Browsing by Author "Narita, Y."

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    Magnetic energy distribution in the four-dimensional frequency and wave vector domain in the solar wind
    (AGU, 2010-04-01) Narita, Y.; Sahraoui, F.; Goldstein,Melvyn; Glassmeier, K.-H.
    We present a measurement of the energy distribution in the four-dimensional (4-D) frequency and wave vector domain of magnetic field fluctuations in the solar wind. The measurement makes use of the wave telescope technique that has been developed particularly for multispacecraft data analysis. We review briefly the theoretical background and then present a numerical test using synthetic data; the technique is then applied to magnetic field data obtained while the Cluster spacecraft was in the solar wind. The energy distribution is determined in the flow rest frame in the frequency range below 0.2 rad/s and the wave number range below 0.0015 rad/km, corrected for the Doppler shift. We find the following properties in the energy distribution in the rest frame: (1) a double anisotropy in the wave vector domain associated with the mean magnetic field and the flow directions, (2) a symmetric distribution with respect to the sign of wave vector, and (3) no evidence for a linear dispersion relation in the frequency and wave number domain. Since the flow direction in the analyzed time interval is close to the normal direction to the bow shock, the anisotropy may well be associated with the bow shock. These results suggest that the solar wind is in a state of well-developed strong turbulence and justifies the theoretical picture of quasi-two-dimensional turbulence that obtains in the presence of a (relatively) strong DC magnetic field. However, the fluctuations are not axisymmetric around the mean field and the energy distribution is extended in the perpendicular direction to the flow or shock normal. Anisotropy associated with the boundary is reminiscent of previously reported magnetosheath turbulence. This study opens a way to investigate solar wind turbulence in the full 4-D frequency and wave vector space.
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    A phase locking mechanism for nongyrotropic electron distributions upstream of the Earth's bow shock
    (AGU, 2005-06-11) Gurgiolo, C.; Goldstein, Melvyn; Narita, Y.; Glassmeier, K.-H.; Fazakerley, A. N.
    Observations of nongyrotropic electron distributions in the region upstream of the Earth's bow shock suggest that there exists a mechanism to lock in their phase, otherwise they would rapidly gyrophase mix into a ring-beam distribution. Measurements by the Plasma Electron and Current Experiment (PEACE) on the Cluster spacecraft have provided a way of determining the rotational period of a nongyrotropic electron distribution. For the time period studied, the rotational period is found to be ≈0.5 Hz, significantly lower than the local Larmor frequency but in line with the frequency of waves observed in the local magnetic field. Detailed wave analysis has revealed that the waves are most likely ordinary right-hand whistler waves. The conclusion is that the waves provide the necessary phase locking mechanism. It is not clear whether those waves are generated by the nongyrotropic distribution or are produced by other means and then cause the observed nongyrotropy.
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    Three-dimensional spatial structures of solar wind turbulence from 10 000-km to 100-km scales
    (EGU, 2011-10-05) Narita, Y.; Glassmeier, K.-H.; Goldstein, Melvyn; Motschmann, U.; Sahraoui, F.
    Using the four Cluster spacecraft, we have determined the three-dimensional wave-vector spectra of fluctuating magnetic fields in the solar wind. Three different solar wind intervals of Cluster data are investigated for this purpose, representing three different spatial scales: 10 000 km, 1000 km, and 100 km. The spectra are determined using the wave telescope technique (k-filtering technique) without assuming the validity of Taylor's frozen-in-flow hypothesis nor are any assumptions made as to the symmetry properties of the fluctuations. We find that the spectra are anisotropic on all the three scales and the power is extended primarily in the directions perpendicular to the mean magnetic field, as might be expected of two-dimensional turbulence, however, the analyzed fluctuations are not axisymmetric. The lack of axisymmetry invalidates some earlier techniques using single spacecraft observations that were used to estimate the percentage of magnetic energy residing in quasi-two-dimensional power. However, the dominance of two-dimensional turbulence is consistent with the relatively long mean free paths of cosmic rays in observed in the heliosphere. On the other hand, the spectra also exhibit secondary extended structures oblique from the mean magnetic field direction. We discuss possible origins of anisotropy and asymmetry of solar wind turbulence spectra.
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    Wave-Vector Dependence of Magnetic-Turbulence Spectra in the Solar Wind
    (APS, 2010-04-28) Narita, Y.; Glassmeier, K.-H.; Sahraoui, F.; Goldstein, Melvyn
    Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|≤1.5×10 ⁻³ rad/km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.