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1.
Many meteoroids burn up between about 120 km and 70 km, deposit metals and dust and form ionized trails which are detected by radars. Model studies about the influence of neutral or positively charged background dust on the ambipolar diffusion indicate that significant smaller decay times should be observed for weak meteor echoes compared to strong meteor echoes which can affect the estimation of temperatures. The variation of meteor decay times in dependence on echo strength, height, and season was studied using radar observations at 69° N, 22° S, and 67° S. Significantly reduced decay times were found for weak echoes below about 88 km at low latitudes throughout the year, and at high latitudes with the exception of summer. In summer at high latitudes, decreasing decay times of weak and strong meteors are observed at altitudes below about 85 km during the appearance of noctilucent clouds. The impact of reduced decay times on the estimation of neutral temperatures from decay times is discussed.  相似文献   

2.
Theoretical studies suggest that Joule dissipation in the ionosphere is the major source of damping for resonant ULF pulsations. The decay rates of transient pulsations (i.e. short-lived pulsations with latitude dependent periods) observed by ground based magnetometers are however generally larger than those predicted, and also larger than those observed in the magnetosphere. We have modelled the integration effects of ground based magnetometers on transient pulsations by considering empirical models of the associated ionospheric currents. The simulated ground magnetometer data show a smearing of the amplitude and period variations, which is more pronounced for smaller scale (specifically latitudinal) variations. The period increase with latitude is reduced, and may even be eliminated over appreciable latitude ranges. For all spatial scales the observed decay rates are typically 2–3 times larger than the true values, due to the additional decay resulting from spatial integration of the incoherent transient pulsations. Estimates of the ionospheric Pedersen conductance based on ground magnetometer observations of decay rates are correspondingly too small, and spurious gradients may be introduced. The present calculations reconcile observed decay rates on the ground with those predicted using the assumption that Joule dissipation is the dominant damping mechanism for toroidal mode resonant oscillations.  相似文献   

3.
The semimajor axis of the Lageos satellite's orbit is decreasing secularly at the rate of 1.1 mm day–1. Ten possible mechanisms are investigated to discover which one (s), if any, might be causing the orbit to decay. Six of the mechanisms, resonance with the Earth's gravitational field, gravitational radiation, the Poynting-Robertson effect, transfer of spin angular momentum to the orbital angular momentum, drag from near-earth dust, and atmospheric drag by neutral hydrogen are ruled out because they are too small or require unacceptable assumptions to account for the observed rate of decay. Three other mechanisms, the Yarkovsky effect, the Schach effect, and terrestrial radiation pressure give perturbations whose characteristic signatures do not agree with the observed secular decrease (terrestrial radiation pressure appears to be too small in any case); hence they are also ruled out. Charged particle drag with the ions at Lageos's altitude is probably the principal cause of the orbital decay. An estimate of charged particle drag based upon laboratory experiments and satellite measurements of ion number densities accounts for 60 percent of the observed rate of decrease in the semimajor axis, assuming a satellite potential of –1V. This figure is in good agreement with other estimates based on charge drag theory. A satellite potential of –1.5V will explain the entire decay rate. Atmospheric drag from neutral hydrogen appears to be the next largest effect, explaining about 10 percent of the observed orbital decay rate.  相似文献   

4.
We present temporal and spectral characteristics of X-ray flares observed from six late-type G–K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM–Newton observatory. All the stars were found to be flaring frequently and altogether a total of 17 flares were detected above the 'quiescent' state X-ray emission which varied from 0.5 to  8.3 × 1029 erg s−1  . The largest flare was observed in a low-activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to 'quiescent' state luminosity of 2. We have studied the spectral changes during the flares by using colour–colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares.  相似文献   

5.
The time profile of two sets of isolated type III bursts, observed in the meter wavelength range at the Trieste Astronomical Observatory, was analyzed using a Fourier transform technique in order to accurately determine the decay constant of the exponential phase and to derive the exciter time profile. The decay constant () was found to be correlated with the exciter duration (D e ), suggesting that the damping of plasma waves is not of collisional origin and confirming results obtained by previous authors at lower frequencies. In particular, two distributions can be identified in the ( – D e ) plane and fitted by two nearly parallel lines, which could be the signature of different decay processes. Moreover, the damping constant observed at higher frequencies (327 and 408 MHz) has the same dependence on exciter duration as that at the lower frequency (237 MHz), also in disagreement with the collisional hypothesis.  相似文献   

