A model current system for the magnetospheric substorm

We propose a model three-dimensional current system for the magnetospheric substorm, which can account for the new findings of the field-aligned and ionospheric currents obtained during the last few years by using new techniques. They include (1) the ionospheric currents at the auroral latitude deduced from the Chatanika incoherent scatter radar data, (2) the field-aligned currents inferred from the vector magnetic field observations by the TRIAD satellite and (3) the global distribution of auroras with respect to the auroral electrojets appearing in DMSP satellite photographs. The model current system is also tested by a computer model calculation of the ionospheric current pattern. It is shown that the auroral electrojets have a strong asymmetry with respect to the midnight meridian. The westward electrojet flows along the discrete aurora in the evening sector, as well as along the diffuse aurora in the morning sector. The eastward electrojet flows equatorward of the westward electrojet in the evening sector. It has a northward component and joins the westward electrojet by turning westward across the Harang discontinuity. Thus, the latitudinal width of the westward electrojet in the morning sector is much larger than that in the evening sector. The field-aligned currents, consisting of two pairs of upward and inward currents (one is located in the morning sector and the other in the evening sector), are closed neither simply by the east-west ionospheric currents nor by the north-south currents, but by a complicated combination of the north-south and east-west paths in the ionosphere. The magnetospheric extension of the current system is also briefly discussed.     

The role of photoemission in the coupling of the Mercury surface and magnetosphere

Photoelectrons are emitted from the surface of Mercury and take part in charge exchanges with the magnetosphere. We investigate the role of photoemission in closing field-aligned currents and in balancing the flow of magnetospheric electrons which precipitate to the surface. The conductance of the photoelectron sheath and sub-surface rock material are estimated and compared with that of the exosphere. It is shown that the loss cone angle is always larger than 30–40° on the dayside, and that significant electron precipitation takes place. It is concluded that the closure of field-aligned current is unlikely because the conductance of the surface environment is too low, but that photoemission may alter the isotropy of the magnetospheric electron energy distribution and induce plasma instabilities.     

Atmospheric expansion through Joule heating by horizontal electric fields

Incoherent scatter measurements made along a magnetic field line into aurora during a period of high electric field in the recovery phase of a substorm show (1) considerably increased electron densities well above the normal F-region maximum, and (2) field-aligned plasma drifts that increase with altitude. A model invoking atmospheric expansion through Joule heating by the horizontal electric field driving the auroral electrojet is used to explain the observations. From this study it is concluded that during magnetically disturbed periods (1) Joule heating by the auroral electrojet raises the neutral temperature and density in the auroral zone ionosphere at F-region heights, (2) ionization formed by the aurora is transported upward by the expanding atmosphere, at times producing an appreciable increase in lower exospheric plasma densities on the field lines containing the aurora, and (3) combined satellite, radar, and optical observations during periods of aurora and high electric field could provide measured F-region collision frequencies.     

Modulation of Jovian middle magnetosphere currents and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: initial response and steady state

We present results from a theoretical model which has been used to investigate the modulation of the magnetosphere-ionosphere coupling currents in the Jovian middle magnetosphere by solar wind-induced compressions and expansions of the magnetosphere. We consider an initial system in which the current sheet field lines extend to 50R_J in the equatorial plane, and where the iogenic plasma in the current sheet undergoes steady outward radial diffusion under the influence of the ionospheric torque which tends to maintain corotation with the planet. We show using typical Jovian parameters that the upward-directed field-aligned currents flowing throughout the middle magnetosphere region in this system peak at values requiring the existence of significant field-aligned voltages to drive them, resulting in large precipitating energy fluxes of accelerated electrons and bright ‘main oval’ UV auroras. We then consider the changes in these parameters which take place due to sudden expansions or compressions of the magnetosphere, resulting from changes in the solar wind dynamic pressure. Two cases are considered and compared, these being first the initial response of the system to the change, determined approximately from conservation of angular momentum of the radially displaced plasma and frozen-in field lines, and second the subsequent steady state of steady outward radial diffusion applied to the compressed or expanded system. We show that moderate inward compressions of the outer boundary of the current sheet field lines, e.g. from 50 to 40R_J, are effective in significantly reducing the coupling currents and precipitation in the initial state, the latter then recovering, but only partly so, during the evolution to the steady state. Strong inward compressions, e.g. to 30R_J cause significant super-corotation of the plasma and a reversal in sense of the current system in the initial state, such that bright auroras may then be formed poleward of the usual ‘main auroral oval’ due to the ‘return’ currents. The sense of the currents subsequently reverts back to the usual direction as steady-state conditions are restored, but they are weak, and so is the consequent electron precipitation. For outward expansions of the current sheet, however, the field-aligned currents and electron precipitation are strongly enhanced, particularly at the poleward border mapping to the outer weak field region of the current sheet. In this case there is little evolution of the parameters between the initial expansion and the subsequent steady state. Overall, the results suggest that the Jovian middle magnetosphere coupling currents and resulting ‘main oval’ auroral acceleration and precipitation will be strongly modulated by the solar wind dynamic pressure in the sense of anti-correlation, through the resulting compressions and expansions in the size of the magnetosphere.     

