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Forbush decrease model for expanding CMEs affecting Earth and Mars

Periodic Reporting for period 1 - ForbMod (Forbush decrease model for expanding CMEs affecting Earth and Mars)

Reporting period: 2017-06-01 to 2019-05-31

Project “Forbush decrease model for expanding CMEs (ForbMod)” aimed to unravel how galactic cosmic rays are influenced by solar storms in the inner solar system (Sun to Mars) by developing a new model and utilizing a number of spacecraft and planetary observation, including those by Mars Curiosity Rover. Our current understanding is that the solar storms, i.e. coronal mass ejections (CMEs), are actually magnetic structures disconnected from its surrounding so that the galactic cosmic rays can only enter inside trough random walk, i.e. diffusion. Since diffusion is slow compared to the speed of initially empty CME while traveling through the interplanetary space, during the passage of the CME over our instruments we observe Forbush decreases, i.e. short-term depressions in the galactic cosmic ray counts. The project generated new knowledge on the properties of galactic cosmic ray decreases by solar storms, which is relevant for space weather, human spaceflight and planetary and exoplanetary atmospheres. The overall objectives were: 1) to use CME observations to constrain the Forbush decrease model and take into account that CME evolves while traveling through interplanetary space, and 2) to compare the modelling results to measurements not only taken from Earth, but also from Mars, recently available with the Mars Curiosity Rover. Both objectives were achieved in the scope of the action, where the analytical Forbush decrease model ForbMod was developed for an arbitrary expansion of a CME and was tested against measurements from near-Earth spacecraft particle detector and Earth ground-based neutron monitors, as well as radiation detector onboard Mars Curiosity Rover. The project results have shown that Forbush decreases can be utilised not only as signatures of CME arrival, but can also provide vital information on CME properties and evolution through combined modelling-observational approach.
The analytical Forbush decrease model ForbMod was developed for an arbitrary expansion of a CME. According to the model, the depression depends on the initial conditions - namely the strength of the magnetic field, but also on the evolutionary properties of the magnetic structure, i.e. expansion. It was shown that the initial conditions can be partly constrained by the 3D CME reconstruction obtained from the stereoscopic coronagraphic measurements and comparison of the model with a statistical observational study has shown good agreement. The model dependency on the evolutionary properties was further investigated and it was found that with simple assumptions ForbMod reverse modeling can be used to estimate CME initial magnetic properties and its expansion. The ForbMod was tested against measurements from near-Earth spacecraft particle detector and Earth ground-based neutron monitors, as well as radiation detector onboard Mars Curiosity Rover, where it was used in a comprehensive study as one of the models to gain insight into the CME evolution. The studies conducted in the scope of the project unravelled the observational aspects of Forbush decreases: while it is expected that Forbush decrease properties indeed depend on the magnetic field inside the CME, due to the evolutionary aspect simple linear trends are not expected. Furthermore, it was shown that ForbMod can provide vital information on CME properties and evolution through combined modelling-observational approach. The results of the action were disseminated via 5 invited seminars at academic institutions and more than 50 presentations at international scientific meetings, out of which 16 are first-author contributions (3 invited and another 10 oral and 3 poster presentations), as well as via 9 scientific publications.
The newly developed Forbush decrease model ForbMod has built on the previous modelling efforts of the diffusion-expansion approach and has for the first time introduced possibility of arbitrary CME expansion, which has shown to be a very important aspect since not all CMEs evolve in the same manner. Testing of the model parameters and comparison with the observation, not only at Earth but also at Mars, enabled us to understand the variety of Forbush decrease observational characteristics and thus to use galactic cosmic ray measurements very effectively as signatures of solar storms. The project results thus represent significant contribution in understanding the evolution of solar storms, which is not only relative for future scientific studies but also from the perspective of forecasting space weather in order to mitigate damages related to hazardous space weather events.
In situ measurements of CME and the corresponding Forbush decrease modelled with ForbMod
A sketch of the ForbMod: solar storm magnetic structure expanding while particles diffuse into it
3D reconstruction of CME using stereoscopic coronagraphic images from 3 different spacecraft