplanetary and geomagnetic disturbances from May 8 to May 13, 2024. During this time, powerful solar flares and fast CMEs were recorded, leading to registration of an extreme geomagnetic storm along with one of the most significant Forbush effects for the entire observation period. All the calculations of cosmic ray characteristics are made using the data of the global neutron monitor network. Unique methods maintained at IZMIRAN – the Global Survey Method and the Ring of Stations Method – are used. It is determined that the magnitude of Forbush effect under study was 15.7% (for particles with 10 GV rigidity) and as an extreme geomagnetic storm was recorded, there was a significant magnetospheric effect observed in the data of neutron monitors (∼4%).