Field-aligned currents above an auroral arc

Simultaneous observations of precipitating electrons and protons in the energy range from 15 eV to 35 keV and magnetic field variations were made onboard a sounding rocket payload launched from the Andoya Rocket Range. The electric current density deduced from the electron precipitation observed during the passage over an auroral arc was comparable to that determined from the magnetic field variations. In addition, a downward current was observed by its magnetic field signature at the northern edge of the arc which was, however, not accompanied by significant particle fluxes in the energy range under consideration. It will be assumed that this current was carried by thermal electrons of ionospheric origin.     

Time-dependent behaviour of a stable auroral red arc excited by an electric field

We examine the electric field hypothesis as a possible explanation of a stable auroral red arc. An electric field perpendicular to the geomagnetic field in the ionosphere heats the ambient F-region electrons and ions. Given large enough electric fields, the electrons can be heated sufficiently to excite the OI (¹D) term of atomic oxygen by electron impact, giving rise to the λ6300 emission characteristic of the red arc. The electron and ion heating rates are determined by the relative drift between the plasma and neutral gas.     

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec⁻¹) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ω_s = ∂ ν_x/ ∂y (28 sec⁻¹). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ω_s, implies an unneutralized electron density 0–1 cm⁻³ and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.     

Solitary auroral arc generation

A theory of the solitary auroral arc generation is suggested. It is based on the existence of inclined currents over the arc and on the generation by those currents of a field-aligned electric field at some distance from the ionosphere.     

Currents over the auroral arc

The three-dimensional current system over an enhanced conductivity strip identified with an auroral arc is calculated for the case of the magnetospheric plasma convection across this strip. The strip produces a stationary Alfvén wave which propagates along magnetic field lines and is carried simultaneously by the convecting plasma. The Alfvén wave generation corresponds to an appearance of field-aligned currents over the arc. The three-dimensional current system generated over the arc is studied, taking into account reflection of the waves from the ionosphere of the opposite hemisphere. The correspondence of the theory with the experimental results is found.     

Explorer 34 magnetic field measurements near the tail current sheet and auroral activity

Explorer 34 (Imp 4) 2.56 s magnetic data during 131 traversals of the tail current sheet are presented along with simultaneous 2.5 min auroral electrojet indices AE and AL. The normal magnetic field,B , satellite crossing times and positions are tabulated for these 131 crossings.B is defined in the center of the sheet: it is the vector magnetic field at the time of field minimum during the crossing (B _x component changes sign). It is remarkable that the only normal components too large in magnitude to be classified as fine structure occur near the time of onset of an AE event. Cases are discussed where the normal component, defined near the plasma sheet edges, has the opposite sign compared to the normal component defined at the sheet center. For quiet times, the current sheet may be only about 1000 km thick within a 3R _e (Earth-radii) plasma sheet, and may carry some 10–15% of the total tail current.     

Transient thermospheric heating and movement caused by an auroral electric field

Two closed form solutions for the velocity distribution of the upper thermosphere were found using magnetohydrodynamic formalism. One corresponds to a constant altitude, is timedependent, and has non-moving boundaries. This case asympotically approaches the steady solution obtained by Cole (1971). The other solution corresponds to a time- and altitude-dependence case with free boundaries. Solutions of electrodynamic (joule) and viscous heating for both cases are given. Some numerical results corresponding to the latter case are presented. It is clearly demonstrated that joule heating is dominant within the electric field region, and that viscous heating becomes important in the neighbourhood of the electric field region. It is also shown that the induced movement extends beyond the electric field region as far as four times the original width of the electric field region.     

A study of the dynamics of a discrete auroral arc

High resolution electric field and particle data, obtained by the S23L1 rocket crossing over a discrete prebreakup arc in January 1979, are studied in coordination with ground observations (Scandinavian Magnetometer Array—SMA, TV and all-sky cameras) in order to clarify the electrodynamics of the arc and its surroundings. Height-integrated conductivities have been calculated from the particle data, including the ionization effects of precipitating protons and assuming a steady state balance between ion production and recombination losses. High resolution optical information of arc location relative to the rocket permitted a check of the validity of this assumption for each flux tube passed by the rocket. Another check was provided by a comparison between calculated (equilibrium values) and observed electron densities along the rocket trajectory. A way to compensate for the finite precipitation time when calculating the electron densities is outlined. The height-integrated HalI-Pedersen conductivity ratio is typically 1.4 within the arc and about 1 at the arc edges, indicative of a relatively softer energy spectrum there. The height-integrated conductivities combined with the DC electric field measurements permitted calculation of the horizontal ionospheric current vectors (J_⊥), Birkeland currents (from div J_⊥) and energy dissipation through Joule heating (Σ_pE²). An eastward current of typically 1 A m^?1 was found to be concentrated mainly to the arc region and equatorward of it. A comparison has been made with the equivalent current system deduced from ground based magnetometer data (SMA) showing a generally good agreement with the rocket results. An intense Pedersen current peak (1.2 A m^?1) was found at the southern arc edge. This edge constituted a division line between a very intense (> 10 μA m^?1) and localized (~ 6 km) downward current sheet to the south, probably carried by upward flowing cold ionospheric electrons and a more extended upward current sheet (> 10 μA m^?2) over the arc carried by measured precipitating electrons. Joule and particle heating across the arc were anticorrelated, consistent with the findings of Evans et al. (1977) with a total value of about 100mW m^?2.     

