Magnetic Field-aligned Currents Research Articles

A statistical study of space hurricanes in the Northern Hemisphere

The space hurricane is a newly discovered large-scale three-dimensional magnetic vortex structure that spans the polar ionosphere and magnetosphere. At the height of the ionosphere, it has a strong circular horizontal plasma flow with a nearly zero-flow center and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. By analyzing the long-term optical observation onboard the Defense Meteorological Satellite Program (DMSP) F16 satellite from 2005 to 2016, we found that space hurricanes in the Northern Hemisphere occur in summer and have a maximum occurrence rate in the afternoon sector around solar maximum. In particular, space hurricanes are more likely to occur in the dayside polar cap at magnetic latitudes greater than 80°, and their MLT (magnetic local time) dependence shows a positive relationship with the IMF (interplanetary magnetic field) clock angle. We also found that space hurricanes occur mainly under dominant positive IMF By and Bz and negative Bx conditions. It is suggested that the stable high-latitude lobe reconnection, which occurs under the conditions of a large Earth’s dipole tilt angle and high ionosphere conductivity in summer, should be the formation mechanism of space hurricanes. The result will give a better understanding of the solar wind–magnetosphere–ionosphere coupling process under northward IMF conditions.

Multiscale Features of Magnetic Field Fluctuations and Field-Aligned Currents in the Polar Ionosphere: A Case Study

Complexity is a typical feature of space plasmas that may involve the formation of multiscale coherent magnetic and plasma structures. The winding features (pseudo-polarization) of magnetic field fluctuations at different spatial scales are a useful quantity in this framework for investigating complexity in space plasma. Indeed, a strong link between pseudo-polarization, magnetic/plasma structures, turbulence and dissipation exists. We present some preliminary results on the link between the polarization of the magnetic field fluctuations and the structure of field-aligned currents in the high-latitude ionosphere. This study is based on high-resolution (50 Hz) magnetic field data collected on board the European Space Agency Swarm constellation. The results show the existence of a clear link between the multiscale coarse-grained structure of pseudo-polarization and intensity of the field-aligned currents, supporting the recent findings according to which turbulence may be capable of generating multiscale filamentary current structures in the auroral ionosphere. This feature is also examined theoretically, along with its significance for the rate of energy deposition and heating in the polar regions.

A space hurricane over the Earth\u2019s polar ionosphere

In Earth’s low atmosphere, hurricanes are destructive due to their great size, strong spiral winds with shears, and intense rain/precipitation. However, disturbances resembling hurricanes have not been detected in Earth’s upper atmosphere. Here, we report a long-lasting space hurricane in the polar ionosphere and magnetosphere during low solar and otherwise low geomagnetic activity. This hurricane shows strong circular horizontal plasma flow with shears, a nearly zero-flow center, and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. Near the center, precipitating electrons were substantially accelerated to ~10 keV. The hurricane imparted large energy and momentum deposition into the ionosphere despite otherwise extremely quiet conditions. The observations and simulations reveal that the space hurricane is generated by steady high-latitude lobe magnetic reconnection and current continuity during a several hour period of northward interplanetary magnetic field and very low solar wind density and speed.

3D Magnetic Holes in Collisionless Plasmas

Recent multispacecraft observations in the Earth’s magnetosphere have revealed an abundance of magnetic holes—localized magnetic field depressions. These magnetic holes are characterized by the plasma pressure enhancement and strongly localized currents flowing around the hole boundaries. There are several numerical and analytical models describing 2D configurations of magnetic holes, but the 3D distribution of magnetic fields and electric currents is studied poorly. Such a 3D magnetic field configuration is important for accurate investigation of charged particle dynamics within magnetic holes. Moreover, the 3D distribution of currents can be used for distant probing of magnetic holes in the magnetosphere. In this study, a 3D magnetic hole model using the single-fluid approximation and a spatial scale hierarchy with the distinct separation of gradients is developed. It is shown that such 3D holes can be obtained as a generalization of 1D models with the plasma pressure distribution adopted from the kinetic approach. The proposed model contains two magnetic field components and field-aligned currents. The magnetic field line configuration resembles the magnetic trap where hot charged particles bounce between mirror points. However, the approximation of isotropic pressure results in a constant plasma pressure along magnetic field lines, and the proposed magnetic hole model does not confine plasma along the field direction.

