Reducing the probability of capture into resonance

A migrating planet can capture planetesimals into mean motion resonances. However, resonant trapping can be prevented when the drift or migration rate is sufficiently high. Using a simple Hamiltonian system for first- and second-order resonances, we explore how the capture probability depends on the order of the resonance, drift rate and initial particle eccentricity. We present scaling factors as a function of the planet mass and resonance strength to estimate the planetary migration rate above which the capture probability drops to less than half. Applying our framework to multiple extrasolar planetary systems that have two planets locked in resonance, we estimate lower limits for the outer planet's migration rate, allowing resonance capture of the inner planet.
Mean motion resonances are comprised of multiple resonant subterms. We find that the corotation subterm can reduce the probability of capture when the planet eccentricity is above a critical value. We present factors that can be used to estimate this critical planet eccentricity. Applying our framework to the migration of Neptune, we find that Neptune's eccentricity is near the critical value that would make its 2 : 1 resonance fail to capture twotinos. The capture probability is affected by the separation between resonant subterms and so is also a function of the precession rates of the longitudes of periapse of both planet and particle near resonance.     

Projection of auroral intensity contours into the magnetosphere

Isointensity contours of 630 nm auroral emission are traced into the magnetosphere, using two different empirical magnetic field models, the Mead-Fairfield model, and the Hedgecock-Thomas model. The auroral data are for a specific ISIS-II satellite pass, and so the starting points are expressed in geographic latitude and longitude coordinates, at a specific universal time. The magnetic field models are constructed from satellite magnetometer measurements, and those used correspond to magnetically quiet times. The projections are found to agree reasonably well with direct plasma measurements of the plasma sheet. The projections of the dayside contour connect to widely different regions of the magnetosphere, providing an interpretation that is consistent with observations of the dayside aurora. It is concluded that field line projections of the aurora into the magnetosphere using these models is a valid procedure, but only under quiet-time conditions.     

The penetration of soft electrons into the ionosphere

Calculations are presented of energy spectra and angular and spatial distributions of electron fluxes in the ionosphere resulting from precipitation ofmonoenergetic (E = 25, 50 and 100 eV) electrons. The incident electrons are assumed to be isotropic over the downward direction. It is found that the resulting steady-state electron fluxes above ca. 300 km are highly anisotropic, and that the pitch angle distribution is energy dependent. About 15 per cent of the incident electrons are backscattered elastically to the protonosphere. A much larger number of electrons escape after they have deposited a part of their energy in the atmosphere. The mean energy of the escaping electrons is about half that of the incident electrons. About 50% of the incident energy is absorbed in the atmosphere, the remainder being returned to the protonosphere. The rate of absorption of energy is a maximum at heights between 300 and 400 km. Most of the energy is absorbed in ionization and excitation of atomic oxygen. An appreciable amount of energy is, however, absorbed as heat by the ambient electron gas. Altitude profiles are presented of the rates of ionization, excitation, and electron heating caused by soft electron precipitation.     

Performance of the veto detector incorporated into the ZEPLIN-III experiment

The ZEPLIN-III experiment is operating in its second phase at the Boulby Underground Laboratory in search of dark matter WIMPs. The major upgrades to the instrument over its first science run include lower background photomultiplier tubes and installation of a plastic scintillator veto system. Performance results from the veto detector using calibration and science data in its first six months of operation in coincidence with ZEPLIN-III are presented. With fully automated operation and calibration, the veto system has maintained high stability and achieves near unity live time relative to ZEPLIN-III. Calibrations with a neutron source demonstrate a rejection of 60% of neutron-induced nuclear recoils in ZEPLIN-III that might otherwise be misidentified as WIMPs. This tagging efficiency reduces the expected untagged nuclear recoil background from neutrons during science data taking to a very low rate of ?0.2 events per year in the WIMP acceptance region. Additionally, the veto detector provides rejection of 28% of γ-ray induced background events, allowing the sampling of the dominant source of background in ZEPLIN-III - multiple scatter γ-rays with rare topologies. Since WIMPs will not be tagged by the veto detector, and tags due to γ-rays and neutrons are separable, this population of multiple scatter events may be characterised without biasing the analysis of candidate WIMP signals in the data.     

