Corotation Electric Field Research Articles

Estimating the corotation lag of the plasmasphere based on the electric field measurements of the Van Allen Probes

The corotation electric field driving the plasmasphere to corotate with the Earth body is important in establishing the topology of the inner magnetosphere and is usually calculated by assuming a 24 h corotation period. However, studies have found that a plasmasphere corotation lag exists, suggesting an overestimation of the electric field driving the plasmasphere to rotate with the planet in previous calculations. In this study, we use electric field measurements from the Van Allen Probes mission from 2012 to 2018 to obtain the distribution of the large-scale electric field in the inner magnetosphere. A new method is developed to extract plasmaspheric rotation information from electric field measurements. Our results show that the electric field driving the plasmaspheric rotation varies with magnetic activity, decreasing with increasing Kp index. It is thus suggested that the corotation lag of the plasmasphere is more significant during magnetically active periods.

A Possible Dust Origin for an Unusual Feature in Io’s Sodium Neutral Clouds

Abstract We report the results of model simulations performed to explain the nature of a sodium emission feature in Io Neutral Clouds. The feature was detected via high-resolution spectroscopic observations from the 3.6 m Italian telescope TNG. The emission feature is blueshifted compared to the main emission (the banana-shaped Neutral Cloud of Io) by a few tens of km s−1, and it is most prominent when Io is a few tens of degrees before eclipse behind Jupiters shadow. The feature’s morphology changes with time, indicative of a geometrical effect. We constrained its direction, velocity, and column density with a model of sodium atom trajectories under the influence of Io’s and Jupiter’s gravity, and solar radiation pressure. The model that best explains this emission feature has the atoms injected into the exosphere from the leading/sub-Jovian hemisphere of Io (45°–68° west longitude), with velocities from 50 to 90 km s−1 relative to Io. These trajectories are consistent with those of negatively charged dust grains (radius ∼10 nm) accelerated by the corotational electric field of Jupiters magnetosphere. We propose that sputtering of sodium atoms from Na-bearing molecules (NaCl and Na2SO4) in these nanodust grains is the process responsible for our emission feature. Both modeling and observational constraints provide an order-of-magnitude estimate of the sodium production rate of ∼1026 s−1. Our work provides another method to monitor the amount of material that Io is supplying to its Neutral Clouds and plasma torus.

Interplanetary magnetic field control of plasma distribution in the polar ionosphere

We used the TEC (Total electron content) data of 5 min resolution obtained from the Madrigal database during solar-maximum winter (Nov. 6, 2000–Feb. 4, 2001) to study statistically the polar ionospheric plasma distribution response to different intensity and orientation of IMF By/Bz components. The sunlit high-density plasma extension from dayside to nightside is favored in negative IMF By and Bz conditions. With the magnitude of the negative Bz increasing, the time range corresponding to the distinct high-density extension feature expands, and the plasma density along the extension path enhances, which can be attributed to the interaction between dayside solar-produced ionization whose poleward limit is decided by terminator and convection extent mainly modulated by IMF Bz component. As for IMF By component influence on the sunlit plasma extension, the combination effect of convection and corotation electric fields is necessary to be considered.

The restrictions on the assumption about conservation of parameters of orbit for submicron particles in the Earth’s plasmasphere in light of the corotational electric field

The influence of the corotational electric field on the possibility of long holding of micro-particles with radii of the order of some hundredths of micrometers and quasi-equilibrium charge moving along weakly elliptic orbits in the plasmasphere of the Earth is considered by analytical and numerical methods. It is shown that, unlike the magnetic field, the corotational electric field causes a slow change in the shape of the orbit.

Energetic Electron Injections Deep Into the Inner Magnetosphere: A Result of the Subauroral Polarization Stream (SAPS) Potential Drop

AbstractIt has been reported that the dynamics of energetic (tens to hundreds of keV) electrons and ions is inconsistent with the theoretical picture in which the large‐scale electric field is a superposition of corotation and convection electric fields. Combining one year of measurements by the Super Dual Auroral Radar Network, DMSP F‐18, and the Van Allen Probes, we show that subauroral polarization streams (SAPSs) are observed when energetic electrons have penetrated below L = 4. Outside the plasmasphere in the premidnight region, potential energy is subtracted from the total energy of ions and added to the total energy of electrons during SAPS onset. This potential energy is converted into radial motion as the energetic particles drift around Earth and leave the SAPS azimuthal sector. As a result, energetic electrons are injected deeper than energetic ions when SAPSs are included in the large‐scale electric field picture, in line with observations.

