Diachronous exhumation of HP–LT metamorphic rocks from south‐western Alps: evidence from fission‐track analysis

New fission‐track ages on zircon and apatite (ZFT and AFT) from the south‐western internal Alps document a diachronous cooling history from east to west, with cooling rates of 15–19 °C Ma⁻¹. In the Monviso unit, the ZFT ages are 19.6 Ma and the AFT ages are 8.6 Ma. In the eastern Queyras, ZFT ages range from 27.0 to 21.7 Ma and AFT ages from 14.2 to 9.4 Ma. In the western Queyras, ZFT ages are between 94.7 and 63.1 Ma and AFT ages are between 22.2 and 22.6 Ma. The Chenaillet ophiolite yields ages of 118.1 Ma on ZFT and 67.9 Ma on AFT. The combination of these new FT data with the available petrological and geochronological data emphasize an earlier exhumation in subduction context before 30 Ma, then in collision associated with westward tilting of the Piedmont zone.     

Convergence structures of the Peru trench between 10°S and 14°S

High resolution seafloor studies of the Peru Trench between 10°S and 14°S with the GLORIA long-range side-scan sonar system show that the Nazca plate is broken by numerous normal faults as it bends into the trench. These bending-induced faults strike subparallel to the trench axis and overprint and cut across spreading fabric structures of the plate. They commonly form grabens having widths and spacings of 3–5 km and extend for as much as 100 km along strike. Vertical displacements are generally 200 m or more by the time they reach the trench axis. Turbidite deposits are found in the trench north of 11.5°S. Both turbidite and pelagic sediments are folded and temporarily accreted to the base of the overriding plate along the length of the trench axis. They are apparently subsequently implaced in the grabens by slumping and subducted with the Nazca plate. The Mendaña Fracture Zone, which intersects the trench between 9°40′S and 10°35′S, appears to be the locus of a seaward propagating rift that is forming in response to subduction-induced extensional stresses in the Nazca plate.     

EIDOSCOPE: particle acceleration at plasma boundaries

We describe the mission concept of how ESA can make a major contribution to the Japanese Canadian multi-spacecraft mission SCOPE by adding one cost-effective spacecraft EIDO (Electron and Ion Dynamics Observatory), which has a comprehensive and optimized plasma payload to address the physics of particle acceleration. The combined mission EIDOSCOPE will distinguish amongst and quantify the governing processes of particle acceleration at several important plasma boundaries and their associated boundary layers: collisionless shocks, plasma jet fronts, thin current sheets and turbulent boundary layers. Particle acceleration and associated cross-scale coupling is one of the key outstanding topics to be addressed in the Plasma Universe. The very important science questions that only the combined EIDOSCOPE mission will be able to tackle are: 1) Quantitatively, what are the processes and efficiencies with which both electrons and ions are selectively injected and subsequently accelerated by collisionless shocks? 2) How does small-scale electron and ion acceleration at jet fronts due to kinetic processes couple simultaneously to large scale acceleration due to fluid (MHD) mechanisms? 3) How does multi-scale coupling govern acceleration mechanisms at electron, ion and fluid scales in thin current sheets? 4) How do particle acceleration processes inside turbulent boundary layers depend on turbulence properties at ion/electron scales? EIDO particle instruments are capable of resolving full 3D particle distribution functions in both thermal and suprathermal regimes and at high enough temporal resolution to resolve the relevant scales even in very dynamic plasma processes. The EIDO spin axis is designed to be sun-pointing, allowing EIDO to carry out the most sensitive electric field measurements ever accomplished in the outer magnetosphere. Combined with a nearby SCOPE Far Daughter satellite, EIDO will form a second pair (in addition to SCOPE Mother-Near Daughter) of closely separated satellites that provides the unique capability to measure the 3D electric field with high accuracy and sensitivity. All EIDO instrumentation are state-of-the-art technology with heritage from many recent missions. The EIDOSCOPE orbit will be close to equatorial with apogee 25-30 RE and perigee 8-10 RE. In the course of one year the orbit will cross all the major plasma boundaries in the outer magnetosphere; bow shock, magnetopause and magnetotail current sheets, jet fronts and turbulent boundary layers. EIDO offers excellent cost/benefits for ESA, as for only a fraction of an M-class mission cost ESA can become an integral part of a major multi-agency L-class level mission that addresses outstanding science questions for the benefit of the European science community.     

The early despinning of the Sun

It is shown that if the Sun passed through a T Tauri stage, then a mass loss of only 15% would be sufficient to despin the Sun to an angular velocity of 0 (10^–5 rad/sec) at 10⁷ years without the additional braking effect of an enhanced magnetic field. Thus the present Sun could have a core rotating at most ten times faster than its surface.     

