To understand the origin of Solar Energetic Particles (SEPs), we must study their injection time relative to other solar eruption manifestations. Traditionally the injection time is determined using the Velocity Dispersion Analysis (VDA) where a linear fit of the observed event onset times at 1 AU to the inverse velocities of SEPs is used to derive the injection time and path length of the first-arriving particles. VDA does not, however, take into account that the particles that produce a statistically observable onset at 1 AU have scattered in the interplanetary space. We use Monte Carlo test particle simulations of energetic protons to study the effect of particle scattering on the observable SEP event onset above pre-event background, and consequently on VDA results. We find that the VDA results are sensitive to the properties of the pre-event and event particle spectra as well as SEP injection and scattering parameters. In particular, a VDA-obtained path length that is close to the nominal Parker spiral length does not imply that the VDA injection time is correct. We study the delay to the observed onset caused by scattering of the particles and derive a simple estimate for the delay time by using the rate of intensity increase at the SEP onset as a parameter. We apply the correction to a magnetically well-connected SEP event of June 10 2000, and show it to improve both the path length and injection time estimates, while also increasing the error limits to better reflect the inherent uncertainties of VDA.
Interplanetary Coronal Mass Ejection (ICME) shocks are known to accelerate particles and contribute significantly to Solar Energetic Particle (SEP) events. We have performed Magnetohydrodynamic-Particle in Cell (MHD-PIC) simulations of ICME shocks to understand the acceleration mechanism. These shocks vary in Alfvénic Mach numbers as well as in magnetic field orientations (parallel \& quasi-perpendicular). We find that Diffusive Shock Acceleration (DSA) plays a significant role in accelerating particles in a parallel ICME shock. In contrast, Shock Drift Acceleration (SDA) plays a pivotal role …
We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic ($\geq$83 keV) protons ($P_{EP}$) is larger than the pressure exerted by the interplanetary magnetic field ($P_{B}$). In the majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when $P_{EP}$ exceeds $P_{B}$ by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of the energetic particle intensities. When solar win…
Solar energetic particles (SEPs) are one of the extreme space weather phenomena. A huge SEP event increases the radiation dose received by aircrews, who should be warned of such events as early as possible. We developed a warning system for aviation exposure to SEPs. This article describes one component of the system, which calculates the temporal evolution of the SEP intensity and the spectrum immediately outside the terrestrial magnetosphere. To achieve this, we performed numerical simulations of SEP transport in interplanetary space, in which interplanetary SEP transport is described by the…
In this work, a gradual solar energetic particle (SEP) event observed by multispacecraft has been simulated. The time profiles of SEP fluxes accelerated by an interplanetary shock in the three-dimensional interplanetary space are obtained by solving numerically the Fokker-Planck focused transport equation. The interplanetary shock is modeled as a moving source of energetic particles. By fitting the 1979/03/01 SEP fluxes observed by Helios 1, Helios 2, and IMP 8 with our simulations, we obtain the best parameters for the shock acceleration efficiency model. And we also find that the particle pe…
The acceleration of charged particles by interplanetary shocks can drain a non-negligible fraction of the upstream ram pressure. For a sample of shocks observed in-situ at 1 AU by the ACE and Wind spacecraft, time-series of the non-Maxwellian components (supra-thermal and higher-energy) of the ion and electron energy spectra were acquired for each event. These were averaged for one hour before and after the time of the shock passage to determine their partial pressure. Using the MHD Rankine-Hugoniot jump conditions, we find that the fraction of the total upstream energy flux transferred to non…
During solar flares fast electron beams generated in the solar corona are non-thermal radio sources in terms of type III bursts. Sometimes they can enter into the interplanetary space, where they can be observed by in-situ measurements as it is done e.g. by the WIND spacecraft. On the other hand, they can be the source of non-thermal X-ray radiation as e.g. observed by RHESSI, if they precipitate toward the dense chromosphere due to bremsstrahlung. Since these energetic electrons are generated in the corona and observed at another site, the study of transport of such electrons in the corona an…
