Formation of episodic magnetically driven radiatively cooled plasma jets in the laboratory

We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a “magnetic tower” jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic “bubble”, confined by the ambient medium. The use of a radial metallic foil instead of the radial wire arrays employed in our previous work allows for the generation of episodic magnetic tower outflows which emerge periodically on timescales of ～30 ns. The subsequent magnetic bubbles propagate with velocities reaching ～300 km/s and interact with previous eruptions leading to the formation of shocks.     

Dynamic Elastic Tides

This is an exploration of dynamic tides on elastic bodies. The body is thought of as a dynamical system described by its modes of oscillation. The dynamics of these modes are governed by differential equations that depend on the rheology. The modes are damped by dissipation. Tidal friction occurs as exterior bodies excite the modes and the modes act back on the tide raising body. The whole process is governed by a closed set of differential equations. Standard results from tidal theory are recovered in a two-timescale approximation to the solution of these differential equations.     

Energetic protons associated with a forward-reverse interplanetary shock pair at 1 A.U.

A forward-reverse interplanetary shock was observed on 25 March 1969 by the magnetometer and plasma detector on the HEOS-1 satellite. This relatively rare event was described by Chao et al (1972) who concluded that the shock pair was formed at a distance 0.10–0.13 A.U. upstream of the Earth as a result of the interaction between a fast and a slow solar wind streams. Simultaneous observations of 1 MeV solar proton fluxes were also performed on HEOS-1. A characteristic intensity peak was observed as the forward shock passed by the spacecraft. The evolution of the proton intensity, together with a detailed analysis of anisotropies and pitch angle distributions show a complex dynamic picture of the effect of the forward shock on the ambient proton population. Significant changes in particle fluxes are seen to be correlated with fluctuations in the magnetic field. It is suggested that simple geometrical models of shock-associated acceleration should be expanded to include the effect of magnetic fluctuations on particle fluxes. The interaction region limited by the forward and reverse shocks contained a large variety of magnetic fluctuations. Following the tangential discontinuity separating the fast solar wind stream from the preceding slow stream, a sunward flow was observed in the proton data, followed by a small but significant drop in intensity prior to the reverse shock.     

Reliability of estimates for the sizes of spots on asteroid surfaces made by the spectral-frequency method

A new spectral-frequency method (SFM) for the study of solid body surfaces is briefly described. This method allows estimation of the sizes of various spots. Estimates for the sizes of spots on asteroid surfaces made by the SFM and other methods are compared and discussed. The sizes of spots on the surface of asteroid 1620 Geographos determined by the SFM are well consistent with those of the craters obtained from radar data. The sizes of hydrosilicate spots on the surface of asteroid 21 Lutetia found by the SFM agree with those of the craters determined by the Rosetta spacecraft. The size of a blue spot on the surface of asteroid 4 Vesta found by the SFM is consistent with the size of the well-known crater on the south pole of the asteroid. It is inferred that the SFM is a promising method for the estimation of the sizes of spots on asteroid surfaces.     

Source rates and ion recycling rates for Na and K in Mercury's atmosphere

The supply rates of Na and K to the atmosphere of Mercury by processes acting on the extreme surface—thermal vaporization, photon-stimulated desorption (PSD), and ion-sputtering—are limited by the rates at which atoms can be supplied to the extreme surface by diffusion from inside the regolith grains. Supply rates to the atmosphere are further regulated by ion retention and by gardening rates that supply new grains to the surface. We consider the limits on supply of sodium and potassium atoms to the atmosphere, and rates of photoion recycling to the surface. Thermal vaporization rates are severely limited by the ability of atoms to diffuse to the surface of the grain. Therefore, the diffusion-limited thermal vaporization rates on Mercury's surface are comparable to or less than the PSD rates. Ion sputtering is primarily due to highly ionized heavy ions, even though they represent a small fraction of the solar wind. We have shown that up to 60% of the Na photoions are deposited on the surface of Mercury. Ion recycling to the surface can have a long-term effect on the regolith abundance if an average recycling pattern persists such that more ions return to a particular area than are launched there. It is unknown whether the formation of latitude bands of >100% ion retention persist on average despite a rapidly changing magnetosphere. The total exospheric column of sodium observed at Mercury between 1997 to 2003 varied by a factor of 2-3 from perihelion to aphelion.     