A corotation electric field model of the Earth derived from Swarm satellite magnetic field measurements

AbstractRotation of the Earth in its own geomagnetic field sets up a primary corotation electric field, compensated by a secondary electric field of induced electrical charges. For the geomagnetic field measured by the Swarm constellation of satellites, a derivation of the global corotation electric field inside and outside of the corotation region is provided here, in both inertial and corotating reference frames. The Earth is assumed an electrical conductor, the lower atmosphere an insulator, followed by the corotating ionospheric E region again as a conductor. Outside of the Earth's core, the induced charge is immediately accessible from the spherical harmonic Gauss coefficients of the geomagnetic field. The charge density is positive at high northern and southern latitudes, negative at midlatitudes, and increases strongly toward the Earth's center. Small vertical electric fields of about 0.3 mV/m in the insulating atmospheric gap are caused by the corotation charges located in the ionosphere above and the Earth below. The corotation charges also flow outward into the region of closed magnetic field lines, forcing the plasmasphere to corotate. The electric field of the corotation charges further extends outside of the corotating regions, contributing radial outward electric fields of about 10 mV/m in the northern and southern polar caps. Depending on how the magnetosphere responds to these fields, the Earth may carry a net electric charge.

The plasmapause formation seen from meridian perspective by KAGUYA

AbstractObservations by the extreme ultraviolet (EUV) imager on board the IMAGE spacecraft revealed that the formation of a sharp plasmapause occurs in the postmidnight sector soon (<1 h) after the convection enhancement. These results cannot be explained simply by the conventional theory of the plasmapause formation that the plasmapause coincides with the last closed equipotential of the convection electric field superposed on the Earth's corotation electric field. However, due to the limitation that the EUV imager provides information on only the azimuthal distribution of the plasmapause, the formation mechanism still remains an open issue. Now global images of the plasmasphere from meridian perspective become available, thanks to the telescope of extreme ultraviolet (TEX) instrument on board the KAGUYA spacecraft. Here we studied the plasmapause formation mechanism by analyzing the sequential TEX images of an erosion event during the geomagnetic disturbance (Kp = 5) on 1–2 May 2008. The temporal evolution of the plasmapause locations at postmidnight observed by TEX agreed with those predicted by the dynamic simulations based on the interchange mechanism. Furthermore, the He+ column density in the nightside plasmasphere decreased by ~30% only at the low latitudes (<20°) during the enhanced convection period. This suggests that the plasmapause formation occurs first near the equatorial region during a geomagnetic disturbance, and it agrees with the plasmapause formation mechanism based on the interchange instability. Although we cannot conclude exclusively for the interchange mechanism, this is the first study to present the plasmapause formation viewed from the meridian perspective.

Survey of pickup ion signatures in the vicinity of Titan using CAPS/IMS

AbstractPickup ion detection at Titan is challenging because ion cyclotron waves are rarely detected in the vicinity of the moon. In this work, signatures left by freshly produced pickup heavy ions (m/q ∼ 16 to m/q ∼ 28) as detected in the plasma data by the Cassini Plasma Spectrometer/Ion Mass Spectrometer (CAPS/IMS) instrument on board Cassini are analyzed. In order to discern whether these correspond to ions of exospheric origin, one of the flybys during which the reported signatures were observed is investigated in detail. For this purpose, ion composition data from time‐of‐flight measurements and test particle simulations to constrain the ions' origin are used. After being validated, the detection method is applied to all the flybys for which the CAPS/IMS instrument gathered valid data, constraining the region around the moon where the signatures are observed. The results reveal an escape region located in the anti‐Saturn direction as expected from the nominal corotation electric field direction. These findings provide new constraints for the area of freshly produced pickup ion escape, giving an approximate escape rate of ions· s−1.

Two-dimensional MHD model of the Jovian magnetodisk

A self-consistent stationary axially symmetric MHD model of the Jovian magnetodisk is constructed. This model is a generalization of the models of plane current sheets that have been proposed earlier in order to describe the structure of the current sheet in the magnetotail of the Earth [1, 2]. The model takes centrifugal force, which is induced by the corotation electric field, and the azimuthal magnetic field into account. The configurations of the magnetic field lines for the isothermic (plasma temperature assumed to be constant) and the isentropic (plasma entropy assumed to be constant) models of the magnetodisk are determined. The dependence of the thickness of the magnetodisk on the distance to Jupiter is obtained. The thickness of the magnetodisk and the magnetic field distribution in the isothermic and isentropic models are similar. The inclusion of a low background plasma pressure results in a considerable reduction in the thickness of the magnetodisk. This effect may be attributed to the fact that centrifugal force prevails over the pressure gradient at large distances from the planet. The mechanism of unipolar induction and the related large-scale current system are analyzed. The direct and return Birkeland currents are determined in the approximation of a weak azimuthal magnetic field. The modeling results agree with theoretical estimates from other studies and experimental data.

