The Dependence of the Geoeffectiveness of Interplanetary Flux Rope on Its Orientation,with Possible Application to Geomagnetic Storm Prediction

Interplanetary magnetic clouds (MCs) are one of the main sources of large non-recurrent geomagnetic storms. With the aid of a force-free flux rope model, the dependence of the intensity of geomagnetic activity (indicated by Dst index) on the axial orientation (denoted by θ and φ in GSE coordinates) of the magnetic cloud is analyzed theoretically. The distribution of the Dst values in the (θ, φ) plane is calculated by changing the axial orientation for various cases. It is concluded that (i) geomagnetic storms tend to occur in the region of θ<0°, especially in the region of θ≲−45°, where larger geomagnetic activity could be created; (ii) the intensity of geomagnetic activity varies more strongly with θ than with φ; (iii) when the parameters B ₀ (the magnetic field strength at the flux rope axis), R ₀ (the radius of the flux rope), or V (the bulk speed) increase, or |D| (the shortest distance between the flux rope axis and the x-axis in GSE coordinates) decreases, a flux rope not only can increase the intensity of geomagnetic activity, but also is more likely to create a storm, however the variation of n (the density) only has a little effect on the intensity; (iv) the most efficient orientation (MEO) in which a flux rope can cause the largest geomagnetic activity appears at φ∼0° or ∼ 180°, and some value of θ which depends mainly on D; (v) the minimum Dst value that could be caused by a flux rope is the most sensitive to changes in B ₀ and V of the flux rope, and for a stronger and/or faster MC, a wider range of orientations will be geoeffective. Further, through analyzing 20 MC-caused moderate to large geomagnetic storms during 1998 – 2003, a long-term prediction of MC-caused geomagnetic storms on the basis of the flux rope model is proposed and assessed. The comparison between the theoretical results and the observations shows that there is a close linear correlation between the estimated and observed minimum Dst values. This suggests that using the ideal flux rope to predict practical MC-caused geomagnetic storms is applicable. The possibility of the long-term prediction of MC-caused geomagnetic storms is discussed briefly.     

Filamentary Structures of Coronal White-Light Images

In the absence of magnetic field measurements of the solar corona, the density structure of white-light images has provided important insight into the coronal magnetic field. Recent work sparked by highly sensitive radio occultation measurements of path-integrated density has elucidated the density structure of unprocessed solar eclipse pictures. This paper does the same for processed images that reveal low-contrast small-scale structures, specifically Koutchmy’s edge-enhanced white-light image of the 11 August 1999 solar eclipse. This processed image provides visual evidence for two important results deduced from radio occultation measurements of small-scale density variations. First, in addition to the closed loops readily seen at the base of the corona in high-resolution EUV and soft X-ray images, open filamentary structures permeate the corona including active regions generally thought to be magnetically closed. Observed at the image resolution, the filamentary structures are 1° wide in latitude and an order of magnitude smaller than polar plumes. Second, although inhomogeneities that are convected along with the solar wind are also present, filamentary structures dominate the image because of their steeper density gradients. The quantitative profile of polarized brightness (pB) at the base of the corona shows that the filamentary structures have transverse density gradients that are proportional to their density. This explains why edge-enhanced images, limited in sensitivity to density gradients, tend to detect filamentary structures more readily in high-density regions (e.g., active regions, streamer stalks, and prominences) than in low-density polar coronal holes, and why filamentary structures seem more prevalent in solar eclipse pictures during solar maximum. The pB profile at the base of the corona also fills the gap in Doppler measurements there, reinforcing that open ultra-fine-scale filamentary structures observed by the radio measurements are predominantly radial and that they are an integral part of the radial expansion of the solar wind.     

<Emphasis Type="Italic">N</Emphasis>-observations and radar orbits

Initial asteriod orbits are determined by a least squares adjustment of an arbitrary number (N) of optical and radar observations. The usual separation, into an orbit determination by three observations and a subsequent differential orbit improvement, is combined into a single algorithm. A priori information is used for very small arcs. Ephemerides very suitable for linking are obtained by strictly linear computations.     

Dark matter in dwarf spheroidals – II. Observations and modelling of Draco

We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude   V ≈19.5  with a mean velocity precision of ≈2 km s⁻¹. As this survey is based on a high-precision photometric target list, it contains many more Draco members at large radii. For the first time, this allows a robust determination of the radial behaviour of the velocity dispersion in a dSph.
We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2 σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood.
We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law  ( v _circ∝ r ^0.17)  . We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3 σ significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.     

The Near-Earth Asteroid Size-Frequency Distribution: A Snapshot of the Lunar Impactor Size-Frequency Distribution

It is shown that the size-frequency distribution (SFD) of a time-averaged projectile population derived from the lunar crater SFD of Neukum and Ivanov (in Hazards Due to Comets and Asteroids (T. Gehrels, Ed.), 1994, pp. 359-416, Univ. of Arizona Press, Tucson) provides a convincing fit to the SFD of the current near-Earth asteroid (NEA) population, as deduced from the results of asteroid search programs. Our results suggest that the shape of the SFD of the impactor flux has remained in a steady state since the late heavy bombardment, so that the current NEA population can be viewed as a snapshot of the flux of impactors on the Moon. The number of bodies in the projectile population with diameters of 1 km or more is 700±130, which is in good agreement with recent estimates of the total number of NEAs in this size range. Our results imply that the contribution to the projectile flux from comets is small for diameters below 10 km.     

Effects of mass loss for highly-irradiated giant planets

We present calculations for the evolution and surviving mass of highly-irradiated extrasolar giant planets (EGPs) at orbital semimajor axes ranging from 0.023 to 0.057 AU using a generalized scaled theory for mass loss, together with new surface-condition grids for hot EGPs and a consistent treatment of tidal truncation. Theoretical estimates for the rate of energy-limited hydrogen escape from giant-planet atmospheres differ by two orders of magnitude, when one holds planetary mass, composition, and irradiation constant. Baraffe et al. [Baraffe, I., Selsis, F., Chabrier, G., Barman, T.S., Allard, F., Hauschildt, P.H., Lammer, H., 2004. Astron. Astrophys. 419, L13-L16] predict the highest rate, based on the theory of Lammer et al. [Lammer, H., Selsis, F., Ribas, I., Guinan, E.F., Bauer, S.J., Weiss, W.W., 2003. Astrophys. J. 598, L121-L124]. Scaling the theory of Watson et al. [Watson, A.J., Donahue, T.M., Walker, J.C.G., 1981. Icarus 48, 150-166] to parameters for a highly-irradiated exoplanet, we find an escape rate ∼¹⁰² lower than Baraffe's. With the scaled Watson theory we find modest mass loss, occurring early in the history of a hot EGP. In this theory, mass loss including the effect of Roche-lobe overflow becomes significant primarily for masses below a Saturn mass, for semimajor axes ?0.023 AU. This contrasts with the Baraffe model, where hot EGPs are claimed to be remnants of much more massive bodies, originally several times Jupiter and still losing substantial mass fractions at present.