Postnoon Sector Research Articles

Coupling of magnetopause‐boundary layer to the polar ionosphere

The ultraviolet auroral images obtained with the Viking satellite often show spatially periodic bright spots which resemble “beads” or “pearls” aligned on the postnoon auroral oval, which may be due to the coupling of the plasma vortices formed in the boundary layer to the polar ionosphere. Geomagnetic pulsations in the ULF range observed in the cusp region may also be associated with the boundary layer vortices. To explain the observed auroral bright spots and the cusp geomagnetic pulsations, we have developed a model for the plasma dynamics in the low‐latitude boundary layer and its interaction with the polar ionosphere via the field‐aligned current (FAC). In the presence of a driven plasma flow along the magnetopause, the Kelvin‐Helmholtz instability can develop, leading to the formation and growth of plasma vortices in the boundary layer. On the other hand, the finite ionospheric conductivity leads to the decay of these vortices. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in a limited region. Several enhanced field‐aligned power density regions associated with the boundary layer vortices and the upward FAC filaments can be found along the postnoon auroral oval. These enhanced field‐aligned power density regions may account for the observed auroral bright spots. In addition, we found that the FACs are stronger in the prenoon sector than in the postnoon sector due to the presence of the field‐aligned potential drop in the postnoon sector. The larger prenoon FACs produce stronger cusp pulsations in the prenoon sector, consistent with observations. The frequency spectrum and polarization of the generated pulsations are also discussed. On the other hand, the vortices in the postnoon boundary layer are found to be stronger than those in the prenoon boundary layer.

Auroral event sequence at the dayside polar cap boundary: Signature of time-varying solar wind-magnetosphere-ionosphere coupling

A specific category of auroral event sequence taking place at the dayside polar cap boundary during southward-directed interplanetary magnetic field (IMF) is exemplified by three cases of combined ground and satellite observations. The detailed temporal and spatial evolution of these optical events were monitored by all-sky TV imagery mapped onto a geographic coordinate system. Profiles along the satellite track of particle precipitation and ionospheric convection in the vicinity of the optical events were important for the determination of the solar wind - magnetosphere coupling mode involved. The observed north-westward event motion in the prenoon sector is typical for periods of positive IMF B Y. Auroral events associated with negative IMF B Y were moving northeastward in the midday and early post-noon sector. The optical events mostly appear as a brightening of the 630.0 nm aurora near the poleward boundary of the persistent cleft/LLBL precipitation at the boundary of the field-of-view of the TV camera. A continuous motion into the polar cap (mantle precipitation) was observed before the optical emission faded out after 10–15 minutes, typically ∼500 km poleward of the continuous cleft arc. The auroral forms typically cover ∼500 km in the east - west direction and ∼200 km in the north - south direction. A glow of 630.0 nm emission spanning ∼500 km in latitudinal extent was observed during the decay phase of the Jan. 12, 1991 case presented here. The optical, particle and ion drift observations in combination with the available IMF and solar wind plasma data indicate that the events represent dynamical structures of IMF B Y-related Dungey cell convection over the polar magnetosphere, initiated by pulses of enhanced merging rate (B n ≠ 0) at the dayside magnetopause.

Magnetopause plasma transients: mapping into the auroral ionosphere

Attention is focused on a specific category of auroral event occurring predominantly during a southward directed interplanetary magnetic field (IMF). Coordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. Both prenoon and postnoon sector events are reported. In one case the auroral event was observed near the zero point potential line separating the morning and postnoon convection cells. The available data indicate that this auroral event sequence is a signature of quasi-periodic bursts of particle entry from the magnetosheath along flux tubes convecting into the polar cap, representing structures of enhanced IMF B/sub Y/-related convection poleward of the persistent cleft aurora. Such events may be initiated by localized patches of enhanced merging rate at the dayside magnetopause.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of Pc 1 waves by the ion temperature anisotropy associated with fast shocks caused by sudden impulses

