Plasma Pressure Gradient Research Articles

Features of the Night-Time F-Region Currents Over Equatorial Africa (August, 2001)

In this paper, we present the features of the equatorial F-region current systems over equatorial Africa during nighttime on 05 th August, 2001 observed at midnight. The observations have inferred from in situ CHAMP satellite measurements of the vector magnetic field. The magnitudes of the current along all the three magnetic field components of the Earth’s magnetic field varied considerably. The Bx-component recorded a diamagnetic effect of about 8nT, By-component recorded a current density of 5mA/m, and Bz-component reached a value of 3mA/m. The F-region dynamo, gravity dynamo and plasmapressure gradients are the possible mechanisms for the occurrence of these currents. The signatures confined to the equatorial region bounded by the Appleton anomaly.

First observation of the anomalous electric field in the topside ionosphere by ionospheric modification over EISCAT

AbstractWe have developed an active ground‐based technique to estimate the steady state field‐aligned anomalous electric field (E*) in the topside ionosphere, up to ~600 km, using the European Incoherent Scatter (EISCAT) ionospheric modification facility and UHF incoherent scatter radar. When pumping the ionosphere with high‐power high‐frequency radio waves, the F region electron temperature is significantly raised, increasing the plasma pressure gradient in the topside ionosphere, resulting in ion upflow along the magnetic field line. We estimate E* using a modified ion momentum equation and the Mass Spectrometer Incoherent Scatter model. From an experiment on 23 October 2013, E* points downward with an average amplitude of ~1.6 μV/m, becoming weaker at higher altitudes. The mechanism for anomalous resistivity is thought to be low‐frequency ion acoustic waves generated by the pump‐induced flux of suprathermal electrons. These high‐energy electrons are produced near the pump wave reflection altitude by plasma resonance and also result in observed artificially induced optical emissions.

On the mechanism of the formation of magnetohydrodynamic vortices in the solar plasma

Based on the magnetohydrodynamic (MHD) equations for an incompressible conductive viscous fluid, the possible mechanism of the formation of giant MHD vortices recently discovered in the solar atmosphere (chromosphere) is analyzed. It is assumed that these vortices arise in the regions of the solar surface (photosphere) with ascending flows of hot plasma that arrives from the inner regions of the Sun as a result of thermal convection and is accelerated upward under the action of the chromospheric plasma pressure gradient. It is shown that, under the assumption of plasma incompressibility and flow continuity, the ascending plasma flows induce converging radial plasma flows, which create the convective and Coriolis nonlinear hydrodynamic forces due to the nonzero initial vorticity of the chromospheric plasma caused by Sun’s rotation. The combined action of these two forces leads to the exponential acceleration of the solid-body rotation of plasma inside the ascending flow, thereby creating a vortex that generates an axial magnetic field, in agreement with astrophysical observations.

Three‐dimensional lunar wake reconstructed from ARTEMIS data

AbstractData from the two‐spacecraft Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun mission to the Moon have been exploited to characterize the lunar wake with unprecedented fidelity. The differences between measurements made by a spacecraft in the solar wind very near the Moon and concurrent measurements made by a second spacecraft in the near lunar wake are small but systematic. They enabled us to establish the perturbations of plasma density, temperature, thermal, magnetic and total pressure, field, and flow downstream of the Moon to distances of 12 lunar radii (RM). The wake disturbances are initiated immediately behind the Moon by the diamagnetic currents at the lunar terminator. Rarefaction waves propagate outward at fast MHD wave velocities. Beyond ~6.5 RM, all plasma and field parameters are poorly structured which suggests the presence of instabilities excited by counter‐streaming particles. Inward flowing plasma accelerated through pressure gradient force and ambipolar electric field compresses the magnetic field and leads to continuous increase in magnitude of magnetic perturbations. Besides the downstream distance, the field perturbation magnitude is also a function of the solar wind ion beta and the angle between the solar wind and the interplanetary magnetic field (IMF). Both ion and electron temperatures increase as a consequence of an energy dispersion effect, whose explanation requires fully kinetic models. Downstream of the Moon, the IMF field lines are observed to bulge toward the Moon, which is unexpected and may be caused by a plasma pressure gradient force or/and the pickup of heavy charged dust grains behind the Moon.

