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The Solar and Heliospheric Observatory (SOHO) spacecraft located at L1 is well outside the Earth's magnetosphere and has been observing the Sun continuously since December 1995, except for relatively brief periods due to spacecraft operational interruptions. While a variety of instruments on the SOHO spacecraft investigate the solar properties important to an improved understanding of the Sun and its effect on space weather, the present work is limited to the observations provided by the Charge, Element, and Isotope Analysis System (CELIAS) proton monitor (PM) and Solar Extreme Ultraviolet Monitor (SEM) instruments and their relationship to other space weather observations. The CELIAS observations consist of particle and EUV/soft X-ray solar flux measurements. A brief description of the CELIAS instrumentation and examples of the precursor information signaling the possibility of coronal mass ejection events observed by the CELIAS/SEM are presented. In addition, the entire SEM database since commissioning is presented on both expanded and compressed timescales in order to provide both the long-term weather trends and short-term storm data. The SEM data presented are full-disk observations and have a 15 s sampling rate.

We present model simulations to estimate the flux of 25-100 keV energetic neutral atoms (ENAs) that can be expected from charge exchange between the ion populations accelerated at corotating interaction regions and the slow neutral atoms penetrating from the local interstellar matter. The extended structure of a corotating interaction region (CIR) evolving in space is a major plasma feature in the inner heliosphere that contains a wealth of information on shock development and charged-particle acceleration. The detection of energetic neutral atoms originating from CIRs, with directional, mass, and energy resolutions, is a way to view CIRs remotely. This global view in ENAs may provide important information complementary to in situ particle and plasma and field observations. We apply a three-dimensional numerical code to model CIRs and simulate the transport of energetic particles accelerated at the forward and reverse shocks of the CIR. Our simulation results are applied to the ongoing ENA observations by the Solar and Heliospheric Observatory (SOHO) and that anticipated by Cassini. We find that a CIR origin of the ENAs detected by the high suprathermal time-of-flight sensor of the charge, element, and isotope analysis system on SOHO cannot be ruled out. Our simulations also reveal that the concentration of local interstellar He atoms focused by the Sun's gravitation contributes significantly to the production of ENAs in the inner regions of the quiet heliosphere.

Using observations of the Charge Time-of-Flight(CTOF) charge and mass spectrometer of the Charge, Element and Isotope Analysis System (CELIAS), and of CELIAS/proton monitor on board the Solar and Heliospheric Observatory (SOHO), we present an overview of speeds and kinetic temperatures of minor ions and protons in the solar wind near solar minimum, covering the Carrington Rotations 1908 to 1912. In the case of a collision-dominated solar wind the speed of minor ions is expected to be lower or equal to the speed of the protons, and all species are expected to have equal temperatures. On the other hand, minor ions can be accelerated and heated by wave-particle interaction. In this case, equal thermal speeds of all species are expected. CTOF data allow the determination of the kinetic parameters of various ions with high accuracy and with high time resolution. The mean O6+ speed of the observed period is 390 km s−1. The speeds of Si7+ and Fe9+ correlate well with O6+, the linear correlation coefficient being 0.96 or higher. Our results also indicate that silicon and iron tend to lag behind oxygen with a speed difference of ∼20 km s−1 at 500 km s−1. At the same time, the kinetic temperature of the ions under investigation exhibit the well-known mass proportionality, which is attributed to wave-particle interactions. During the period of low solar activity in consideration, many cases are observed where the kinetic temperature is extraordinarily low (104 K for O6+).

Solar wind abundance ratios of magnesium isotopes measured with the high resolution Mass Time-of-Flight spectrometer (MTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) are presented. MTOF, as part of CELIAS, is, because of its high time and mass resolution, an excellent tool for isotope abundance measurements in the solar wind. From the data analysis we have found that the isotopic composition of magnesium in the solar wind agrees with the terrestrial composition within the experimental uncertainty. We have obtained isotopic ratios of 24Mg/25Mg = 7.7 ± 0.4 and 24Mg/26Mg = 7.0 ± 0.5. These values are consistent with the terrestrial values of 24Mg/25Mg = 7.90 ± 0.01 and 24Mg/26Mg = 7.17 ± 0.03. Furthermore, these investigations also show that with the given uncertainties the abundance ratios do not vary significantly within a solar wind velocity range from 375 km/s to 530 km/s.

The proton monitor, a small subsensor in the Charge, Element, and Isotope Analysis System/Mass Time-of-Flight (CELIAS/MTOF) experiment on the SOHO spacecraft, was designed to assist in the interpretation of measurements from the high mass resolution main MTOF sensor. In this paper we demonstrate that the proton monitor data may be used to generate reasonably accurate values of the solar wind proton bulk speed, density, thermal speed, and north/south flow direction. Correlation coefficients based on comparison with the solar wind measurements from the SWE instrument on the Wind spacecraft range from 0.87 to 0.99. On the basis of the initial 12 months of observations, we find that the proton momentum flux is almost invariant with respect to the bulk speed, confirming a previously published result. We present observations of two interplanetary shock events, and of an unusual solar wind density depletion. This large density depletion, and the correspondingly large drop in the solar wind ram pressure, may have been the cause of a nearly simultaneous large increase in the flux of relativistic magnetospheric electrons observed at geosynchronous altitudes by the GOES 9 spacecraft. Extending our data set with a 10-year time span from the OMNIWeb data set, we find an average frequency of about one large density depletion per year. The origin of these events is unclear; of the 10 events identified, 3 appear to be corotating and at least 2 are probably CME related. The rapidly available, comprehensive data coverage from SOHO allows the production of near-real time solar wind parameters that are now accessible on the World Wide Web.

