Stochastic Hill's equations for the study of errant rocket burns in orbit

The problem of errant rocket burns in low Earth orbit is of growing interest, especially in the area of safety analysis of nuclear powered spacecraft. The development of stochastic Hill's equations provides a rigorous mathematical tool for the study of such errant rocket maneuvers. These equations are analyzed within the context of a theory of linear dynamical systems driven by a random white noise. It is established that the trajectories of an errant rocket are realizations of a Gauss-Markov process, whose mean vector is given by the solution of a deterministic rocket problem. The time-dependent covariance matrix of the process is derived in an explicit form.     

The impulsive phase of a large solar limb flare of June 20, 1989

On 1989 June 20, we observed in H the impulsive phase of a 3B/X1.6 limb flare with high temporal resolution. Line profiles have been acquired every 2.3 s with an imaging spectrograph. During the eruption of a filament we observed in H a moving plasma blob from what we believe to be a second loop which correlated spatially and temporally with a microwave source at 1.4 GHz observed by VLA. A magnetodynamic model is used to understand the development of the moving plasma blob.     

The identification and interaction of network,intranetwork, and ephemeral-region magnetic fields

Network magnetic fields, ephemeral active regions, and intranetwork magnetic fields are illustrated and discussed in several contexts. First, they are presented in relation to the appearance and disappearance of magnetic flux. Second, their properties in common with all solar magnetic features are discussed. Third, their distinguishing characteristics are emphasized. Lastly, their interactions are illustrated.Network magnetic fields are no longer considered to be just the aged remnants of active regions. The network is the dynamic product of the merging and cancelling of intranetwork fields, ephemeral regions, and the remnants of active regions. Intranetwork fields are magnetic fields of mixed polarity that appear to originate continuously from localized source sites in between the network. The intranetwork magnetic fields are characterized by flow of successive fragments in approximately radial patterns away from their apparent source sites and by the relative weakness of their magnetic fields. Ephemeral active regions are small, new bipoles that grow as a unit or a succession of bipolar units and whose poles move in opposite directions from their apparent site of origin. Large ephemeral regions are not distinguishable from small active regions.Solar Cycle Workshop Paper.     

Conditions for the Formation and Maintenance of Filaments – (Invited Review)

Observational conditions for the formation and maintenance of filaments are reviewed since 1989 in the light of recent findings on their structure, chirality, inferred magnetic topology, and mass flows. Recent observations confirm the necessary conditions previously cited: (1) their location at a boundary between opposite-polarity magnetic fields (2) a system of overlying coronal loops, (3) a magnetically-defined channel beneath, (4) the convergence of the opposite-polarity network magnetic fields towards their common boundary within the channel and (5) cancellation of magnetic flux at the common polarity boundary. Evidence is put forth for three additional conditions associated with fully developed filaments: (A) field-aligned mass flows parallel with their fine structure (B) a multi-polar background source of small-scale magnetic fields necessary for the formation of the filament barbs and (C) a handedness property known as chirality which requires them to be either of two types, dextral or sinistral. One-to-one relationships have been established between the chirality of filaments and the chirality of their filament channels and overlying coronal arcades. These findings reinforce earlier evidence that every filament magnetic field is separate from the magnetic field of the overlying arcade but both are parts of a larger magnetic field system. The larger system has at least quadrupolar footprints in the photosphere and includes the filament channel and subphotospheric magnetic fields, This ‘systems’ view of filaments and their environment enables new perspectives on why arcades and channels are invariable conditions for their existence. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005026814076     

