Nina Dresing

Statistical survey of widely spread out solar electron events observed with STEREO and ACE with special attention to anisotropies

Context. In February 2011, the two STEREO spacecrafts reached a separation of 180 degrees in longitude, offering a complete view of the Sun for the first time ever. When the full Sun surface is visible, source active regions of solar energetic particle (SEP) events can be identified unambiguously. STEREO, in combination with near-Earth observatories such as ACE or SOHO, provides three well separated viewpoints, which build an unprecedented platform from which to investigate the longitudinal variations of SEP events. Aims. We show an ensemble of SEP events that were observed between 2009 and mid-2013 by at least two spacecrafts and show a remarkably wide particle spread in longitude (wide-spread events). The main selection criterion for these events was a longitudinal separation of at least 80 degrees between active region and spacecraft magnetic footpoint for the widest separated spacecraft. We investigate the events statistically in terms of peak intensities, onset delays, and rise times, and determine the spread of the longitudinal events, which is the range filled by SEPs during the events. Energetic electron anisotropies are investigated to distinguish the source and transport mechanisms that lead to the observed wide particle spreads. Methods. According to the anisotropy distributions, we divided the events into three classes depending on different source and transport scenarios. One potential mechanism for wide-spread events is efficient perpendicular transport in the interplanetary medium that competes with another scenario, which is a wide particle spread that occurs close to the Sun. In the latter case, the observations at 1 AU during the early phase of the events are expected to show significant anisotropies because of the wide injection range at the Sun and particle-focusing during the outward propagation, while in the first case only low anisotropies are anticipated. Results. We find events for both of these scenarios in our sample that match the expected observations and even different events that do not agree with the scenarios. We conclude that probably both an extended source region at the Sun and perpendicular transport in the interplanetary medium are involved for most of these wide-spread events.

THE SOLAR ENERGETIC PARTICLE EVENT ON 2013 APRIL 11: AN INVESTIGATION OF ITS SOLAR ORIGIN AND LONGITUDINAL SPREAD

We investigate the solar phenomena associated with the origin of the solar energetic particle (SEP) event observed on 2013 April 11 by a number of spacecraft distributed in the inner heliosphere over a broad range of heliolongitudes. We use Extreme UltraViolet (EUV) and white-light coronagraph observations from the Solar Dynamics Observatory (SDO), the SOlar and Heliospheric Observatory (SOHO) and the twin Solar TErrestrial RElations Observatory spacecraft (STEREO-A and STEREO-B) to determine the angular extent of the EUV wave and coronal mass ejection (CME) associated with the origin of the SEP event. We compare the estimated release time of SEPs observed at each spacecraft with the arrival time of the structures associated with the CME at the footpoints of the field lines connecting each spacecraft with the Sun. Whereas the arrival of the EUV wave and CME-driven shock at the footpoint of STEREO-B is consistent, within uncertainties, with the release time of the particles observed by this spacecraft, the EUV wave never reached the footpoint of the field lines connecting near-Earth observers with the Sun, even though an intense SEP event was observed there. We show that the west flank of the CME-driven shock propagating at high altitudes above the solar surface was most likely the source of the particles observed near Earth, but it did not leave any EUV trace on the solar disk. We conclude that the angular extent of the EUV wave on the solar surface did not agree with the longitudinal extent of the SEP event in the heliosphere. Hence EUV waves cannot be used reliably as a proxy for the solar phenomena that accelerates and injects energetic particles over broad ranges of longitudes.

CIRCUMSOLAR ENERGETIC PARTICLE DISTRIBUTION ON 2011 NOVEMBER 3

Late on 2011 November 3, STEREO-A, STEREO-B, MESSENGER, and near-Earth spacecraft observed an energetic particle flux enhancement. Based on the analysis of in situ plasma and particle observations, their correlation with remote sensing observations, and an interplanetary transport model, we conclude that the particle increases observed at multiple locations had a common single-source active region and the energetic particles filled a very broad region around the Sun. The active region was located at the solar backside (as seen from Earth) and was the source of a large flare, a fast and wide coronal mass ejection, and an EIT wave, accompanied by type II and type III radio emission. In contrast to previous solar energetic particle events showing broad longitudinal spread, this event showed clear particle anisotropies at three widely separated observation points at 1 AU, suggesting direct particle injection close to the magnetic footpoint of each spacecraft, lasting for several hours. We discuss these observations and the possible scenarios explaining the extremely broad particle spread for this event.

