Angels Aran

Radial and Longitudinal Dependence of Solar 4-13 MeV and 27-37 MeV Proton Peak Intensities and Fluences: Helios and IMP 8 Observations

We study the radial and longitudinal dependence of 4-13 and 27-37 MeV proton peak intensities and fluences measured within 1 AU of the Sun during intense solar energetic particle events. Data are from the IMP 8 and the two Helios spacecraft. We analyze 72 events and compute the total event fluence (F) and the peak intensity (J), distinguishing between the events absolute maximum intensity and that neglecting local increases associated with the passage of shocks or plasma structures. Simultaneous measurements of individual events by at least two spacecraft show that the dominant parameter determining J and F is the longitudinal separation () between the parent active region and the footpoint of the field line connecting each spacecraft with the Sun, rather than the spacecraft radial distance (R). We perform a multiparameter fit to the radial and longitudinal distributions of J and F for events with identified solar origin and that produce intensity enhancements in at least two spacecraft. This fit determines simultaneously the radial and longitudinal dependences of J and F. Radial distributions of events observed by at least two spacecraft show ensemble-averaged variations ranging from R-2.7 to R-1.9 for 4-13 and 27-37 MeV proton peak intensities, and R-2.1 to R-1.0 for 4-13 and 27-37 MeV proton event fluences, respectively. Longitudinal distributions of J and F are approximated by the form e, where 0 is the distribution centroid and k is found to vary between ~1.3 and ~1.0 rad-2. Radial dependences are less steep than both those deduced from diffusion transport models by Hamilton et al. in 1990 and those recommended by Shea et al. in 1988 to extrapolate J and F from R = 1 to R < 1 AU.

Modeling and forecasting solar energetic particle events at Mars : the event on 6 March 1989

Context. Large solar energetic particle events are able to enhance the radiation intensity present in interplanetary space by several orders of magnitude. Therefore the study, modeling and prediction of these events is a key factor to understand our space environment and to protect manned space missions from hazardous radiation. Aims. We model an intense solar energetic particle event observed simultaneously on the 6 of March 1989 by the near-Earth orbiting spacecraft IMP-8 and by the Phobos-2 spacecraft in orbit around Mars (located 72° to the East of the Earth and at 1.58 AU from the Sun). This particle event was associated with the second largest X-ray flare in solar cycle 22. The site of this long-duration X15/3B solar flare was N35E69 (as seen from the Earth) and the onset of the 1-8 A X-ray emission occurred at 1350 UT on 6 March 1989. A traveling interplanetary shock accompanied with <15 MeV proton intensity enhancements was observed by IMP-8 at 1800 UT on 8 March and by Phobos-2 at 2015 UT on 9 March. This shock determines the particle intensities at both spacecraft. Methods. We use an MHD code to model the propagation of the associated shock to both spacecraft and a particle transport code to model the proton intensities measured by IMP-8 and Phobos-2. By assuming that energetic particles are continuously accelerated by the traveling shock, and that the injection rate of these particles, Q, into the interplanetary medium is related to the upstream-to-downstream velocity ratio, VR, at the point of the shock front that connects with the observer, we perform predictions of the solar energetic particle intensities observed at Mars from those measured at Earth. Results. We reproduce not only the arrival times of the shock at both spacecraft but also the measured jump discontinuity of solar wind speed, density and magnetic field. Also, we reproduce the 0.5-20 MeV proton intensities measured by both spacecraft. Functional dependences such as the Q(VR) relation deduced here allow us to predict the proton intensities measured at Phobos-2 for this event. Applications of this model for future predictions of solar energetic particle fluxes at Mars are discussed.

SEPEM: A tool for statistical modeling the solar energetic particle environment

Solar energetic particle (SEP) events are a serious radiation hazard for spacecraft as well as a severe health risk to humans traveling in space. Indeed, accurate modeling of the SEP environment constitutes a priority requirement for astrophysics and solar system missions and for human exploration in space. The European Space Agencys Solar Energetic Particle Environment Modelling (SEPEM) application server is a World Wide Web interface to a complete set of cross-calibrated data ranging from 1973 to 2013 as well as new SEP engineering models and tools. Both statistical and physical modeling techniques have been included, in order to cover the environment not only at 1 AU but also in the inner heliosphere ranging from 0.2 AU to 1.6 AU using a newly developed physics-based shock-and-particle model to simulate particle flux profiles of gradual SEP events. With SEPEM, SEP peak flux and integrated fluence statistics can be studied, as well as durations of high SEP flux periods. Furthermore, effects tools are also included to allow calculation of single event upset rate and radiation doses for a variety of engineering scenarios.

Three frontside full halo coronal mass ejections with a nontypical geomagnetic response

After analyzing the source regions of these halo CMEs, it was found that the halo associated with the strongest geomagnetic disturbance was the one that initiated farther away from disk center (source region at W66); while the other two CMEs originated closer to the central meridian but had weaker geomagnetic responses. Therefore, these three events do not fit into the general statistical trends that relate the location of the solar source and the corresponding geoeffectivity. We investigate possible causes of such a behavior. Nonradial direction of eruption, passage of the Earth through a leg of an interplanetary flux rope, and strong compression at the eastern flank of a propagating interplanetary CME during its interaction with the ambient solar wind are found to be important factors that have a direct influence on the resulting north-south interplanetary magnetic field (IMF) component and thus on the CME geoeffectiveness. We also find indications that interaction of two CMEs could help in producing a long-lasting southward IMF component. Finally, we are able to explain successfully the geomagnetic response using plasma and magnetic field in situ measurements at the L1 point. We discuss the implications of our results for operational space weather forecasting and stress the difficulties of making accurate predictions with the current knowledge and tools at hand.