Solar energetic particles (SEPs), accelerated during solar eruptions, propagate in turbulent solar wind before being observed with in situ instruments. In order to interpret their origin through comparison with remote-sensing observations of the solar eruption, we thus must deconvolve the transport effects due to the turbulent magnetic fields from the SEP observations. Recent research suggests that the SEP propagation is guided by the turbulent meandering of the magnetic fieldlines across the mean magnetic field. However, the lengthening of the distance the SEPs travel, due to the fieldline me…
Fast interplanetary coronal mass ejections (interplanetary CMEs, or ICMEs) are the drivers of strongest space weather storms such as solar energetic particle events and geomagnetic storms. The connection between space weather impacting solar wind disturbances associated with fast ICMEs at Earth and the characteristics of causative energetic CMEs observed near the Sun is a key question in the study of space weather storms as well as in the development of practical space weather prediction. Such shock-driving fast ICMEs usually expand at supersonic speed during the propagation, resulting in the …
Energetic particle-mediated interplanetary shocks observed by Solar Orbiter
European Space Agency (ESA), European Space Astronomy Centre (ESAC)（欧洲空间局（ESA），欧洲空间天文中心（ESAC））；Heliospheric Physics Laboratory, Heliophysics Division, NASA Goddard Space Flight Center（NASA戈达德太空飞行中心，日球物理部，日球物理实验室）；Max-Planck-Institut für Kernphysik（马克斯·普朗克核物理研究所）；Johns Hopkins University Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；Istituto per la Scienza e Tecnologia dei Plasmi (ISTP), Consiglio Nazionale delle Ricerche（意大利国家研究委员会等离子体科学与技术研究所（ISTP））；Department of Physics and Astronomy, Queen Mary University of London（伦敦大学玛丽女王学院物理与天文学系）；Lunar and Planetary Laboratory, University of Arizona（亚利桑那大学月球与行星实验室）；Institute of Experimental and Applied Physics, Kiel University（基尔大学实验与应用物理研究所）
We calculate the interplanetary magnetic field path lengths traveled by electrons in solar electron events detected by the WIND 3DP instrument from $1994$ to $2016$. The velocity dispersion analysis method is applied for electrons at energies of $\sim$ $27$ keV to $310$ keV. Previous velocity dispersion analyses employ the onset times, which are often affected by instrumental effects and the pre-existing background flux, leading to large uncertainties. We propose a new method here. Instead of using the peak or onset time, we apply the velocity dispersion analysis to the times that correspond t…
During solar flares a large amount of electrons with energies greater than 20 keV is generated with a production rate of typically $10^{36}$ s$^{-1}$. A part of them is able to propagate along open magnetic field lines through the corona into interplanetary space. During their travel they emit radio radiation which is observed as type III radio bursts in the frequency range from 100 MHz down to 10 kHz by the WAVES radio spectrometer aboard the spacecraft WIND, for instance. From the drift rates of these bursts in dynamic radio spectra the radial propagation velocity $V_r$ of the type III burst…
The acceleration of charged particles by interplanetary shocks (IPs) can drain a non-negligible fraction of the plasma pressure. In this study, we have selected 17 IPs observed in-situ at $1\,\text{au}$ by the Advanced Composition Explorer (ACE) and the Wind spacecraft, and 1 shock at $0.8\,\text{au}$ observed by Parker Solar Probe (PSP). We have calculated the time-dependent partial pressure of supra-thermal and energetic particles (smaller and greater than $50\,\text{keV}$ for protons and $30\,\text{keV}$ for electrons, respectively) in both the upstream and downstream regions. The particle …
Forecasting Solar Energetic Particle (SEP) fluence, as integrated over an SEP event, is an important element when estimating the effect of solar eruptions on humans and technology in space. Current real-time estimates are based on SEP measurements at a single location in space. However, the interplanetary magnetic field corotates with the Sun approximately 13$^\circ$ each day with respect to Earth, thus in 4 days a near-Earth spacecraft will have changed their connection about 60$^\circ$ from the original SEP source. We estimate the effect of the corotation on particle fluence using a simple p…
We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1AU: standard solar wind or interplanetary coron…
An X3.4 solar flare and a fast halo coronal mass ejection (CME) occurred on 2006 December 13, accompanied by a high flux of energetic particles recorded both in near-Earth space and at ground level. Our purpose is to provide evidence of flare acceleration in a major solar energetic particle (SEP) event. We first present observations from ACE/EPAM, GOES, and the Apatity neutron monitor. It is found that the initial particle release time coincides with the flare emission and that the spectrum becomes softer and the anisotropy becomes weaker during particle injection, indicating that the accelera…
