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| On nonstationarity and rippling of the quasiperpendicular zone of the Earth bow shock: Cluster observations |
| A new method for remote sensing of the quasiperpendicular part of the bow shock surface is presented. The method is based on analysis of high frequency electric field fluctuations corresponding to Langmuir, upshifted, and downshifted oscillations in the electron foreshock. Langmuir waves usually have maximum intensity at the upstream boundary of this region. All these waves are generated by energetic electrons accelerated by quasiperpendicular zone of the shock front. Nonstationary behavior of the shock, in particular due to rippling, should result in modulation of energetic electron fluxes, thereby giving rise to variations of Langmuir waves intensity. For upshifted and downshifted oscillations, the variations of both intensity and central frequency can be observed. For the present study, WHISPER measurements of electric field spectra obtained aboard Cluster spacecraft are used to choose 48 crossings of the electron foreshock boundary with dominating Langmuir waves and to perform for the first time a statistical analysis of nonstationary behavior of quasiperpendicular zone of the Earth's bow shock. Analysis of hidden periodicities in plasma wave energy reveals shock front nonstationarity in the frequency range 0.33 fBi < f < fBi, where fBi is the proton gyrofrequency upstream of the shock, and shows that the probability to observe such a nonstationarity increases with Mach number. The profiles observed aboard different spacecraft and the dominating frequencies of the periodicities are usually different. Hence nonstationarity and/or rippling seem to be rather irregular both in space and time rather than resembling a quasiregular wave propagating on the shock surface. |
| Publication date: 23 Sep 2008 |
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| Weaker solar wind from the polar coronal holes and the whole Sun |
| Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in-ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long-term variations in solar wind output from the entire Sun. The significant, long-term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period. |
| Publication date: 18 Sep 2008 |
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| The ULF wave in the pile-up and dipolarization proces |
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| Publication date: 15 Sep 2008 |
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| Low-frequency fluctuations in the magnetosheath: Double Star TC-1 and Cluster observations |
| The magnetic field variations are analyzed in the range of time periods from 4 s to 240 s in the magnetosheath observed by the Double Star TC-1 and Cluster in 2004. The characteristics of the magnetic field fluctuations are strongly controlled by the angle between the upstream interplanetary magnetic field (IMF) and the normal of the bow shock. Generally speaking, the magnetic field fluctuations in the quasi-parallel magnetosheath are more intense than those in the quasi-perpendicular ones. Almost purely compressional waves are found in the quasi-perpendicular magnetosheath. With the increase of the local plasma beta, both the magnitude and direction of the magnetic field fluctuate more intensely. There exists an inverse correlation between the local temperature anisotropy Tperp./Tparallel and the plasma beta. |
| Publication date: 14 Sep 2008 |
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| High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements |
| In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes >80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00-15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ~-10° to -15° clock angle for the North (South) Hemisphere. - Remainder of abstract truncated - |
| Publication date: 12 Sep 2008 |
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| Target Acquisition for MIRI Coronagraphs |
| Coronagraphs are powerful instruments to reduce diffraction from a bright source in order to detect planets. Four coronagraphs will be installed in MIRI, the Mid-InfraRed Instrument of the James Webb Space Telescope. To further reduce the diffraction in addition to the coronagraph, a calibration of the residual speckle pattern can be obtained, for instance, with a reference star (or alternatively on the target star at a different roll angle). For this calibration to be accurate, the diffraction pattern of the two coronagraphic images must be as similar as possible. We study the accuracy of the star image positioning onto the coronagraph to reach acceptable performance: we proved that pointing reproducibility must be better than 5 mas rms per axis while the absolute pointing can be relaxed to 10 mas rms. The choice of algorithm is driven by the level of accuracy to be reached in the presence of a nonlinear system like the coronagraph. We first study their bias, and then we estimate their sensitivity to different sources of noises in the context of MIRI. And finally, for practical matter, we derive the necessary exposure time to obtain the centroid on an actual star. |
| Publication date: 04 Sep 2008 |
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