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The search for life in the Universe is a fundamental problem of astrobiology and modern science. The current progress in the detection of terrestrial-type exoplanets has opened a new avenue in the characterization of exoplanetary atmospheres and in the search for biosignatures of life with the upcoming ground-based and space missions. To specify the conditions favourable for the origin, development and sustainment of life as we know it in other worlds, we need to understand the nature of global (astrospheric), and local (atmospheric and surface) environments of exoplanets in the habitable zones (HZs) around G-K-M dwarf stars including our young Sun. Global environment is formed by propagated disturbances from the planet-hosting stars in the form of stellar flares, coronal mass ejections, energetic particles and winds collectively known as astrospheric space weather. Its characterization will help in understanding how an exoplanetary ecosystem interacts with its host star, as well as in the specification of the physical, chemical and biochemical conditions that can create favourable and/or detrimental conditions for planetary climate and habitability along with evolution of planetary internal dynamics over geological timescales. A key linkage of (astro)physical, chemical and geological processes can only be understood in the framework of interdisciplinary studies with the incorporation of progress in heliophysics, astrophysics, planetary and Earth sciences. The assessment of the impacts of host stars on the climate and habitability of terrestrial (exo)planets will significantly expand the current definition of the HZ to the biogenic zone and provide new observational strategies for searching for signatures of life. The major goal of this paper is to describe and discuss the current status and recent progress in this interdisciplinary field in light of presentations and discussions during the NASA Nexus for Exoplanetary System Science funded workshop ‘Exoplanetary Space Weather, Climate and Habitability’ and to provide a new roadmap for the future development of the emerging field of exoplanetary science and astrobiology.
To assess the diagnostic role of mean platelet volume in tonsillitis with and without peritonsillar abscess.
Mean platelet volume and other laboratory data were retrospectively investigated.
Mean platelet volume was significantly lower in the tonsillitis group (7.8 per cent ± 0.7 per cent) than in the control group (8.7 per cent ± 0.6 per cent; p < 0.0001), and it was significantly lower in the abscess group (7.5 per cent ± 0.6 per cent) than in the no abscess group (8.0 per cent ± 0.7 per cent; p = 0.0277). White blood cell counts and C-reactive protein levels were not significantly different between patients with an abscess and those without. The mean platelet volume cut-off values for the diagnosis of tonsillitis and peritonsillar abscess were 7.95 fl and 7.75 fl, respectively.
Our results suggest that a decreased mean platelet volume is associated with the development and severity of tonsillitis. This finding provides useful diagnostic information for physicians treating patients with tonsillitis.
A two dimensional MHD code is used to study the nonlinear evolution of the Parker instability in isolated horizontal magnetic flux imbedded in (or below) the solar photosphere. It is found that the magnetic loop expands self-similarly in the nonlinear stage. Numerical results explain many features observed in emerging flux regions.
Two-dimensional MHD simulations are performed to study the nonlinear evolution of the Parker instability in galactic gas disks. When the most unstable mode grows, magnetic field lines kink across the equatorial plane of the disk and thin spur-like structures are formed above dense regions in magnetic pockets. In low β (= ρgas/ρmag < 3) disks, shock waves are produced at the footpoint of magnetic loops, while in high β (> 3) disks, nonlinear oscillations are excited and the loop length increases with time up to λc ≃ (3.5β + 6)H, where H is the half-thickness of the disk.
We propose a mechanism of amplification of magnetic fields and plasma heating in clusters of galaxies. Recent observations indicate the existence of ~ μG magnetic fields in clusters of galaxies (e.g., Kronberg 1994). There should be some mechanism which locally amplify magnetic fields. In clusters of galaxies, individual motions of galaxies may create locally strong field region by stretching and tangling the magnetic fields threading the galaxies. Magnetic reconnection taking place in the tangled magnetic fields may convert the kinetic energy of the galaxy motion into the inter-galactic plasma heating (Makishima 1996).