LONGITUDINAL PROPERTIES OF A WIDESPREAD SOLAR ENERGETIC PARTICLE EVENT ON 2014 FEBRUARY 25: EVOLUTION OF THE ASSOCIATED CME SHOCK

We investigate the solar phenomena associated with the origin of the solar energetic particle (SEP) event observed on 2014 February 25 by a number of spacecraft distributed in the inner heliosphere over a broad range of heliolongitudes. These include spacecraft located near Earth; the twin Solar TErrestrial RElations Observatory spacecraft, STEREO-A and STEREO-B, located at ∼1 au from the Sun 153° west and 160° east of Earth, respectively; the MErcury Surface Space ENvironment GEochemistry and Ranging mission (at 0.40 au and 31° west of Earth); and the Juno spacecraft (at 2.11 au and 48° east of Earth). Although the footpoints of the field lines nominally connecting the Sun with STEREO-A, STEREO-B and near-Earth spacecraft were quite distant from each other, an intense high-energy SEP event with Fe-rich prompt components was observed at these three locations. The extent of the extreme-ultraviolet wave associated with the solar eruption generating the SEP event was very limited in longitude. However, the white-light shock accompanying the associated coronal mass ejection extended over a broad range of longitudes. As the shock propagated into interplanetary space it extended over at least ∼190° in longitude. The release of the SEPs observed at different longitudes occurred when the portion of the shock magnetically connected to each spacecraft was already at relatively high altitudes (2 Re above the solar surface). The expansion of the shock in the extended corona, as opposite to near the solar surface, determined the SEP injection and SEP intensity-time profiles at different longitudes.

Proton intensity spectra during the solar energetic particle events of May 17, 2012 and January 6, 2014

Context. Ground-level enhancements (GLEs) are solar energetic particle events that show a significant intensity increase at energies that can be measured by neutron monitors. The most recent GLE-like events were recorded on May 17, 2012 and January 6, 2014. They were also measured by sophisticated instrumentation in space such as PAMELA and the Electron Proton Helium INstrument (EPHIN) onboard SOHO. Since neutron monitors are only sensitive to protons above 400 MeV with maximum sensitivity at 1 to 2 GeV, the spectra of such weak GLE-like events (January 6, 2014) can only be measured by space instrumentation. Aims. We show that the SOHO/EPHIN is capable of measuring the solar energetic particle proton event spectra between 100 MeV

MULTI-SPACECRAFT OBSERVATIONS AND TRANSPORT MODELING OF ENERGETIC ELECTRONS FOR A SERIES OF SOLAR PARTICLE EVENTS IN AUGUST 2010

During August 2010 a series of solar particle events, origin ating from the adjacent active regions 11093 and 11099, was observed by the two STEREO as well as by ne ar-Earth spacecraft. For the events occurring on the August 7 and 18 we combine in-situ and remote-sensing observations with predictions from our model of three-dimensional aniso tropic particle propagation in order to investigate the physical processes which cause the large an gular spreads of the energetic particles during these events. In particular, we address the effects o f lateral transport of the electrons in the solar corona as well as due to diffusion perpendicular to the average magnetic field in the interplanetary medium. We also study the influence of two Cor onal Mass Ejections and associated shock waves on the electron propagation, and a possible long itudinal variation in space of the transport conditions during the above period. For the Augus t 18 event we additionally utilize electron observations from the MESSENGER spacecraft at a di st nce of 0.31 AU from the Sun for an attempt to separate between radial and longitudinal d ependencies in the transport process.

