Browsing by Subject "CUSP"

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  • Kurylev, Yaroslav; Lassas, Matti; Uhlmann, Gunther (2018)
    We study two inverse problems on a globally hyperbolic Lorentzian manifold (M, g). The problems are: Passive observations in spacetime: consider observations in an open set . The light observation set corresponding to a point source at is the intersection of V and the light-cone emanating from the point q. Let be an unknown open, relatively compact set. We show that under natural causality conditions, the family of light observation sets corresponding to point sources at points determine uniquely the conformal type of W. Active measurements in spacetime: we develop a new method for inverse problems for non-linear hyperbolic equations that utilizes the non-linearity as a tool. This enables us to solve inverse problems for non-linear equations for which the corresponding problems for linear equations are still unsolved. To illustrate this method, we solve an inverse problem for semilinear wave equations with quadratic non-linearities. We assume that we are given the neighborhood V of the time-like path and the source-to-solution operator that maps the source supported on V to the restriction of the solution of the wave equation to V. When M is 4-dimensional, we show that these data determine the topological, differentiable, and conformal structures of the spacetime in the maximal set where waves can propagate from and return back to mu.
  • Järvinen, R.; Vainio, R.; Palmroth, M.; Juusola, L.; Hoilijoki, S.; Pfau-Kempf, Y.; Ganse, U.; Turc, L.; von Alfthan, S. (2018)
    We report ion acceleration by flux transfer events in the terrestrial magnetosheath in a global two-dimensional hybrid-Vlasov polar plane simulation of Earth's solar wind interaction. In the model we find that propagating flux transfer events created in magnetic reconnection at the dayside magnetopause drive fast-mode bow waves in the magnetosheath, which accelerate ions in the shocked solar wind flow. The acceleration at the bow waves is caused by a shock drift-like acceleration process under stationary solar wind and interplanetary magnetic field upstream conditions. Thus, the energization is not externally driven but results from plasma dynamics within the magnetosheath. Energetic proton populations reach the energy of 30 keV, and their velocity distributions resemble time-energy dispersive ion injections observed by the Cluster spacecraft in the magnetosheath.
  • Walsh, B. M.; Komar, C. M.; Pfau-Kempf, Y. (2017)
    Multispacecraft measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission are used to probe the spatial extent of an X line at the dayside magnetopause. A case study from 21 April 2014 is presented where two THEMIS spacecraft have a near-simultaneous encounter with the equatorial dayside magnetopause separated by 3.9 Earth radii. Both spacecraft observe similar steady inflow conditions with southward interplanetary magnetic field and a high magnetic shear angle at the magnetopause (133 degrees) boundary. One spacecraft observes clear fluid and kinetic signatures of active magnetic reconnection, while the other spacecraft does not observe reconnection. The predicted location of reconnection across the magnetopause is found using several theoretical models and a Block Adaptive Tree Solarwind Roe-type Upwind Scheme (BATS-R-US) MHD simulation. Each model predicts a continuous X line passing close to the two spacecraft, suggesting both would observe reconnection, if active. Using the constraints of the multipoint measurements, the extent or length L of the reconnection is estimated to be 2.4L <5.2h in local time or 6L<14 R-E.
  • Palmroth, M.; Laitinen, T. V.; Anekallu, C. R.; Pulkkinen, T. I.; Dunlop, M.; Lucek, E. A.; Dandouras, I. (2011)