Interplanetary Magnetic Field Variations Research Articles

Three-dimensional cosmic-ray simulations: Heliographic latitude and current-sheet tilt

We present the results from a study of the variations of the cosmic-ray intensity with time, heliographic latitude, and longitude, and for varying interplanetary conditions, using our three-dimensional, time-dependent computer code for cosmic-ray transport in the heliosphere. Our code also produces a solar-wind and interplanetary magnetic field (IMF) configuration which is compared with observations. Because of the fully three-dimensional nature of the model calculations, we are able to model time variations which would be expected to be observed along Ulysses’s trajectory as it moves to high latitudes. In particular we can model the approximately 13-and 26-day solar-rotation induced variations in cosmic rays, solar wind and IMF, as a function of increasing heliographic latitude, as one moves poleward of the interplanetary current sheet. Our preliminary model results seem to be in general form quite similar to published data, but depend on the physical parameters used such as cosmic-ray diffusion coefficients, boundary conditions, and the nature of the solar wind and IMF and current sheet.

Coupling of IMF BY variations into the polar ionospheres through interplanetary field‐aligned currents

Studies of the correlation between the geomagnetic fields recorded at high‐latitude observatories and the interplanetary magnetic field (IMF) observed from the IMP 8 satellite in the solar wind have indicated that variations in the IMF BY component may generate perturbations of similar appearance in the geomagnetic north (H) components observed in the noon sector of the dayside polar cap. During southward interplanetary field conditions (IMF BZ < 0) these perturbations are observed to progress poleward from the cusp region across a large fraction of the dayside polar cap, changing only slightly in shapes and amplitudes. These progressing geomagnetic variations are interpreted as the footprints of the variable IMF BY component present in the solar wind volume which is magnetically connected to the polar cap ionosphere. It is suggested in this work that both the IMF BY variations and the corresponding variations in the H(ground) components are related to interplanetary field‐aligned currents. The “open” magnetospheric topology, resulting from the merging of the geomagnetic field and the interplanetary field during southward interplanetary field conditions, allows these interplanetary currents to flow along the merged field lines to the northern and southern polar cap ionospheres. Here they generate the convection‐related Hall currents responsible for the observed geomagnetic perturbations. The closure from one polar cap to the other can be partly in the form of ionospheric Pedersen currents and partly in the form of field‐aligned currents along closed field lines in the inner magnetosphere.

Cusp latitude magnetic impulse events: 2. Interplanetary magnetic field and solar wind conditions

The interplanetary magnetic field (IMF) conditions and solar wind plasma parameters prevailing during the magnetic impulse events identified by Lanzerotti et al. (1991) at the near cusp latitude stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica, are examined. The impulse events are found to occur during periods of high IMF variability. The prevailing IMF orientation, averaged over 11‐min periods during the events, shows no distinct anisotropies. However, by examining the IMF orientation when the IMF variability is small a south‐east anisotropy is found. Examining the IMF orientation on timescales of less than 1 min reveals that the impulse events are characterized by a more southward oriented IMF than is to be expected statistically. The duration of the events is found to be controlled by both solar wind speed and by the IMF By. The IMF By control of the duration of nonconjugate events is in accordance with the effect expected from the presence of magnetic tension in a reconnection setting. The amplitude of the events is found to be strongly related to solar wind speed. These findings suggest that sporadic reconnection is among the mechanisms producing the events. On the basis of the amplitudes of the impulse events, it is argued that the fractional changes in dynamic pressure must be of the order of 1 to be able to associate the impulse events with changes in the dynamic pressure. Such variability in the dynamic pressure at the time of the magnetic impulse events is not observed. It is concluded that a change in the solar wind dynamic pressure, while not among the primary mechanisms generating the magnetic impulse events, may account for between 15% and 30% of them. On the other hand, reconnection processes are able to produce impulse events of the required magnitude. It is argued then that magnetic reconnection is likely responsible for generating a minimum of 50%‐70% to a maximum of 90% of the events. However, this conclusion leaves parts of the picture unclear. Lanzerotti et al. (1991) found that magnetic impulse events with a unipolar deflection in the vertical component of the magnetic field possessed a local time dependence; positive deflections occurred in local morning, while negative deflections occurred in local afternoon. This dependence can be explained by postulating that the magnetic impulse events are associated with vortices generated by the Kelvin‐Helmholtz instability in the low‐latitude boundary layer. As such, it is suggested that the Kelvin‐Helmholtz instability may mediate the process by which sporadic reconnection produces a ground signature.

