Maria Gerontidou

Three years after GLE73 (2021 Oct 28), three distinct GLEs took place before the end of 2024. The first event, GLE74, was registered on 2024 May 11, followed by GLE75 on 2024 Jun 8 and GLE76 on 2024 Nov 21. Each event exhibited unique characteristics in terms of intensity, temporal evolution, and spectral properties. Notably, GLE74 occurred during an irregular Forbush decrease with sharp amplitude reduction and rapid recovery. Specifically, a 15 % decrease was recorded, followed by an exceptionally fast 10 % recovery within 1.5 hours, as observed by the Oulu neutron monitor station. This event triggered the strongest geomagnetic storm in two decades (2024 May 10–11). GLE75 was a rather weak event which evolved in a quiet background as did GLE76. In this study, apart from the solar, interplanetary and geomagnetic conditions existing during these events, the response of the GLE Alert++ system, implemented by A.Ne. Mo.S. of the National and Kapodistrian University of Athens, to these Ground Level Enhancements is also being presented and analyzed, offering insights into their implications for space weather forecasting and radiation hazard.

During the NMDB Meeting 2025, which took place in March 2025 in Athens, Greece, more than 40 contributions (oral and e-posters) were presented. Particular emphasis was given to the intense events of solar, interplanetary and geomagnetic nature that occurred in 2024. The topics of the NMDB upgrade and expansion (with new neutron measurements and other scientific programs) as well as the importance of cosmic ray intensity measurements over time and the development of new applications were addressed. In the following overview, a brief description of the meeting’s contributions is provided.

In this work, we present a statistical analysis of selected properties of all recorded GLE events to date. More specifically, we study the association between GLEs and the corresponding probable sources – the solar flares. We investigate the time delay between the GLE events onset time and the solar flare onset time, as well as the dependence of the occurrence and amplitude of the GLE events on solar flare characteristics: the longitude and the class of the flare. From the available data of 48 events, we find an average time delay of 37 minutes while 83% of GLE events were caused by X Class Solar Flares. From the analysis of 40 events, we calculated the average solar flare longitude to be 43.6° West, which is within the well-connected region of the Sun with 75% of events originating from the 20°–90° West heliolongitude range. Additionally, we find that the average linear speed of GLE-associated CMEs is 1762.05 km/s with a wide distribution. Finally, we also present the distribution of GLEs across all phases of the solar cycle.

Precursors (pre-decreases and/or pre-increases of the cosmic ray intensity) are observed preceding Forbush decreases. The large Forbush decreases (magnitude > 5%) accompanied by geomagnetic storms (i.e., Dst < -100 nT and 5 ≤ Kp-index ≤ 9) have been studied in regard to precursors. The threshold value of the equatorial anisotropy one hour before the onset of the event (Axyb, %) was set at 0.8%. In total, 59 events with Axyb > 0.8% (test group) and 68 events with Axyb < 0.8% (control group), from 1957–2023, were examined. The ›Forbush Effects and Interplanetary Disturbances‹ database was used for selecting events and analyzing the solar, interplanetary, and geomagnetic conditions during each event. The method ›Ring of Stations‹ was also used. The chosen threshold is well set at 0.8%, since precursors are clearly present in the test group and principally absent in the control group.

During a period of intense solar activity and highly disturbed geomagnetic conditions, a large Forbush decrease began on 10 May 2024 accompanied by a historic geomagnetic storm that lasted for four days. This extreme geomagnetic disturbance classified as G5 according to “NOAA Space Weather Scale for Geomagnetic Storms” is referred to in the literature as the Mother’s Day Storm. This resulted from multiple, at least seven, Coronal Mass Ejections (CMEs) that had been occurring since 7 May. In addition, on 11 May, a powerful X5.8 class solar flare, reaching its maximum at 01:32 UT, was followed by an abrupt increase in proton flux with energies > 100 MeV (with onset on 11 May at 01:45 UT and peaking at 02:45 UT), as recorded by GOES satellites. This resulted in a Ground Level Enhancement (GLE), identified as GLE74, occurring on 11 May 2024 during the recovery phase of the deep Forbush decrease (~15%). This Solar Energetic Particle (SEP) event consisted of both impulsive and gradual components, where the high-energy tail of the gradual component was recorded by several stations of the worldwide ground-based neutron monitor network. Approximately 15 minutes after the onset of the SEP event and 40 minutes prior to its peak, an alert was issued by the GLE Alert++ system of the Athens Neutron Monitor Station of the National and Kapodistrian University of Athens (NKUA), available as a federated product on the ESA SWE Portal under the Space Radiation Expert Service Centre. In this paper, a description of the solar activity, i.e., solar flares and CMEs, occurring during this time period is given. Moreover, recordings of cosmic ray data obtained by ground-based neutron monitors are used to perform a detailed analysis of GLE74. Finally, the response of the NKUA GLE Alert++ system to GLE74 is thoroughly presented.

