http://hdl.handle.net/2122/104 Search the Channel search http://www.earth-prints.org/simple-search http://hdl.handle.net/2122/3015 Title: Micromagnetic coercivity distributions and interactions in chondrules with implications for paleointensities of the early solar system <br/> <br/>Authors: Acton, G.; University of California, Davis,; Yin, Q.-Z.; University of California, Davis,; Verosuv, K. L.; University of California, Davis,; Jovane, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Roth, A.; University of California, Davis,; Jacobsen, B.; University of California, Davis,; Denton, S. E.; American Museum of Natural History, New York, <br/> <br/>Abstract: Chondrules in chondritic meteorites record the earliest stages of formation of the solar system, potentially providing information about the magnitude of early magnetic fields and early physical and chemical conditions. Using first-order reversal curves (FORCs), we map the coercivity distributions and interactions of 32 chondrules from the Allende, Karoonda, and Bjurbole meteorites. Distinctly different distributions and interactions exist for the three meteorites. The coercivity distributions are lognormal shaped, with Bjurbole distributions being bimodal or trimodal. The highest-coercivity mode in the Bjurbole chondrules is derived from tetrataenite, which interacts strongly with the lower-coercivity grains in a manner unlike that seen in terrestrial rocks. Such strong interactions have the potential to bias paleointensity estimates. Moreover, because a significant portion of the coercivity distributions for most of the chondrules is <10 mT, low-coercivity magnetic overprints are common. Therefore paleointensities based on the REM method, which rely on ratios of the natural remanent magnetization (NRM) to the saturation isothermal remanent magnetization (IRM) without magnetic cleaning, will probably be biased. The paleointensity bias is found to be about an order of magnitude for most chondrules with low-coercivity overprints. Paleointensity estimates based on a method we call REMc, which uses NRM/IRM ratios after magnetic cleaning, avoid this overprinting bias. Allende chondrules, which are the most pristine and possibly record the paleofield of the early solar system, have a mean REMc paleointensity of 10.4 μT. Karoonda and Bjurbole chondrules, which have experienced some thermal alteration, have REMc paleointensities of 4.6 and 3.2 μT, respectively. Mon, 26 Feb 2007 22:58:59 GMT http://hdl.handle.net/2122/1683 Title: Interplanetary magnetic field and its possible effects on the mid-latitude ionosphere III <br/> <br/>Authors: Tulunay, Y.; Middle East Technical University, Department of Aeronautical Engineering, Ankara, Turkey <br/> <br/>Abstract: Using critical frequencies, f0F2 from the Lannion, Slough, Poitiers, Garchy, Dourbes, Rome, Juliusrud, Gibilmanna, Pruhonice, Uppsala, Kaliningrad, Miedzeszyn, Sofia, Athens and Kiev ionosonde stations, the possible effects of the orientation of the Interplanetary Magnetic Field (IMF) on mid-latitude ionosphere are further investigated. This time, only the southward polarity changes in IMF Bz with seasonal effects were considered. The same method of analysis was employed to facilitate a comparison between the recent results presented here with those which appeared in the preceding papers in the series. That is, the regular diurnal, seasonal and solar cycle variations in the f0F2 data were removed by subtracting the mean of the f0F2 for the same UT on all magnetically quite days (Ap < 6) within 15 days around the IMF Bz turnings (Tulunay, 1994). This last paper also includes the seasonal effects on the ionospheric data. The results confirm that much of the day-to-day variability of the mid-latitude ionosphere may be related to the orientation of the southward IMF Bz , characterized by the ionospheric winter anomaly. Day-to-day ionospheric variability becomes more significant towards higher latitudes. Mon, 29 Jul 1996 22:58:59 GMT http://hdl.handle.net/2122/1667 Title: Influence of the interplanetary magnetic field on the variability of the mid-latitude F2-layer <br/> <br/>Authors: Bremer, J.; Institute of Atmospheric Physics, Kühlungsborn, Germany; Lastovicka, J.; Institute of Atmospheric Physics, Prague, Czech Republic; Tulunay, Y.; Middle East Technical University, Department of Aeronautical Engineering, Ankara, Turkey <br/> <br/>Abstract: The structure of the Interplanetary Magnetic Field (IMF) is responsible for an essential part of the variability of the ionospheric plasma as demonstrated by investigations of the influence of IMF sector boundary crossings as well as of ?Bz-changes (defined from satellite observations) to the maximal electron density of the F2-layer at different stations in mid-latitudes. It could be shown that negative Bz-values cause distinct negative ionospheric effects. Maximal effects were detected at high geomagnetic latitudes (ionospheric response decreases with decreasing latitude), high solar/geomagnetic activity, equinoxes and night-time conditions. Mon, 29 Jul 1996 22:58:59 GMT