Radar observations of the Geminid meteoroid stream

Continuous radio-wave monitoring of the Geminid activity in December 1992 and 1993 by using a forward scatter (FS) bistatic radar over the Bologna-Lecce baseline (700 km) in Italy, reveals peculiar structural aspects of the stream in terms of signal amplitude-rate and duration-rate dependence. The observational results of the Geminid display obtained in the two consecutive years with differentiated peak levels of transmitted power, exhibit different time distributions of underdense meteors against the signal received power. Both sets of the data relative to the peak activity in December 12–14, show reflection properties of Geminids which are atypical if compared with echoes from cometary-type showers, with really high echo counts at mid-upper levels of the peak received power. An assymmetric curve of activity of the Geminids complex is evidenced, with the peak flux of smaller particles occurring earlier than that of larger ones.     

Fractal space-time and black-body radiation

We discuss the possibility of setting upper limits to deviations of the dimensionD of space from the standard integer value of 3. The method we suggest — a reanalysis of the Planck law for the black-body radiation in aD-dimensional space — is less sensitive than other methods previously suggested. It offers, however, the possibility of probing the fractal dimension of space-time over cosmological distances.     

Study of landslides at the archaeological site of Abakainon necropolis (NE Sicily) by geomorphological and geophysical investigations

This paper reports on the investigation of the old landslide body on which the necropolis of Abakainon, an Italian archaeological site, was built about 2400 years ago. This site is located in a geologically complex area of northeastern Sicily, where active tectonics has strongly modified the morphological landscape, as testified by the meandering path of watercourses, widespread presence of paleosurfaces, and deviations of crest line. Previous studies have hypothesized that the necropolis, suddenly destroyed by a catastrophic event in historical times, could have been built on an old landslide body. This represents the starting point of this paper, focused on geomorphological analysis and geophysical surveys, which allowed discovering peculiar elements in support of the landslide theory. In particular, the elaboration of a Digital Terrain Model allowed locating the main morphostructural elements, thus highlighting how the occurrence of landslides in the study area is strongly linked to recent tectonics. Passive seismic surveys, carried out through the acquisition of ambient noise, allowed detecting the presence of impedance contrasts, which were associated to peculiar features, such as the landslide body itself. Moreover, local strong directional effects testify the presence of faults, affecting even the necropolis area. Such results, further corroborated by the outcomes of an electrical resistivity survey, lay the foundations for interesting consideration on the possible cause of the destruction of this site, probably related to the occurrence of a historical earthquake in the first century AD.     

Science-based approach for credible accounting of mitigation in managed forests

Background

The credibility and effectiveness of country climate targets under the Paris Agreement requires that, in all greenhouse gas (GHG) sectors, the accounted mitigation outcomes reflect genuine deviations from the type and magnitude of activities generating emissions in the base year or baseline. This is challenging for the forestry sector, as the future net emissions can change irrespective of actual management activities, because of age-related stand dynamics resulting from past management and natural disturbances. The solution implemented under the Kyoto Protocol (2013–2020) was accounting mitigation as deviation from a projected (forward-looking) “forest reference level”, which considered the age-related dynamics but also allowed including the assumed future implementation of approved policies. This caused controversies, as unverifiable counterfactual scenarios with inflated future harvest could lead to credits where no change in management has actually occurred, or conversely, failing to reflect in the accounts a policy-driven increase in net emissions. Instead, here we describe an approach to set reference levels based on the projected continuation of documented historical forest management practice, i.e. reflecting age-related dynamics but not the future impact of policies. We illustrate a possible method to implement this approach at the level of the European Union (EU) using the Carbon Budget Model.