Estimation of the current density and analysis of the geometry of the current system surrounding the Earth

The results of an investigation of the distribution of plasma pressure, pressure gradients, and magnetic field near the equatorial plane in the plasma ring surrounding the Earth under magneto-quiet conditions are presented. Observational data obtained during the international THEMIS mission are used. The picture of the distribution of transverse-current density near the equatorial plane was obtained under assumption of observing the magnetostatic balance condition at geocentric distances from 6 to 12RE. In estimating the integral transverse current it was accepted that in daytime sector the magnetic-field minima on magnetic field lines are not localized in the equatorial plane. Estimates of the integral transverse current were obtained, which demonstrate the possibility of closing nighttime transverse currents at geocentric distances of up to ∼12RE inside the magnetosphere, which form a high-latitudinal continuation of the ring current.

Turbulence-induced diffusion analysis of national Spherical Torus Experiment based on the gyrocenter shift

The radial current generated by ion-neutral momentum exchange is suggested to be one of the methods for generating the radial electric field (E r ), the turbulence transport, and the low-confinement-mode (L-mode) to high-confinement-mode (H-mode) transitions at the edge of tokamak plasmas. In this analysis of the gyrocenter shift, the plasma pressure gradient, the neutral density gradient and the neutral velocity are the major driving mechanisms of the radial current and the electric field. When there is turbulence, small-scale $$\tilde E \times B$$ eddies induce cross-field transport. The confinement time of the National Spherical Torus Experiment is compared with the density fluctuation level to verify the turbulence-induced energy diffusion coefficient from the theory of the gyrocenter shift.

Non-symmetric magnetohydrostatic equilibria: a multigrid approach

Aims. Linear magnetohydrostatic (MHS) models of solar magnetic fields balance plasma pressure gradients, gravity and Lorentz forces where the current density is composed of a linear force-free component and a cross-field component that depends on gravitational stratification. In this paper, we investigate an efficient numerical procedure for calculating such equilibria. Methods. The MHS equations are reduced to two scalar elliptic equations - one on the lower boundary and the other within the interior of the computational domain. The normal component of the magnetic field is prescribed on the lower boundary and a multigrid method is applied on both this boundary and within the domain to find the poloidal scalar potential. Once solved to a desired accuracy, the magnetic field, plasma pressure and density are found using a finite difference method. Results. We investigate the effects of the cross-field currents on the linear MHS equilibria. Force-free and non-force-free examples are given to demonstrate the numerical scheme and an analysis of speed-up due to parallelization on a graphics processing unit (GPU) is presented. It is shown that speed-ups of x30 are readily achievable.

ELM control strategies and tools: status and potential for ITER

Operating ITER in the reference inductive scenario at the design values of Ip = 15 MA and QDT = 10 requires the achievement of good H-mode confinement that relies on the presence of an edge transport barrier whose pedestal pressure height is key to plasma performance. Strong gradients occur at the edge in such conditions that can drive magnetohydrodynamic instabilities resulting in edge localized modes (ELMs), which produce a rapid energy loss from the pedestal region to the plasma facing components (PFC). Without appropriate control, the heat loads on PFCs during ELMs in ITER are expected to become significant for operation in H-mode at Ip = 6–9 MA; operation at higher plasma currents would result in a very reduced life time of the PFCs.Currently, several options are being considered for the achievement of the required level of ELM control in ITER; this includes operation in plasma regimes which naturally have no or very small ELMs, decreasing the ELM energy loss by increasing their frequency by a factor of up to 30 and avoidance of ELMs by actively controlling the edge with magnetic perturbations. Small/no ELM regimes obtained by influencing the edge stability (by plasma shaping, rotational shear control, etc) have shown in present experiments a significant reduction of the ELM heat fluxes compared to type-I ELMs. However, so far they have only been observed under a limited range of pedestal conditions depending on each specific device and their extrapolation to ITER remains uncertain. ELM control by increasing their frequency relies on the controlled triggering of the edge instability leading to the ELM. This has been presently demonstrated with the injection of pellets and with plasma vertical movements; pellets having provided the results more promising for application in ITER conditions. ELM avoidance/suppression takes advantage of the fact that relatively small changes in the pedestal plasma and magnetic field parameters seem to have a large stabilizing effect on large ELMs. Application of edge magnetic field perturbation with non-axisymmetric fields is found to affect transport at the plasma edge and thus prevent the uncontrolled rise of the plasma pressure gradients and the occurrence of type-I ELMs. This paper compiles a brief overview of various ELM control approaches, summarizes their present achievements and briefly discusses the open issues regarding their application in ITER.

