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1.
From the gyroresonance brightness temperature spectrum of a sunspot, one can determine the magnetic field strength by using the property that microwave brightness is limited above a frequency given by an integer-multiple of the gyrofrequency. In this paper, we use this idea to find the radial distribution of magnetic field at the coronal base of a sunspot in the active region, NOAA 4741. The gyroresonance brightness temperature spectra of this sunspot are obtained from multi-frequency interferometric observations made at the Owens Valley Radio Observatory at 24 frequencies in the range of 4.0–12.4 GHz with spatial resolution 2.2″–6.8″. The main results of present study are summarized as follows: first, by comparison of the coronal magnetic flux deduced from our microwave observation with the photospheric magnetic flux measured by KPNO magnetograms, we show that theo-mode emission must arise predominantly from the second harmonic of the gyrofrequency, while thex-mode arises from the third harmonic. Second, the radial distribution of magnetic fieldsB(r) at the coronal base of this spot (say, 2000–4000 km above the photosphere) can be adequately fitted by $$B(r) = 1420(1 \pm 0.080)\exp \left[ { - \left( {\frac{r}{{11.05''(1 \pm 0.014)}}} \right)^2 } \right]G,$$ wherer is the radial distance from the spot center at coronal base. Third, it is found that coronal magnetic fields originate mostly from the photospheric umbral region. Fourth, although the derived vertical variation of magnetic fields can be approximated roughly by a dipole model with dipole moment 1.6 × 1030 erg G?1 buried at 11000 km below the photosphere, the radial field distribution at coronal heights is found to be more confined than predicted by the dipole model. 相似文献
2.
G.S. Odin V. Barbin A.J. Hurford H. Baadsgaard B. Galbrun P.-Y. Gillot 《Earth and Planetary Science Letters》1991,106(1-4)
A comparison between conventional KAr (biotite) ages and fission track (zircon and apatite) and UPb (zircon) ages obtained from stratigraphically well-constrained Priabonian (Late Eocene) volcano-sedimentary deposits of northern Italy is presented. Two sections at Priabona (one level) and Possagno (two levels) were dated. The application of fission track dating appears fruitful for obtaining reasonably precise (±4 to 5% 2σ errors) ages useful for time-scale calibration. The concordancy of apatite and zircon fission track ages, and the reproducibility of results provide the time of volcanic eruption and deposition. The UPb analysis of the zircons has not been unsuccessful, but discordancy does not permit accurate dating. Significant dates obtained from Possagno are: KAr method, 35.0 ± 0.5 Ma (duplicate analysis on K-rich biotite from the same level); fission track dating method, 35.8 ± 1.4 Ma (weighted mean age on 2 apatite and 3 zircon separates from the same level); UPb method, 36.7 ± 1.0 Ma (maximum age of discordant zircons from the same level). The comparison between the present results and recent multi-method and multi-laboratory results obtained from time equivalent Priabonian (Late Eocene) biotite-rich layers from the Apennines shows perfect agreement and supports the location of a Priabonian stage between about 37.5 Ma and about 33.7 (±0.5) Ma; the alternative ages preferred by the Decade of North American Geology convention should be abandoned and a large portion of this scale revised accordingly. 相似文献
3.
Krucker Säm Christe Steven Lin R.P. Hurford Gordon J. Schwartz Richard A. 《Solar physics》2002,210(1-2):445-456
The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty
High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and
the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May,
1:40–2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come
from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources,
while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted
better with a thermal model plus power law above ∼ 6–7 keV than with a thermal only. The decay phase sometimes can be fitted
with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration
after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal
power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents
between −5 and −8, and imply a total non-thermal electron energy content between 1026–1027 erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated
microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive
to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy
input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.)
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022404512780 相似文献
4.
Microwave observations in the range 1 to 18 GHz with high spectral resolution (40 frequencies) have shown that many events display a complex microwave spectrum. From a set of 14 events with two or more spectral components, we find that two different classes of complex events can be distinguished. The first group (4 events) is characterized by a different temporal evolution of the spectral components, resulting in a change of the spectral shape. These events probably can be explained by gyrosynchrotron emission from two or more individual sources. The second class (10 events) has a constant spectral shape, so that the two spectral components vary together in intensity. For all ten events in this second class, the ratio of primary to secondary peak frequencies is remarkably similar, exhibiting an average value of 3.4, and both components show a common circular polarization. These properties suggest either a common source for the different spectral components or several sources which are closely coupled. An additional example of this class of burst was observed interferometrically to provide spatial resolution. This event suggests that the primary and secondary components have a similar location, but that the surface area of the secondary component is larger.Swiss National Science Foundation Fellow from the University of Bern. 相似文献
5.
