[1] We study the acceleration of magnetosheath plasma using a semi-analytical magnetic string approach for a range of solar wind Alfvén Mach numbers, M A , between 2 and 20. We work with an IMF vector perpendicular to the solar wind velocity, V sw , and pointing north. We do not invoke magnetic reconnection. Our results indicate that magnetosheath speeds can exceed the solar wind speed, and the ratio V/V sw increases with decreasing M A . Analyzing the dependence of this ratio on M A , we find that for M A = 2, maximum V/V sw ≈ 1.6, and for M A = 10-20, maximum V/V sw varies from 1.21 to 1.13. Maximum speeds occur a few Earth radii (R E ) tailward of the dawndusk terminator. The thickness of the accelerated flow layer varies as M A −2 . Taking the magnetopause subsolar distance as 10 R E , we find typical values for the thickness of ∼4 R E for M A = 3 and 0.35 R E for M A = 10. The physical mechanism is that of draping of the magnetic field lines around the magnetosphere, and the associated magnetic tension and total pressure gradient forces acting on the flow. For lower M A the plasma depletion is stronger, and thus the acceleration produced by the pressure gradient is larger. An additional acceleration is produced by the magnetic tension, which is stronger for smaller M A . At the dayside the pressure gradient and magnetic tension forces both act in the same direction. But tailward of the terminator the magnetic tension starts to act in the opposite direction to the pressure gradient. When the resulting force vanishes, the highest speed is attained. Citation: Erkaev,
Abstract. Acceleration of magnetosheath plasma resulting from the draping of the interplanetary magnetic field (IMF) around the magnetosphere can give rise to flow speeds that exceed that of the solar wind (V SW ) by up to ∼ 60 %. Three case event studies out of 34 identified events are described. We then present a statistical study of draping-related accelerations in the magnetosheath. Further, we compare the results with the recent theory of Erkaev et al. (2011Erkaev et al. ( , 2012. We present a methodology to help distinguish draping-related accelerations from those caused by magnetic reconnection. To rule out magnetopause reconnection at low latitudes, we focus mainly on the positive B z phase during the passage of interplanetary coronal mass ejections (ICMEs), as tabulated in Richardson and Cane (2010) for 1997-2009, and adding other events from 2010. To avoid effects of high-latitude reconnection poleward of the cusp, we also consider spacecraft observations made at low magnetic latitudes. We study the effect of upstream Alfvén Mach number (M A ) and magnetic local time (MLT) on the speed ratio V /V SW . The comparison with theory is good. Namely, (i) flow speed ratios above unity occur behind the dawn-dusk terminator, (ii) those below unity occur on the dayside magnetosheath, and (iii) there is a good general agreement in the dependence of the V ratio on M A .
We discuss the temporal variations and frequency distributions of solar wind and interplanetary magnetic field parameters during the solar minimum of 2007 -2009 from measurements returned by the IMPACT and PLASTIC instruments on STEREO-A. We find that the density and total field strength were significantly weaker than in the previous minimum. The Alfvén Mach number was higher than typical. This reflects the weakness of magnetohydrodynamic (MHD) forces, and has a direct effect on the solar wind-magnetosphere interactions. We then discuss two major aspects that this weak solar activity had on the magnetosphere, using data from Wind and ground-based observations: i) the dayside contribu-The Sun 360
Magnetic core materials with low loss, high saturation magnetization, large permeability, and operating frequency above 1 MHz are in high demands for the next generation of miniaturized power electronics. Amorphous FeHfB ribbons with thickness around 20 μm have been fabricated through melt-spinning. Different heat treatments were performed on the FeHfB ribbons, and the relations among heat treatments, microstructure, and magnetic properties have been explored. Properties such as coercivity (Hc) of 2.0 Oe and saturation magnetic flux density (BS) of 1.2 T have been achieved in samples with exchange coupling. The losses can be minimized by balancing the hysteretic and eddy current losses and can be further reduced with additional magnetic field annealing. At 5 MHz with peak magnetic flux density of 20 mT, the materials show core losses comparable to the best ferrites, but with higher permeability value of about 200 and superior saturation induction of more than 1 T.
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