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A study by Sauvaud et al. (1987) on the overall magnetospheric response to a southward turning of the IMF on March 4, 1979 is discussed. Because the coupling function epsilon for the most intense substorm in their study did not show any appreciable intensification prior to the occurrence of the substorm, Sauvaud et al. concluded that the substorm was driven by a magnetospheric reservoir. It is suggested that the coupling function epsilon does not necessarily well represent the magnetospheric energization due to possible combinations of solar wind parameters that are expected to influence the coupling. Therefore, it is concluded that the main substorm studied by Sauvaud et al. was probably driven by the solar wind.
An examination of ISEE-3 data from 1978 reveal 25 electron heat flux dropout events ranging in duration from 20 min to over 11 hours. The heat flux dropouts are found to occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. It is suggested that the heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the sun and instead are connected to the outer heliosphere at both ends.
The distribution of the convection potential around the polar cap boundary was investigated under a variety of different IMF conditions, using plasma flow data from the AE-C, AE-D, and DE 2 satellites. The data examined reveal that the potential drop at the duskside boundary is essentially always larger than that at the dawnside boundary. It was found that the sinusoidal and arc-tangent fits represent the data equally well, implying that the convection 'throat' is wide and/or that it moves randomly in MLT. In response to the IMF B(y) component, the zero potential line was found to be displaced toward the prenoon or postnoon sector. Finally, it was found that, as the IMF changes from the 'garden hose' to the 'ortho-garden hose' condition, a slight duskward shift of the pattern occurs.
Magnetic disturbances related to the passage of the asteroid 2201 Oljato are described. It is pointed out that the process leading to these disturbances must be associated with material in the orbit of Oljato and not the Oljato asteroid itself. Using data from latest space observations, the statistical association of these disturbances with the orbit of 2201 Oljato is updated, and the possible mechanisms for causing the disturbances are discussed. The phenomenon is considered to be associated with comet-like processes in the solar wind.
Data from the polar-orbiting satellite DE 2 are used to calculate one-dimensional electrostatic potential distributions across the polar cap region. Using passes that lie within + or - 3 hours MLT of the dawn-dusk line, various parameters of the polar potential distribution (location and magnitude of the maxima and minima, location of the zero potential point, etc.) are analyzed in relation to each other and to the IMF. The resulting dependences are used to derive a two-dimensional model of the distribution of the electrostatic potential in the high-latitude ionosphere during times of southward IMF. This model can be generated using as inputs either the ionospheric potential parameters or, based on the relationships analyzed here, the IMF conditions. The capabilities of the resulting mathematical model are illustrated, and the importance of retaining a flexibility in the model to accommodate individual observations is emphasized.
Voyager-2 measurements indicate that, for the period day 190, 1987 to day 345, 1988, the large-scale fluctuations of the IMF between 23.3 AU and 27.8 AU have the symmetry properties of a multifractal over scales from 16 hours to 21 days. This suggests the existence of scaling symmetries for the higher moments of the fluctuations of the magnetic field strength.
Intensity-time profiles of solar energetic particle enhancements generally show an asymmetry with respect to the heliolongitude of the associated solar event. Particles arrive promptly from events to the west of an observer because of good magnetic connection, whereas particle enhancements from poorly connected eastern source regions usually show much slower onsets. However, some 15 percent of eastern events do show prompt onsets. Two prompt particle enhancements associated with eastern solar events are studied using data from the Goddard Space Flight Center instruments on the ISEE 3 and IMP 8 spacecraft. In both events the prompt particle onset was observed when the spacecraft were in a postshock plasma region, apparently within a magnetic bottle. It is suggested that the magnetic bottle extended back to the sun and served as a channel for fast particle propagation to the spacecraft. Particles accelerated at an expanding coronal shock initiated by the eastern event could be injected onto field lines in the foot of the bottle.
The overall properties of Venus ionospheric tail rays (such as density, spatial extent, and distribution) and their relationship to the draped magnetic field configuration behind the planet were investigated using measurements obtained by the Pioneer Venus Orbiter Langmuir probe, a magnetometer, and a plasma-wave detector. The results suggest that tail rays are a normal feature of the steady solar wind interaction with Venus and are not generally associated with a central tail plasma sheet. The statistics of the tail rays occurrence point toward the existence of a distributed terminator ionosphere source, consistent with findings of Brace et al. (1990).
