A methodology to construct a reduced chemical scheme for 2D-3D photochemical models: Application to Saturn

We present a methodology to build a reduced chemical scheme adapted to the study of hydrocarbons in the atmospheres of giant planets and Titan. As an example, we have built a reduced chemical scheme, containing only 25 compounds and 46 reactions (including photolysis), which is well adapted to compute the abundance of the main hydrocarbons observed so far in the atmosphere of Saturn (CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₈ and C₄H₂). This scheme gives similar results, within the error bars of the model, as a 1D photochemical model using an initial chemical scheme containing 90 compounds and more than 600 reactions. As a consequence, such a methodology can be used to build a reduced scheme well adapted to future 2D (or 3D) photochemical models and GCMs.     

The Evolution of Quasi-Separatrix Layer in Two Solar Eruptive Events

Quasi-separatrix layer, also called as QSL, is a region where magnetic connectivity changes drastically, and mostly well coincides with the location of flare ribbons in observations. The research on the relations of this topological structure with the 3-dimensional magnetic reconnection, and solar flares has attracted more and more attention. In this paper, using the theory of QSL we investigate a C5.7 classical two-ribbon solar flare (event 1) which occurred at AR11384 on 2011 December 26, and an M6.5 solar flare (event 2) which occurred at AR12371 on 2015 June 22, respectively. Combining the multi-wavelength data of AIA (Atmospheric Imaging Assembly) and vector magnetogrames of HMI (Helioseismic and Magnetic Imager) onboard SDO (Solar Dynamics Observatory), we extrapolate the coronal magnetic field using the PF (Potential Field) and NLFFF (Nonlinear Force Free Field) models, and calculate the evolution of the AR (Active Region) magnetic free energy. Then, we calculate the logarithmic distribution of Q-factors (magnetic squashing factor) at different heights above the solar photosphere with the results of the PF and NLFFF extrapolations, in order to determine the location of QSL. Afterward, we investigate the evolutionary relation between the QSLs at different heights above the solar photosphere and the flare ribbons observed at the corresponding heights. Finally, we study the multi-wavelength evolution features of the 2 flare events, and obtain by calculation the mean slip velocities of magnetic lines in the event 2 at 304 Å and 335 Å to be 4.6 km s^-1 and 6.3 km s^-1, respectively. We find that the calculated location of QSL in the chromosphere and corona is in good agreement with the location of flare ribbons at the same height, and the QSLs at different heights have almost the same evolutionary behavior in time as the flare ribbons of the corresponding heights, which highlights the role of QSL in the research of 3D magnetic reconnection and solar flare, and we suggest that the energy release in the flare of event 2 may be triggered by the magnetic reconnection at the place of QSL. We also suggest that the QSL is very important for us to study the essential relation between the 3D and 2D magnetic reconnections.     

Magnetic Fields and Solar Activities

We discuss the study of solar magnetic fields based on the photospheric vector magnetograms of solar active regions which were obtained at Huairou Solar Observing Station near Beijing in the period of 22nd and 23th solar cycles. The measurements of the chromospheric magnetic field and the spatial configuration of the field at the lower solar atmosphere inferred by the distribution of the solar photospheric and chromospheric magnetic field. After the analysis on the formation process of delta configuration in some super active regions based on the photospheric vector magnetogram observations, some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, proposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and the relationship with magnetic shear in some delta active regions completely. (3) The proposition is that the large-scale delta active regions are formed from contribution by highly sheared non-potential magnetic flux bundles generated in the subatmosphere. We present some results of a study of the magnetic helicity. We also compare these results with other data sets obtained by magnetographs (or Stokes polarimeters) at different observatories, and analyze the basic chirality of the magnetic field in the solar atmosphere.     