Pulsar electrodynamics revisited

AbstractThe inductive electric field is unjustifiably neglected in most models for pulsar electrodynamics; it cannot be screened by the magnetospheric plasma, and it is not small in comparison with the corotation electric field. The perpendicular component of the inductive electric field implies a drift motion that is inconsistent with corotation at any angular velocity. Some implications of the inductive electric field and the associated drift motion are discussed.

Variational symplectic algorithm for guiding center dynamics in the inner magnetosphere

Charged particle dynamics in magnetosphere has temporal and spatial multiscale; therefore, numerical accuracy over a long integration time is required. A variational symplectic integrator (VSI) [H. Qin and X. Guan, Phys. Rev. Lett. 100, 035006 (2008) and H. Qin, X. Guan, and W. M. Tang, Phys. Plasmas 16, 042510 (2009)] for the guiding-center motion of charged particles in general magnetic field is applied to study the dynamics of charged particles in magnetosphere. Instead of discretizing the differential equations of the guiding-center motion, the action of the guiding-center motion is discretized and minimized to obtain the iteration rules for advancing the dynamics. The VSI conserves exactly a discrete Lagrangian symplectic structure and has better numerical properties over a long integration time, compared with standard integrators, such as the standard and adaptive fourth order Runge-Kutta (RK4) methods. Applying the VSI method to guiding-center dynamics in the inner magnetosphere, we can accurately calculate the particles’orbits for an arbitrary long simulating time with good conservation property. When a time-independent convection and corotation electric field is considered, the VSI method can give the accurate single particle orbit, while the RK4 method gives an incorrect orbit due to its intrinsic error accumulation over a long integrating time.

Transport and loss of the inner plasma sheet electrons: THEMIS observations

[1] Using the electron data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements from 2007 to 2009, we derived global phase space density (PSD) distributions of plasma sheet electrons (2–100 eV/nT) to examine the transport process of the electrons to the inner magnetosphere and possible loss mechanisms of plasma sheet electrons during the convective transport. The inner boundaries of the electron plasma sheet were determined by the observed global distributions and compared with the Alfven boundaries that were calculated by the sum of the simple corotation and convection electric field models. This comparison confirms the previous results that the large-scale convection electric field controls the electron transport to the inner magnetosphere. The gradual decrease in PSD is observed from the dawn to the dayside sector, indicating the existence of some loss mechanisms in the morning sector. The loss time scales estimated from the PSD distributions were compared with the theoretical ones based on the quasi-linear diffusion theory using an empirical wave model of whistler mode chorus. We also estimated the required wave amplitudes that can explain the estimated loss time scales. It is shown that whistler mode chorus has a sufficient power to scatter the plasma sheet electrons, and the required wave amplitudes are roughly consistent with the CRRES statistical survey of the chorus wave amplitude. We suggest that the loss of plasma sheet electrons in the morning sector is mainly induced by pitch angle scattering by whistler mode chorus.

Properties of Ganymede's magnetosphere inferred from improved three‐dimensional MHD simulations

We describe a three‐dimensional single‐fluid MHD simulation of Ganymede's magnetosphere that accords extremely well with the Galileo particles and fields measurements. Major improvements to our previously published model involve the modification of the inner boundary condition and the implementation of an anomalous resistivity model. The improved model couples the moon's ionosphere (with finite Pedersen conductance) with the magnetosphere self‐consistently. The previous model applied only in the limit of unreasonably high ionospheric conductivity. We illustrate in detail the global convection pattern inferred from the new model and demonstrate some features of the convection that differ from that of the Earth's magnetosphere because Ganymede lacks a corotation electric field. Our new model does a better job of reproducing magnetic field and plasma observations from multiple Galileo passes, which sampled different external conditions and different regions of the magnetosphere. In particular, for a critical upstream pass (G8) during which the Galileo spacecraft entered onto closed field lines, the simulated magnetosphere provides an excellent fit to the measurements without the need for tuning the spacecraft trajectory. In comparison with the plasma measurements of the G2 flyby, our model also yields good agreement with the Galileo PLS observations and supports the conclusion reached by Vasyliūnas and Eviatar (2000) that the observed ionospheric outflow consists of oxygen ions. For constant external conditions, dynamic variations associated with magnetic reconnection on timescales of the order of tens of seconds are found over a large region near the magnetopause in the simulations. Future applications of our model, such as test particle tracing and investigating the behavior of the cross polar cap potential under different external and ionospheric conditions, will provide a more comprehensive understanding of Ganymede's magnetospheric environment.