Key Links to Space Weather: Forecasting Solar-Generated Shocks and Proton Acceleration

Forecasting the arrival of solar-generated shocks and accelerated protons anywhere in the heliosphere presents an awesome challenge in the new field of space weather. Currently, observations of solar wind plasmas and interplanetary magnetic fields are made at the sun-Earth libration point, L1, about 0.01 astronomical units (∼245 Earth radii) sunward of our planet. An obvious analogy is the pilot tube that protrudes ahead of a supersonic vehicle. The Advanced Composition Explorer and Solar and Heliospheric Observatory spacecraft, currently performing this function, provide about -1 h advance notice of impending arrival of interplanetary disturbances. The signatures of these disturbances may be manifested as interplanetary shock waves and/or coronal mass ejecta. We describe a first-generation procedure, based on first-principles numerical modeling, that provides the key links required to increase the advance notice (or lead time) to days, or even weeks. This procedure, instituted at the start of the present solar cycle 23, involves three separate models, used in real time, to predict the arrival of solar-event-initiated interplanetary shock waves at the L1 location. We present statistical results, using L1 observations as ground truth for 380 events. We also briefly discuss how one of these models (Hakamada-Akasofu-Fry version 2) may be used with a model that predicts the flux and fluence of energetic particles, for energies up to 100 MeV, that are generated by these propagating interplanetary shock waves.

MODELING OF SOLAR ENERGETIC PARTICLES IN INTERPLANETARY SPACE

Solar energetic particles (SEPs) in the interplanetary (IP) medium are transported under the influence of electromagnetic fields of the solar wind. These fields consists of the smooth background fields, which can be modeled by the MHD equations governing the expansion of the solar wind, and of the small-scale fluctuations (waves or turbulence) that scatter the particles in pitch angle and act as agents enabling their acceleration at IP shock waves. We review theoretical models of SEP transport and acceleration in the IP medium. We start from the simple analytical approaches (diffusion models), which assume quasi-isotropic particle distributions, and then continue to the more accurate numerical approaches based on the focused transport equation, not making this simplifying assumption. A careful analysis of two SEP events, an impulsive and a gradual one, is presented and the spatial scaling of their peak intensities, differential fluences and time-integrated net fluxes is discussed. We conclude that rather simple scaling laws for these quantities can be obtained for impulsive events but no simple scaling laws can be expected to govern the gradual SEP events

Charged Particle Transport in the Interplanetary Medium

The scenario and fundamentals of the physics of charged particle interplanetary transport are briefly introduced. Relevant characteristics of solar energetic particle (SEP) events and of the interplanetary magnetic field are described. Next, the motion of a charged particle and the main assumptions leading to the description of the focused and diffusive particle transport equations utilised in the next chapters are discussed. Finally, two different models are applied to interpret SEP events.

Modelling of Shock-Accelerated Gamma-Ray Events

Solar γ-ray events recently detected by the Fermi/LAT instrument at energies above 100 MeV have presented a puzzle for solar physicists as many of such events were observed lasting for many hours after the associated flare/coronal mass ejection (CME) eruption. Data analyses suggest the γ-ray emission originate from decay of pions produced mainly by interactions of high-energy protons deep in the chromosphere. Whether those protons are accelerated in the associated flare or in the CME-driven shock has been under active discussion. In this chapter, we present some modelling efforts aimed at testing the shock acceleration hypothesis. We address two γ-ray events: 2012 January 23 and 2012 May 17 and approach the problem by, first, simulating the proton acceleration at the shock and, second, simulating their transport back to the Sun.

Clarifying some issues on the geoeffectiveness of limb halo CMEs

A recent study by Cid et al. (2012) showed that full halo coronal mass ejections (CMEs) coming from the limb can disturb the terrestrial environment. Although this result seems to rise some controversies with the well established theories, the fact is that the study encourages the scientific community to perform careful multidisciplinary analysis along the Sunto- Earth chain to fully understand which are the solar triggers of terrestrial disturbances. This paper aims to clarify some of the polemical issues arisen by that paper.

Proton Flux Prediction: First Version of an Operational Code

Solar energetic particle (SEP) events belong to the major causes of prejudicial effects to space systems and pose a serious hazard to astronauts. The characterization and modelling of flux profiles of SEP events in several locations of interplanetary space are among the key parameters for space weather effects (Feynmann et al., 2000, JGR, 105, 10543). We have developed a preliminary version of an operational code which provides proton flux profiles at 0.5 MeV and 2 MeV as well as the cumulative fluence for gradual SEP events, for spacecrafts located at 1 AU and 0.4 AU. This code is based on the particle transport model by Lario et al. (1998, ApJ, 509, 415) and the 2 1/2-D MHD shock propagation model of Wu et al. (1983, Sol. Phys., 84, 395). SEP events profiles strongly depend on the characteristics of the interplanetary shock and on the position of the observer in space (Cane et al., 1988, JGR, 93, 9555).