Description: Publication date: Available online 9 May 2018 Source: Planetary and Space Science Author(s): V. Petropoulou, M. Lazzarin, I. Bertini, P. Ochner, F. La Forgia, A. Siviero, F. Ferri, G. Naletto The potentially hazardous asteroid (PHA) 2014 JO25 made a close approach to Earth on 19 April 2017 at the distance of 0.0118 AU, giving the opportunity to study its nature with large spatial resolution from ground-based observatories. Radar observations revealed an object with a clear bilobate shape. We aimed at the characterization of the surface properties of the object, the determination of its spectral class and the search for possible surface heterogeneities, possibly linked to the two lobes, during different rotational phases. Fourteen low-resolution spectra were acquired with the Boller & Chivens optical spectrograph mounted on the 1.22–m Asiago telescope. The spectra covered an entire rotation period of the asteroid. Data were reduced using IRAF and analyzed with the M4AST tool to infer the best fit Bus-Demeo taxonomy type. We found the asteroid belonging to the S/Q complex of silicaceous asteroids, complex comprising the vast majority of NEAs. This is the tenth bilobate NEA to be taxonomically classified and only the seventh bilobate PHA. We found no clear proof of surface heterogeneity, but there is an interesting periodicity in the best spectral match which appears to be synchronous with the rotation period of the object.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): Haokun Kang, Yu Jiang, Hengnian Li This paper is focused on the pseudo bifurcations and the variety of periodic ratio in the periodic orbits near the primary of binary irregular asteroid system (22) Kalliope, which would help on trajectory design for asteroid missions and give a practical insight into the generation and dynamic behaviour of binary asteroid systems. In this paper, we find three basic pseudo bifurcations in the periodic orbit families near (22) Kalliope during the numerical continuation with the variation of Jacobian constant. We also discover a nonuple mixed bifurcations which possess the highest multiplicity of bifurcations ever found and consist of three pseudo tangent bifurcations, two period-doubling bifurcations, one pseudo period-doubling bifurcation, one Neimark-Sacker bifurcation, one pseudo Neimark-Sacker bifurcation, and one real saddle bifurcation. Moreover, we find that the periodic ratio in the periodic orbit family may change during the continuation. Based on plenty of numerical evidences, we summarize three astonishing and exciting conclusions about the relationship of the periodic ratio and (pseudo) bifurcations in the periodic motion near (22) Kalliope. Firstly, the pseudo period-doubling bifurcation shows up when the periodic ratio comes near a half-integer (i.e. 1.5:1, 2.5:1, 3.5:1). Furthermore, almost all the cuspidal changes of the periodic ratio are accompanied with tangent bifurcation or pseudo tangent bifurcation. In addition, an integer can be admitted as the asymptotic value of the periodic ratio at the end of continuation, if the Jacobian constant isn't stuck into its local extremum, yet a half-integer can not.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): S.D. Xiao, T.L. Zhang Although there is no intrinsic magnetic field at Venus, the convected interplanetary magnetic field piles up to form an induced magnetosphere around the planetary ionosphere. Previous investigations show that the magnetic barrier, the part of the induced magnetosphere in the dayside inner magnetosheath, can act as an effective obstacle to the solar wind during solar maximum, and the magnetic barrier can stop the solar wind even during solar minimum. In this study, we perform a comprehensive statistical study of the magnetic barrier near the terminator during almost a complete solar cycle by using Venus Express magnetic data. The magnetic barrier configuration is located at the dayside even near the terminator and a hemispheric asymmetry exists during the whole solar cycle. We also demonstrate that the general magnetic barrier configuration is controlled by the interplanetary magnetic field orientation and solar cycle dependent. The magnetic barrier under IMF quasi-perpendicular to the solar wind flow is stronger than quasi-parallel to the solar wind flow during the solar cycle, and this difference becomes larger with the increase in solar activity.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): D. Mackler, L. Avanov, A. Barrie, D. Chornay, D. Gershman, B. Giles, C. Pollock, A. Rager, S. Smith Microchannel Plate (MCP) gain and its stability in time predominantly determine the success of any experiment in long-term space plasma missions using such a detector. This is of particular importance for the Fast Plasma Investigation (FPI) suite on the Magnetospheric Multiscale Mission (MMS) because FPI operates 64 MCP assemblies that require consistent inter-calibration over years on orbit. Due to the massive amount of MCP chevron assemblies FPI did not have enough time to fully precondition the flight detectors prior to launch. This process extracts charge until the gain change in time is significantly reduced. Preconditioning ensures that variation in countrate is environmental and not detector drift. Therefore knowledge of the gain change in time in a predictable manner is vital for the quality of data acquired by FPI. To understand the long-term behavior of the FPI MCPs we have conducted extended lifetime experiments with flight-grade MCP assemblies for both ion and electron detectors. These lifetime experiments demonstrate that the gain begins to stabilize after ≈ 1.0 C/ c m 2 has been extracted from an MCP assembly. It is expected, based of the lifetime experiments, that the FPI MCPs will operate for 20 + years with stable performance without gain degradation.