Observations have reported on the high correlation of Pc 1 events with magnetospheric compressions. A number of mechanisms have been suggested for the generation of the Pc 1 waves. In this paper, we propose a new mechanism which leads to the generation of Pc 1 waves. The interaction of a dynamic pressure pulse (Δρυ²) with the Earth's bow shock leads to the formation of a weak fast mode shock propagating into the magnetosheath. The shock wave can pass right through a tangential discontinuity (magnetopause) and into the magnetosphere, without disturbing either of the structures. In a quasi‐perpendicular geometry, the shock wave exhibits anisotropic heating with T⊥ &gt; T∥. This anisotropy drives unstable ion cyclotron waves which we believe can contribute to the generation of the Pc 1 waves which are detected. The viability of the mechanism is demonstrated with simulations. This mechanism could explain peak in the occurrence of observed Pc 1 waves in the postnoon sector where a field‐aligned discontinuity in the solar wind would most often be parallel to the magnetopause surface due to the average Parker spiral magnetic field configuration.

AMPTE CCE observations of Pc 3–4 pulsations at L = 2–6

Very few spacecraft observations of magnetic Pc 3–4 pulsations at L &lt; 4 have been reported in the past despite the fact that Pc 3–4 pulsations are a common occurrence on the ground at L &lt; 4. In order to fill this observational gap in the magnetosphere, we have initiated an analysis of magnetic field data acquired by the AMPTE CCE spacecraft. A procedure has been developed to reduce the artificial modulation and noise in magnetic field measurements, with the resultant data useful for studying magnetic pulsations with amplitude less than 1 nT in the 1000 nT (L ∼ 3) background and periods longer than 12 s. From a survey of 41 CCE passes in L = 2–6 and in the postnoon sector, we find that the most commonly observed pulsations in this region are the east‐west oscillations of magnetic field with clearly L‐dependent frequencies. These are identified to be the toroidal mode standing Alfvén waves. From a comparison of CCE and GOES 5 magnetic field measurements there is evidence that the resonances can be excited by the same mechanism as those previously found at geostationary orbit or beyond. Abrupt changes in resonance frequency, as a function of L, are not usually seen, supporting previous reports that the mass or number density profile on the dayside does not have a steep plasmapause structure.

Viking observations above a postnoon aurora

This is the first paper that combines observations of the electric and magnetic fields, particles, wave emissions and UV images taken at intermediate altitude (two Earth radii) above the acceleration region of a 14 MLT aurora. The observations were made by the Swedish Viking satellite. The aurora appeared in the region of large‐scale reversal of the perpendicular electric field. Strong fluctuations in the electric and magnetic field indicated that the aurora consisted of many arcs of varying latitudinal extent. Spin‐averaged data show that an upward current sheet was connected with the observed UV aurora, with an ionospheric current density of 4 µA/m² in the center of the sheet and 7 µA/m² near the equatorward edge. The field‐aligned potential drop V∥ was determined from the mean energy of the upward ion beams and the width of the electron loss cone. V∥ increased from several hundred volts near the equatorward edge of the current sheet to at least 3 kV in the center. Auroral kilometric radiation (AKR) and the widened electron loss cone indicated that the top of the acceleration region with V∥ ≥ 3 kV was located near one Earth radius. The critical field‐aligned potential drop, V∥c, which is strong enough to accelerate the bulk of the electrons into the loss cone, was derived from the characteristic electron energy T and the ratio of the magnetic field in the ionosphere to that at the top of the acceleration region, Bi/Bv. The source plasma parameters were typical for magnetosheath plasma with a characteristic electron energy of about 200 eV. This means that in the center of the current sheet V∥c was 1.4 kV, and thus the observed field‐aligned potential drop, V∥ ≥ 3 kV, was greater than V∥c, indicating a saturation current. At the edges V∥c was higher than the measured potential drop, indicating a linear relationship with current density. There was a density depletion in the region of saturation current, and upward ion conies were observed with energies above V∥c. The electron source density determined from (1) the particle data and (2) the local plasma frequency agrees fairly well with that calculated from the current density observed by the magnetometer, the characteristic electron energy, and the altitude of the top of the acceleration region. This fact provides further evidence for the existence of a saturation current. Comparisons with earlier work indicate that the observed values for T, Bi/Bv, and V∥ are not exceptions but rather typical for the postnoon sector. We therefore conclude that a saturation current may be a common phenomenon for intense postnoon aurora. Similarities with the current‐voltage relationship for strong laboratory double layers are discussed.

