Evidence for enhanced hydration on the northern flank of Olympus Mons, Mars

In this paper we report about a small region on the northern scarp of Olympus Mons showing an increase of the 3 μm hydration band in the OMEGA spectra, together with low superficial temperatures. Although water ice clouds can occurs on the flank of big martian volcanoes, radiative transfer modeling indicates that atmospheric water ice alone cannot justify the shape of the observed band. A fit of the 1.9–3 μm absorption features is obtained by hypothesizing that the study region consists of a mixture of dust and water ice covered by an optically thin (τ=0.08 at 3 μm) layer of dust. Thermal modeling also suggests that water ice in this region may be stable during most of the martian year due to the saturation of the atmosphere. If water ice is responsible for the observed spectral behavior, it might consist of a number of ice or snow patches possibly deposited in small depressions.     

The investigation of nova-like variable MV Lyr during the 1999–2001 years

A peculiar nova-like MV Lyr was investigated. The CCD-observations of MV Lyr were continued in Crimea during the outbursts and quiescent states. Last year its behavior became non-typical for “anti-dwarf novae”. The length of its first deep minimum was 10 years. A modern state is characterized by very strong outbursts and very often changes of the stages. The periods 0. ^d 128 and 0. ^d 136 were obtained for 1998 and 1999 years accordingly. Possibly, a relation between the photometric period and the brightness exists.     

Partial Reduction in the N-Body Planetary Problem using the Angular Momentum Integral

We present a new set of variables for the reduction of the planetary n-body problem, associated to the angular momentum integral, which can be of any use for perturbation theory. The construction of these variables is performed in two steps. A first reduction, called partial is based only on the fixed direction of the angular momentum. The reduction can then be completed using the norm of the angular momentum. In fact, the partial reduction presents many advantages. In particular, we keep some symmetries in the equations of motion (d'Alembert relations). Moreover, in the reduced secular system, we can construct a Birkhoff normal form at any order. Finally, the topology of this problem remains the same as for the non-reduced system, contrarily to Jacobi's reduction where a singularity is present for zero inclinations. For three bodies, these reductions can be done in a very simple way in Poincaré's rectangular variables. In the general n-body case, the reduction can be performed up to a fixed degree in eccentricities and inclinations, using computer algebra expansions. As an example, we provide the truncated expressions for the change of variable in the 4-body case, obtained using the computer algebra system TRIP.     

Hard X-ray Microflares down to 3 keV

The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May, 1:40–2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted better with a thermal model plus power law above ∼ 6–7 keV than with a thermal only. The decay phase sometimes can be fitted with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents between −5 and −8, and imply a total non-thermal electron energy content between 10²⁶–10²⁷ erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.) Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022404512780     

RHESSI Observation of Atmospheric Gamma Rays from Impact of Solar Energetic Particles on 21 April 2002

The RHESSI high-resolution spectrometer detected γ-ray lines and continuum emitted by the Earth's atmosphere during impact of solar energetic particles in the south polar region from 16:00–17:00 UT on 21 April 2002. The particle intensity at the time of the observation was a factor of 10–100 weaker than previous events when gamma-rays were detected by other instruments. This is the first high-resolution observation of atmospheric gamma-ray lines produced by solar energetic particles. De-excitation lines were resolved that, in part, come from ¹⁴N at 728, 1635, 2313, 3890, and 5106 keV, and the ¹²C spallation product at ∼ 4439 keV. Other unresolved lines were also detected. We provide best-fit line energies and widths and compare these with moderate resolution measurements by SMM of lines from an SEP event and with high-resolution measurements made by HEAO 3 of lines excited by cosmic rays. We use line ratios to estimate the spectrum of solar energetic particles that impacted the atmosphere. The 21 April spectrum was significantly harder than that measured by SMM during the 20 October 1989 shock event; it is comparable to that measured by Yohkoh on 15 July 2000. This is consistent with measurements of 10–50 MeV protons made in space at the time of the γ-ray observations.     

Chromospheric Height and Density Measurements in a Solar Flare Observed with RHESSI – II. Data Analysis

We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50 keV energy range, viz., z(ε)≈2.3(ε/20 keV)^−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in Paper I, which follows the power-law function n _e(z)=1.25×10¹³(z/1 Mm)^−2.5 cm⁻³. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources. At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs.     

Secular contraction in extreme helium stars and the future of V4334 Sgr

Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated as the precursors of extreme helium stars (EHes). The question arises to whether Sakurai's Object will evolve down the [WC]-PG1159 evolution track, or the RCB-EHe-HesdO track. From a number of IUE observations, we have measured thesecular contraction rates and pulsation masses of several EHes. These are inconsistent with the predicted contraction rates for helium-shell burning giants produced by a final helium-shell flash (the [WC]-PG1159 track). Although there may be some similarities between Sakurai's Object and some RCBs, if the RCB-EHe conection is valid then these must be superficial rather than structural.     

Estimation of Fried's Parameter From Specklegrams of Solar Features

A few methods of estimating Fried's parameter (r ₀) from specklegrams of solar features are described. Some of these methods were used to estimate r ₀ for the speckle data obtained from Kodaikanal Observatory (KO), Uttar Pradesh State Observatory (UPSO) and Udaipur Solar Observatory (USO). The average value of r ₀ was found to be 3 cm at USO and UPSO during our observations. At KO, values of r ₀ ranging from 6 to 10 cm were estimated.