6.
Schrijver  C. J. 《Solar physics》1989,122(2):193-208
This paper studies how the properties of large-scale convection affect the decay of plages. The plage decay, caused by the random-walk dispersion of flux tubes, is suggested to be severely affected by differences between the mean size of cellular openings within and around plages. The smaller cell size within a plage largely explains the smaller diffusion coefficient within plages as compared to that of the surrounding regions. Moreover, the exchange of flux tubes between the inner regions of the plage and the surrounding network is suggested to be modified by this difference in cell size, and the concept of a partially transmitting plage periphery is introduced: this periphery preferentially turns back flux parcels that are moving out of the plage and preferentially lets through flux parcels that are moving into the plage, thus confining the flux tubes to within the plage. This semi-permeability of the plage periphery, together with the dependence of the diffusion coefficient on the flux-tube density, can explain the observed slow decay of plages (predicting a typical life time of about a month for a medium-sized plage), the existence of a well-defined plage periphery, and the observed characteristic mean magnetic flux density of about 100 G. One effect of the slowed decay of the plage by the semi-permeability of the plage periphery is the increase of the fraction of the magnetic flux that can cancel with flux of the opposite polarity along the neutral line to as much as 80%, as compared to at most 50% in the case of non-uniform diffusion. This may explain why only a small fraction of the magnetic flux is observed to escape from the plage into the surrounding network.  相似文献   

7.
Tail emission of the prompt gamma-ray burst (GRB) is discussed using a multiple emitting sub-shell (inhomogeneous jet, sub-jets or mini-jets) model, where the whole GRB jet consists of many emitting sub-shells. One may expect that such a jet with angular inhomogeneity should produce spiky tail emission. However, we found that the tail is not spiky but is decaying roughly monotonically. The global decay slope of the tail is not so much affected by the local angular inhomogeneity but affected by the global sub-shell energy distribution. The fact that steepening GRB tail breaks appeared in some events prefers the structured jets. If the angular size of the emitting sub-shell is around 0.01–0.02 rad, some bumps or fluctuations appear in the tail emission observed frequently in long GRBs. If the parameter differences of sub-shell properties are large, the tail has frequent changes of the temporal slope observed in a few bursts. Therefore, the multiple emitting sub-shell model has the advantage of explaining the small-scale structure in the observed rapid decay phase.  相似文献   

8.
We present a comprehensive multiwavelength temporal and spectral analysis of the 'fast rise exponential decay' GRB 070419A. The early-time emission in the γ-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 102 up to 106 s). We investigate the light-curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at ∼450 s with the fireball deceleration time results in a Lorenz factor  Γ≈ 350  at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until   t ∼ 4 × 106 s  , inconsistent with the observed decay of the optical emission from   t ∼ 104 s  . A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light-curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves, consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of γ-ray bursts.  相似文献   

9.
The sizes and shapes of X-ray emitting loops brightened by flares and other coronal transients have been derived from the Skylab S-054 photographs. This information has been combined with estimates of temperature and emission measure derived from the photographs and from Solrad data to compute brightness decay times attributable to various coronal energy loss mechanisms. The computed decay times are compared to those actually observed. Examples are presented of the brightness decay of soft X-ray flare kernels, post-flare loops, and the coronal X-ray enhancement asssociated with an H filament disappearance.The computed decay time due to conductive losses is always found to be much more rapid than that due to radiative losses in the corona. However, the observed soft X-ray brightness decay times are always much longer than those computed from conductive cooling.The role of geometrical inhibition of conduction as discussed by Antiochos and Sturrock (1976a) is examined for these events. It is shown that this mechanism might be adequate to account for the observed results in two of the five cases examined, but it is inadequate in the other three. The possible breakdown of classical collisional thermal conductivity (Forslund, 1970) is examined and it is shown that this mechanism is not applicable to the cases presented here. Confirmation of the existence of the very high conductive fluxes predicted by the coronal flare conductive cooling models is sought from EUV and H observations. No evidence is found which unequivocally demonstrates the presence, at lower levels in the atmosphere, of very high conductive fluxes. The soft X-ray results are consistent with the continuation of evaporation driven by thermal conduction (Antiochos and Sturrock, 1976b) late into the decay phase of the event. In this case, no source of continued magnetic energy dissipation after the initial stages of the flare is required to explain the lifetime of the X-ray emitting loops.  相似文献   

