Height-integrated Pedersen Conductivity Research Articles

Plasma dynamics in Saturn's middle-latitude ionosphere and implications for magnetosphere-ionosphere coupling

A multifluid model is used to investigate how Saturn's magnetosphere affects ionosphere. The model includes a magnetospheric plasma temperature of 2eV as a boundary condition. The main results are: (1) H+ ions are accelerated along magnetic field lines by ambipolar electric fields and centrifugal force, and have an upward velocity of about 10km/s at 8000km; (2) the ionospheric plasma temperature is 10,000K at 5000km, and is significantly affected by magnetospheric heat flow at high altitudes; (3) modeled electron densities agree with densities from occultation observations if the maximum neutral temperature at a latitude of 54˚ is about 900K or if electrons are heated near an altitude of 2500km; (4) electron heating rates from photoelectrons (≈100K/s) can also give agreement with observed electron densities when the maximum neutral temperature is lower than 700K (note that Cassini observations give 520K); and (5) the ion temperature is high at altitudes above 4000km and is almost the same as the electron temperature. The ionospheric height-integrated Pedersen conductivity, which affects the magnetospheric plasma velocity, varies with local time with values between 0.4 and 10S. We suggest that the sub-corotating ion velocity in the inner magnetosphere depends on the local time, because the conductivity generated by dust–plasma interactions in the inner magnetosphere is almost comparable to the ionospheric conductivity. This indicates that magnetosphere–ionosphere coupling is highly important in the Saturn system.

Height-integrated Pedersen conductivity in both E and F regions from COSMIC observations

Altitudinal distribution of Joule heating is very important to the thermosphere and ionosphere, which is roughly proportional to the Pedersen conductance at high latitudes. Based on the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites observations from 2008 to 2011, the height-integrated Pedersen conductivities in both E (100–150km) and F (150–600km) regions and their ratio γP (∑PE/∑PF) have been calculated. The result shows that the maximum ratio in the northern summer hemisphere is ~5.5, which is smaller than that from the Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIE-GCM v1.94) simulation (~9). This indicates that the energy inputs into the F region may be underestimated in the model. The seasonal variations of the ratio have been investigated for both hemispheres, and an interhemispheric asymmetry has been identified. The variational trend of the ratio is similar in both hemispheres, which reaches minimum at local summer and maximum at local winter. However, the difference of the ratio from local summer to local winter in the southern hemisphere is larger than that in the northern hemisphere.

Hybrid Kelvin-Helmholtz/Rayleigh-Taylor instability in the plasma sheet

[1] This paper is to generalize the previous linear analysis of the hybrid Kelvin-Helmholtz/Rayleigh-Taylor (KH/RT) instability in a system of the magnetosphere-ionosphere (M-I) coupling, which is valid in the long-wavelength limit of Lky ≪ 2, where ky is the azimuthal wave number and L stands for the scale length of a latitudinal profile in azimuthal velocity or energy density. For the first time, the linear growth rate profile of a hybrid wave can be revealed in an essentially overall range of ky. When the gradient of the energy density is directed inward (toward inner magnetic shells), the hybrid growth rate is positive (meaning growth) in a certain range of ky: 0 < Lky < m. As the inertial relaxation rate v approaches zero, the growth rate curve is close to the KH “spectrum” with m = 2, where v ≡ ΣP/Cm, the ratio between the inertial capacitance of the magnetospheric plasma and the height-integrated Pedersen conductivity in the ionosphere. Without a velocity shear in the background flow, the spectrum much broadens, namely, the RT spectrum appears. The hybrid growth rate is shown to have a spectrum intermediate between the two ones of the corresponding KH and RT “root” instabilities. It is also found that the electrostatic KH instability is completely suppressed when the KH growth rate, estimated excluding the M-I coupling, is less than v, which is a straightforward extension of that previous work. On the basis of the evaluation of Cm in the Tsyganenko model, electrostatic waves (with wavelengths greater than a few tens of kilometers at the ionospheric height) are unlikely to grow by the KH instability alone in the nighttime plasma sheet. Instead, hybrid waves can grow at the places where the particle energy density has an inward gradient. This fact may account for the frequent appearance of periodic auroral luminosity in a high-latitude part of the nighttime oval as well as the formation of omega bands typically in the recovery phase of a substorm.

Ionospheric feedback effects on the quasi-stationary coupling between LLBL and postnoon/evening discrete auroral arcs

Abstract. We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.

Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution

Abstract. Based on Astrid-2 satellite data, results are presented from a statistical study on subauroral ion drift (SAID) occurrence. SAID is a subauroral phenomenon characterized by a westward ionospheric ion drift with velocity greater than 1000m/s, or equivalently, by a poleward-directed electric field with intensity greater than 30mV/m. SAID events occur predominantly in the premidnight sector, with a maximum probability located within the 20:00 to 23:00 MLT sector, where the most rapid SAID events are also found. They are substorm related, and show first an increase in intensity and a decrease in latitudinal width during the expansion phase, followed by a weakening and widening of the SAID structures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID events were calculated. The results reveal that the strongest SAID electric field peaks are associated with the lowest Pedersen conductivity minimum values. Clear modifications are seen in the ionospheric Pedersen conductivity distribution associated with the SAID structure as time evolves: the SAID peak is located on the poleward side of the corresponding region of reduced Pedersen conductivity; the shape of the regions of reduced conductivity is asymmetric, with a steeper poleward edge and a more rounded equatorward edge; the SAID structure becomes less intense and widens with evolution of the substorm recovery phase. From the analysis of the SAID occurrence relative to the mid-latitude trough position, SAID peaks are seen to occur relatively close to the corresponding mid-latitude trough minimum. Both these features show a similar response to magnetospheric disturbances, but on different time scales - with increasing magnetic activity, the SAID structure shows a faster movement towards lower latitudes than that of the mid-latitude trough. From the combined analysis of these results, we conclude that the SAID generation mechanism cannot be regarded either as a pure voltage generator or as a pure current generator, applied to the ionosphere. While the anti-correlation between the width and the peak intensity of the SAID structures with substorm evolution indicates a magnetospheric source acting as a constant voltage generator, the ionospheric modifications and, in particular the reduction in the conductivity for intense SAID structures, are indicative of a constant current system closing through the ionosphere. The ionospheric feedback mechanisms are seen to be of major importance for sustaining and regulating the SAID structures. Key words. Ionosphere (mid-latitude ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)

Experimental study of the formation of inverted-V structures and their stratification using AUREOL-3 observations

Abstract. Multiple inverted-V structures are commonly observed on the same auroral zone crossing by a low-altitude orbiting satellite. Such structures appear grouped and apparently result from an ionospheric and/or magnetospheric mechanism of stratification. More than two years of AUREOL-3 satellite observations were analyzed to study their properties and their formation in the framework of the ionosphere-magnetosphere coupling model proposed by Tverskoy. This model predicts some natural periodicity in the electrostatic potential profile (and subsequently in the field-aligned current profiles) that could account for oscillations experimentally observed in the auroral zone, such as successive inverted-Vs. Experimental results obtained during quiet or moderately active periods demonstrate that the number of structures observed within a given event is well described by a 'scaling' parameter provided by the hot plasma stratification theory and expressed in terms of the field-aligned current density, the total width of the current band, the plasma sheet ion temperature, and the height-integrated Pedersen conductivity of the ionosphere. The latitudinal width, in the order of 100–200 km at ionospheric altitudes, is relatively independent of the current density, and is determined not only by the existence of a potential difference above the inverted-Vs, but also by basic oscillations of the ionosphere-magnetosphere coupling system predicted by Tverskoy. The large number of cases studied by the AUREOL-3 satellite provides reliable statistical trends which permits the validation of the model and the inference that the multiple structures currently observed can be related directly to oscillations of the magnetospheric potential (or the pressure gradients) on a scale of ~1000-2000 km in the near-Earth plasma sheet. These oscillations arise in the Tverskoy model and may naturally result when the initial pressure gradients needed to generate a large-scale field-aligned current have a sufficiently wide equatorial scale, of about 1 RE or more.Key words: Magnetospheric physics (current systems; energetic particles, precipitating; magnetosphere-ionosphere interactions)

Polar spacecraft based comparisons of intense electric fields and Poynting flux near and within the plasma sheet‐tail lobe boundary to UVI images: An energy source for the aurora

In this paper, we present measurements from two passes of the Polar spacecraft of intense electric and magnetic field structures associated with Alfven waves at and within the outer boundary of the plasma sheet at geocentric distances of 4–6 RE near local midnight. The electric field variations have maximum values exceeding 100 mV/m and are typically polarized approximately normal to the plasma sheet boundary. The electric field structures investigated vary over timescales (in the spacecraft frame) ranging from 1 to 30 s. They are associated with strong magnetic field fluctuations with amplitudes of 10–40 nT which lie predominantly in the plane of the plasma sheet and are perpendicular to the local magnetic field. The Poynting flux associated with the perturbation fields measured at these altitudes is about 1–2 ergs cm−2 s−1 and is directed along the average magnetic field direction toward the ionosphere. If the measured Poynting flux is mapped to ionospheric altitudes along converging magnetic field lines, the resulting energy flux ranges up to 100 ergs cm−2s−1. These strongly enhanced Poynting fluxes appear to occur in layers which are observed when the spacecraft is magnetically conjugate (to within a 1° mapping accuracy) to intense auroral structures as detected by the Polar UV Imager (UVI). The electron energy flux (averaged over a spatial resolution of 0.5° ) deposited in the ionosphere due to auroral electron beams as estimated from the intensity in the UVI Lyman‐Birge‐Hopfield‐long filters is 15–30 ergs cm−2s−1. Thus there is evidence that these electric field structures provide sufficient Poynting flux to power the acceleration of auroral electrons (as well as the energization of upflowing ions and Joule heating of the ionosphere). During some events the phasing and ratio of the transverse electric and magnetic field variations are consistent with earthward propagation of Alfven surface waves with phase velocities of 4000–10000 km/s. During other events the phase shifts between electric and magnetic fields suggest interference between upward and downward propagating Alfven waves. The E/B ratios are about an order of magnitude larger than typical values of c/Σp, where Σp is the height integrated Pedersen conductivity. The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1–2 orders of magnitude larger than that estimated from the large‐scale steady state convection electric field and field‐aligned current system.

Experimental test of a hot stratification theory for inverted-V events using AUREOL-3 observations

We present AUREOL-3 satellite observations of multiple inverted-V structures and a statistical analysis of their formation. Their number, one to four inverted-V structures, is compared to the predictions of a hot plasma stratification theory based on assumption that plasma in the central plasma sheet is in magnetostatic equilibrium, and that plasma sheet ion motion is unmagnetized. According to this theory, only upward field-aligned current bands can split into inverted-V structures. Their number is determined by a parameter Γ that depends on current density, current latitudinal width, hot magnetospheric ion temperature, and height-integrated Pedersen conductivity. In the majority of the cases the theory predicts rather well the number of observed structures.

Extrapolating EISCAT Pedersen conductances to other parts of the sky using ground-based TV auroral images

Abstract. Ionospheric conductivity is not very easily measured directly. Incoherent scatter radars perhaps offer the best method but can only measure at one point in the sky at any one time and are limited in their time resolution. Statistical models of average conductivity are available but these may not be applied to individual case studies such as substorms. There are many instances where a real-time estimate of ionospheric conductivity over a large field-of-view is highly desirable at a high temporal and spatial resolution. We show that it is possible to make a reasonable estimate of the noctural height-integrated Pedersen conductivity, or conductance, with a single all-sky TV camera operating at 557.7 nm. This is not so in the case of the Hall conductance where at least two auroral wavelengths should be imaged in order to estimate additionally the energy of the precipitating particles.Key words. Atmospheric composition and structure (airglow and aurora) · Magnetospheric physics (auroral phenomena · instruments and techniques)

Ionospheric conductance distribution and MHD wave structure: observation and model

Abstract. The ionosphere influences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are different eigenvalues and eigensolutions of the three dimensional toroidal wave equation if the height integrated Pedersen conductivity exceeds a critical value, namely the wave conductance of the magnetosphere. As a result a jump in frequency can be observed in ULF pulsation records. This effect mainly occurs in regions with gradients in the Pedersen conductances, as in the auroral oval or the dawn and dusk areas. A pulsation event recorded by the geostationary GOES-6 satellite is presented. We explain the observed change in frequency as a change in the wave structure while crossing the terminator. Furthermore, selected results of numerical simulations in a dipole magnetosphere with realistic ionospheric conditions are discussed. These are in good agreement with the observational data.Key words. Ionosphere · (Ionosphere · magnetosphere interactions) · Magnetospheric physics · Magnetosphere · ionosphere interactions · MHD waves and instabilities.

