Th. Zurbuchen

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.

The suprathermal seed population for corotating interaction region ions at 1 AU deduced from composition and spectra of H + , He ++ , and He + observed on Wind

We have measured H + , He ++ , and He + distribution functions over the solar wind through the suprathermal energy range during two corotating interaction region (CIR) events o b- served by the STICS, MASS, and STEP instruments on board the Wind spacecraft at 1 AU during April and May 1995. The major properties we find are as follows : In the suprathermal energy range (~10-500 keV/nucleon), the particle intensities peak inside the CIR itself, in the compressed and decelerated fast solar wind, in contrast to the situation at MeV energies, where the peak inte n- sities are observed outside the CIR in the fast solar wind. The distribution functions of solar wind H + and He ++ change smoothly from the core at solar wind speeds to a power law or exponential form at higher energies, with no turnover observed at intermediate energies. CIR He + is observed with an abundance ratio He + /He ++ ~ 16-17%, orders of magnitude higher than that in the bulk solar wind but nevertheless lower than that observed in CIRs at 4.5 AU. The H + , He ++ , and He + spectra have similar slopes above speeds of ~2.5-3 times the solar wind speed ( Vsw) in the space- craft frame. The ion speed at which the CIR He ++ /H + ratio changes from typical solar wind va l- ues of 4-5% to the higher (>10%) value typical of CIRs is ~1.5-1.7 Vsw, measured in the space- craft reference frame. Analyzing these observations in the context of previous global observations and simple models of CIR acceleration and transport ( Fisk and Lee, 1980), we conclude the fol- lowing: (1) Suprathermal CIR ions at 1 AU originated close (within ~0.5 AU) to the point of o b- servation, not in the outer heliosphere; (2) the injection/acceleration mechanism is not especially sensitive to charge-to-mass ratio over the range 0.25-1.0; (3) since the particles are locally accel - erated, the low-energy ion populations we observe contain the seed population; (4) the bulk solar wind itself is not the source of the energetic ions; rather, the source is in the suprathermal tail, with an injection threshold in the spacecraft frame of ~1.8-2.5 times the solar wind speed; and (5) in at least one of these CIRs, suprathermal particle acceleration is not shock associated and must therefore be associated with a statistical mechanism or co mpression in the solar wind.

Interception of comet Hyakutake's ion tail at a distance of 500 million kilometres

Remote sensing observations and the direct sampling of material from a few comets have established the characteristic composition of cometary gas. This gas is ionized by solar ultraviolet radiation and the solar wind to form ‘pick-up’ ions, ions in a low ionization state that retain the same compositional signatures as the original gas. The pick-up ions are carried outward by the solar wind, and they could in principle be detected far from the coma. (Sampling of pick-up ions has also been used to study interplanetary dust, Venus’ tail and the interstellar medium.) Here we report the serendipitous detection of cometary pick-up ions, most probably associated with the tail of comet Hyakutake, at a distance of 3.4 au from the nucleus. Previous observations have provided a wealth of physical and chemical information about a small sample of comets, but this detection suggests that remote sampling of comet compositions, and the discovery of otherwise invisible comets, may be possible.

Limits of the efficiency of isotope fractionation processes in the solar wind derived from the magnesium isotopic composition as observed with the WIND/MASS experiment

Abstract From geochemical evidence it is assumed that the isotopic composition of solar magnesium agrees within fractions of a per mill with terrestrial magnesium. This provides a unique opportunity to derive upper limits on the efficiency of isotope fractionation processes in the solar wind. We analyze results from the first year of operation of the WIND/MASS instrument, and we find that the isotopic composition of magnesium in the solar wind agrees within the experimental uncertainty of a few percent with the terrestrial composition. We observe no significant variations between different solar wind regimes. Our results are discussed in the context of recent theoretical models of solar wind feeding and acceleration.

Abundance of solar wind magnesium isotopes determined with WIND/MASS

We present first results of the abundance ratios of Mg isotopes in the solar wind using the high resolution mass spectrometer on the WIND spacecraft. The isotopic composition of Mg in the solar wind is consistent with terrestrial values. Our preliminary result is 24Mg:25Mg:26Mg=(0.792±0.006):(0.095±0.005):(0.113±0.005).

Coronal hole differential rotation rate observed with SWICS/Ulysses

We discuss the latitude variation of the coronal hole differential rotation investigating persistent structures in high speed streams as observed from SWICS Ulysses during its first passage of the southern polar hole in 1993–1994. We find a slower rotation rate near the ecliptic than what is inferred from averaged photospheric features, e.g. from sunspots. At intermediate latitudes we find a rate similar to the equatorial rotation rate indicating a quasi-rigid rotation of the polar coronal hole. At latitudes >65° no persistent structures to determine the polar rotation have been observed. For the passage of the southern heliosphere in 1993/94 we find a latitudinal dependence of the sidereal rotation rate of the coronal hole which can be approximated by ωSW=[13.13+1.94 sin2(Θ)]°/day, where Θ denotes the solar latitude.

Features of small-scale solar wind mass flux fluctuations

We analyze solar wind mass flux data measured with the PROGNOZ 8 satellite. The temporal resolution of the data reaches 0.02 sec, such that also the range is probed where finite Larmor radius effects become increasingly important. We find a general tendency for steepening of the power spectra towards higher frequencies, similar to what is observed in spectra of magnetic field fluctuations. A significant peak is found near f=3.2 Hz in a high-resolution spectrum. Possible causes for this feature are discussed. Using the same data we also study the practical applicability of the wavelet transform as a tool for the analysis of non-stationary data with the emphasis on the search for transient spatial structures which are often not easily resolved using spectral analysis.

Test particle study of minor ions in the solar wind

Using numerical simulations of test particles in a MHD turbulence model we investigate the influence of nonlinear effects in the interaction of MHD turbulence with minor ions. We conclude that non-linear interactions lead to an efficient heating of minor ions.