Polar Cap Electric Field Research Articles

Fluid and kinetic aspects of magnetic reconnection and some related magnetospheric phenomena

Magnetic field reconnection plays a key role in determining the magnetic field topology and in the conversion of magnetic energy into kinetic energy in cosmic plasmas. Magnetic reconnection was proposed by solar physicists in an attempt to explain solar flares. The concept of magnetic reconnection was extended to Earth’s magnetosphere to explain auroral substorms. In this review, we first briefly review the classical fluid steady models with small separatrix angle. We then report magnetic reconnection with a large separatrix angle, and the time-dependent multiple X line reconnection (MXR) associated with flux transfer events. The transition from a single X line reconnection to MXR is examined. Global three-dimensional (3-D) magnetic reconnection is reviewed. The particle pressure gradient associated mainly with the electron (ion) off-diagonal pressure tensor terms, $$P_{xy}$$ and $$P_{zy}$$ , is shown to play an important role for electron (ion) dynamics to balance the reconnection electric field ( $$E_{y}$$ ) near the X line. In addition, various plasma waves and instabilities may occur near the ion diffusion and electron diffusion regions, including the upper hybrid drift waves, lower hybrid drift waves, ion-ion beam instability, electron beam instability, whistler waves and kinetic Alfven waves. The layered structure of outflow region of magnetic reconnection is examined. These layered structures include slow shocks, rotation discontinuities, expansion waves and plasma jets. Recent satellite observations of magnetic reconnection in the Earth and planetary magnetospheres are also discussed. The polar cap electric field associated with magnetic reconnection at the Earth's magnetopause and the energy flow in the polar cap region are examined.

Activation of the tail open part during the magnetospheric storm

In each polar cap (PC) we mark out “old PC” observed during quiet time before the event under consideration, and “new PC” that emerges during the substorm framing the old one and expanding the PC total area. Old and new PCs are the areas for the magnetosphere old and new tail lobes, respectively. The new lobe variable magnetic flux Ψ1 is usually assumed to be active, i.e. it provides the electromagnetic energy flux (Poynting flux) ɛ′ transport from solar wind (SW) into the magnetosphere. The old lobe magnetic flux Ψ2 is supposed to be passive, i.e. it remains constant during the disturbance and does not participate in the transporting process which would mean the old PC electric field absolute screening from the convection electric field created by the magnetopause reconnection. In fact, screening is observed, but far from absolute. We suggest a model of screening and determine its quantitative characteristics in the selected superstorm. The coefficient of a screening is the β = Ψ2/Ψ02, where Ψ02 = const is open magnetic flux through the old PC measured prior to the substorm, and Ψ2 is variable magnetic flux through the same area measured during the substorm. We consider three various regimes of disturbance. In each, the coefficient β decreased during the loading phase and increased at the unloading phase, but the rates and amplitudes of variations exhibited a strong dependence on the regime. We interpreted decrease in β as a result of involving the old PC magnetic flux Ψ2, which was considered to be constant earlier, in the Poynting flux ɛ′ transport process from solar wind into the magnetosphere. Transport process weakening at the subsequent unloading phase creates increase in β. Estimates showed that coefficient β during each regime and the computed Poynting flux ɛ′ varied manifolds. In general, unlike the existing substorm conception, the new scenario describes an unknown earlier of tail lobe activation process during a substorm growth phase that effectively increases the accumulated tail energy for the expansion and recovery phases.

Polar cap electric field saturation during interplanetary magnetic field Bz north and south conditions

We report the results of an investigation of the saturation of the polar cap electric field during periods of large northward and southward interplanetary magnetic field (IMF). While it has been demonstrated that saturation can occur for both northward and southward IMF, a direct comparison between the two regimes during saturated driving has not been performed. We use solar wind measurements to search for events between 1998 and 2007 when the IMF is stable for 50 min. The selected intervals are binned according to interplanetary electric field (−VSW × B). SuperDARN Doppler radar velocity vectors from high‐latitude antisunward looking beams are averaged to determine the approximate polar cap electric field. Results show that sunward convection under northward IMF is stronger in summer than in winter, but that antisunward convection under southward IMF exhibits the opposite seasonal behavior. One explanation is that, as the earth tilts near solstice, lobe reconnection is less effective in the winter hemisphere.

