Efficiency of particle acceleration at interplanetary shocks: Statistical study of STEREO observations

Context. Among others, shocks are known to be accelerators of energetic charged particles. However, many questions regarding the acceleration efficiency and the required conditions are not fully understood. In particular, the acceleration of electrons by shocks is often questioned. Aims. In this study we determine the efficiency of interplanetary shocks for $<$100 keV electrons, and for ions at $\sim$0.1 and $\sim$2 MeV energies, as measured by the Solar Electron and Proton Telescope (SEPT) instruments aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. Methods. We employ an online STEREO in situ shock catalog that lists all shocks observed between 2007 and mid 2014 (observed by STEREO A) and until end of 2013 (observed by STEREO B). In total 475 shocks are listed. To determine the particle acceleration efficiency of these shocks, we analyze the associated intensity increases (shock spikes) during the shock crossings. For the near-relativistic electrons, we take into account the issue of possible ion contamination in the SEPT instrument. Results. The highest acceleration efficiency is found for low energy ions (0.1 MeV), which show a shock-associated increase at 27% of all shocks. The 2 MeV ions show an associated increase only during 5% of the shock crossings. In the case of the electrons, the shocks are nearly ineffective. Only five shock-associated electron increases were found, which correspond to only 1% of all shock crossings

The longitudinal distribution of energetic particles in the inner heliosphere - multi-point observations with STEREO-

The datasets of the STEREO spacecraft, utilized in this thesis, built an unprecedented platform to investigate longitudinal particle distributions in the inner heliosphere. In contrast to previous space missions these two nearly identical spacecraft fly on Earth-like orbits around the Sun with successively increasing longitudinal separation to Earth. One spacecraft, therefore, runs ahead of the Earth, STEREO A, while the other one, STEREO B, trails behind. Beside plenty of up to date instruments carried by the STEREO spacecraft, one of the main advantages of the mission is the radial distance to the Sun which is nearly the same as that of the Earth. This is of great use when investigating energetic particles because radial effects can almost be neglected. Another important advance is that both spacecraft do not orbit the Sun ’blindly’: In addition to a number of in-situ experiments they are also equipped with several remote sensing instruments. These provide optical observations of the Sun and the corona at different wavelengths which were only available from the Earth’s viewpoint previous to the STEREO mission. In the region of STEREO’s orbit, two energetic particle populations lend themselves for longitudinal investigations: Energetic particles associated with corotating interaction regions (CIRs), and solar energetic particles (SEPs). The first part of this thesis deals with the former and the different effects which can cause variable CIR observations and associated particle increases. For this purpose, the special configuration of the STEREO spacecraft enables us to disentangle temporal and spatial effects. Furthermore, local interactions with transient structures were observed which obviously favor the local particle acceleration. The second part of this thesis presents investigations of solar energetic particle events which show remarkably wide particle spreads of up to 360 degrees in longitude in the inner heliosphere. These so-called wide-spread events are of special interest because the processes, yielding to these unexpectedly wide distributions, are not completely understood yet. On the basis of a detailed study of such an event and comparison with a 3D propagation model, we conclude that particle transport perpendicular to the mean magnetic field in the interplanetary medium cannot be neglected for wide spread events. In a second study we identified 21 of such wide-spread events and investigate these in a statistical manner. As a key characteristic the longitudinal anisotropy distribution is also investigated. By means of this information different types of events could be distinguished. In contrast to the first study where strong perpendicular diffusion was supposed to play the main role, several observations require a pre-distribution of the particles over large angular ranges close to the Sun. The second study, therefore, concludes that it is likely that both processes, perpendicular diffusion in the interplanetary medium as well as a lateral distribution in the corona (due to coronal transport, a shock or another so far unknown process) must be present at the same time to explain the majority of our observations

Long-lasting injection of solar energetic electrons into the heliosphere

The main sources of solar energetic particle (SEP) events are solar flares and shocks driven by coronal mass ejections (CMEs). While it is generally accepted that energetic protons can be accelerated by shocks, whether or not these shocks can also efficiently accelerate solar energetic electrons is still debated. In this study we present observations of the extremely widespread SEP event of 26 Dec 2013. To the knowledge of the authors, this is the widest longitudinal SEP distribution ever observed together with unusually long-lasting energetic electron anisotropies at all observer positions. Further striking features of the event are long-lasting SEP intensity increases, two distinct SEP components with the second component mainly consisting of high-energy particles, a complex associated coronal activity including a pronounced signature of a shock in radio type-II observations, and the interaction of two CMEs early in the event. The observations require a prolonged injection scenario not only for protons but also for electrons. We therefore analyze the data comprehensively to characterize the possible role of the shock for the electron event. Remote-sensing observations of the complex solar activity are combined with in-situ measurements of the particle event. We also apply a Graduated Cylindrical Shell (GCS) model to the coronagraph observations of the two associated CMEs to analyze their interaction. We find that the shock alone is likely not responsible for this extremely wide SEP event. Therefore we propose a scenario of trapped energetic particles inside the CME-CME interaction region which undergo further acceleration due to the shock propagating through this region, stochastic acceleration, or ongoing reconnection processes inside the interaction region. The origin of the second component of the SEP event is likely caused by a sudden opening of the particle trap.Comment: Reproduced with permission from Astronomy & Astrophysics, \c{opyright} ES