Ionospheric plasma acceleration at Mars: ASPERA-3 results

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on-board the Mars Express spacecraft (MEX) measured penetrating solar wind plasma and escaping/accelerated ionospheric plasma at very low altitudes (250 km) in the dayside subsolar region. This implies a direct exposure of the martian topside atmosphere to solar wind plasma forcing leading to energization of ionospheric plasma. The ion and electron energization and the ion outflow from Mars is surprisingly similar to that over the magnetized Earth. Narrow “monoenergetic” cold ion beams, ion beams with broad energy distributions, sharply peaked electron energy spectra, and bidirectional streaming electrons are particle features also observed near Mars. Energized martian ionospheric ions (O⁺, O⁺₂, CO⁺₂, etc.) flow in essentially the same direction as the external sheath flow. This suggests that the planetary ion energization couples directly to processes in the magnetosheath/solar wind. On the other hand, the beam-like distribution of the energized plasma implies more indirect energization processes like those near the Earth, i.e., energization in a magnetized environment by waves and/or parallel (to B) electric fields. The general conditions for martian plasma energization are, however, different from those in the Earth's magnetosphere. Mars has a weak intrinsic magnetic field and solar wind plasma may therefore penetrate deep into the dense ionospheric plasma. Local crustal magnetization, discovered by Acuña et al. [Acuña, M.J., Connerey, J., Ness, N., Lin, R., Mitchell, D., Carlsson, C., McFadden, J., Anderson, K., Rème, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Science 284, 790-793], provide some dayside shielding against the solar wind. On the other hand, multiple magnetic anomalies may also lead to “hot spots” facilitating ionospheric plasma energization. We discuss the ASPERA-3 findings of martian ionospheric ion energization and present evidences for two types of plasma energization processes responsible for the low- and mid-altitude plasma energization near Mars: magnetic field-aligned acceleration by parallel electric fields and plasma energization by low frequency waves.     

Debris-covered piedmont glaciers along the northwest flank of the Olympus Mons scarp: Evidence for low-latitude ice accumulation during the Late Amazonian of Mars

We use Viking and new MGS and Odyssey data to characterize the lobate deposits superimposed on aureole deposits along the west and northwest flanks of Olympus Mons, Mars. These features have previously been interpreted variously as landslide, pyroclastic, lava flow or glacial features on the basis of Viking images. The advent of multiple high-resolution image and topography data sets from recent spacecraft missions allow us to revisit these features and assess their origins. On the basis of these new data, we interpret these features as glacial deposits and the remnants of cold-based debris-covered glaciers that underwent multiple episodes of advance and retreat, occasionally interacting with extrusive volcanism from higher on the slopes of Olympus Mons. We subdivide the deposits into fifteen distinctive lobes. Typical lobes begin at a theater-like alcove in the escarpment at the base of Olympus Mons, interpreted to be former ice-accumulation zones, and extend outward as a tongue-shaped or fan-shaped deposit. The surface of a typical lobe contains (moving outward from the basal escarpment): a chaotic facies of disorganized hillocks, interpreted as sublimation till in the accumulation zone; arcuate-ridged facies characterized by regular, subparallel ridges and interpreted as the ridges of surface debris formed by the flow of underlying ice; and marginal ridges interpreted as local terminal moraines. Several lobes also contain a hummocky facies toward their margins that is interpreted as a distinctive type of sublimation till shaped by structural dislocations and preferential loss of ice. Blocky units are found extending from the escarpment onto several lobes; these units are interpreted as evidence of lava-ice interaction and imply that ice was present at a time of eruptive volcanic activity higher on the slopes of Olympus Mons. Other than minor channel-like features in association with lava-ice interactions, we find no evidence for the flow of liquid water in association with these lobate features that might suggest: (1) near-surface groundwater as a source for ice in the alcoves in the lobe source region at the base of the scarp, or (2) basal melting and drainage emanating from the lobes that might indicate wet-based glacial conditions. Instead, the array of features is consistent with cold-based glacial processes. The glacial interpretations outlined here are consistent with recent geological evidence for low-latitude ice-rich features at similar positions on the Tharsis Montes as well as with orbital dynamic and climate models indicating extensive snow and ice accumulation associated with episodes of increased obliquity during the Late Amazonian period of the history of Mars.     

Production of hot nitrogen atoms in the martian thermosphere

The energy and altitude distributions for nitrogen atoms produced from photodissociation of N₂ and dissociative recombination of N⁺₂ have been computed for the low and high solar activity martian thermospheres. We find that the fraction of nitrogen atoms with E>Eesc at the exobase is ∼2.5% for photodissociation as compared to ∼22.6% for dissociative recombination. However, the peak magnitudes of the production rate profiles for dissociative recombination are a factor of ∼1000 and ∼200 times smaller than those of photodissociation at low and high solar activities, respectively. Thus, our findings suggest that photodissociation of N₂ is the dominant escape mechanism for atomic nitrogen for both the low and high solar activity martian thermospheres. The general consensus in the field prior to these research results was that dissociative recombination of N⁺₂ was the main escape mechanism. We find that it is the dissociation of N₂ along repulsive states that results in the production of very energetic nitrogen atoms.     