Ionospheric responses to the October 2003 superstorm: Longitude/local time effects over equatorial low and middle latitudes

Ionospheric responses to the major magnetic storm disturbances of October 2003 are investigated using database selected in the Brazilian and Japanese‐Asian longitude sectors. Data obtained from latitudinally spaced digisondes in the equatorial and low‐latitude sites in Brazil and from the Asian and Japanese ionosonde network, the total electron content data from the extensive Japanese GPS receiver chain, and magnetometer data from the Pacific equatorial electrojet stations are analyzed during the period 28–31 October. Prompt penetrating (PP) dawn‐dusk polar cap electric fields produce large F region plasma uplift on the dayside and eveningside, while the associated westward electric field on the nightside produces large downdraft of the F region plasma, and causes development of westward electrojet current, observed for the first time. Episodes of PP electric field effects appear to be of larger intensity over Brazil than over Asian longitudes. Equatorial anomaly, development due to undershielding as well as overshielding electric fields, was observed in the Brazilian and in the Asian sectors. Disturbance dynamo electric field causes large nighttime F layer uplifts that are modulated by strong meridional winds in both sectors. The disturbance electric field local time variation patterns are compared with the results of recent global model (MTIEGCM) simulation by Richmond et al. (2003) and validated in some cases. Transients of transequatorial winds, flipping direction from southward to northward, in the widely separated longitude sectors, were diagnosed to be present toward the final recovery phase of the storm. These results are presented and discussed in this paper.

Spike‐like change of the vertical E × B drift in the equatorial region during very large geomagnetic storms

We investigate the characteristics of the storm‐time disturbance electric fields in the equatorial region using the measurements of the vertical ion velocity from the Defense Meteorological Satellite Program F13 and the first Republic of China Satellite during the selected 43 geomagnetic storms of Dst <−100 nT in 2000–2004. The distinguishing feature in both observations is the occurrence of the spike‐like vertical E × B drift during the main phase of very large storms (Dst <−150 nT). The spikes occur simultaneously at different local times but the direction of the spikes shows a dependence on the local time. The spikes are upward on the dayside and downward on the nightside. The observed characteristics of the spikes can be explained by the penetration of the dawn‐to‐dusk polar cap electric field to low latitude.

IMF effect on sporadic-E layers at two northern polar cap sites: Part I – Statistical study

Abstract. In this paper we investigate the relationship between polar cap sporadic-E layers and the direction of the interplanetary magnetic field (IMF) using a 2-year database from Longyearbyen (75.2 CGM Lat, Svalbard) and Thule (85.4 CGM Lat, Greenland). It is found that the MLT distributions of sporadic-E occurrence are different at the two stations, but both are related to the IMF orientation. This relationship, however, changes from the centre of the polar cap to its border. Layers are more frequent during positive By at both stations. This effect is particularly strong in the central polar cap at Thule, where a weak effect associated with Bz is also observed, with positive Bz correlating with a higher occurrence of Es. Close to the polar cap boundary, at Longyearbyen, the By effect is weaker than at Thule. On the other hand, Bz plays there an equally important role as By, with negative Bz correlating with the Es occurrence. Since Es layers can be created by electric fields at high latitudes, a possible explanation for the observations is that the layers are produced by the polar cap electric field controlled by the IMF. Using electric field estimates calculated by means of the statistical APL convection model from IMF observations, we find that the diurnal distributions of sporadic-E occurrence can generally be explained in terms of the electric field mechanism. However, other factors must be considered to explain why more layers occur during positive than during negative By and why the Bz dependence of layer occurrence in the central polar cap is different from that at the polar cap boundary.

Polar cap index comparisons with AMIE cross polar cap potential, electric field, and polar cap area

The polar cap index (PCI) has been shown to be quite useful as an index of the electrodynamic processes within the high‐latitude ionosphere. Here we use between 150 and 180 magnetometers to model the high‐latitude electrodynamics with the assimilative mapping of ionospheric electrodynamics (AMIE) technique for all of 1997–2001. We compare AMIE calculations of the cross polar cap potential (CPCP), polar cap electric field, and polar cap area to the Northern PCI and the existing PCI‐based relationships for these quantities. We find that the existing PCI estimates underestimate all of the AMIE derived quantities. The PCI is best correlated with the CPCP, while it correlates least with the polar cap electric field. In addition, there is a seasonal effect in the relationship between all AMIE quantities and the PCI. It is most pronounced in the electric field. The polar cap area is shown to have a linear fit in the summer and an asymptotic fit in the winter. These differences indicate that there may be a conductance difference in the techniques. We derive new relationships between the Northern PCI and the AMIE derived polar cap area, CPCP, and polar cap electric field which include the seasonal dependence.

