Torsten Neubert

Bio: Torsten Neubert is an academic researcher from Technical University of Denmark. The author has contributed to research in topics: Electron & Sprite (lightning). The author has an hindex of 35, co-authored 194 publications receiving 4364 citations. Previous affiliations of Torsten Neubert include Danish Space Research Institute & Georgia Institute of Technology.

Seasonal variations of high‐latitude field‐aligned currents inferred from Ørsted and Magsat observations

Abstract: [1] In this paper we report on field-aligned currents inferred from high-precision three-component geomagnetic field observations made on board the Danish satellite Orsted over polar regions. Because of a slow drift in local time of the satellite orbit through the “noon-midnight” sector, we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field-aligned current systems over the Northern and Southern Hemispheres. We find an average over-the-pole distance between dayside and nightside currents of 32° during summer but 37° during winter and 36° during equinox. The decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes. For comparison, the dawn-dusk cross-polar distance of the Region 1/Region 2 field-aligned currents has been determined from high-precision data observed by Magsat, a satellite flown in 1979–1980 in a “dawn-dusk” orbit. The latter results show that the dawn-dusk distance between R1/R2 currents exhibits little seasonal dependence and amounts to ∼34° for all seasons in both polar caps. The seasonal dependence is confirmed for the high-latitude field-aligned intensities; they are larger by a factor of 1.5–1.8 in the sunlit (summer) polar cap in comparison with the winter hemisphere. Our results suggest that the R1/R2 and dayside field-aligned currents are well balanced between the pairs of downward/upward currents for all seasons as well as between hemispheres during equinox. We were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity.

A new model of field‐aligned currents derived from high‐precision satellite magnetic field data

Abstract: [1] A model of field-aligned currents is derived from high-precision magnetic field measurements from the Orsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox. The high-precision data allow the model to be determined directly by a simple 2-D curl technique combined with fitting of spherical harmonic functions. New elements of the model are: (a) the FAC patterns are determined separately for both polar regions, resolving the seasonal dependence of interhemispheric asymmetries, (b) the IMF ∼ 0, ground-state patterns are also resolved; these elements are obtained for the first time. From the model, the total upward/downward currents have been determined for various IMF conditions. The ratio of the summer/winter currents is ∼1.35 and the equinox currents ∼1. The model allows FAC mapping for IMF |B| ≤ 12 nT, except during magnetic storms and substorms.

Generation of a Small-Scale Quasi-Static Magnetic Field and Fast Particles during the Collision of Electron-Positron Plasma Clouds

Abstract: We present the results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counterstreaming instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and shows that, in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted into magnetic field energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collisionless change of structure, leading to large-scale, long-living structures and the production of high-energy particles. These processes may be important for understanding the production of high-energy particles in the region where two pulsar winds collide.

Ørsted Initial Field Model

Abstract: Magnetic measurements taken by the Orsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

The Observation of Gravitational Waves from a Binary Black Hole Merger

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160) Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Femtosecond pulse shaping using spatial light modulators

Abstract: We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

International Geomagnetic Reference Field: the 12th generation

Abstract: The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic field.

Contemporary particle-in-cell approach to laser-plasma modelling

Abstract: Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects.

Overview of plasma-based accelerator concepts

Abstract: An overview is given of the physics issues relevant to the plasma wakefield accelerator, the plasma beat-wave accelerator, the laser wakefield accelerator, including the self-modulated regime, and wakefield accelerators driven by multiple electron or laser pulses. Basic properties of linear and nonlinear plasma waves are discussed, as well as the trapping and acceleration of electrons in the plasma wave. Formulas are presented for the accelerating field and the energy gain in the various accelerator configurations. The propagation of the drive electron or laser beams is discussed, including limitations imposed by key instabilities and methods for optically guiding laser pulses. Recent experimental results are summarized.