Wave Phenomena and Beam-Plasma Interactions at the Magnetopause Reconnection Region.

This paper reports on Magnetospheric Multiscale observations of whistler mode chorus and higher‐frequency electrostatic waves near and within a reconnection diffusion region on 23 November 2016. The diffusion region is bounded by crescent‐shaped electron distributions and associated dissipation just upstream of the X‐line and by magnetic field‐aligned currents and electric fields leading to dissipation near the electron stagnation point. Measurements were made southward of the X‐line as determined by southward directed ion and electron jets. We show that electrostatic wave generation is due to magnetosheath electron beams formed by the electron jets as they interact with a cold background plasma and more energetic population of magnetospheric electrons. On the magnetosphere side of the X‐line the electron beams are accompanied by a strong perpendicular electron temperature anisotropy, which is shown to be the source of an observed rising‐tone whistler mode chorus event. We show that the apex of the chorus event and the onset of electrostatic waves coincide with the opening of magnetic field lines at the electron stagnation point.

Electric potential structures of auroral acceleration region border from multi-spacecraft Cluster data

This paper studies an auroral event using data from three spacecraft of the Cluster mission, one inside and two at the poleward edge of the bottom of the Auroral Acceleration Region (AAR). The study reveals the three-dimensional profile of the region’s poleward boundary, showing spatial segmentation of the electric potential structures and their decay in time. It also depicts localized magnetic field variations and field-aligned currents that appear to have remained stable for at least 80 s. Such observations became possible due to the fortuitous motion of the three spacecraft nearly parallel to each other and tangential to the AAR edge, so that the differences and variations can be seen when the spacecraft enter and exit the segmentations, hence revealing their position with respect to the AAR.

Electron cyclotron maser instability (ECMI) in strong magnetic guide field reconnection

Abstract. The ECMI model of electromagnetic radiation from electron holes is shown to be applicable to spontaneous magnetic reconnection. We apply it to reconnection in strong current-aligned magnetic guide fields. Such guide fields participate only passively in reconnection, which occurs in the antiparallel components to both sides of the guide-field-aligned current sheets with current carried by kinetic Alfvén waves. Reconnection generates long (the order of hundreds of electron inertial scales) electron exhaust regions at the reconnection site X point, which are extended perpendicular to the current and the guide fields. Exhausts contain a strongly density-depleted hot electron component and have properties similar to electron holes. Exhaust electron momentum space distributions are highly deformed, exhibiting steep gradients transverse to both the reconnecting and guide fields. Such properties suggest application of the ECMI mechanism with the fundamental ECMI X-mode emission beneath the nonrelativistic guide field cyclotron frequency in localized source regions. An outline of the mechanism and its prospects is given. Potential applications are the kilometric radiation (AKR) in auroral physics, solar radio emissions during flares, planetary emissions and astrophysical scenarios (radiation from stars and compact objects) involving the presence of strong magnetic fields and field-aligned currents. Drift of the exhausts along the guide field maps the local field and plasma properties. Escape of radiation from the exhaust and radiation source region still poses a problem. The mechanism can be studied in 2-D particle simulations of strong guide field reconnection which favours 2-D, mapping the deformation of the electron distribution perpendicular to the guide field, and using it in the numerical calculation of the ECMI growth rate. The mechanism suggests also that reconnection in general may become a source of the ECMI with or without guide fields. This is of particular interest in extended turbulent plasmas where reconnection serves as an integral dissipation mechanism of turbulent energy in myriads of small-scale current filaments.