Mapping observations of nearby large-extended clusters of galaxies (Coma, Perseus, Virgo, etc.) are being performed with ASCA. Such clusters allow us to map physical parameters of hot gas in the clusters, such as temperature, metal abundance, and X-ray surface brightness. To determine such parameters at each part of a cluster, one should take careful care of X-ray contamination from outside of a pointed field, which is mainly due to “stray-light” X-rays (Honda et al. 1997). For this reason, the only way to obtain the distribution of hot gas parameter is to process the whole cluster data in a self-consistent way. For this purpose, we are developing the new analysis system called TERRA.
We present the results of axisymmetric, two-dimentional magnetohydrodynamic (MHD) simulations of weakly ionized gas torus threaded by large scale vertical magnetic fields. The gas torus corresponds to the 100pc scale circumnuclear torus observed by HST in nearby AGNs (e.g. NGC4261) or 1010M⊙ circumnuclear gas found by CO observations in luminous IR galaxies and quasars (e.g. Scoville et al. 1991). The initial state of simulation is a constant angular momentum polytropic torus threaded by uniform vertical magnetic fields. The torus is assumed to be rotating in a static, spherical hot halo. The model parameters are Eth = vs02/(γvk02) = 5 ×10−3 and Eth = vA02/vK02 = 6.6×10−6 where γ is the adiabatic index and vs0 and va0 are the sound speed and the Alfvén speed at r = r0 respectively.
We present the results of 2.5-dimensional MHD simulations for jet formation from accretion disks in a situation such that not only ejection but also accretion of disk plasma are also included self-consistently. Although the jets in nonsteady MHD simulations (e.g., Uchida & Shibata 1985, Shibata & Uchida 1986, Matsumoto et al. 1996) have often been referred to as transient phenomena resulting from a special choice of initial conditions, we found that the characteristics of the nonsteady jets are very similar to those of steady jets: (1) The ejection point of the jet, which corresponds to slow magnetosonic point in steady MHD jet theory, is determined by the effective potential which results from the gravitational force and the centrifugal force along a field line (Blandford & Payne 1982). (2) The dependence of the velocity (vz) and mass outflow rate (Ṁω) on the initial magnetic field strength is about Ṁω ∝ B0 and vz ∝ (Ω2FB20/Ṁω)1/3, where B0 is an initial poloidal magnetic field strength, and ΩF is an ‘angular velocity of the field line’ which is nearly the same as the Keplerian angular velocity where the jet is ejected. These are consistent with those of 1D steady solution (Kudoh & Shibata 1997), although the explanation is a little complicated in the 2.5D case because of an avalanche-like accretion. We also confirm that the velocity of the jet is of order of the Keplerian velocity of the disk for a wide range of parameters. We conclude that the ejection mechanism of nonsteady jets found in the 2.5-dimensional simulations are understood with a previous theory which is studied on the assumption of steady state even when nonsteady avalanche-like accretions occur along the surface of disks.
We present a scenario for the origin of the hot plasma in our Galaxy, as a model of a strong X-ray emission (LX(2 – 10keV) ~ 1038 erg s−1), called Galactic Ridge X-ray Emission (GRXE), which has been observed near the Galactic plane. GRXE is thermal emission from hot component (~ 7 keV) and cool component (~ 0.8 keV). Observations suggest that the hot component is diffuse, and is not escaping away freely. Both what heats the hot component and what confines it in the Galactic ridge are still remained puzzling, while the cool component is believed to be made by supernovae. We propose a new scenario: the hot component of GRXE plasma is heated by magnetic reconnection, and confined in the helical magnetic field produced by magnetic reconnection or in the current sheet and magnetic field. We solved also the 2-dimensional magnetohydrodynamic (MHD) equations numerically to study how the magnetic reconnection creates hot plasmas and magnetic islands (helical tubes), and how the magnetic islands confine the hot plasmas in Galaxy. We conclude that the magnetic reconnection is able to heat up the cool component to hot component of GRXE plasma if the magnetic field is localized into intense flux tube with Blocal ~ 30 μG (the volume filling factor of f ~ 0.1).