Effects of wind-induced ionization layers on ionospheric electrodynamics

Abstract The electrodynamic consequences of ionization layers produced by zonal and meridional winds are examined by investigating the height integrated Pedersen conductivity, wind driven currents, and their spatial gradients, in the presence and absence of such layers. We find that a wind induced layer itself has little effect on the height integrated Pedersen conductivity when the process of layer creation involves depletion as well as enhancement of the nominal background profile. The altitude gradients in the conductivity associated with the layer are both positive and negative and in the presence of neutral winds with long vertical wavelengths, they contribute little to the height integrated current divergence. However, short vertical wavelength meridional winds responsible for layer formation drive zonal currents with a divergence that depends on the horizontal scale of the layer itself and is likely to be small compared to the vertical gradients. A significant contribution to the gradient in the height integrated meridional current is produced by the combined action of the layer conductivity and a short vertical wavelength zonal wind. In this case the presence of a layer can easily change this gradient by a factor of 2 and thus significantly effect the global electrodynamics.

Ionospheric conductance distribution and MHD wave structure: observation and model

The ionosphere influences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are different eigenvalues and eigensolutions of the three dimensional toroidal wave equation if the height integrated Pedersen conductivity exceeds a critical value, namely the wave conductance of the magnetosphere. As a result a jump in frequency can be observed in ULF pulsation records. This effect mainly occurs in regions with gradients in the Pedersen conductances, as in the auroral oval or the dawn and dusk areas. A pulsation event recorded by the geostationary GOES-6 satellite is presented. We explain the observed change in frequency as a change in the wave structure while crossing the terminator. Furthermore, selected results of numerical simulations in a dipole magnetosphere with realistic ionospheric conditions are discussed. These are in good agreement with the observational data.Key words. Ionosphere · (Ionosphere · magnetosphere interactions) · Magnetospheric physics · Magnetosphere · ionosphere interactions · MHD waves and instabilities.

Mercury's exosphere: origin of surface sputtering and implications

The existence of gaseous species H, He, O, Na and K, established by euv spectroscopic observations on Mariner 10 for the first three and groundbased observations for the last two species with column contents of the order of 10 12 cm −2 or less, qualify the gaseous envelope of Mercury as an exosphere (column content per definition ≤ 10 14 cm −2). Whereas H and He seem to have its origin in the direct supply from the solar wind, the heavier constituents can arise from particle and photon interactions with surface materials. Some observations have suggested that Na extends to 700 km above the surface at the subsolar point, implying the existence of non-thermal atomic velocities of at least 2 km s −1. Although Mercury has a small intrinsic magnetic field, solar wind protons have access to parts of its surface. Since particle sputtering liberates volatiles from the surface at energies greater than the mean thermal energy, the sputtered particle profile is governed by a scale height much larger than the barometric scale height. The surface densities of these sputtered volatiles then exhibit a lower exponential decay than would be expected from a barometric law using the exospheric (surface) temperature. Mercury's exosphere is a low attenuation medium for solar euv radiation which therefore can penetrate to its surface. Under these circumstances no ionospheric “layer” can be formed. The ionization of exospheric species leads to electron-ion pair formation which is balanced by chemical loss and transport processes. Since radiative recombination is an extremely slow loss process for atomic ions, plasma diffusion under gravity, which has a short time constant, controls the ionized species. Thus, the densities of the ionized exospheric component do not exceed a few electrons/ions cm −3. This yields an extremely low height-integrated Pedersen conductivity, which must be taken into account in the understanding of convection processes in the magnetosphere.