Solar Energetic Particles (SEPs), a major component of space weather, propagate through the interplanetary medium strongly guided by the Interplanetary Magnetic Field (IMF). In this work, we analyse the implications a flat Heliospheric Current Sheet (HCS) has on proton propagation from SEP release sites to the Earth. We simulate proton propagation by integrating fully 3-D trajectories near an analytically defined flat current sheet, collecting comprehensive statistics into histograms, fluence maps and virtual observer time profiles within an energy range of 1--800 MeV. We show that protons exp…
This paper investigates the onset time of Solar Energetic Particle (SEP) events with numerical simulations, and analyses the accuracy of the Velocity Dispersion Analysis (VDA) method. Using a 3-dimensional focused transport model, we calculate the fluxes of protons observed in the ecliptic at 1 AU in the energy range between 10 MeV and 80 MeV. In particular, three models are used to describe different SEP sources produced by flare or coronal shock, and the effects of particle perpendicular diffusion in the interplanetary space are also studied. We have the following findings: When the observer…
Energetic Particle Acceleration and Transport: Interplanetary Coronal Mass Ejections and Shocks
National Observatory of Athens, IAASARS（雅典国家天文台）；California Institute of Technology（加州理工学院）；University of Arizona（亚利桑那大学）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体物理科学系）；European Space Agency (ESA), European Space Astronomy Centre (ESAC)（欧洲空间局（ESA），欧洲空间天文中心（ESAC））；Universidad de Alcalá, Space Research Group (SRG-UAH)（阿尔卡拉大学，空间研究小组（SRG-UAH））；The Johns Hopkins University, Applied Physics Laboratory（约翰斯·霍普金斯大学，应用物理实验室）；Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel（基尔大学实验与应用物理研究所）；National Key Laboratory of Deep Space Exploration/School of Earth and Space Sciences, University of Technology and Science of China（中国科学技术大学深空探测全国重点实验室/地球与空间科学学院）；Institute of Physics, University of Graz（格拉茨大学物理研究所）；NASA, Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；University of New Hampshire（新罕布什尔大学）；Southwest Research Institute（西南研究院）；CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China（中国科学院比较行星学卓越创新中心，中国科学技术大学）
本综述利用多航天器多点多仪器观测，结合Solar Orbiter与Parker Solar Probe的近太阳高能粒子测量，研究行星际日冕物质抛射和激波对高能粒子加速与传输的作用，以解决相关未决问题。
The random walk of magnetic field lines is an important ingredient in understanding how the connectivity of the magnetic field affects the spatial transport and diffusion of charged particles. As solar energetic particles (SEPs) propagate away from near-solar sources, they interact with the fluctuating magnetic field, which modifies their distributions. We develop a formalism in which the differential equation describing the field line random walk contains both effects due to localized magnetic displacements and a non-stochastic contribution from the large-scale expansion. We use this formalis…
Relating in situ measurements of relativistic solar particles to their parent activity in the corona requires understanding the magnetic structures that guide them from their acceleration site to the Earth. Relativistic particle events are observed at times of high solar activity, when transient magnetic structures such as Interplanetary Coronal Mass Ejections (ICMEs) often shape the interplanetary magnetic field (IMF). They may introduce interplanetary paths that are longer than nominal, and magnetic connections rooted far from the nominal Parker spiral. We present a detailed study of the IMF…
Sheaths ahead of coronal mass ejections (CMEs) are large heliospheric structures that form with CME expansion and propagation. Turbulent and compressed sheaths contribute to the acceleration of particles in the corona and in interplanetary space, but the relation of their internal structures to particle energization is still relatively little studied. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open problem. This work seeks to provide new insights on the internal structure of CME sheaths with regard to energetic particle enhan…
Small 3He-rich solar energetic particle (SEP) events have been commonly associated with extreme-ultraviolet (EUV) jets and narrow coronal mass ejections (CMEs) which are believed to be the signatures of magnetic reconnection involving field lines open to interplanetary space. The elemental and isotopic fractionation in these events are thought to be caused by processes confined to the flare sites. In this study we identify 32 3He-rich SEP events observed by the Advanced Composition Explorer near the Earth during the solar minimum period 2007-2010 and examine their solar sources with the high r…
The event-averaged charge state of heavy ion Solar Energetic Particles (SEPs), measured at 1 AU from the Sun, typically increases with the ions' kinetic energy. The origin of this behaviour has been ascribed to processes taking place within the acceleration region. In this paper we study the propagation through interplanetary space of SEP Fe ions, injected near the Sun with a variety of charge states that are uniformly distributed in energy, by means of a 3D test particle model. In our simulations, due to gradient and curvature drifts associated with the Parker spiral magnetic field, ions of d…