Isophot Mapping Observations of Carbon Stars

Mapping observations have been made toward five carbon stars in the far-infrared using ISOPHOT, an imaging photo-polarimeter on board the Infrared Space Observatory. Cold, very extended dust shells are clearly revealed in two of them. Y CVn is surrounded by a very extended, detached dust shell, which indicates a sudden decline of the mass-loss by two orders of magnitude in the last (1-2) × 104 years on a short time scale. The Hipparcos parallax reinforces our previous conclusion that Y CVn is a J-type carbon star on the asymptotic giant branch. U Ant shows a double shell structure, a compact dust shell surrounded by a very extended one. The outer shell has a brightness comparable to the dust shell of Y CVn. The structure indicates that there were two different high mass-loss phases separated by a period with a much lower mass-loss rate in between the two. The structure is consistent with the double dust shell proposed for this star by Izumiura et al. (1997) based on a detailed investigation of IRAS survey data. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects on UV lines observations of stationary plasma flows confined in coronal loops

We describe a method which uses a rather detailed model of coronal loop hosting a siphon flow as a diagnostic tool to interpret solar UV and X-ray observations in selected bands and lines. We apply the method to investigate the deviations from ionization equilibrium induced by stationary plasma flows confined in coronal loops and their effects on the UV and EUV emission lines observed by the instruments on board SOHO. We present results on the detailed synthesis of loop emission in a set of selected emission lines observed by CDS and SUMER, taking into account the non-equilibrium of ionization effects.     

The formation of an eccentric gap in a gas disc by a planet in an eccentric orbit

We investigate the effect of a planet on an eccentric orbit on a two-dimensional low-mass gaseous disc. At a planet eccentricity above the planet's Hill radius divided by its semimajor axis, we find that the disc morphology differs from that exhibited by a disc containing a planet in a circular orbit. An eccentric gap is created with eccentricity that can exceed the planet's eccentricity and precesses with respect to the planet's orbit. We find that a more massive planet is required to open a gap when the planet is on an eccentric orbit. We attribute this behaviour to spiral density waves excited at corotation resonances by the eccentric planet. These act to increase the disc's eccentricity and exert a torque opposite in sign to that exerted by the Lindblad resonances. The reduced torque makes it more difficult for waves driven by the planet to overcome viscous inflow in the disc.     

On the integral equation method for radiative transfer past a rotating plate in unsteady flow

The transient effect on the flow of a thermally-radiating and electrically-conducting compressible gas in a rotating medium bounded by a vertical flat plate, is studied when the radiative flux satisfies the exact integral expression. The transience is provoked by a time-dependent perturbation on a constant plate temperature. The solution is constructed for the flow near and away from the plate by the Laplace transform method. The results are compared with the recent work of Bestman and Adjepong (1988).     

Research on the Application of Fast-steering Mirror in Stellar Interferometers

In stellar interferometers, the fast-steering mirror (FSM) is widely utilized to correct the wavefront tilt caused by the atmospheric turbulence and internal instrumental vibration, because of its high resolution and fast response frequency. In this study, the non-coplanar error between the FSM and the actuator deflection axis introduced by the manufacturing, assembly, and adjustment is analyzed systematically. Via a numerical method, the additional optical path difference (OPD) caused by the above factors is studied, and its effect on the fringe tracking accuracy of a stellar interferometer is also discussed. On the other hand, the starlight parallelism between the beams of two arms is one of the main factors for the loss of fringe visibility. By analyzing the influence of wavefront tilt caused by the atmospheric turbulence on fringe visibility, a simple and efficient real-time correction scheme of starlight parallelism is proposed based on a single array detector. The feasibility of this scheme is demonstrated by a laboratory experiment. The results show that after the correction of fast-steering mirror, the starlight parallelism meets preliminarily the requirement of a stellar interferometer on the wavefront tilt.     

Modelling the turbulent magnetized ISM

Briefly stated, the interstellar matter (hereafter ISM) is a highlyturbulent flow, the turbulence mainly being driven by the violentstirring by supernova explosions, penetrated by dynamicallysignificant magnetic fields both on small and largescales. Sophisticated numerical models of such a turbulent flow cangive important information on some of the complicated processesoccurring in the ISM, e.g. on the regulation of the observedmultiphase structure, properties of interstellar turbulence and thegeneration and maintenance of magnetic fields by the flow itself; inthis paper such models and their most important results are reviewed.     

