| 1. |
- Eshagh, Mehdi, 1977-, et al.
(författare)
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On the integral inversion of satellite-to-satellite velocity differences for local gravity field recovery : A theoretical study
- 2016
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Ingår i: Celestial mechanics & dynamical astronomy. - : Springer Science and Business Media LLC. - 0923-2958 .- 1572-9478. ; 124:2, s. 124-144
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Tidskriftsartikel (refereegranskat)abstract
- The gravity field can be recovered locally from the satellite-to-satellite velocity differences (VDs) between twin-satellites moving in the same orbit. To do so, three different integral formulae are derived in this paper to recover geoid height, radial component of gravity anomaly and gravity disturbance at sea level. Their kernel functions contain the product of two Legendre polynomials with different arguments. Such kernels are relatively complicated and it may be impossible to find their closed-forms. However, we could find the one related to recovering the geoid height from the VD data. The use of spectral forms of the kernels is possible and one does not have to generate them to very high degrees. The kernel functions are well-behaving meaning that they reduce the contribution of far-zone data and for example a cap margin of 7∘ is enough for recovering gravity anomalies. This means that the inversion area should be larger by 7∘ from all directions than the desired area to reduce the effect of spatial truncation error of the integral formula. Numerical studies using simulated data over Fennoscandia showed that when the distance between the twin-satellites is small, higher frequencies of the anomalies can be recovered from the VD data. In the ideal case of having short distance between the satellites flying at 250 km level, recovering radial component of gravity anomaly with an accuracy of 7 mGal is possible over Fennoscandia, if the VD data is contaminated only with the spatial truncation error, which is an ideal assumption. However, the problem is that the power of VD signal is very low when the satellites are close and it is very difficult to recognise the signal amongst the noise of the VD data. We also show that for a successful determination of gravity anomalies at sea level from an altitude of 250 km mean VDs with better accuracy than 0.01 mm/s are required. When coloured noise at this level is used for the VDs at 250 km with separation of 300 km, the accuracy of recovery will be about 11 mGal over Fennoscandia. In the case of using the real velocities of the satellites, the main problems are downward/upward continuation of the VDs on the mean orbital sphere and taking the azimuthal integration of them.
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| 2. |
- Petit, Antoine C.
(författare)
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An integrable model for first-order three-planet mean motion resonances
- 2021
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Ingår i: Celestial Mechanics and Dynamical Astronomy. - : Springer Science and Business Media LLC. - 0923-2958 .- 1572-9478. ; 133:8
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Tidskriftsartikel (refereegranskat)abstract
- Recent works on three-planet mean motion resonances (MMRs) have highlighted their importance for understanding the details of the dynamics of planet formation and evolution. While the dynamics of two-planet MMRs are well understood and approximately described by a one-degree-of-freedom Hamiltonian, little is known of the exact dynamics of three-body resonances besides the cases of zeroth-order MMRs or when one of the bodies is a test particle. In this work, I propose the first general integrable model for first-order three-planet mean motion resonances. I show that one can generalize the strategy proposed in the two-planet case to obtain a one-degree-of-freedom Hamiltonian. The dynamics of these resonances are governed by the second fundamental model of resonance. The model is valid for any mass ratio between the planets and for every first-order resonance. I show the agreement of the analytical model with numerical simulations. As examples of application, I show how this model could improve our understanding of the capture into MMRs as well as their role in the stability of planetary systems.
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| 3. |
- Queck, Martina, et al.
(författare)
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Velocity distributions among colliding asteroids
- 2007
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Ingår i: Celestial Mechanics and Dynamical Astronomy. - : Springer Science and Business Media LLC. - 0923-2958 .- 1572-9478. ; :99, s. 169-
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Tidskriftsartikel (refereegranskat)
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| 4. |
- Rickman, Hans, et al.
