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Assessing the radiation exposure of aviators and frequent flyers requires the study of the cosmic ray showers inside the Earth’s atmosphere. DYASTIMA / DYASTIMA-R is a Geant4 based software application, implemented by the Athens Cosmic Ray Group which allows the study of the evolving secondary particles cascades inside the atmosphere, as well as radiation dosimetry calculations (ambient dose equivalent rate) at different atmosphe - ric altitudes, geographic coordinates and magnetic cut-off rigidity. Results for various scenarios, as calculated by DYASTIMA/DYASTIMA-R, are provided as a federated product through the European Space Agency Space Situational Awareness of the Space Radiation Service Centre Network, while the DYASTIMA software is provi- ded through the Athens Neutron Monitor Station (A.Ne.Mo.S.) portal. Initial results for the assessment of the radiation exposure during the last Solar Cycles 23 and 24 are presented in this work, covering the most usual flying altitudes. The results indicate the dependence of the dose rate on the magnetic cut-off rigidity threshold, with higher dose rates at high geographic latitudes, as well as the anti-correlation of cosmic ray intensity with the solar activity, as higher dose rates are observed during solar minimum conditions.

Many previous studies have shown that before the beginning of a Forbush Decrease (FD) of the cosmic ray intensity, a precursor signal can be observed. All these surveys were focused on FDs that are associated with a sudden storm com- mencement (SSC). In this work we demonstrate that precursors could also be observed in events without a SSC that is determined by an abrupt increase of the interplanetary magnetic field. The type of precursory signals and their diversity among the events are the main purpose of this study. We try to figure out similarities and differences on the signals and the associated events from both categories in the last fifty years, from 1969 to 2019, using the same selection criteria of the under investigation FDs. Simultaneously the orientation of the upcoming solar disturbances in comparison to the way they configure the increase of the interplanetary magnetic field and create these signals are discussed.

In this investigation the different features and characteristics of Forbush decreases, with emphasis on large For- bush decreases (≥4%) and their association to solar sources, are being examined. According to the heliolongitude of the solar source, the events under study were separated into three subcategories: western (21º ≤ heliolongitude ≤ 60º), eastern (-60º ≤ heliolongitude ≤ -21º) and central (-20º ≤ heliolongitude ≤ 20º). The selected events cover the time period 1967 - 2017. The ‘Global Survey Method’ was used for analyzing the Forbush decreases, along with data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. In ad - dition, the superimposed epoch method was applied in order to plot the time profiles for the aforementioned group of events. This detailed analysis reveals interesting results concerning the features of cosmic ray decreases in re- gard to the heliolongitude of the solar sources. Moreover, it is also shown that large Forbush decreases, regardless of the heliolongitude of the solar source, are accompanied by increased geomagnetic activity and increased aniso- tropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases.