Ionospheric Conductivity Gradients Research Articles

Statistical study of geomagnetic field line resonances observed by CHAMP and on the ground

Studies of single events showing the signatures of Pc3 pulsation field line resonances (FLRs) observed in magnetic field data by CHAMP above the E‐region ionosphere and at conjugate ground stations at L < 2 have previously been reported. Notable features of FLRs observed in these investigations were a Doppler shift in the satellite azimuthal component and a 90° rotation of the polarization ellipse between CHAMP and the ground. In this report, we extend the previous work by means of the first statistical investigation of the same features for a large number of events using CHAMP and ground magnetic field data. Our statistical study confirms the Doppler shifts to higher or lower frequencies relative to the ground FLR frequencies for equatorward or poleward passes of CHAMP over the ground station. Our statistical investigation shows the amount of rotation of the FLR polarization ellipses between the ionosphere and the ground to be ~100° on average, rather than the theoretically predicted 90°. This is explained by the polarization ellipses on the ground being oriented close to N‐S irrespective of the orientation above the ionosphere, which is −13° on average. We find no evidence of ionospheric conductivity gradients associated with sunrise and sunset affecting the amount of rotation of the polarization ellipses as predicted by more recent theoretical advances. We are not able to provide conclusive evidence on the relationship between the polarization ellipse rotation and azimuthal wave number as predicted in recent numerical modeling.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti-parallel to one another. Our results show that in general the equivalent ionospheric currents are anti-parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti-parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value-added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere-ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti‐parallel to one another. Our results show that in general the equivalent ionospheric currents are anti‐parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti‐parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value‐added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere‐ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Statistical study of O+ transport from the cusp to the lobes with Cluster CODIF data

Ionospheric origin O+ accelerated in the cusp/cleft region convects over the polar cap and flows along open field lines to the tail lobes. Some O+ in the lobes enters the near‐Earth plasma sheet where it is then convected to the inner magnetosphere, while some O+ ends up in the distant tail where it is lost. In order to understand the transport of ionospheric O+ to the plasma sheet so as to understand its contribution to the formation of geomagnetic storms, we have determined the occurrence frequency of cusp source O+ over the polar caps and in the lobes to determine where and when it is observed. The results show that the probability of observing O+ along the transport path is high even during nonstorm times, although, as expected, the highest probability is found during storm times. It was also found that when interplanetary magnetic field (IMF) By is positive, O+ from the northern cusp/cleft tends to stream toward the dawnside tail lobe while O+ from the south are observed on the duskside. The transport path for negative IMF By is more symmetric, but shows some evidence for a reversed asymmetry when IMF By is strongly negative. IMF Bz has little influence on the asymmetry. The asymmetry for positive By and lack of mirror symmetry between positive and negative By most likely result from the combination of convection driven by the solar wind and coupling with the ionosphere. Similar asymmetries have been observed in the convection patterns over the polar caps, which are attributed to a day‐night ionospheric conductivity gradient adding to the IMF By effect. However, there are some disagreements between the asymmetries observed in polar cap potential patterns and the asymmetries observed in the O+ spatial distribution, indicating there may be other causes for the symmetry breaking, in addition to the day‐night conductivity gradient.

Small and meso‐scale properties of a substorm onset auroral arc

We present small and meso‐scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground‐based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth‐phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted‐V structure together with a strong Alfvén wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field‐aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso‐scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope‐like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfvén resonator that can drive structuring in luminosity over the range of scales observed.

Plasma convection jets near the poleward boundary of the nightside auroral oval and their relation to Pedersen conductivity gradients

Abstract. In this work, we have shown that the ionospheric azimuthal plasma velocity jets near the open-closed field line boundary on the nightside can be associated with the peak in the ionospheric conductivity gradient. Both model and DMSP observations have been utilized to conduct this investigation. The model tests show that when the gradient of conductivity in the poleward boundary becomes sharper, convection peaks appear around the poleward edge of the aurora. The model results have been confirmed by DMSP observations. Hundreds of large ion flow events are identified from one year DMSP observations, with flow speed larger than 500 m/s that occurred poleward of the aurora. Among them, 280 (74%) events are found to be associated with conductivity gradient peaks. Most of the convection jets occur in winter when conductivity gradients are expected to be large. The convection jets tend to occur at later local times (21:00–22:00 MLT) at 70°–72° MLat. These events are preceded by increasing of the merging electric field suggesting that they occur after the expansion of the polar cap. Both observation and model results show that the conductivity gradient at the polar cap boundary are one of the important elements in establishing the convection jets.

