Ionospheric Convection Electric Field Research Articles

Estimating the Ionospheric Induction Electric Field Using Ground Magnetometers

AbstractThe ionospheric convection electric field is often assumed to be a potential field. This assumption is not always valid, especially when the ionosphere changes on short time scales min. We present a technique for estimating the induction electric field using ground magnetometer measurements. The technique is demonstrated on real and simulated data for sudden increases in solar wind dynamic pressure of 1 and 10 nPa, respectively. For the real data, the ionospheric induction electric field is 0.15 0.015 mV/m, and the corresponding compressional flow is 2.5 0.3 m/s. For the simulated data, the induction electric field and compressional flow reach 3 mV/m and 50 m/s, respectively. The induction electric field can locally constitute tens of percent of the total electric field. Inclusion of the induction electric field increased the total Joule heating by 2.4%. Locally the Joule heating changed by tens of percent. This corresponds to energy dissipation that is not accounted for in existing models.

Separation and Quantification of Ionospheric Convection Sources: 1. A New Technique

AbstractThis paper describes a novel technique that allows separation and quantification of different sources of convection in the high‐latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high‐latitude reconnection associated with lobe cells from the low‐latitude reconnection associated with Dungey two‐cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models.

Simulated ion velocity distribution and its incoherent scattering spectrum in the high-latitude ionosphere

If the relatively larger convection electric field and small the collision frequency between ions and neutrals in the high-latitude ionosphere are considered, the integral solution of the non-Maxwellian ion velocity distribution function for the relaxation collision model is simplified, and a torus distribution function of the ion velocity is obtained. Also, the feature of the torus velocity distribution function is analyzed and simulated with different parameters in high-latitude ionosphere. Furthermore, by using the electromagnetic radiation theory of Sheffield and taking into account the ion temperature anisotropy and its variation with the electric field, we simulate the incoherent scattering spectrum of ions in different directions. The research shows that the ionospheric convection electric field, the ion-neutral collision frequency and the ion anisotropic temperature distribution all have an effect on the incoherent scattering spectrum.

Response of the magnetospheric convection to sudden interplanetary magnetic field changes as deduced from the evolution of partial ring currents

We examined the response of the partial ring current (PRC) to sudden changes in the interplanetary magnetic field (IMF) Bz for two PRC events, in order to investigate the response of the convection in the near‐Earth magnetotail to IMF changes. We found that ground magnetic changes due to the PRC started to develop simultaneously at all local times with a time lag of 5–11 min from the growth of the ionospheric convection as detected by the polar cap magnetometers. The ground magnetometers furthermore showed that the asymmetric ring current changed into a symmetric ring current with no significant time lag (<1 min) when the convection electric field started to decay because of the northward turning of the IMF. The temporal and spatial evolution of the plasma pressure and the ground magnetic perturbations due to the PRC are simulated for the events involved using a particle simulation model. The simulation results agree well with the observations in the temporal variations and their local time dependence. The quick development and reconfiguration of the observed PRC suggest that the ionosphere played a crucial role in driving the convection in the near‐Earth magnetotail and in the inner magnetosphere. In particular, the immediate reconfiguration of the PRC caused by the northward turning of the IMF implies nearly instantaneous propagation of the ionospheric convection electric field to the inner magnetosphere and subsequent quick change in the ring current configuration. We discuss the propagation of the convection electric field to the near‐Earth magnetotail through the ionosphere after its propagation along the magnetic field lines from the generator in the dayside magnetosphere.

The convection electric field in auroral substorms

Dynamics Explorer 2 (DE 2) electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge‐type auroral substorms. Thirty‐one individual DE 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained by DE 1. The selected passes, which occurred during substorm expansion phase, maximum, or early recovery phase, cover the entire nighttime substorm. The organization of the data used the method developed by Fujii et al. [1994], which divided the data into six local time sectors covering the nighttime substorm region. Following the procedures employed in the paper by Gjerloev and Hoffman [2000b], the latitudinal width and location of each auroral oval crossing was then adjusted to fit the sector average. In addition to the detailed study of the characteristics of the field within each sector this database enabled us to compile a model of the ionospheric convection electric field. The characteristics of the premidnight convection reversal show a pronounced local time dependency. Far west of the surge it is a fairly well defined point reversal or convection shear. Approaching the surge and within the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields. Therefore we adopt the term Harang region rather than the Harang discontinuity for the premidnight convection reversal. A relatively narrow convection channel is coincident with the highest conductances located just poleward of the Harang region. This channel drives the substorm current wedge component of the westward electrojet in the surge and middle surge sectors. It is present in all premidnight passes and consequently is an integral part of the three‐dimensional substorm current wedge system.