The ion energy spectra in the ring current during the geomagnetic storm of February, 1986

Using data from the CHEM instrument on the AMPTE/CCE spacecraft, we follow the development of the ring current energy spectra (1–300 keV/e) of the ion species H +, O +, He +, and He ++ in the post-noon and pre-noon local time sectors during the geomagnetic storm of February 1986. By comparing displays of phase space density, f, vs. magnetic moment, μ, we can distinguish between enhancements due to newly injected ions and those due to adiabatic energization of a pre-existing population. In both the local time sectors, the initial drop in Dst is associated with enhanced phase space densities of all species. The spectra observed during the pass when the Dst dropped to a minimem of −312 nT show a strong local time asymmetry. In the post-noon sector, the spectra showed the influx of a new population of ions, rich in O + and He ++. In the pre-noon sector, the flux increase was consistent with adiabatic energization of the ion population injected earlier in the storm. This local time difference is consistent with a greatly enhanced convection electric field which brings a new population from the magnetotail to the post-noon, but not the pre-noon local time sector.

Irregular magnetic pulsations in the polar cleft caused by traveling ionospheric convection vortices

Irregular magnetic pulsations with periods of 5–8 minutes analyzed using data from an array of temporary stations around Sondre Stromfjord, Greenland have been found to correspond to traveling convection vortices. The vortices can be sorted into different classes corresponding to the following types: 1) isolated twin vortices, 2) single vortices and 3) multiple vortices. The vortices are observed in the pre-noon sector as well in the post-noon sector although more frequently in the pre-noon sector. The motion is generally antisunward along the auroral oval. Although magnetic variations like those discussed here have been proposed to be ionospheric signatures of flux transfer events (FTE's) the spatial and temporal resolution of the Greenland magnetometer observations show that none of the observed vortices may be explained by existing models of flux transfer events.

Radar and satellite measurements of an F‐region ionization enhancement in the post‐noon sector

A post‐noon enhancement in F‐region ionization has been studied using data from the Sondrestrom radar and the NOAA‐7 and Triad satellites. This enhancement is seen quite often in Sondrestrom radar data. It is extended in the east‐west direction and is the site of elevated electron temperatures. Data from the Triad satellite indicate that the enhancement observed on 9 September 1983 was collocated with intense upward field‐aligned currents. The NOAA‐7 satellite measured an electron spectrum in association with the event that was peaked at about 1 keV. The appearance of the spectrum suggests that the electrons were accelerated through a potential drop of 1.3 kV. The temperature and density of the source electrons was estimated to be 350 eV and 3.5 cm−3, respectively. The spatial relationship between the currents, electric field and electron precipitation is similar to that observed in evening sector auroral arcs. We argue that the F‐region arc on the dayside is produced by the same process as the more intense evening sector arcs except that the accelerating potential above the arc is lower. This difference in accelerating potential is a result of the higher source densities in the magnetosphere to which field lines from the dayside arc connect.

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field ( B z ). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the Δ H = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.

Optical measurements of a nightside poleward expanding aurora

Coordinated optical observations were performed from the poleward side of the midnight auroral oval. Height measurements of the auroral emissions at 4278, 5577 and 6300 Å, as well as their intensity ratios in the poleward expanded auroral substorm, have been carried out. The findings indicate a significantly softened electron spectrum compared with similar data from the equatorward part of this substorm. Typical values for the poleward expanded aurora are 300 eV and lower, while keV particles dominate the auroras at 10° lower latitudes. Emission altitudes and spectral characteristics are comparable to the transient burst emissions frequently observed from the same site in the post-noon sector, i.e. within the cusp. The 6300 Å atomic oxygen emission is used as a tracer of F-region wind and temperature. Interferometer observations show that there exists a prevailing crosspolar antisunward wind, increasing with geomagnetic activity to several hundred m s −1. The temperature shows an increase of 150 K associated with high geomagnetic activity.

