The Geotail spacecraft carries a high-resolution Magnetic Field Experiment to provide magnetic field data in the frequency range below 50 Hz. This experiment includes dual fluxgate magnetometers and a search coil magnetometer. Fluxgate sensors are mounted at distances of 4 m and 6 m from the spacecraft on a deployable mast to reduce spacecraft-generated noises. Both outboard and inboard fluxgate magnetometers have 7 automatically switchable ranges from ±16 nT to ±65536 nT (full scale) and resolutions equivalent to a 15-bit A/D conversion in each range. The basic sampling rate for the A/D conversion is 128 Hz for both magnetometers

The GEOTAIL Magnetic Field Experiment.

The physical processes which give rise to convection in the earth's magnetosphere have been the subject of active debate for many years. Most of the discussion has centered on two basic pictures, one in which closed magnetospheric flux tubes are transported from dayside to nightside in a boundary layer around the flanks of the magnetosphere by a ‘viscouslike’ process occurring at the magnetopause and the other in which open flux tubes are transported over the poles of the earth after reconnection has taken place with the interplanetary magnetic field. These processes may coexist on a continuous basis, and the question then arises as to their relative contributions to usual total cross-magnetospheric voltages of ∼40–100 kV. The first detailed observations of plasmas and fields in the vicinity of the dayside magnetopause have recently been made during the International Magnetospheric Study by the ISEE 1 and 2 spacecraft and are discussed in this paper in relation to this question. Observations which relate to the occurrence of ‘quasi-steady’ as well as impulsive flux transfer event (FTE) reconnection are reviewed in detail in the paper, together with measurements of the properties of the boundary layer on the magnetospheric flanks. Particular emphasis is given to the interpretation of these data in terms of the physical processes occurring. It is argued that the ISEE observations of quasi-steady reconnection are indeed compatible with the process playing a major role in magnetospheric dynamics. The observed frequency of these events at the magnetopause indicates that they are associated either with infrequent intervals of intense magnetospheric convection or with more frequent contributions of lesser intensity (i.e., ∼40 kV whenever IMF Bz is negative). The latter seems the more likely situation at the present time. In addition, a rough estimate of ∼20 kV is made for the contribution due to FTE's, this figure having the nature of a lower limit. It is therefore argued that the in situ ISEE observations pertaining to reconnection processes are consistent with the view that these provide a major contribution to magnetospheric flows. A preliminary picture is suggested from our inferences based on ISEE data in which quasi-steady subsolar reconnection occurs essentially continuously in a band ∼2 hours LT wide when IMF Bz ≤ 0, centered often near the noon meridian, while at other local times, reconnection also occurs but in an unsteady, sporadic manner. Examination of the unsteady flows observed in the boundary layers on the flanks of the magnetosphere yields voltages of typically ∼5–20 kV for dusk and dawn layers combined. Similar values are obtained from studies of low-altitude spacecraft observations, although there is some disagreement in the literature concerning the exact identification of the boundary layer in such data. This latter topic is also reviewed. It is also argued that part, at least, of the boundary layer flow could occur on open field lines and indeed that some of the boundary layer observations correspond closely to what one would expect for the magnetospheric counterpart of magnetosheath FTE's. Finally, recent analyses of the variation of the transpolar voltage (measured at low altitudes) with solar wind conditions suggest an average ∼30-kV contribution from the boundary layers, a rather higher value than is indicated by the in situ measurements. However, all these estimates suggest that the layers generally provide a small but sometimes significant contribution to the magnetospheric voltage. We conclude that dayside reconnection is the dominant contributor under usual conditions, in agreement with conclusions reached previously on the basis of less direct information.

The causes of convection in the Earth's magnetosphere: A review of developments during the IMS

The plasma particle velocity distributions observed in the solar wind generally show enhanced (non-Maxwellian) suprathermal tails, decreasing as a power law of the velocity and well described by the family of Kappa distribution functions. The presence of non-thermal populations at different altitudes in space plasmas suggests a universal mechanism for their creation and important consequences concerning plasma fluctuations, the resonant and nonresonant wave - particle acceleration and plasma heating. These effects are well described by the kinetic approaches where no closure requires the distributions to be nearly Maxwellian. This paper summarizes and analyzes the various theories proposed for the Kappa distributions and their valuable applications in coronal and space plasmas.

