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Enhancements of suprathermal particles observed at 1AU often can be related to Corotating Interaction Regions (CIRs). The compression regions associated with CIRs and their driven shocks which typically form at a few AU distance to the Sun can efficiently accelerate particles. If accelerated at the trailing edge of a CIR these particles can travel sunward along the ambient magnetic field and thus enhanced fluxes can be observed even if the acceleration region has passed over the spacecraft. We have analysed a CIR that has been observed at L1 by ACE/SWICS and SOHO/CELIAS/STOF on days 207 and 208 in 2003. The combination of SWICS and STOF data allowed us to study suprathermal Helium ranging from its onset at solar wind bulk energies up to 330 keV/nuc. Here we present our results for the temporal evolution of the flux, energy spectra and the He+/He++ ratio. In particular we present observational evidence for a turnover of the energy spectra at lower energies after the CIR passage which has been theoretically predicted but never been observed so far. [ABSTRACT FROM AUTHOR]

Supra-thermal neutral atoms such as H and He have been detected with the particle instrument CELIAS/HSTOF onboard SOHO since 1996. While the observed energetic neutral hydrogen (ENH) fluxes are well distinct from background ion fluxes, the helium (ENHe) fluxes have to be corrected for the local He ion and alpha particle fluxes in the same energy region. The latter ion background originates from well known sources such as solar particle events or co-rotating interaction regions and the ion particle flux and spectrum is a function of time. The resulting H/He ratio in the supra-thermal energy region is lower than expected from the anomalous cosmic ray abundances. Heavier neutral particles are not identified in the CELIAS/HSTOF dataset. We will discuss the neutral energetic flux measurements in view of the H/He ratio and potential source regions from the heliosheath region and beyond. [ABSTRACT FROM AUTHOR]

The particle instrument CELIAS/HSTOF onboard SOHO is observing the energetic neutral atom (ENA) flux in the ecliptic plane since 1996. During quiet time periods, the ENA flux is well separated from the energetic ion flux, originating in the solar corona and/ or heliosphere. The instrument is capable to measure the energetic neutral hydrogen (ENH) and helium (ENHe) fluxes along the line-of-sight of the instrument field-of-view. We will discuss the neutral energetic flux measurements of hydrogen and helium in the 28 to 88 keV/n energy/mass range of CELIAS/HSTOF, focusing on the frequently increased fluxes from the tail direction of the heliosphere. Potential source regions might be co-rotating interaction regions (CIRs) and the helium cone in the inner heliosphere, solar energetic particle events, accelerated pick-up ions in the outer heliosphere and energetic neutral particles originating from the heliosheath region and beyond. [ABSTRACT FROM AUTHOR]

In the years 2004 and 2007, the instruments onboard the Voyager 1&2 spacecraft delivered unprecedented data on the structure of the solar wind termination shock. This shock has been assumed to be responsible for the acceleration of the anomalous component of cosmic rays (ACRs) from a pick-up ion seed population derived from the interstellar neutral gas penetrating the heliosphere. In this article, we present model results on the evolution of solar wind turbulence and suprathermal tails with heliocentric distance to support the hypothesis that stochastic acceleration all over the heliosphere but in particular in the heliosheath is an important process to energize the ACRs if not the most important one. [ABSTRACT FROM AUTHOR]