Long‐lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere‐ionosphere‐thermosphere model

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.

Polar cap index (PC) as a proxy for ionospheric electric field in the near‐pole region

The ion drift measurements made by a number of DMSP satellites during some intervals in 1991, 1997, and 1998 are utilized for estimation of the ionospheric electric fields over the near‐pole region; these estimates are then compared with the Polar Cap (PC) magnetic activity index obtained from ground geomagnetic observations at Qaanaaq (former Thule, Greenland) and Vostok (Antarctica). The analysis shows that the polar cap electric field is primarily controlled by variations in the near‐Earth's interplanetary electric field. The relationship between the polar cap ionospheric electric field and the PC‐index can be approximated by a quadratic polynomial. The polar cap ionospheric electric field tends to saturate at the asymptote of ∼45–50 mV/m when the PC index reaches large positive values (PC>10); the residual electric field (for near‐zero interplanetary electric field applied to the Earth's magnetosphere) is ∼12 mV/m. It is concluded that the PC‐index can serve as a proxy of the ionospheric electric fields in the near‐pole region.

High‐altitude polar cap electric field responses to southward turnings of the interplanetary magnetic field

Interplanetary electric field coupling with the magnetosphere has been analyzed predominantly using data from the Wind magnetometer and the Polar electric field instrument. The coupling was investigated using the Polar Electric Field Instrument (EFI) to measure the electric field in the northern polar cap immediately following sharp southward turnings of the IMF as observed by Wind. Southward turnings were chosen which exhibited a sudden change of the IMF north–south component from BZ > 0 to BZ < 0 (GSM coordinates) after an hour or more of relatively stable conditions, and for which Polar was in the northern polar cap. These BZ changes correspond to EY changes in the interplanetary electric field. For each of the 30 identified events, a time was estimated for the arrival of the IMF change at the magnetopause using the solar wind speed observed by the Wind Solar Wind Experiment (SWE), and Polar electric field data were examined to identify responses. For many of the selected events (about one third), abrupt changes of state in the magnetospheric electric field were evident with timing that matched the expected solar wind arrival time at Earth. For events for which additional data were available, we conducted in‐depth examination of the individual events using IMP 8, Geotail, and GOES 9. In one such event, GOES 9 data showed a substorm growth phase and onset which also corresponded to features in the solar wind observed by Wind, Geotail, and IMP 8. In addition to the individual event studies, a superposed epoch analysis of all available events revealed a consistent rise in the mean polar cap electric field about 15 min following sharp IMF southward turnings.

Austral thermospheric wind circulation and interplanetary magnetic field orientation

Ground‐based high‐resolution spectral measurements of the OI ¹D emission at 15,867 K (630 nm; 1 K = 1 cm−1) from thermospheric altitudes at the geographic south pole are used to determine the relationship between the southern hemisphere high‐latitude thermospheric wind circulation and the interplanetary magnetic field (IMF) during the austral winter of 1989. A clear dependence is shown between the thermospheric wind direction and magnitude and the IMF. In the midnight sector, the zonal wind magnitude is dependent on By, and the meridional component on Bz. The magnetic local times of the largest polar cap electric fields are also inferred for either sign of By. In addition, it is shown that the field angle Φ of the IMF in the Y–Z plane is also useful for ordering the neutral wind data. These ground‐based wind measurements also reflect the polarity and magnitude of the IMF, at least near the observing station's magnetic midnight.

High‐latitude dayside electric fields and currents during strong northward interplanetary magnetic field: Observations and model simulation

On July 23, 1983, the Interplanetary Magnetic Field turned strongly northward, becoming about 22 nT for several hours. Using a combined data set of ionospheric convection measurements made by the Sondre Stromfjord incoherent scatter radar and convection inferred from Greenland magnetometer measurements, we observe the onset of the reconfiguration of the high‐latitude ionospheric currents to occur about 3 min following the northward IMF encountering the magnetopause. The large‐scale reconfiguration of currents, however, appears to evolve over a period of about 22 min. Using a computer model in which the distribution of field‐aligned current in the polar cleft is directly determined by the strength and orientation of the interplanetary electric field, we are able to simulate the time‐varying pattern of ionospheric convection, including the onset of high‐latitude “reversed convection” cells observed to form during the interval of strong northward IMF. These observations and the simulation results indicate that the dayside polar cap electric field observed during strong northward IMF is produced by a direct electrical current coupling with the solar wind.