IMF data made on November 1, 1984, by three spatially well-separated spacecraft in the solar wind are presented. The IMF measured by each of the spacecraft is found to consist of a multiplicity of structures within which the magnetic field varies in parallel planes. The orientations of these planes at the three spacecraft locations are similar. The planes are inclined at a large angle to the ecliptic, and they lie almost perpendicular to the nominal Parker spiral direction in the ecliptic. Intercomparisons of the measurements at the various spacecraft show that the IMF features at one spacecraft are clearly reproduced at another, with time delays required for signal propagation. From these time delays and the mutual separations of the spacecraft, it is inferred that the structures are convecting with the ambient flow. Simultaneous observations made downstream of the bow shock in the magnetosheath reveal that the magnetosheath magnetic field, too, is planar.
A model for the formation of the low-boundary layer in which a magnetosheath flux tube reconnects in the north and south beyond the cusp when the IMF is strongly northward is presented. For northward IMF the geomagnetic field captures solar wind flux tubes through intermittent reconnection at the cusp region. These newly captured flux tubes shorten and sink into the magnetosphere while the flux tubes reorient themselves as they become assimilated into the magnetosphere. There is no significant acceleration for the particles within the flux tubes. In the magnetosphere the interchange instability disperses the flux tube azimuthally along the magnetopause to form a boundary layer. Subsequent reconnection forms sublayers of the boundary layer and different sublayers represent different ages after reconnection. The interchange instability is stable radially, which keeps sharp boundaries between sublayers and between the boundary layer and magnetosphere.
New evidence is presented to support the hypothesis that the 11-yr solar cycle modulation of galactic cosmic rays is caused by strong diffusion inside long-lived merged interaction regions. To test this hypothesis, the 1D force-field approximation of the cosmic ray modulation equation is solved. It is assumed that a constant solar wind speed convects magnetic field compressions and rarefactions unchanged through a model heliosphere. The result is a reasonable simulation of the integrated high-energy cosmic ray intensity profile from about 1982 to mid-1989. This period encompasses both the full recovery portion of the last profile from about 1982 to mid-1989. This model responds to the Voyager 2 magnetic field data by correctly timing the beginning of the new modulation cycle in late 1987. It is concluded that the present hypothesis is consistent with the results of this simulation.
Two cases of observations on board the IMP 8 spacecraft of upstream energetic ions were used to test the viability of the Fermi mechanism as an efficient accelerator of particles with E not less than 50 keV at the earth's bow shock, under conditions which are more constrained than those applied in all previously published detailed analyses of upstream ion events. In addition to the nearly radial IMF, the following particle and field conditions were present: (1) in situ cyclotron-resonant wave activity, (2) a seed energetic particle population, and (3) small (not above 25 deg) theta(Bn) at the points of connection of the spacecraft to the bow shock. The analysis of data from days 67, 1979 and 303, 1980 showed that, despite these conditions, no ion enhancements attributable to the Fermi process could be detected.
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Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north-south changes of the IMF. During the 6-h interval in question, IMF Bz oscillates between +/- 7 nT with about a 1-h period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5 deg magnetic latitude, but separated by about 2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (about 10 MLT) and then spread outward toward dawn and dusk with a phase speed of about 5 km/s over the longitude range about 8 to 12 MLT, slowing to about 2 km/s outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds.
Strong slow mode waves in the Pc 3-4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renormalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3-4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18 percent of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosphere.
Twenty years of solar magnetograph and in-ecliptic IMF measurements are used to verify that the radial IMF component (B sub r) can be approximated by a superposition of the sun's inclined magnetic dipole moment and a current sheet normal to the dipole axis. The net field is found to be about three times stronger along the dipole axis than near the current sheet. Over the sunspot cycle, the absolute value of B sub r should undergo the least modulation near the heliographic equator and the greatest modulation above the sun's poles, where, at 1 AU, it varies systematically between about 10 nT near sunspot minimum and about 1 nT near sunspot maximum. The latitudinal gradients in the absolute value of B sub r are expected to be steepest around sunspot minimum and flattest near sunspot maximum. It is predicted that Ulysses will encounter very strong fields when it flies over the solar poles during the declining phase of sunspot cycle 22.
Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter at about 0.7 AU and IMP 8 at 1.0 AU are used to compare the long-term behavior of the IMF at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle (as compared to solar maximum or minimum), slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvenic and magnetosonic Mach numbers, and a remarkably consistent sector structure. Differences include the temporal behavior of the high-field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.