Corotation Electric Field Research Articles

Effects of the large‐scale electric field on particle drifts in the near‐Earth tail

Large‐scale electric field contributes to the charged particle drift motion directly through the E × B drift and indirectly through energization caused by magnetic drifts parallel to the electric field and through pitch angle changes due to magnetic field gradients perpendicular to the electric field. In this paper, we describe these effects on particle drifts in the near‐Earth tail, where the electric field pattern can be expected to be complex, especially near the inner edge of the plasma sheet. We use the Tsyganenko 1989 model to describe the near‐Earth tail magnetic field. The ionospheric Heppner‐Maynard model is mapped to the magnetosphere to model various electric field structures that are not included in a simple model that combines the corotation electric field with a constant cross‐tail convection electric field. The simple model is used as a reference when the results deduced from the Heppner‐Maynard model are discussed. Because these field models are numerically tedious, the bounce‐averaged drift equations are used. They also provide a practical way to include complex electric field models. In addition, we study the electric field mapping in a stretched magnetic field configuration typical for substorm growth phases. Furthermore, it is shown that the inductive electric field associated with temporal stretching during substorm growth phases can be comparable to the typical values of the convective electric field due to earthward plasma convection.

Giant pulsations: An explanation for their rarity and occurrence during geomagnetically quiet times

It is generally agreed that giant pulsations (Pgs) are the result of a particle instability that occurs inside the magnetosphere rather than the consequence of an external stimulus. Previous studies have suggested that protons with energies ∼5–30 keV play a role in Pg excitation. It is shown that protons with energies ∼5–30 keV, injected into the inner magnetosphere on the nightside, will only drift westward around the Earth on enclosed paths if the E × B drifts due to the magnetospheric convection and corotation electric fields are small. This is the case when the magnetosphere is quiet. If the E × B drifts are large, as is the case for more disturbed times, then their influence may overcome that of the gradient‐curvature drift for these lower energy protons, detrapping them from their enclosed paths and allowing them to follow convective paths to the dayside magnetopause. At these times, the lower energy protons which may be an important factor in Pg generation will not reach the early morning sector where Pgs occur, This phenomena can explain the rarity and occurrence during quiet times of Pgs. It can also explain the quashing of Pg activity during substorms and the tendency for Pgs to occur on successive days, 24 hours apart. A similar reasoning can also explain why radially polarized waves with large azimuthal wave numbers, thought to be generated by the bounce resonance mechanism, are frequently observed in the afternoon/evening sector of the magnetosphere but occur infrequently in the morning sector.

Photochemistry and Vertical Transport in Io's Atmosphere and Ionosphere

We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. SO2is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as Na2S or Na2O released by sputtering or venting from the surface. Photochemical products include SO, O2, S, O, Na, NaO, NaS, and Na2. We consider both “thick” and “thin” SO2atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that O2and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O2limits the column abundance of O2to ∼104less than that calculated by Kumar (J. Geophys. Res.87, 1677–1684, 1982; 89(A9), 7399–7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of NaO2or Na2O were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of O2. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, Na+, is lost primarily by charge exchange with Na2O and/or Na2S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, O2, and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density “background” SO2atmosphere, while a localized high density “volcanic” SO2atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.

Hot ions in Jupiter's magnetodisc: A model for Voyager 2 low‐energy charged particle measurements

The Low‐Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy‐dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two‐species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest‐energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near‐Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton‐Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower‐energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.

On the Vertical Thermal Structure of Io's Atmosphere

A radiative-thermal conduction model for the vertical thermal structure of Io's atmosphere is developed with solar heating by SO2 absorption in UV and near-IR bands and non-LTE cooling by SO2v1 , v2, v3, vibrational bands and rotational lines. The model predicts the existence of a mesopause in Io's atmosphere when the surface pressure exceeds ∼10 nbar. The radiative time constant for establishing a mesosphere/mesopause on Io is only ∼20 min, whereas the thermaspheric radiative time constant is about 1 hr. These time constants are significantly shorter than the diurnal time scale and competitive with dynamical time scales. In the thermosphere when solar UV heating dominates, the asymptotic thermospheric temperature is ∼270 K, only 140 K greater than the surface temperature because at high altitudes non-LTE cooling by SO2 rotation lines exceeds cooling in the v2 vibrational band. Solar-heating-only models are incapable of generating warm enough atmospheres to satisfy the observational inferences from UV and especially millimeter-wave measurements. Joule heating driven by the penetration of Jupiter's corotational electric field into Io's conducting ionosphere is demonstrated to be the dominant heating mechanism in the subnanobar regions of Io's atmosphere with temperatures ranging from 150 to 1000 K as a function of decreasing pressure from 1 to 0.1 nbar. The asymptotic thermaspheric temperature can attain a value as high as 1800 K.

