Imaging the Earth's magnetosphere by using ground-based magnetometer arrays is still one of the major techniques for investigating the dynamical features of solar wind-magnetosphere interactions. The organized ground network data of magnetic fields make it possible (1) to study the magnetospheric processes by distinguishing between temporal changes and spatial variations in the phenomena, (2) to clarify the global latitudinal structures and propagation characteristics of magnetic variations from high to equatorial latitudes along the magnetic meridian (MM), and (3) to understand the global generation mechanism of magnetospheric phenomena. During the international Solar Terrestrial Energy Program (STEP) period of 1990-1997, multinationally coordinated magnetic observations are being conducted along the 190°, 210°, and 250° MMs from high latitudes through middle and low latitudes to the equatorial region, spanning L = 8.50-1.00, in cooperation with 29 organizations in Australia, Indonesia, Japan, Papua New Guinea, Philippines, Russia, Taiwan, and the United States. In this paper, we review the 210° MM Magnetic Observation Project and its initial results.

View full abstract

A magnetometer system for unmanned long-term observation in equatorial regions has been developed and is available. Because this system is designed for the study of both micropulsations and the equatorial electrojet, it must have accurate timing, high sensitivity, a wide measurement range, and a long-term recording capability. The equatorial regions we are interested in are quite remote far from sufficient technical and human support, and the breakdown of a public power supply is frequent. So, the system is requested to be compact and work successively in low power consumption with battery backup under equatorial environments. This system is successfully developed in low cost compared with commercial magnetometer system with similar properties.

View full abstract

A time source for a data acquisition system was designed to use the fluxgate magnetometer for the measurement of long-distance phase propagation of magnetic pulsations at very low latitudes. The time source consists of a very low frequency (VLF) signal receiver, antenna, time code generator with crystal oscillator, and microcomputer chip. Omega navigation signals received by the VLF receiver are utilized for the time base. The computer-aided system can choose the Omega station with the greatest signal strength at the observation site and can decode the transmitted signals to utilize them as the time source of the internal clock. The prominent feature of this system is software-aided fully automatic operation at any location in the world. The overall accuracy of the clock is expected to be ±30 ms.

View full abstract

A compact data recorder using nonvolatile flash memory cards has been developed and is available for use in long-term and unmanned magnetometer data acquisition systems. A recently available flash memory card that consists of semiconductor memories is suitable for unmanned long-term recording in remote places difficult to visit frequently. This recorder collects 200 days of three-component 3-s-sampling geomagnetic data by using five pieces of a commonly sold 4-MB flash memory card and can be used for longer-term recording by using larger-capacity cards. Processing data from the cards to standard-format computer files is quick and smart, and no data are lost because of reading errors caused by physical damage.

View full abstract

Magnetic field data from the 210° magnetic meridian chain of stations have been analyzed to examine which component of the DL and DP fields dominates main impulses of sc and si observed on the ground. The DL and DP fields correspond to transfer processes from the magnetopause through the magnetosphere to the Earth's surface and through the polar ionosphere into the equatorial ionosphere, respectively. Northern and Southern Hemisphere asymmetry of sc and si disturbances appears allow and middle latitudes. Amplitude ranges of the se and si main impulses observed at low-latitude conjugate pairs (L = 2.1 and 1.6) in the summer hemisphere are about twice as wide as those in the winter hemisphere. The equatorial enhancement of se and si main impulses can be seen at Yap island (Ψ =-0.3°) during the daytime at 0600-1800 LT. These observational facts can be interpreted by invoking an asymmetry in the Northern and Southern Hemisphere twin-vortex-type ionospheric currents driven by the DP field, i.e., by invoking enhanced ionospheric conductivities in the summer hemisphere. The instantaneous penetration of sc and si-associated DP fields through the ionosphere from the polar region to the equator must play an important role in the energy transfer of sc and si disturbances from the solar-wind region to the magnetic equator.

