PARADISE: a model for energetic particle transport in the solar wind

Abstract

Our modern society is becoming increasingly dependent on the constant stream of information sent out by satellites orbiting Earth. However, highly energetic rays traversing the near-Earth space environment pose a constant threat to the electronics on board these satellites. In addition, humankind renewed interest in manned missions to the Moon and Mars requires the protection of the spacecraft crew members against the high radiation doses that they will encounter during their interplanetary voyage. A profound understanding of the ever changing radiation conditions in our solar system is thus crucial, in order to efficiently protect our space-based assets and to succeed in our future ventures in space.

A major part of the high intensity radiation in interplanetary space has a solar origin, and consists of electrons, protons, and heavier ions that have undergone significant acceleration during solar eruptive events such as flares and coronal mass ejections (CMEs). These particles are referred to as solar energetic particles (SEPs), and are typically much more energetic than solar wind particles. The solar wind is a continuous stream of charged particles originating from the Sun and filling up the entire heliosphere.

The transport and acceleration of energetic particles through the solar wind can be described by the focused transport equation (FTE). This equation can be derived from the Vlasov equation, under the assumptions that energetic particles travel in a collisionless manner through a turbulent background plasma. The effects of the turbulence are modelled through a set of different diffusion processes in phase space. Moreover, as illustrated in this thesis, the effect of the GC drifts can be included in the FTE if an appropriate transformation is made to GC coordinates, which in addition leads to a cross-field diffusion process.

In this thesis, we develop a numerical model that calculates energetic particle intensities in the inner heliosphere by solving the FTE under realistic solar wind conditions. This model is called PARADISE, an acronym for Particle Radiation Asset Directed at Interplanetary Space Exploration. The PARADISE model solves the FTE by integrating a set of Itô stochastic differential equations, while assuming a solar wind configuration obtained from a magnetohydrodynamic (MHD) model. In this work, the time-dependent MHD module of the EUropean Hliospheric FORecasting Asset (EUHFORIA) is used to calculate different solar wind configurations.

By using PARADISE and EUHFORIA, we investigate the transport of SEPs in a solar wind containing a corotating interaction region (CIR), which is a region of compressed plasma that is formed when a fast solar wind stream overtakes a preceding slow stream. It is illustrated how a CIR can alter SEP distributions by acting as an efficient magnetic mirror and by re-accelerating particles in its associated compression and shock waves. Moreover, it is shown that the intricate magnetic topology near and within the CIR can strongly affect the efficiency of cross-field diffusion in spreading particles through interplanetary space.

Moreover, we study the effects of the magnetic field curvature and gradient drifts on the decay phase of SEP events. It is illustrated that even for low energy protons (<36 MeV), drifts can potentially inhibit the phenomenon known as the SEP flood, unless they are mitigated through e.g., a sufficiently strong cross-field diffusion process. Particle drift effects are shown to become increasingly important in non-nominal solar wind conditions, such as near a CIR.

Finally, the transport and acceleration of 100~keV protons in a solar wind containing a CME are modelled. This is done by using the cone CME model of EUHFORIA, and illustrates the current capabilities of PARADISE to model the evolution of energetic particle distributions in a dynamic solar wind containing a large-scale transient structure.

Les partícules d’alta energia que travessen l’entorn espacial terraqui són una amenaça tant per als satèl·lits com per futures missions tripulades a la Lluna o Mart. Per tant, conèixer detalladament l’entorn de radiació de partícules en el sistema solar és crucial. Aquesta radiació es compon principalment d’electrons, protons i altres ions accelerats en erupcions solars: fulguracions i ejeccions coronals de massa (CMEs). El transport i l’acceleració d’aquestes partícules energètiques solars (SEPs) en el vent solar es descriu amb l’equació de transport focalitzat (FTE). Aquesta equació s’obté de l’equació de Vlasov tot suposant que les SEPs viatgen acol·lisionalment en el si d’un plasma turbulent, els efectes del qual es modelitzen via procesos de difusió. A més, demostrem que les derives del centre-guia es poden incloure en l’FTE, el que mena a un procés de difusió transversal. Hem desenvolupat el model numèric PARADISE que calcula intensitats de les SEPs a l’heliosfera interna en un vent solar realista. PARADISE resol l’FTE tot integrant un conjunt d’equacions diferencials estocàstiques d’Itô. Per a això, empra un vent solar generat per un model magnetohidrodinàmic; en particular usem el model EUHFORIA. Hem investigat el transport de SEPs en un vent solar que conté una regió corrotacional interactiva (CIR). Demostrem com la CIR altera la distribució de les SEPs tot actuant com un mirall magnètic eficient i accelerant partícules en les seves ones de xoc i zona de compressió. A més, la topologia complexa de la CIR afecta l’eficiència dels processos de difusió transversal a l’hora d’escampar partícules a l’heliosfera. Hem estudiat també els efectes de deriva deguts a la curvatura i gradient del camp magnètic en la fase de decaïment dels SEPs. Fins i tot per a protons de baixa energia (< 36 MeV), les derives poden inhibir el fenomen de reservori-SEP, si és que no són contrarestades per la difusió transversal. Mostrem que els efectes de les derives es veuen enfortits en presència d’estructures com les CIRs. Finalment, demostrem la capacitat de PARADISE de modelitzar el transport i acceleració de SEPs en vents solars dinàmics on hi ha grans estructures transitòries com les CMEs.