Torsten Neubert

Ørsted Initial Field Model

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 Earths 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.

Observations of the relationship between sprite morphology and in-cloud lightning processes

[1] During a thunderstorm on 23 July 2003, 15 sprites were captured by a LLTV camera mounted at the observatory on Pic du Midi in the French Pyrenees. Simultaneous observations of cloud-to-ground (CG) and intracloud (IC) lightning activity from two independent lightning detection systems and a broadband ELF/VLF receiver allow a detailed study of the relationship between electrical activity in a thunderstorm and the sprites generated in the mesosphere above. Results suggest that positive CG and IC lightning differ for the two types of sprites most frequently observed, the carrot- and column-shaped sprites. Column sprites occur after a short delay (<30 ms) from the causative +CG and are associated with little VHF activity, suggesting no direct IC action on the charge transfer process. On the other hand, carrot sprites are delayed up to about 200 ms relative to their causative +CG stroke and are accompanied by a burst of VHF activity starting 25–75 ms before the CG stroke. While column sprites associate with short-lasting (less than 30 ms) ELF/VLF sferics, carrot sprites associate with bursts of sferics initiating at the time of the causative +CG discharge and persisting for 50 to 250 ms, indicating extensive in-cloud activity. One carrot event was found to be preceded by vigorous IC activity and a strong, long-lived cluster of ELF/VLF sferics but lacking a +CG. The observations of ELF/VLF sferic clusters associated with lightning and sprites form the basis for a discussion of the reliability of lightning detection systems based on VHF interferometry.

Ørsted satellite captures high‐precision geomagnetic field data

Space-based, high-precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earths main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Orsted, Denmarks first satellite, was launched on February 23, 1999 into a polar, low-Earth orbit to provide the first near-global set of high-precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earths magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high-precision vector data since August 1999, and the mission is continuing well beyond its nominal 14-month lifetime into 2001.

A PIC-MCC code for simulation of streamer propagation in air

A particle code has been developed to study the distribution and acceleration of electrons in electric discharges in air. The code can follow the evolution of a discharge from the initial stage of a single free electron in a background electric field to the formation of an electron avalanche and its transition into a streamer. The code is in 2D axi-symmetric coordinates, allowing quasi 3D simulations during the initial stages of streamer formation. This is important for realistic simulations of problems where space charge fields are essential such as in streamer formation. The charged particles are followed in a Cartesian mesh and the electric field is updated with Poissons equation from the charged particle densities. Collisional processes between electrons and air molecules are simulated with a Monte Carlo technique, according to cross section probabilities. The code also includes photoionisation processes of air molecules by photons emitted by excited constituents. The paper describes the code and presents some results of streamer development at 70km altitude in the mesosphere where electrical discharges (sprites) are generated above severe thunderstorms and at ~10km relevant for lightning and thundercloud electrification. The code is used to study acceleration of thermal seed electrons in streamers and to understand the conditions under which electrons may reach energies in the runaway regime. This is the first study in air, with a particle model with realistic spatial dependencies of the electrostatic field. It is shown that at 1atm pressure the electric field must exceed ~7.5 times the breakdown field to observe runaway electrons in a constant electric field. This value is close to the field where the electric force on an electron equals the maximum frictional force on an electron - found at ~100eV. It is also found that this value is reached in a negative streamer tip at 10km altitude when the background electric field equals ~3 times the breakdown field. At higher altitudes, the background electric field must be relatively larger to create a similar field in a streamer tip because of increased influence of photoionisation. It is shown that the role of photoionization increases with altitude and the effect is to decrease the space charge fields and increase the streamer propagation velocity. Finally, effects of electrons in the runaway regime on negative streamer dynamics are presented. It is shown the energetic electrons create enhanced ionization in front of negative streamers. The simulations suggest that the thermal runaway mechanism may operate at lower altitudes and be associated with lightning and thundercloud electrification while the mechanism is unlikely to be important in sprite generation at higher altitudes in the mesosphere.

Sprites over Europe

Results are presented from the first European campaign for observation of sprites, conducted during the summer of 2000 from the French astronomical observatory, Observatoire Midi-Pyrenees. The primary objective was to establish if sprites are generated over Europe and to identify the characteristics of the associated thunderstorms. During the one-month campaign local weather conditions allowed observations approximately half of the nights. Sprites were observed two nights over the Alps and one night over southeastern France in connection with cold fronts moving in from the Atlantic. In all, 40 sprites were recorded, including dancing sprites, multiple carrot sprites and c-sprites. The weather conditions were almost identical during the 3 nights, with the active area forming on the front-side of the cold fronts. The storms are not of the same magnitude as active systems often observed over the North American plains. Even so, sprites seem to be a common occurrence also over Europe.