View full abstract

On February 21, 1994, a large coronal mass ejection (CME) passed by the Earth. In the solar wind, the CME front was noted by very large changes in the solar wind dynamic pressure, flow direction, and magnetic field, as measured by several spacecraft. At the Earth's surface, many ground stations also recorded the passage of this CME, as a storm sudden commencement onset. We examine here several interesting features of the surface magnetic field response to this sudden commencement onset. In particular, we use the magnetometer records to examine the propagation time of the initial impulse along the Earth's surface. We also study in detail the initial amplitude change at several stations, and note the variation in the response as a function of magnetic latitude. In addition, we crudely estimate the relative contributions of the Chapman-Ferraro and the ionospheric current system to the surface response. Also of interest is the polarization and period of Psc oscillations superposed on the main impulse at several stations, including polarization reversals as a function of magnetic latitude. Corresponding ionospheric currents at stations along the 210° MM chain are also computed, and very large and sudden changes in the ionospheric current flow patterns are reported during the initial minutes of the sudden commencement onset. These and other results are interpreted in terms of previous models, paradigms, and statistical results.

View full abstract

A modem digital ionosonde (Digisonde 256) recording the amplitude, angle-of-arrival, Doppler shift, and polarisation of MF and HF (1-10 MHz) echoes from the ionosphere was operated at the mid-latitude station of Beveridge (geographic 37.5°S, 144.9°E), located 40 km north of Melbourne, Australia, during the intense auroral storms of August 22 and 23, 1990. We changed the digisonde antenna design to permit the alternate recording of near-vertical angle-of-arrival ionograms, and southward-looking Doppler ionograms sensitive to oblique echoes at great ranges. This experimental design permitted us to track the equatorward migration of radio aurora associated with the poleward wall of the mid-latitude trough, the small-scale “slant-F” irregularities and medium-scale travelling ionospheric disturbances (TIDs) detected within the trough, and distinctive oblique spread-F patches located in proximity to the equatorward edge of the auroral zone; all arriving in response to large auroral electrojet activity. These observations graphically illustrate the processes by which the magnetosphere dumps energy into the high-latitude ionosphere when the geomagnetic index is high (ΣK_p = 46 on August 23), leading to the generation of large-scale TIDs which precede the arrival of, and possibly contribute to the production of, the aforementioned menagerie of high mid-latitude irregularities. We illustrate the five forms (broadly speaking) of spread-F observed and offer plausible explanations for their cause. We corroborate our interpretation of oblique ionogram measurements by using DMSP/F8 and F9 precipitating particles satellite data, magnetometer measurements at Canberra (Δ = 45.9°S) (geomagnetic latitudes hereafter) and at Macquarie Is. (64.4°S) in the auroral zone, and routine ionograms (i.e., limited sensitivity to oblique echoes, and no angle-of-arrival or Doppler shift information) recorded at other mid-latitude stations. For example, during the night of August 23, radio aurora were observed to move equatorwards towards Beveridge (48.0°S) where a severe spread-F event occurred due to TIDs generated in the region defined by Macquarie Island north to Hobart (53.7°) (i.e., the source location probably changed in latitude as the aurora moved equatorward). The TIDs observed at Beveridge were also observed at Canberra (where mild spread-F was observed), and as far north as Norfolk Island (36.0°S) where they had little impact on the ionosphere.

View full abstract

Auroral observations using automatic optical instruments have been conducted during the Solar Terrestrial Energy Program (STEP) period, at several latitudes along, lower, and higher than the auroral zone, together with the chain stations along 210° magnetic meridian. This paper gives details of the instrumentation and the locations of stations. In an initial analysis, auroral data from an auroral zone station at Kotzebue, Alaska, and magnetic field data from both high- and low-latitude stations along the 210° and 250° magnetic meridians were used to demonstrate the relations between auroral substorm onsets and multiple Pi 2 magnetic pulsation. The correspondence between each Pi 2 onset and each small- or large-scale brightening and poleward expansion of auroras during an auroral substorm with multiple onsets was good.

