S. Hefti

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self-consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi-stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing-in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well-known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low-FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low-FIP bias in the slow wind and a weaker (and perhaps conceptually different) low-FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi-stationary solar wind types.

Unusual composition of the solar wind in the 2–3 May 1998 CME observed with SWICS on ACE

Elemental, isotopic and charge state abundances provide valuable information about the source and acceleration mechanism of Coronal Mass Ejections (CMEs). Even though the kinetic properties of the plasma might be subject to changes because of dynamic effects occurring during the expansion of the CME, the composition of the solar wind remains unchanged after it leaves the low corona. Data from the Solar Wind Ion Composition Spectrometer (SWICS) on ACE are used to study the elemental and charge state composition of He, O, C, N, and Fe as well as the isotopic ratio of He during the very large CME of May 2–3, 1998. We find in this CME anomalously large enrichment of ³He++/4He++, He/O and Fe/O. During the 28 hour long cloud portion of the CME unusually cold material (4He+ and very low charge state heavy ions) was observed together with hot (high charge state ions) and normal solar wind plasma.

The solar origin of corotating interaction regions and their formation in the inner heliosphere, Report of Working Group 1

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind.

Solar wind measurements with SOHO: The CELIAS/MTOF proton monitor

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.

Isotopic composition of solar wind neon measured by CELIAS/MTOF on board SOHO

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.

Kinetic properties of solar wind minor ions and protons measured with SOHO/CELIAS

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+).

Magnetic structure of the slow solar wind: Constraints from composition data

The elemental and ionic composition of the solar wind is determined very close to the Sun. Composition data therefore contain direct information about the structure of the corona and the processes which lead to the generation and acceleration of solar wind and can constrain processes occurring in the heliosphere. We use Advanced Composition Explorer (ACE)-Solar Wind Ion Composition Spectrometer (SWICS) composition measurements of the solar wind to examine first the structure of the slow solar wind. We find that the statistical properties of the data suggest the presence of a large number of composition boundaries, where compositional patterns change abruptly. These discontinuities are interpreted here as a consequence of the structure of the solar wind source region. Using the experimental constraints of these observed composition boundaries, we derive direct constraints for turbulent diffusion of magnetic field lines in the heliosphere. We then discuss consequences for the propagation of energetic particles.

Isotopic Composition of Solar Wind Nitrogen: First In Situ Determination with the CELIAS/MTOF Spectrometer on board SOHO

Using the high-resolution Mass Time-of-Flight (MTOF) spectrometer of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO), we have determined the solar wind isotope abundance ratio 14N/15N = 200±55 (1 σ error), suggesting that the relative abundance of 15N in the terrestrial atmosphere is lower than in solar matter. This result is compatible with the hypothesis that terrestrial N (14N/15N = 272) and also N found in lunar surface material are a mixture of a heavy component that is identical to solar N and an unspecified light component. The large variations of 14N/15N in solar system matter is caused by special isotope enrichment processes, as in the case of Mars, as well as by varying contributions of isotopically different components.

On the origin of microscale magnetic holes in the solar wind

Magnetic holes are sudden changes in the magnetic field intensity |B| from typical interplanetary values (∼10 nT) to less than 1 nT in a matter of seconds. The intensity then recovers within seconds or up to ∼30 min later. These |B| dropouts can be seen daily. Less often observed, but even more dramatic, are magnetic field depletions that last for up to several hours. We use selected periods of magnetic flux dropouts observed with various sensors of the Advanced Composition Explorer (ACE), which has a unique combination of magnetic field, plasma, and composition experiments, to establish the origin of these peculiar objects. We conclude that these microscale magnetic holes very likely develop in the heliosphere and are not of direct solar origin. We also suggest a possible formation mechanism associated with magnetic reconnection close to the Sun.

The Transition Between Fast and Slow Solar Wind from Composition Data

The transition between coronal hole associated fast solar wind and slow solar wind is studied using data from the high resolution mass spectrometer SWICS on ACE. We discuss the data in the framework of a recent theory about the global heliospheric magnetic field and conclude that the data are consistent with magnetic connections between field-lines in the fast and in the slow wind.