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): Tatsuaki Okada, Tetsuya Fukuhara, Satoshi Tanaka, Makoto Taguchi, Takehiko Arai, Hiroki Senshu, Hirohide Demura, Yoshiko Ogawa, Toru Kouyama, Naoya Sakatani, Jun Takita, Tomohiko Sekiguchi, Jorn Helbert, Thomas G. Mueller, Axel Hagermann Thermal Infrared Imager TIR on Hayabusa2 has proven its in-flight performance to detect celestial bodies during the cruise phase, especially by the observations of the Earth and the Moon before and after the Earth swing-by on 3rd of December 2015. The result indicates that the target C-type asteroid 162173 Ryugu will be detected from the distance of 3000 km at the beginning of the approach phase, and that a C-type small moon larger than 1 m will be detected from the home position, 20 km from the asteroid, if it orbits around the asteroid.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): William G. Read, Leslie K. Tamppari, Nathaniel J. Livesey, R.Todd Clancy, François Forget, Paul Hartogh, Scot C.R. Rafkin, Goutam Chattopadhyay Atmospheric limb sounding is a well-established technique for measuring atmospheric temperature, composition, and wind. The theoretical capabilities of a submillimeter limb sounder placed in low Mars orbit are quantified, with a particular focus on the ability to make profile measurements of line-of-sight wind, temperature, water vapor, deuterated water vapor, several isotopes of carbon monoxide, oxygen-18 carbon dioxide, ozone, and hydrogen peroxide. We identify cases where all such measurements can be made within a single 25–70 GHz wide region of the submillimeter spectrum, enabling use of a single state-of-the-art submillimeter receiver. Six potential spectral regions, approximately centered at 335 GHz, 450 GHz, 550 GHz, 900 GHz, 1000 GHz, and 1130 GHz are found, any one of which can provide a complete measurement suite. The expected precision and vertical resolution of temperature, composition, and wind measurements from instruments in each range are quantified. This work thus follows on from that of Urban et al. (2005), Kasai et al. (2012), and earlier studies, expanding them to consider many alternative observing frequency regions. In general, performance (in terms of measurement precision and vertical resolution) is improved with increasing observation frequency. In part this is due to our choice to assume the same antenna size for each frequency, thus providing a narrower field of view for the higher frequency configurations. The general increase in emission line strengths with increasing frequency also contributes to this improved performance in some cases. However, increased instrument power needs for the higher frequency configurations may argue against their choice in some mission scenarios.

Description: Publication date: Available online 8 May 2018 Source: Planetary and Space Science Author(s): R. Brunetto, C. Lantz, Z. Dionnet, F. Borondics, A. Aléon-Toppani, D. Baklouti, M.A. Barucci, R.P. Binzel, Z. Djouadi, K. Kitazato, C. Pilorget Space weathering produces variations in the optical properties of small solar system body surfaces. Laboratory studies have been performed by several groups on different meteorite classes, aimed at simulating space weathering by using ion irradiation and laser ablation. Here we present new FTIR spectral imaging data of ion irradiated carbonaceous chondrites (CV Allende and CM Murchison meteorites), simulating solar wind irradiation on primitive asteroids. Irradiation conditions (40 keV He + and Ar + up to 3.10 16 ion/cm 2 ) were reported in detail in previous studies. The new analyses were performed with a FTIR micro-spectrometer equipped with an FPA (Focal Plane Array) detector. Micro-reflectance spectra were acquired in the 2.5–12 μm range using a spot size of 20 μm and scanning large areas (from mm 2 to cm 2 ) of the meteorite pellets, both on virgin areas and on areas irradiated with different doses. Spectral maps allow us characterizing the heterogeneity of the meteorites at the 20 μm spatial scale. In the irradiated areas, we observe spectral modifications of both anhydrous and hydrated silicates. Spectral shifts are observed after irradiation, larger for anhydrous than hydrated silicates. Compositional heterogeneity of the pristine materials and irradiation effects are compared to each other as a function of the irradiation dose, to determine which spectral features are more sensitive to space weathering. Results are then compared with the IR spectral capabilities of instruments onboard the Hayabusa2/JAXA and OSIRIS-REx/NASA spacecrafts, to provide these missions with spectral criteria on how to distinguish space weathering from compositional heterogeneity effects at the asteroid, and in view of surface selection for sample return.