A model study of how electric field structures affect the polar cap F region

A three‐dimensional time‐dependent ionospheric model was used to study how electric field structures affect the polar F region. The electric field structures are represented by elongated Volland two‐cell models whose dimensions range from tens to 1000 km. These model structures are intended to represent the polar cap electric field for IMF Bz northward conditions. A statistical method is used to generate a set of these structures. Their electric field strength and polarity are varied in order to study the F region's dependence on this magnetospheric input. For electric field structures whose size and electric field strengths are consistent with Bz northward observations, several ionospheric dependencies were found. The NmF2 parameter, in general, decreased due to the presence of the higher electric fields associated with the structures. Decreases ranging from a few percent to a factor of 4 were obtained from the simulations. This decrease in NmF2 was frther complicated by the initial NmF2 conditions, i.e., past history of the flux tube, being quite different when the structure is present. HmF2 in the vicinity of a structure can be raised or lowered by up to 100 km, however, this change is not uniquely dependent upon the vertical induced drift. The role of enhanced reaction rates due to elevated ion temperatures is very important. The F region becomes spatially very structured. The modulation of this structure in NmF2 is up to an order of magnitude in this study.

IMF By and day‐night conductivity effects in the expanding polar cap convection model

During southward Bz periods the open field line region in the ionosphere (polar cap) expands due to increased dayside merging. Ionospheric plasma flow patterns result which can be classified by the sign of the interplanetary magnetic field (IMF) By component. In this paper we construct a time‐dependent ionospheric convection model to simulate these flows. The model consists of a spiral boundary with a gap in it. The sign of the IMF By component determines the geometry of the gap. A potential is applied across the gap and distributed around the boundary. A flow results which enters the polar cap through the gap and uniformly pushes the boundary outward. Results of the model show that By effects are greatest near the gap and virtually unnoticeable on the nightside of the polar cap. Adding a day‐night ionospheric conductivity gradient concentrates the polar cap electric field toward dawn. The resulting flow curvature gives a sunward component that is independent of By. These patterns are shown to be consistent with published observations.

By‐dependent convection patterns during northward interplanetary magnetic field

Observations of Birkeland currents, electric fields, and auroral forms in the dayside polar regions during periods of northward interplanetary magnetic field (IMF) can be unified to fit into consistent convection patterns ordered principally by the IMF By component. During periods of weak IMF By, two polar convection cells are symmetrically located on either side of the noon‐midnight meridian, producing sunward convection over the pole, as previously reported. As the IMF By becomes significantly positive (negative), the dawn (dusk) convection cell expands across the northern polar cap, whereas the other cell shrinks. In the southern polar cap it is the dusk (dawn) cell which expands. During strong By this cell expansion gives the appearance of a single convection cell in the polar region. At the “collapsed” cell a large convective flow gradient is developed where reversal of the northward Bz (NBZ) Birkeland current system, antisunward polar ionospheric current, and the most intense polar cap electric fields are statistically observed. We suggest that this convection gradient region is also associated with sun‐aligned arcs and the transpolar arc of the theta aurora, which are observed in the polar regions during northward IMF. The convection patterns proposed here are consistent with the antiparallel merging model in which the IMF merges with the geomagnetic field lines in the tail lobe during northward IMF intervals. The resulting convection in the tail brings plasma from the plasma sheet to the tail. This process divides the open field lines of the lobes and produces an area of closed field lines in the polar cap associated with the transpolar arc of the theta aurora.