Since the discovery of fading X-rays from Gamma-Ray Bursts (GRBs) with BeppoSAX (Piro et al. 1997, Costa et al. 1997), world-wide follow-up observations in optical band have achieved the fruitful results. The case of GRB 970228, there was an optical transient, coincides with the BeppoSAX position and faded (Paradijs et al. 1997, Sahu et al. 1997). These optical observations also confirmed the extended component, which was associated with the optical transient. The new transient are fading with a power-law function in time and the later observation of HST confirmed the extended emission is stable (Fruchter et al. 1997). This extended object seems to be a distant galaxy and strongly suggests to be the host.
Black hole candidates sometimes show a transition between the high (or soft) state and the low (or hard) state. In the low state, low frequency time variations are much larger than the high state. A possible mechanism of the large-amplitude, sporadic time variabilities in the low-state is the magnetic energy release in low-β (β = Pgas/Pmag < 1) disks (Mineshige, Kusunose & Matsumoto 1995). It had been thought that low-β disks cannot exist because buoyant escape of magnetic flux due to the Parker instability may set the lower limit for β inside the disk. Shibata, Tajima & Matsumoto (1990), however, pointed out that in accretion disks, once a low-β disk is formed, it can stay in low-β state partly because the growth rate of the Parker instability decreases when β < 1. They suggested that magnetic accretion disks fall into two types; high-β disks and low-β disks.
Intra-cluster spaces are filled with intra-cluster medium (ICM), whose typical temperature and density are TICM ~ 107.5 K and nICM ~ 10−3 cm−3, respectively (e.g., Sarazin 1988). Recent Faraday rotation measurements have revealed the existence of magnetic fields in ICM with few − 10 μG (e.g., Ge & Owen 1993). In ICM, the plasma β (the ratio of gas pressure to magnetic pressure) is almost “equipartition” value as follows:
We made a search of quiescent X-ray counterparts of two Gamma-Ray Bursts (GRBs), GRB930131 and GRB940217. These GRBs were detected with BATSE, EGRET, COMPTEL on board CGRO together with the GRB detector on Ulysses spacecraft, then they were localized in small error regions. These observations showed that the bursts were remarkably bright accompanying delayed high energy gamma-rays. ASCA observations have found a single X-ray source for each GRB on the possible location determined with the above instruments.
We performed 2.5-dimensional, nonsteady MHD numerical simulations to investigate the acceleration and collimation of magnetically driven outflows from accretion disks, including the accretion process itself, consistently. As an initial condition, we used a paraboloidal magnetic field line that is produced by electric current on the equatorial plane. We found that the outflow ejected from the accretion disk is collimated by the pinch effect of the toroidal component of the magnetic field that is produced by the rotation of the disk.
Magnetically driven jets from accretion disks are considered to be the most promising models of astrophysical jets. Uchida & Shibata (1985) and Shibata & Uchida (1986) first carried out two-dimensional nonlinear MHD simulations of jet formation from a magnetized disk. Matsumoto et al. (1996) applied the Uchida-Shibata model to a gas torus in active galactic nuclei and showed that the surface layer of the torus accretes faster than the equatorial region like an avalanche because magnetic braking most effectively extracts angular momentum from that layer. A magnetized torus subjects to global non-axisymmetric instabilities (Curry & Pudritz 1996) and local magnetorotational instability (Balbus & Hawley 1991). We carried out three-dimensional global MHD simulations to show the non-axisymmetric effects on the torus, avalanche flow and jet formation.
The Sun’s activity has been evolving in the ascending phase of Solar Cycle 23 since 1996. Similarly, the research on solar activity is also in the ascending phase of a new active period. Numerous new results have been obtained from a large amount of space and ground observations covering a wide spectral range. In particular, observations with YOHKOH, SOHO, and TRACE have revealed a multitude of phenomena and processes in the solar atmosphere which provide us a new picture of the Sun.