Origin of the diffuse galactic gamma-ray emission at low and medium gamma-ray energies

Recently the galactic plane has been observed in the low and medium energy gamma-ray range in the directions towards the center and anticenter. Spectral measurements are now available at those energies, where the contribution from π°-decay gamma rays can be neglected. The high MeV-fluxes observed in both parts of the Galaxy are an indication of either a strong electron induced component or a high contribution from unresolved sources. Several interstellar cosmic-ray electron spectra have been used to calculate the contribution from electron bremsstrahlung and inverse Compton collisions with optical, infrared and 2.7 K black-body photons. From these calculations restrictions on the interstellar electron spectrum are derived.     

Thermal model and thermo-mechanical stresses in cometary nuclei

Spherically symmetric radial temperature profiles of cometary nuclei have been determined numerically (and for simplified models analytically) in dependence on the orbital position of the periodic comet Halley. These temperature fields in the nucleus are connected with thermal stress fields which have been calculated with the assumption of elastic properties of cometary matter. The remarkable result is the possible existence of stresses, strong enough to cause internal cracking of the nucleus and break-ups of the cometary surface. This may be essential understanding normal cometary activity as well as outbursts and splits.     

The Galactic Environment of the Sun: Interstellar Material Inside and Outside of the Heliosphere

Interstellar material (ISMa) is observed both inside and outside of the heliosphere. Relating these diverse sets of ISMa data provides a richer understanding of both the interstellar medium and the heliosphere. The galactic environment of the Sun is dominated by warm, low-density, partially ionized interstellar material consisting of atoms and dust grains. The properties of the heliosphere are dependent on the pressure, composition, radiation field, ionization, and magnetic field of ambient ISMa. The very low-density interior of the Local Bubble, combined with an expanding superbubble shell associated with star formation in the Scorpius-Centaurus Association, dominate the properties of the local interstellar medium (LISM). Once the heliosphere boundaries and interaction mechanisms are understood, interstellar gas, dust, pickup ions, and anomalous cosmic rays inside of the heliosphere can be directly compared to ISMa outside of the heliosphere. Our understanding of ISMa at the Sun is further enriched when the circumheliospheric interstellar material is compared to observations of other nearby ISMa and the overall context of our galactic environment. The IBEX mission will map the interaction region between the heliosphere and ISMa, and improve the accuracy of comparisons between ISMa inside and outside the heliosphere.     

GRACE accelerometer calibration by high precision non-gravitational force modeling

The precise modeling and knowledge of non-gravitational forces acting on satellites is of big interest to many scientific tasks and missions. Since 2002, the twin GRACE satellites have measured these forces in a low Earth orbit with highly precise accelerometers, for about 15?years. Besides the significance for the GRACE mission, these measurement data allow the evaluation of modeling approaches and the improvement of force models. Unfortunately, before any scientific usage, the accelerometer measurements need to be calibrated, namely scale factor and bias have to be regularly estimated.In this study we demonstrate an accelerometer calibration approach, solely based on high precision non-gravitational force modeling without any use of empirically or stochastically estimated parameters, using our in-house developed satellite simulation tool XHPS. The aim of this work is twofold, first we use the accelerometer data and the residuals resulting from the calibration to quantitatively analyze and validate different non-gravitational force model approaches. In a second step, we compare the calibration results to three different calibration methods from different authors, based on gravity field recovery, GPS-based precise orbit determination, and based on modeled accelerations.We consider atmospheric drag forces and winds, as well as radiation forces due to solar radiation pressure, albedo, Earth infrared and thermal radiation (TRP) of the satellite itself. For TRP, we investigate different transient temperature calculation approaches for the satellite surfaces with absorbed power from the aforementioned radiation sources. A detailed finite element model of the satellite is utilized for every force, considering orientation, material properties and shadowing conditions for each element.For cross-track and radial direction, which are mainly affected by the radiative forces, our calibration residuals are quite small when drag is not super dominant (1–3?$\text{nm}/{\text{s}}^2$ for total accelerations around $\pm$50?$\text{nm}/{\text{s}}^2$). For these directions the calibration seems to perform better than the other compared methods, where some bigger differences were found. For the drag dominated along-track direction it is vice versa, here our method is not sensitive enough because the difference between modeled and measured drag is bigger (e.g. residuals around 10?$\text{nm}/{\text{s}}^2$ for total accelerations around $\pm$70?$\text{nm}/{\text{s}}^2$ for low solar activity). In along-track direction the orbit determination based methods are more sensitive and produce more reliable results. Results for the complete GRACE mission time span from 2003 to 2017 are shown, covering different seasonal environmental conditions.     