Detection of dusty plasma near the E-ring of Saturn

We present several independent in-situ measurements, which provide evidence that charged dust in the E-ring interacts collectively with the dense surrounding plasma disk of Saturn, i.e., form a system of dust-plasma interaction. The results are based on data sampled by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini, which allows for interferometry of plasma density inhomogeneities (δ n/ n) with two antenna elements and a Langmuir probe sensor. The interferometer experiment detects two ion populations; one co-rotating with the planetary magnetic field and another moving with near Keplerian speed around Saturn. The full range of RPWS measurements indicates that the Keplerian population consists of colder ions ( T i<few eV), which would interact with the electrical potential cavities associated with the few volts negatively charged E-ring water-rich dust grains. The presence of the charged dust inhibits E× B pick-up of freshly ionized particles by the rotating magnetic field of Saturn, since the local potential gradients near the dust grains are stronger than the large-scale co-rotation electric field. Even so, most ions are eventually energised to energies above the dust potentials of a few volts and become part of a hotter co-rotating ion population. The observed results have direct relevance to the interactions occurring in planet forming accretion disks around young stars, since the physical environment is similar.

Plasma and fields in the wake of Rhea: 3-D hybrid simulation and comparison with Cassini data

Abstract. Rhea's magnetospheric interaction is simulated using a three-dimensional, hybrid plasma simulation code, where ions are treated as particles and electrons as a massless, charge-neutralizing fluid. In consistency with Cassini observations, Rhea is modeled as a plasma absorbing obstacle. This leads to the formation of a plasma wake (cavity) behind the moon. We find that this cavity expands with the ion sound speed along the magnetic field lines, resulting in an extended depletion region north and south of the moon, just a few Rhea radii (RRh) downstream. This is a direct consequence of the comparable thermal and bulk plasma velocities at Rhea. Perpendicular to the magnetic field lines the wake's extension is constrained by the magnetic field. A magnetic field compression in the wake and the rarefaction in the wake sides is also observed in our results. This configuration reproduces well the signature in the Cassini magnetometer data, acquired during the close flyby to Rhea on November 2005. Almost all plasma and field parameters show an asymmetric distribution along the plane where the corotational electric field is contained. A diamagnetic current system is found running parallel to the wake boundaries. The presence of this current system shows a direct corelation with the magnetic field configuration downstream of Rhea, while the resulting j×B forces on the ions are responsible for the asymmetric structures seen in the velocity and electric field vector fields in the equatorial plane. As Rhea is one of the many plasma absorbing moons of Saturn, we expect that this case study should be relevant for most lunar-type interactions at Saturn.

Energization of charged particles in planetary magnetospheres

A model is presented to describe the energiza- tion of charged particles in planetary magnetospheres. The model is based on the stochastic acceleration produced by a random electric field that is induced by the magnetic field fluctuations measured within the magnetospheres. The sto- chastic behavior of the electric field is simulated through a Monte Carlo method. We solve the equation of motion for a single charged particle—which comprises the stochastic acceleration due to the stochastic electric field, the Lorentz acceleration (containing the local magnetic field and the corotational electric field) and the gravitational planetary ac- celeration of the particle—under several initial conditions. The initial conditions include the ion species and the ve- locity distribution of the particles which depends on the sources they come from (solar wind, ionospheres, rings and satellites). We applied this model to Saturn's inner magne- tosphere using a sample of particles (H + ,H 2O + ,N + ,O + and OH + ) initially located on Saturn's north pole, above the C-Ring, on the south pole of Enceladus, in the north pole of Dione and above the E-Ring. The results show that the particles tend to increase the value of their energy with time reaching several eV in a few seconds and the large en- ergization is observed far from the planet. We can distin- guish three main energization regions within Saturn's inner magnetosphere: minimum (Saturn's ionosphere), intermedi- ate (Dione) and high-energy (Enceladus and the E-ring). The resulting energy spectrum follows a power-law distribution