Ulysses observations of anti-sunward flow on Jovian polar cap field lines

We examine the energetic (∼MeV) ion data obtained by the Anisotropy Telescopes instrument of the Ulysses COSPIN package during two northern high-latitude excursions prior to closest approach to Jupiter, when the spacecraft left the region of trapped fluxes on closed magnetic field lines at lower latitudes and entered a region of open field lines which we term the polar cap. During these intervals the ion fluxes dropped by 4–5 orders of magnitude to low but very steady values, and the ion spectrum was consistent with the observation of an essentially unprocessed interplanetary population. Ion anisotropies observed at these distances (within ∼16 R J, of Jupiter) indicate that in the low-latitude, high-flux regions the flows are principally azimuthail and in the sense of corotation, with speeds which are within a factor of ∼2 (in either direction) of rigid corotation. In the higher latitude trapped flux regions the flows rotate to become northward as the polar cap is approached, while in the polar cap itself the flows rotate further to become anti-corotational (and anti-sunward in the morning sector) and northward. These results provide primary evidence of the existence of solar wind-driven flows in the outer Jovian magnetosphere mapping to the high-latitude ionosphere. Investigation of concurrent magnetic data for the signatures of related field-aligned currents reveals only weak signatures with an amplitude of order ∼1 nT. The implication is that the height-integrated Pedersen conductivity of the ionosphere to which the spacecraft was connected was low, of order 0.01 mho or less. We also examine the ion observations during the two northern high-latitude excursions previous to those discussed above. These data indicate that the spacecraft approached but did not penetrate the open flux region during these intervals.

Neutral winds and thermosphere/ionosphere coupling and energetics during the geomagnetic disturbances of March 6–10, 1989

Observations of electron density made using the fully steerable 46‐m‐diameter antenna at Millstone Hill have been used to derive the peak electron density (NmF2) and the peak height of the F2 region (hmF2) as a function of latitude during the March 6–10, 1989, period. This period was characterized by varying levels of geomagnetic activity, with a magnetic storm commencing near 1800 UT on March 8. The radar data set presented for this period provides a detailed example of the mid‐latitude ionospheric response to geomagnetic disturbances. The derived hmF2 values are combined with measurements of electric field‐induced ion drifts and the MSIS‐86 model to estimate the meridional neutral winds at thermospheric heights over the geodetic latitude range 30° to 56°N. Strong postmidnight surges in the neutral wind were observed on March 7, 9, and 10 which reached well equatorward of Millstone Hill. The nighttime electron density trough was above Millstone Hill during the disturbances and hmF2 exceeded 500 km in the trough on March 7 and 9. A dusk enhancement in NmF2 followed the magnetic storm commencement on March 8. This is associated with a large increase in westward ion velocity due to the equatorward penetration of magnetospheric electric fields. Large daytime decreases in NmF2, apparently due to a neutral composition disturbance zone, were observed on March 9 and 10, with a sharp gradient on March 9, and a stronger equatorward penetration of the NmF2 decreases on March 10. The Joule heating as a function of latitude is estimated for March 7 and March 9 from calculations of height‐integrated Pedersen conductivity and incoherent scatter electric field measurements. In spite of considerably more Joule heating input at high latitudes on March 9, the postmidnight surge is stronger on March 7. This is explained by a combination of Coriolis and ion drag effects.

Electrodynamics of the polar cusp ionosphere; A case study

By using continuous ground‐based observations of polar cusp auroras and electromagnetic parameters obtained from satellites in polar orbit, electrodynamic processes in the polar cusp have been investigated. HILAT satellite measurements of magnetic field, precipitating electrons and ion drift components above auroral structures observed from the ground permit detailed investigations of the cusp ionosphere. Information on Birkeland currents, Poynting flux, height integrated Pedersen conductivity and Joule heat dissipation rate has been derived. Similar latitudinal structures were observed from HILAT (electron precipitation/ion drift) and in the simultaneous photometer profiles obtained from the ground ∼200 km to the east of the satellite pass. Intermediate scale (tens of kilometers) structures of enhanced electron precipitation (peak energy flux ∼10 ergs/cm²s (0.01 W/m²) and average energy ∼0.1 keV), strong northward electric fields (peak values ∼200 mV/m), and Birkeland current densities ∼10 µA/m² were observed. Rather complex patterns of Birkeland currents were connected to these structures. From correlated latitudinal variations in electric and magnetic field components the downward directed Poynting flux and the ionospheric Joule heat dissipation rate within one of the auroral forms could be estimated. The dissipation rate in the center of this structure was found to be a factor of 2 higher than the electron energy input rate. The satellite pass occurred within an interval (interplanetary magnetic field (IMF) Bz &lt; 0) characterized by a series of poleward moving auroral structures (east‐west motion not resolved by the present technique), each appearing at the cusp equatorward boundary. The auroral forms were dominated by red oxygen emission (I630.0 nm/I557.7 nm ≃5 and I630.0 nm ≃5 kR), which is typical in the cusp region.