Magnetic Turbulence in the Geospace Environment

Magnetic turbulence is found in most space plasmas, including the Earth’s magnetosphere, and the interaction region between the magnetosphere and the solar wind. Recent spacecraft observations of magnetic turbulence in the ion foreshock, in the magnetosheath, in the polar cusp regions, in the magnetotail, and in the high latitude ionosphere are reviewed. It is found that: 1. A large share of magnetic turbulence in the geospace environment is generated locally, as due for instance to the reflected ion beams in the ion foreshock, to temperature anisotropy in the magnetosheath and the polar cusp regions, to velocity shear in the magnetosheath and magnetotail, and to magnetic reconnection at the magnetopause and in the magnetotail. 2. Spectral indices close to the Kolmogorov value can be recovered for low frequency turbulence when long enough intervals at relatively constant flow speed are analyzed in the magnetotail, or when fluctuations in the magnetosheath are considered far downstream from the bow shock. 3. For high frequency turbulence, a spectral index α?2.3 or larger is observed in most geospace regions, in agreement with what is observed in the solar wind. 4. More studies are needed to gain an understanding of turbulence dissipation in the geospace environment, also keeping in mind that the strong temperature anisotropies which are observed show that wave particle interactions can be a source of wave emission rather than of turbulence dissipation. 5. Several spacecraft observations show the existence of vortices in the magnetosheath, on the magnetopause, in the magnetotail, and in the ionosphere, so that they may have a primary role in the turbulent injection and evolution. The influence of such a turbulence on the plasma transport, dynamics, and energization will be described, also using the results of numerical simulations.     

Correction to: Water in the Earth’s Interior: Distribution and Origin

Space Science Reviews - Correction to: Space Sci Rev DOI This article has been corrected. Figure 3 was initially published with erroneous axis titles in Fig. 3B and 3D where the $x$ axis should be...     

Ages and Geologic Histories of Martian Meteorites

We review the radiometric ages of the 16 currently known Martian meteorites, classified as 11 shergottites (8 basaltic and 3 lherzolitic), 3 nakhlites (clinopyroxenites), Chassigny (a dunite), and the orthopyroxenite ALH84001. The basaltic shergottites represent surface lava flows, the others magmas that solidified at depth. Shock effects correlate with these compositional types, and, in each case, they can be attributed to a single shock event, most likely the meteorite's ejection from Mars. Peak pressures in the range 15 – 45 GPa appear to be a "launch window": shergottites experienced ～30 – 45 GPa, nakhlites ～20 ± 5 GPa, Chassigny ～35 GPa, and ALH84001 ～35 – 40 GPa. Two meteorites, lherzolitic shergottite Y-793605 and orthopyroxenite ALH84001, are monomict breccias, indicating a two-phase shock history in toto: monomict brecciation at depth in a first impact and later shock metamorphism in a second impact, probably the ejection event. Crystallization ages of shergottites show only two pronounced groups designated S₁ (～175 Myr), including 4 of 6 dated basalts and all 3 lherzolites, and S₂ (330 – 475 Myr), including two basaltic shergottites and probably a third according to preliminary data. Ejection ages of shergottites, defined as the sum of their cosmic ray exposure ages and their terrestrial residence ages, range from the oldest (～20 Myr) to the youngest (～0.7 Myr) values for Martian meteorites. Five groups are distinguished and designated S_Dho (one basalt, ～20 Myr), S_L (two lherzolites of overlapping ejection ages, 3.94 ± 0.40 Myr and 4.70 ± 0.50 Myr), S (four basalts and one lherzolite, ～2.7 – 3.1 Myr), S_DaG (two basalts, ～1.25 Myr), and S_E (the youngest basalt, 0.73 ± 0.15 Myr). Consequently, crystallization age group S₁ includes ejection age groups S_L, S_E and 4 of the 5 members of S, whereas S₂ includes the remaining member of S and one of the two members of S_DaG. Shock effects are different for basalts and lherzolites in group S/S₁. Similarities to the dated meteorite DaG476 suggest that the two shergottites that are not dated yet belong to group S₂. Whether or not S₂ is a single group is unclear at present. If crystallization age group S₁ represents a single ejection event, pre-exposure on the Martian surface is required to account for ejection ages of S_L that are greater than ejection ages of S, whereas secondary breakup in space is required to account for ejection ages of S_E less than those of S. Because one member of crystallization age group S₂ belongs to ejection group S, the maximum number of shergottite ejection events is 6, whereas the minimum number is 2. Crystallization ages of nakhlites and Chassigny are concordant at ～1.3 Gyr. These meteorites also have concordant ejection ages, i.e., they were ejected together in a single event (NC). Shock effects vary within group NC between the nakhlites and Chassigny. The orthopyroxenite ALH84001 is characterized by the oldest crystallization age of ～4.5 Gyr. Its secondary carbonates are ～3.9 Gyr old, an age corresponding to the time of Ar-outgassing from silicates. Carbonate formation appears to have coincided with impact metamorphism, either directly, or indirectly, perhaps via precipitation from a transient impact crater lake. The crystallization age and the ejection age of ALH84001, the second oldest ejection age at 15.0 ± 0.8 Myr, give evidence for another ejection event (O). Consequently, the total number of ejection events for the 16 Martian meteorites lies in the range 4 – 8. The Martian meteorites indicate that Martian magmatism has been active over most of Martian geologic history, in agreement with the inferred very young ages of flood basalt flows observed in Elysium and Amazonis Planitia with the Mars Orbital Camera (MOC) on the Mars Global Surveyor (MGS). The provenance of the youngest meteorites must be found among the youngest volcanic surfaces on Mars, i.e., in the Tharsis, Amazonis, and Elysium regions.     

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.