Observation of Birkeland currents with the TRIAD satellite

The intimate connection between geomagnetic storms and the aurora was appreciated by many early scientists including Edmund Halley and Anders Celsius, but the first serious study of this phenomena was made by Kristian Birkeland who, during his polar expeditions of 1902–1903, determined that large-scale ionospheric current were associated with the aurora. Birkeland was also the first to suggest that these currents originated far from the Earth and that they flowed into and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned orBirkeland currents was widely disputed because it was not possible to unambiguously identify current systems that are field-aligned (Alfvén, 1939, 1940) and those which are completely contained in the ionosphere (Vestine and Chapman, 1938) only from a study of surface magnetic field measurements. During the last decade, the presence of Birkeland currents has been absolutely confirmed with particle and magnetic field observations acquired from a variety of rocket and satellite instruments. The vector magnetometer on the low-altitude (800 km) polar orbiting TRIAD satellite has been used to determine for the first time the flow direction, spatial distribution, and intensities of Birkeland currents in the north and south auroral regions. These observations support the mechanism originally proposed by Alfvén (1939, 1940)-later expanded by Shieldet al. (1969)-to drive Birkeland currents in the auroral regions, and they demonstrate the important role that these intense currents (ranging between 10⁶ and 10⁷ amperes) play in the coupling of energy between the magnetosphere and the lower ionosphere and atmosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Riometer absorption events at SANAE,L=4.0

The absorption of cosmic radio noise passing through the ionosphere may be described as a function of radio wave frequencyA(f _e ) f _e ^-n , with n 2.0 for spatially uniform precipitation of electrons and n < 2.0 for spatially nonuniform precipitation. Using multifrequency riometer recordings at SANAE, the following observations are reviewed: (1) The frequency distribution of the power index, n, obtained from 4 min averaged absorptions during 1983, shows a most probable value around n 1.5, indicating that mostly energetic electrons are precipitated spatially structured onto the upper atmosphere, as in optical aurora. (2) Multifrequency riometer recordings suggest that field-aligned ionospheric irregularities have scattered additional cosmic radio waves from the central region of the Galaxy into the fields of views of the riometer antennae during an auroral absorption event in the early morning hours of 27 July, 1982. With the power reflectivity by ionospheric irregularities inversely proportional to the fourth power of radio wave frequency, as required by the Bragg condition, an estimated 70% increase in the 20 MHz radio flux at 01:22 UT, at the strong absorption peak, can explain the strongly reduced absorption observed in 20 MHz relative to 30 and 51.4 MHz. (3) Gradual increases in absorptions observed at all three riometer frequencies from onset at 11:50 UT of the largest solar proton ground level enhancement on 29 September, 1989, until 18:00 UT, suggest diffusion of the much more intense low energy protons from the polar cap to the L=4.0 geomagnetic field shell and subsequent precipitation at SANAE due to the South Atlantic Geomagnetic Anomaly. (4) The flux of electron energy deposited per second at SANAE is closely related to geomagnetic activity, but has a lower maximum during the years 1971 and 1980 of solar polar magnetic reversals than in the years 1976 and 1986/87 of minimum solar activity. (5) A significant correlation has been found between the arrival of single-hop whistlers and 30 MHz riometer absorption events, using point statistics. The maximum absorption at 30 MHz was 0.04 dB with a delay of 3 ± 2 s relative to the whistler.     

