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Aims. The space radiation environment conditions and the maximum expected coronal mass ejection (CME) speed are being assessed through the investigation of scaling laws between the peak proton flux and fluence of Solar Energetic Particle (SEP) events with the speed of the CMEs. Methods. We utilize a complete catalog of SEP events, covering the last ~25 years of CME observations (i.e. 1997 to 2017). We calculate the peak proton fluxes and integrated event fluences for those events reaching an integral energy of up to E> 100 MeV, covering the period of the last ~25 years of CME observations. For a sample of 38 strong SEP events, we first investigate the statistical relations between the recorded peak proton fluxes (IP) and fluences (FP) at a set of integral energies of E >10 MeV, E>30 MeV, E>60 MeV, and E>100 MeV versus the projected CME speed near the Sun (VCME) obtained by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO). Based on the inferred relations, we further calculate the integrated energy dependence of both IP and FP, assuming that they follow an inverse power-law with respect to energy. By making use of simple physical assumptions, we combine our derived scaling laws to estimate the upper limits for VCME, IP, and FP focusing on two cases of known extreme SEP events that occurred on February 23, 1956 (GLE05) and in AD774/775, respectively. Given physical constraints and assumptions, several options for the upper limit VCME, associated with these events, are investigated. Results. A scaling law relating IP and FP to the CME speed as V^5CME for CMEs ranging between ~3400-5400 km/s is consistent with values of FP inferred for the cosmogenic nuclide event of AD774/775. At the same time, the upper CME speed that the current Sun can provide possibly falls within an upper limit of VCME <= 5500 km/s.

Aims. We studied the first multi-spacecraft high-energy solar energetic particle (SEP) event of solar cycle 25, which triggered a ground level enhancement (GLE) on 28 October 2021, using data from multiple observers that were widely distributed throughout the heliosphere. Methods. We performed detail modelling of the shock wave and investigated the magnetic connectivity of each observer to the solar surface and examined the shock magnetic connection. We performed 3D SEP propagation simulations to investigate the role of particle transport in the distribution of SEPs to distant magnetically connected observers. Results. Observations and modelling show that a strong shock wave formed promptly in the low corona. At the SEP release time windows, we find a connection with the shock for all the observers. PSP, STA, and Solar Orbiter were connected to strong shock regions with high Mach numbers, whereas the Earth and other observers were connected to lower Mach numbers. The SEP spectral properties near Earth demonstrate two power laws, with a harder (softer) spectrum in the low-energy (high-energy) range. Composition observations from SIS (and near-Earth instruments) show no serious enhancement of flare-accelerated material. Conclusions. A possible scenario consistent with the observations and our analysis indicates that high-energy SEPs at PSP, STA, and Solar Orbiter were dominated by particle acceleration and injection by the shock, whereas high-energy SEPs that reached near-Earth space were associated with a weaker shock; it is likely that efficient transport of particles from a wide injection source contributed to the observed high-energy SEPs. Our study cannot exclude a contribution from a flare-related process; however, composition observations show no evidence of an impulsive composition of suprathermals during the event, suggestive of a non-dominant flare-related process.

Aims The possible influence of solar superflares on the near-Earth space radiation environment are assessed through the investigation of scaling laws between the peak proton flux and fluence of Solar Energetic Particle (SEP) events with the solar flare soft X-ray peak photon flux. Methods We compiled a catalog of 65 well-connected (W20-90) SEP events during the last three solar cycles covering a period of $\sim$34 years (1984-2020) that were associated with flares of class $\geq$C6.0 and investigated the statistical relations between the recorded peak proton fluxes ($I_{P}$) and the fluences ($F_{P}$) at a set of integral energies from E $>$10; $>$30; $>$60; to $>$100 MeV versus the associated solar flare peak soft X-ray flux in the 1$-$8 A band ($F_{SXR}$). Based on the inferred relations, we calculate the integrated energy dependence of the peak proton flux ($I_{P}$) and fluence ($F_{P}$) of the SEP events, assuming that they follow an inverse power-law with respect to energy. Finally, we make use of simple physical assumptions, combining our derived scaling laws, and estimate the upper limits for $I_{P}$ and $F_{P}$ focusing on the flare associated with the strongest GLE yet directly observed (GLE 05 on 23 February 1956), and that inferred for the cosmogenic radionuclide based SEP event of AD774/775. Results A scaling law relating $I_{P}$ and $F_{P}$ to the solar soft X-ray peak intensity ($F_{SXR}$) as $\propto$~$F_{SXR}^{5/6}$ for a flare with a $F_{SXR}$ = X600 (in the revised scale) is consistent with values of $F_{P}$ inferred for the cosmogenic nuclide event of AD774/775.

