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921.
Luca Maltagliati Dmitrij V. Titov Thérèse Encrenaz Francois Forget Jean-Pierre Bibring 《Icarus》2011,213(2):480-1871
We present here the annual behavior of atmospheric water vapor on Mars, as observed by the OMEGA spectrometer on board Mars Express during its first martian year. We consider all the different features of the cycle of water vapor: temporal evolution, both at a seasonal and at a diurnal scale; longitudinal distribution; and the vertical profile, through the variations in the saturation height. We put our results into the context of the current knowledge on the water cycle through a systematic comparison with the already published datasets. The seasonal behavior is in very good agreement with past and simultaneous retrievals both qualitatively and quantitatively, within the uncertainties. The average water vapor abundance during the year is ∼10 pr. μm, with an imbalance between northern and southern hemisphere, in favor of the first. The maximum of activity, up to 60 pr. μm, occurs at high northern latitudes during local summer and shows the dominance of the northern polar cap within the driving processes of the water cycle. A corresponding maximum at southern polar latitudes during the local summer is present, but less structured and intense. It reaches ∼25 pr. μm at its peak. Global circulation has some influence in shaping the water cycle, but it is less prominent than the results from previous instruments suggest. No significant correlation between water vapor column density and local hour is detected. We can constrain the amount of water vapor exchanged between the surface and the atmosphere to few pr. μm. This is consistent with recent results by OMEGA and PFS-LW. The action of the regolith layer on the global water cycle seems to be minor, but it cannot be precisely constrained. The distribution of water vapor on the planet, after removing the topography, shows the already known two-maxima system, over Tharsis and Arabia Terra. However, the Arabia Terra increase is quite fragmented compared with previous observations. A deep zone of minimum separates the two regions. The saturation height of water vapor is mainly governed by the variations of insolation during the year. It is confined within 5-15 km from the surface at aphelion, while in the perihelion season it stretches up to 55 km of altitude. 相似文献
922.
R. Chandra B. Schmieder C. H. Mandrini P. Démoulin E. Pariat T. Török W. Uddin 《Solar physics》2011,269(1):83-104
We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501
on 20 November 2003. Both flares displayed four homologous Hα ribbons and were both accompanied by coronal mass ejections
(CMEs). The central flare ribbons were located at the site of an emerging bipole in the centre of the active region. The negative
polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ≈ 110° within 32 hours.
We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time
to each flare. In particular, we calculate the location of quasi-separatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501,
the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points.
For both flares, the photospheric traces of QSLs are similar and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology.
However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable
during each flare, so that magnetic reconnection proceeds differently in both events. 相似文献
923.
We present a statistical study of the characteristics of type-II radio bursts observed in the metric (m) and deca-hectometer
(DH) wavelength range during 1997–2008. The collected events are divided into two groups: Group I contains the events of m-type-II
bursts with starting frequency ≥ 100 MHz, and group II contains the events with starting frequency of m-type-II radio bursts
< 100 MHz. We have analyzed both samples considering three different aspects: i) statistical properties of type-II bursts, ii) statistical properties of flares and CMEs associated with type-II bursts, and iii) time delays between type-II bursts, flares, and CMEs. We find significant differences in the properties of m-type-II bursts
in duration, bandwidth, drift rate, shock speed and delay between m- and DH-type-II bursts. From the timing analysis we found
that the majority of m-type-II bursts in both groups occur during the flare impulsive phase. On the other hand, the DH-type-II
bursts in both groups occur during the decaying phase of the associated flares. Almost all m-DH-type-II bursts are found to
be associated with CMEs. Our results indicate that there are two kinds of shock in which group I (high frequency) m-type-II
bursts seem to be ignited by flares whereas group II (low frequency) m-type-II bursts are CME-driven. 相似文献
924.
The activity minimum between the end of cycle 23 and the beginning of cycle 24 was the longest and deepest since at least
the beginning of the 20th century. This has led to speculation that the Sun is changing its behaviour. The sunspot number
and 10.7-cm solar radio flux indices have traditionally been highly correlated, so a change in the relationship between them
might flag at such a change. An examination of this relationship suggests a significant change in the relationship between
activity in the photosphere and in the chromosphere/corona happened soon after the maximum of cycle 23 and has continued into
cycle 24. However, there are indications of change as early as 1980. 相似文献
925.
