The Ultraviolet Spectrograph on NASA’s Juno Mission

The ultraviolet spectrograph instrument on the Juno mission (Juno-UVS) is a long-slit imaging spectrograph designed to observe and characterize Jupiter’s far-ultraviolet (FUV) auroral emissions. These observations will be coordinated and correlated with those from Juno’s other remote sensing instruments and used to place in situ measurements made by Juno’s particles and fields instruments into a global context, relating the local data with events occurring in more distant regions of Jupiter’s magnetosphere. Juno-UVS is based on a series of imaging FUV spectrographs currently in flight—the two Alice instruments on the Rosetta and New Horizons missions, and the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter mission. However, Juno-UVS has several important modifications, including (1) a scan mirror (for targeting specific auroral features), (2) extensive shielding (for mitigation of electronics and data quality degradation by energetic particles), and (3) a cross delay line microchannel plate detector (for both faster photon counting and improved spatial resolution). This paper describes the science objectives, design, and initial performance of the Juno-UVS.     

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.     

Bursty Bulk Flow Driven Turbulence in the Earth’s Plasma Sheet

During the last several years significant progress has been made in understanding MHD turbulence in the Earth’s plasma sheet. Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results, Alfvénic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma sheet.     

The Magnetic Field Pile-up and Density Depletion in the Martian Magnetosheath: A Comparison with the Plasma Depletion Layer Upstream of the Earth's Magnetopause

Using magnetometer and electron observations from the Mars Global Surveyor (MGS) and the Wind spacecraft we show that the region of magnetic field pile-up and density decrease located between the Martian ionosphere and bow shock exhibit strong similarities with the plasma depletion layer (PDL) observed upstream of the Earth's magnetopause in the absence of magnetic reconnection when the magnetopause is a solid obstacle in the solar wind. A PDL is formed upstream of the terrestrial magnetopause when the magnetic field piles up against the obstacle and particles in the pile-up region are squeezed away from the high magnetic pressure region along the field lines as the flux tubes convect toward the magnetopause. We here discuss the possibility that at least part of the region of magnetic field pile-up and density depletion upstream of Mars may be formed by the same physical processes which generate the PDL upstream of the Earth's magnetopause. More complete ion, electron, and neutral measurements are needed to conclusively determine the relative importance of the plasma depletion process versus exospheric processes.     

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.     

The Search Coil Magnetometer for THEMIS

THEMIS instruments incorporate a tri-axial Search Coil Magnetometer (SCM) designed to measure the magnetic components of waves associated with substorm breakup and expansion. The three search coil antennas cover the same frequency bandwidth, from 0.1 Hz to 4 kHz, in the ULF/ELF frequency range. They extend, with appropriate Noise Equivalent Magnetic Induction (NEMI) and sufficient overlap, the measurements of the fluxgate magnetometers. The NEMI of the searchcoil antennas and associated pre-amplifiers is smaller than 0.76 pT $/\sqrt{\mathrm{Hz}}$ at 10 Hz. The analog signals produced by the searchcoils and associated preamplifiers are digitized and processed inside the Digital Field Box (DFB) and the Instrument Data Processing Unit (IDPU), together with data from the Electric Field Instrument (EFI). Searchcoil telemetry includes waveform transmission, FFT processed data, and data from a filter bank. The frequency range covered depends on the available telemetry. The searchcoils and their three axis structures have been precisely calibrated in a calibration facility, and the calibration of the transfer function is checked on board, usually once per orbit. The tri-axial searchcoils implemented on the five THEMIS spacecraft are working nominally.     

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.     

Using Precise Solar Limb Shape Measurements to Study the Solar Cycle

Despite 20 years of total solar irradiance measurements from space, the lack of high precision spatially resolved observations limits definitive answers to even simple questions like ``Are the solar irradiance changes caused solely by magnetic fields perturbing the radiative flux at the photosphere?" More subtle questions like how the aspheric structure of the sun changes with the magnetic cycle are only now beginning to be addressed with new tools like p-mode helioseismology. Solar 5-min oscillation studies have yielded precise information on the mean radial interior solar structure and some knowledge about the rotational and thermal solar asphericity. Unfortunately this progress has not been enough to generate a self-consistent theory for why the solar irradiance and luminosity vary with the magnetic cycle. We need sharper tools to describe and understand the sun's global aspheric response to its internal dynamo, and we need to be able to measure the solar cycle manifestation of the magnetic cycle on entropy transport from the interior to the photosphere in much the same way that we study the fundamentally more complex problem of magnetic flux transport from the solar interior. A space experiment called the Solar Physics Explorer for Radius, Irradiance and Shape (SPHERIS) and in particular its Astrometric and Photometric Telescope (APT) component will accomplish these goals.

     

Common envelope evolution of binary stars

We discuss the common envelope phase in the evolution of binary systems. The problem of the efficiency of energy deposition into envelope ejection is treated in some detail. We describe the implications of common envelope evolution for the shaping of planetary nebulae with close binary nuclei and for double white dwarf systems, considered to be the progenitors of Type I supernovae.     

Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System

The absolute ages of cratered surfaces in the inner solar system, including Mars, are derived by extrapolation from the impact flux curve for the Moon which has been calibrated on the basis of absolute ages of lunar samples. We reevaluate the lunar flux curve using isotope ages of lunar samples and the latest views on the lunar stratigraphy and the principles of relative and absolute age dating of geologic surface units of the Moon. The geological setting of the Apollo and Luna landing areas are described as far as they are relevant for this reevaluation. We derive the following best estimates for the ages of the multi-ring basins and their related ejecta blankets and present alternative ages for the basin events (in parentheses): 3.92 ± 0.03 Gyr (or 3.85 ± 0.05 Gyr) for Nectaris, 3.89 ± 0.02 Gyr (or 3.84 ± 0.04 Gyr) for Crisium, 3.89 ± 0.01 Gyr (or 3.87 ± 0.03 Gyr) for Serenitatis, and 3.85 ± 0.02 Gyr (or 3.77 ± 0.02 Gyr) for Imbrium. Our best estimates for the ages of the mare landing areas are: 3.80 ± 0.02 Gyr for Apollo 11 (old surface), 3.75 ± 0.01 Gyr for Apollo 17, 3.58 ± 0.01 Gyr for Apollo 11 (young surface), 3.41 ± 0.04 Gyr for Luna 16, 3.30 ± 0.02 Gyr for Apollo 15, 3.22 ± 0.02 Gyr for Luna 24, and 3.15 ± 0.04 Gyr for Apollo 12. The ages of Eratosthenian and Copernican craters remain: ～ 2.1 (?) Gyr (Autolycus), 800 ± 15 Myr (Copernicus), 109 ± 4 Myr (Tycho), 50.3 ± 0.8 (North Ray crater, Apollo 16), and 25.1 ± 1.2 (Cone crater, Apollo 14). When plotted against the crater densities of the relevant lunar surface units, these data result in a revised lunar impact flux curve which differs from the previously used flux curve in the following respects: (1) The ages of the stratigraphically most critical impact basins are notably younger, (2) the uncertainty of the calibration curve is decreased, especially in the age range from about 4.0 to 3.0 Gyr, (3) any curve for ages older than 3.95 Gyr (upper age limit of the Nectaris ejecta blanket) is abandoned because crater frequencies measured on such surface formations cannot be correlated with absolute ages obtained on lunar samples. Therefore, the impact flux curve for this pre-Nectarian time remains unknown. The new calibration curve for lunar crater retention ages less than about 3.9 Gyr provides an updated standard reference for the inner solar system bodies including Mars.