Th. Zurbuchen

Universität Bern, Berna, Bern, Switzerland

Are you Th. Zurbuchen?

Claim your profile

Publications (9)0 Total impact

  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Dec 1997 · Physics and Chemistry of The Earth
  • Th. Zurbuchen · P. Bochsler · H. Politano · A. Pouquet
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    No preview · Article · Jul 1996
  • Th. Zurbuchen · P. Bochsler · R. von Steiger
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 omegaSW=[13.13+1.94 sin2(Theta)]°/day, where Theta denotes the solar latitude.
    No preview · Article · Jul 1996
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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).
    No preview · Article · Jul 1996
  • Th. Zurbuchen · G. Zastenker · P. Eiges · P. Bochsler · L. Avanov · N. Astafyeva
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    No preview · Article · Jul 1996
  • Th. Zurbuchen · P. Bochsler · R. von Steiger
    [Show abstract] [Hide abstract]
    ABSTRACT: We characterize the dynamic properties of He ions of the solar wind. Because of the non-negligible abundance and the significant fraction of momentum flux inherent in helium ions, this species has an influence on the state of turbulence. Especially, we analyze the helium dynamic properties of different solar wind types. After a discussion of the influence of measurement errors on the statistical analysis of He bulk velocities, we investigate the structure function dependency on the solar wind state. We find a self-similar sealing in the range of minutes to days with characteristic structure function slopes deviating from the canonical Kolmogorov values. For comparison with previous studies, we also analyze H structure functions of the same time periods and discuss differences of coinciding He and H structure functions in the framework of the concept of intermittency.
    No preview · Article · Jul 1995
  • [Show abstract] [Hide abstract]
    ABSTRACT: The MASS instrument on WIND contains the first isochronous time-offlight spectrometer to be flown in the solar wind. The first spectra obtained with this instrument has demonstrated its capability to measure the abundances of several high-and low-FIP elements in the solar wind. The derivation of these abundances requires a careful calibration of the charge exchange efficiencies of the relevant ions in carbon foils. These efficiencies and the corresponding instrument functions have been determined in extensive calibration campaigns at different institutions. We present first and preliminary results obtained in slow solar wind streams and we compare these results with those obtained from previous investigations of solar wind abundances and of coronal abundances as derived from Solar Energetic Particles. Recent models of the FIP related fractionation effect predict a depletion of a factor of typically 4 to 5 for high-FIP elements (He, N, O, Ne, Ar, etc.) relative to low-FIP elements (Mg, Fe, Si, etc.). We also compare our results with the detailed predictions of the different models and we discuss the resulting evidence to validate or to invalidate different physical scenarios explaining the feeding and the acceleration of slow stream solar wind.
    No preview · Article · Jul 1995
  • G. Zastenker · P. Eiges · L. Avanov · N. Astafyeva · Th. Zurbuchen · P. Bochsler
    [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated small-scale variations of the solar wind ion flux measured with Faraday cups onboard the Prognoz-8 satellite. These measurements have a high time resolution of 1.24 seconds for intervals with a duration of several hours and as high as 0.02 seconds for some periods of about 1 hour duration. The main goal of this work is the determination of the quantitative features of fast ion flux fluctuations using mainly spectral analysis but also other methods. We also identify their association with interplanetary plasma parameters. Particularly, it is shown that the slope of the power spectra in the frequency range from 1E-4 to 6E-2 Hz is close to the classical Kolmogorov (-5/3) law. We also discuss some intervals with a very high level of the relative amplitude of flux fluctuations (10-20 percent) which were observed near the Earth's bow shock in the foreshock region. The use of the wavelet method for the long time series allows us to study the temporal evolution of power spectra.
    No preview · Article · Jul 1995
  • Th. Zurbuchen · P. Bochsler · H. Politano · A. Pouquet
    [Show abstract] [Hide abstract]
    ABSTRACT: We perform a parameter study of the temporal evolution of a test particle distribution function in MHD turbulence. The turbulent fields are calculated using a pseudo-spectral method and periodic boundary conditions on a regular grid of 180(exp 3) points, appropriate for incompressible, homogeneous and isotropic turbulence. Initially, the kinetic and the magnetic energy are equal on the average. Both, deterministic and random initial conditions are used, in the former case with zeros of the magnetic field located at grid points, in the latter case located by interpolation between grid points. The evolution of the minor ion distribution function is studied in detail as these turbulent fields evolve, developing strong current and vorticity sheets. Using the full collisionless equation of motion for the test particles, the efficiency of nonlinear interactions can be studied. The results are compared to theoretical predictions and are then discussed in connection with the observations of the dynamical properties of solar wind minor ions derived from in situ observations.
    No preview · Article · Jul 1995