Description: Publication date: Available online 4 May 2018 Source: Planetary and Space Science Author(s): A.M. Du, K.T. Wang, H. Luo, B.T. Tsurutani, Jesper Gjerloev, Wei Sun, Y. Wang, Jiaming Ou, Yasong Ge It is found that the coupling between the solar wind and polar geomagnetic activity cause the annual and semiannual variation of auroral electrojet. The wavelet spectrum of the SuperMAG index, SML and SMU shows that both of the semiannual and annual variations are permanent features, and spectral power of the annual period of SML and SMU are stronger than that of the semiannual period. Due to coupling of the annual variation and the semiannual variation of SML, the diurnal variation of SML in summer months is not well correlated with results anticipated from the previous principle hypotheses [McIntosh, 1959; Russell and McPherron, 1973; Svalgaard, 1977; Cliver et al., 2000; Newell et al., 2013]. The annual/semiannual variation of SML and SMU is typically coupled together and is dependent on the local time and latitudinal variations of ionospheric conductivity and the electric field. A new index, the annual-corrected SMO index (SMO*), was suggested to indicate the semiannual variation of the westward auroral electrojet (SML). The seasonal-UT distribution of SMO* has good correlation with that predicted by the equinoctial hypothesis [McIntosh, 1959; Svalgaard, 2011].

Description: Publication date: Available online 4 May 2018 Source: Planetary and Space Science Author(s): D. Gamborino, P. Wurz In most surface-bound exospheres Na has been observed at altitudes above what is possible by thermal release. Photon stimulated desorption of adsorbed Na on solid surfaces has been commonly used to explain observations at high altitudes. We investigate three model velocity distribution functions (VDF) that have been previously used in several studies to describe the desorption of atoms from a solid surface either by electron or by photon bombardment, namely: the Maxwell-Boltzmann (M-B) distribution, the empirical distribution proposed by [1] for PSD, and the Weibull distribution. We use all available measurements reported by [2, 3] to test these distributions and determine which one fits best (statistically) and we discuss their physical validity. Our results show that the measured VDF of released Na atoms are too narrow compared to Maxwell-Boltzmann fits with supra-temperatures as suggested by [3]. We found that a good fit with M-B is only achieved with a speed offset of the whole distribution to higher speeds and a lower temperature, with the offset and the fit temperature not showing any correlation with the surface temperature. From the three distributions we studied, we find that the Weibull distribution provides the best fits using the temperature of the surface, though an offset towards higher speeds is required. This work confirms that Electron-Stimulated Desorption (ESD) and Photon-Stimulated Desorption (PSD) should produce non-thermal velocity (or energy) distributions of the atoms released via these processes, which is expected from surface physics. We recommend to use the Weibull distribution with the shape parameter κ  = 1.7, the speed offset v 0  = 575 m/s, and the surface temperature to model PSD distributions at planetary bodies.

Description: Publication date: Available online 1 May 2018 Source: Planetary and Space Science Author(s): E. Chané, B. Palmaerts, A. Radioti Recently, a transient localized brightness enhancement has been observed inJupiter's main auroral emission close to noon by Palmaerts et al. (2014). We use results from three-dimensional global MHD simulations to understand what is causing this localized peak in the main emission. In the simulations, the peak occurs every rotation period and is due to shearing motions in the magnetodisk. These shearing motions are caused by heavy flux-tubes being accelerated to large azimuthal speeds at dawn. The centrifugal force acting on these flux-tubes is then so high that they rapidly move away from the planet. When they reach noon, their azimuthal velocity decreases, thus reducing the centrifugal force, and allowing the flux-tubes to move back closer toJupiter. The shearing motions associated with this periodic phenomenon locally increase the field aligned currents in the simulations, thus causing a transient brightness enhancement in the main auroral emission, similar to the one observed by Palmaerts et al. (2014).