EUHFORIA : European heliospheric forecasting information asset

The implementation and first results of the new space weather forecasting-targeted inner heliosphere model "European heliospheric forecasting information asset" (EUHFORIA) are presented. EUHFORIA consists of two major components: a coronal model and a heliosphere model including coronal mass ejections. The coronal model provides data-driven solar wind plasma parameters at 0.1AU by constructing a model of the coronal large-scale magnetic field and employing empirical relations to determine the plasma state such as the solar wind speed and mass density. These are then used as boundary conditions to drive a three-dimensional time-dependent magnetohydrodynamics model of the inner heliosphere up to 2 AU. CMEs are injected into the ambient solar wind modeled using the cone model, with their parameters obtained from fits to imaging observations. In addition to detailing the modeling methodology, an initial validation run is presented. The results feature a highly dynamic heliosphere that the model is able to capture in good agreement with in situ observations. Finally, future horizons for the model are outlined.Peer reviewe

Magnetohydrodynamic modeling of large-amplitude waves in the solar corona

Solar eruptions are a consequence of the complex dynamics occurring in the tenuous, hot magnetized plasma that characterizes the solar corona. From a socio-economic viewpoint, solar eruptions can be argued to be the most important manifestation of the magnetic activity of the Sun due to their role as the main drivers of space weather, i.e., conditions in space that can have an adverse impact on space- as well as ground-based technologies such as telecommunication, electric power systems and satellite navigation. The launch of new space-based solar observatories during the past two decades has resulted in a dramatic improvement of the instrumentation monitoring the inner heliosphere. In spite of the advances in the observational capabilities, the physics of the solar eruptions as well as the nature of the various transient large-scale coronal phenomena observationally associated with the eruptions remain elusive. Constructing models capable of simulating the coronal and heliospheric dynamics is a viable path for gaining a more complete understanding of these phenomena. This thesis is concerned with developing a simulation tool based on the magnetohydrodynamic (MHD) description of plasmas and employing it for studying the characteristics of large-scale waves and shocks launched into the solar corona by the lift-off of a solar eruption such as a coronal mass ejection (CME). A particular focus is on discussing the role of large-amplitude waves in producing transient phenomena such as EIT waves and solar energetic particle (SEP) events that are known to appear in conjunction with CMEs. A suite of MHD models of the solar corona are constructed that allow the study of the coronal dynamics in varying environments at several stages of the eruption. For this purpose, novel robust numerical methods for solving the equations of magnetohydrodynamics in orthogonal curvilinear geometries in multiple dimensions are derived, forming the basis of the numerical simulation tool developed for the thesis. The results show that a dynamically intricate global shock front degenerating to a fast-mode MHD wave towards the surface of the Sun is an essential and natural part of the eruption complex that plays a key role in the generation of eruption-related transient phenomena. For instance, the close resemblance between the on-disk signatures produced by the fast-mode wave and EIT waves suggest a wave interpretation of the latter. The simulations also reveal that a highly non-trivial evolution of the shock properties on coronal field lines occurs even for simple coronal conditions, highlighting the need for more sophisticated models of particle acceleration than generally used so far. The results of the thesis are of particular importance for the continuing efforts to construct reliable physics-based models of the inner heliosphere for use in space weather applications.I det tunna och heta magnetiserade plasmat, som kännetecknar solens korona, uppkommer soleruptioner som en följd av den komplicerade dynamiken i plasmat. I och med eruptionernas roll som huvudorsakaren av rymdvädret kan man ur en socioekonomisk synvinkel anse att soleruptioner är den viktigaste manifestationen av solens magnetiska aktivitet. Med begreppet rymdväder förstås sådana förhållanden i rymden som negativt kan påverka teknologiska system såväl i rymden som på jorden, till exempel telekommunikation, elsystem och satellitnavigering. I och med att nya rymdbaserade solobservatorier tagits i bruk de senaste två decennierna har instrumentationen som observerar den inre heliosfären förbättrats dramatiskt. Trots framstegen i observationsmöjligheterna är fysiken bakom soleruptionerna samt karaktären av diverse tillfälliga storskaliga fenomen som observeras i koronan i samband med eruptionerna fortfarande svårfångad. Ett sätt att uppnå en mera komplett förståelse av dessa fenomen är att konstruera modeller som kan simulera koronans och heliosfärens dynamik. I denna avhandling har ett simulationsverktyg utvecklats och tillämpats för att studera globala vågor och chockvågor i solens korona orsakade av soleruptioner så som koronamassutkast. Simulationsverktyget grundar sig på den magnetohydrodynamiska (MHD) beskrivningen av plasmor. Ett bärande tema är att diskutera rollen av dessa vågor i uppkomsten av fenomen som observeras i samband med koronamassutkast, exempelvis så kallade EIT vågor samt utbrott av energetiska partiklar. En svit av MHD modeller av solens korona har konstruerats i avhandlingen. Modellerna möjliggör studiet av koronans dynamik i varierande förhållanden och olika skeden av eruptionen. För detta ändamål har numeriska metoder som löser magnetohydrodynamikens ekvationer i ortogonala kroklinjiga geometrier i flera dimensioner utvecklats. Dessa numeriska metoder utgör grunden för det nya simulationsverktyget. Resultaten av modelleringen visar att en dynamiskt invecklad global chockfront, som övergår till en snabb magnetosonisk våg i närheten av solens yta, är en väsentlig och naturlig del av eruptionskomplexet. Dessa spelar en avgörande roll för uppkomsten av fenomen relaterade till eruptionen. Ett exempel är likheten mellan EIT vågen och den snabba magnetosoniska vågen på solens skiva. Detta antyder en vågtolkning av EIT vågorna. Simulationerna visar också att en icke-trivial evolution av chockvågens egenskaper på magnetiska fältlinjer i koronan förekommer även under relativt enkla omständigheter, vilket belyser behovet av att utveckla mera sofistikerade modeller för partikelacceleration i chockvågor i koronan. Avhandlingens resultat är av särskild betydelse för de fortsatta ansträngningarna att konstruera pålitliga fysikbaserade modeller av den inre heliosfären för applikationer med anknytning till rymdvädret