Propagation of the geomagnetic density and temperature effect into the exosphere

The propagation of the geomagnetic effect into the exosphere is investigated based on a free-flight particle kinetic model of exospheric densities and temperatures. Exobasic neutral gas conditions and their variations during a geomagnetic storm occurrence are adopted as given by the OGO-6 model. The contributions of particles originating at different exobasic locations to the density and temperature at exospheric regions are taken into account according to the time needed to reach these regions. A short-time geomagnetic variation of exobasic conditions is simulated by a Gaussianshaped A_p -index variation with an FWHM of 20 min. It is then shown that the relative amplitude and the half width of the geomagnetic density variation increase strongly with exospheric heights. The density peak and the main temperature peak are shown to be delayed by more than one and two hours, respectively, at heights above 10,000 km. The temperature variation changes from a singlepeaked to a double-peaked structure at greater exospheric heights. It is shown that the exospheric density response to geomagnetic disturbances is detectable in observations of the geocoronal He-1-584 Å resonance radiation.     

Continual matter accretion into the solar system

Rapid accretion of matter takes place while the solar system resides in a dark cloud of high density. The mass thus accreted may be comparable to the mass of the planetary system. Since the elemental abundances in a dark cloud are considered to be heterogeneous due to matter processed inside stars and then ejected, the continual accretion causes elemental heterogeneities in the solar system.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.     

Influence of the magnetosheath on solar proton penetration into the magnetosphere

The observation of solar protons (1–9 MeV) aboard HEOS-2 in the high-latitude magnetotail and magnetosheath on 9 June 1972, and their comparison with simultaneous measurements on Explorers 41 and 43, both in interplanetary space, indicate the existence of a distinct region of the inner magnetosheath (about 3 Earth radii thick) near the high-latitude magnetopause in which the solar particle flow is almost reversed with respect to the flow observed in interplanetary space. The region can also be seen by comparing magnetic field measurements on the three spacecraft. The observations in the outer layer of the magnetotail show solar protons predominantly entering the magnetosphere somewhere near the Earth, perhaps the cusp region.     

Digging into the surface of the icy dwarf planet Eris

We describe optical spectroscopic observations of the icy dwarf planet Eris with the 6.5-m MMT telescope and the Red Channel Spectrograph. We report a correlation, that is at the edge of statistical significance, between blue shift and albedo at maximum absorption for five methane ice bands. We interpret the correlation as an increasing dilution of methane ice with another ice component, probably nitrogen, with increasing depth into the surface. We suggest a mechanism to explain the apparent increase in nitrogen with depth. Specifically, if we are seeing Eris 50 degrees from pole-on [Brown, M.E., Schaller, L., 2008. Science 316, 1585], the pole we are seeing now at aphelion was in winter darkness at perihelion. Near perihelion, sublimation could have built up atmospheric pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the dark (winter) hemisphere, where the winds would condense. Because nitrogen is more volatile and scarcer than methane, it sublimated from the sunlit hemisphere relatively early in the season, so the early summer atmosphere was nitrogen rich, and so was the ice deposited on the winter pole. Later in the season, much of the nitrogen was exhausted from the summer pole, but there was plenty of methane, which continued to sublimate. At this point, the atmosphere was more depleted in nitrogen, as was the ice freezing out on top of the earlier deposited nitrogen rich ice. Our increasing nitrogen abundance with depth apparently contradicts the Licandro et al. [Licandro, J., Grundy, W.M., Pinilla-Alonso, N., Leisy, P., 2006. Astron. Astrophys. 458, L5-L8] result of a decreasing nitrogen abundance with depth. A comparison of observational, data reduction, and analysis techniques between the two works, suggests the difference between the two works is real. If so, we may be witnessing the signature of weather on Eris. The work reported here is intended to trigger further observational effort by the community.     