Model of photoelectron impact ionization within the high latitude ionosphere at Mars: Comparison of calculated and measured electron density

The production rate, ion density and electron density are calculated between longitudes 0° and 360° E due to incident radiation of wavelength range 1-102.57 nm in the dayside atmosphere of Mars. These calculations are made by using global analytical yield spectrum (AYS) model at solar zenith angle 80° between latitudes 50° and 70° N for spring equinox and medium solar activity condition. These conditions are appropriate for Mars Global Surveyor (MGS) Phase 2 aerobraking period during which both the accelerometer and the radio occultation data are used. The calculated results are compared with MGS radio occultation measurements carried out at different latitudes (64.7°-67.3° N) and longitudes (0°-360° E) in December 1998 between solar zenith angle 78° and 81°. This measurement shows primary and secondary ionization peaks, which are varying with longitudes. Our calculation suggests that first peak is produced by photoionization and photoelectron impact ionization processes due to absorption of solar EUV radiation (9-102.57 nm). The second peak is produced by photoelectron impact ionization of soft X-ray photon (1-9 nm). There is a good agreement between our calculation and measurement as far as the maximum and the minimum values of primary peak altitude/peak density of electrons are concerned. However, the calculated values of secondary peak density and peak altitude are higher than the measured values by a factor of 1.5-2.0 and 1.1, respectively. The secondary peak is brought into agreement with the measurement using low X-ray flux by a factor of 2 to 3 below 9 nm. The longitudinal distribution of calculated and measured peak density and peak altitude are fitted by least-square method with 0.95 confidence limits.     

Analytical orbit predictions with air drag using KS uniformly regular canonical elements

A new nonsingular analytical theory for the motion of near Earth satellite orbits with the air drag effect is developed for long term motion in terms of the KS uniformly regular canonical elements by a series expansion method, by assuming the atmosphere to be symmetrically spherical with constant density scale height. The series expansions include up to third order terms in eccentricity. Only two of the nine equations are solved analytically to compute the state vector and change in energy at the end of each revolution, due to symmetry in the equations of motion. Numerical comparisons of the important orbital parameters semi major axis and eccentricity up to 1000 revolutions, obtained with the present solution, with KS elements analytical solution and Cook, King-Hele and Walker's theory with respect to the numerically integrated values, show the superiority of the present solution over the other two theories over a wide range of eccentricity, perigee height and inclination.     

Vertical pressure profile of Titan—observations of the PPI/HASI instrument

The Huygens entry probe descended through the atmosphere of Titan and provided an excellent set of observations of the atmosphere and the surface of Titan. During the 150-min descent the Huygens Atmospheric Structure Instrument (HASI) observed a comprehensive set of variables, including pressure, temperature, density and atmospheric electricity. The atmospheric pressure profile was recorded by the Pressure Profile Instrument (PPI), provided by Finnish Meteorological Institute (FMI). The instrument started measurements at an altitude of 150 km, and produced about 28 bits of data per second. Data were also obtained through the time of 31 min beyond the time of surface impact. The first-order scientific analysis of the PPI results has been performed. The observations together with hydrostatic assumption and in combination with other measurements have provided the first atmospheric pressure profile and the surface pressure (of approximately ) for Titan's atmosphere. To carry out the pressure profile reconstruction we developed a real gas formulation, which is applicable also for other Titan atmospheric investigations. The altitude versus time speed of the descent was calculated and the results were compared with the direct altitude observations by the radar altimeter during the last 40 km of the descent. The fit was excellent demonstrating the high-quality level of the PPI observations as well as the utilized investigation methods.     

用Monte Carlo方法拟合超新星SN 1993J的早期光谱

通过把从光球辐射出来的连续谱能量分成大量的光子包，然后用MonteCarlo方法跟踪这些光子包从光球向超新星包层外运动的过程，这一方法能很好地解决超新星的光谱拟合中的谱线覆盖问题，由于同时编制了用于构造超新星包层中的密度结构和丰度分布的程序，以及大量的用于比较观测和理论拟合的绘图和支持程序，使我们的MonteCarlo光谱拟合软件包能够很好地运用于超新星的早期光谱研究。本文给出了SN1993J在1993年4月13日的观测光谱的运行实例。