Results

Using EU country data, we show that forest sinks between 2013 and 2016 were greater than that assumed in the 2013–2020 EU reference level under the Kyoto Protocol, which would lead to credits of 110–120 Mt CO₂/year (capped at 70–80 Mt CO₂/year, equivalent to 1.3% of 1990 EU total emissions). By modelling the continuation of management practice documented historically (2000–2009), we show that these credits are mostly due to the inclusion in the reference levels of policy-assumed harvest increases that never materialized. With our proposed approach, harvest is expected to increase (12% in 2030 at EU-level, relative to 2000–2009), but more slowly than in current forest reference levels, and only because of age-related dynamics, i.e. increased growing stocks in maturing forests.

Conclusions

Our science-based approach, compatible with the EU post-2020 climate legislation, helps to ensure that only genuine deviations from the continuation of historically documented forest management practices are accounted toward climate targets, therefore enhancing the consistency and comparability across GHG sectors. It provides flexibility for countries to increase harvest in future reference levels when justified by age-related dynamics. It offers a policy-neutral solution to the polarized debate on forest accounting (especially on bioenergy) and supports the credibility of forest sector mitigation under the Paris Agreement.

     

Accurate ionospheric delay model for real-time GPS-based positioning of LEO satellites using horizontal VTEC gradient estimation

Ionospheric delays compensation is a mandatory step for precise absolute and relative positioning of Low Earth Orbit Satellites (LEO) by GPS measurements. The most frequently used ionosphere model for real-time GPS-based navigation in LEO is an isotropic model proposed by Lear, which uses the Vertical Total Electron Content (VTEC) above the receiver and a mapping function for TEC evaluation along a given ray path. Based on significant assessed results available for ground-based GPS receivers, we propose the use of a different model relying on the thin shell assumption and a bilinear horizontal variation of the VTEC as a function of latitude and longitude in the shell. It is expected that this model is capable of better describing horizontal gradients in the ionosphere, thus improving ionospheric delay estimation, especially in intense ionospheric conditions. This model is referred to as Linear Thin Shell (LTS). LTS performance in estimating undifferenced and double-differenced ionospheric delays is checked by comparing measured and predicted delays computed using flight data from the GRACE mission. Results show that the LTS always outperforms the isotropic model, especially in case of high solar activity. Moreover, the LTS model provides a higher performance uniformity over a wide range of ionospheric delays, thus ensuring good performance in different conditions. The results obtained demonstrate that the LTS model improves the ionosphere delays estimation accuracy by 20 and 40% for undifferenced and double-differenced delays, respectively. This suggests the LTS model can effectively contribute to improving precision in LEO positioning applications.     

Groundwater resource degradation in coastal plains: The example of the Cecina area (Tuscany – Central Italy)

The paper describes the degradation of the groundwater resources in the Cecina area, where seawater intrusion, B contamination and NO₃ pollution are all affecting the heavily exploited Pleistocene aquifer. Over-pumping has brought water levels to about 0 m.a.s.l. as far as about 7 km from the shore line, thereby promoting the seawater intrusion. The intrusion, which is characterized by cation exchange phenomena and Ca–Cl type waters, enters the plain mostly through the shallower horizons. The saline front, which advanced from 0.5 to 1 km in 4 a, has by now reached the foot of the hills to the east of the town, where it is also affecting wells of the local aqueduct. Boron contamination, linked to past discharge of industrial waste transported downstream by the river, reached concentrations as high as 3.5 mg/L in the mid-1980s. Although a decreasing trend is now under way, B content is still close to 1 mg/L. The presence of high NO₃, which, together with the seawater intrusion, represents a major issue for groundwater management in the area, is linked to the widespread utilization of fertilizers. Nitrate concentration, which reaches a maximum of about 300 mg/L in the shallow aquifer horizons and then decreases rather regularly with depth, is strongly influenced by precipitation. However, irrigation also contributes significantly to transporting the NO₃ contamination to depth, as clearly shown by δ¹⁸O data. The severe decline in the quality of the groundwater resource in the Cecina area is further compounded by an overall decrease in water availability in the region of Tuscany, as evidenced by long-term monitoring of precipitation and fluvial discharge.     

