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H -U Schmincke 《Bulletin of Volcanology》1991,53(4):323-325

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Initial phase of chromospheric evaporation in a solar flare     

E. Antonucci  B. R. Dennis  A. H. Gabriel  G. M. Simnett 《Solar physics》1985,96(1):129-142

In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 10⁷ K was moving toward the observer with a mean velocity of about 80 km s^-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s^-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 10¹⁰ erg s^-1 cm^-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona.  相似文献   

Application of beam forming techniques to measurements of acoustic scattering from the ocean bottom     

Berkson  J. Kloosterman  H. Akal  T. Berrou  J. 《Oceanic Engineering, IEEE Journal of》1985,10(3):299-302

Signals from an explosive source backscattered from the seafloor and received at long range by hydrophones of a towed array are processed to estimate the directional distribution of energy for a given time increment. As assembly of these data shows the time and amplitude of scattering features, and after conversion to distance, the geographic location of the return. A frequency-domain beam-forming procedure is used in which beam levels are averaged over a given band of a broad-band source. The processing is applied to experimental data obtained in the southern Tyrrhenian Sea. The major backscattering occurred at the Baconi Seamounts and the coastal margin of Sardinia.  相似文献   

The Early Pleistocene Praetiglian and Ludhamian pollen stages in the North Sea Basin and their relationship to the marine isotope record     

T. Meijer  P. Cleveringa  D. K. Munsterman  R. M. C. H. Verreussel 《第四纪科学杂志》2006,21(3):307-310

In this paper, the currently accepted correlation of the Early Pleistocene Ludhamian stage of England with the Tiglian‐A sub‐stage of the Netherlands is challenged. Recent investigations of Early Pleistocene marine North Sea deposits from a borehole near Noordwijk (the Netherlands) yielded evidence from molluscs, dinoflagellate cysts and sporomorphs for an alternation of warm‐temperate and arctic intervals within the Praetiglian and Tiglian stages. Marine equivalents of the terrestrial‐based pollen sub‐stages Tiglian A and B have been recognised in the upper part of the sequence. A Praetiglian age can be assigned to the lower part of the sequence on the basis of mollusc analysis. Within the Praetiglian, an alternation of warm and cold phases has been recognised from both the dinoflagellate cyst and molluscan records. Three cold phases within the Praetiglian are tentatively correlated with marine isotope stages (MIS) 96–100. The molluscan assemblages provide evidence for climate forcing of the sea level: highest sea levels are reached in the warm‐temperate intervals. Within the Praetiglian, an interval with an acme zone of the dinoflagellate cyst Impagidinium multiplexum, is correlated with the Ludhamian and tentatively linked to MIS 97 and/or MIS 96. The cold molluscan assemblages from the Noordwijk borehole include an acme zone of Megayoldia thraciaeformis, the first and only occurrence of this North Pacific bivalve in the North Sea Basin. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

The variability of Mercury's exosphere by particle and radiation induced surface release processes     

H Lammer  P Wurz  R Killen  S Massetti  A Milillo 《Icarus》2003,166(2):238-247

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (P_surface<1×10⁻⁸ Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N?10¹⁴ cm⁻²) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na⁺ ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H⁺ are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10⁸ cm⁻² s⁻¹ calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8-1.0×10⁷ cm⁻² s⁻¹ and for Na approximately 1.3-1.6×10⁵ cm⁻² s⁻¹ depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0-1.4×10⁴ cm⁻² s⁻¹. By taking an average open magnetic field line area of about 2.8×10¹⁶ cm² modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10²¹ s⁻¹ and for O of about 2.5×10²³ s⁻¹. The particle sputter rate for K atoms is about 3.0×10²⁰ s⁻¹ assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10⁶ cm⁻² s⁻¹. The average source rates for Na particles released by PSD are about 1×10²⁴ s⁻¹. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873-5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10⁴ cm⁻² s⁻¹ along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).  相似文献   

An Overview of the Performance of the Chandra X-ray Observatory     

M. C. Weisskopf  T. L. Aldcroft  M. Bautz  R. A. Cameron  D. Dewey  J. J. Drake  C. E. Grant  H. L. Marshall  S. S. Murray 《Experimental Astronomy》2003,16(1):1-68

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Chandra and RXTE spectroscopy of the Galactic microquasar XTE J1550−564 in outburst     

J. M. Miller  H. L. Marshall  R. Wijnands †  T. Di Matteo  D. W. Fox  J. Kommers  D. Pooley  T. Belloni  J. Casares  P. A. Charles  A. C. Fabian  M. van der Klis  W. H. G. Lewin 《Monthly notices of the Royal Astronomical Society》2003,338(1):7-13

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A search for the optical and near-infrared counterpart of the accreting millisecond X-ray pulsar XTE J1751−305     

P. G. Jonker  G. Nelemans  Z. Wang  A. K. H. Kong  D. Chakrabarty  M. Garcia  P. J. Groot  M. van der Klis  T. Kerr  B. Mobasher  M. Sullivan  T. Augusteijn  B. W. Stappers  P. Challis  R. P. Kirshner  J. Hjorth  A. Delsanti 《Monthly notices of the Royal Astronomical Society》2003,344(1):201-206

We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of   R > 23.1, I > 21.6, Z > 20.6, J > 19.6  and   K > 19.2  . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system.  相似文献   

MS 1603.6+2600: an accretion disc corona source?     

P. G. Jonker  M. van der Klis  C. Kouveliotou  M. Méndez  W. H. G. Lewin  T. Belloni 《Monthly notices of the Royal Astronomical Society》2003,346(2):684-688

We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.  相似文献   

The dynamical structure of dwarf elliptical galaxies     

W.W. Zeilinger  S. De Rijcke  H. Dejonghe  G.K.T. Hau 《Astrophysics and Space Science》2003,284(2):607-610

Deep imaging and long-slit spectroscopy was obtained for a sample of dwarf ellipticals in the Fornax cluster, NGC 5044 and NGC 5898 groups using the ESO VLT. The observational data extend out to typically 1.5–2 effective radii and indicate a kinematic dichotomy in the family of ellipticals. The observed stellar kinematics indicate a luminosity–velocity dispersion relation largely supporting Supernova-driven stellar mass loss scenarios for the formation of dwarf ellipticals. Stellar dynamical models favour dark matter halos with typical mass-to-light ratios in the range of 3 to 9 solar units. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献