Loss of ring current O+ ions due to interaction with Pc 5 waves

The behavior of ring current ions in low‐frequency geomagnetic pulsations is investigated analytically and numerically. We focus primarily on ring current O+ ions, whose flux increases dramatically during geomagnetic storms and decays at a rate which is not fully explained by collisional processes. This paper presents a new loss mechanism for the O+ ions due to the combined effects of convection and corotation electric fields and interaction with Pc 5 waves (wave period: 150–600 s) via a magnetic drift‐bounce resonance. A test particle code has been developed to calculate the motion of the ring current O+ ions in a time‐independent dipole magnetic field, and convection and corotation electric fields, plus Pc 5 wave fields, for which a simple analytical model has been formulated based on spacecraft observations. For given fields, whether a particle gains or loses energy depends on its initial kinetic energy, pitch angle at the equatorial plane, and the position of its guiding center with respect to the azimuthal phase of the wave. The ring current O+ ions show a dispersion in energies and L values with decreasing local time across the day side, and a bulk shift to lower energies and higher L values. The former is due to the wave‐particle interaction causing the ion to gain or lose energy, while the latter is due to the convection electric field. Our simulations show that, due to interaction with the Pc 5 waves, the particle's kinetic energy can drop below that required to overcome the convection potential and the particle will be lost to the dayside magnetopause by a sunward E × B drift. This may contribute to the loss of O+ ions at intermediate energies (tens of keV) observed during the recovery phase of geomagnetic storms.

Effects of quiet time plasmaspheric electric fields on electron and proton precipitation

The effect of the corotation and the quiet time ionospheric dynamo electric fields on the dynamics of energetic electrons and protons (with energies between 50 keV and 1 MeV), inside the inner magnetosphere, is investigated, using the concept of drift paths of constant effective potential. The quiet time ionospheric dynamo electric field is given by an analytic empirical model based on scatter radar observations. The corotation electric field and the Earth's magnetic field are given by analytic expressions corresponding to a centered and an eccentric dipole model. The main conclusion is that the quiet time electric fields cause a diurnal modulation of the electron and proton precipitation. At low L values the locus in longitude of the maximum precipitation is shown to be energy dependent.

On the transport of ions released in the magnetotail by the AMPTE-IRM satellite

The Ba and Li ions that were released in the magnetotail in the Spring, 1985, by the AMPTE-IRM satellite were not observed subsequently in the inner magnetosphere with the AMPTE-CCE satellite. These results were studied by using a Monte Carlo code to compute the transport of the ions. For each release several hundred ion guiding-center trajectories were computed under simulated magnetospheric conditions, using the Tsyganenko-Usmanov magnetic field model and the Millstone-Hill convection electric field model. The corotation and convection electric fields were mapped to altitudes above the ionosphere assuming the magnetic field lines to be equipotentials. The initial conditions of the ions, at the times at which the ions were picked up by the electric field, were estimated by taking into consideration the release conditions and the early-time collective effects. The results indicate that the Ba + ions were not observed because the CCE satellite was not along the drift paths of the ions, and the Li + ions were not observed because their fluxes at the satellite were too low.

Large scale structures of the convection inferred from coordinated measurements by EISCAT and GEOS 2

The equipotential lines of the ionospheric convection electric field can be derived from F-region ion drift measurements performed by EISCAT during 24 h latitudinal scans across the auroral zone. The plasmapause location can be identified with the outermost closed equipotential line from the flow pattern, taking into account the corotation electric field. This indirect prediction is compared with plasmapause crossings simultaneously observed at the geostationary orbit by the spacecraft GEOS 2 located in the vicinity of the apex of the EISCAT magnetic field line. The agreement between these two experimental results validates the method inferred from EISCAT F-region ion drift measurements. In addition, it demonstrates the capability of the EISCAT system to reproduce correctly the large scale structures of the magnetospheric convection over a wide equatorial region from about 4 to 10 Earth radii.