Methods for the analysis of data from the Planetary Fourier Spectrometer on the Mars Express Mission

This work presents an algorithm for the scientific analysis of individual calibrated measurements from the Planetary Fourier spectrometer (PFS).The instrument, included in the scientific payload of the ESA Mars Express mission to Mars, acquires spectra in the range between 250 and 8200 cm⁻¹, with a sampling step of ∼1 cm⁻¹ and an effective resolution of ∼2 cm⁻¹. The observed radiance depends on several parameters of the atmosphere and surface of Mars as described by the radiative transfer equation. Adopting the very general formalism of Bayesian analysis, we determined which quantities are actually retrievable from individual measurements. Namely, they are: the surface temperature, the column density of dust and water ice aerosols in the atmosphere, the air temperature as a function of altitude (in the indicative range 5-45 km above the surface), the surface pressure, and the column density of water vapor and carbon monoxide. These evaluations are carried out taking into account the noise equivalent radiance (NER) of the instrument and the natural variabilities of the investigated parameters in the Martian environment, as estimated from the expectations of the European Martian Climate Dataset v3.1 (EMCD). Other parameters included in the radiative transfer equation shall be assumed as known, because they are not retrievable from individual measurements due to the instrumental NER or an underconstrained inverse problem: the surface emissivity in the thermal infrared, the optical properties of suspended dust and the analytical shape of dust concentration vs. altitude.During the development of the algorithm devoted to these studies, different approaches were evaluated on the basis of formal, computational and scientific considerations, with the aim to develop the general design of an integrated software package.The resulting code was extensively tested on a wide set of simulated PFS spectra. These spectra were computed from the atmospheric and surface conditions extracted from the EMCD, assumed to be representative of the Martian environment for different values of latitude, local time and season. Their comparison with the retrievals from simulated observations allowed us to evaluate the systematic and random errors affecting the procedures with respect to the different quantities involved. The code evaluates the surface temperature with an error in the order of 1 K, while the vertical air temperature profile is computed with an uncertainty less than 2 K from in the region between 5 and 20 km above the surface, increasing up to 7 K at 50 km. The column opacity of dust, measured in terms of integrated optical thickness at 1100 cm⁻¹, is computed with an error of around 0.13. The surface pressure determination is carried out with a typical uncertainty of 0.2-0.3 millibar. Several auxiliary tests allowed us to study the correlations between the different retrieval errors and the possible causes of incorrect PFS data interpretation. The choice of a suitable model for the dust optical properties is demonstrated to be particularly critical.This paper also presents the first discussion about application of the procedure to actual PFS Martian data. Despite the calibration issues still affecting the determination of absolute radiance in the near-infrared, the algorithm is able to achieve a satisfactory modeling of observations in a wide range of situations.     

The role of the land use, land use change and forestry sector in achieving Annex I reduction pledges

Annex I Parties may receive credits or debits from Land Use, Land Use Change and Forestry (LULUCF) activities, contributing to achieving individual emission reduction targets. In the Durban climate negotiations, Parties agreed new LULUCF accounting rules for the second commitment period of the Kyoto Protocol (CP2). By using these new rules, this paper presents key differences among Parties at the minimum (assuming no additional action) and potential (assuming additional actions) contribution of the forest-related LULUCF activities in achieving the pledges for 2020. Overall, the potential contribution of LULUCF is relatively modest (up to about 2 % of 1990 emissions) for the EU, the Annex I Parties likely joining the CP2, and for the Annex I Parties that joined the CP1 as a whole. However, for specific Parties, LULUCF can make a substantial contribution to achieving the pledges. For New Zealand, for instance, the potential contribution of future LULUCF credits may equal 33 % of its 1990 emission level. For Australia, the pledges are expressed relative to 2000 emission levels including LULUCF emissions. Given that LULUCF emissions have strongly declined between 1990 and 2000, and a further decline in foreseen by 2020 (based on Australia’s projections), the minimum contribution of LULUCF to meet the Australian pledges appears to be about 19 % and 7 % relative to its 1990 and 2000 emission level, respectively. A further 3 % potential contribution is estimated from additional actions.     