The Probabilistic Solar Particle Event foRecasting (PROSPER) model predicts the probability of occurrence and the expected peak flux of Solar Energetic Particle (SEP) events. Predictions are derived for a set of integral proton energies (i.e. E$>$10, $>$30 and $>$100 MeV) from characteristics of solar flares (longitude, magnitude), coronal mass ejections (width, speed) and combinations of both. Herein the PROSPER model methodology for deriving the SEP event forecasts is described and the validation of the model, based on archived data, is presented for a set of case studies. The PROSPER model has been incorporated into the new operational Advanced Solar Particle Event Casting System (ASPECS) tool to provide nowcasting (short term forecasting) of SEP events as part of ESA's future SEP Advanced Warning System (SAWS). ASPECS also provides the capability to interrogate PROSPER for historical cases via a run on demand functionality.

Aims. The first relativistic solar proton event of solar cycle 25 (SC25) was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors onboard spacecraft in the near-Earth space. This is the first ground level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in-situ and remote-sensing measurements. Methods. In-situ proton observations from a few MeV to $\sim$500 MeV were combined with the detection of a solar flare in soft X-rays (SXRs), a coronal mass ejection (CME), radio bursts and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed and a relation to the solar sources was outlined. Results. GLE73 reached a maximum particle rigidity of $\sim$2.4 GV and is associated with type III, type II, type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to an anisotropic nature of the event. The peak flux at E$>$10 MeV was only $\sim$30 pfu and remained at this level for several days. The release time of $\geq$1 GV particles was found to be $\sim$15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies ($\gamma$ $\sim$5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed

Aims. We present observations of the first coronal mass ejection (CME) observed at the Solar Orbiter spacecraft on April 19, 2020, and the associated Forbush decrease (FD) measured by its High Energy Telescope (HET). This CME is a multispacecraft event also seen near Earth the next day. Methods. We highlight the capabilities of HET for observing small short-term variations of the galactic cosmic ray count rate using its single detector counters. The analytical ForbMod model is applied to the FD measurements to reproduce the Forbush decrease at both locations. Input parameters for the model are derived from both in situ and remote-sensing observations of the CME. Results. The very slow (~350 km/s) stealth CME caused a FD with an amplitude of 3 % in the low-energy cosmic ray measurements at HET and 2 % in a comparable channel of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter, as well as a 1 % decrease in neutron monitor measurements. Significant differences are observed in the expansion behavior of the CME at different locations, which may be related to influence of the following high speed solar wind stream. Under certain assumptions, ForbMod is able to reproduce the observed FDs in low-energy cosmic ray measurements from HET as well as CRaTER, but with the same input parameters, the results do not agree with the FD amplitudes at higher energies measured by neutron monitors on Earth. We study these discrepancies and provide possible explanations. Conclusions. This study highlights that the novel measurements of the Solar Orbiter can be coordinated with other spacecraft to improve our understanding of space weather in the inner heliosphere. Multi-spacecraft observations combined with data-based modeling are also essential to understand the propagation and evolution of CMEs as well as their space weather impacts.

White light images of Coronal Mass Ejections (CMEs) are projections on the plane-of-sky (POS). As a result, CME kinematics are subject to projection effects. The error in the true (deprojected) speed of CMEs is one of the main causes of uncertainty to Space Weather forecasts, since all estimates of the CME Time-of-Arrival (ToA) at a certain location within the heliosphere require, as input, the CME speed. We use single viewpoint observations for 1037 flare-CME events between 1996-2017 and propose a new approach for the correction of the CME speed assuming radial propagation from the flare site. Our method is uniquely capable to produce physically reasonable deprojected speeds across the full range of source longitudes. We bound the uncertainty in the deprojected speed estimates via limits in the true angular width of a CME based on multiview-point observations. Our corrections range up to 1.37-2.86 for CMEs originating from the center of the disk. On average, the deprojected speeds are 12.8% greater than their POS speeds. For slow CMEs (VPOS < 400 km/s) the full ice-cream cone model performs better while for fast and very fast CMEs (VPOS > 700 km/s) the shallow ice-cream model gives much better results. CMEs with 691-878 km/s POS speeds have a minimum ToA mean absolute error (MAE) of 11.6 hours. This method, is robust, easy to use, and has immediate applicability to Space Weather forecasting applications. Moreover, regarding the speed of CMEs, our work suggests that single viewpoint observations are generally reliable.

Upcoming missions, including the James Webb Space Telescope, will soon characterize the atmospheres of terrestrial-type exoplanets in habitable zones around cool K- and M-type stars searching for atmospheric biosignatures. Recent observations suggest that the ionizing radiation and particle environment from active cool planet hosts may be detrimental for exoplanetary habitability. Since no direct information on the radiation field is available, empirical relations between signatures of stellar activity, including the sizes and magnetic fields of starspots, are often used. Here, we revisit the empirical relation between the starspot size and the effective stellar temperature and evaluate its impact on estimates of stellar flare energies, coronal mass ejections, and fluxes of the associated stellar energetic particle events.

Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate? (2) How do solar transients drive heliospheric variability? (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere? (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans (SOOPs), resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime.

We study the solar eruptive event on 2017 September 10 that produced long-lasting $>$100 MeV $\gamma$-ray emission and a ground level enhancement (GLE72). The origin of the high-energy ions producing late-phase gamma-ray emission (LPGRE) is still an open question, but a possible explanation is proton acceleration at coronal shocks produced by coronal mass ejections. We examine a common shock acceleration origin for both the LPGRE and GLE72. The $\gamma$-ray emission observed by the Fermi-Large Area Telescope exhibits a weak impulsive phase, consistent with that observed in hard X-and $\gamma$-ray line flare emissions, and what appear to be two distinct stages of LPGRE. From a detailed modeling of the shock wave, we derive the 3D distribution and temporal evolution of the shock parameters, and we examine the shock wave magnetic connection with the visible solar disk. The evolution of shock parameters on field lines returning to the visible disk, mirrors the two stages of LPGRE. We find good agreement between the time history of $>$100 MeV $\gamma$-rays and one produced by a basic shock acceleration model. The time history of shock parameters magnetically mapped to Earth agrees with the rates observed by the Fort Smith neutron monitor during the first hour of the GLE72 if we include a 30% contribution of flare-accelerated protons during the first 10 minutes, having a release time following the time history of nuclear $\gamma$-rays. Our analysis provides compelling evidence for a common shock origin for protons producing the LPGRE and most of the particles observed in GLE72.

Solar Energetic Particles (SEP) are an integral part of the physical processes related with Space Weather. We present a review for the acceleration mechanisms related to the explosive phenomena (flares and/or CMEs) inside the solar corona. For more than 40 years, the main 2D cartoon representing our understanding of the explosive phenomena inside the solar corona remained almost unchanged. The acceleration mechanisms related to solar flares and CMEs also remained unchanged and were part of the same cartoon. In this review, we revise the standard cartoon and present evidence from recent global MHD simulations that supports the argument that explosive phenomena will lead to the spontaneous formation of current sheets in different parts of the erupting magnetic structure. The evolution of the large scale current sheets and their fragmentation will lead to strong turbulence and turbulent reconnection during solar flares and turbulent shocks. In other words, the acceleration mechanism in flares and CME-driven shocks may be the same, and their difference will be the overall magnetic topology, the ambient plasma parameters, and the duration of the unstable driver.

The Radiation Assessment Detector (RAD), on board Mars Science Laboratory's (MSL) Curiosity rover, has been measuring ground level particle fluxes along with the radiation dose rate at the surface of Mars since August 2012. Similar to neutron monitors at Earth, RAD sees many Forbush decreases (FDs) in the galactic cosmic ray (GCR) induced surface fluxes and dose rates. These FDs are associated with coronal mass ejections (CMEs) and/or stream/corotating interaction regions (SIRs/CIRs). Orbiting above the Martian atmosphere, the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has also been monitoring space weather conditions at Mars since September 2014. The penetrating particle flux channels in the Solar Energetic Particle (SEP) instrument onboard MAVEN can also be employed to detect FDs. For the first time, we study the statistics and properties of a list of FDs observed in-situ at Mars, seen both on the surface by MSL/RAD and in orbit detected by the MAVEN/SEP instrument. Such a list of FDs can be used for studying interplanetary CME (ICME) propagation and SIR evolution through the inner heliosphere. The magnitudes of different FDs can be well-fitted by a power-law distribution. The systematic difference between the magnitudes of the FDs within and outside the Martian atmosphere may be mostly attributed to the energy-dependent modulation of the GCR particles by both the pass-by ICMEs/SIRs and the Martian atmosphere.

The Athens Neutron Monitor Data Processing (ANMODAP) Center recorded an unusual Forbush decrease with a sharp enhancement of cosmic ray intensity right after the main phase of the Forbush decrease on 16 July 2005, followed by a second decrease within less than 12 h. This exceptional event is neither a ground level enhancement nor a geomagnetic effect in cosmic rays. It rather appears as the effect of a special structure of interplanetary disturbances originating from a group of coronal mass ejections (CMEs) in the 13-14 July 2005 period. The initiation of the CMEs was accompanied by type IV radio bursts and intense solar flares (SFs) on the west solar limb (AR 786); this group of energetic phenomena appears under the label of Solar Extreme Events of July 2005. We study the characteristics of these events using combined data from Earth (the ARTEMIS IV radioheliograph, the Athens Neutron Monitor (ANMODAP)), space (WIND/WAVES) and data archives. We propose an interpretation of the unusual Forbush profile in terms of a magnetic structure and a succession of interplanetary shocks interacting with the magnetosphere.