•We show that Mercury’s calcium exosphere, which is observed to vary seasonally about that planet’s orbit, can be attributed toimpact vaporization by interplanetary dust.•A comparison of models to ...MESSENGER observations shows that the seasonal variations in that Ca signal result from the planet’s sizable orbital eccentricity and inclination which cause that planet to experience significant radial and vertical excursions through the interplanetary dust cloud.•The model developed here also requires an additional source localized at 25±5° degrees after Mercury’s perihelion, and that may be due to a meteor stream possibly associated with the nearby Comet Encke.•Impact vaporization can explain the source rate and true anomaly angle variations in the calcium exosphere but an additional mechanism must be invoked to explain the extreme temperature.
Mercury’s calcium exosphere varies in a periodic way with that planet’s true anomaly. We show that this pattern can be explained by impact vaporization from interplanetary dust with variations being due to Mercury’s radial and vertical excursions through an interplanetary dust disk having an inclination within 5 degrees of the plane of Mercury’s orbit. Both a highly inclined dust disk and a two-disk model (where the two disks have a mutual inclination) fail to reproduce the observed variation in calcium exospheric abundance with Mercury true anomaly angle. However, an additional source of impacting dust beyond the nominal dust disk is required near Mercury’s true anomaly (ν) 25°±5°. This is close to but not coincident with Mercury’s true anomaly (ν=45°) when it crosses Comet 2P/Encke’s present day orbital plane. Interestingly, the Taurid meteor storms at Earth, which are also due to Comet Encke, are observed to occur when Earth’s true anomaly is ±20 or so degrees before and after the position where Earth and Encke orbital planes cross. The lack of exact correspondence with the present day orbit of Encke may indicate the width of the potential stream along Mercury’s orbit or a previous cometary orbit. The extreme energy of the escaping calcium, estimated to have a temperature >50,000K if the source is thermal, cannot be due to the impact process itself but must be imparted by an additional mechanism such as dissociation of a calcium-bearing molecule or ionization followed by recombination.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We use N-body simulations to examine the consequences of Neptune's outward migration into the Kuiper Belt, with the simulated end states being compared rigorously and quantitatively to the ...observations. These simulations confirm the 2003 findings of Chiang and coworkers, who showed that Neptune's migration into a previously stirred-up Kuiper Belt can account for the Kuiper Belt objects (KBOs) known to librate at Neptune's 5 : 2 resonance. We also find that capture is possible at many other weak, high-order mean-motion resonances, such as 11 : 6, 13 : 7, 13 : 6, 9 : 4, 7 : 3, 12 : 5, 8 : 3, 3 : 1, 7 : 2, and 4 : 1. The more distant of these resonances, such as the 9 : 4, 7 : 3, 5 : 2, and 3 : 1, can also capture particles in stable, eccentric orbits beyond 50 AU, in the region of phase space conventionally known as the 'Scattered Disk.' Indeed, 90% of the simulated particles that persist over the age of the solar system in the Scattered-Disk zone never had a close encounter with Neptune but instead were promoted into these eccentric orbits by Neptune's resonances during the migration epoch. This indicates that the observed Scattered Disk might not be so scattered. This model also produced only a handful of Centaurs, all of which originated at Neptune's mean-motion resonances in the Kuiper Belt. However, a noteworthy deficiency of the migration model considered here is that it does not account for the observed abundance of Main Belt KBOs having inclinations higher than 15°. In order to rigorously compare the model end state with the observed Kuiper Belt in a manner that accounts for telescopic selection effects, Monte Carlo methods are used to assign sizes and magnitudes to the simulated particles that survive over the age of the solar system. If the model considered here is indeed representative of the outer solar system's early history, then the following conclusions are obtained: (1) The observed 3 : 2 and 2 : 1 resonant populations are both depleted by a factor of ~20 relative to model expectations; this depletion is likely due to unmodeled effects, possibly perturbations by other large planetesimals. (2) The size distribution of those KBOs inhabiting the 3 : 2 resonance is significantly shallower than the Main Belt's size distribution. (3) The total number of KBOs having radii R > 50 km and orbiting interior to Neptune's 2 : 1 resonance is N ~ 1.7 X 105; these bodies have a total mass of M ~ 0.08(r/1 g cm-3)(p/0.04)-3/2 M{circled plus}, assuming they have a material density r and an albedo p. We also report estimates of the abundances and masses of the Belt's various subpopulations (e.g., the resonant KBOs, the Main Belt, and the so-called Scattered Disk) and provide upper limits on the abundance of Centaurs and Neptune's Trojans, as well as upper limits on the sizes and abundances of hypothetical KBOs that might inhabit the a > 50 AU zone.