10.
The theory of velocity dependent inertial induction, based upon extended Mach’s principle, has been able to generate many interesting results related to celestial mechanics and cosmological problems. Because of the extremely minute magnitude of the effect its presence can be detected through the motion of accurately observed bodies like Earth satellites. LAGEOS I and II are medium altitude satellites with nearly circular orbits. The motions of these satellites are accurately recorded and the past data of a few decades help to test many theories including the general theory of relativity. Therefore, it is hoped that the effect of the Earth’s inertial induction can have any detectable effect on the motion of these satellites. It is established that the semi-major axis of LAGEOS I is decreasing at the rate of 1.3 mm/d. As the atmospheric drag is negligible at that altitude, a proper explanation of the secular change has been wanting, and, therefore, this paper examines the effect of the Earth’s inertial induction effect on LAGEOS I. Past researches have established that Yarkovsky thermal drag, charged and neutral particle drag might be the possible mechanisms for this orbital decay. Inertial induction is found to generate a perturbing force that results in 0.33 mm/d decay of the semi major axis. Some other changes are also predicted and the phenomenon also helps to explain the observed changes in the orbits of a few other satellites. The results indicate the feasibility of the theory of inertial induction i.e. the dynamic gravitation phenomenon of the Earth on its satellites as a possible partial cause for orbital decay.  相似文献   

11.
Dark matter is explained within our scenario of the formation of the universe from the decay of macroscopic superstrings. The observed increase of the fraction of dark matter with the hierarchical rank of astronomical objects is explained by assuming that about 90% of superstrings of each rank decay into unbound fragments. The dark matter consists essentially of asteroids and smaller objects which are not bound to stars.  相似文献   

12.
In the set of 236 gamma-ray burst (GRB) afterglows observed by Swift between 2005 January and 2007 March, we identify 30 X-ray light-curves that have power-law fall-offs that exhibit a steepening ('break') at 0.1–10 d after they are triggered, to a decay steeper than t −1.5. For most of these afterglows, the X-ray spectral slope and the decay indices before and after the break can be accommodated by the standard jet model although a different origin of the breaks cannot be ruled out. In addition, there are 27 other afterglows which have X-ray light-curves that may also exhibit a late break to a steep decay, but the evidence is not that compelling. The X-ray emissions of 38 afterglows decay slower than t −1.5 until after 3 d, half of them exhibiting such a slow decay until after 10 d. Therefore, the fraction of well-monitored Swift afterglows with potential jet breaks is around 60 per cent, whether we count only the strongest cases for each type or all of them. This fraction is comparable to the 75 per cent of pre-Swift afterglows which have optical light-curves that displayed similar breaks at ∼1 d. The peak energy of the GRB spectrum of Swift afterglows with light-curve breaks shows the same correlations with the burst isotropic output (Amati relation) and with the burst collimated output (Ghirlanda relation) as previously found for pre- Swift optical afterglows with light-curve breaks. However, we find that the Ghirlanda relation is largely a consequence of Amati's and that the use of the jet-break time leads to a stronger Ghirlanda correlation only when the few objects that do not satisfy the Amati relation are included.  相似文献   

13.
Using a multi-component model to describe the γ-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra. The detection of π0 emisson alone can indicate much about the energy distribution and spectral variation of the protons accelerated to pion producing energies. Here both the intensity and detailed spectral shape of the Doppler-broadened π0 decay feature are used to determine the spectral form of the accelerated proton energy distribution. The Doppler width of this γ-ray emission provides a unique diagnostic of the spectral shape at high energies, independent of any normalisation. To our knowledge, this is the first time that this diagnostic has been used to constrain the proton spectra. The form of the energetic proton distribution is found to be severely limited by the observed intensity and Doppler width of the π0 decay emission, demonstrating effectively the diagnostic capabilities of the π0 decay γ-rays. The spectral index derived from the γ-ray intensity is found to be much harder than that derived from the Doppler width. To reconcile this apparent discrepancy we investigate the effects of introducing a high-energy cut-off in the accelerated proton distribution. With cut-off energies of around 0.5–0.8 GeV and relatively hard spectra, the observed intensities and broadening can be reproduced with a single energetic proton distribution above the pion production threshold.  相似文献   

14.
The central compact object for some gamma-ray bursts (GRBs) may be a strongly magnetized millisecond pulsar. It can inject energy to the outer shock of the GRB by through the magnetic dipole radiation, and therefore causes the shallow decay of the early afterglow. Recently, from a large number of GRB X-ray afterglows observed by Swift/XRT(X-ray telescope), it is revealed that many of them exhibit the shallow decay about 102∼104 s after the burst prompt emission. We have fitted the X-ray afterglow light curves of 11 GRBs by using the energy injection model of a magnetar with the rotation period in the millisecond order of magnitude. The obtained result shows the validity and universality of the magnetar energy injection model in explaining the shallow decay of afterglows, and simultaneously provides some constraints on the magnetic field strength and rotation period of the central magnetar.  相似文献   

