The magnetic and magnetotransport properties of granular GussFerS prepared by mechanical alloying have been investigated. The sample was prepared by grinding tine powders of copper and iron in a high energy ball mill. Zero field cooled and field cooled susceptibilities showed a behavior that is typical of a superparamagnet. The blocking temperature TB of 20 K was determined from ac susceptibility. The magnetoresistance reached 5.5% at 4.5 K in a field of 5 T. It was increased to 7.6% after the sample was annealed at 300 "G for 20 min. The hysteresis loop was measured for both magnetization and magnetoresistance. There was a clear correlation between the two. The magnetoresistance is due to the scattering associated with iron nanoparticles present in the samples and its dependence on particle size is discussed.
We report observations of dynamically unstable strong wind shear (Richardson number < 0.25) capable of inducing Kelvin‐Helmholtz instability in the polar mesospheric summer echoes (PMSE) layer (80–90 km) using very high frequency radar measurements in Kiruna (67.8°N, 20.4°E), Sweden, in 2006. The unstable strong wind shear can play an important role in producing PMSE by inducing turbulence and adiabatic cooling. We find that the strong shears take up 64% of the observed wind profiles and are frequently composed of systematically single‐shear/multishear (layer) structures, which gradually or abruptly vary in wind directions, so‐called wind shifts, through heights at intervals of 4–8 km. The strong shear rate normalized by PMSE counts has a good correlation (R = 0.7) in day‐to‐day variation with energetic electron (>30 keV) precipitations that were related to high‐speed solar wind streams. The observations of strong wind shear can be supported by satellite‐measured temperature modulations matching with the peaks of the first three high‐speed solar wind stream events. This study suggests that PMSE production is closely associated with the strong shear that is in turn linked to the effects of energetic electron precipitation.
A new 40.8 MHz coherent scatter radar was built in Daejeon, South Korea (36.18°N, 127.14°E, dip latitude: 26.7°N) on 29 December 2009 and has since been monitoring the occurrence of field‐aligned irregularities (FAIs) in the northern middle latitudes. We report on the occurrence climatology of the F region FAIs as observed by the Daejeon radar between 2010 and 2014. The F region FAIs preferentially occur around 250–350 km at 18:00–21:00 local time (postsunset FAI), around 350–450 km near midnight (nighttime FAI), around 250–350 km before sunrise (presunrise FAI), and around 160–300 km after 05:00 local time (postsunrise FAI). The occurrence rates of nighttime and presunrise FAIs are maximal during summer, though the occurrence rates of postsunset and postsunrise FAIs are maximal during the equinoxes. FAIs rarely occur during local winter. The occurrence rate of F region FAIs increases in concert with increases in solar activity. Medium‐scale traveling ionospheric disturbances (MSTIDs) are known as an important source of the F region FAIs in middle latitudes. The high occurrence rate of the nighttime FAIs in local summer is consistent with the high occurrence rate of MSTIDs in that season. However, the dependence of the FAI activity on the solar cycle is inconsistent with the MSTID activity. The source of the F region FAIs in middle latitudes is an open question. Our report of different types of FAIs and their occurrence climatology may provide a useful reference for the identification of the source of the middle latitude FAIs.
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