View full abstract

In order to understand the characteristics of lower-latitude Pi 2 geomagnetic pulsations along a magnetic meridian, we have analyzed 1 year's data from the Adelaide (geomagnetic latitude Ψ=-46.72°), Birdsville (-37.08°), Weipa (-23.06°), Guam (9.02°), Chichijima (20.65°), Kagoshima (25.23°), and Moshiri (37.76°) stations along the 210° magnetic meridian. The characteristics of the lower-latitude (|Ψ| < 50°) Pi 2 pulsations are summarized as follows: (1) 80% of H- and D-component Pi 2 events show very similar wave forms and the same frequency component; (2) not only H-component Pi 2 events but also D-component Pi 2 events are observed within ±5 s; (3) H-component Pi 2 amplitudes are of almost the same order, but D-component Pi 2 pulsations have a minimum near the magnetic equator and increase exponentially from lower to higher latitudes; (4) the observed amplitudes of both H- and D-component Pi 2 pulsations are larger in the summer hemisphere than in the winter hemisphere. Results 1 and 2 show that lower-latitude Pi 2 pulsations are not propagating-mode but global-mode oscillations in the inner magnetosphere. Result 4 cannot be explained by the ordinary screening effect in the ionosphere but gives a clue to the unresolved generation mechanism of lower-latitude Pi 2 pulsations.

View full abstract

The occurrence of Pi 2 pulsations (20-150 s) at very-low-latitude stations is compared with the energetic-particle and magnetic-field variations at geosynchronous orbit during the PROMIS (Polar Region and Outer Magnetosphere International Study) period (March 10-June 16, 1986). Ground magnetometer data from Huancayo, Peru (dip equator, 75°W), and Kuju, Japan (L = 1.2, 135°E), are utilized to monitor the magnetic pulsations, while the particles (electron, 30-300 keV; proton, 145-560 keV) and magnetic fields in space are monitored by geosynchronous satellites S/C1982-019, S/C1984-129, S/C1984-037, GOES-5, and GOES-6. Of 175 Pi 2 events surveyed during the PROMIS period, 110 events are accompanied by electron flux enhancement, and 51 events have a concurrent onset of the field-aligned current (FAC). The FAC events correlated with ground Pi 2 pulsations indicate a local time change in polarities. If currents are assumed to flow along the field lines tailward of the geosynchronous orbit, the polarities are upward in the premidnight sector and downward in the postmidnight sector. We classified Pi 2 events in the nightside sector in several different ways. The results imply that if Pi 2 events in the present study can be assumed to be of substorm origin, then 47% of Pi 2 events occur in regions close to geosynchronous altitude.

View full abstract

A few characteristics of Pc 3-4 geomagnetic pulsation recorded at a station close to the equatorial electrojet in the Indian region under varied ionospheric conditions are reported. The polarization characteristics of these pulsations during counter electrojet condition are found to change in their ellipticities and orientation axes. These are attributed to either height distortions of ionospheric electric fields during the counter electrojet time or to changes of gyrotropic MHD waves that might be endogenously generated by the turbulence of equatorial electrojet.

View full abstract

Two basic but novel equations directly describing the generation of shear Alfvén and compressional waves in the inner magnetosphere filled with a cold magnetized plasma are derived. The shear Alfvéen wave is characterized by the field-aligned current and the compressional wave by the compressional component of the magnetic field. Such a generation arises from the effects of inhomogeneous Alfvén speed and curvilinear field line. Around the magnetic equator, if the Alfvén speed is inversely proportional to a power of the geocentric distance, these effects have magnitudes of the same order and their signs are identical. Considered in the present study is a situation that the earthward propagating compressional wave is launched from a large scale oscillating current wedge centered at midnight and symmetric about the magnetic equator. Then, it is found that the field-aligned current excited around the equator by the compressional wave has opposite senses in direction in the northern and southern hemispheres, in the pre- and post-midnight sectors as well as just inside the plasmapause and in its surrounding regions. As a result of the excitation of shear Alfvén wave, two types of oscillations appear on a field line: One is a forced oscillation and the other is an eigenoscillation. Although a modulation of the compressional wave may be caused locally (or microscopically) around the equator by the eigenoscillation of field line, the modulation can be globally (or macroscopically) neglected. So far as the propagation along the source longitude (source-earth line) around the equator is concerned, the coupling between compressional and shear Alfvén waves can be almost neglected and so one-dimensional response of the inner magnetosphere around the equator plays a significant role in the compressional oscillation.

View full abstract