Exploring the coronal evolution of AR 12473 using time-dependent, data-driven magnetofrictional modelling

Erratum: 10.1051/0004-6361/202038925e.Aims. We present a detailed examination of the magnetic evolution of AR 12473 using time-dependent, data-driven magnetofrictional modelling.Methods. We used maps of the photospheric electric field inverted from vector magnetogram observations, obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO), to drive our fully time-dependent, data-driven magnetofrictional model. Our modelled field was directly compared to extreme ultraviolet observations from the Atmospheric Imaging Assembly, also onboard SDO. Metrics were also computed to provide a quantitative analysis of the evolution of the magnetic field.Results. The flux rope associated with the eruption on 28 December 2015 from AR 12473 was reproduced by the simulation and found to have erupted due to a torus instability.Peer reviewe

Time-dependent Data-driven Modeling of Active Region Evolution Using Energy-optimized Photospheric Electric Fields

In this work, we present results of a time-dependent data-driven numerical simulation developed to study the dynamics of coronal active region magnetic fields. The evolving boundary condition driving the model, the photospheric electric field, is inverted using a time sequence of vector magnetograms as input. We invert three distinct electric field datasets for a single active region. All three electric fields reproduce the observed evolution of the normal component of the magnetic field. Two of the datasets are constructed so as to match the energy input into the corona to that provided by a reference estimate. Using the three inversions as input to a time-dependent magnetofrictional model, we study the response of the coronal magnetic field to the driving electric fields. The simulations reveal the magnetic field evolution to be sensitive to the input electric field despite the normal component of the magnetic field evolving identically and the total energy injection being largely similar. Thus, we demonstrate that the total energy injection is not sufficient to characterize the evolution of the coronal magnetic field: coronal evolution can be very different despite similar energy injections. We find the relative helicity to be an important additional metric that allows one to distinguish the simulations. In particular, the simulation with the highest relative helicity content produces a coronal flux rope that subsequently erupts, largely in agreement with extreme-ultraviolet imaging observations of the corresponding event. Our results suggest that time-dependent data-driven simulations that employ carefully constructed driving boundary conditions offer a valuable tool for modeling and characterizing the evolution of coronal magnetic fields.Peer reviewe

MAFIAT : Magnetic field analysis tools

Peer reviewe

The Spheromak Tilting and How it Affects Modeling Coronal Mass Ejections

Spheromak-type flux ropes are increasingly used for modeling coronal mass ejections (CMEs). Many models aim at accurately reconstructing the magnetic field topology of CMEs, considering its importance in assessing their impact on modern technology and human activities in space and on the ground. However, so far there is little discussion about how the details of the magnetic structure of a spheromak affect its evolution through the ambient field in the modeling domain and what impact this has on the accuracy of magnetic field topology predictions. If the spheromak has its axis of symmetry (geometric axis) at an angle with respect to the direction of the ambient field, then the spheromak starts rotating so that its symmetry axis finally aligns with the ambient field. When using the spheromak in space weather forecasting models, this tilting can happen already during insertion and significantly affects the results. In this paper, we highlight this issue previously not examined in the field of space weather and we estimate the angle by which the spheromak rotates under different conditions. To do this, we generated simple purely radial ambient magnetic field topologies (weak/strong, positive/negative) and inserted spheromaks with varying initial speed, tilt, and magnetic helicity sign. We employ different physical and geometric criteria to locate the magnetic center of mass and axis of symmetry of the spheromak. We confirm that spheromaks rotate in all investigated conditions and their direction and angle of rotation depend on the spheromak's initial properties and ambient magnetic field strength and orientation.Peer reviewe

