M. Sugiura

Electrodynamic parameters in the nighttime sector during auroral substorms

The characteristics of the large-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulge-type auroral emission region has been deduced from auroral images taken by the Dynamics Explorer 1 (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with little net current in the surge horn and the west of bulge. The downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are associated with narrow, intense antisunward convection, corresponding to an equatorward directed spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region 1 FAC. In the surge, the convection reversal is near the low-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region 1 and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents.

Characterization of the IMF By ‐dependent field‐aligned currents in the cleft region based on DE 2 observations

The interplanetary magnetic field (IMF) By-dependent distribution of field-aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellites crossing of the relevant field-aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field-aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By-dependent field-aligned currents in the equatorward and the poleward sheets are referred to as the low-latitude cleft current (LCC) and the high-latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field-aligned current regime. The proposed pair of cleft field-aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi-steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field-aligned currents when By is positive; the pair of field-aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field-aligned current direction in the LCC and HCC regions.

Conjugate occurrence of the electric field fluctuations in the nighttime midlatitude ionosphere

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature variations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by field-aligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power α ƒ −n, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high-latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field-aligned irregularities or spread-F, observed by ground-based methods such as the MU radar, but the relationship is not clear.

Correlation between magnetic and electric field perturbations in the field-aligned current regions deduced from DE 2 observations

Satellite observations have shown high correlation between magnetic and electric field perturbations in the high-latitude field-aligned current regions. The high correlation has been interpreted by two models. In the first, the Static model, the observed perturbations are regarded as being static spatial variations, and the ratio of the orthogonal magnetic and electric field components ΔBz/Ex represents the height-integrated ionospheric Pedersen conductivity ΣP. In the second, Alfven wave model, the observed perturbations are interpreted as being Doppler-shifted Alfven waves, and the inverse of the ratio gives the Alfven wave velocity VA. In this paper we investigate changes of this ratio with spatial scale length, using the DE 2 observations. The ratio ΔBz/Ex is found to change little with scale length for variations of scale lengths longer than 64 km, or 8.0 s in time. While for variations of smaller scale lengths, which are obtained using numerical filters with cutoff periods shorter than 4.0 s, the same ratio shows a significant dependence on scale length. The calculated ratios are nearly equal to ΣP based on an ionospheric model for long-wavelength structures and to 1/VA for short-wavelength variations. The transition from the former to the latter usually begins around 4.0-8.0 s on the time scale. On the dayside the correlation between ΔBz and Ex is generally high, and the transition is clearly seen. Thus the static model is applicable to valuations of scale lengths greater than 8.0 s (or 64 km); while the Alfven wave effect becomes increasingly dominant for scale lengths less than 4.0 s (or 32 km). For scale lengths below about 5 km (∼0.6 s) the short-circuiting effect at ionospheric altitudes higher than the altitudes at which the horizontal Pedersen closure current usually flows becomes appreciable. However, this effect alone cannot explain the observed decrease in the ratio ΔBz/μ0Ex. The relation between the ratio ΔBz/μ0Ex and the solar zenith angle is consistent with the relationship between the Pedersen conductivity and the solar zenith angle in the published conductivity models.

Characteristics of the field‐aligned current system in the nighttime sector during auroral substorms

Fujii et al. (1994) obtained characteristics of the electrodynamic parameters, that is, field-aligned currents, electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, through a unique analysis that places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. In this paper we investigate in considerably more detail the characteristics of the field-aligned currents using data from the same set of passes as the previous study. We show for the first time that the net upward field-aligned currents throughout the surge and surge horn are sufficient to account for most if not all of the converging currents of the auroral electrojets. Current densities are largest in the surge and surge horn. Current region continuity does not appear to exist across the substorm bulge region. Much of the auroral substorm field-aligned current is composed of filamentary currents and finite current segments at large angles to each other. The westward electrojet may contain large gradients in intensity both in local time and latitude due to sets of localized field-aligned currents. The net downward current for several hours to the west of the surge is insufficient to account for the eastward electrojet, consistent with the concept that this electrojet originates primarily on the dayside. Our pattern of field-aligned currents associated with the surge has common features and also differs significantly from the patterns previously derived from data from radars and ground-based magnetometer arrays. Our pattern is considerably more complex, probably due to the much higher resolution in latitude of the satellite data. It is also larger in area, since our average substorm is much larger than those pertaining to the previous patterns, giving a substorm wedge considerably wider than that obtained from the radar and array data.