UIT: Ultraviolet surface photometry of the spiral galaxy M74 (NGC 628)

Ultraviolet photometry, obtained from Ultraviolet Imaging Telescope (UIT) images at 1520 A (far-UV; magnitudes m(152)) and 2490 A (near-UV; magnitudes m(249)), of the spiral galaxy M74 (NGC 628) is compared with H-alpha, R, V, and B surface photometry and with models. M74's surface brightness profiles have a central peak with an exponential falloff; the exponential scale lengths of the profiles increase with decreasing wavelength for the broad-band images. The slope of the continuum-subtracted H-alpha profile is intermediate between those of far-UV and near-UV profiles, consistent with the related origins of H-alpha and UV emission in extreme Population I material. M74's color profiles all become bluer with increasing radius. The (m(152) - m(249)) color as measured by UIT averages near 0.0 (the color of an A0 star) over the central 20 sec radius and decreases from approximately -0.2 to approximately -0.4 from 20 sec to 200 sec. The spiral arms are the dominant component of the surface photometry colors; interarm regions are slightly redder. In the UV, M74's nuclear region resembles its disk/spiral arm material in colors and morphology, unlike galaxies such as M81. No UV 'bulge' is apparent. The m(152) - m(249) colors and models of M74's central region clearly demonstrate that there is no significant population of O or B stars present in the central 10 sec. M74's UV morphology and (m(152) - m(249)) color profiles are similar to those of M33, although M74 is approximately 0.5 mag redder. M81 has a smooth UV bulge which is much redder than the nuclear regions of M74 and M33. M74 is approximately 0.4 mag bluer than M81 in its outer disk, although M81 has bright UV sources only in spiral arms more than 5 kpc from its center. We investigate possible explanations for the color profiles of the galaxies and the differences among the galaxies: abundances; reddening due to internal dust; interplanetary magnetic field (IMF) variations, and the history of formation of the dominant generations of stars. Abundance and IMF variations do not produce large enough m(152) - m(249) or UV - V color differences. Comparing model UV/optical colors with those of M74 shows that M74's disk has undergone significant star formation over the past 500 Myr, and that either the star-formation history or the extinction varies systematically across M74's disk. Comparison of M74, M33, and M81 (UV - V) colors shows that M74 colors range from the bluest of M33's colors to the bluest of M81's. The failure of reddening models to cover the range of colors, and the known abundance range in such material, leads to the conclusion that star-formation history varies significantly as a function of radius in these galaxies, and that such variation is required to explain the range of colors observed in M74, M33, and M81.

Correction to “Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988”

Correction to “Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988”

Galactic cosmic ray modulation and interplanetary medium perturbations due to a long‐living active region during October 1989

During October 1989, three very energetic flares were ejected by the same active region at longitudes 9° E, 32° W, and 57° W, respectively. The shape of the galactic cosmic ray variations suggests the presence of large magnetic cloud structures (Nagashima et al., 1990) following the shock‐associated perturbations. In spite of long data gaps the interplanetary observations at IMP 8 (near the Earth) and ICE (∼1 AU, ∼65° W) confirm this possibility for the event related to the 9° E flare; the principal axes analysis shows that the interplanetary magnetic field variations at both spacecraft locations are mainly confined on a meridian plane. This result suggests that the western longitudinal extension of this cloud is indeed very large (≥75°). The nonnegligible depression in the cosmic ray intensity observed inside the possible cloud related to the 57° W flare indicates that also the eastern extension could be very wide. The analysis of neutron monitor data shows clearly the cosmic ray trapping effect of magnetic clouds; this mechanism seems to be responsible for the enhanced diurnal effect often observed during the recovery phase of Forbush decreases. We give an interpretation for the anisotropic cosmic ray peak occurring in the third event, and, related to that, we suggest that the Forbush decrease modulated region at the Earth's orbit could be somewhat wider than the magnetic cloud, as already anticipated by Nagashima et al. (1990). By this analysis, based mainly on cosmic ray data, we show that it is possible to do reasonable inferences on the large‐scale structure of flare‐related interplanetary perturbations when interplanetary medium data are not completely present.

Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high‐latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground‐based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon‐to‐morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn‐to‐dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn‐to‐dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single‐cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

The relationship between coronal and interplanetary magnetic fields

The morphology of the interplanetary magnetic field (IMF) is being used increasingly often to diagnose the state and solar origin of interplanetary plasma. For example, the heliospheric current sheet has been used to locate the magnetic equator and relate it to the coronal streamer belt. More often, recently, has been use of variance analysis of the IMF to infer the topology of apparent magnetic loops, magnetic draping, and relationships between the IMF and photospheric fields. These time-dependent IMF variations open new horizons for relating the IMF to the coronal field. Here, I review applications of coronal magnetic field models to predicting the quasi-steady IMF morphology and review recent applications of IMF variance analysis to diagnose origins and history of solar wind plasma.

A 22-yr variation of geomagnetic activity and interplanetary magnetic field

The 22-yr variation in geomagnetic activity has been reported by Chernosky [(1966) J. geophys. Res. 71, 965]. It has been suggested that the observed 22-yr variation in the geomagnetic activity is related to the 22-yr cycle of the solar magnetic field. In order to provide a quantitative understanding of this relationship, we first calculate 〈 B z 〉 GSEQ the annual average of the z-component of the interplanetary magnetic fields (IMF) in GSEQ (Solar Equatorial) coordinates, observed at 1 A.U. from 1967 to 1988. It is found that 〈 B z 〉 GSEQ does not correlate with the geomagnetic activity. We then calculate 〈 B y 〉 GSEQ , the y-component of the IMF in GSEQ coordinates and 〈 B z 〉 gsm , the z-component of IMF in GSM coordinates and found both of them to correlate with the 22-yr variation of the geomagnetic activity. We then suggest that the cause for the 22-yr variation of the geomagnetic variation is because 〈 B y 〉 GSEQ , the y-component of the IMF in the solar equatorial plane, has a 22-yr variation. In comparison with the 22-yr variation of geomagnetic activity with the effective component of IMF for geomagnetic activity, we calculated 〈 B〉 GSM and found that southward of 〈 B〉 GSM correlates with a higher geomagnetic activity. Then, adopting a simple model, we assume the IMF consists of a spiral magnetic field in the x− y plane of the GSEQ coordinates with no z-component (i.e. 〈 B z 〉 GSEQ = 0). However, the effective 〈 B z 〉 GSM which derives mainly from 〈 B y 〉 GSEQ does give a 〈 B z 〉 GSM ranging from −0.08 to 0.15 nT. This simple model assumes that the effective B z z( GSM) is related to geomagnetic activity in which there is no geomagnetic activity for B z( Gsm) > 0 and the geomagnetic activity is proportional to ∥ B z ∥ for B z( GSM) < 0. It is found from this simple model that the calculated geomagnetic activity due to the 22-yr variation of 〈 B z 〉 GSM can explain the phase and amplitude of the observed 22-yr variation of geomagnetic activity.

Flux transfer events: Spontaneous or driven?

Since flux transfer events (FTEs) occur predominantly under conditions of southward interplanetary magnetic field (IMF), one might expect that FTEs could be associated with the southward turning of the IMF which could lead to a transient increase in the merging rate. Lockwood and Wild [1993] recently suggested, in fact, that FTEs are driven by the IMF variations in which the Bz becomes more southward and rather than arising spontaneously during periods of steady IMF. To test this hypothesis, we have surveyed observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE spacecraft. The upstream solar wind data from IMP‐8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE to determine the time lag between them, which enables us to survey the simultaneous solar wind data during the FTEs. We find no evidence that FTEs are directly correlated with southward turning of the IMF. Rather, the FTEs more frequently occur when the IMF Bz is steady. A survey of 10‐minute intervals of simultaneous IMF data preceding each FTE show that the IMF fluctuates between northward and southward for less than 20% events and few of these turnings appear to be consistent with triggering the FTE. For majority of events, the IMF stays southward with fluctuations of clock angle less than 30° over the 10‐minute period. Thus we conclude that FTEs do occur spontaneously for steady southward IMF and are not simply directly driven by the observed fluctuations in the IMF Bz component.