Non-equilibrium of Magnetic Flux Tubes emerging into the Solar Corona

A lack of equilibrium of twisted magnetic flux tubes emerging from the photosphere into the corona is considered. Assuming mass and flux conservation in the tube and an isothermal tube that is in thermal equilibrium with the surrounding plasma, it is shown that a sufficently rapid temperature increase through the transition zone may lead to the loss of magnetohydrostatic equilibrium of the emerging flux tube due to the enhancement of the plasma pressure inside the tube. The non-equilibrium leads to a rapid expansion of the tube to reach a new equilibrium state. The rise and expansion of the tube before and after the non-equilibrium are accompanied by an increase in the twist of the magnetic field. This may lead to the field exceeding the threshold for the onset of the kink instability and a subsequent explosive release of magnetic energy.     

On penetration depth of the Shoemaker-Levy 9 fragments into the Jovian atmosphere

The actual penetration depth of the Shoemaker-Levy 9 fragments into the Jovian atmosphere is still an open question. From fundamental equations of meteoric physics with variable cross-section, a new analytic model of energy release of the fragments is presented. In use of reasonable parameters, a series of results are calculated for different initial mass of the fragments. The results show that the largest fragment explodes above pressure levels of 3 bars and does not penetrate into the H₂O cloud layer of the Jovian atmosphere, and that airburst of smaller fragments occur even above the upper cloud layer.     

High-resolution calculations of asteroid impacts into the Venusian atmosphere.

We present results from a number of 2D high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus. These cover a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 2-3 km diameter asteroids, as these are responsible for most of the impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain enough impetus to make large craters when they meet the surface. We find that smaller impactors (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impactors (>2-3 km) fragmentation by mechanical ablation is preferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities spurs us to develop a simple heuristic model of the mechanical ablation of fragments based on the growth rates of Rayleigh-Taylor instabilities. Although in principle the model has many free parameters, most of these have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters.     

Expansion of an X-ray coronal arch into the outer corona

An asymmetric, expanding arch, photographed in the inner corona with an X-ray telescope on 13 August, 1973, is identified as the source of the mass ejected in a white light transient in the outer corona. The morphology, angular position, estimated mass and apparent rate of upward acceleration of the lower coronal arch are similar to those of the arch seen passing through the outer corona. The mass of material removed from the lower corona is estimated at 2 × 10¹⁵ g, and the upward movement is consistent with a constant acceleration of 12.5 m s^–2 between 1.3 and 5 R.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Insights into the petrogenetic history of the Northwest Africa 7635 augite-rich shergottite

The petrogenesis of the Northwest Africa (NWA) 7635 Martian meteorite involved the entrainment of xenocrystic olivine grains into a relatively magnesian and oxidized melt, followed by a redox-dependent reaction between olivine and melt that resulted in the crystallization of orthopyroxene and magnetite. Subsequent crystallization of the melt began with augite, plagioclase, and magnetite phenocrysts, and was followed by crystallization of augite, plagioclase, magnetite, ilmenite, and pyrrhotite in the groundmass, which took place under more rapid conditions of cooling, as reflected in the groundmass grain size. The petrogenetic history of NWA 7635 is similar in many ways to that of NWA 8159; this observation, coupled with similarities in geochemical and isotopic characteristics from other studies, suggests that the parent melts of the two rocks—as represented by all minerals except the xenocrystic olivine—were one and the same. The main distinctions between the two rocks are that their parent melts entrained xenocrystic olivine of different composition, and the cooling rate of the groundmass of NWA 7635 was more rapid than that of NWA 8159. The conclusion that the redox reaction took place between olivine and melt is in contrast to other work that suggests the reaction took place in the subsolidus, and has implications for the nature of the reaction in both NWA 7635 and NWA 8159.     

Thermal effects of insolation propagation into the regoliths of airless bodies

We have investigated thermal models for planetary surfaces composed of particles that are bright and optically thin in the visual, and dark and opaque in the thermal infrared. The models incorporate the assumption that insolation is absorbed over a finite distance in the regolith, predicting lower daytime and higher nighttime temperatures than those predicted if the insolation were a absorbed only at the surface. The magnitude of the effect depends on the scale length for absorption of insolation relative to the diurnal skin depth for thermal diffusion, and can be significant when insolation penetrates to a depth comparable to the diurnal skin depth. In particular, for bodies like Enceladus and Europa, the maximum daytime temperature depression and nighttime temperature elevation can be 10°K or more for penetration-depth scales ∼ 1.5 cm. If insolation penetrates deeply enough into a surface, and the thermal-infrared opacity of its constituent particles is very high (e.g., in a regolith composed of particles of water ice), a solid-state greenhouse can result! This has important implications for geophysical models of high-albedo, icy bodies because actual boundary-layer temperatures may in fact be significantly higher than those assumed in previous studies, making it easier to melt the interiors of such bodies. Another important implication of the models is that where insolation- penetration is significant, thermal inertias inferred from models that do not allow for this effect will be upper limits to the real thermal inertia.     