Unipolar induction in the moon and a lunar limb shock mechanism

The unipolar induction mechanism is employed to calculate electric field profiles in the interior of a chemically homogeneous Moon possessing a steep radial thermal gradient characteristic of long-term radioactive heating. The thermal models used are those of Fricker, Reynolds, and Summers. From the magnetic field, the magnetic back pressure upon the solar wind is found. The electric field profile is shown to depend only upon the activation energy,E _o, of the geological material and the radial gradient of the reciprocal temperature. The current is additionally dependent upon the coefficient of the electrical conductivity function but only by a scale factor. Since the Moon is experimentally known to correspond to the case of weak interaction with the solar wind, the magnetic back pressure is calculated without the need for an iterative procedure. The results indicate that a hot Moon can yield sufficient current flow so that the magnetic back pressure is observable as a vestigial limb shock wave using an activation energy of about 2/3 eV together with a conductivity coefficient of about 10³ mhos/m. Such matter is approximated by diabase-like composition, although the result that both the activation energy and coefficient enter into the current determination does not rule out the possibility of a match with other similar substances. The calculations are entirely consistent with earlier results which indicated a model where the unipolar current density is dominated by a high impedance surface layer and a strong shock wave is inhibited. In addition to the magnetic back pressure, the integration of the current continuity equation permits current densities and joule heating rates to be calculated, though the magnitude of the latter for present solar wind conditions is not thermally important.On leave from NASA Ames Research Center     

Flare fragmentation and type III productivity in the 1980 June 27 flare

We present observations of the solar flare on 1980 June 27, 16:14–16:33 UT, which was observed by a balloon-borne 300 cm² phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200–400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of 7.6 × 10²⁷ photons keV^–1 per type III burst at 20 keV. The occurrence of 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile.The generation of a superhot isothermal HXR component in the decay phase of the flare coincides with the fade-out of type III production.Universities Space Research Associates.ST Systems Corporation.     

Correlated fast time structures at millimeter waves and hard X-rays during a solar burst

We present an analysis of the time profiles detected during a solar impulsive flare, observed at one-millimeter radio frequency (48 GHz) and in three hard X-ray energy bands (25–62, 62–111, and 111–325 keV) with high sensitivity and time resolution. The time profiles of all emissions exhibit fast time structures of 200–300 ms half power duration which appear in excess of a slower component varying on a typical time scale of 10 s. The amplitudes of both the slow and fast variations observed at 48 GHz are not proportional to those measured in the three hard X-ray energy bands. However, the fast time structures detected in both domains are well correlated and occur simultaneously within 64 ms, the time resolution of the hard X-ray data. In the context of a time-of-flight flare model, our results put strong constraints on the acceleration time scales of electrons to MeV energies.     

Submillimeter observations from space: A space station submillimeter facility (SSSF)

There are many important scientific problems to be attacked in the submillimeter wavelength region including the astrophysics of star formation, the dynamics of protoplanetary systems, the physics of interstellar gas, mass loss from stars, supernovae, the chemical evolution of galaxies, the star formation rates in galaxies, the astrophysics of active galactic nuclei, the deuterium abundance in different astrophysical environments, and the distribution of the cosmic background radiation. However, to effectively explore this wavelength range requires going into space since atmospheric absorption precludes most observations from the ground. As in most areas of astronomy, the twin needs for sensitivity and high redsolution indicate use of an imaging interferometer, but the needed baselines of a few tens-of-meters require a large physical structure. The planned Space Station will provide, for the first time, a platform which is large enough to accommodate a forefront submillimeter synthesis instrument. Such a telescope would open an entirely new wavelength regime to astronomy with the attendant possibilities for unexpected new discoveries.A submillimeter array would also be technically well suited for operation on the Space Station. Second-of-are resolution at submillimeter wavelengths requires only relatively short baselines; pointing accuracy and tracking stability requirements are relatively crude being determined by the single dish size rather than the array resolution; radio frequency interference (RFI) susceptibility is very low due to the large frequency separation from normal communications bands; emissions from co-orbiting debris, dust, gas, and water vapour are uncorrelated between interferometer elements; baseline stability requirements, while severe, are less stringent than for optical/IR interferometers and can almost certainly be satisfied for existing phase correction and phaseless image restoration techniques; and the technology for the dishes, mounts, receivers, LO/IF systems, and correlators either exists or is a reasonable extrapolation of what already is available on the ground.We consider the applications and possible design of a Space Station based submillimeter array which could be mounted along the main (Y) axis of the Space Station and use orbital revolution and precession to produce high-resolution synthesis mapping in much the same way ground-based linear arrays do by Earth rotation synthesis.     

Particle interactions with Alfvén solitons

We compute velocity and the corresponding energy changes due to non-resonant interactions of protons with Alfvén solitons. It is seen that the protons heat in the perpendicular direction but associated with this is a cooling in the parallel direction.ICTP (Trieste) Fellow under programme: Training and Research in Italian Laboratories.Instituto di Fisicia dello Spazio Interplanetario CNR, via G. Galilei, CP 27, 00044 Frascati, Italy.