(författare)
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Injection of Oort Cloud comets : the fundamental role of stellar perturbations
- 2008
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Ingår i: Celestial mechanics & dynamical astronomy. - : Springer Science and Business Media LLC. - 0923-2958 .- 1572-9478. ; 102:1-3, s. 111-132
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Tidskriftsartikel (refereegranskat)abstract
- We present Monte Carlo simulations of the dynamical evolution of the Oort cloud over the age of the Solar System, using an initial sample of one million test comets without any cloning. Our model includes perturbations due to the Galactic tide (radial and vertical) and passing stars. We present the first detailed analysis of the injection mechanism into observable orbits by comparing the complete model with separate models for tidal and stellar perturbations alone. We find that a fundamental role for injecting comets from the region outside the loss cone (perihelion distance q > 15 AU) into observable orbits (q < 5 AU) is played by stellar perturbations. These act in synergy with the tide such that the total injection rate is significantly larger than the sum of the two separate rates. This synergy is as important during comet showers as during quiescent periods and concerns comets with both small and large semi-major axes. We propose different dynamical mechanisms to explain the synergies in the inner and outer parts of the Oort Cloud. We find that the filling of the observable part of the loss cone under normal conditions in the present-day Solar System rises from < 1% for a < 20 000 AU to about 100% for a greater than or similar to 100 000 AU.
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| 5. |
- Malmberg, Bo, et al.
(författare)
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Residential Segregation of European and Non-European Migrants in Sweden: 1990–2012
- 2018
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Ingår i: European Journal of Population. - : Springer Science and Business Media LLC. - 0168-6577 .- 1572-9885. ; 34:2, s. 169-193
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we analyse how a migrant population that is both expanding and changing in composition has affected the composition of Swedish neighbourhoods at different scales. The analysis is based on Swedish geocoded individual-level register data for the years 1990, 1997, 2005, and 2012. This allows us to compute and analyse the demographic composition of neighbourhoods that range in size from encompassing the nearest 100 individuals to the nearest 409,600 individuals. First, the results confirm earlier findings that migrants, especially those from non-European countries, face high levels of segregation in Sweden. Second, large increases in the non-European populations in combination with high levels of segregation have increased the proportion of non-European migrants living in neighbourhoods that already have high proportions of non-European migrants. Third, in contrast to what has been the established image of segregation trends in Sweden, and in an apparent contrast to the finding that non-European migrants increasingly live in migrant-dense neighbourhoods, our results show that segregation, when defined as an uneven distribution of different populations across residential contexts, is not increasing. On the contrary, for both European migrants from 1990 and non-European migrants from 1997, there is a downward trend in unevenness as measured by the dissimilarity index at all scale levels. However, if segregation is measured as differences in the neighbourhood concentration of migrants, segregation has increased.
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| 6. |
- Karimi, Fariba, 1981-, et al.
(författare)
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Cascades in real interbank markets
- 2016
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Ingår i: Computational Economics. - : Springer Science and Business Media LLC. - 0927-7099 .- 1572-9974. ; 47:1, s. 46-66
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Tidskriftsartikel (refereegranskat)abstract
- We analyze cascades of defaults in an interbank loan market. The novel feature of this study is that the network structure and the size distribution of banks are derived from empirical data. We find that the ability of a defaulted institution to start a cascade depends on an interplay of shock size and connectivity. Further results indicate that the interbank loan network is structurally less stable after the financial crisis than it was before. To evaluate the influence of the network structure on market stability, we compare simulated cascades from the empirical network with results from different network models. The results show that the empirical network has non-random features, which cannot be captured by randomized networks. The analysis also reveals that simulations that assume homogeneity for banks and loan size tend to overestimate the fragility of the interbank market.
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| 7. |
- Konstantopoulos, Takis
(författare)
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A review of Burke's theorem for Brownian motion
- 2016
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Ingår i: Queueing systems. - : Springer Science and Business Media LLC. - 0257-0130 .- 1572-9443. ; 83:1-2, s. 1-12
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Forskningsöversikt (refereegranskat)abstract
- Burke's theorem is a well-known fundamental result in queueing theory, stating that a stationary M/M/1 queue has a departure process that is identical in law to the arrival process and, moreover, for each time t, the following three random objects are independent: the queue length at time t, the arrival process after t and the departure process before t. Burke's theorem also holds for a stationary Brownian queue. In particular, it implies that a certain "complicated" functional derived from two independent Brownian motions is also a Brownian motion. The aim of this overview paper is to present an independent complete explanation of this phenomenon.
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