E and F region study of the evening sector auroral oval: A Chatanika/Dynamics Explorer 2/NOAA 6 comparison

Simultaneous data obtained with the Chatanika incoherent scatter radar and the Dynamics Explorer 2 (DE 2) and NOAA 6 satellites are used to relate the locations of the precipitating particles, field-aligned currents, and E and F region ionization structures in the evening-sector auroral oval. The auroral E layer observed by the radar extends about 2° equatorward of the electron precipitation region, and its equatorward edge coincides with the equatorward edges of the region 2 field-aligned current and intense convection region (E ≃ 50 mV/m). It is shown that precipitating protons are responsible for part of the E region ionization within the electron precipitation region as well as south of it. E region density profiles calculated from ion spectra measured by the DE 2 and NOAA 6 satellites are in fairly good agreement with the Chatanika data. In the F region, a channel of enhanced ionization density, elongated along the east-west direction and having a width of about 100 km, marks the poleward edge of the main trough. It is colocated with the equatorward boundary of the electron precipitation from the central plasma sheet. Although enhanced fluxes of soft electrons are observed at this boundary, the energy input to the ionospheric electron gas, calculated from the radar data, shows that this ionization channel is not locally produced by this soft precipitation, but that it is rather a convected feature. In fact, both the trough and the ionization channel are located in a region where the plasma flows sunward at high speed, but the flux tubes associated with these two features have different convective time histories. Keeping in mind that several processes operate together in the F region, our data set is consistent with the following trough and ionization channel formation mechanisms. (1) The mid-latitude trough, located equatorward of the electron precipitation region, is mainly the result of transport and enhanced recombination due to large electric fields. Flux tubes on the low-latitude edge of the trough have most probably corotated eastward before flowing sunward at higher latitudes where magnetospheric convection predominates. The trough thus forms by recombination during the long time the flux tubes stagnate in the region where the flow reverses. (2) Flux tubes associated with the ionization channel have drifted antisunward in the polar cap before drifting sunward in the auroral zone. Its formation results from the distortion of polar cap F region ionization structures, due to the incompressibility of the flow.

Global energetic neutral atom (ENA) measurements and their association with the Dst index

We present a new global magnetospheric index that measures the intensity of the Earths ring current through energetic neutral atoms (ENAs). We have named it the Global Energetic Neutral Index (GENI), and it is derived from ENA measurements obtained by the Imaging Proton Spectrometer (IPS), part of the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment on the POLAR satellite. GENI provides a simple orbit-independent global sum of ENAs measured with IPS. Actual ENA measurements for the same magnetospheric state look different when seen from different points in the POLAR orbit. In addition, the instrument is sensitive to weak ion populations in the polar cap, as well as cosmic rays. We have devised a method for removing the effects of cosmic rays and weak ion fluxes, in order to produce an image of “pure” ENA counts. We then devised a method of normalizing the ENA measurements to remove the orbital bias effect. The normalized data were then used to produce the GENI. We show, both experimentally and theoretically the approximate proportionality between the GENI and the Dst index. In addition we discuss possible implications of this relation. Owing to the high sensitivity of IPS to ENAs, we can use these data to explore the ENA/Dst relationship not only during all phases of moderate geomagnetic storms, but also during quiescent ring current periods.