Rocket-borne and groundbased observations of coincident field-aligned currents,electron beams,and plasma density enhancements in the afternoon auroral oval

Results are reported from a rocket experiment conducted at Søndre Strømfjord, Greenland, on 22 August 1976, at 16.00 M.L.T. A series of plasma, particles, and fields and wave experiments were carried on board the payload, and the venture was supported by data from the AE-C satellite and by groundbased ionosondes and magnetometers at the launch site and at Godhavn. Two regions of field-aligned electron precipitation, electron density and temperature enhancements, and field-aligned upflowing current sheets were intercepted by the rocket. The density enhancements were also observed by groundbased ionosondes. Significant discrepancies were found between the currents carried by the streaming electrons in the 0.15–10 keV range and the upflowing currents seen by the on board magnetometer, suggesting that the upflowing current could not be the primary driver of the electron acceleration mechanism. The E-region was unstable to the combined Gradient-Drift and Farley-Buneman instability, and plasma turbulence was observed in situ, but the absolute density fluctuations were too small to return detectable HF-radar power to the ground.     

Hubble observations of Jupiter’s north–south conjugate ultraviolet aurora

Comparisons of the northern and southern far ultraviolet (UV) auroral emissions of Jupiter from the Hubble Space Telescope (HST) or any other ultraviolet imager have mostly been made so far on a statistical basis or were not obtained with high sensitivity and resolution. Such observations are important to discriminate between different mechanisms responsible for the electron acceleration of the different components of the aurora such as the satellite footprints, the «main oval» or the polar emissions. The field of view of the ACS and STIS cameras on board HST is not wide enough to provide images of the full jovian disk. We thus compare the morphology of the north and south aurora observed 55 min apart and we point out similarities and differences. On one occasion HST pointed successively the two polar regions and auroral images were seen separated by only 3 min. This makes it possible to compare the emission structure and the emitted FUV power of corresponding regions. We find that most morphological features identified in one hemisphere have a conjugate counterpart in the other hemisphere. However, the power associated with conjugate regions of the main oval, diffuse or discrete equatoward emission observed quasi-simultaneously may be different in the two hemispheres. It is not directly nor inversely proportional to the strength of the B-field as one might expect for diffuse precipitation or field-aligned acceleration with equal ionospheric electron density in both hemispheres. Finally, the lack of symmetry of some polar emissions suggests that some of them could be located on open magnetic field lines.     

Field-aligned currents and parallel electric field potential drops at Mars. Scaling from the Earth’ aurora

The observations of electron inverted ‘V’ structures by the MGS and MEX spacecraft, their resemblance to similar events in the auroral regions of the Earth, and the discovery of strong localized magnetic field sources of the crustal origin on Mars, raised hypotheses on the existence of Martian aurora produced by electron acceleration in parallel electric fields. Following the theory of this type of structures on Earth we perform a scaling analysis to the Martian conditions. Similar to the Earth, upward field-aligned currents necessary for the generation of parallel potential drops and peaked electron distributions can arise, for example, on the boundary between ‘closed’ and ‘open’ crustal field lines due to shears of the flow velocity of the magnetosheath or magnetospheric plasmas. A steady-state configuration assumes a closure of these currents in the Martian ionosphere. Due to much smaller magnetic fields as compared to the Earth case, the ionospheric Pedersen conductivity is much higher on Mars and auroral field tubes with parallel potential drops and relatively small cross scales to be adjusted to the scales of the localized crustal patches may appear only if the magnetosphere and ionosphere are decoupled by a zone with a strong E_∥. Another scenario suggests a periodic short-circuit of the magnetospheric electric fields by a coupling with the conducting ionosphere.     

The diffuse aurora

A study of ground-based all-sky photographs substantiates the presence of the diffuse auroral belt as seen by the ISIS-2 (polar orbiting satellite) scanning auroral photometer. The intensity of the diffuse aurora increases when discrete auroras become active; in particular the diffuse aurora is most clearly seen equatorward of westward travelling surges. However, in the morning sector, it may or may not be detectable near eastward drifting patches in all-sky photographs. Some of what has been previously identified visually and in all-sky photographs as the proton aurora probably is a part of what we identify here as the diffuse aurora.The diffuse aurora appears sometimes to branch out into two, one along the auroral oval and the other along a constant geomagnetic latitude circle. The latter probably corresponds to the mantle aurora and the drizzle zone precipitation.     