Observations of the flow of H+ and He+ along magnetic field lines in the plasmasphere

The dynamics of earth's plasmasphere is affected by gradients in the plasma pressure, the convection electric field, and the corotation electric field. The Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer 1 measures the pitch angles of this plasma from which the total plasma velocity vector can be determined. Using assumptions concerning the relative importance of convection and corotation, the velocity of ions along the magnetic field line can be determined. These results represent the first direct observations of ion velocities along magnetic field lines at high altitudes in the plasmasphere. Data from October 1981 through January 1982 have revealed several interesting aspects of the field‐aligned flow of H+ and He+. Downward directed velocities observed in the northern hemisphere within 30° of the equatorial plane are interpreted as interhemispheric flows. Typical velocities are of the order of a few hundred meters per second. Higher‐speed (up to 1 km s−1) flows observed in the outer plasmasphere appear to be associated with refilling. Counterstreaming of H+ and He+ has been observed in the autumn and winter hemisphere with He+ flowing upward and H+ flowing downward. Differences between these results and theoretical work can probably be attributed to differences in the season and the density level within the flux tube.

Large plasmaspheric electric fields at L ∼ 2 measured by the S3‐3 satellite during strong geomagnetic activity

Large plasmaspheric electric fields at L ∼ 2 measured by the S3‐3 satellite during strong geomagnetic activity are reported in this letter. Since these measurements have amplitudes comparable to those of the local corotation electric field, during such events the plasmasphere is expected to get strongly altered even at such low L‐values. Furthermore, those measurements could contribute to the understanding of the physics of the convection‐electric field penetration to the low latitude plasmasphere as well as of the “disturbed” dynamo, during strong geomagnetic activity. For this purpose, critical parameters related to geomagnetic activity are also presented for the reported electric field events.

Universal time effects on plasma convection in the geomagnetic frame

Coordinate frames for description of plasma convection in the ionosphere at high latitudes, together with the transformations between the frames, are presented for a centred dipole magnetic field. The Earth's corotation velocity is shown to depend on universal time in the geomagnetic frame; the dependence is most marked at the solstices. The UT-dependent corotation electric field (in the geomagnetic frame) is added to a magnetospheric convection electric field model that is UT-independent in the geomagnetic frame and plasma velocities are calculated. Under these conditions, the plasma velocity observed in the magnetic frame sufficiently poleward of the plasmapause does not vary significantly with UT. In the transplasmapause region where the convection and corotation electric fields become comparable, the plasma velocity at a point varies markedly with UT. The resulting plasma trajectories, along a fixed equipotential, have markedly different shapes if they start at different universal times. Trajectories starting from the same location but at different times can be very different. The UT variations can cause the plasma to make multiple traversais of the polar cap boundary and can cause the formation of eddies in convection paths, thus affecting plasma densities; there may be UT-dependence of the latitude of the dusk and evening mid-latitude trough. The ‘last closed equipotential’ is time-dependent even for a steady magnetospheric convection electric field. Thus the plasmapause may be located in a region of plasma with behaviour intermediate between that in the plasmasphere and that at higher latitudes, even under steady conditions. Plasmatails may form even in the absence of a disturbance. The conclusions are not critically dependent on the forms assumed for the convection electric field and for the magnetic field.

Effects of tailward stretching geomagnetic field on drift motion of plasma particles in the magnetospheric equatorial plane

Drift paths of electrons and protons are calculated in the equatorial plane of the superquiet (SQ) and superdisturbed(SD) Mead-Fairfield magnetospheric models in order to study effects of tailward stretching geomagnetic field on drift motion of plasma particles. Magnetospheric electric fields in the equatorial plane are given by vector sums of uniform dawn-dusk electric field of 0.1 mV/m or 0.4 mV/m and corotation electric field projected onto the equatorial plane from the earth's surface along the MF geomagnetic field lines. All particles start from a geocentric circle of 17 earth radii (R E) in the tail equatorial plane. The late-evening stagnation point for zero-energy particles and low-energy electrons is produced by the anticlockwise corotation drift larger than the sunward electric-field drift in the outer magnetosphere. The late-morning stagnation point for energetic protons and the late-afternoon stagnation point for energetic electrons result from small radial gradient of the MF geomagnetic field in the late-morning and the late-afternoon outer magnetosphere respectively.