The CELIAS particle experiment on SOHO contains the Charge Time Of Flight (CTOF) mass spectrometer which measures the ionic and elemental composition of minor ions in the solar wind. In this paper we present iron freeze-in temperatures derived with a time resolution of 5 min. They indicate that some of the filamentary structures of the inner corona observed in Hα survive in the interplanetary medium as far as 1 AU.

Using solar wind particle data from the CELIAS/MTOF sensor on the SOHO mission, we studied the abundance of the elements O, Ne, Mg, Si, S, Ca, and Fe for the time period around the January 6, 1997, coronal mass ejection event (CME). In the interstream and coronal hole regions before and after this event we found elemental abundances consistent with the expected abundance patterns of the respective flow regimes. However, during the passage of the CME and during the passage of the erupted filament, which followed the CME, we found that the elemental composition differed markedly from the interstream and coronal hole regions before and after this event. During the passage of the CME and the passage of the erupted filament we found a mass-dependent element fractionation, with a monotonic increase toward heavier elements. We observed Si/O and Fe/O abundance ratios of about one half during these time periods, which is significantly higher than for typical solar wind.

We present first results taken from the high-resolution mass time-of-flight spectrometer (MTOF) of the charge, element, and isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft launched in December 1995, concerning the abundance ratios of neon isotopes in the solar wind. We obtain the isotopic ratios 20Ne/22Ne = (13.8 ± 0.7) and 20Ne/21Ne = (440 ± 110), which agree with the values obtained from the Apollo foil solar wind experiments and which have been derived from measurements of solar particles implanted in lunar and meteoritic samples.

[1] Neutral hydrogen is indicative of the behavior of the main solar wind component formed by protons out to at least 5 R ⊙ . In fact, beyond this distance, the characteristic time for charge exchange between hydrogen atoms and protons becomes larger than the coronal expansion timescale, causing the neutrals to decouple from the charged solar wind. The mean free path of the neutral component rapidly increases with the radial distance so that neutrals generated at heliocentric distances >24 R ⊙ fly unperturbed and eventually are detected by Solar Orbiter (perihelion at approximately 48 R ⊙ ), since their mean free path is long enough to let neutrals reach the neutral solar wind detector. However, the computation of the differential flux shows that the bulk of the flux detected at the Solar Orbiter vantage point mainly comes from about 9 R ⊙ . Neutrals retain information on the three-dimensional distribution of hydrogen at the level where they are generated as the proton velocity distribution is frozen within the generated neutrals and transferred up to the Solar Orbiter position. In the present study, we report our preliminary results from our simulation of the neutral solar wind distribution as predicted at the Solar Orbiter position and considering the evolution of a coronal hole-emerging solar wind whose major parameters are estimated by the Solar and Heliospheric Observatory (SOHO) Ultraviolet Coronagraph Spectrometer (UVCS) experiment. The synergy between corona remote sensing and in situ neutral particle observations will enable us to infer the degree of anisotropy, if any, in the neutral and charged coronal hydrogen close to the Sun.

[1] The Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight ((H)STOF) mass spectrometer measures the elemental composition and the charge state distribution of ions with suprathermal energies in the range 10–4000 keV amu−1 from above the solar wind particle distribution up to low-energy flare particle energies. We analyze the time period around the large coronal mass ejection event on 2–3 May 1998. Using data from (H)STOF, we study the charge state distribution of He, the CNO group, and Fe. In addition, we investigate the energy dependence of the mean ionic charge state of Fe in the energy-per-nucleon range 12–100 keV amu−1. Furthermore, we also report energy spectra of H+, He++, and He+ and the variations in the elemental abundance ratios He/H, He/CNO, and Fe/CNO. These observations greatly extend the energy range in which particles associated with this time period have been measured. Because of their elevated energies, suprathermal particles are the prime seed population for further acceleration in gradual events. We identify interstellar pickup ions as an important nonsolar seed population. The observed low mean ionic charge states of Fe and the small Fe/CNO ratio are typical for large gradual events. The energy dependence of the Fe charge states may be a result of the presence of different iron populations or of the thermal history of the accelerated material. The temporal variations of Fe/CNO and He/CNO indicate a stronger confinement of low-rigidity particles at the acceleration site.

A statistical study of diffuse energetic ion events and their related waves upstream of the Earth's bow shock was performed using data from the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) satellite over two 5-month periods in 1984 and 1985. The data set was used to test the assumption in the self-consistent model of the upstream wave and particle populations by Lee (1982) that the particle acceleration through hydromagnetic waves and the wave generation are directly coupled. The comparison between the observed wave power and the wave power predicted on the observed energetic particle energy density and solar wind parameters results in a high correlation coefficient of about 0.89. The intensity of diffuse ions falls off approximately exponentially with the distance upstream from the bow shock parallel to the magnetic field with e-folding distances which vary from approximately 3.3 R(sub E) to approximately 11.7 R(sub E) over the energy range from 10 keV/e to 67.3 keV/e for both protons and alpha particles. After normalizing the upstream particle densities to zero bow shock distance by using these exponential variations, a good correlation (0.7) of the density of the diffuse ions with the solar wind density was found. This supports the suggestion that the solar wind is the source of the diffuse ions. Furthermore, the spectral slope of the diffuse ions correlates well with the solar wind velocity component in the direction of the interplanetary magnetic field (0.68 and 0.66 for protons and alpha particles) which concurs with the notion that the solar wind plays an important role in the acceleration of the upstream particles.