Tailward stretching of geomagnetic field lines in the presence of an enhanced ionospheric convection electric field

When the interplanetary magnetic field turns southward, dayside reconnection leads to an increase of open magnetic flux and an increase of the electric potential drop across the polar cap. The enhanced electrostatic field in the ionosphere can be mapped to the closed field region of magnetotail, contributing to a dawn‐to‐dusk electrostatic component of electric field in the plasma sheet. In the presence of this driving electric field, an adiabatic evolution of the plasma sheet is formulated in A‐space, where A is the magnetic flux function. It is found that the adiabatic evolution leads to the observed enhancement of the entropy content in the near‐earth plasma sheet and to a tailward stretching of geomagnetic field lines. As a result, the cross‐tail current in the near‐earth plasma sheet is strongly enhanced.

Centrifugal acceleration of the polar wind

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis (1986). However, this effect is typically neglected in “self‐consistent” models of polar plasma outflow, apart from the fluid simulation by Swift [1990]. Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time‐dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H+ and O+. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as Fcent,pole = 1.5m(Ei/Bi)²(r²/ri³) where m is ion mass, r is geocentric distance; and Ei, Bi and ri refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitidinal convection along a constant L shell the parallel force is Fcent,long = Fcent,pole [1‐(r/(riL)]3/2/[1‐3r/(4riL)]5/2. For high latitudes the difference between these two cases is relatively unimportant below ∼5 RE. We find that the steady state O+ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km s−1 at 4000 km altitude to ∼10 km s−1 at 5 RE geocentric distance for a 50‐mV/m ionospheric convection electric field. However, the centrifugal effect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low‐base temperatures (Ti = Te = 3000 K) the O+ density at high altitudes increases greatly with convection, while for higher base temperatures (Ti = 5000 K, Te = 9000 K), the high‐altitude O+ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O+ flux, especially for cool exobase conditions; as referenced to the 4000‐km altitude, the steady state O+ flux increases from 105 ions cm−2 s−1 when the ionospheric convection field Ei = 0 mV/m to ∼107 ions cm−2 s−1 when Ei = 100 mV/m. The centrifugal effect also decreases the time scale for approach to steady‐state. For example, in the plasma expansion for Ti = Te = 3000 K, the O+ density at 7 RE reaches only 10−7 of its final value ∼1.5 hours after expansion onset for Ei = 0. For meridional convection driven by Ei = 50 mV/m, the density at the same time after initial injection is 30‐50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to ∼10 km s−1 or more) O+ outflow velocities observed in the midaltitude polar magnetosphere with the Dynamics Explorer 1 and Akebono spacecraft.

Storm time plasma transport at middle and high latitudes

Associated with the large‐scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar‐produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm‐enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large‐scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid‐ or low‐latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ∼750 m s−1 with a flux of 1014 m−2 s−1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions.

Observations of a giant pulsation across an extended array of ground magnetometers and on auroral radar

It has often been suggested that energetic protons are involved in the generation of giant pulsations (Pgs). A Pg event occurring in the early morning of 29 December 1987 has been observed by magnetometers in Scandinavia, the Faroe Islands and Iceland. The Pg has also been observed by the SABRE coherent radar system. The azimuthal drift velocity of the Pg disturbance region across the available longitudinal extent has been measured using the magnetometer data and the radar data have been used to estimate the ionospheric convection electric field. The energies of protons that would drift at this velocity in the region of the magnetosphere where the Pg occurred have been deduced to be ∼ 10–20 keV, depending on the pitch angle of the protons. Using azimuthal wavenumbers calculated on the EISCAT magnetometer cross and the SABRE radar for this event, the energies of protons that would satisfy the bounce resonance condition for this Pg have been calculated to be ∼ 5–18 keV. The Pg is thought to be driven by protons associated with a substorm particle injection that occurred on 28 December 1987. The energies of protons that would drift westward from this injection region and reach the Pg disturbance region have been calculated to be ∼ 11–24 keV. This study therefore presents good evidence to suggest that the bounce resonance interaction with protons is the mechanism responsible for the Pg and that these protons originated from a substorm particle injection.

Multi-radar mapping of auroral convection

Simultaneous radar azimuth scans with the Millstone Hill, Sondrestrom, and EISCAT incoherent scatter radars produce maps of the ionospheric convection electric field at high latitudes which span 10 hours of magnetic local time. A series of convection “snapshots” made during an interval of increasing geomagnetic activity indicate that the large-scale convection pattern maintains a two-cell character during substorm onset and that average convection models derived from radar data provide a reasonable representation of the large-scale convection pattern during dynamically varying conditions.