Electric field around an accreting star

The accretion of matter on a neutron star is considered. The electric field is studied in the case of a null magnetic field and of a dipole field. The relevance of the results for models of X-ray sources is examined.     

The aries auroral modelling campaign: characterization and modelling of an evening auroral arc observed from a rocket and a ground-based line of meridian scanners

An auroral arc system excited by soft electrons was studied with a combination of in situ rocket measurements and optical tomographic techniques, using data from a photometer on a horizontal, spinning rocket and a line of three meridian scanning photometers. The ground-based scanner data at 4709, 5577, 8446 and 6300 Å were successfully inverted to provide a set of volume emission rate distributions in the plane of the rocket trajectory, with a basic time resolution of 24 s. Volume emission rate profiles, derived from these distributions peaked at about 150 km for 5577 and 4709 Å, while the 8446 Å emission peaked at about 170 km with a more extended height distribution. The rocket photometer gave comparable volume emission rate distributions for the 3914 Å emission as reported in a separate paper by McDade et al. (1991, Planet. Space Sci. 39, 895). Instruments on the rocket measured the primary electron flux during the flight and, in particular, the flux precipitating into the auroral arc overflown at apogee (McEwen et al., 1991; in preparation). The local electron density and temperature were measured by probes on the rocket (Margot and McNamara (1991; Can. J. Phys. 69, 950). The electron density measurements on the downleg were modelled using ion production rate data derived from the optical results. Model calculations of the emission height profile based on the measured electron flux agree with the observed profiles. The height distribution of the N₂⁺ emission in the equatorward band, through which the rocket passed during the descent, was measured by both the rocket and the ground-based tomographic techniques and the results are in good agreement. Comparison of these profiles with model profiles indicates that the exciting primary spectrum may be represented by an accelerated Maxwellian or a Gaussian distribution centered at about 3 keV. This distribution is close to what would be obtained if the electron flux exciting the poleward form were accelerated by a 1–2 kV upward potential drop. The relative height profiles for the volume emission rate of the 5577 Å OI emission and the 4709 Å N₂⁺ emission were almost indistinguishable from each other for both the forms measured, with ratios in the range 38–50; this is equivalent to I(5577)/I(4278) ratios of 8–10. The auroral intensities and intensity ratios measured in the magnetic zenith from the ground during the period before and during the rocket flight are consistent with the primary electron fluxes and height distributions measured from the rocket. Values of I(5577)/I(4278) in the range 8–10 were also measured directly by the zenith ground photometers over which the arc system passed. These values are slightly higher than those reported by Gattinger and Vallance-Jones (1972) and this may possibly indicate an enhancement of the atomic oxygen concentration at the time of the flight. Such an enhancement would be consistent with our result, that the observed values of I(5577) and I(8446) are also significantly higher than those modelled on the basis of the electron flux spectrum measured at apogee.     

Bi-dimensional observation of waves near the mesopause at auroral latitudes

During a campaign of optical observations at high latitude, a bi-dimensional study of the wave structure of the OH layer has been performed in December 1981 from Sodankyla (Finland). This site is one of the three stations of the EISCAT ionosphere sounding system. It has been found that a wave field covering an area of 1 million km² may extend to latitudes as high as 70°N. The OH wave structure shows many similarities with noctilucent clouds. The fairly large horizontal wavelength, of the order of 40 km cannot easily be explained by a wave motion at an interface. The observed wave structure seems to be a result of the propagation of an internal gravity wave in the 80–100 km region. This wave structure was often recorded during the same time as an active aurora was present. As a result, it appears that the perturbation might be correlated with particle precipitations at auroral latitudes.     