Fourier analysis of polar cap electric field and current distributions

A theoretical study of high‐latitude electric fields and currents, using analytic Fourier analysis methods, is conducted. A two‐dimensional planar model of the ionosphere with an enhanced conductivity auroral belt and field‐aligned currents at the edges is employed. Two separate topics are treated. A field‐aligned current element near the cusp region of the polar cap is included to investigate the modifications to the convection pattern by the east‐west component of the interplanetary magnetic field. It is shown that a sizable one‐cell structure is induced near the cusp which diverts equipotential contours to the dawnside or duskside, depending on the sign of the cusp current. This produces characteristic dawn‐dusk asymmetries to the electric field that have been previously observed over the polar cap. The second topic is concerned with the electric field configuration obtained in the limit of perfect shielding, where the field is totally excluded equatorward of the auroral oval. When realistic field‐aligned current distributions are used, the result is to produce severely distorted, crescent‐shaped equipotential contours over the cap. Exact, analytic formulae applicable to this case are also provided.

Eddy intrusion of hot plasma into the polar cap and formation of polar‐cap arcs

We present plasma and electric field data obtained by the S3‐3 satellite over the polar caps. We demonstrate that: (1) plasma signatures in the polar cap arc formation region near 5000 km altitude show clear intrusions of plasma sheet ( ∼ keV) and magnetosheath ( ∼ 100 eV) plasma into a background of low‐energy polar cap plasma; (2) the combined plasma and electric field signatures (electron inverted‐V, ion beam and ▽ • < 0) are exactly the same as in the evening discrete arc. We interpret this equivalence of polar cap and evening discrete arc signatures as indication that their formation processes are identical. The spatial structures of polar cap electric fields and the associated plasma signatures are consistent with the hypothesis that plasma intrusion into the polar cap takes the form of multiple cellular eddies. This hypothesis provides a unifying view of arc formation and arc configurations.

UHF scintillation activity over polar latitudes

Polar cap ionospheric irregularities have been monitored for several years from Thule Air Base, Greenland using 250 MHz satellite beacon scintillation measurements. The seasonal pattern of the polar cap irregularities shows very high intensity levels during the winter and lower levels during the summer (sunlit) months. This behavior is similar to in‐situ polar cap electric field measurements which show larger fluctuations in the winter than in the summer, an effect which may be related to E layer conductivity changes. A striking contrast was noted between high scintillation levels observed during 1979/80, a year of high solar flux, and much lower levels observed during 1975, a year of low solar flux. This variation may be related to a corresponding solar cycle variation in polar cap F layer electron density. The data reveal little difference between periods of high and low Kp, and only a weak diurnal variation in any season. Direct optical and ionosonde measurements indicate that these scintillations are produced by ionospheric irregularities in the polar cap F‐region. Results of spaced receiver drift measurements indicate that the small scale of irregularity drift was antisunward. Intense irregularities are associated with discrete sun aligned F layer auroras. A weak background level of scintillation persisted in the high solar flux years.

Quantitative dependence of the polar cap electric field on the IMF Bz-component and solar wind velocity

Ten years data set is used to separate the influence of IMF B z -component and solar wind speed on the dawn-dusk component of magnetic variations in the summer polar cap. The reference level was chosen from most quiet periods of winter solstices (small polar cap and auroral zone conductivity) to exclude the inner source component. The linear regression analysis was then used to calculate the PC variation response to B z under different ranges of solar wind speed. As a result, taking into account the value of polar cap conductivity and effects of induced currents, the response of dawn-dusk electric field component to B z and V was obtained and the potential difference across the polar cap was estimated to be Δϕ( kV) ≈ 6( V 300 ) 2 − 9B z(γ) for B z ≲ + 1 γ. The results give a proof for simultaneous operation in the magnetosphere of two electric field generation mechanisms, related to the boundary layer processes and magnetic field reconnection. The above-mentioned functional form was shown to correlate effectively with AE index ( R = 0.73).

Influence of the Bx component of the interplanetary magnetic field on magnetopause reconnection

A first clear evidence of an effect of the Bx component (in Solar Magnetospheric Coordinates) of the Interplanetary Magnetic Field (IMF) on magnetopause reconnection has been obtained from polar ionospheric electric fields measured with balloons during an away sector of the IMF. This Bx ‐ effect is such that larger electric field values were measured when Bx was small, and vice versa, during intervals of time with Bz ≤ O, and while the transverse component of the IMF and the solar wind speed had approximately constant values. For those cases, changes of Bx form 2 to 8 gammas are found to account for as much as a factor of two decrease in the magnitude of the convection electric field at the polar cap. When Bz > O, there is also some indication that the measured dusk to dawn polar cap electric field (reversed convection) became larger when |Bx| was small and vice versa.