Kappa Distributions: Theory and Applications in Space Plasmas

▶ Gathers original articles on topics in earth and planetary sciences ▶ Coverage includes geomagnetism, aeronomy, space science, seismology, volcanology, geodesy and planetary science ▶ Official journal of the Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan, and The Japanese Society for Planetary Sciences

「Earth,Planets and Space」のオープンアクセス出版

[1] First results are presented of an empirical modeling of the Earth's inner and near magnetosphere (X ≥ −15 RE), using a new set of data and new methods, described in a companion paper. The modeling database included 5-min average B field data, taken in a wide range of altitudes and latitudes by the International Solar Terrestrial Physics spacecraft Polar (1996–1999) and Geotail (1994–1999), as well as by earlier missions, ISEE 2 (1984–1987), Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE (1984–1988), AMPTE/Ion Release Module (1984–1986), CRRES (1990–1991), and DE 1 (1984–1990). To take into account the delayed response of the magnetosphere to the solar wind and interplanetary magnetic field (IMF), each data record in the data set was tagged by a “trail” of 5-min averages of the IMF, solar wind, and Dst field data, covering the preceding 2-hour interval. The axisymmetric ring current (SRC) and the partial one (PRC), both parameterized by the corrected Dst* index and the solar wind pressure Pd, were found to vary in strikingly different ways. While under quiet conditions the PRC is much weaker than the SRC, it dramatically grows in magnitude and rotates to the dusk sector with rising |Dst*| and Pd, significantly exceeding the SRC even during moderate storms, in excellent agreement with recent particle simulations. The innermost part of the cross-tail current is quite sensitive to the southward IMF and yields ∼90% of the tail's contribution to the Dst index, in contrast with the more distant tail current, which responds mainly to the solar wind pressure and provides no appreciable contribution to Dst. In response to southward IMF conditions, Region 1 and 2 Birkeland currents rapidly grow in magnitude and expand to lower latitudes, while their peaks shift slightly in local time toward noon. The coefficient of the IMF penetration inside the magnetosphere was found to dramatically increase with growing IMF clock angle: while quite small (∼0.1) for northward IMF, it rises to ∼0.6 as the IMF turns southward. Priorities and challenges for future data-based modeling studies are discussed.

https://ccmc.gsfc.nasa.gov/modelweb/magnetos/data-based/Paper220.pdf

A model of the near magnetosphere with a dawn-dusk asymmetry 2. Parameterization and fitting to observations

Seasonal variations of large-scale Birkeland currents are examined in a study of the source mechanisms and the closure of the three-dimensional current systems in the ionosphere. Vector magnetic field data acquired by the TRIAD satellite in the Northern Hemisphere were analyzed for the statistics of single sheet and double sheet Birkeland currents during 555 passes during the summer and 408 passes during the winter. The single sheet currents are observed more frequently in the dayside of the auroral zone, and more often in summer than in winter. The intensities of both the single and double dayside currents are found to be greater in the summer than in the winter by a factor of two, while the intensities of the double sheet Birkeland currents on the nightside do not show a significant difference from summer to winter. Both the single and double sheet currents are found at higher latitudes in the summer than in the winter on the dayside. Results suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region, and that the currents close via the polar cap when the conductivity there is sufficiently high. It is also concluded that an important source of these currents must be a voltage generator in the magnetosphere.

Seasonal dependence of large-scale Birkeland currents

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 assoc iated with narrow, intense antisunwqard 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 protion 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, 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.

Electrodynamic parameters in the nighttime sector during auroral substorms

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| 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 > 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.

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

Magnetospheric Current Systems

In order to investigate the characteristics of generation mechanisms of large-scale Birkeland currents from the standpoint of relative importance of a voltage source and a current source, we have determined quantitatively the relationships between large-scale Birkeland current intensities and ionospheric conductivities under geomagnetic quiet conditions. The large-scale Birkeland currents are the well-defined region 1 and region 2 systems that were determined from transverse magnetic disturbances acquired with the Magsat satellite over the entire polar regions except for the midnight sector. The ionospheric conductivities were determined by solar EUV ionization. The principal results include the following: (1) The region 1 and region 2 systems exhibit different dependence on ionospheric conductivities. The region 1 current intensities are proportional linearly to the conductivities at all MLTs. The region 2 currents also show a dependence on ionospheric conductivities, but they have in general poorer correlations at all MLTs than the region 1 currents have. (2) The correlations with conductivities are improved by using the region 1 current density and the region 2 current latitudinal width. (3) The degree of the coupling between the region 1 system and the region 2 system seems to have no correlation with conductivities. (4) The above-mentioned characteristics are common both in the northern and southern polar regions. These findings suggest that the large-scale region 1 system is primarily driven by voltage generators in the magnetosphere. On the other hand, the region 2 system seems to be driven by a combination of voltage and current generators.

Ryoichi Fujii

Papers

Seasonal dependence of large-scale Birkeland currents

Electrodynamic parameters in the nighttime sector during auroral substorms

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

Magnetospheric Current Systems

Control of the ionospheric conductivities on large-scale Birkeland current intensities under geomagnetic quiet conditions