Magnetic field-aligned plasma currents in gravitational fields

Abstract. Analytical models are presented for currents along vertical magnetic field lines due to slow bulk electron motion in plasmas subject to a gravitational force. It is demonstrated that a general feature of this problem is a singularity in the plasma pressure force that develops at some finite altitude when a plasma that is initially in static equilibrium is set into slow motion. Classical fluid models thus do not allow general steady-state solutions for field-aligned currents. General solutions have to be non-stationary, varying on time scales of many periods of a plasma equivalent to the Brunt–Väisälä frequency. Except for very special choices of parameters, a steady-state solution exists only in an average sense. The conditions at large altitudes turn out to be extremely sensitive to even small changes in parameters at low altitudes. Low frequency fluctuations detected at large altitudes in the polar regions need not be caused by local low frequency instabilities, but merely reflect small fluctuations in conditions at low altitudes.

Low-frequency electrostatic waves in a magnetized, current-free, heavy negative ion plasma

AbstractWe report experimental observations of a low-frequency (≪ ion gyrofrequency) electrostatic wave mode in a magnetized cylindrical (Q machine) plasma containing positive ions, very few electrons and a relatively large fraction (n−/ne > 103) of heavy negative ions (m−/m+ ≈ 10), and no magnetic field-aligned current. The waves propagate nearly perpendicular to B with a multiharmonic spectrum. The maximum wave amplitude coincided spatially with the region of largest density gradient suggesting that the waves were excited by a drift instability in a nearly electron-free positive ion–negative ion plasma

ORIGIN OF ELECTRON CYCLOTRON MASER INDUCED RADIO EMISSIONS AT ULTRACOOL DWARFS: MAGNETOSPHERE-IONOSPHERE COUPLING CURRENTS

A number of ultra-cool dwarfs emit circularly polarised radio waves generated by the electron cyclotron maser instability. In the solar system such radio is emitted from regions of strong auroral magnetic field-aligned currents. We thus apply ideas developed for Jupiter's magnetosphere, being a well-studied rotationally-dominated analogue in our solar system, to the case of fast-rotating UCDs. We explain the properties of the radio emission from UCDs by showing that it would arise from the electric currents resulting from an angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by an extremely powerful analogue of the process which causes Jupiter's auroras. Such a velocity gradient indicates that these bodies interact significantly with their space environment, resulting in intense auroral emissions. These results strongly suggest that auroras occur on bodies outside our solar system.

Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating

Abstract. Time-dependent, three-dimensional numerical simulations of the reduced MHD model describing shear Alfvén waves in the magnetosphere provide an interesting prediction superficially similar to results of several ionospheric heating experiments conducted at high altitudes. In these experiments, heating of the ionospheric F-region with a constant/zero-frequency beam of HF waves causes luminous structures in the ionosphere in the form of a ring or a solid spot with a characteristic size comparable to the size of the heated spot. Simulations suggest that spots/rings or similar optical appearance might be associated with a magnetic field-aligned current system produced by the ionospheric heating. Two of the most interesting features of this current system are (1) strong localization across the ambient magnetic field and (2) distinctive non-symmetrical luminous signatures (ring/spot) in magnetically conjugate locations in the ionosphere.

Spontaneous Electromagnetic Emission from a Strongly Localized Plasma Flow

Laboratory observations of electromagnetic ion-cyclotron waves generated by a localized transverse dc electric field are reported. Experiments indicate that these waves result from a strong E×B flow inhomogeneity in a mildly collisional plasma with subcritical magnetic field-aligned current. The wave amplitude scales with the magnitude of the applied radial dc electric field. The electromagnetic signatures become stronger with increasing plasma β, and the radial extent of the power is larger than that of the electrostatic counterpart. Near-Earth space weather implications of the results are discussed.