Plasma flow channels at the dawn/dusk polar cap boundaries: momentum transfer on old open field lines and the roles of IMF <I>B<sub>y</sub></I> and conductivity gradients

Abstract. Using DMSP F13 data in conjunction with IMF data we investigate the newly discovered channels of enhanced (1.5–3 km/s) antisunward convection occurring at the dawn (06:00–09:00 MLT) or dusk (15:00–18:00 MLT) flanks of the polar cap for different combinations of IMF By polarity, hemisphere (NH/SH) and the dawn/dusk MLTs. Dawn-side cases where this flow channel appears occur for the following combinations: NH-dawn/By>0 and SH-dawn/By<0. The dusk-side cases are: NH-dusk/By<0 and SH-dusk/By>0. The flow channels are placed in the context of particle precipitation regimes/boundaries and ionospheric conductivity gradients. They are found to be threaded by "old open field lines" ("time since reconnection" >10 min) characterized by polar rain precipitation. In the dawn-side cases (NH-dawn/By>0 and SH-dawn/By<0) and in a Parker spiral field, the polar rain contains the "solar wind strahl" component. The convection enhancement is attributed to the Pedersen current closure of Birkeland current sheets (C1 and C2) in the polar cap (C1) and at the polar cap boundary (C2). The low ionospheric conductivity in the polar cap, particularly in the winter hemisphere, is compensated by an enhanced electric field driving the flow channel there. This is momentum transfer from the solar wind via dynamo action taking place in the combined current system of the high- and low-latitude boundary layers (HBL/LLBL). The conductivity gradient at the polar cap boundary contributes to establishing the convection channel and the associated enhancement of the dawn-dusk convection asymmetry extending beyond the dawn-dusk terminator during intervals of nonzero IMF By component. The HBL/LLBL-ionosphere coupling via Birkeland currents C1/C2 is a source of dawn-dusk convection asymmetry and Svalgaard-Mansurov effect which must be added to the effect of magnetic tension acting on "newly open field lines".

A statistical study of the local time asymmetry of Pc 5 ULF wave characteristics observed at midlatitudes by SAMNET

A total of 129 Pc 5 events were identified in more than a year of mid‐latitude magnetometer data from the United Kingdom Sub‐Auroral Magnetometer Network (SAMNET). The horizontal polarization and azimuthal wavenumbers associated with these events were estimated, and their variations with magnetic local time were studied. A marked local time asymmetry was apparent in the Pc 5 wave characteristics with distinctly different polarization and azimuthal wave characteristics occurring on either side of noon. The morning sector observations of predominantly clockwise polarization and westward phase propagation suggest that the majority of the morning sector Pc 5 waves are the midlatitude signature of solar wind driven field line resonances occurring at higher latitudes. It is suggested that the most likely energy sources for these waves are the Kelvin‐Helmholtz instability at the magnetopause boundary or cavity/waveguide modes prompted by impulsive variations in the solar wind. The large variation in the polarization azimuths (ellipse orientations) observed in the morning sector was interpreted as a result of the large ionospheric conductivity gradients that occur around and after dawn, which alter the ionospheric rotation of Alfvén waves from 90°. It is suggested that the afternoon sector Pc 5 waves are the ground signature of compressional waves in the magnetosphere, mainly cavity/waveguide modes. There is little evidence of field line resonances in the afternoon sector. The possibly dominant role of cavity/waveguide modes in the magnetosphere is heightened by the observation of preferential frequencies in both the morning and afternoon sectors.

The polarization of Pc5 ULF waves around dawn: A possible ionospheric conductivity gradient effect

Abstract. In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.

The polarization of Pc5 ULF waves around dawn: A possible ionospheric conductivity gradient effect

In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.

Dependence of the region II field-aligned currents on ionospheric conductivity gradients

The effect of the ionospheric conductivity distribution on the field-aligned currents in the polar ionosphere is systematically studied. It is shown that the intensity and the location of the Region II field-aligned current in the magnetosphere-ionosphere coupling are critically dependent on the latitudinal gradient of the Pedersen conductivity in the morning and afternoon sectors around the auroral oval. Moreover, the Region II current density is also found to increase as the conductance ratio Σ H/Σ P is reduced by increasing Σ P.