Magnetospheric boundary dynamics: DE 1 and DE 2 observations near the magnetopause and cusp

A broad spectrum of particle and field measurements was taken near local noon by the Dynamics Explorer satellites during the magnetic storm of September 6, 1982. While at apogee, DE 1 sampled the magnetospheric boundary layer at mid southern latitudes and, due to the passage of an intense solar wind burst, briefly penetrated into the magnetosheath. In the boundary layer and the adjacent magnetosheath the plasma flow was directed toward dawn. Variance and de Hoffmann-Teller analyses of electric and magnetic field data during the magnetopause crossing showed the magnetopause structure to be that of a rotational discontinuity or an intermediate shock with a substantial normal magnetic field component. This is consistent with an open magnetosphere model in which significant magnetic merging occurs at the local time of the spacecraft. The orbit of DE 2 carried it through the morning sector of the low-altitude, southern cusp. The measurements show a well-defined, cusp current system occurring on open magnetic field lines. At both cusp and subcusp latitudes the electric field was equatorward indicating a strongly eastward plasma flow. The boundary between these two regions was marked by the onset of magnetosheath precipitation and an electric field structure containing both poleward and equatorward spikes. The poleward spike has associated field-aligned currents which are closed by Pedersen currents and, from force balance considerations, is interpreted as the signature of a magnetic merging event at the magnetopause. The equatorward spike has the characteristics of a down-coming and reflected Alfven wave packet of finite dimensions. The high-altitude measurements suggest that the dayside boundary layer is made up of closed magnetic flux tubes, a large fraction of which drift to the magnetopause where merging with the IMF occurs. The merging line maps to the ionosphere as a “gap” across which the polar cap potential is applied to the magnetosphere. The potential is applied from a magnetosheath generator to the polar ionosphere by means of the cusp, field-aligned current system. The electric fields provide an ionospheric indicator of the mapping of the merging line location.

A strong dawn/dusk asymmetry in Pc5 pulsation occurrence observed by the DE‐1 satellite

Using the magnetic field data obtained by the DE-1 polar orbiting satellite, statistical characteristics of transverse Pc5 pulsations in the inner magnetosphere are examined. The occurrence distribution is found to have a strong dawn/dusk asymmetry. The occurrence is most frequent in the region around 72° invariant latitude (ILAT) between 0800 and 1000MLT. The distribution shifts to lower ILAT both in the early morning and in the afternoon sectors. Our results are generally consistent with the occurrence distributions obtained by previous studies except for appreciable differences in the local time of the peak occurrence. The dependence of the transverse Pc5 occurrence on solar wind velocity is also investigated. Results show that the higher the solar wind velocity, the more frequent the Pc5 occurrence. This fact suggests that the energy source of these Pc5s is in the Kelvin-Helmholtz instability (KHI) on the magnetopause. To explain the strong dawn/dusk asymmetry, the dependence of the Pc5 occurrence on the angle between the solar wind velocity and the IMF in the ecliptic plane is investigated. It is found that this angle controls the magnetic local time of Pc5 appearance when the solar wind velocity is small. This dependence may be explained by the low threshold of KHI due to an influence of a quasi-parallel bow shock.

Electron precipitation accompanying Pc 5 pulsations observed by the DE satellites and at a ground station

Using data from the polar orbiting Dynamic Explorer (DE) −1 and −2 satellites and a ground-based station, we investigated electron precipitation phenomena accompanying Pc 5 pulsations. DE-2 observed oscillatory disturbances in the magnetic and electric fields in the upper ionosphere at the geomagnetic footprint of the high altitude region in which transverse Pc 5 pulsations were detected by DE-1. DE-2 observed electrons precipitating into the ionosphere with energies of several keV to several tens of keV. These electrons were accelerated in the direction of the ambient magnetic field. When Pc 5 pulsations in the H-component and periodic variations of cosmic radio noise absorption (CNA pulsations) were observed at Syowa Station, DE-2 which was in geomagnetic conjunction with Syowa Station also observed oscillatory disturbances in the magnetic and electric fields. These oscillatory disturbances are caused by small-scale field-aligned currents each with width of 0.5°–1.4° invariant latitude. This suggests that Pc 5 pulsations have a small-scale resonance structure in the radial direction. The resonance structure has a small scale comparable to the ion acoustic gyroradius, then kinetic Alfven waves having electric fields parallel to the ambient magnetic field can arise. The parallel electric field generates a field-aligned potential drop of about 3–5 kV. Electrons accelerated by these kinetic Alfven waves would cause CNA pulsations, the phase of which leads that of the H-component of the Pc 5 pulsations by 90° in the southern hemisphere. This is consistent with the observations at Syowa Station.