Drifts are known to play a role in galactic cosmic ray transport within the heliosphere and are a standard component of cosmic ray propagation models. However, the current paradigm of Solar Energetic Particle (SEP) propagation holds the effects of drifts to be negligible, and they are not accounted for in most current SEP modelling efforts. We present full-orbit test particle simulations of SEP propagation in a Parker spiral interplanetary magnetic field which demonstrate that high energy particle drifts cause significant asymmetric propagation perpendicular to the interplanetary magnetic fiel…
The spatial and temporal invariance in the spectra of energetic particles in the gradual solar events is reproduced in the simulations. Based on a numerical solution of the focused transport equation, we obtain the intensity time profiles of solar energetic particles (SEPs) accelerated by an interplanetary shock in the three-dimensional interplanetary space. The shock is treated as a moving source of energetic particles with a distribution function. The time profiles of particle flux with different energies are calculated in the ecliptic at $1$ AU. According to our model, we find that shock ac…
Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for Solar Energetic Particles (SEPs). As a result they have so far been ignored in SEP propagation modelling and data analysis. We examine drift velocities in the Parker spiral within single particle first-order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy rang…
We investigate the acceleration source of the impulsive solar energetic particle (SEP) events on 2007 January 24. Combining the in situ electron measurements and remote-sensing solar observations, as well as the calculated magnetic fields obtained from a potential-field source-surface model, we demonstrate that the jets associated with the hard X-ray flares and type-III radio bursts, rather than the slow and partial coronal mass ejections, are closely related to the production of interplanetary electron streams. The jets, originated from the well-connected active region (AR 10939) whose magnet…
A functional form I_{max}(R)=kR^{-α}, where R is the radial distance of spacecraft, was usually used to model the radial dependence of peak intensities I_{max}(R) of solar energetic particles (SEPs). In this work, the five-dimensional Fokker-Planck transport equation incorporating perpendicular diffusion is numerically solved to investigate the radial dependence of SEP peak intensities. We consider two different scenarios for the distribution of spacecraft fleet: (1) along the radial direction line, (2) along the Parker magnetic field line. We find that the index αin the above expression varie…
Understanding the transport of Solar Energetic Particles (SEPs) from acceleration sites at the Sun into interplanetary space and to the Earth is an important question for forecasting space weather. The Interplanetary Magnetic Field (IMF), with two distinct polarities and a complex structure, governs energetic particle transport and drifts. We analyse for the first time the effect of a wavy Heliospheric Current Sheet (HCS) on the propagation of SEPs. We inject protons close to the Sun and propagate them by integrating fully 3D trajectories within the inner heliosphere in the presence of weak sc…
Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensiona…
We investigate multi-spacecraft observations of the January 17, 2010 solar energetic particle event. Energetic electrons and protons have been observed over a remarkable large longitudinal range at the two STEREO spacecraft and SOHO suggesting a longitudinal spread of nearly 360 degrees at 1AU. The flaring active region, which was on the backside of the Sun as seen from Earth, was separated by more than 100 degrees in longitude from the magnetic footpoints of each of the three spacecraft. The event is characterized by strongly delayed energetic particle onsets with respect to the flare and onl…
The dearth of observations between 1 au and 3 au limits our understanding of energetic particle acceleration processes in interplanetary space. We present the first-of-their-kind observations of the energetic particle acceleration in a Corotating Interaction Region (CIR) using data from two vantage points, 1 au (near Earth) and 1.5 au (near Mars). The CIR event of June 2015 was observed by the particle detectors aboard the Advanced Composition Explorer (ACE) satellite as well as the SEP (Solar Energetic Particle) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft s…
We draw a comparison between a solar energetic particle event associated with the release of a slow coronal mass ejection close to the sun, and the energetic particle population produced in high current density field-aligned current structures associated with auroral phenomena in planetary magnetospheres. We suggest that this process is common in CME development and lift-off in the corona, and may account for the electron populations that generate Type III radio bursts, as well as for the prompt energetic ion and electron populations typically observed in interplanetary space.