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.     

Birkeland currents in the Earth's magnetosphere

As a result of his polar expeditions at the beginning of this century, Kristian Birkeland determined that intense ionospheric currents were associated with the aurora. Birkeland suggested that these currents originated far from the Earth and that they flowed ointo and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned or Birkeland currents was disputed because it was not possible to unambiguously identify current systems that are field-aligned (as suggested by Alfvén, 1939, 1940) and those which are completely contained in the ionosphere (as developed by Vestine and Chapman, 1938) with surface magnetic field observations. The presence of Birkeland currents has been absolutely confirmed with satellite-borne particle and magnetic field experiments conducted over the past two decades. These satellite observations have determined the large-scale patterns, flow directions, and intensities of Birkeland currents in the auroral and polar regions, and their relationship to the orientation and magnitude of the interplanetary magnetic field. The Birkeland currents are directly associated with visible and UV auroral forms observed with satellites. The results obtained from a variety of recently launched satellites are discussed here. These include Sweden's first satellite, VIKING, which has provided evidence for resonant Alfvén waves on the same geomagnetic field lines that guide stationary Birkeland currents. These observations demonstrate the important role that these currents play in the coupling of energy between the interplanetary medium and the lower ionosphere and atmosphere.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Time-dependent studies of the aurora: Effects of particle precipitation on the dynamic morphology of ionospheric and atmospheric properties

A self-consistent, time-dependent numerical model of the aurora and high-latitude ionos-phere has been developed. It is used to study the response of ionospheric and atmospheric properties in regions subjected to electron bombardment. The time history of precipitation events is arbitrarily specified and computations are made for a variety of electron spectral energy distributions and flux magnitudes. These include soft electron precipitation, such as might occur on the poleward edge of the auroral oval and within the magnetospheric cleft, and harder spectra representative of particle precipitation commonly observed within and on the equatorward edge of the auroral oval. Both daytime and night-time aurorae are considered. The results of the calculations show that the response of various ionospheric and atmospheric parameters depends upon the spectral energy distribution and flux magnitudes of the precipitating electrons during the auroral event. Various properties respond with different time constants that are influenced by coupling processes described by the interactive model. The soft spectrum aurora affects mainly the ionospheric F region, where it causes increases in the electron density, electron temperature and the 6300 Å red line intensity from normal quiet background levels during both daytime and night-time aurora. The fractional variation is greater for the night-time aurora. The hard spectrum aurorae, in general, do not greatly affect the F-2 region of the ionosphere; however, in the F-1 and E regions, large increases from background conditions are shown to occur in the electron and ion temperatures, electron and ion densities, airglow emission rates and minor neutral constituent densities during the build-up phase of the auroral event. During the decay phase of the aurora, most of these properties decrease at nearly the same rate as the specified particle precipitation flux. However, some ionospheric and atmospheric species have a long memory of the auroral event. The odd nitrogen species N(⁴S) and NO probably do not ever reach steady-state densities between auroral storms.     

Electric fields and currents in the ionosphere generated by field-aligned currents observed by TRIAD

On the basis of the experimental data on the ionospheric conductivities and field-aligned currents the electric fields and currents in the ionosphere generated by the field-aligned currents were computated for various magnetic activity conditions. The model of the ionospheric conductivities by Vanyan and Osipova (1975) was used taking into account the influence of the universal time seasons and magnetic activity. The field-aligned current patterns and their change with magnetic activity was set on the basis of the TRIAD data. It is shown that the calculated patterns of the ionospheric electric fields and currents are in agreement with the measured electric fields and the equivalent current systems of the magnetic disturbances in high latitudes. The conclusion is made that the magnetospheric field-aligned currents are the main sources of the presently known polar magnetic disturbances.     

Short-wavelength drift dissipative instability in the presence of field-aligned currents

Sharp density gradients coupled with field-aligned currents can give rise to short wavelength (?15 m) drift waves due to collisional effects in the F-region of the auroral ionosphere. In this wavelength range, ion-ion collisions at altitudes of 300–450 km render the ions unmagnetized and a field-aligned current can drive a drift wave, propagating almost transverse to the magnetic field, unstable due to the resistance in electron parallel motion arising from electron collisions.     