Reply to Kamide and Richmond

Detailed empirical models of the ionospheric convection electric field and auroral‐latitude conductance are used to show that at 04 magnetic local time (MLT) the electric field magnitude peaks at least 5° poleward of the Hall conductance and is the stronger contributor to the poleward half of the westward electrojet current in the morning sector. At 20 MLT the eastward electrojet current is centered on the electric field and conductance profiles, which have a similar peak latitude and extent.

An interplanetary magnetic field dependent model of the ionospheric convection electric field

A mathematical model of the magnetospheric electric field imposed upon the ionosphere is presented. The model provides an interplanetary magnetic field (IMF) dependent description of the magnetospheric electric field at ionospheric altitudes for global ionospheric and thermospheric modelers. Although many theoretical and empirical ionospheric convection models have been published, none give both a quantitative and a general Kp, IMF (Bx, By, Bz) dependent description. The need for such a model is particularly pressing with the success of the Dynamic Explorer (DE) mission. As a result of this mission, extensive data sets of thermospheric and ionospheric parameters for well‐defined IMF orientations have been acquired. These data sets, especially for northward IMF orientations, are so complex that global models of both the thermosphere and ionosphere are required to help unravel the physics. Our convection model is an empirical model based upon a synthesis of published observations and current understanding. It describes the convection electric field by a set of simple functions which are dependent on Kp and IMF (Bx, By, Bz). These simple functions create a framework by which current and future empirical data can be used to adjust the model. Since no recent major statistical surveys have been made of ionospheric electric fields, these functions (based upon a few published case studies) are exceedingly simple.

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.

Ionospheric convection associated with discrete levels of particle precipitation

A precipitation index is described which quantifies the intensity and spatial extent of high‐latitude particle precipitation based on observations made along individual satellite passes. By sorting plasma convection data according to this index, average patterns of the ionospheric convection electric field have been derived from a data set consisting of five years' observations by the Millstone Hill radar. Reference to the instantaneous precipitation index, and the average patterns keyed to it, provides a means of characterizing the global precipitation and convection patterns throughout an event.

ELF wave production by an electron beam emitting rocket system and its suppression on auroral field lines: Evidence for Alfven and drift waves

ELF electric disturbances in the frequency range from DC to 1 kHz produced in the auroral ionosphere by the injection of powerful electron beams have been studied by orthogonal probes on a free‐flying plasma diagnostics payload. Frequency spectrograms have been produced for various pitch angles, pulsing characteristics and other properties of the injected beams. The large‐scale DC ionospheric convection electric field has also been measured, as well as the auroral particle precipitation, the visual auroral forms and the ionospheric parameters. The convection field was strongly attenuated as the experiment system moved northward across discrete arcs but recovered to a higher value north of the arcs where the particle precipitation ceased. The production of ELF waves by beam injection was also strongly reduced in the arcs, and like the DC field, recovered to an even greater intensity north of the arcs. This behavior did not extend into the lower hybrid or whistler mode region. The DC field decrease has been explained by the high Pederson conductivity caused by auroral electron precipitation in the E‐region below the vehicle. It is postulated that the observed ELF waves are in the Alfven and drift modes and are generated by the positive vehicle potential during beam injection. More effective vehicle neutralization in the auroral arcs lowers the vehicle potential and, therefore, reduces wave production. Most features of the experimental observations can be qualitatively explained by this mechanism.

Electrostatic potential in the auroral ionosphere derived from Chatanika Radar observations

A technique is described for determining the latitudinal variation of the electrostatic potential associated with the ionospheric convection electric field. Using the north‐south electric field component derived from radar convection velocity experiments, the integral of E• dl is taken northward along the magnetic meridian, starting at low latitudes. The radar data consist of up to 40 independent measurements of plasma convection spanning 15° of invariant latitude centered on Chatanika, Alaska (65°Λ), with half‐hour temporal resolution. It has been found that (1) the electric field contributions to the potential at and below 60°Λ are small under most circumstances and (2) the latitudinal variation of the potential is smooth and regular, permiting the potentials to be contoured across local time. It is found from the experiments that the potential often varies uniformly over 10° latitude at dawn and dusk. Electric fields of 50 mV/m are common. It is also noted that the latitude of the greatest negative potential in the premidnight sector coincides with the Harang discontinuity in ionospheric currents. The potentials calculated from the measured plasma drifts exhibit a regular local time variation. Equipotential contours derived from the latitude‐local time potential field obtained with the long‐duration radar experiments, while not providing a snapshot of the instantaneous pattern, elucidate the large‐scale diurnal variation of the electrostatic potential at auroral latitudes. From such contours it is found that a two‐cell convection pattern with varying degrees of asymmetry is consistently present at auroral latitudes, that a cross‐polar cap potential drop of 70–120 kV is present in moderately disturbed conditions, and that substorms perturb the potential pattern at all local times.