Uncovering erosion effects on magnetic flux rope twist

Context. Magnetic clouds (MCs) are transient structures containing large-scale magnetic flux ropes from solar eruptions. The twist of magnetic field lines around the rope axis reveals information about flux rope formation processes and geoeffectivity. During propagation MC flux ropes may erode via reconnection with the ambient solar wind. Any erosion reduces the magnetic flux and helicity of the ropes, and changes their cross-sectional twist profiles. Aims. This study relates twist profiles in MC flux ropes observed at 1 AU to the amount of erosion undergone by the MCs in interplanetary space. Methods. The twist profiles of two clearly identified MC flux ropes associated with the clear appearance of post eruption arcades in the solar corona are analyzed. To infer the amount of erosion, the magnetic flux content of the ropes in the solar atmosphere is estimated, and compared to estimates at 1 AU. Results. The first MC shows a monotonically decreasing twist from the axis to the periphery, while the second displays high twist at the axis, rising twist near the edges, and lower twist in between. The first MC displays a larger reduction in magnetic flux between the Sun and 1 AU, suggesting more erosion than that seen in the second MC. Conclusions. In the second cloud the rising twist at the rope edges may have been due to an envelope of overlying coronal field lines with relatively high twist, formed by reconnection beneath the erupting flux rope in the low corona. This high-twist envelope remained almost intact from the Sun to 1 AU due to the low erosion levels. In contrast, the high-twist envelope of the first cloud may have been entirely peeled away via erosion by the time it reaches 1 AU.Peer reviewe

Validation scheme for solar coronal models : Constraints from multi-perspective observations in EUV and white-light

Context. In this paper, we present a validation scheme to investigate the quality of coronal magnetic field models, which is based on comparisons with observational data from multiple sources. Aims. Many of these coronal models may use a range of initial parameters that produce a large number of physically reasonable field configurations. However, that does not mean that these results are reliable and comply with the observations. With an appropriate validation scheme, which is the aim of this work, the quality of a coronal model can be assessed. Methods. The validation scheme was developed with the example of the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) coronal model. For observational comparison, we used extreme ultraviolet and white-light data to detect coronal features on the surface (open magnetic field areas) and off-limb (streamer and loop) structures from multiple perspectives (Earth view and the Solar Terrestrial Relations Observatory - STEREO). The validation scheme can be applied to any coronal model that produces magnetic field line topology. Results. We show its applicability by using the validation scheme on a large set of model configurations, which can be efficiently reduced to an ideal set of parameters that matches best with observational data. Conclusions. We conclude that by using a combined empirical visual classification with a mathematical scheme of topology metrics, a very efficient and objective quality assessment for coronal models can be performed.Peer reviewe

Flux-tube-dependent propagation of Alfvén waves in the solar corona

Context. Alfven-wave turbulence has emerged as an important heating mechanism to accelerate the solar wind. The generation of this turbulent heating is dependent on the presence and subsequent interaction of counter-propagating Alfven waves. This requires us to understand the propagation and evolution of Alfven waves in the solar wind in order to develop an understanding of the relationship between turbulent heating and solar-wind parameters. Aims. We aim to study the response of the solar wind upon injecting monochromatic single-frequency Alfven waves at the base of the corona for various magnetic flux-tube geometries. Methods. We used an ideal magnetohydrodynamic model using an adiabatic equation of state. An Alfven pump wave was injected into the quiet solar wind by perturbing the transverse magnetic field and velocity components. Results. Alfven waves were found to be reflected due to the development of the parametric decay instability (PDI). Further investigation revealed that the PDI was suppressed both by efficient reflections at low frequencies as well as magnetic flux-tube geometries.Peer reviewe

Studying the spheromak rotation in data-constrained CME modelling with EUHFORIA and assessing its effect on the Bz prediction

A key challenge in space weather forecasting is accurately predicting the magnetic field topology of interplanetary coronal mass ejections (ICMEs), specifically the north-south magnetic field component (Bz) for Earth-directed CMEs. Heliospheric MHD models typically use spheromaks to represent the magnetic structure of CMEs. However, when inserted into the ambient interplanetary magnetic field, spheromaks can experience a phenomenon reminiscent of the condition known as the "spheromak tilting instability", causing its magnetic axis to rotate. From the perspective of space weather forecasting, it is crucial to understand the effect of this rotation on predicting Bz at 1 au while implementing the spheromak model for realistic event studies. In this work, we study this by modelling a CME event on 2013 April 11 using the "EUropean Heliospheric FORecasting Information Asset" (EUHFORIA). Our results show that a significant spheromak rotation up to 90 degrees has occurred by the time it reaches 1 au, while the majority of this rotation occurs below 0.3 au. This total rotation resulted in poor predicted magnetic field topology of the ICME at 1 au. To address this issue, we further investigated the influence of spheromak density on mitigating rotation. The results show that the spheromak rotation is less for higher densities. Importantly, we observe a substantial reduction in the uncertainties associated with predicting Bz when there is minimal spheromak rotation. Therefore, we conclude that spheromak rotation adversely affects Bz prediction in the analyzed event, emphasizing the need for caution when employing spheromaks in global MHD models for space weather forecasting.Comment: Accepted for publication in The Astrophysical Journal Supplement (ApJS) serie