Rädler effect in a collisionless plasma and field-aligned currents in the magnetotail

The mean electromotive force perpendicular to the mean current (Rädler effect) by random hydromagnetic waves in a collisionless plasma is derived. The results are applied to the field-aligned currents in the Earth's magnetotail. It is shown that the Rädler-effect electric field is large enough to give the observed value of the field-aligned currents and can be identified as a possible source for the field-aligned currents.     

Precipitation of auroral particles in the central polar cap during a sudden commencement magnetic storm

All-sky camera observations from two stations in the inner (northern) polar cap and an auroral zone station are combined with photometer records from the polar cap station Nord in a study of the brilliant auroral display following the ssc of the storm of 7 November 1970. This display is the large, poleward expanding bulge of a substorm triggered by the ssc. It is composed of brilliant discrete forms embedded in low-intensity diffuse electron and proton aurora. The poleward edge of the diffuse electron aurora is 5° north of the discrete auroras and 3° north of the proton aurora. The intensity of the discrete aurora varies as the strength of the auroral electrojet as shown by magnetograms from auroral zone stations. Succeeding the retreating display a subvisible low-energy electron precipitation, which may be identified as the polar squall (Winningham and Heikkila, 1974) is observed over the polar cap during the main phase of the storm.In the early morning sector already existing diffuse auroras broaden towards the equator from the time of the ssc and at least during the following half hour.Ssc-triggered displays have been found (Feldstein, 1959) to withdraw from the inner polar cap as the initial (positive H) phase of the storm ends. A comparison of the records from seven low-latitude stations shows that during this particular storm the positive phase appears to be composed by two overlapping disturbances, i.e. the proper initial phase, which is generally thought to be due to compression of the inner magnetosphere and a series of positive bays accompanying the negative bays in auroral latitudes. These positive bays are observable over a great range of longitudes with a maximum of amplitude near midnight. As judged from the dayside magnetograms the initial (compression) phase ends at an early stage of the substorm. The observed coincidence between the withdrawal of the display and the cessation of the positive H phase of the storm is a consequence of the fact that the second component—the positive bays—and the auroral display over the polar cap are both signatures of the substorm activity.     

The Auroral electrojet and field-aligned current

The location of field-aligned currents in the evening sector with respect to the auroral electrojets is examined. The tri-axial TRIAD satellite data and the simultaneous ground magnetometer data from along the Alaska meridian are analysed. It is shown that an intense upward fieldaligned current flows out from the region of the westward electrojet where discrete auroras are located. The downward flowing current exists in the region further equatorward, namely in the region of the eastward electrojet. However, the downward current is present even when there is no eastward electrojet. The boundary between the upward and the downward currents coincides, in most cases, with the boundary between the westward and the eastward auroral electrojets. Thus, the Harang discontinuity, a narrow area separating the positive and negative H bays, is the region where there is no field-aligned current.     

Electron precipitation equatorward of the auroral oval and the mantle aurora in the midday sector

In the midday sector, the hard electron precipitation and the associated patchy aurora at geomagnetic latitude ～65° are the only auroral features (? 20 keV) located equatorward of the dayside auroral oval during intense and moderately disturbed geomagnetic conditions. We identify the patchy luminosity in the midday and late morning sectors as the active mantle aurora. The mantle aurora was found by Sanford (1964) using the IGY-IGC auroral patrol spectrographs and which was thought to be non-visual. The precipitating electrons reside mostly at energies greater than several keV with an energy flux of ? 0.1 erg cm^?2 s^?1 sr^?1 during geomagnetic active periods. This hard precipitation occurs in a region which is asymmetric in L.T. with respect to the noon meridian. The region extends from the morning sector to only early afternoon (13–14 M.L.T.) along the geomagnetic latitude circle of about 65–70°. The model calculation indicates that the mantle aurora is produced by the precipitation of the energetic electrons which drift azimuthally from the plasma sheet at the midnight sector to the dayside magnetopause during magnetospheric substorms.     

Plasma sheet particle precipitation: A kinetic model

Ionospheric and plasma sheet particle densities, fluxes and bulk velocities along an auroral magnetic field line have been calculated for an ion-exosphere model. It is shown that such a collisionless model accounts for many features observed above the auroral regions. Except for very strong plasma sheet electron precipitation, no large potential difference is needed along the magnetic field lines to account for the usual proton and electron fluxes, their pitch angle distributions, and auroral field aligned currents.