Thermospheric atomic oxygen density estimates using the EISCAT Svalbard Radar

AbstractCoupling between the ionized and neutral atmosphere through particle collisions allows an indirect study of the neutral atmosphere through measurements of ionospheric plasma parameters. We estimate the neutral density of the upper thermosphere above ~250 km with the European Incoherent Scatter Svalbard Radar (ESR) using the year‐long operations of the International Polar Year from March 2007 to February 2008. The simplified momentum equation for atomic oxygen ions is used for field‐aligned motion in the steady state, taking into account the opposing forces of plasma pressure gradients and gravity only. This restricts the technique to quiet geomagnetic periods, which applies to most of the International Polar Year during the recent very quiet solar minimum. The method works best in the height range ~300–400 km where our assumptions are satisfied. Differences between Mass Spectrometer and Incoherent Scatter and ESR estimates are found to vary with altitude, season, and magnetic disturbance, with the largest discrepancies during the winter months. A total of 9 out of 10 in situ passes by the CHAMP satellite above Svalbard at 350 km altitude agree with the ESR neutral density estimates to within the error bars of the measurements during quiet geomagnetic periods.

The dependence of magnetic reconnection on plasma β and magnetic shear: Evidence from magnetopause observations

We have performed a statistical study of THEMIS spacecraft crossings of the asymmetric dayside magnetopause to test the prediction that the diamagnetic drift of the X‐line due to a plasma pressure gradient across the magnetopause can suppress magnetic reconnection. The study includes crossings both when reconnection exhausts were present and when they were absent in the current sheet. When we restrict the survey to the subsolar region (10 < MLT < 14), we find that for low Δβ (the difference of plasma β on the two sides of the current sheet) the majority of reconnection events occurred over a large range of magnetic shears, whereas when Δβ was high reconnection events occurred only for high shears. Furthermore, nonreconnection events occurred primarily in the Δβ‐shear regime in which reconnection is predicted to be suppressed, in good agreement with theory. The Δβ‐shear condition should have general consequences for the occurrence of reconnection in space and laboratory plasmas.

The influence of hemispheric asymmetries on field‐aligned ion drifts at the geomagnetic equator

In the topside ionosphere, equatorial field‐aligned ion drifts are driven by field‐aligned gradients in the plasma pressure. These gradients form due to hemispheric asymmetries in plasma temperature and density, which are caused by the sun and the neutral atmosphere. The role of the neutral atmosphere in creating a field‐aligned plasma pressure gradient and driving ion drifts across the geomagnetic equator is examined for several local times during solar minimum. Using satellite observations and complimentary modeled data, the field‐aligned ion drifts are found to correlate with the vertical displacement between conjugate ion density peaks in both the observed and modeled data sets. In the modeled data sets, an even stronger correlation is seen between the field‐aligned ion drift and the total neutral wind along the magnetic meridian at the height of the ion density peak. This correlation is seen to improve in the absence of anE × B drift.

Plasma confinement regimes and collective modes characterizing them

A unified theory is presented for the modes that are excited at the edge of the plasma column and are important signatures of the advanced confinement regimes into which magnetically confined plasmas can be driven. In particular, the so-called EDA H-Regime, the Elmy H-Regime, and the I-Regime are considered. The modes that are identified theoretically have characteristics that are consistent with or have anticipated those of the modes observed experimentally for each of the investigated regimes. The phase velocities, the produced transport processes, the frequencies, the wavelengths, and the consistency with the direction of spontaneous rotation are the factors considered for comparison with the relevant experiments. The quasi-coherent mode [I. Cziegler, Ph.D. dissertation, Massachusetts Institute of Technology, Cambridge, MA, 2011] that is present in the EDA H-Regime has a phase velocity in the direction of the ion diamagnetic velocity in the plasma reference frame. Consequently, this is identified as a ballooning mode near finite Larmor radius marginal stability involving the effects of transverse ion viscosity and other dissipative effects. In this regime, impurities are driven outward by the combined effects of the local temperature gradients of the impurities and their thermal conductivity, while in the Elmy H-Regime impurities are driven toward the center of the plasma column. In the I-Regimes, the excited “Heavy Particle” modes [B. Coppi and T. Zhou, Phys. Plasmas 19, 012302 (2012); Phys. Lett. A 375, 2916 (2011)] are not of the ballooning kind and are shown to expel the impurities toward the plasma edge in the presence of significant fluctuations. These modes can have a finite frequency of oscillation with a phase velocity in the direction of the electron diamagnetic velocity or they can be nearly purely growing, explaining why there are I-Regimes where fluctuations are not observed. Instead, the modes considered for the Elmy H-Regime are of the ballooning kind. They are driven by the combined effects of the plasma pressure gradient and the magnetic field curvature, are close to conditions under which the acquired growth rates are proportional to half power of the relevant dissipation parameters, involve the effects of finite magnetic diffusivity and finite electron thermal conductivity, and can have phase velocities in either direction.