Based on observations near Earth’s orbit with the CELIAS/STOF sensor on board SOHO and with the ACE magnetometer, and on the model of Zank et al. we calculate the evolution of transport parameters of energetic particles over heliocentric distance. From the calculated transport parameters we estimate the spatial distribution of termination shock energetic particles (TSPs) and of the Anomalous Cosmic Rays (ACRs) near the termination shock and in the heliosheath. In agreement with Voyager 1 observations we find that the TSP/ACR pressure is of the order of a significant fraction of the dynamical pressure of the neutral gas component of the interstellar medium. Therefore, the solar wind termination shock should be a cosmic-ray mediated shock. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Energetic hydrogen and helium atoms have been identified and their fluxes monitored by the High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 since 1996. The potential sources of energetic neutral atoms (ENAs) in the heliosphere are co-rotating interaction regions (CIRs), solar energetic particle events, pre-accelerated pickup ions as well as low-energy (up to few hundred keV) anomalous cosmic ray (ACR) ions in the outer heliosphere, close to and beyond the solar wind termination shock. The ENAs, neutralized via charge transfer reactions, can penetrate into the inner solar system, unaffected by the interplanetary magnetic field. The observed ENA fluxes in the apex direction of the heliosphere set limits on potential theories of the dominant sources of the energetic neutral atoms and we discuss implications of these ENA observations on modeling of the heliosheath. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The observations on magnetic field fluctuations and suprathermal ion spectra near the shocks driven by the coronal mass ejections during the time periods of the Bastille Day 2000 event and the Halloween 2003 events are summarized. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We summarize observational data and present theoretical considerations in order to estimate the relative contributions of suprathermal pick-up ions neutralized inside and outside the termination shock to the energetic neutral atom (ENA) flux in the energy range 55 – 80 keV/amu detected by the SOHO/CELIAS/HSTOF sensor near Earth’s orbit. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A model for the acceleration and transport of suprathermal ions associated with interplanetary coronal mass ejections (CMEs) is presented. The combined mechanisms of stochastic acceleration in the turbulence near interplanetary shocks, first-order Fermi acceleration, stationary spatial diffusion in field irregularities of magnetic clouds, and time-dependent propagation in the upstream solar wind are described. Ions from the bulk solar wind are considered as source populations of suprathermal ions. The results of the model are compared to the spectra of suprathermal ions observed with SOHO/CELIAS/HSTOF during the Bastille Day CME of 14–16 July 2000. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

Since the launch of the Solar and Heliospheric Observatory (SOHO) on Dec. 2, 1995, the Suprathermal Time-Of-Flight (STOF) energetic particle sensor of the Charge, Element and Isotope Analysis System (CELIAS) has observed gradual and impulsive solar particle events from solar activity cycle minimum to maximum. The instrument CELIAS/STOF has the capability of detecting energetic ions and determine the mass, energy and charge of each particle. We report on the measurements of the detected energetic particle events from 1996 to 2001, i.e. the velocity dispersion of energetic He ions in the energy/charge range between 30 and 800 keV/nuc. For energetic particle events we analyse the velocity dispersion of the suprathermal He and O ions (78 and 43 events, resp.) and estimate the onset time of the solar event and set limits on the propagation length along the connecting magnetic field line. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present SOHO/CELIAS/HSTOF data on suprathermal H, He, CNO, and Fe ions in the energy range 0.035–2 MeV/amu associated with the Bastille Day coronal mass ejection event, July 14–16, 2000. We observe a complicated evolution of the spectra in the plasma upstream from the strong interplanetary shock on July 15, in the downstream turbulent compression region, and in the magnetic cloud following the compression region. The spectra of suprathermal H, CNO, and Fe ions from the solar wind source fit a scheme of ordering by their charge-to-mass (Q/A) ratio. This scheme suggests that the ions are stochastically accelerated in the turbulence region downstream from the shock and then injected into first-order Fermi acceleration. The suprathermal He flux consists of solar wind 4He2+ ions matching the Q/A-ordering scheme, and an additional component. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Sun is the largest reservoir representing the matter of the early solar nebula. Its isotopic composition for the elements N, Ne, Mg, Si, At, and Fe has been determined by measuring solar wind abundances with the CELIAS experiment on board SOHO and other spacecraft instruments. These measurements and theoretical considerations indicate that the solar wind, in particular the coronal-hole type solar wind, is much less isotopically fractionated than solar energetic particles. The data give evidence that the isotopic abundance ratios typically vary by only ∼1 - 2% per amu in different solar wind regimes such as coronal-hole type and streamer-belt associated solar wind, ejecta of coronal mass ejections (CMEs), or so-called 'blobs'. [ABSTRACT FROM AUTHOR]