Tectonic patterns on Europa are influenced by tidal stress. An important well-recognized component is associated with the orbital eccentricity, which produces a diurnally varying stress as Jupiter's apparent position in Europa's sky oscillates in longitude. Cycloidal lineaments seem to have formed as cracks propagated in this diurnally varying stress field. Maps of theoretical cycloid patterns capture many of the characteristics of the observed distribution on Europa. However, a few details of the observed cycloid distribution have not been reproduced by previous models. Recently, it has been shown that Europa has a finite forced obliquity, so Jupiter's apparent position in Europa's sky will also oscillate in latitude. We explore this new type of diurnal effect on cycloid formation. We find that stress from obliquity may be the key to explaining several characteristics of observed cycloids such as the shape of equator-crossing cycloids and the shift in the crack patterns in the Argadnel Regio region. All of these improvements of the fit between observation and theory seem to require Jupiter crossing Europa's equatorial plane 45° to 180° after perijove passage, suggestive of complex orbital dynamics that locks the direction of Europa's pericenter with the direction of the ascending node at the time these cycloids were formed. 相似文献
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7.
Variations in diurnal tidal stress due to Europa’s eccentric orbit have been considered as the driver of strike-slip motion along pre-existing faults, but obliquity and physical libration have not been taken into account. The first objective of this work is to examine the effects of obliquity on the predicted global pattern of fault slip directions based on a tidal-tectonic formation model. Our second objective is to test the hypothesis that incorporating obliquity can reconcile theory and observations without requiring polar wander, which was previously invoked to explain the mismatch found between the slip directions of 192 faults on Europa and the global pattern predicted using the eccentricity-only model. We compute predictions for individual, observed faults at their current latitude, longitude, and azimuth with four different tidal models: eccentricity only, eccentricity plus obliquity, eccentricity plus physical libration, and a combination of all three effects. We then determine whether longitude migration, presumably due to non-synchronous rotation, is indicated in observed faults by repeating the comparisons with and without obliquity, this time also allowing longitude translation. We find that a tidal model including an obliquity of 1.2°, along with longitude migration, can predict the slip directions of all observed features in the survey. However, all but four faults can be fit with only 1° of obliquity so the value we find may represent the maximum departure from a lower time-averaged obliquity value. Adding physical libration to the obliquity model improves the accuracy of predictions at the current locations of the faults, but fails to predict the slip directions of six faults and requires additional degrees of freedom. The obliquity model with longitude migration is therefore our preferred model. Although the polar wander interpretation cannot be ruled out from these results alone, the obliquity model accounts for all observations with a value consistent with theoretical expectations and cycloid modeling. 相似文献
8.
Spectral maps of Mimas’ daytime thermal emission show a previously unobserved thermal anomaly on Mimas’ surface. A sharp V-shaped boundary, centered at 0°N and 180°W, separates relatively warm daytime temperatures from a cooler anomalous region occupying low- to mid-latitudes on the leading hemisphere. Subsequent observations show the anomalous region is also warmer than its surroundings at night, indicating high thermal inertia. Thermal inertia in the anomalous region is , compared to < outside the anomaly. Bolometric Bond albedos are similar between the two regions, in the range 0.49-0.70. The mapped portion of the thermally anomalous region coincides in shape and location to a region of high-energy electron deposition from Saturn’s magnetosphere, which also has unusually high near-UV reflectance. It is therefore likely that high-energy electrons, which penetrate Mimas’ surface to the centimeter depths probed by diurnal temperature variations, also alter the surface texture, dramatically increasing its thermal inertia. 相似文献
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10.
Cycloidal crack patterns on Europa are influenced by tides induced by orbital eccentricity, which in turn is driven by the Laplace orbital resonance. Their shapes potentially record the location of their formation (relative to the direction of Jupiter), as well as the parameters of crack formation. Hoppa et al. [Hoppa, G., Tufts, B.R., Greenberg, R., Geissler, P., 1999a. Icarus 141, 287-298] modeled several cycloid chains using a fixed set of material parameters, but some details did not fit. We now allow material parameters to vary for each arc of an observed cycloid. In general, with minimal variation of model parameters between the arcs, fits are greatly improved. Furthermore, accounting for tidal stress accumulated during non-synchronous rotation, in addition to diurnal stress, allows even better fits. Even with the added freedom in the model our fits allow us to better constrain the location where each cycloid may have formed. Our results support Hoppa et al.'s finding that only a few cracks form ridges per cycle of non-synchronous rotation in the region examined, probably because cracking relieves built up stress until further substantial rotation occurs. 相似文献