Interpretation of combined infrared,submillimeter, and millimeter thermal flux data obtained during the Rosetta fly-by of Asteroid (21) Lutetia

The European Space Agency’s Rosetta spacecraft is the first Solar System mission to include instrumentation capable of measuring planetary thermal fluxes at both near-IR (VIRTIS) and submillimeter–millimeter (smm–mm, MIRO) wavelengths. Its primary mission is a 1 year reconnaissance of Comet 67P/Churyumov–Gerasimenko beginning in 2014. During a 2010 close fly-by of Asteroid 21 Lutetia, the VIRTIS and MIRO instruments provided complementary data that have been analyzed to produce a consistent model of Lutetia’s surface layer thermal and electrical properties, including a physical model of self-heating. VIRTIS dayside measurements provided highly resolved 1 K accuracy surface temperatures that required a low thermal inertia, I < 30 J/(K m² s^0.5). MIRO smm and mm measurements of polar night thermal fluxes produced constraints on Lutetia’s subsurface thermal properties to depths comparable to the seasonal thermal wave, yielding a model of I < 20 J/(K m² s^0.5) in the upper few centimeters, increasing with depth in a manner very similar to that of Earth’s Moon. Subsequent MIRO-based model predictions of the dayside surface temperatures reveal negative offsets of ～5–30 K from the higher VIRTIS-measurements. By adding surface roughness in the form of 50% fractional coverage of hemispherical mini-craters to the MIRO-based thermal model, sufficient self-heating is produced to largely remove the offsets relative to the VIRTIS measurements and also reproduce the thermal limb brightening features (relative to a smooth surface model) seen by VIRTIS. The Lutetia physical property constraints provided by the VIRTIS and MIRO data sets demonstrate the unique diagnostic capabilities of combined infrared and submillimeter/millimeter thermal flux measurements.     

PFS/MEX observations of the condensing CO2 south polar cap of Mars

The condensing CO₂ south polar cap of Mars and the mechanisms of the CO₂ ice accumulation have been studied through the analysis of spectra acquired by the Planetary Fourier Spectrometer (PFS) during the first two years of ESA's Mars Express (MEX) mission. This dataset spans more than half a martian year, from Ls∼330° to Ls∼194°, and includes the southern fall season which is found to be extremely important for the study of the residual south polar cap asymmetry. The cap expands symmetrically and with constant speed during the fall season. The maximum extension occurs sometime in the 80°-90° Ls range, when the cap edges are as low as −40° latitude. Inside Hellas and Argyre basins, frost can be stable at lower latitudes due to the higher pressure values, causing the seasonal cap to be asymmetric. Within the seasonal range considered in this paper, the cap edge recession rate is approximately half the rate at which the cap edge expanded. The longitudinal asymmetries reduce during the cap retreat, and disappear around Ls∼145°. Two different mechanisms are responsible for CO₂ ice accumulation during the fall season, especially in the 50°-70° Ls range. Here, CO₂ condensation in the atmosphere, and thus precipitation, is allowed exclusively in the western hemisphere, and particularly in the longitudinal corridor of the perennial cap. In the eastern hemisphere, the cap consists mainly of CO₂ frost deposits, as a consequence of direct vapor deposition. The differences in the nature of the surface ice deposits are the main cause for the residual south polar cap asymmetry. Results from selected PFS orbits have also been compared with the results provided by the martian general circulation model (GCM) of the Laboratoire de Météorologie dynamique (LMD) in Paris, with the aim of putting the observations in the context of the global circulation. This first attempt of cross-validation between PFS measurements and the LMD GCM on the one hand confirms the interpretation of the observations, and on the other hand shows that the climate modeling during the southern polar night on Mars is extremely sensitive to the dynamical forcing.