Rotational remanent magnetization and the torque exerted on a rotating rock in an alternating magnetic field

Summary. Using an air turbine at rotation frequencies of between 1.5 and 275 revolutions per second (rps), the dependence of rotational remanent magnetization (RRM) on rotation frequency has been investigated for two igneous samples in and alternating field of 51 mT peak at 50 Hz. The same experimental arrangement has also been used to measure the dependence on rotation frequency of the torque exerted by the alternating field on the rock samples. The dependence of torque and RRM on peak field has also been measured at a rotation frequency of 112 rps and a linear relationship between RRM and torque has been demonstrated.
In an attempt to elucidate the way in which RRM arises, analytical and numerical models of the rock have been developed in order to calculate the torque curves and these agree quite closely with those observed experimentally. While the precise factor responsible for RRM has not yet been identified from the numerical model it is suggested that RRM may arise as a result of particle moments suddenly flipping into the field direction, and thus by virtue of their intrinsic angular momentum acquiring a transient component of magnetic moment antiparallel to the rotation vector describing the flip. This component, due to the hysteresis of the assembly of particles, will not then entirely disappear when the alternating field is removed. An estimate of the transient axial field which can be considered to deflect each moment towards the rotation axis during the flip yields a value of the order of 1 mT.     

Two‐hourly surface change on supra‐tidal rock (Marengo,Victoria, Australia)

A traversing micro‐erosion meter was used to measure rock surface micro‐topography over 40 cm² on a supra‐tidal cliff face from early morning to late evening in late spring. From 06:00 hours to 22:00 hours the relative heights of 188 coordinates were obtained using the meter at 2‐hour intervals, resulting in a data set of 1607 readings. Monitoring shows that rock surfaces are dynamic entities, with significant rise and fall relative to the first measurement at shorter timescales than previously reported. The maximum positive rise between readings was 0·261 mm and lowering was 0·126 mm. The pattern of change did not relate as expected to environmental variables such as temperature or insolation. Rather, the surface showed greater surface change in the early morning and late afternoon. It is hypothesized that this pattern relates to the expansion and contraction of lichen thalli as moisture is absorbed during higher humidity in the morning and late afternoon. The implications of these results for weathering studies are considered. Copyright © 2006 John Wiley & Sons, Ltd.     

A revised orbital ephemeris for HAT-P-9b

We present here three transit observations of HAT-P-9b taken on 14 February 2010 and 05 April 2010 UT from the University of Arizona’s 1.55 m Kuiper telescope on Mt. Bigelow. Our two light curves were obtained in the Arizona-I filter for all our observations, and underwent the same reduction process. Both transits occurred approximately 24 min earlier than expected from the ephemeris of Shporer et al. (2009). However, due to the large time span between our observed transits and those of Shporer et al. (2009), a 6.5 s shift downwards in orbital period from the value of Shporer et al. (2009) is sufficient to explain all available transit data. We find a new period of 3.922814 ± 0.000002 days for HAT-P-9b, an order of magnitude more precise than previous measurements, with no evidence for significant nonlinearities in the transit period.