Hermann Opgenoorth

Bio: Hermann Opgenoorth is an academic researcher from Umeå University. The author has contributed to research in topics: Ionosphere & Substorm. The author has an hindex of 31, co-authored 97 publications receiving 2783 citations. Previous affiliations of Hermann Opgenoorth include University of Leicester & Swedish Institute of Space Physics.

EISCAT observations of topside ionospheric ion outflows during auroral activity: Revisited

Abstract: New EISCAT observations of large field-aligned bulk ion outflows from the topside ionosphere during auroral activity are presented. The ions (mainly O+) start their outflows from a variable altitude and may reach field-aligned outward velocities of up to 1500 m s−1 in the altitude region 900–1500 km. The observed ion fluxes are about a factor of 10 larger than previously observed reaching 2×1014 m−2 s−1, and in some cases is nonconstant with altitude. Two different types of ion outflows have been identified. The first type is related to periods of strong perpendicular electric fields, enhanced and anisotropic ion temperatures, and low electron densities below 300 km, indicating small amounts of auroral precipitation. The second type is related to auroral arcs and enhanced electron temperatures. The exact mechanism causing the ion outflows is still not yet understood, but additional mechanisms other than thermal expansion are required to explain the observations presented here.

Three-dimensional current flow and particle precipitation in a westward travelling surge (observed during the barium-GEOS rocket experiment)

Abstract: During the Barium-GEOS rocket experiment on September 24, 1979 the passage of a westward travelling surge (WTS) was observed over Scandinavia. Extended instrument networks in that area, viz., the Scandinavian Magnetometer Array, the STARE radars, all-sky cameras, and riometers, allowed simultaneous observation of the two-dimensional character of magnetic and electric field disturbances and particle precipitation associated with the auroral feature. By combination of the different datasets and additional information from particle and electric field measurements aboard the Barium-GEOS rocket it was possible to derive the two-dimensional distribution of ionospheric electric fields and conductivities and to model the three-dimensional current flow in the vicinity of the westward travelling surge. The main feature of the resulting model current system is the presence of a localized upward field-aligned current directed out of the head of the surge and fed by a westward electrojet, which is composed of both Hall and Pedersen currents. Secondary ionospheric currents, e.g. a counterclockwise loop of mainly Hall currents around the leading edge of the surge, are found to be responsible for most of the transient effects observed by ground-based magnetometers as the WTS passed overhead. The most energetic particle precipitation as inferred from cosmic noise absorption measurements and triangulation of auroral arc altitudes is found to be confined to the leading part and central regions of the surge and to travel westward with the visual auroral form.

Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto: Comparison of its Forbush decreases at 1.4, 3.1, and 9.9 AU

Abstract: We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn.

Characteristics of eastward drifting omega bands in the morning sector of the auroral oval

Abstract: The STARE radars and the Scandinavian networks of magnetometers, all-sky cameras, and riometers recorded during the night of October 21/22, 1979, the occurrence of a fairly regular sequence of auroral omega bands and associated magnetic and electric field variations. The combined two-dimensional data are used to derive a realistic model for the three-dimensional current flow associated with the auroral forms. In the model calculations the observed structure in the particle precipitation is accounted for by an inhomogeneous ionospheric conductivity distribution. The main resulting feature of the model current system is a sequence of east–west orientated pairs of upward and downward directed field-aligned currents, associated with the bright and dark areas of the visual aurora, respectively. The major source of magnetic disturbances on the ground is a “meandering” ionospheric Hall current, composed of a westward background electrojet and circular Hall current vortices around the locations of the localized field-aligned currents. The total magnetic disturbance observed on the ground during different events appears, however, to be strongly dependent on the Hall to Pedersen conductivity ratio and the degree of inhomogeneity in the conductivity distribution. The three-dimensional current system associated with the auroral omega bands drifts eastward with a velocity comparable to an E×B drift within the general southward directed electric background field. However, complete agreement was not found at all times.

Neutral line model of substorms: Past results and present view

Abstract: The near-Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere-tail coupling, large-scale mapping, onset trigger- ing, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near-tail, midtail, and deep- tail features of substorms. Nonetheless, the NENL model still seems to provide the best avail- able framework for ordering the complex, global manifestations of substorms.

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

Abstract: The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.

The terrestrial ring current: Origin, formation, and decay

Abstract: The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ;10,000 - 60,000 km. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which are known as geomagnetic storms. Intense geomagnetic storms have severe effects on technological systems, such as disturbances or even permanent damage to tele- communication and navigation satellites, telecommuni- cation cables, and power grids. The main carriers of the storm ring current are positive ions, with energies from ;1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magneto- spheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while geospace activity tends to increase the abundance (both absolute and relative) of O 1 ions, which are of ionospheric origin. During intense magnetic storms, the O 1 abundance increases dramatically, resulting in a rapid intensification of the ring current and an O 1 dominance around storm maximum. This compositional change affects, among other processes, the decay of the ring current through the species- and energy-dependent charge exchange and wave-particle scattering loss. En- ergetic neutral atoms, products of charge exchange, en- able global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, the effects of composi- tional variations in the ring current, the effects of sub- storms on ring current growth, and the dynamics of ring current decay with an emphasis on the process of charge exchange and the potential for wave scattering loss.

Plasma sheet instability related to the westward traveling surge

Abstract: The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.

The cluster magnetic field investigation

Abstract: The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster