Showing papers by "Ramon Lopez published in 1990"

A multisatellite case study of the expansion of a substorm current wedge in the near-earth magnetotail

Abstract: Results are presented on observations from four satellites (GOES 5, GOES 6, AMPTE CCE, and AMPTE IRM) and two ground stations (San Juan and Tucson) on a substorm that occurred on April 19, 1985 at about 0830 UT. The four spacecraft were arrayed in a configuration that made it possible to separate the effects arising from the longitudinal versus the radial expansion of the substorm current wedge and thus to examine its spatial evolution. The sequence of events that was observed suggests that, during this substorm, the disruption of the cross-tail current sheet, the formation of the substorm current wedge, and the expansion of the plasma sheet began in the near-earth region, and subsequently spread tailward as well as longitudinally.

The energetic ion substorm injection boundary

Abstract: In this paper, 167 dispersionless energetic ion injections which were observed by AMPTE CCE are identified. The radial and local time distribution of the events as a function of kp is qualitatively similar to that envisioned in the injection boundary model of Mauk and McIlwain (1974). It is argued that particles observed during dispersionless injections are locally energized during the disruption of the cross-tail current sheet. Therefore, the injection boundary is identified as derived from the spatial distribution of dispersionless injections, with the earthward edge of the region of the magnetotail which undergoes current sheet disruption during the substorm expansion phase. It is shown that this qualitative model for the generation of the injection boundary can provide an explanation for the dispersionless nature, the double spiral shape, and the Kp dependence of the boundary.

Multipoint observations of a small substorm

Abstract: Results are presented of multipoint observations of a small substorm which occurred at about 0110 UT on April 25, 1985, carried out by AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7, as well as by ground auroral stations and midlatitude stations. These data yield information on the latitudinal extent of the polar cap and provide visual identification of substorm aurorae, magnetic perturbations produced directly beneath aurorae, and the situ magnetic field. In addition, they provide magnetic-particle observations of the disruption of the cross-tail current sheet and observations concerning the spatial expansion of the current disruption region. Evidence is presented that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa.

Solar wind properties observed during high-latitude impulsive perturbation events

Abstract: The data base from the 1985-86 Balloon Campaign at South Pole Station, Antarctica, has been searched for impulsive magnetic and electric field events that occurred when IMP 8 was in the solar wind and data were acquired. Two events are shown that appear to be similar to published examples of high latitude twin vortex current systems. Neither event was preceded by a dynamic pressure pulse in the solar wind. These examples disprove the hypothesis that all such high-latitude impulses arise solely from dynamic pressure pulses produced by the solar wind. Further, on a statistical basis, a total of 42 events were found for which IMP 8 data are available. Of these events, 12 occurred when the Z{sub GSM} component (B{sub Z}) of the interplanetary magnetic field (IMF) was northward and 30 occurred when B{sub Z} was southward. Only 3 of the B{sub Z} north cases and only 5 of the B{sub Z} south cases were preceded by pressure pulses greater than 0.4 nPa in amplitude.

The position of the magnetotail neutral sheet in the near‐Earth region

Abstract: The position of the near-earth neutral sheet throughout the nightside region between R = 5 R(E) and R = 8.8 R(E) is examined statistically using magnetic field data from AMPTE/CCE. As has been demonstrated previously, the latitudinal position of the neutral sheet is primarily controlled by the dipole tilt angle. The position also depends on local time, radial distance, and magnetic activity. These dependencis may be represented by the empirical formula MLAT = - (0.14 Kp + 0.69) /cos(Phi)/exp 1/3 (0.065 R exp 0.8 - 0.16) DTA, where MLAT is the position in degrees of the neutral sheet relative to the magnetic equator, Kp is the 3-hourly magnetic activity index, Phi is the magnetic local time in degrees (midnight = 0 deg), R is the radius in R(E), and DTA is the dipole tilt angle in degrees.

Import Dependency and Structural Adjustment in Sub-Saharan Africa

Abstract: One of the effects of structural adjustment programs in sub - Saharan Africa has been the reduction of imports in the face of scarce foreign exchange. This article takes the analysis of import demand beyond the traditional income and price determinants to account for factors likely to be important to sub - Saharan African countries in the 1990s. First, the effect of demand on imports is reflected by the level of absorption rather than the less direct income variable. Second, because adjustment programs may cut government consumption and, through increases in interest rates, reduce investment, these components of absorption are also considered independently to assess their differential effect on imports. Third, import barriers are often set in dollar terms to limit the use of foreign exchange. Because reliable and complete data for import restrictions are not available, the ratio of exports to debt is included as an indicator of foreign exchange availability to reflect its effect on trade barriers and thus imports. The findings suggest that this more comprehensive assessment of import demand will be needed if the size and even direction of changes in import demand in response to policy reform is to be understood and anticipated.

The structure of the Birkeland current system in the post-midnight plasma sheet

Abstract: Simultaneous magnetic field and particle measurements by AMPTE/CCE with magnetic field measurements by GOES 5 and GOES 6 clarify the structure of the field-aligned Birkeland current system in the post-midnight region of the near-Earth magnetotail. The current system, consisting of current sheets flowing in opposite directions, exists near the boundary region of the plasma sheet; its magnetic effects are confined mainly to the current region because of its structure. During the expansion phase of substorms, a single-sheet current system is formed in the boundary region of the plasma sheet and net current is probably present in one meridian plane. This type of current system produces the substorm-related transient variations in the azimuthal (D) component normally observed at synchronous orbit.