Introduction to the physics of solar eruptions and their space weather impact
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Research
Article submitted to journal
Subject Areas:
Astrophysics, Plasma Physics, SpaceWeather
Keywords:
Sun, magnetic fields, Coronal MassEjections
Author for correspondence:
V. Archontise-mail: [email protected]
Introduction to the physics ofsolar eruptions and theirspace weather impact
Vasilis Archontis and Loukas Vlahos St Andrews University, School of Mathematics andStatistics,St Andrews KY 16 9SS, UK Department of Physics, Aristotle University,54124 Thessaloniki, Greece
The physical processes, which drive powerful solareruptions, play an important role in our understandingof the Sun-Earth connection. In this Special Issue, wefirstly discuss how magnetic fields emerge from thesolar interior to the solar surface, to build up activeregions, which commonly host large-scale coronaldisturbances, such as coronal mass ejections (CMEs).Then, we discuss the physical processes associatedwith the driving and triggering of these eruptions, thepropagation of the large-scale magnetic disturbancesthrough interplanetary space and the interaction ofCMEs with Earth’s magnetic field. The accelerationmechanisms for the solar energetic particles related toexplosive phenomena (e.g. flares and/or CMEs) in thesolar corona are also discussed. The main aim of thisIssue, therefore, is to encapsulate the present state-of-the-art in research related to the genesis of solareruptions and their space-weather implications.This article is part of the theme issue ”Solareruptions and their space weather impact”. c (cid:13) The Authors. Published by the Royal Society under the terms of theCreative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author andsource are credited. a r X i v : . [ a s t r o - ph . S R ] M a y r s t a . r o y a l s o c i e t y pub li s h i ng . o r g P h il . T r an s . R . S o c ..................................................................
1. Introduction
The ultimate driver of eruptive solar phenomena (e.g. flares, coronal mass ejection, CMEs), whichhave a major impact on space weather, is the turbulent plasma flows in the solar convection zonethat shape the structure and dynamics of the solar atmosphere. It is important to highlight thateruptive phenomena are dynamically very complex and interrelated. Typically, they originatefrom solar active regions (ARs) and their physical properties and dynamics vary during thelifetime of an AR.Space Weather predominantly encompasses the impact of solar eruptions in the heliosphere[1]. The causal chain of physical processes that shape the structure and dynamics of eruptiveevents lies at the heart of the predictability of the solar magnetic activity and is vital to quantitativeSpace Weather forecasting. The progress needed to improve both the short and the long-termforecasting is enormous and it challenges our entire understanding of the problem [2]. In thisissue, we address four interrelated topics, namely: (i) the driving and triggering mechanisms ofsolar eruptions [3,4], (ii) the prediction of the geoeffective properties of CMEs [2], (iii) the SolarEnergetic Particles (SEPs) [5,6] and (iv) the interaction of CMEs with Earth’s magnetic field [7–9].The authorship of the review articles mainly stems from the members of the Hellenic NationalSpace Weather Research Network (HNSWR) ( http://proteus.space.noa.gr/~hnswrn/ )co-funded by the European Union and Greece. This Special Issue comprises the culmination ofthat research effort.
