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Solar flare electron acceleration is an extremely efficient process, but the method of acceleration is not well constrained. Two of the essential diagnostics, electron anisotropy (velocity angle to the guiding magnetic field) and the high energy cutoff (highest energy electrons produced by the acceleration conditions: mechanism, spatial extent, and time), are important quantities that can help to constrain electron acceleration at the Sun but both are poorly determined. Here, by using electron and X-ray transport simulations that account for both collisional and non-collisional transport processes, such as turbulent scattering and X-ray albedo, we show that X-ray polarization can be used to constrain the anisotropy of the accelerated electron distribution and the most energetic accelerated electrons together. Moreover, we show that prospective missions, for example CubeSat missions without imaging information, can be used alongside such simulations to determine these parameters. We conclude that a fuller understanding of flare acceleration processes will come from missions capable of both X-ray flux and polarization spectral measurements together. Although imaging polarimetry is highly desired, we demonstrate that spectro-polarimeters without imaging can also provide strong constraints on electron anisotropy and the high energy cutoff. [ABSTRACT FROM AUTHOR]

After decades of observations of solar energetic particles from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼30° heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere. [ABSTRACT FROM AUTHOR]

Context. The HELIOS solar observation probes provide unique data regarding their orbit and operation time. One of the onboard instruments, the Experiment 6 (E6), is capable of measuring ions from 4 to several hundred MeV nucleon−1. Aims. In this paper we aim to demonstrate the relevance of the E6 data for the calculation of galactic cosmic ray (GCR), anomalous cosmic ray (ACR), and solar energetic particle (SEP) fluxes for different distances from the sun and time periods. Methods. Several corrections have been applied to the raw data: determination of the Quenching factor of the scintillator, correction of the temperature dependent electronics, degradation of the scintillator as well as the effects on the edge of semi-conductor detectors. Results. Fluxes measured by the E6 are in accordance with the force field solution for the GCR and match models of the anomalous cosmic ray propagation. GCR radial gradients in the inner heliosphere show a different behaviour than in the outer heliosphere. [ABSTRACT FROM AUTHOR]

Aims. We study how a fast solar wind stream embedded in a slow solar wind influences the spread of solar energetic protons in interplanetary space. In particular, we aim at understanding how the particle intensity and anisotropy vary along interplanetary magnetic field (IMF) lines that encounter changing solar wind conditions such as the shock waves bounding a corotating interaction region (CIR). Moreover, we study how the intensities and anisotropies vary as a function of the longitudinal and latitudinal coordinate, and how the width of the particle intensities evolves with the heliographic radial distance. Furthermore, we study how cross-field diffusion may alter these spatial profiles. Methods. To model the energetic protons, we used a recently developed particle transport code that computes particle distributions in the heliosphere by solving the focused transport equation (FTE) in a stochastic manner. The particles are propagated in a solar wind containing a CIR, which was generated by the heliospheric model, EUHFORIA. We study four cases in which we assume a delta injection of 4 MeV protons spread uniformly over different regions at the inner boundary of the model. These source regions have the same size and shape, yet are shifted in longitude from each other, and are therefore magnetically connected to different solar wind conditions. Results. The intensity and anisotropy profiles along selected IMF lines vary strongly according to the different solar wind conditions encountered along the field line. The IMF lines crossing the shocks bounding the CIR show the formation of accelerated particle populations, with the reverse shock wave being a more efficient accelerator than the forward shock wave. The longitudinal intensity profiles near the CIR are highly asymmetric in contrast to the profiles obtained in a nominal solar wind. For the injection regions that do not cross the transition zone between the fast and slow solar wind, we observe a steep intensity drop of several orders of magnitude near the stream interface (SI) inside the CIR. Moreover, we demonstrate that the longitudinal width of the particle intensity distribution can increase, decrease, or remain constant with heliographic radial distance, reflecting the underlying IMF structure. Finally, we show how the deflection of the IMF at the shock waves and the compression of the IMF in the CIR deforms the three-dimensional shape of the particle distribution in such a way that the original shape of the injection profile is lost. [ABSTRACT FROM AUTHOR]

Context. The Parker Solar Probe and the incoming Solar Orbiter mission will provide measurements of solar energetic particle (SEP) events at close heliocentric distances from the Sun. Up to present, the largest data set of SEP events in the inner heliosphere are the observations by the two Helios spacecraft. Aims. We re-visit a sample of 15 solar relativistic electron events measured by the Helios mission with the goal of better characterising the injection histories of solar energetic particles and their interplanetary transport conditions at heliocentric distances <1 AU. Methods. The measurements provided by the E6 instrument on board Helios provide us with the electron directional distributions in eight different sectors that we use to infer the detailed evolution of the electron pitch-angle distributions. The results of a Monte Carlo interplanetary transport model, combined with a full inversion procedure, were used to fit the observed directional intensities in the 300–800 keV nominal energy channel. Unlike previous studies, we have considered both the energy and angular responses of the detector. This method allowed us to infer the electron release time profile at the source and determine the electron interplanetary transport conditions. Results. We discuss the duration of the release time profiles and the values of the radial mean free path, and compare them with the values reported previously in the literature using earlier approaches. Five of the events show short injection histories (<30 min) at the Sun and ten events show long-lasting (>30 min) injections. The values of mean free path range from 0.02 AU to 0.27 AU. Conclusions. The inferred injection histories match with the radio and soft X-ray emissions found in literature. We find no dependence of the radial mean free path on the radial distance. In addition, we find no apparent relation between the strength of interplanetary scattering and the size of the solar particle release. [ABSTRACT FROM AUTHOR]