Unexpected spatial intensity distributions and onset timing of solar electron events observed by closely spaced STEREO spacecraft

We present multi-spacecraft observations of four solar electron events using measurements from the Solar Electron Proton Telescope (SEPT) and the Electron Proton Helium INstrument (EPHIN) on board the STEREO and SOHO spacecraft, respectively, occurring between 11 October 2013 and 1 August 2014, during the approaching superior conjunction period of the two STEREO spacecraft. At this time the longitudinal separation angle between STEREO-A (STA) and STEREO-B (STB) was less than 72°. The parent particle sources (flares) of the four investigated events were situated close to, in between, or to the west of the STEREO’s magnetic footpoints. The STEREO measurements revealed a strong difference in electron peak intensities (factor ≤12) showing unexpected intensity distributions at 1 AU, although the two spacecraft had nominally nearly the same angular magnetic footpoint separation from the flaring active region (AR) or their magnetic footpoints were both situated eastwards from the parent particle source. Furthermore, the events detected by the two STEREO imply a strongly unexpected onset timing with respect to each other: the spacecraft magnetically best connected to the flare detected a later arrival of electrons than the other one. This leads us to suggest the concept of a rippled peak intensity distribution at 1 AU formed by narrow peaks (fingers) superposed on a quasi-uniform Gaussian distribution. Additionally, two of the four investigated solar energetic particle (SEP) events show a so-called circumsolar distribution and their characteristics make it plausible to suggest a two-component particle injection scenario forming an unusual, non-uniform intensity distribution at 1 AU.

SEPServer catalogues of solar energetic particle events at 1 AU based on STEREO recordings: 2007–2012

The Solar Terrestrial Relations Observatory (STEREO) recordings provide an unprecedented opportunity to study the evolution of solar energetic particle (SEP) events from different observation points in the heliosphere, allowing one to identify the effects of the properties of the interplanetary magnetic field (IMF) and solar wind structures on the interplanetary transport and acceleration of SEPs. Two catalogues based on STEREO recordings, have been compiled as a part of the SEPServer project, a three-year collaborative effort of eleven European partners funded under the Seventh Framework Programme of the European Union (FP7/SPACE). In particular, two instruments on board STEREO have been used to identify all SEP events observed within the descending phase of solar cycle 23 and the rising phase of solar cycle 24 from 2007 to 2012, namely: the Low Energy Telescope (LET) and the Solar Electron Proton Telescope (SEPT). A scan of STEREO/LET protons within the energy range 6–10 MeV has been performed for each of the two STEREO spacecraft. We have tracked all enhancements that have been observed above the background level of this particular channel and cross-checked with available lists of interplanetary coronal mass ejections (ICMEs), stream interaction regions (SIRs), and shocks, as well as with the reported events in literature. Furthermore, parallel scanning of the STEREO near relativistic electrons has been performed in order to pinpoint the presence (or absence) of an electron event in the energy range of 55–85 keV, for all of the aforementioned proton events included in our lists. We provide the onset and peak time as well as the peak value of all events for both protons and electrons, the relevant solar associations in terms of electromagnetic emissions, soft and hard X-rays (SXRs and HXRs). Finally, a subset of events with clear recordings at both STEREO spacecraft is presented together with the parent solar events of these multispacecraft SEP events.

Perpendicular diffusion of solar energetic particles: model results and implications for electrons

The processes responsible for the effective longitudinal transport of solar energetic particles (SEPs) are still not completely understood. We address this issue by simulating SEP electron propagation using a spatially 2D transport model that includes perpendicular diffusion. By implementing, as far as possible, the most reasonable estimates of the transport (diffusion) coefficients, we compare our results, in a qualitative manner, to recent observations {at energies of 55 -- 105 keV}, focusing on the longitudinal distribution of the peak intensity, the maximum anisotropy and the onset time. By using transport coefficients which are derived from first principles, we limit the number of free parameters in the model to: (i) the probability of SEPs following diffusing magnetic field lines, quantified by

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

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