Field‐aligned currents in the south polar cusp and their relationship to the interplanetary magnetic field

The large‐scale field‐aligned currents (FAC) as reported earlier from the Triad magnetometer data received at College, Alaska (lijima and Potemra, 1976a, b), are also observed in the south polar region. From the analysis of vector magnetometer data of more than 230 passes of Triad that were recorded at McMurdo, Antarctica, the principal characteristics of the cusp region FAC in the dayside south polar region that have been determined are as follows: (1) The cusp region FAC (defined as the currents appearing poleward of, and adjacent to, the region 1 FAC of Iijima and Potemra (1976a)), are most often observed in the midday sector of ∼0900–1500 MLT (magnetic local time) in the south polar region. These currents generally flow into the ionosphere in the postnoon sector (∼1200–1500 MLT) and flow away from the ionosphere in the prenoon sector (∼0900–1200 MLT). The demarcation line between the prenoon cusp region FAC and the postnoon cusp region FAC scatters statistically around the magnetic noon meridian. At a given MLT the flow of the cusp region FAC is the opposite of the flow of the region 1 FAC. (2) During weakly disturbed polar geomagnetic conditions (|;AL| &lt; 100 nT) the cusp region FAC are statistically confined to the invariant latitude range of ∼75°‐80°. This area shifts equatorward and spreads in longitude (toward ∼0800 MLT and ∼1500 MLT) during enhanced polar geomagnetic conditions. (3) The average intensity of the cusp region FAC in the south polar region during the period from the end of March to August 1974 and during weakly disturbed polar geomagnetic conditions (|AL| &lt; 100 nT) is ∼0.16 A/m. (4) In the south polar region the prenoon cusp region FAC (flowing away from the ionosphere) are observed about twice as often during periods when the interplanetary magnetic field is directed toward the sun (By &lt; 0) as during periods when the interplanetary magnetic field is directed away from the sun (By &gt; 0). Conversely, the postnoon cusp region FAC (flowing into the ionosphere) are observed about twice as often during periods when the interplanetary magnetic field is directed away from the sun (By &gt; 0) as during periods when the By is negative. In contrast, the region 1 FAC show less of a systematic occurrence tendency which depends upon the polarity of the interplanetary magnetic By component. (5) There is no special preference of the polarity of the interplanetary magnetic Bz component associated with the occurrence of the cusp region FAC. We suggest that the region 1 FAC and the cusp region FAC have basically independent generators, probably in the magnetosphere, and that the cusp region FAC may be directly related to regions of the magnetosphere which mark the transitions of plasma and magnetic fields from the magnetosheath type to the internal‐to‐the‐magnetosphere type. The more intense cusp region FAC are associated with increases of Bz in the southward direction, as determined for the northern hemisphere (Iijima and Potemra, 1976b). But the larger number of events studied here show a wide spread of FAC intensities for a given Bz, and there is not a close correlation between the intensities and Bz. There is no close correlation between the FAC intensities and the By component.

Thermal plasma enhancements in the topside ionosphere and their relationship to the auroral electrojets

Data from the Topside sounder aboard ISIS II are used to calculate the latitudinal distribution of thermal plasma at 1400 km when the satellite moves along a line of constant geographic longitude. The distribution of thermal plasma is related to the latitudinal regime of the auroral electrojets as inferred from data from a meridian line of magnetometers. It is found that thermal plasma at high altitude tends to be found above the poleward portion of the auroral electrojet. This finding is explained in terms of the spectrum of precipitating electrons across the auroral oval. It is found that the thermal plasma distribution in the post-noon sector is distinctly different from that in the premidnight sector despite the fact that both quadrants feature clear latitudinally confined eastward current flow. This difference can be used to define whether or not the polar cleft penetrates into the local time sector traversed by the satellite. On the nightside, the peak in thermal plasma poleward of the ionospheric trough can be used to identify the magnetic field lines which map to the boundary between the tail lobe and the plasma sheet.