Some investigations into the atmospheric drag problem

This paper calls into question the validity of the well-known formulae for the perturbations in the Keplerian elements, over one revolution of an orbit, for the motion of a drag-perturbed artificial satellite. These formulae are derived from Gauss's form of the planetary equations, by averaging over a single revolution of the orbit, and using the eccentric anomaly as the independent variable.It is shown that for light balloon-type satellites in near-circular orbits neither the eccentric anomaly nor the true longitude is a suitable choice of independent variable for the averaging procedure. Under these circumstances, it would seem that simple formulae for the variations in the elements cannot be derived from Gauss's equations.     

Partial penetration of IMF By into the magnetosphere

Recent observations of partial penetration of the IMF B_y into the magnetosphere (Fairfield, 1979; Cowley and Hughes, 1983) are shown to agree with the idea of a magnetopause current K_y, induced by IMF B_y (Primdahl and Spangslev, 1983). The slow decay of K_y, caused by Joule heat losses in the cusp ionospheres is responsible for the appearance (on the average) of a fraction of IMF B_y inside the magnetosphere, and this also explains the large statistical scatter of the data. A decay time constant of 4–5 days is derived from the average fraction of IMF B_y observed inside the magnetosphere in good agreement with the 7-day time constant previously proposed.     

Looking into Pulsar Magnetospheres

Diffractive and refractive magnetospheric scintillations may allow direct probing of the plasma inside the pulsar light cylinder. The unusual electrodynamics of the strongly magnetized electron-positron plasma allows separation of the magnetospheric and interstellar scattering. The most distinctive feature of the magnetospheric scintillations is their independence of frequency. Diffractive scattering due to small scale inhomogeneities produces a scattering angle that may be as large as 0.1radians, and a typical decorrelation time of 10^-8 seconds. Refractive scattering due to large scale inhomogeneities is also possible, with atypical angle of 10^-3 radians and a correlation time of the order of10^-4 seconds. Some of the magnetospheric propagation effects may have already been observed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Propagation of tropospheric gravity waves into the upper atmosphere of Mars

We study the propagation of gravity waves in the martian atmosphere using a linearized one-dimensional full-wave model. Calculations are carried out for atmospheric parameters characteristic of Mars Orbiter Laser Altimeter (on Mars Global Surveyor MGS) observations of apparent gravity waves in high latitude clouds and MGS radio occultation measurements of temperature variations with height suggestive of gravity wave activity. Waves that reach the thermosphere produce fluctuations in density comparable in amplitude with the density variations detected in Mars Odyssey aerobraking data. Gravity waves of modest amplitude are found to deposit momentum and generate significant heating and cooling in the martian atmosphere. The largest heating and cooling effects occur in the thermosphere, at altitudes between about 130 and 150 km, with heating occurring at the lower altitudes and cooling taking place above.     

Integrating advanced visualization technology into the planetary Geoscience workflow

Recent advances in computer visualization have allowed us to develop new tools for analyzing the data gathered during planetary missions, which is important, since these data sets have grown exponentially in recent years to tens of terabytes in size. As part of the Advanced Visualization in Solar System Exploration and Research (ADVISER) project, we utilize several advanced visualization techniques created specifically with planetary image data in mind. The Geoviewer application allows real-time active stereo display of images, which in aggregate have billions of pixels. The ADVISER desktop application platform allows fast three-dimensional visualization of planetary images overlain on digital terrain models. Both applications include tools for easy data ingest and real-time analysis in a programmatic manner. Incorporation of these tools into our everyday scientific workflow has proved important for scientific analysis, discussion, and publication, and enabled effective and exciting educational activities for students from high school through graduate school.