Characteristics of plasma ring, surrounding the Earth at geocentric distances ∼7–10RE, and magnetospheric current systems

There are strong experimental evidences of the existence of plasma domain forming a closed plasma ring around the Earth at geocentric distances ∼7–10RE. In this work, we analyze the main properties of this ring, using the data of the THEMIS satellite mission, acquired between April 2007 and September 2011. We also analyze the contribution of this ring to the storm dynamics. In particular, it is shown that the distribution of plasma pressure at ∼7–10RE is nearly azimuthally symmetric. However, the daytime compression of the magnetic field lines and the shift of the minimal value of the magnetic field to higher latitudes lead to the spreading of the transverse current along field lines and splitting of the daytime integral transverse current into two branches in Z direction. The CRC is the high latitude continuation of the ordinary ring current (RC), generated by plasma pressure gradients, directed to the Earth. We evaluated the contribution of the azimuthally symmetric part of the plasma ring to the Dst index for strong geomagnetic storms using the AMPTE/CCE radial profiles of plasma pressure published before, and showed that the contribution of the ring current including both RC and CRC is sufficient to obtain the observed Dst variation without the necessity to include the tail current system.

Sources of low‐latitude ionospheric E × B drifts and their variability

The complete mechanism of how upward propagating tropospheric tides connect to the upper atmosphere is not yet fully understood. One proposed mechanism is via ionospheric wind dynamo. However, other sources can potentially alter the vertical E × B drift: gravity and plasma pressure gradient driven current, the geomagnetic main field, and longitudinal variation in the conductivities. In this study we examine the contribution to the vertical drift from these sources, and compare them. We use March equinox results from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model. We found that the gravity and plasma pressure gradient driven current and the longitudinal variation of the conductivities excluding the variation due to the geomagnetic main field do not change the longitudinal variation of the vertical drift significantly. Modifying the geomagnetic main field will change the vertical drift at 5–6 LT, 18–19 LT and 23–24 LT at almost all longitudes. In general the influence of the geomagnetic main field on the vertical drift is larger, with respect to the maximum difference, at 18–19 LT and 23–24 LT, equal at 5–6 LT, and smaller at 14–15 LT than the influence due to nonmigrating tidal components in the neutral winds. Examination of the contribution from E‐ and F‐region neutral winds to the vertical drift shows that their importance depends on the local time and the solar activity. This implies that the vertical drift has to be analyzed at specific local times to examine the relation between the wave number in the vertical drift and in the neutral winds.

Solar Force-free Magnetic Fields

The structure and dynamics of the solar corona is dominated by the magnetic field. In most areas in the corona magnetic forces are so dominant that all non-magnetic forces like plasma pressure gradient and gravity can be neglected in the lowest order. This model assumption is called the force-free field assumption, as the Lorentz force vanishes. This can be obtained by either vanishing electric currents (leading to potential fields) or the currents are co-aligned with the magnetic field lines. First we discuss a mathematically simpler approach that the magnetic field and currents are proportional with one global constant, the so-called linear force-free field approximation. In the generic case, however, the relation between magnetic fields and electric currents is nonlinear and analytic solutions have been only found for special cases, like 1D or 2D configurations. For constructing realistic nonlinear force-free coronal magnetic field models in 3D, sophisticated numerical computations are required and boundary conditions must be obtained from measurements of the magnetic field vector in the solar photosphere. This approach is currently of large interests, as accurate measurements of the photospheric field become available from ground-based (for example SOLIS) and space-born (for example Hinode and SDO) instruments. If we can obtain accurate force-free coronal magnetic field models we can calculate the free magnetic energy in the corona, a quantity which is important for the prediction of flares and coronal mass ejections. Knowledge of the 3D structure of magnetic field lines also help us to interpret other coronal observations, e.g., EUV-images of the radiating coronal plasma.