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

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.

Production of runaway electrons by negative streamer discharges

[1] In this paper we estimate the probability that cold electrons can be accelerated by an ambient electric field into the runaway regime, and discuss the implications for negative streamer formation. The study is motivated by the discovery of ms duration bursts of g‐rays from the atmosphere above thunderstorms, the so‐called Terrestrial Gamma‐Ray Flashes. The radiation is thought to be bremsstrahlung from energetic (MeV) electrons accelerated in a thunderstorm discharge. The observation goes against conventional wisdom that discharges in air are carried by electrons with energies below a few tens of eV. Instead the relativistic runaway electron discharge has been proposed which requires a lower threshold electric field; however, seed electrons must be born with energies in the runaway regime. In this work we study the fundamental problem of electron acceleration in a conventional discharge and the conditions on the electric field for the acceleration of electrons into the runaway regime. We use particle codes to describe the process of stochastic acceleration and introduce a novel technique that improves the statistics of the relatively few electrons that reach high energies. The calculation of probabilities for electrons to reach energies in the runaway regime shows that even with modest fields, electrons can be energized in negative streamer tips intothe runaway regime, creating abeamed distribution in front of thestreamer that affects its propagation. The results reported here suggest that theories of negative streamers and spark propagation should be reexamined with an improved characterization of the kinetic effects of electrons. Citation: Chanrion, O., and T. Neubert (2010), Production of runaway electrons by negative streamer discharges, J. Geophys. Res., 115, A00E32, doi:10.1029/2009JA014774.

Recent results from studies of electron beam phenomena in space plasmas

Abstract Experiments involving the ejection of beams of electrons from spacecraft have been performed for more than 2 decades in order to study fundamental plasma physical processes as well as for a range of diagnostic- and application-oriented purposes. This paper reviews some of the key issues that have been pursued in the past 10 years. These include questions regarding spacecraft charging and beam dynamics, the interaction of beams with neutral gas and plasmas, and the electromagnetic radiation generated by continuous and pulsed electron beams. It is shown how our understanding of these phenomena has matured, thereby providing a solid foundation for future experiments involving the use of electron beams.

A unified theory of ionosphere-plasmasphere transport of suprathermal electrons

The authors present a model of suprathermal electrons in the ionosphere and plasmasphere based on a solution to the kinetic equation along the entire length of a closed magnetic field line, that is, simultaneously for the two conjugate ionospheres and the plasmasphere. They call this the unified approach. It allows the determination of the distribution in energy and pitch-angle of photoelectrons along the complete length of the field line thereby avoiding the introduction of artificial boundaries between the ionosphere and magnetosphere and, consequently, avoids problems introduced by the uncertainty of these boundary conditions. In addition, it automatically accounts for back-scattered electrons in the atmosphere and plasmasphere, and avoids splitting photoelectrons into a loss-cone and a trapped population. The method is not limited to specific situations such as conjugated sunrise or symmetrical illumination of hemispheres, but is equally applicable to arbitrary illumination conditions. >

Solar wind-magnetosphere interaction as simulated by a 3-D EM particle code

We have studied the solar wind-magnetosphere interaction using a 3-D electromagnetic particle code. The results for an unmagnetized solar wind plasma streaming past a dipole magnetic field show the formation of a magnetopause and a magnetotail, the penetration of energetic particles into cusps and radiation belt and dawn-dusk asymmetries. The effects of interplanetary magnetic field (IMF) have been investigated in a similar way as done by MHD simulations. The simulation results with a southward IMF show the shrunk magnetosphere with great particle entry into the cusps and nightside magnetosphere. This is a signature of a magnetic reconnection at the dayside magnetopause. After a quasi-stable state is established with an unmagnetized solar wind we switched on a solar wind with an northward IMF. In this case the significant changes take place in the magnetotail. The waving motion was seen in the magnetotail and its length was shortened. This phenomena are consistent with the reconnections which occur at the high latitude magnetopause. In our simulations kinetic effects will determine the self-consistent anomalous resistivity in the magnetopause that causes reconnections.