In-flight calibration of the HIFI diplexers

HIFI is a heterodyne spectrometer aboard the Herschel Space Observatory, providing high-spectral-resolution capabilities. Of its seven frequency bands, four (bands 3, 4, 6, and 7) employ Martin-Puplett diplexers to combine sky signal and local oscillator at the two linear polarizations H and V, prior to feeding them into the mixers (receivers). The optical path difference in each of these 8 diplexers must be tuned to the observed frequency. The required actuator currents were determined in flight before the start of routine science observations. We here report on regular (roughly quarterly) engineering test observations to validate the repeatability of the HIFI diplexers during the routine phase of Herschel operations. We find the optical path difference to be stable to within 0.4 % of the relevant wavelength, typically at the sub-micron level. We conclude that the repeatability and precision of the diplexer tuning mechanism are so high that science data are in no way negatively affected. With the diplexer calibration established and validated, this line of reasoning can be reversed, and the diplexers can be used as relative spectrometers to measure the local-oscillator frequency, i.e., to check the spectral purity of the local oscillator across the diplexer bands. This was done from before launch out to the last months of cryogenic operations in space.     

Late Pleistocene and Holocene environments in the Nile basin

Owing to the very gently sloping nature of the flood plain in the lower White Nile valley, which is underlain by a former lake-bed, the depositional record in that area is unusually well preserved. In Egypt and along the Blue Nile phases of erosion have destroyed segments of the sedimentary record, but the White Nile sequence is a good proxy for both the main Nile and the Blue Nile. During the last 15 ka, at least, times of high flow in the Blue Nile and main Nile were synchronous with those in the White Nile.Not all the White Nile flood deposits have been preserved but calibrated radiocarbon dates obtained on fossil freshwater and amphibious Pila shells and fish bones indicate that White Nile levels were high around 14.7–13.1 ka, 9.7–9.0 ka, 7.9–7.6 ka, 6.3 ka and 3.2–2.8 ka. The Blue Nile record is more fragmentary and that of the main Nile even more so except for the Holocene Nile delta. Calibrated radiocarbon ages for high Blue Nile flows indicate very high flood levels towards 13.9–13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka.Incision by the Blue Nile and main Nile has caused progressive incision in the White Nile amounting to at least 4 m since the terminal Pleistocene  15 ka ago and at least 2 m over the past 9 ka. The Blue Nile seems to have cut down at least 10 m since  15 ka and at least 4 m since 9 ka. The time-transgressive and relatively late inception of plant domestication in the Nile valley may partly reflect this history of incision. Nile incision would propagate upstream into the White Nile valley, draining previously swampy areas along the valley floor, which would then become accessible to cultivation.     

Ion composition and chemistry in the coma of Comet 1P/Halley—A comparison between Giotto's Ion Mass Spectrometer and our ion-chemical network

In order to understand the cometary plasma environment it is important to track the closely linked chemical reactions that dominate ion evolution. We used a coupled MHD ion-chemistry model to analyze previously unpublished Giotto High Intensity Ion Mass Spectrometer (HIS-IMS) data. In this way we study the major species, but we also try to match some minor species like the CH_x and the NH_x groups. Crucial for this match is the model used for the electrons since they are important for ion-electron recombination. To further improve our results we included an enhanced density of supersonic electrons in the ion pile-up region which increases the local electron impact ionization. In this paper we discuss the results for the following important ions: C⁺, CH⁺, CH⁺₂, CH⁺₃, N⁺, NH⁺, NH⁺₂, NH⁺₃, NH⁺₄, O⁺, OH⁺, H₂O⁺, H₃O⁺, CO⁺, HCO⁺, H₃CO⁺, and CH₃OH⁺₂. We also address the inner shock which is very distinctive in our MHD model as well as in the IMS data. It is located just inside the contact surface at approximately 4550 km. Comparisons of the ion bulk flow directions and velocities from our MHD model with the data measured by the HIS-IMS give indication for a solar wind magnetic field direction different from the standard Parker angle at Halley's position. Our ion-chemical network model results are in a good agreement with the experimental data. In order to achieve the presented results we included an additional short lived inner source for the C⁺, CH⁺, and CH⁺₂ ions. Furthermore we performed our simulations with two different production rates to better match the measurements which is an indication for a change and/or an asymmetric pattern (e.g. jets) in the production rate during Giotto's fly-by at Halley's comet.     

