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  • 1
    Digitale Medien
    Digitale Medien
    Coronal Faraday rotation during solar occultation of PSR0525 + 21 (1980)
    [s.l.] : Nature Publishing Group
    Nature 283 (1980), S. 459-460 
    Details
    ISSN: 1476-4687
    Quelle: Nature Archives 1869 - 2009
    Thema: Biologie , Chemie und Pharmazie , Medizin , Allgemeine Naturwissenschaft , Physik
    Notizen: [Auszug] Pulsars were recognised early as potential sources of probe signals for coronal investigations1,2. Group delay time measurements, from which one may determine coronal dispersion measure (DM) by recording arrival times of pulses in well separated frequency bands, were first performed using the Crab ...
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1038/283459a0
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  • 2
    Digitale Medien
    Digitale Medien
    The structure of Venus’ middle atmosphere and ionosphere (2007)
    [s.l.] : Nature Publishing Group
    Nature 450 (2007), S. 657-660 
    Details
    ISSN: 1476-4687
    Quelle: Nature Archives 1869 - 2009
    Thema: Biologie , Chemie und Pharmazie , Medizin , Allgemeine Naturwissenschaft , Physik
    Notizen: [Auszug] The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the ...
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1038/nature06239
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  • 3
    Digitale Medien
    Digitale Medien
    The vertical profile of winds on Titan (2005)
    [s.l.] : Nature Publishing Group
    Nature 438 (2005), S. 800-802 
    Details
    ISSN: 1476-4687
    Quelle: Nature Archives 1869 - 2009
    Thema: Biologie , Chemie und Pharmazie , Medizin , Allgemeine Naturwissenschaft , Physik
    Notizen: [Auszug] One of Titan's most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titan's atmospheric pressure and temperature and evidence for the presence of strong zonal winds. It was realized that ...
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1038/nature04060
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  • 4
    Digitale Medien
    Digitale Medien
    Effects of diverging coronal fields on the solar wind expansion (1976)
    Springer
    Astrophysics and space science 43 (1976), S. 19-33 
    Details
    ISSN: 1572-946X
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The expansion of the solar wind in divergent flux tubes is calculated by taking into account a ‘magnetic acceleration’ of the particles, analogous to the magnetic mirror effect. The resulting force term included in the magnetohydrodynamical equations describes a conversion of thermal into kinetic energy. This causes an additional acceleration of the solar wind plasma which has never been taken into account before. The force is directed opposite to the magnetic field gradient. Consequently, in this case the solar wind velocity increases faster to its asymptotic value than it does for corresponding nonmagnetic solutions. Therefore inside and close to the solar corona markedly higher velocities are found. Compared to strictly hydrodynamical models, the critical point is shifted towards the Sun, and the radial decrease of the ratio of thermal to kinetic energy is faster. The necessary prerequisites for these calculations are (a) that the gyroperoid τ g of the plasma particles is much shorter than the Coulomb collision time τ c , and (b) that the collision time τ c is shorter than the characteristic time τ d in which an appreciable amount of thermal anisotropy is built up. Thus it is (a) insured that the particles have established magnetic moments and follow the guiding center approximation, and (b) an almost isotropic velocity distribution function is maintained which, in this first approximation of a purely radial expansion, justifies the use of isotropic pressures and temperatures. Both (a) and (b) are shown to be fulfilled in a region around the Sun out to about 20R ⊙, and thermal anisotropies developing outside of this region could explain the observed magnetically aligned anisotropies at 1 AU.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00640552
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  • 5
    Digitale Medien
    Digitale Medien
    The effects of diverging coronal fields on the solar wind expansion (1977)
    Springer
    Astrophysics and space science 46 (1977), S. L11 
    Details
    ISSN: 1572-946X
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The calculations made by Fahret al. (1976) have been subjected to a re-examination, the results of which are described in this letter.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00644395
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  • 6
    Digitale Medien
    Digitale Medien
    Quasi-harmonic faraday-rotation fluctuations of radio waves when sounding the outer solar corona (2000)
    Springer
    Astronomy letters 26 (2000), S. 544-552 
    Details
    ISSN: 1562-6873