Generation of field-aligned currents in magnetosphere-ionosphere coupling in a MHD plasma

From numerical modeling, the ionospheric closure of the Region I field-aligned currents is found to be governed by the vorticity of the ionospheric convection while the closure of Region II currents is governed by the ionospheric conductivity gradients. From the MHD equations, it is found that the magnetospheric generation of the Region I field-aligned currents is likely to be dominated by J · ( ω × V) V 2 A which depends on the angular acceleration (ω × V) in the MHD momentum equation where ω( = Δ × V ) is the vorticity, V is the flow velocity, V A is the Alfvén speed and J is the current density. The Region II currents are commonly accepted as generated by 2 J · Δ⊥ B B . One part of the Region I currents, termed “source” current, originates from the dynamo regions (i) on open field lines and (ii) on closed field lines in the antisunward flow region of the plasma sheet. The other part, termed “response” current, originates from the motor region in the sunward flow region of the plasma sheet. Vorticity can be generated by the J × B force through the field-aligned currents called for by the ionosphere except possibly in the low-latitude boundary layer where the vorticity may be generated by the viscous force to produce field-aligned currents.

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.

A time-dependent analytical model with day-night ionospheric conductivity gradient for magnetospheric convection

A time-dependent analytical model has been used to study the effect of the day-night ionospheric conductivity gradient on the magnetosphere-ionosphere coupling and field-aligned currents distribution. The basic idea of the work is the same as that of Senior-Blanc's model. The mathematical difficulties caused by the conductivity gradient have been overcome and analytical solutions have been deduced for a finite driving potential φ( r 0) which is applied to a magnetosphere initially at rest and maintain invariant later on. It is shown that when the day-night gradient of conductivity is taken into account, the time constant of the evolution toward the steady state will vary with local time. Under some special conditions the steady state may even be unattainable within a certain time period around noon. The distribution of the electric potential calculated from the model used here indicates a dawn-dusk asymmetric pattern even in the polar cap and auroral zone, and the calculated field aligned currents show that, even for a driving potential that only has sin ϑ component, the two peaks of the region-1 currents are not at dawn and dusk. They are before noon and after noon. The feature predicted by the model calculation for the field-aligned currents and potential distribution is in agreement with the statistical picture of measurements. This supports the linear approximation of ring-current motions and the analytical description of the ionosphere/magnetosphere circuit suggested by Senior and Blanc. This also confirms the significance of the effect of the day-night conductivity gradient on the circuit.

A study of high-latitude E-field patterns using simple models with day-night ionospheric conductivity gradient

Simple models with the effect of the day-night ionospheric conductivity gradient are used to discuss the high-latitude convective electric field pattern. The exponential function exp ( −x L ) is used to express the day-night variation in conductivity. Three types of models are used in this work: Model 1: The electropotential distribution is a sine function at the polar cap boundary if we neglect effects of region-2 field-aligned current and the conductivities enhancement within the auroral zone; Model 2: With two regions of field-aligned currents, and without conductivity enhancement in the auroral zone; Model 3: Both the two regions of field-aligned currents and the enhancement of conductivity within the auroral zone are taken into account. The comparison between the different models helps us to understand the convective pattern under different geomagnetic conditions. It also shows the combined effects of the field-aligned currents and the variation of the conductivities (both the day-night gradient and the enhancement within the auroral zone). The results can also offer reasonable explanations to some phenomena observed, such as the apparent discrepancy between different observations concerning the dawn-dusk asymmetry of the convective pattern, and some features of the field-aligned currents.

Effects of Birkeland current limitation on high‐latitude convection patterns

It is shown how the high‐latitude convection pattern may be modified by substorm‐enhanced polarization electric fields. These are generated whenever the flow of those Birkeland currents which are associated with ionospheric conductivity gradients is limited. Such Birkeland currents are fed mainly by the enhanced Pedersen current in the evening and morning sectors of the auroral oval and by the enhanced Hall current around local midnight. As the current limitation increases, the ionospheric potential, represented here by a symmetric two‐cell pattern, will rotate clockwise and deform, as will the associated Birkeland current distribution, which will attain roughly a symmetric shape relative to the noonmidnight magnetic meridian. The resulting patterns are shown to agree well with observations. A pronounced westward intrusion of the equipotential contours occurs in the auroral oval and may be associated with the westward traveling surge. This feature does not, however, require any assumed longitudinal conductivity gradients. Rather, it falls out naturally when the closure of the enhanced Pedersen current is also taken into account.