•Saturn’s Maxwell ringlet is home to a density wave.•Cassini occultation observations reveal complex structure of the wave.•The wave is identified as an m=2 outer Lindblad resonance.•The likely ...origin of the wave is from Saturn internal oscillations.•N-body simulations reproduce complex structure seen in the wave.
The eccentric Maxwell ringlet in Saturn’s C ring is home to a prominent wavelike structure that varies strongly and systematically with true anomaly, as revealed by nearly a decade of high-SNR Cassini occultation observations. Using a simple linear “accordion” model to compensate for the compression and expansion of the ringlet and the wave, we derive a mean optical depth profile for the ringlet and a set of rescaled, background-subtracted radial wave profiles. We use wavelet analysis to identify the wave as a 2-armed trailing spiral, consistent with a density wave driven by an m=2 outer Lindblad resonance (OLR), with a pattern speed Ωp=1769.17°d−1 and a corresponding resonance radius ares=87530.0km. Estimates of the surface mass density of the Maxwell ringlet range from a mean value of 110.25em0exg0.25em0excm−2 derived from the self-gravity model to 5−12gcm−2, as inferred from the wave’s phase profile and a theoretical dispersion relation. The corresponding opacity is about 0.120.25em0excm20.25em0exg−1, comparable to several plateaus in the outer C ring (Hedman, M.N., Nicholson, P.D. 2014. Mont. Not. Roy. Astron. Soc. 444, 1369–1388). A linear density wave model using the derived wave phase profile nicely matches the wave’s amplitude, wavelength, and phase in most of our observations, confirming the accuracy of the pattern speed and demonstrating the wave’s coherence over a period of 8years. However, the linear model fails to reproduce the narrow, spike-like structures that are prominent in the observed optical depth profiles. Using a symplectic N-body streamline-based dynamical code (Hahn, J.M., Spitale, J.N. 2013. Astrophys. J. 772, 122), we simulate analogs of the Maxwell ringlet, modeled as an eccentric ringlet with an embedded wave driven by a fictitious satellite with an OLR located within the ring. The simulations reproduce many of the features of the actual observations, including strongly asymmetric peaks and troughs in the inward-propagating density wave. We argue that the Maxwell ringlet wave is generated by a sectoral normal-mode oscillation inside Saturn with ℓ=m=2, similar to other planetary internal modes that have been inferred from density waves observed in Saturn’s C ring (Hedman, M.N., Nicholson, P.D. 2013. Astron. J. 146, 12; Hedman, M.N., Nicholson, P.D. 2014. Mont. Not. Roy. Astron. Soc. 444, 1369–1388). Our identification of a third m=2 mode associated with saturnian internal oscillations supports the suggestions of mode splitting by Fuller et al. (Fuller, J., Lai, D., Storch, N.I. 2014. Icarus 231, 34–50) and Fuller (Fuller, J. 2014. Icarus 242, 283–296). The fitted amplitude of the wave, if it is interpreted as driven by the ℓ=m=2f-mode, implies a radial amplitude at the 1bar level of ∼ 50cm, according to the models of Marley and Porco (Marley, M.S., Porco, C.C. 1993. Icarus 106, 508).
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A numerical model of a circumstellar debris disk is developed and applied to observations of the circumstellar dust orbiting {beta} Pictoris. The model accounts for the rates at which dust is ...produced by collisions among unseen planetesimals, and the rate at which dust grains are destroyed due to collisions. The model also accounts for the effects of radiation pressure, which is the dominant perturbation on the disk's smaller but abundant dust grains. Solving the resulting system of rate equations then provides the dust abundances versus grain size and dust abundances over time. Those solutions also provide the dust grains' collisional lifetime versus grain size, and the debris disk's optical depth and surface brightness versus distance from the star. Comparison to observations then yields estimates of the unseen planetesimal disk's radius, and the rate at which the disk sheds mass due to planetesimal grinding. The model can also be used to measure or else constrain the dust grain's physical and optical properties, such as the dust grains' strength, their light-scattering asymmetry parameter, and the grains' efficiency of light scattering Q{sub s}. The model is then applied to optical observations of the edge-on dust disk orbiting {beta} Pictoris, and good agreement is achieved when the unseen planetesimal disk is broad, with 75 {approx}< r {approx}< 150 AU. If it is assumed that the dust grains are bright like Saturn's icy rings (Q{sub s} = 0.7), then the cross section of dust in the disk is A{sub d} {approx_equal} 2 x 10{sup 20} km{sup 2} and its mass is M{sub d} {approx_equal} 11 lunar masses. In this case, the planetesimal disk's dust-production rate is quite heavy, M-dot {sub d{approx}}9 M {sub +} Myr{sup -1}, implying that there is or was a substantial amount of planetesimal mass there, at least 110 Earth masses. If the dust grains are darker than assumed, then the planetesimal disk's mass-loss rate and its total mass are heavier. In fact, the apparent dearth of any major planets in this region, plus the planetesimal disk's heavy mass-loss rate, suggests that the 75 {approx}< r < 150 AU zone at {beta} Pic might be a region of planetesimal destruction, rather than a site of ongoing planet formation.