Sudden geomagnetic field variations in the night-side cusp-region during magnetospheric substorms accompanied by electron precipitation in the auroral zone

The relationship between sudden geomagnetic field changes in the nightside cusp region and impulsive electron precipitation events in the auroral zone is investigated. The investigations are based on magnetic field measurements from the spacecraft Explorer 35, Explorer 33 and OGO-5 and on X-ray measurements with balloon-borne instruments from Kiruna/Sweden. The sudden field changes are characterized by a decrease of the field strength and a rotation of the field direction. The precipitation events represent strong flux increases within a few minutes. The field changes were accompanied by impulsive precipitation not only in the midnight sector but also on the dayside. They can be regarded as a manifestation of the unsteady magnetospheric processes during the expansion phase. Whereas both phenomena occurred simultaneously on the nightside, the increase of precipitation was delayed by ca. 5 min on the dayside. It is assumed that the simultaneous occurrence on the nightside can be related to the formation of a neutral line with a considerable length in dawn-dusk-direction. Mechanisms are also discussed which could be responsible for the time delay on the dayside.     

Note on tidal evolution of the Earth-Moon system

The need is pointed out of a re-discussion of the past tidal evolution of the Earth-Moon system as a boundary-value problem on the time-scale indicated by radiometric dating of lunar soils returned by successive space missions from different localities on the Moon's surface.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Atmospheric control of the cooling rate of impact melts and cryolavas on Titan’s surface

As on Earth, Titan’s atmosphere plays a major role in the cooling of heated surfaces. We have assessed the mechanisms by which Titan’s atmosphere, dominantly N₂ at a surface pressure of 1.5 × 10⁵ Pa, cools a warm or heated surface. These heated areas can be caused by impacts generating melt sheets and (possibly) by endogenic processes emplacing cryolavas (a low-temperature liquid that freezes on the surface). We find that for a cooling cryolava flow, lava lake, or impact melt body, heat loss is mainly driven by atmospheric convection. Radiative heat loss, a dominant heat loss mechanism with terrestrial silicate lava flows, plays only a minor role on Titan. Long-term cooling and solidification are dependent on melt sheet or flow thickness, and also local climate, because persistent winds will speed cooling. Relatively rapid cooling caused by winds reduces the detectability of these thermal events by instruments measuring surface thermal emission. Because surface temperature drops by ≈50% within ≈1 day of emplacement, fresh flows or impact melt may be difficult to detect via thermal emission unless an active eruption is directly observed. Cooling of flow or impact melt surfaces are orders of magnitude faster on Titan than on airless moons (e.g., Enceladus or Europa).Although upper surfaces cool fast, the internal cooling and solidification process is relatively slow. Cryolava flow lengths are, therefore, more likely to be volume (effusion) limited, rather than cooling-limited. More detailed modeling awaits constraints on the thermophysical properties of the likely cryomagmas and surface materials.     

Small-Scale Energy Release Driven by Supergranular Flows on the Quiet Sun

We present data and modelling for the quiet Sun that strongly suggest a ubiquitous small-scale atmospheric heating mechanism that is driven solely by converging supergranular flows. A possible energy source for such events is the power transfer to the plasma via the work done on the magnetic field by photospheric convective flows, which exert drag on the footpoints of magnetic structures. We present evidence of small-scale energy release events driven directly by the hydrodynamic forces that act on the magnetic elements in the photosphere, as a result of supergranular-scale flows. We show strong spatial and temporal correlation between quiet-Sun soft X-ray emission (from Yohkoh SXT) and SOHO MDI-derived flux removal events driven by deduced photospheric flows. We also present a simple model of heating generated by flux submergence, based on particle acceleration by converging magnetic mirrors. In the near future, high resolution soft X-ray images from XRT on the Hinode satellite will allow definitive, quantitative verification of our results. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Zur Dynamotheorie kosmischer Magnetfelder. I. Gleichungen für sphärische Dynamomodelle

This paper deals with the mathematical treatment of special models of hydromagnetic dynamos. For the models considered here the conducting medium occupies a spherical region surrounded by vacuum. Both laminar and turbulent dynamos are included. The partial vector differential equations governing the models are transformed into an infinite set of differential equations for scalar functions by applying a method previously used by BULLARD and GELLMAN and on the basis of a representation of vector fields as a sum of a poloidal and a toroidal part. The scalar functions depend only on a radial coordinate and possibly on the time. In both the stationary and the periodic case, an infinite set of ordinary differential equations results which may be treated numerically. A series of relations for computing various dynamo models is prepared.     