The ionospheric gravity and diamagnetic current systems

[1] Large-scale currents in the ionosphere are driven by a variety of sources, including neutral winds, gravity, and plasma pressure gradients. While the stronger daytime wind-driven currents have been extensively studied, gravity and diamagnetic currents in the ionosphere have received very little attention but can have substantial effects even during the night. With the availability of a new generation of magnetic field models based on high-accuracy satellite magnetic measurements, it becomes increasingly important to account for these smaller current systems. In this work, we use the stand-alone NCAR TIEGCM electrodynamics solver along with empirical density, wind, and temperature inputs to model the global current systems caused by gravity and diamagnetism in the F region ionosphere and calculate their magnetic perturbations. These results allow us for the first time to visualize the global structure of these currents and quantify their magnetic perturbations. We find a significantly higher gravity-driven current during the night than one would expect from the lower conductivity which is primarily due to a Pedersen current driven by polarization charges in the predawn sector. We find some discrepancies between the diamagnetic perturbation and a theoretical prediction which could be a result of magnetic tension due to the curvature of the geomagnetic field lines. These results will allow geomagnetic field modelers to account for these important current systems and create more accurate models. This work will also be crucial in analyzing ionospheric magnetic field measurements from upcoming satellite missions such as Swarm.

Relations between multiple auroral streamers, pre-onset thin arc formation, and substorm auroral onset

[1] Recent ground-based imager observations have provided evidence of precursor auroral activity leading to substorm auroral onset, where the precursor is initiated by a poleward boundary intensification (PBI) followed by an auroral streamer moving equatorward toward the onset latitude leading to substorm auroral onset. However, since many streamers do not lead to substorms, the question arises as to what conditions are required for streamers to lead to onset. Using 382 events detected by the THEMIS all-sky imagers during 2007–2009, we examined the properties of latitudinally thin, quiet arcs that eventually break up during the substorm auroral onset and the relationship of such quiet arcs to streamers. We found that a pre-existing latitudinally thin quiet arc that leads to auroral onset is much brighter than prior thin arcs that do not lead to onset, and that streamers that do not lead to onset form or intensify such quiet arcs. The newly formed or intensified quiet arc remains bright for a few to tens of minutes (∼20 min on average) until a subsequent streamer leads to substorm auroral onset along the pre-existing arc. The pre-onset sequence proposed here suggests that both types of streamers, which do and do not lead to substorms, enhance auroral luminosity near the equatorward boundary of the oval, and that a sufficiently intense, quiet time thin arc near the poleward edge of proton precipitation, likely corresponding to a large plasma pressure gradient in the near-Earth plasma sheet, reflects important pre-conditions for a precursor flow burst to trigger substorm auroral onset.

Generation of a Sheared Plasma Rotation by Emission, Propagation, and Absorption of Drift Wave Packets

Collisional electron drift wave turbulence generates drift wave packet structures with density and vorticity fluctuations in the central plasma pressure gradient region of a linear plasma device. Tracking these packets reveals that they follow an outward directed spiral-shaped trajectory in the (r,θ) plane, are azimuthally stretched, and develop anisotropy as they approach an axisymmetric, radially sheared azimuthal flow located at the plasma boundary. Nonlinear energy transfer measurements and time-delay analysis confirm that structure absorption amplifies the sheared flow. Similar mechanisms likely operate at the edge of confined toroidal plasmas and should lead to the amplification of sheared flows at the boundary of these devices as well.

Near-Earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset

[1] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near-Earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region. We take advantage of two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R ∼10–12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1–5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.

The dayside magnetospheric boundary layer at Mercury

Magnetic field and plasma data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on the outbound portions of the first (M1) and second (M2) flybys of Mercury reveal a region of depressed magnetic field magnitude and enhanced proton fluxes adjacent to but within the magnetopause, which we denote as a dayside boundary layer. The layer was present during both encounters despite the contrasting dayside magnetic reconnection, which was minimal during M1 and strong during M2. The overall width of the layer is estimated to be between 1000 and 1400 km, spanning most of the distance from the dayside planetary surface to the magnetopause in the mid-morning. During both flybys the magnetic pressure decrease was ∼1.6 nPa, and the width of the inner edge was comparable to proton gyro-kinetic scales. The maximum variance in the magnetic field across the inner edge was aligned with the magnetic field vector, and the magnetic field direction did not change markedly, indicating that the change in field intensity was consistent with an outward plasma-pressure gradient perpendicular to the magnetic field. Proton pressures in the layer inferred from reduced distribution observations were 0.4 nPa during M1 and 1.0 nPa during M2, indicating either that the proton pressure estimates are low or that heavy ions contribute substantially to the boundary-layer plasma pressure. If the layer is formed by protons drifting westward from the cusp, there should be a strong morning–afternoon asymmetry that is independent of the interplanetary magnetic field (IMF) direction. Conversely, if heavy ions play a major role, the layer should be strong in the morning (afternoon) for northward (southward) IMF. Future MESSENGER observations from orbit about Mercury should distinguish between these two possibilities.