Context. 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. Aims. 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. Methods. 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. Results. 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. [ABSTRACT FROM AUTHOR]

Context: The PAMELA experiment observed galactic proton and electron spectra down to 70 MV and 400 MV, respectively, from mid-2006 to the end of 2009 during what is called an A < 0 solar magnetic polarity cycle. During this period, solar activity was at its lowest level since the beginning of the space exploration era. This provides the opportunity to study charge-sign-dependent modulation under very quiet heliospheric conditions. Aims. Drift theory for the solar modulation of cosmic rays predicts that the intensity of protons at the Earth is expected to show a different rate of recovery towards solar minimum when compared to electrons during A < 0 cycles. These charge-sign related differences are investigated. Methods: The solutions of a comprehensive three-dimensional drift model are compared to PAMELA spectra to authenticate the modelling approach and then to make predictions of how electrons and protons are differently modulated down to 1 MeV, based on new very local interstellar spectra. Results: The comparison of observations and modelling provides insight into how the rigidity dependence of the three major diffusion coefficients changes during such quiet modulation conditions. How drift effects dissipate above several GeV and below 100 MeV is illustrated for both protons and electrons. The modulation that occurred at the Earth during this quiet period is shown as a function of rigidity and time. The e-/p ratio is computed from 10 MV to 50 GV for this period and a prediction is made for what may be observed in terms of spectra during the next A > 0 solar minimum. Conclusions: The presence of drifts during this quiet period is established based on the presented modelling and PAMELA measurements. Drift effects for protons and electrons are quantified in terms of their rigidity and temporal development from 2006 to 2009. [ABSTRACT FROM AUTHOR]

Solar electron spike events are a special subclass of near-relativistic electron events characterized by their short duration, symmetric time profile, and their strong anisotropic pitch angle distribution. All previous studied events were only observed by a single spacecraft (s/c). For the first time, we present measurements of this kind of an electron spike event that was observed simultaneously by both close-spaced STEREO s/c on 2 May 2014. The longitudinal separation angle between STEREO-A (STA) and STEREO-B (STB) was 38°. The spikes at STA and STB are characterized by very short durations of ≤12 min at full-width at half maximum (FWHM) and almost identical and symmetric time profiles. They exhibit similar properties in durations, pitch angle distributions (PADs), energy spectra, and peak intensities. The spike parent source was situated close to the STA nominal magnetic footpoint and was separated by 48° from STB's footpoint. The intensity distribution and the relative onset timing behaved opposite to what is expected: STB measured a higher intensity and an earlier onset than the nominally better connected STA. We suggest that the spike electrons undergo a substantial nonradial injection into interplanetary (IP) space from the parent source, a flaring active region (AR), and propagate in a strong nonradial diverging magnetic field. In addition we show that because of the spike properties it is evident that, irrespective of how the electrons were injected into the IP medium, the en route particle scattering conditions along the paths to STA and STB were very similar. Evidently both s/c detected different parts of the same beam whose angular extension was at least 38° at 1 AU, comparable to the longitudinal separation between STA and STB. [ABSTRACT FROM AUTHOR]

The detectors of the Ultraviolet Spectrographs (UVS) on Voyager 1/2 are recording a background intensity that was earlier assigned mainly to disintegrations in the radio-isotope thermoelectric generator and systematically subtracted from the signal to infer photon counting. Here, we show that it arises instead from galactic cosmic rays (GCRs). We show the GCR flux measured by UVS on Voyager 1 from 1992 to August 2013, and by comparing to data from the GCR dedicated detectors, we estimate the energy range responsible for this UVS signal, which is around 300 MeV, and the response of UVS to the GCR anisotropy. After the abrupt jumps of May and August 2012, the count rate has been only slightly fluctuating around a constant value. However, comparing it to the data from the Low Energy Charge Particle Experiment (LECP) and the Cosmic Ray Subsystem (CRS) shows that those small variations are only responses to a varying anisotropy and not to a flux change. Taking advantage of the similarity in energy range to one of the products of the CRS instrument suite, we use the ratio between the two independent signals as a proxy for the temporal evolution of the GCR spectral slope around the 300 MeV range. We show that this slope remained unchanged since August 2012 and find strong evidence that it will no longer vary, implying the end of the heliospheric modulation at those energies, and that Voyager 1 at this date is near or past the heliopause. The origin of this unexpectedly narrow and stagnating inner heliosheath is still unclear, and we discuss the potential effects of low solar wind speed episodes and subsequent self-amplified charge-exchange with interstellar neutrals, as a source of deceleration and collapse. We suggest that the quasi-static region encountered by Voyager 1 may be related to such effects, triggered by the strong solar-maximum variability. This did not happen for Voyager 2 due to its trajectory at an angle further from the heliosphere axis and a later termination shock crossing. The existence on the upwind side of a mixing layer formed by charge transfer instead of a pure plasma contact discontinuity could explain various Voyager 1 observations. [ABSTRACT FROM AUTHOR]