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21.
22.
Richard De Grijs Robert W. O'Connell John S. Gallagher III 《Astrophysics and Space Science》2001,276(2-4):397-404
We present high-resolution optical and near-infrared HST observations of two adjacent regions in the fossil starburst region
inM82, M82 B1 and B2. The presence of both the active and the fossil starburst in M82 provides a unique physical environment
to study the stellar and dynamical evolution of star cluster systems. The cluster population in B2 is more heavily affected
by internal extinction than that in B1, amounting to an excess extinction in B2 of AV,excess≃1.1±0.3 mag. Preliminary age estimates date the cluster population in the fossil starburst between ∼2× 108 and ∼ 109 years. The radial luminosity profiles of the brightest clusters are more closely approximated by power laws than by a Gaussian
model, in particular in their wings, which favours a slow star formation scenario.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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24.
Daniel F. Ryan Aidan M. O’Flannagain Markus J. Aschwanden Peter T. Gallagher 《Solar physics》2014,289(7):2547-2563
We test the compatibility and biases of multi-thermal flare DEM (differential emission measure) peak temperatures determined with AIA with those determined by GOES and RHESSI using the isothermal assumption. In a set of 149 M- and X-class flares observed during the first two years of the SDO mission, AIA finds DEM peak temperatures at the time of the peak GOES 1?–?8 Å flux to have an average of T p=12.0±2.9 MK and Gaussian DEM widths of log10(σ T )=0.50±0.13. From GOES observations of the same 149 events, a mean temperature of T p=15.6±2.4 MK is inferred, which is systematically higher by a factor of T GOES/T AIA=1.4±0.4. We demonstrate that this discrepancy results from the isothermal assumption in the inversion of the GOES filter ratio. From isothermal fits to photon spectra at energies of ?≈6?–?12 keV of 61 of these events, RHESSI finds the temperature to be higher still by a factor of T RHESSI/T AIA=1.9±1.0. We find that this is partly a consequence of the isothermal assumption. However, RHESSI is not sensitive to the low-temperature range of the DEM peak, and thus RHESSI samples only the high-temperature tail of the DEM function. This can also contribute to the discrepancy between AIA and RHESSI temperatures. The higher flare temperatures found by GOES and RHESSI imply correspondingly lower emission measures. We conclude that self-consistent flare DEM temperatures and emission measures require simultaneous fitting of EUV (AIA) and soft X-ray (GOES and RHESSI) fluxes. 相似文献
25.
Bryant J. McAvaney Robert R. Dahni Robert A. Colman James R. Fraser 《Global and Planetary Change》1995,10(1-4)
Two sensitivity experiments, in which CO2 is instantaneously doubled, have been performed with a general circulation model to determine the influence of the convective parametrization on simulated climate change. We have examined the spatial structure of changes in the annual mean and annual cycle for surface temperature and precipitation for both experiments; similarly we have examined changes in the variance for these two fields. We have also computed a range of test statistics in order to obtain reliable measures of the signal-to-noise ratio in the climate change signal from each experiment. We have computed test statistics for the entire globe and for five different region and we contrast the global response with the response in the Australian region taken as a representative sample.We find that the highest signal-to-noise ratios in the change from 1 * CO2 to 2 * CO2 are for the change in surface temperature for both experiments with little difference in the global averages between the experiments. Globally averaged precipitation shows a greater noise level but perhaps the greatest contrast between experiments. There are generally significant increases in the temporal and spatial variability of precipitation in the change from the 1 * CO2 to 2 * CO2 and with some differences apparent between the two experiments. The temporal variability of surface temperature does not change significantly in any of the 2 * CO2 cases, and there is little difference between the experiments. There is a significant decrease in the spatial variability of surface temperature in all 2 * CO2 experiments in all cases and with significant differences in the seasonal variations between different experiments. The spatial variability of precipitation increases in all 2 * CO2 cases and also with substantial differences in the seasonal variations between the experiments. There are accompanying significantly different spatial pattern correlations for both surface temperature and precipitation. In general we find that the global changes are fairly robust with the differences associated with convective parametrization schemes being very small. However, at the regional level, there are marked differences between experiments with changes both in the means and in the spatial and temporal variances but often with low levels of significance. 相似文献
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We have found sorted stone circles and polygons near the equator of Mars, using new 25 cm/pixel NASA HiRISE (High Resolution Imaging Science Experiment) images. The sorted circles occur in geologically recent catastrophic flood deposits in the equatorial Elysium Planitia region, and are diagnostic of periglacial processes: sorted polygons do not form from volcanic activity, as has been suggested for non-sorted polygons in this region. These landforms indicate that (i) a long-lived, geologically recent, active cryoturbation layer of ground ice was present in the regolith, (ii) there was some degree of freeze-thaw, and thus (iii) there were sustained period(s), likely within the last 10 Ma, in which the martian climate was 40 to 60 K warmer than current models predict. 相似文献
29.
Solar flares occur due to the sudden release of energy stored in active-region magnetic fields. To date, the precursors to
flaring are still not fully understood, although there is evidence that flaring is related to changes in the topology or complexity
of an active-region’s magnetic field. Here, the evolution of the magnetic field in active region NOAA 10953 was examined using
Hinode/SOT-SP data over a period of 12 hours leading up to and after a GOES B1.0 flare. A number of magnetic-field properties and
low-order aspects of magnetic-field topology were extracted from two flux regions that exhibited increased Ca ii H emission during the flare. Pre-flare increases in vertical field strength, vertical current density, and inclination angle
of ≈ 8° toward the vertical were observed in flux elements surrounding the primary sunspot. The vertical field strength and
current density subsequently decreased in the post-flare state, with the inclination becoming more horizontal by ≈ 7°. This
behavior of the field vector may provide a physical basis for future flare-forecasting efforts. 相似文献
30.
J. Ireland C. A. Young R. T. J. McAteer C. Whelan R. J. Hewett P. T. Gallagher 《Solar physics》2008,252(1):121-137
Two different multiresolution analyses are used to decompose the structure of active-region magnetic flux into concentrations
of different size scales. Lines separating these opposite polarity regions of flux at each size scale are found. These lines
are used as a mask on a map of the magnetic field gradient to sample the local gradient between opposite polarity regions
of given scale sizes. It is shown that the maximum, average, and standard deviation of the magnetic flux gradient for α,β,β
γ, and β
γ
δ active-regions increase in the order listed, and that the order is maintained over all length scales. Since magnetic flux
gradient is strongly linked to active-region activity, such as flares, this study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length scales in the active-region, and not just those length scales
at which the strongest flux gradients are found. Further, it is also shown that the average gradients in the field, and the
average length-scale at which they occur, also increase in the same order. Finally, there are significant differences in the
gradient distribution, between flaring and non-flaring active regions, which are maintained over all length scales. It is
also shown that the average gradient content of active-regions that have large flares (GOES class “M” and above) is larger
than that for active regions containing flares of all flare sizes; this difference is also maintained at all length scales.
All of the reported results are independent of the multiresolution transform used. The implications for the Mt. Wilson classification
of active-regions in relation to the multiresolution gradient content and flaring activity are discussed. 相似文献