Context. Interplanetary (IP) shocks are known to be accelerators of energetic charged particles observed in-situ in the heliosphere. However, the acceleration of near-relativistic electrons by shocks in the interplanetary medium is often questioned. On 9 August 2011 a Corotating Interaction Region (CIR) passed STEREO B (STB) that resulted in a flux increase in the electron and ion channels of the Solar Electron and Proton Telescope (SEPT). Because electron measurements in the few keV to several 100 keV range rely on the so-called magnet foil technique, which is utilized by SEPT, ions can contr…
Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion. Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters. Methods. We us…
Type III bursts and hard X-rays are both produced by flare energetic electron beams. The link between both emissions has been investigated in many previous studies, but no statistical studies have compared both coronal and interplanetary type III bursts with X-ray flares. Using coronal radio events above 100 MHz exclusively from type III bursts, we revisited long-standing questions: Do all coronal type III bursts have X-ray counterparts. What correlation, if any, occurs between radio and X-ray intensities. What X-ray and radio signatures above 100 MHz occur in connection with interplanetary ty…
We investigate the spatial distribution of charged particles accelerated by non-relativistic oblique fast collisionless shocks using three-dimensional test-particle simulations. We find that the density of low-energy particles exhibit a localised enhancement at the shock, resembling the "spike" measured at interplanetary shocks. In contrast to previous results based on numerical solutions to the focused transport equation, we find a shock spike for any magnetic obliquity, from quasi-perpendicular to parallel. We compare the pitch-angle distribution with respect to the local magnetic field and …
We report on the space weather events of solar cycle 24 in comparison with those during a similar epoch in cycle 23. We find major differences in all space weather events: solar energetic particles, geomagnetic storms, and interplanetary shocks. Dearth of ground level enhancement (GLE) events and major geomagnetic storms during cycle 24 clearly standout. The space weather events seem to reflect the less frequent solar eruptions and the overall weakness of solar cycle 24.
Solar Energetic Particles (SEPs) are an important component of Space Weather, including radiation hazard to humans and electronic equipment, and the ionisation of the Earth's atmosphere. We review the key observations of SEPs, our current understanding of their acceleration and transport, and discuss how this knowledge is incorporated within Space Weather forecasting tools. Mechanisms for acceleration during solar flares and at shocks driven by Coronal Mass Ejections are discussed, as well as the timing relationships between signatures of solar eruptive events and the detection of SEPs in inte…
We compare the characteristics of flare-accelerated energetic electrons at the Sun with those injected into interplanetary space. We have identified 17 energetic electron events well-observed with the SEPT instrument aboard STEREO which show a clear association with a hard X-ray (HXR) flare observed with the RHESSI spacecraft. We compare the spectral indices of the RHESSI HXR spectra with those of the interplanetary electrons. Because of the frequent double-power-law shape of the in situ electron spectra, we paid special attention to the choice of the spectral index used for comparison. The ti…
We study the characteristics of moving type IV radio bursts which extend to the hectometric wavelengths (interplanetary type IV or type IV IP bursts) and their relationship with energetic phenomena on the Sun. Our dataset comprises 48 interplanetary type IV bursts observed with Wind/WAVES in the 13.825 MHz-20 kHz frequency range. The dynamic spectra of the RSTN, the Nancay Decametric Array (DAM), the ARTEMIS-IV, the Culgoora, Hiraiso and Izmiran Radio Spectrographs were used to track the evolution of the events in the low corona. These were supplemented with SXR flux data from the GOES and CME…