Quasi‐stationary auroral patches observed at the South Pole Station

We present quasi‐stationary auroral patches (QSAPs) observed by an all‐sky imager at the South Pole Station (−74.3° CGLAT) on the dayside between ∼0900 and ∼1400 MLT. QSAPs appeared in a closed field line region where I(557.7 nm) ≫ I(630.0 nm) and each patch tends to preserve its form, luminosity, and location for up to several hours. The quasi‐stationary nature of the QSAPs cannot be explained by the traditional role of the corotation electric field because the South Pole is located on the axis of Earth's rotation. The most plausible scenario is that a corotation electric potential, which dominates the convection potential, results in a trapping region of the magnetospheric cold plasmas. On the analogy of the formation of the plasmasphere, the trapping region would capture cold plasmas originated from the ionosphere and result in a locally enhanced plasma density (miniplasmasphere) that may lead to electron scattering through cyclotron wave‐particle interactions. This circumstance may occur when the South Pole is located on a closed field line and the local electric potential is dominated by the corotation potential. QSAPs were also accompanied with periodic fluctuations in the Pc 5 range, which coincide closely with ground magnetic fluctuations. Assuming that the pulsation is associated with field line resonances, we could estimate the equatorial mass density of thermal plasmas, which gradually increased from 0.13 to 0.44 amu cm−3. The increase in the thermal plasma density would support the hypothesis that a miniplasmasphere exists, though further investigation is necessary to confirm it.

Storm‐time electric field distribution in the inner magnetosphere

The distribution of the storm‐time electric field in the inner magnetosphere has been investigated by using the Akebono/EFD data within a period from 1989 to 1995. During the main phase of geomagnetic storms, the strong electric field appears between L = 2 and L = 6 in both dawn and dusk sectors with the magnitude from 2 to 4 mV/m, and the maximum value appears at L = 3 in the dusk sector. During the recovery and weakly disturbed period, the region with intense electric field moves outward with decreasing the amplitude. This structure of the localized electric field is quite different from the Volland‐Stern electric field model, suggesting the existence of another electric field source other than the large‐scale convection electric field. During quiet periods, although the electric field fairly follows the corotation electric field, the distribution at L = 3.5 indicates a small deviation from pure corotation.

Magnetospheric Eclipses in the Double‐Pulsar System PSR J0737−3039

We argue that eclipses of radio emission from the millisecond pulsar A in the double-pulsar system PSR J0737-3039 are due to synchrotron absorption by plasma in the closed field line region of the magnetosphere of its normal pulsar companion B. On the basis of a plausible geometric model, pulsar A's radio beam only illuminates pulsar B's magnetosphere for about 10 minutes surrounding the time of eclipse. During this time it heats particles at r ≳ 10^9 cm to relativistic energies and enables extra plasma, beyond that needed to maintain the corotation electric field, to be trapped by magnetic mirroring. An enhancement of the plasma density by a factor of ~10^2 is required to match the duration and optical depth of the observed eclipses. The extra plasma might be supplied by a source near B through Bγ pair creation by energetic photons produced in B's outer gap. Relativistic pairs cool by synchrotron radiation close to where they are born. Reexcitation of their gyrational motions by cyclotron absorption of A's radio beam can result in their becoming trapped between conjugate mirror points in B's magnetosphere. Because the trapping efficiency decreases with increasing optical depth, the plasma density enhancement saturates even under steady state illumination. The result is an eclipse with finite, frequency-dependent optical depth. After illumination by A's radio beam ceases, the trapped particles cool and are lost. The entire cycle repeats every orbital period. We speculate that the asymmetries between eclipse ingress and egress result in part from the magnetosphere's evolution toward a steady state when illuminated by A's radio beam. We predict that A's linear polarization varies with both eclipse phase and B's rotational phase.

Global dynamics of dust grains in magnetic planets

We study the dynamics of a charged particle orbiting a rotating magnetic planet. The system is modelled by the Hamiltonian of the two-body problem perturbed by an axially-symmetric potential. The perturbation consists in a magnetic dipole field and a corotational electric field. After an averaging process we arrive at a one degree of freedom Hamiltonian system for which we obtain its relative equilibria and bifurcations. It is shown that the system exhibits a complex and rich dynamics. In particular, dramatic changes in the phase flow take place in the vicinity of a circular equatorial orbit, that in the case of Saturn is located inside the E-ring.