Field-aligned current reversals and fine structure in a dayside auroral arc

A serendipitous event is reported in which the MAGSAT satellite intercepted an auroral arc over Svalbard, Norway where an all-sky television camera, a photographic camera and a meridian scanning photometer were making continuous measurements. The high time resolution of the optical measurements and the high spatial resolution of the magnetometer data are combined to investigate the relationship between the fine structure in the field-aligned current reversals and the temporal and spatial morphology of the auroral structure. Meridian scans of several optical emissions in the auroral arc, which had its upper portion in sunlight, are utilized to derive the total energy input and the intensity of the precipitating energetic electrons. The MAGSAT satellite apparently intercepted a fold within an extended intense upward current sheet. The current carried by the energetic electrons responsible for the optical aurora is found to be smaller than the field-aligned current derived from the magnetic perturbations, implying that there may be a large flux of low energy particles in this arc. Within the spatial-temporal constraints of this event there is a suggestion that the rayed structure is related to the field-aligned current reversals.     

Kinetic Alfvén waves on auroral field lines

Several observations near moving arcs require particle acceleration in nonstationary electric fields. We suggest that kinetic Alfvén waves play a significant role in the acceleration process. The characteristic properties of kinetic Alfvén waves are summarized and the Hasegawa and Mima (1976) solitary kinetic Alfvén waves are described. The resonant coupling of large-scale surface waves to the kinetic Alfvén wave is discussed. Finally, we show that kinetic Alfvén waves can reasonably well explain the observations of what has hence been called “electrostatic” shocks.     

Large-scale auroral distribution and the open field line region

An example of global auroral distribution is presented which is clearly more circular than oval and is thus fit to an offset circle. The area surrounded by the aurora is also compared with the open region constructed by a model of the open magnetosphere for the IMF condition about 1 h prior to the auroral observation.     

Umbral oscillations measured in the Stokes-V inversion point

The inversion point of the circular Zeeman polarization profile (V-Stokes) parameter is used to observe umbral Doppler oscillations free from disturbing influences of parasitic light. In a second step, purely umbral lines are used to avoid remaining influences from the V-profile of the (oscillating) penumbra. Among a total of nine sunspot umbrae, three exhibit oscillations within the various 1.5 to 2.5 hr samples. The periods differ significantly from 300 s, vary with time, and occur within time intervals of high tranquility thus explaining the lack of oscillations in the remaining sunspots.     

Influences of the interplanetary magnetic field on the auroral dynamics

This paper reports the study concerning the latitudinal dispalacement of the auroral oval as a function of the northward orientation of the B_z-component IMF and the relation between southward B_z and the auroral dynamics in the night sector.     

Transient heating of the upper atmosphere by auroral electric field

Magnetohydrodynamic formulation has been used to deduce the velocity distribution of the upper atmospheric movement caused by the auroral electric field at the thermospheric height. The expressions for Joule heating and viscous heating are obtained. Numerical analysis has been made to estimate their magnitudes as well as the rate of their variations with time. The results are presented graphically.     

Some aspects of electric field mapping in the auroral ionosphere

The mapping of the spectra of electrostatic field below 300 km altitude is theoretically calculated for a horizontally stratified auroral ionosphere. Perpendicular electric fields of large scale size are the same for different altitudes of the ionosphere. However, electric fields of small scale size vary with altitude and decrease drastically when the scale size is smaller than a certain value which depends on altitude. These results are similar to those observed by satellites above 300 km altitude. In the case of a homogeneous anisotropic ionosphere, analytical results are obtained for the penetration of electric field into the ionosphere as a function of wavenumber. The “smoothing” of the electric field when penetrating a horizontally stratified ionosphere is demonstrated. The smallest possible scale of parallel electric field structure within the ionosphere is found. Also presented is a method of finding the smallest horizontal length with which the electric field can penetrate the ionosphere with little distortion. For an average conductivity model, this length is found to be about 1 km. Finally, the mapping of packets of electric field to the ground is constructed.     

Configuration of the auroral oval and the interplanetary magnetic field

The poleward boundary of the auroral oval, whose footline forms the periphery of the polar cap, is calculated, based on a model in which the geomagnetic field is interpermeated with the interplanetary field. It is shown that the calculated auroral oval size varies with the strength and direction of the interplanetary magnetic field, in agreement with recent observations of the location of large-scale nightside auroras.     

Artificial oscillations of geomagnetic field line

Forced oscillations of a geomagnetic field tube are theoretically studied. The oscillation source is the ionosphere the conductivity of which is modulated artificially. The amplitude-frequency characteristics of the tube are calculated in the Pc3–5 frequency range.