Precursory Signs of Large Forbush Decreases: The Criterion of Anisotropy

The study of precursors preceding Forbush decreases belongs to the applied side of space research and to a relatively new area of modern science, that of Space Weather. Moreover, it is a pioneering and innovative research field with interesting results. In the framework of the above, the Athens Cosmic Ray Group of the National and Kapodistrian University of Athens (NKUA) and the Cosmic Ray Group of the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN) have collaborated in investigating predecreases and/or preincreases of the cosmic-ray intensity before the development of a Forbush decrease, that could serve as precursory signs of the upcoming event and consequently play a significant role in the prediction of cosmic-ray and geomagnetic activity. In this work, the criterion of the increased anisotropy one hour before the onset of the event ( $A_{\mathit{xyb}}$ $A_{\mathit{xyb}}$ , %) is being examined for large Forbush decreases. Specifically, Forbush decreases with magnitude greater than 5%, accompanied with geomagnetic storms (i.e., geomagnetic index Dst < −100 nT and 5 ≤ Kp-index ≤ 9) and characterized by $A_{\mathit{xyb}}\ge$ $A_{\mathit{xyb}}\ \geq $ 0.8% were analyzed. The catalog of Forbush Effects and Interplanetary Disturbances of IZMIRAN was used for analyzing the solar, interplanetary, and geomagnetic conditions during each event. Additionally, for a visual inspection of the precursory signs in each event the Ring of Stations method (i.e., asymptotic longitude–time diagram) was applied. Results revealed that the increased anisotropy one hour before the main phase of the Forbush decrease is a valid and reliable criterion of precursors that can be eventually used in the development of a Forbush decrease prognosis application tool.