Dynamo process governing solar wind-magnetosphere energy coupling

Based on the method of dimensional analysis, the energy transfer rate from the solar wind into the magnetosphere can be characterized by a magnetic coupling parameter α on open field lines and by a viscous coupling parameter β on closed field lines. By assuming that the energy transfer rate can be monitored by the total energy dissipation rate of the magnetosphere, the histogram of α is constructed and is found to peak around ?0.1 < α < 0.1. This result implies that the energy transfer is governed primarily by the MHD dynamo process on open field lines and indicates that the ? function obtained by Perreault and Akasofu is verified as the first approximation of the solar wind-magnetosphere energy coupling function.     

Gravitational microlensing of gamma-ray bursts at medium optical depth

Gravitational lensing of a gamma-ray burst (GRB) by a single point mass will produce a second, delayed signal. Several authors have discussed using microlensed GRBs to probe a possible cosmological population of compact objects. We analyse a closely related phenomenon: the effect of microlensing by low to medium optical depth in compact objects on the averaged observed light curve of a sample of GRBs. We discuss the cumulative measured flux as a function of time resulting from delays caused by microlensing by cosmological compact objects. The time-scale and curvature of this function describe unique values for the compact object mass and optical depth. For GRBs with durations larger than the detector resolution, limits could be placed on the mass and optical depth of cosmological compact objects. The method does not rely on the separation of lensed bursts from those that are spatially coincident.     

Stokes II — A new polarimeter for solar observations

A Stokes polarimeter has been built at the High Altitude Observatory to obtain line profiles in both linear and circular polarization in solar spectral lines. These measurements are interpreted using the theory of radiative transfer in the presence of a magnetic field to obtain vector magnetic fields on the solar disk and using the theory of resonance scattering and the Hanle effect to obtain vector magnetic fields in prominences. The polarimeter operates on the Sacramento Peak Observatory 40 cm coronagraph. It is an extensively modified and improved version of an earlier instrument.Polarization modulation is achieved by two KD^*P Pockels cells at the coronagraph prime focus and demodulation is by a microprocessor. The instrument control and data handling is done by a minicomputer. Silicon photodiode 128 element line array detectors have replaced the two photomultipliers used on the earlier instrument. This gives a speed increase of a factor of 50.A polarization scrambler provides a chop to a reference beam of unpolarized light by time scrambling the polarization of the solar beam. This device improves sensitivity to polarizations less than 0.01%. The polarization measurements are photon noise limited in most cases. This noise is 0.1% for a typical three second observation which is about one gauss on the longitudinal field and 10 gauss on the transverse field.The National Center for Atmospheric Research is sponsored by The National Science Foundation.     

The photoevaporation of discs around young stars in massive clusters

We present models in which the photoevaporation of discs around young stars by an external ultraviolet source (as computed by Adams et al.) is coupled with the internal viscous evolution of the discs. These models are applied to the case of the Orion Nebula Cluster (ONC), where the presence of a strong ultraviolet field from the central OB stars, together with a detailed census of circumstellar discs and photoevaporative flows, is well established. In particular we investigate the constraints that are placed on the initial disc properties in the ONC by the twin requirement that most stars possess a disc on a scale of a few astronomical unit (au), but that only a minority (<20 per cent) are resolved by Hubble Space Telescope ( HST ) at a scale of 50 au. We find that these requirements place very weak constraints on the initial radius distribution of circumstellar discs: the resulting size distribution readily forgets the initial radius distribution, owing to the strong positive dependence of the photoevaporation rate on disc radius. Instead, the scarcity of large discs reflects the relative scarcity of initially massive discs (with mass  >0.1 M_⊙  ). The ubiquity of discs on a small scale, on the other hand, mainly constrains the time-span over which the discs have been exposed to the ultraviolet field (<2 Myr). We argue that the discs that are resolved by HST represent a population of discs in which self-gravity was important at the time that the dominant central OB star switched on, but that, according to our models, self-gravity is unlikely to be important in these discs at the present time. We discuss the implications of our results for the so-called proplyd lifetime problem.