    Schlagwort(e): Sun ; fluctuations ; radio waves ; spectra ; magnetic fields ; Alfvén waves
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v1=4 mHz (fundamental frequency) and v2=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ1,2] distribution relative to the v=v1, 2 peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1134/1.1306991
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  • 7
    Digitale Medien
    Digitale Medien
    K-band Radio Observations of Comet Hale-Bopp: Detections of Ammonia and (Possibly) Water (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 21-28 
    Details
    ISSN: 1573-0794
    Schlagwort(e): Radio line emission ; comets ; comet Hale-Bopp
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract K-band radio observations of comet Hale-Bopp (C/1995 O1) were conducted in March/April 1997 at the 100-m Telescope of the Max-Planck-Institut für Radioastronomie. Emission was firmly detected from the five lowest metastable (J = K)inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of NH3, we derive a rotational temperature of 104 ± 30 K and an ammonia production rate at perihelion of6.6 ± 1.3 × 1028 s-1.The updated ammonia-to-water abundance ratio is found to be of the order of 1.0%. We also report a marginal detection of the 616–523transition line of water at λ = 1.35 cm.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1006246606131
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  • 8
    Digitale Medien
    Digitale Medien
    The mean coronal magnetic field determined from HELIOS Faraday rotation measurements (1971)
    Springer
    Solar physics 109 (1971), S. 91-105 
    Details
    ISSN: 1573-093X
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r −α, where α = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 −5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00167401
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  • 9
    Digitale Medien
    Digitale Medien
    White-light and radio sounding observations of coronal transients (1985)
    Springer
    Solar physics 98 (1985), S. 341-368 
    Details
    ISSN: 1573-093X
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A concerted search for coronal transients was conducted with the ‘Solwind’ coronagraph during the solar occultations of the two Helios spacecraft in October/November 1979. The polarization angle and bandwidth of the linearly polarized S-band downlink signal were monitored at the three 64-m tracking stations of the NASA Deep Space Network to determine coronal Faraday rotation and spectral broadening. A one-to-one correspondence could be established between abrupt disturbances in the two signal parameters and the passage of a white-light transient through the signal ray path from spacecraft to Earth. The white-light morphology and the additional information provided by the radio sounding coverage are presented for each of the five distinct events recorded. Although no specific example could be observed in sufficient detail in both white light and Faraday rotation to derive the small-scale magnetic structure, some qualitative descriptions of the orientation and rough estimates of the magnitude of the transient magnetic field could be made.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00152465
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  • 10
    Digitale Medien
    Digitale Medien
    Characteristics of Coronal Alfvén Waves Deduced from Helios Faraday Rotation Measurements (1997)
    Springer
    Solar physics 176 (1997), S. 387-402 
    Details
    ISSN: 1573-093X
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract A statistical study of Faraday rotation fluctuations (FRF) has been performed using polarization angle data of S-band (f = 2.3 GHz) radio spacecraft signals. The measurements were recorded during the recurring superior conjunctions of the Helios probes, during which the solar proximate point of the radio ray path reached heliocentric distances between 3 and 34 R⊙. The most commonly found temporal FRF spectra are power laws with an average spectral index ∼1.5 over the frequency range from 1 mHz 〈 v 〈 10 mHz. The FRF variance decreases with heliocentric distance, the falloff exponent being ∼8 for R 〈 6 R⊙ and ∼3 for distances 8 〈 R 〈 6 R 〈 16 R⊙. The results are interpreted under the assumption that the FRF are produced by Alfvén waves propagating in the coronal plasma. For the applicable range of heliocentric distances it is shown that Alfvén waves are in a regime of free propagation and probably transfer much of their energy to the kinetic energy of the solar wind. The spatial power spectrum of magnetic field fluctuations is inferred to be strongly anisotropic, the irregularities extending along the background magnetic field with axial ratios of the order of 10.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1004965310604
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