A Ground Level Enhancement (GLE) event can be observed as an increase in the background of ground‐based neutron monitor observations and is often associated with an increase of >500 MeV space‐based proton flux measurements. GLE events begin as very high‐energy SEP events associated with GeV protons. For such events to be detected at sea level, proton energies must exceed about 433 MeV. Since the increased flux of such particles can be a major problem to technology in space and on Earth and pose a threat for human health, developing real time warning systems is of great importance. GLE Alert++ is a product, built by the Athens Cosmic Ray Group of the National and Kapodistrian University of Athens, that issues alerts when a GLE event starts to register and is based on ground‐based neutron monitor observations. From the space‐based approach, the HESPERIA UMASEP‐500 product, jointly developed within the framework of the EU HORIZON2020 HESPERIA project by the Universidad de Malaga, Spain and National Observatory of Athens, Greece, provides forecasts of GLE events and >500 MeV protons relying on GOES satellite Soft X‐Ray and high energy proton observations. These two products are fully integrated as federated products on the ESA SWE Portal and are provided as part of the ESA Space Safety Program Space Weather Service Network. In this paper we present how the products were built, provide examples of their outputs as seen on the ESA SWE Portal, and show how the products complement each other and how using them together can in some instances provide more information for users of these services.

A novel tool utilizing machine learning techniques was designed to forecast ap index values for the next three consecutive days (24 values). The tool employs time series data from the 3 h ap index of solar cycles 23 and 24 to train the Long Short-Term Memory (LSTM) model, predicting ap index values for the next 72 h at three-hour intervals. During periods of quiet geomagnetic activity, the LSTM model’s performance is sufficient to yield favorable outcomes. Nevertheless, during geomagnetically disturbed conditions, such as geomagnetic storms of different levels, the model needs to be adapted in order to provide accurate ap index results. In particular, when coronal mass ejections occur, the ap Prediction tool is modulated by inserting predominant features of coronal mass ejections such as the date of the event, the estimated time of arrival and the linear speed. In the present work, this tool is described thoroughly; moreover, results for G2 and G3 geomagnetic storms are presented.

As the current solar cycle 25 progresses and moves towards solar maxima, solar activity is increasing and extreme space weather events are taking place. Two severe geomagnetic storms accompanied by two large Forbush decreases in galactic cosmic ray intensity were recorded in March and May, 2024. More precisely, on 24 March 2024, a G4 (according to the NOAA Space Weather Scale for Geomagnetic Storms) geomagnetic storm was registered, with the corresponding geomagnetic indices Kp and Dst equal to 8 and −130 nT, respectively. On the same day, the majority of ground-based neutron monitor stations recorded an unusual Forbush decrease. This event stands out from a typical Forbush decrease because of its high amplitude decrease phase and rapid recovery phase, i.e., 15% decrease and an extremely rapid recovery of 10% within 1.5 h, as recorded at the Oulu neutron monitor station. Furthermore, on 10–13 May 2024, an unusual G5 geomagnetic storm (geomagnetic indices Kp = 9 and Dst = −412 nT) was registered (the last G5 storm had been observed in 2003). In addition, the polar neutron monitor stations recorded a Ground Level Enhancement (GLE74) during the recovery phase of a large Forbush decrease of 15%, which started on 10 May 2024. In this study, a detailed analysis of these two severe events in regard to the accompanying solar activity, interplanetary conditions and solar energetic particle events is provided. Moreover, the results of the NKUA “GLE Alert++ system”, the NKUA/IZMIRAN “FD Precursory Signals” method and the NKUA “ap Prediction tool” concerning these events are presented.

Forbush decreases are usually characterized by increased values of cosmic ray anisotropy. The precursory signs, i.e., pre-increases and especially pre-decreases of the cosmic ray intensity, are highly anisotropic phenomena that ordinarily forewarn of such events. Two Cosmic Ray Groups from the National and Kapodistrian University of Athens (NKUA) and the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN) have been investigating the existence of precursory signs preceding Forbush decreases in relation to different solar phenomena, interplanetary parameters, and geomagnetic conditions. In this study, large Forbush decreases (magnitude > 5%) accompanied by geomagnetic storms (i.e., geomagnetic index Dst < −100 nT and 5 ≤ Kp-index ≤ 9) and characterized by an equatorial anisotropy 1 h before the onset of the event (A_xyb, %) less than 0.8% were examined regarding precursors. In total, 50 events with the aforementioned features were selected and analyzed from the IZMIRAN’s Forbush Effects and Interplanetary Disturbances database concerning the time period from 1969 until 2023. The Ring of Stations method, which depicts the cosmic ray variations for various asymptotic longitudes in relation to time, was applied on each event. The results revealed that clear signs of pre-decreases were not present for the majority of the events. Since particularly strong events were considered, most of them still showed some precursory signs, albeit mainly weak. Despite this, the value of A_xyb = 0.8% proves to be a good threshold for the manual selection of FDs with well-expressed precursors.

Over the last years the Athens Cosmic Ray Group of the National & Kapodistrian University of Athens has implemented a warning tool called GLE Alert, which is a highly credible application that issues alerts when a ground level enhancement (GLE) starts due to very high energy solar energetic particles reaching the Earth. This application warns of a high intensity solar energetic particle event up to several minutes before it reaches near the near-Earth space environment. In this work, an assessment of the latest updated version of GLE Alert, GLE Alert++, is presented. GLE Alert++ is a federated product of the ESA S2P SWE Space Radiation Expert Service Centre, which is part of the ESA Space WEather Service NETwork (SWESNET) project. The assessment of the GLE Alert++, which was finalized in October 2022, focused on: (a) the availability of the real-time data provided by the neutron monitor stations that contribute to the GLE Alert++, (b) the behaviour of each station regarding the different Alert levels status (Watch, Warning and Alert), and (c) the definition of the real-time assessment index. The results of this work are of essential importance since they ensure a reliable and trustworthy warning tool, and can be highly useful in protecting humans during extreme solar energetic events.