Auroral electrodynamics with current and voltage generators

The electrodynamic structure of auroral currents is studied in the steady state by coupling Maxwell's equations with effective Ohm's laws for ionospheric Pedersen currents, field‐aligned currents and currents in the generator region. An effective generator conductivity is introduced which allows consideration of a variety of states from pure current (ΣG = 0) to pure voltage (ΣG → ∞) generators. On short time scales when Alfvén waves are important, this conductivity mapped to the ionosphere is lower than the ionospheric conductivity, implying a currentlike generator. This is in contrast with previous electrodynamic models of the aurora, which have assumed voltage‐fixed generators. Current generators differ from the voltage generators in that the natural scale size is smaller for current generators, indicating that the narrow discrete arcs may be current driven, while the overall large‐scale auroral currents are voltage driven. In addition, reversals of the electric field occur naturally in the current‐driven case, leading to V‐shaped potential structures. Examination of the effects of ionospheric conductivity gradients and comparison with a time‐dependent MHD model of auroral currents indicate that either regions of nonlinear resistivity or generator motions can decouple the ionospheric currents from the generator. In this case the effective conductivity becomes the Alfvén wave conductivity, which, being small compared to the ionospheric conductivity, leads to a structure typical of a current‐driven system. This implies that the current structure is not affected by ionospheric conductivity gradients, while the ionospheric electric field is enhanced on the low‐conductivity side.

Concerning the structure of Pi 2 pulsations

We look at some simple theoretical considerations relevant to the origin of Pi 2 geomagnetic pulsations observed on the ground with the onset of geomagnetic activity. Using a basic model, we examine how subsidiary current flows are created by auroral ionospheric conductivity enhancements in the electrojet regions. These current flows, which include field‐aligned currents, are associated with reflected shear Alfvén waves localized to the field lines which thread ionospheric conductivity gradients. Thus the ionosphere can create distinct latitudinal structure in the signal. We discuss the origin of the observed polarization pattern and predominantly westward phase propagation of Pi 2 pulsations in the light of our results. We conclude that a superposition of two travelling waves of differing amplitude is required to produce these characteristics.

A Modified Composite Wave Technique for OMEGA

A proposal is made for improvement of J. A. Pierce's composite wave technique for the reduction of diurnal and solar flare effects on OMEGA navigation signals. Based on calculations of a fictitious free-space transmission time Tc and multi-frequency phase measurements, a general formalism is presented for the compensation of phase shifts due to changes in the earth-ionosphere waveguide parameters. In principle, phase measurements at (n + 1) discrete frequencies are necessary to compensate for variations in n parameters such as reference height, ionospheric conductivity gradient, etc. For n = 1 (compensation of height changes), explicit expressions for Tc are derived from various models of the earth-ionosphere waveguide using phase measurements recorded at the basic OMEGA frequencies of 10.2 and 13.6 kHz. For a flat earth with perfectly conducting boundaries, the results are very similar to those of Pierce. More realistic waveguide models (spherical earth with exponential ionosphere) result in a considerable reduction of diurnal shifts and anomalies due to solar disturbances. It is also shown that no further improvements result from the addition of a third signal because of unrealistic accuracy requirements for phase measurements.

A note on lower ionosphere equivalent height diurnal variation

Five months of very-low-frequency phase measurements along the propagation paths Balboa (Panama)-Sao Paulo and Criggion (GB)-Sao Paulo, were studied in order to find apparent and equivalent diurnal height variation for the upper boundary of the earth-ionosphere waveguide. The results suggest that an explanation of the apparent height variation dependence on the wavelength and propagation path may be found if the second order mode of propagation is considered during nighttime and the diurnal change of the ionospheric conductivity gradient is taken into account. The results fit well with an “average ionosphere” model presenting nighttime conductivity gradient of 0.5 km ?1 and daytime gradient of 0.3 km ?1 . As a consequence, the equivalent diurnal height variation found is situated near 10 km, the assumption of a single waveguide configuration for both propagation paths being valid. DOI: 10.1111/j.2153-3490.1968.tb00411.x