Jovian magnetospheric neutral wind and auroral precipitation flux

We propose a mechanism for the transport of energy from the rotation of Jupiter into the heavy ions whose precipitation into the atmosphere is responsible for the intense ultraviolet aurora. The process involves charge exchange of heavy ions in the Io torus, their flight to the outer magnetosphere as neutrals, and their reionization in a region where the pickup magnetic moment is large. The particles diffuse back inward adiabatically and are precipitated at 9 RJ with an energy exceeding 10 keV/nucleon. A field and flow model based on Voyager observations is used that defines a source region extending from 32 RJ to 80 RJ on the nightside. This region is capable of delivering 4×1013 W of power to the auroral region, well within the realm of the power required by observations. We also demonstrate that this mechanism provides a neutral planetary wind from Jupiter. The predicted wind will consist of two beams: one intense (5×1028 s−1) at 25 eV/nucleon and the other tenuous (1×1025 s−1) at 600 eV/nucleon and above. These neutral particles will appear as a suprathermal heavy ion component in the solar wind.

Dawn‐dusk electric field asymmetry of the Io plasma torus

We consider the effects of a combined convection and corotation electric field across the Io plasma torus. A dawn‐to‐dusk electric field Ec will modify the orbits of charged particles shifting them toward dawn. The radial drift imposed by the perturbed orbits implies a local time‐dependent modulation of low‐energy ion and electron temperatures with particles hotter at dusk than at dawn. With Ec ≃ 4 mV/m, the orbits near 6 RJ would be shifted by approximately 0.2 RJ. Then the electron temperature would be 20% higher at dusk than at dawn, an effect which could explain the local time asymmetry of EUV intensity found by the Voyager ultraviolet spectrometer. The source of the convection electric field is internal to the magnetosphere and is attributed to the tailward escape of Iogenic and Jovian plasma beyond the Alfven surface.

Ring current energy injection rate and solar wind-magnetosphere energy coupling

Assuming that the formation of the ring current belt is a direct consequence of an enhanced convection of plasma sheet protons, the expression for the energy injection rate U R is formulated as a function of the cross-tail potential drop φ CT for a simple electric field-magnetic field model. It is shown that an approximate expression for U R thus formulated consists of two parts: (i) the first part U R1 , which is linearly proportional to φ CT , is supplied by the corotation electric field and (ii) the second part U R>2 , which is proportional to φ 2 CT , is supplied by the solar wind energy input to the magnetosphere. The second part U R2 dominates the ring current energy input when the cross-tail potential drop φ CT is greater than ∼ 95 keV, namely during disturbed periods. An important finding is that the second part U R2 of the ring current energy input is shown to be proportional to the solar wind-magnetosphere energy coupling function ϵ, recovering the observationally established relationship. Therefore, the present study verifies that an enhanced convection is the cause of the ring current formation.

The structure of Titan's wake from plasma wave observations

During the Voyager 1 flyby of Saturn's moon Titan, the plasma wave instrument detected several types of plasma wave emissions. On the inbound leg a broad region of intense low‐frequency noise was detected on the side of Titan facing away from Saturn. This noise has characteristics similar to the electric field turbulence observed in the magnetosheath at the earth and the ionosheath at Venus and is believed to be generated by newly created ions that are being accelerated in the vicinity of Titan by the corotational electric field. During the pass through the induced magnetic tail of Titan, a series of upper hybrid resonance emissions were observed. The electron density profile inferred from these emissions shows three distinct peaks with densities of ∼40 cm−3, the first peak corresponding to the entry into the magnetic tail, the second peak corresponding to the neutral sheet crossing from the northern to the southern tail lobe, and the third, somewhat smaller, peak corresponding to the outbound exit from the tail. Large depressions in the magnetic field strength are observed coincident with each of the density peaks. These effects indicate that a dense plume of plasma is being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn. By equating the magnetic field pressure in the tail lobe to the plasma pressure in the neutral sheet, the temperature of the plasma is estimated to be about 8600°K. This low temperature suggests that the plasma originates from the ionosphere of Titan, probably forming a plume of plasma with a θ or H cross section extending downstream from Titan. Within the tail lobes, a second type of low‐frequency electric field noise was observed, with characteristics very similar to a type of noise called broadband electrostatic noise, which is found in the earth's magnetic tail. As in the case of the earth, this noise is most intense near the outer boundary of the plasma sheet and is almost completely absent in the high‐density region near the neutral sheet.