To determine where to search for life in our solar system or in other extrasolar systems, the concept of habitability has
been developed, based on the only sample we have of a biological planet—the Earth. Habitability can be defined as the set
of the necessary conditions for an active life to exist, even if it does not exist. In astronomy, a habitable zone (HZ) is
the zone defined around a sun/star, where the temperature conditions allow liquid water to exist on its surface. This habitability
concept can be considered from different scientific perspectives and on different spatial and time scales. Characterizing
habitability at these various scales requires interdisciplinary research. In this article, we have chosen to develop the geophysical,
geological, and biological aspects and to insist on the need to integrate them, with a particular focus on our neighboring
planets, Mars and Venus. Important geodynamic processes may affect the habitability conditions of a planet. The dynamic processes,
e.g., internal dynamo, magnetic field, atmosphere, plate tectonics, mantle convection, volcanism, thermo-tectonic evolution,
meteorite impacts, and erosion, modify the planetary surface, the possibility to have liquid water, the thermal state, the
energy budget, and the availability of nutrients. They thus play a role in the persistence of life on a planet. Earth had
a liquid water ocean and some continental crust in the Hadean between 4.4 and 4.0 Ga (Ga: billions years ago), and may have
been habitable very early on. The origin of life is not understood yet; but the oldest putative traces of life are early Archean
(~3.5 Ga). Studies of early Earth habitats documented in the rock record hosting fossil life traces provide information about
possible habitats suitable for life beyond Earth. The extreme values of environmental conditions in which life thrives today
can also be used to characterize the “envelope” of the existence of life and the range of potential extraterrestrial habitats.
The requirement of nutrients by life for biosynthesis of cellular constituents and for growth, reproduction, transport, and
motility may suggest that a dynamic and rocky planet with hydrothermal activity and formation of relief, liquid water alteration,
erosion, and runoff is required to replenish nutrients and to sustain life (as we know it). The concept of habitability is
very Earth-centric, as we have only one biological planet to study. However, life elsewhere would most probably be based on
organic chemistry and leave traces of its past or recent presence and metabolism by modifying microscopically or macroscopically
the physico-chemical characteristics of its environment. The extent to which these modifications occur will determine our
ability to detect them in astrobiological exploration. Looking at major steps in the evolution of life may help determining
the probability of detecting life (as we know it) beyond Earth and the technology needed to detect its traces, be they morphological,
chemical, isotopic, or spectral. 相似文献
926.
Martin Nekola René Hudec Martin Jelínek Matůš Kocka Petr Kubánek Filip Münz Cyril Polášek Vojtěch Šimon Jan Štrobl 《Experimental Astronomy》2010,28(1):79-85
We report on two small aperture robotic telescopes called BART and D50 operated in Ondřejov. Both telescopes are capable of
automatic observation of gamma ray burst (GRB) optical afterglows. Coordinates of GRBs are taken from alerts distributed via
Internet. Telescopes observe other interesting high energy sources when there is not any alert. The smaller telescope BART
has aperture D = 254 mm. The bigger telescope D50 has a primary mirror of diameter D = 500 mm. Both telescopes are controlled
by free software package RTS2 and are accessible through Internet. We describe the two telescopes and related software and
show some results such as our first observed optical counterpart of GRB. 相似文献
927.
Bengt Gustafsson Jorge Meléndez Martin Asplund David Yong 《Astrophysics and Space Science》2010,328(1-2):185-191
The question whether the solar chemical composition is typical for solar-type stars is analysed by comparing the Sun with different stellar samples, including a sample of stars with very similar parameters, solar twins. Although typical in terms of overall metallicity for stars of solar age and galactic orbit, the solar atmosphere is found to have abundances, as compared with solar twins, that indicate that its gas has once been affected by dust formation and dust separation. It is concluded that this may be related to the formation of the solar planetary system and its special properties. 相似文献
928.
In understanding the nucleosynthesis of the elements in stars, one of the most important quantities is the reaction rate and
it must be evaluated in terms of the stellar temperature T, and its determination involves the knowledge of the excitation function σ(E) of the specific nuclear reaction leading to the final nucleus. In this paper, the effect of thermonuclear reaction rates
to the pre-main sequence evolution of low mass stars having masses 0.7, 0.8, 0.9 and 1M⊙ are studied by using our modified Stellar Evolutionary Program. 相似文献
929.
M. M. Bisi A. R. Breen B. V. Jackson R. A. Fallows A. P. Walsh Z. Mikić P. Riley C. J. Owen A. Gonzalez-Esparza E. Aguilar-Rodriguez H. Morgan E. A. Jensen A. G. Wood M. J. Owens M. Tokumaru P. K. Manoharan I. V. Chashei A. S. Giunta J. A. Linker V. I. Shishov S. A. Tyul’bashev G. Agalya S. K. Glubokova M. S. Hamilton K. Fujiki P. P. Hick J. M. Clover B. Pintér 《Solar physics》2010,265(1-2):49-127
We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time (Dst) index reaching ?263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation. 相似文献
930.
Zdeněk ?vestka 《Solar physics》2010,267(2):235-250
I recount my career in solar physics beginning at Ondřejov Observatory in 1948 and ending with my ∼30 year stay at the Laboratory
of Space Research in Utrecht. 相似文献