A new symplectic N-body integrator is introduced, one designed to calculate the global 360degrees evolution of a self-gravitating planetary ring that is in orbit about an oblate planet. The great ...advantage of this approach is that the perturbing forces arise from smooth wires of ring matter rather than discreet particles, so there is very little gravitational scattering and so only a modest number of particles are needed to simulate, say, the scalloped edge of a resonantly confined ring or the propagation of spiral density waves. Cassini observations show that the B ring-edge has several free normal modes, which are long-lived disturbances of the ring-edge that are not driven by any known satellite resonances. These simulations also indicate that impulsive disturbances at a ring can excite long-lived normal modes, which suggests that an impact in the recent past by perhaps a cloud of cometary debris might have excited these disturbances, which are quite common to many of Saturn's sharp-edged rings.
GaN Unleashed: The Benefits of Microfluidic Cooling Ditri, John; Pearson, Robert R.; Cadotte, Roland ...
IEEE transactions on semiconductor manufacturing,
2016-Nov., 2016-11-00, 20161101, Volume:
29, Issue:
4
Journal Article
Peer reviewed
Gallium nitride (GaN) transistors, first introduced in the 1990s, have become key building blocks for many commercial and military electronics systems. The high radio frequency (RF) power generating ...capability of GaN is one of its most attractive features, but if they are not properly thermally managed, severe RF performance degradation can occur, such as reduced gain, reduced output power, and lower efficiency. This paper introduces a recently developed microfluidic cooling technique and presents experimental results quantifying the RF performance benefits to a high frequency RF GaN high power amplifier. It is shown that, when compared to state-of-the-art conventional liquid cooling, the new microfluidic cooling technique improved a GaN monolithic microwave integrated circuit amplifier's gain by over 4 dB, its maximum output power by over 8 dB and its power added efficiency by 3% to 5% points.
The Lagrange planetary equations are used to study secular evolution of a small, eccentric satellite that orbits within a narrow gap in a broad, self- gravitating planetary ring. These equations show ...that the satellite's secular perturbations of the ring will excite a very long wavelength spiral density wave that propagates away from the gap's outer edge. The amplitude of these waves, as well as their dispersion relation, are derived here. That dispersion relation reveals that a planetary ring can sustain two types of density waves: long waves that, in Saturn's A ring, would have wavelengths of image km and short waves that tend to be very nonlinear and are expected to quickly damp. The excitation of these waves also transports angular momentum from the ring to the satellite in a way that damps the satellite's eccentricity e, which also tends to reduce the amplitude of subsequent waves. The rate of eccentricity damping due to this wave action is then compared to the rates at which the satellite's Lindblad and corotation resonances alter the satellite's e. These results are then applied to the gap-embedded Saturnian satellites Pan and Daphnis, and the long-term stability of their eccentricities is assessed.
During the 1994 Clementine lunar mapping mission, portions of 25 orbits were dedicated to a search for lunar horizon glow (LHG) using the spacecraft star tracker navigation cameras. Previous putative ...detections of LHG were believed to result from forward scattering of sunlight by exospheric dust grains with radii ≈ 0.1 µm, observable above the limb from within the shadow of the Moon near orbital sunrise or sunset. We have examined star tracker image sequences from five Clementine orbits in which the limb occulted the Sun, and was at least partially shadowed from earthshine, minimizing the chance of stray light contamination. No LHG appears in the image data, or in any of the net brightness images, after subtraction of a reference zodiacal light model. However, some of the images display faint excess limb brightness that appears to be solar streamer structure. Therefore, we derive upper limits for the amount of dust in the lunar exosphere that could be hidden by these brightness fluctuations using a dust‐scattering simulation code and simple exponential dust profiles defined by surface concentration n0 and scale height H. Simulations using grains of radius 0.1 µm show that fluctuations in the observed excess brightness can be matched by a dust exosphere with a vertical column abundance n0H of 5–30 cm−2 and overlying mass <10−12 g cm−2. These dust upper limit estimates are highly dependent on assumed grain size due to the rapid increase in per‐grain brightness with grain radius.
Key Points
Lunar dust exosphere searched for using Clementine star trackersResults show no evidence of a dust exosphereUpper limits for dust abundance were obtained from the measurements
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
PDF