2. Main topics
In the following, we briefly describe the context of the contributed articles associated with thefour main topics presented in this Special Issue. (i) Mechanisms of Solar eruptions
One of the key physical process making solar magnetic activity possible is the emergence ofmagnetic flux from the interior of the Sun towards the surface and the outer solar atmosphere.The first review [4] discusses the mechanisms of solar eruptions originating from emerging fluxregions. Observational examples of eruptive events and numerical simulations of magnetic fluxemergence present some of the most recent developments and advances related to multi-scaleeruptions and ejections of hot and cool magnetized plasma (e.g. CMEs, jets) into the heliosphere.The second review [3] focuses on the fundamental properties of CME sources and highlights acertain causal and irreversible sequence of events that occur whenever a strong (flux-massiveand sheared) magnetic polarity inversion line develops in the coronal base of eruptive ARs. Thisirreversibility makes eruptions inevitable when certain thresholds of magnetic energy due toelectric currents (that is, available for release) and magnetic helicity are crossed in these regions.This finding, originally presented in [3], may explain why strong polarity inversion lines cannotdisappear without hosting at least one eruption. (ii) Geoffective properties of solar eruptions
In this topic, the review by Vourlidas et al. [2] presents some of the key CME impact parameters,which determine their geoeffectivness, such as: Time-of-Arrival (ToA), Speed of Arrival (SoA),Momentum of CME when impact the Earth’s magnetic field, duration of impact, etc. The reviewarticle focuses on how far we are form the “acceptable” predictions of the CME geoffectiveparameters, discusses the reasons that prevent us to do better at the moment and providestrategies to overcome the current open problems and challenges. r s t a . r o y a l s o c i e t y pub li s h i ng . o r g P h il . T r an s . R . S o c .................................................................. (iii) Solar Energetic Particles Solar Energetic Particles (SEP) are an integral part of the physical processes related to SpaceWeather. The review by [5] presents the acceleration mechanisms related to the explosivephenomena (flares and/or CMEs) inside the solar corona. Solar eruptions cause a large scalere-configuration of the AR coronal magnetic field and host at least two well known particleacceleration mechanisms: turbulent reconnection and turbulent shocks.SEP events are the outcome of an impulsive and a gradual component: the former relates tothe early, impulsive eruption stages dominated by magnetic reconnection and the latter is clearlythe signature of the CME shock acceleration. Impulsive and gradual events cannot easily bedistinguished, as the impulsive component can be hidden under the much more intense shock-accelerated component of a gradual event. Eruptions driven by the emerging magnetic flux areideal sources for the impulsive injection of SEP particles in the interplanetary space [5]. The articleby Anastasiadis et al. The review by [ ? ] points out the current challenges to advance our physicalunderstanding of the SEP events to the short- and long-term forecasting using empirical andphysics-based methods. A list of open questions and suggestions for future work on the subjectare also presented in this review. (iv) Magnetospheric, Ionospheric and Thermoshperic response to solar eruptions It is now accepted that Van Allen belts consist of two belts of energetic particles. The outer beltis very dynamic, composed entirely by electrons. Occasionally, a third (electron) belt is present.The outer belt electrons have been observed to reach ultra relativistic energies. The review by [7]discusses briefly the possible acceleration mechanisms for these electrons and their relation to thesolar eruptions or the fast solar wind streamers. These electrons are a serious hazard to spacecraftsand have earned the dubious reputation of the “satellite killers”. Electric fields and plasmawaves, which are driven by solar eruptions, propagating through the interplanetary space andimpacting the Earth are believed to be the sources of the ultra-relativistic particles. The detailedunderstanding of the acceleration of the ultra-relativistic electrons in the outer Van Allen belt is akey issue for Space Weather and many other astrophysical applications.The review by [8] discusses the ionospheric response to interplanetary disturbances driven bysolar eruptions, using two years worth of data from the Swarm fleet of satellites surveying theEarth’s top side ionosphere and measuring magnetic and electric fields. They present how to usein-situ data to study the occurrence of plasma instabilities and their impact on Space Weather.The review article by [9] is a brief overview of the key processes coupling the ionosphereand thermosphere with the magnetosphere, especially when the large interplanetary disturbancesdriven by solar eruptions reach and interact with Earth’s magnetic field. The challenges from thestate-of-the-art modeling, the missing data and our current understanding of how the impulsivelydriven ITM system operates is outlined. Some of the most challenging open questions from theITM coupling and its Space Weather impact are also presented.
3. Summary
In this Special Issue we present the ongoing research, the main advances and the openquestions shaping our understanding of solar eruptions and their Space Weather impact. Thephenomena associated with Space Weather are a paradigm for many astrophysical plasmas.Many of the problems addressed in this issue are important space physics problems onmagnetic stability, propagation of heliospheric disturbances and their interaction with planetarymagnetospheres. The topics presented in this issue include contributions from the three tenets ofbasic research (observations, theory and modelling) and extend into the largely unexplored areas(in Heliophysics) of research-to-operations issues, including forecasting tools and assessment,forecasting and now-casting, near-real time modeling and its challenges. r s t a . r o y a l s o c i e t y pub li s h i ng . o r g P h il . T r an s . R . S o c .................................................................. Funding.
Part of this study was funded by the European Union (European Social Fund) and the Greeknational funds through the Operational Program “Education and Lifelong Learning” of the National StrategicReference Frame Work Research Funding Program: Thales. Investing in Knowledge society through theEuropean Social Fund.
Acknowledgements.
We thank all the Authors for their insightful contributions, diligence and patience inhelping us to prepare this Special Issue. The Guest Editors thank the Senior Publishing Editor Bailey Fallonfor his sustained efforts to achieve a timely publication.
References
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