The self-consistent parallel electric field due to electrostatic ion-cyclotron turbulence in downward auroral-current regions of the Earth’s magnetosphere. IV

The physical processes that determine the self-consistent electric field (E∥) parallel to the magnetic field have been an unresolved problem in magnetospheric physics for over 40 years. Recently, a new multimoment fluid theory was developed for inhomogeneous, nonuniformly magnetized plasma in the guiding-center and gyrotropic approximation that includes the effect of electrostatic, turbulent, wave-particle interactions (see Jasperse et al. [Phys. Plasmas 13, 072903 (2006); Jasperse et al., Phys. Plasmas13, 112902 (2006)]). In the present paper and its companion paper [Jasperse et al., Phys. Plasmas 17, 062903 (2010)], which are intended as sequels to the earlier work, a fundamental model for downward, magnetic field-aligned (Birkeland) currents for quasisteady conditions is presented. The model includes the production of electrostatic ion-cyclotron turbulence in the long-range potential region by an electron, bump-on-tail-driven ion-cyclotron instability. Anomalous momentum transfer (anomalous resistivity) by itself is found to produce a very small contribution to E∥; however, the presence of electrostatic, ion-cyclotron turbulence has a very large effect on the altitude dependence of the entire quasisteady solution. Anomalous energy transfer (anomalous heating and cooling) modifies the density, drift, and temperature altitude profiles and hence the generalized parallel-pressure gradients and mirror forces in the electron and ion momentum-balance equations. As a result, |E∥| is enhanced by nearly a factor of 40 compared to its value when turbulence is absent. The space-averaged potential increase associated with the strong double layer at the bottom of the downward-current sheet is estimated using the FAST satellite data and the multimoment fluid theory.

Current driven instability in collisional dusty plasmas

The current driven electromagnetic instability in a collisional, magnetized, dusty medium is considered in the present work. It is shown that in the presence of the magnetic field aligned current, the low-frequency waves in the medium can become unstable if the ratio of the current to the ambient field is larger than the light speed times the wave number. The growth rate of the instability depends upon the ratio of the Alfvén to the dust cyclotron frequency as well as on the ratio of the current density J to the dust charge density Zend, where Z is the number of electronic charge on the grain, e is the electron charge, and nd is the dust number density. The typical growth rate of this instability is on the order of Alfvén frequency which compares favorably with the electrostatic, cross-field current driven, Farley–Buneman instability and thus could play an important role in the Earth’s ionosphere.

Narrowing of the discrete auroral arc by the ionosphere

[1] We investigate the role of the ionosphere in the development of intense, narrow discrete auroral arcs. Our study shows that interactions between a pair of downward and upward magnetic field-aligned currents (FACs) and the ionosphere can lead to the narrowing of the upward current channel and broadening of the downward current channel such that the total width of the initial current pair remains the same. In this case the intensity of the upward current increases, and the intensity of the large-scale downward current decreases. Conditions promoting this asymmetry between the upward and downward FACs include low ionospheric conductivity (≤1 mho) and a moderate magnitude of the current density (<5 μA/m 2 ). Simulations show that the ionosphere causes significant asymmetry not only in the structure and amplitude of the currents but also in the structure and amplitude of the corresponding parallel electric fields. The dynamics of the parallel electric field in the upward current channel is similar to the dynamics of the current itself, but the dynamics of the field in the downward current channel differs from the dynamics of the current. The major difference is that the width of the downward current channel becomes broader with time but the scale size of the parallel electric field becomes smaller inside the channel. This effect is quite important for understanding the parallel electron acceleration in the auroral zone. In particular, it may explain why the so-called black auroral arcs, which are produced by the electrons flowing along the magnetic field lines from the ionosphere, always appear in the form of very narrow, discrete dark lines.

The global morphology of wave Poynting flux: powering the aurora.

Large-scale, electric currents flowing along magnetic field lines into the polar regions of Earth are thought to be the main contributors of the energy that powers the ionospheric aurora. However, we have found evidence for global contributions from electromagnetic waves (Alfvén waves). Data that were collected from the Polar spacecraft over the course of 1 year show that the flow of wave electromagnetic energy at altitudes of 25,000 to 38,000 kilometers delineates the statistical auroral oval. The Poynting flux of individual events distributed along the auroral oval was larger than 5 ergs per square centimeter per second, which is sufficient to power auroral acceleration processes. This evidence suggests that in addition to magnetic field-aligned currents, the dayside and nightside aurora is globally powered by the energy flow of these high-altitude Alfvén waves.