15.
Commencing at 0825 +3 –1 UT on January 28, 1967, a large and prolonged increase in the intensity of penetrating charged particles was observed by balloon-borne instruments floating over Byrd Station, Antarctica. (80°S, 120°W). A peak intensity of approximately 60 protons per cm2-secsteradian with E> 100 MeV occurred at about 1230 UT on the 28th. The event was under observation almost continuously over a period of about 100 hours until the intensity decayed below cosmic-ray background on February 1. The initial decay was rapid but, some 40 hours after onset, went over into a slow exponential decay characterized by a 20 hour time-constant. The decay phase of an additional, though considerably less intense, event was observed on February 3 and 4. Presumably both events had their origins in major disturbances on the far side of the sun since neither event has been definitely linked to any feature which existed on the visible disk within an appropriate time interval.Results pertaining to the time-intensity profile and to the energy spectrum for protons E> 100 MeV are presented for the January 28 event. Comparison of the balloon results with neutron-monitor and satellite measurements and with models of interplanetary diffusion has led to some conclusions regarding the role of small-angle scattering by irregularities and by the random walk of magnetic lines of force relative to the mean interplanetary field within the orbit of earth.  相似文献   

16.
This paper considers the torques causing spin decay in cylindrical rocket bodies in orbit. Eddy current torques, due to the Earth's magnetic field, are estimated using Smith's (1962) model—a hollow cylindrical conducting shell, tumbling about a transverse axis. Air torques are estimated by numerical integration of aerodynamic moments over the rocket surface. It is shown that for Cosmos rockets, 7.4 m long and 2.4 m in diameter, eddy current torques outweigh air torques by several orders of magnitude at altitudes near 500 km, and that they are dominant at altitudes down to 160 km. Visual observations of several such rockets illustrate a variation of spin decay time with altitude which supports this conclusion. The same observations suggest that a few Cosmos rockets may be special cases, different from the rest in construction.  相似文献   

17.
C. C. Harvey 《Solar physics》1976,46(2):509-509
An attempt is made to explain the observed frequency-time profiles of type III solar radiobursts in terms of a rapid plasma wave decay rate combined with the exciter model recently proposed by the author. The decay rate is assumed to be sufficiently rapid for the plasma wave energy density profile to be similar to the excitor power density time profile; this is consistent with the exciter model, the rapid decay being caused by Landau damping on the electrons of the modified high energy tail of the ambient plasma electron velocity distribution. The model is compared with radio observations by making simple assumptions about the dependence of the radio intensity upon the plasma wave energy. A comparison is made with simultaneous radio and electron observations by further assuming a simple power-law velocity distribution for the electrons at their point of ejection from the Sun.  相似文献   

18.
The Ulysses Unified Radio and Plasma Wave Experiment (URAP) has observed Langmuir, ion-acoustic and associated solar type III radio emissions in the interplanetary medium. Bursts of 50–300 Hz (in the spacecraft frame) electric field signals, corresponding to long-wavelength ion-acoustic waves are often observed coincident in time with the most intense Langmuir wave spikes, providing evidence for the electrostatic decay instability. Langmuir waves often occur as envelope solitons, suggesting that strong turbulence processes, such as modulational instability and soliton formation, often coexist with weak turbulence processes, such as electrostatic decay, in a few type III burst source regions.  相似文献   

19.
We present the first statistical analysis of 27 Ultraviolet Optical Telescope (UVOT) optical/ultraviolet light curves of gamma-ray burst (GRB) afterglows. We have found, through analysis of the light curves in the observer's frame, that a significant fraction rise in the first 500 s after the GRB trigger, all light curves decay after 500 s, typically as a power law with a relatively narrow distribution of decay indices, and the brightest optical afterglows tend to decay the quickest. We find that the rise could be either produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising light curve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8 per cent confidence, there is a correlation, in the observed frame, between the apparent magnitude of the light curves at 400 s and the rate of decay after 500 s. However, in the rest frame, a Spearman rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle θ or within a core of a uniform energy density  θc  . We also produced logarithmic luminosity distributions for three rest-frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT light curves with the X-ray Telescope (XRT) light-curve canonical model. The range in decay indices seen in UVOT light curves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model, there is no indication of the rising behaviour observed in the UVOT light curves.  相似文献   

20.
Observations on board Helios 1 and 2 have shown that, in 0.3–0.9 AU, the decay of the Alfvenic fluctuations is greater than is calculated by the WBK solution of the propagation of Alfven waves. The additional decay can be expressed by an exponential factor with a “dissipation length” λ, which varies with the frequency but is approximately constant for frequencies higher than 0.01 Hz. In this paper, we deduce the analytical expression for the dissipation length in both the wave energy cascade model, given in a previous work [7] and the viscous decay model [8]. After inserting the observed values of the solar wind parameters, we find the dissipation length given by the cascade model agrees with the observed length both in size and in its variation with frequency, while the one given by the viscous decay model differs greatly from the observations.  相似文献   

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