Morning sector ion precipitation following substorm injections

The auroral ion measurements (0.4–30 keV) made from the polar satellite Aureol 1 have revealed two types of precipitation with different energy characteristics in the auroral and subauroral nighttime zone: (1) In the entire nighttime auroral zone a proton precipitation with an average energy ranging from ≃5 keV to at least 25 keV, which coincides with the region of auroral Balmer series lines emission as observed from the ground (Crasnier et al., 1974). The large scale characteristics of this precipitation reflect the adiabatic acceleration of the protons in the plasma sheet and their drift motions in the outer magnetosphere (Galperin et al., 1976, 1978). (2) A subauroral precipitation of ions with energy less than 12 to 15 keV limited to the morning sector between approximately 0000 and 0500–0600 MLT. This newly observed structure is sporadic and takes place in the hours following the injections of plasma sheet particles into the inner magnetosphere at the onset of magnetospheric substorms. The ionospheric region affected by this precipitation corresponds, in the equatorial plane, to the zone in which Alfvén layers result from the ion drift pattern (from the added effects of convection and corotation electric fields and from magnetic field gradient). It is shown that the electric and magnetic field models derived by McIlwain (1972) from the energy dispersion of the particles injected at 6.6 RE at the substorm onset accounts for the main spatial and energy characteristics of these low‐energy ion subauroral precipitation. This result is obtained by extending the logic used to study the proton ‘noses’ (Smith and Hoffman, 1974; Konradi et al., 1976) from the evening side to the morning side of the inner magnetosphere, and this makes it possible to obtain, from the injection boundary concept, a coherent approach to the substorm injection features at different local time sectors.

Io's interaction with the plasma torus

A new model for the interaction of Io with the dense corotating plasma of the Io torus is described which involves pickup and Alfvén waves carrying a field‐aligned current. Pickup refers to the process whereby ions freshly created near Io are accelerated by the corotational electric field. It is shown that the pickup current is connected to a field‐aligned current carried by an Alfvén wave. The combination of currents allows for a self‐consistent determination of the electric field in Io's vicinity. Using a simplified solution for the electric field, we calculate the total pickup rate, field‐aligned current, enhancement of plasma density in Io's vicinity, thermal energy injected into the torus, UV radiation power, and inertial drag on the magnetosphere. All observations made by Voyager are compatible with an electron impact ionization mechanism and a neutral SO2 Io atmosphere with a density of 109 cm−3.

Ion convection and the formation of the mid‐latitude F region ionization trough

Measurements of total ion concentration and ion drift velocity from Atmosphere Explorer C have been used to examine the convective flow of plasma in the vicinity of the mid‐latitude F region ionization trough. It is found that the low‐latitude portion of the premidnight trough is a region of slow eastward plasma drift while the poleward portion is characterized by generally westward flow. The reversal from eastward to westward flow typically occurs on the low‐latitude edge of the trough where Ni is observed to vary smoothly. We propose that the plasma flux tubes within the premidnight trough have corotated eastward into the evening local time sector from dusk before stagnating and flowing westward at higher latitudes in the nighttime sector. In the absence of ionization sources, rather deep troughs can be formed by F region recombination during the long time required for the flow to stagnate and reverse. The proposed drift paths can result quite naturally from the combined effects of the corotation and magnetospheric electric fields. Large plasma convection velocities that act to increase the F region decay rate are not required to produce a trough but may cause the plasma composition changes that are occasionally observed within the trough.