Auroral and magnetic variations in the polar cusp and cleft — Signatures of magnetopause boundary-layer dynamics

By combining continuous ground-based observations of polar cleft/cusp auroras and local magnetic variations with electromagnetic parameters obtained from satellites in polar orbit (low-altitude cleft/cusp) and in the magnetosheath/interplanetary space, different electrodynamic processes in the polar cleft/cusp have been investigated. One of the more controversial questions in this field is related to the observed shifts in latitude of cleft/cusp auroras and the relationship with the interplanetary magnetic field (IMF) orientation, local magnetic disturbances (DP2 and DPY modes) and magnetospheric substorms. A new approach which may contribute to clarifying these complicated relationships — simultaneous ground-based observations of the midday and evening-midnight sectors of the auroral oval — is illustrated. A related topic is the spatial relationship between the cleft/cusp auroras and the ionospheric convection currents. A characteristic feature of the polar cusp and cleft regions during negative IMF B Z is repeated occurrence of certain short-lived auroral structures which seem to move in accordance with the local convection pattern. Satellite measurements of particle precipitation, magnetic field and ion drift components permit detailed investigations of the electrodynamics of these cusp/cleft structures. Information on electric field components, Birkeland currents, Poynting flux, height-integrated Pedersen conductivity, and Joule heat dissipation rate has been derived. These observations are discussed in relation to existing models of temporal plasma injections from the magnetosheath.

Birkeland currents correlated with direct-current electric fields observed during the CENTAUR Black Brant X rocket experiment

Vector magnetic-field observations from two rocket flights in the winter polar cusp during the CENTAUR project show several groups of upward and downward flowing small-scale Birkeland current sheets in the afternoon region of the cusp, whereas none are detected at noon. High correlation with electric-field observations allow calculation of the height-integrated Pedersen conductivity, and an estimate is made of the ionospheric input power density associated with the Birkeland currents. This input power density is comparable to that of a similar summertime experiment at nearly the same magnetic local time and under almost the same interplanetary magnetic-field conditions, despite an order-of-magnitude difference in ionospheric conductivity. Indications are that the magnetospheric energy source for these two events more closely resembles a constant-power generator rather than a constant-voltage or a constant-current generator.

Ionospheric Joule dissipation as a damping mechanism for high latitude ULF pulsations: Observational evidence

On 9 January 1979 an SI-excited pulsation event was observed by the Scandinavian Magnetometer Array. The pulsation period shows a clear variation with latitude which suggests decoupled oscillations of individual magnetic field shells. The pulsation amplitudes exhibit an e-fold decay with the damping rate γ varying both in longitudinal and latitudinal directions. Assuming Joule heating in the ionosphere as the dominant damping mechanism (and thus γ ∼ Σ −1 p ) approximate height-integrated Pedersen conductivity profiles were calculated which fit well with previously observed Σ p distributions. This is interpreted as observational evidence for ionosopheric Joule dissipation as the major damping mechanism for high-latitude ULF-pulsations.

A new theoretical approach to the modelling of toroidal Pc5 pulsations

A model is developed to represent a toroidal mode of Pc5 geomagnetic pulsations. It is shown that this model is consistent in its predictions, such as the latitude profiles of amplitude and phase and their dependence on the height integrated Pedersen conductivity, Σ p , with those of Walker's (1980) theory. It is also shown that this theory is relatively easily capable of accommodating (i) a variety of field line plasma mass density distributions, (ii) a variety of external excitation schemes, (iii) unequal Σ p 's at each end of the field lines and (iv) non-dipolar geomagnetic fields. The theory yields the transient as well as the steady state response, an important feature permitting application to short-lived events or to those for which the generator is amplitude modulated. It is shown, for instance, that the amplitude-latitude profile varies during the transient. It is also shown that the steady state latitude profiles of amplitude and phase are the dual of those observed as a function of frequency when the excitation frequency is scanned through a resonance. A more realistic steady state energy flow from a generator along the field lines to the ionosphere is inherent in this theory compared with that from the mode to the ionosphere which is inherent in Walker's theory.