Jovian auroral signature of Io's corotational wake

Auroral images of Jupiter reveal a distinctive signature of the downstream corotational wake of Io, as well as of Io itself. The wake signature can be understood as a product of the plasma current that accelerates Io's mass‐loaded flux tube up to (almost) corotation with Jupiter. The radially directed acceleration current in the magnetosphere is linked by Birkeland (magnetic field aligned) currents to Jupiter's ionosphere where closure is provided by a meridional Pedersen current. The Pedersen current is proportional to the velocity of the plasma relative to the corotating frame, while the acceleration current is proportional to (minus) the time rate of change of that velocity. Thus the requirement of current closure implies a simple exponential decay of the relative velocity, with a time constant that is proportional to the ratio of flux‐tube mass content to ionospheric Pedersen conductance. For expected values of these parameters, the time constant is ∼ 0.5 hours, implying a length scale ∼ 1 RJ (= Jupiter's radius) for the variation of velocity with downstream distance in the wake, or ∼12° of longitude. This length scale is remarkably close to that of the observed brightness of the wake auroral signature.

Acceleration of charged particle fluxes by turbulent electrostatic solitary structures in auroral plasma

A turbulent acceleration of charged particle fluxes near a substantial magnetic field-aligned electric current is considered. Conditions in the auroral magnetosphere are known to be favourable for an excitation of plasma turbulence in a form of oscillating solitary electrostatic structures. The model of turbulent structures and its statistics is selected to be confirmed with known results of experimental observation and numerical calculations. The solution of stationary kinetic equation for particle fluxes in turbulent nonuniform layer is obtained. It is shown that electron fluxes accelerated by turbulent electrostatic structures move almost along the magnetic field while ion distributions have considerable perpendicular dispersion. Conditions of a formation of particle beams with selected energies are determined. Obtained theoretical results are in a qualitative agreement with the known experimental observations of particle fluxes up to energies of several keV in the disturbed auroral magnetosphere.

The importance of conductivity gradients in ground-based field-aligned current studies

Abstract Magnetosphere-ionosphere coupling is achieved primarily through magnetic field-aligned currents. Such currents are difficult to measure directly and are usually inferred from satellite magnetometer recordings or from ground-based measurements of the divergence of ionospheric electric fields. The latter technique requires a knowledge of the ionospheric conductance distribution. Although it is possible to obtain the ionospheric electric field distribution over large spatial areas with good temporal resolution from coherent backscatter radars, these instruments cannot measure conductivity. Since the equation for computing field-aligned currents explicitly requires the gradient in conductance to be known, the use of statistically averaged models is excluded for case studies. If a dense enough array of magnetometers is available, these data may be used in combination with radar data to produce a measured conductance distribution within the overlapping fields of view. This has been done for data obtained in northern Scandinavia. Comparing field-aligned currents, computed with and without knowing the ionospheric conductance distribution, shows that gradients in conductance can not be ignored, even for quiet geomagnetic conditions.

Statistical average estimates of high latitude field-aligned currents from the STARE and SABRE coherent VHF radar systems

Abstract Two bistatic VHF radar systems, STARE and SABRE, have been employed to estimate ionospheric electric fields in the geomagnetic latitude range 61.1 – 69.3° (geographic latitude range 63.8 – 72.6°) over northern Scandinavia. 173 days of good backscatter from all four radars have been analysed during the period 1982 to 1986, from which the average ionospheric divergence electric field versus latitude and time is calculated. The average magnetic field-aligned currents are computed using an AE-dependent empirical model of the ionospheric conductance. Statistical Birkeland current estimates are presented for high and low values of the Kp and AE indices as well as positive and negative orientations of the IMF B z component. The results compare very favourably to other ground-based and satellite measurements.