On the Role of Coronal Shocks for Accelerating Solar Energetic Electrons

We study the role of coronal mass ejection (CME) driven shocks in the acceleration of solar energetic electrons. Using observations by the two STEREO spacecraft, we correlate electron peak intensities of solar energetic particle events measured in situ with various parameters of the associated coronal shocks. These shock parameters were derived by combining 3D shock reconstructions with global modeling of the corona. This modeling technique provides also shock properties in the specific shock regions that are magnetically connected to the two STEREO spacecraft. We find significant correlations between the peak intensities and the Mach number of the shock with correlation coefficients of about 0.7, which are similar for electrons at similar to 1 MeV and protons at >60 MeV. Lower-energy electrons with <100 keV show a smaller correlation coefficient of 0.47. The causal relationship between electron intensities and the shock properties is supported by the vanishing correlations when peak intensities at STEREO A are related with the Alfvenic Mach number at the magnetic footpoint of STEREO B and vice versa, which yields correlation coefficients of 0.03 and -0.13 for similar to 1 MeV and <100 keV electron peak intensities, respectively. We conclude that the high-energy electrons are accelerated mainly by the shock, while the low-energy electrons are likely produced by a mixture of flare and shock-related acceleration processes

Solar energetic particle time series analysis with Python

Peer reviewe

Global energetics of solar powerful events on 6 September 2017

Solar flares and coronal mass ejections (CMEs) are thought to be the most powerful events on the Sun. They can release energy as high as 10^32 erg in tens of minutes,and could produce solar energetic particles (SEPs) in the interplanetary space. We explore global energy budgets of solar major eruptions on 6 September 2017, including the energy partition of a powerful solar flare, the energy budget of the accompanied CME and SEPs. In the wavelength range shortward of 222 nm, a major contribution of the flare radiated energy is in the soft X-ray (SXR) 0.1-7 nm domain. The flare energy radiated at wavelengths of Ly-alpha and middle ultraviolet is larger than that radiated in the extreme ultraviolet wavelength, but it is much less than that radiated in the SXR waveband. The total flare radiated energy could be comparable to the thermal and nonthermal energies. The energies carried by the major flare and its accompanied CME are roughly equal, and they are both powered by the magnetic free energy in the AR NOAA 12673. Moreover, the CME is efficient in accelerating SEPs, and that the prompt component (whether it comes from the solar flare or the CME) contributes only a negligible fraction.Comment: accepted for publication in Research in Astronomy and Astrophysic

Tracking a beam of electrons from the low solar corona into interplanetary space with the Low Frequency Array, Parker Solar Probe and 1 au spacecraft

Type III radio bursts are the result of plasma emission from mildly relativistic electron beams propagating from the low solar corona into the heliosphere where they can eventually be detected in situ if they align with the location of a heliospheric spacecraft. Here we observe a type III radio burst from 0.1-16 MHz using the Parker Solar Probe (PSP) FIELDS Radio Frequency Spectrometer (RFS), and from 10-80 MHz using the Low Frequency Array (LOFAR). This event was not associated with any detectable flare activity but was part of an ongoing noise storm that occurred during PSP encounter 2. A deprojection of the LOFAR radio sources into 3D space shows that the type III radio burst sources were located on open magnetic field from 1.6-3 $R_\odot$ and originated from a specific active region near the East limb. Combining PSP/RFS observations with WIND/WAVES and Solar Terrestrial Relations Observatory (STEREO)/WAVES, we reconstruct the type III radio source trajectory in the heliosphere interior to PSP's position, assuming ecliptic confinement. An energetic electron enhancement is subsequently detected in situ at the STEREO-A spacecraft at compatible times although the onset and duration suggests the individual burst contributes a subset of the enhancement. This work shows relatively small-scale flux emergence in the corona can cause the injection of electron beams from the low corona into the heliosphere, without needing a strong solar flare. The complementary nature of combined ground and space-based radio observations, especially in the era of PSP, is also clearly highlighted by this study.Comment: 17 pages, 10 figures, Submitted to ApJ, April 15 202

Three-dimensional modelling of the shock-turbulence interaction

The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully-developed plasma turbulence, employing a novel combination of magnetohydrodynamic (MHD) and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed.We find that the behaviour of shock ripples, i.e., shock surface fluctuations with short (a few ion skin depths, $d_i$) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multi-spacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterised by the presence of shock waves interacting with plasma turbulence.Comment: Submitted to MNRA