The haze and methane distributions on Uranus from HST-STIS spectroscopy

We analyzed a unique, three-dimensional data set of Uranus acquired with the STIS Hubble spectrograph on August 19, 2002. The data covered a full afternoon hemisphere at 0.1 arc-sec spatial resolution between 300 and 1000 nm wavelength at 1 nm resolution. Navigation was accurate to 0.002 arc-sec and 0.02 nm. We tested our calibration with WFPC2 images of Uranus and found good agreement. We constrained the vertical aerosol structure with radiative transfer calculations. The standard types of models for Uranus with condensation cloud layers did not fit our data as well as models with an extended haze layer. The dark albedo of Uranus at near-infrared methane windows could be explained by methane absorption alone using conservatively scattering aerosols. Ultraviolet absorption from small aerosols in the stratosphere was strongest at high southern latitudes. The uppermost troposphere was almost clear, but showed a remarkable narrow spike of opacity centered on the equator to 0.2° accuracy. This feature may have been related to influx from ring material. At lower altitudes, the feature was centered at 1-2° latitude, suggesting an equatorial circulation toward the north. Below the 1.2 bar level, the aerosol opacity increased some 100 fold. A comparison of methane and hydrogen absorptions contradicted the standard interpretation of methane band images, which assumes that the methane mixing ratio is independent of latitude and attributes reflectivity variations to variations in the aerosol opacity. The opposite was true for the main contrast between brighter high latitudes and darker low latitudes, probing the 1-3 bar region. The methane mixing ratio varied between 0.014 and 0.032 from high to low southern latitudes, while the aerosol opacity varied only moderately with latitude, except for an enhancement at −45° latitude and a decrease north of the equator. The latitudinal variation of methane had a similar shape as that of ammonia probed by microwave observations at deeper levels. The variability of methane challenges our understanding of Uranus and requires reconsideration of previous investigations based on a faulty assumption. Below the 2 bar level, the haze was thinning somewhat. Our global radiative transfer models with 1° latitude sampling fit the observed reflectivities to 2% rms. The observed spectra of two discrete clouds could be modeled by using the background model of the appropriate latitude and adding small amounts of additional opacity at levels near 1.2 bar (southern cloud) and levels as high as 0.1 bar (northern cloud). These clouds may have been methane condensation clouds of low optical depth (∼0.2).     

The Northwest Africa (NWA) 5790 meteorite: A mesostasis‐rich nakhlite with little or no Martian aqueous alteration

Northwest Africa (NWA) 5790 is the most recently discovered member of the nakhlite group. Its mineralogy differs from the other nakhlites with a high abundance mesostasis (38.1 ± 3.6 vol%) and scarcity of olivine (4.0 ± 2.2 vol%). Furthermore, zoning of augite phenocrysts, and other petrographic and chemical characteristics suggest that NWA 5790 samples the chilled margin of its parent lava flow/sill. NWA 5790 contains calcite and rare clay minerals that are evidence for its exposure to liquid water. The calcite forms a cement to coatings of dust on the outer surface of the find and extends into the interior of the meteorite within veins. The presence of microbial remains within the coating confirms that the dust and its carbonate cement are terrestrial in origin, consistent with the carbon and oxygen isotope composition of the calcite. The clay minerals are finely crystalline and comprise ~0.003 vol% of the meteorite. δD values of the clay minerals range from ?212 ± 109‰ to ?96 ± 132‰, and cannot be used to distinguish between a terrestrial or Martian origin. As petrographic results are also not definitive, we conclude that secondary minerals produced by Martian groundwaters are at best very rare within NWA 5790. The meteorite has therefore sampled a region of the lava flow/sill with little or no exposure to the aqueous solutions that altered other nakhlites. This isolation could relate to the scarcity of olivine in NWA 5790 because dissolution of olivine in other nakhlites by Martian groundwaters enhanced their porosity and permeability, and provided solutes for secondary minerals.