Comment [on “Solar wind control of the magnetopause shape, location, and motion” by D. G. Sibeck, R. E. Lopez, and E. C. Roelof

The methodology employed in the paper 'Solar wind control of the magnetopause shape, location, and motion' by Sibeck et al. (1991), which quantifies the magnetospheric response to solar wind dynamic pressure and interplanetary magnetic field variations, is commented on, with emphasis on how the shape and position of the magnetopause boundary can be specified, within some uncertainty, for particular simultaneous values of the solar wind pressure in the magnetic field. The reply of Sibeck et al. included a data set of 1821 magnetopause crossings, each associated with an hourly averaged solar wind dynamic pressure and/or north-south component of the interplanetary magnetic field. A least squares fit to an ellipsoid of revolution to subsets of the data was performed.

The response of the quiet-time auroral configuration to short- and long-term interplanetary magnetic field variations

Observations from the IMP-8 satellite of the interplanetary magnetic field (IMF) are compared with areas of the polar region bounded by the aurora as observed by the Viking spacecraft during quiet-time conditions (IMF Bz northward). A variety of energy-coupling functions are investigated and it is determined that the auroral distribution can be best described by the inclusion of azimuthal terms in addition to standard energy-coupling functions. The auroral distributions for Bz northward support antiparallel merging as a mechanism whereby energy is transferred to the magnetosphere from the solar wind. As well, however, when the polar angle is small the region bounded by the auroras expands and may be controlled more by wave interaction at the magnetopause. Observations by the Viking spacecraft indicate a dominance of dusk sector polar arcs in the spring time and dawn sector arcs in the fall (post equinox). Two alternative mechanisms can explain the observations. One involves the ordering of the IMF in a solar equatorial coordinate system while the other involves the Sun's polarity and the traversal of the Earth's orbit through different heliographic latitudes. A test is proposed whereby the two hypotheses can be investigated during the next solar cycle.

Flux transfer events at the dayside magnetopause: Transient reconnection or magnetosheath dynamic pressure pulses?

The suggestion is discussed that characteristic particle and field signatures at the dayside magnetopause, termed “flux transfer events” (FTEs), are, in at least some cases, due to transient solar wind and/or magnetosheath dynamic pressure increases, rather than time‐dependent magnetic reconnection. It is found that most individual cases of FTEs observed by a single spacecraft can, at least qualitatively, be explained by the pressure pulse model, provided a few rather unsatisfactory features of the predictions are explained in terms of measurement uncertainties. The most notable exceptions to this are some “two‐regime” observations made by two satellites simultaneously, one on either side of the magnetopause. However, this configuration has not been frequently achieved for sufficient time, such observations are rare, and the relevant tests are still not conclusive. The strongest evidence that FTEs are produced by magnetic reconnection is the dependence of their occurrence on the north‐south component of the interplanetary magnetic field (IMF) or of the magnetosheath field. The pressure pulse model provides an explanation for this dependence (albeit qualitative) in the case of magnetosheath FTEs, but this does not apply to magnetosphere FTEs. The only surveys of magnetosphere FTEs have not employed the simultaneous IMF, but have shown that their occurrence is strongly dependent on the north‐south component of the magnetosheath field, as observed earlier/later on the same magnetopause crossing (for inbound/outbound passes, respectively). This paper employs statistics on the variability of the IMF orientation to investigate the effects of IMF changes between the times of the magnetosheath and FTE observations. It is shown that the previously published results are consistent with magnetospheric FTEs being entirely absent when the magnetosheath field is northward: all crossings with magnetosphere FTEs and a northward field can be attributed to the field changing sense while the satellite was within the magnetosphere (but close enough to the magnetopause to detect an FTE). Allowance for the IMF variability also makes the occurrence frequency of magnetosphere FTEs during southward magnetosheath fields very similar to that observed for magnetosheath FTEs. Conversely, the probability of attaining the observed occurrence frequencies for the pressure pulse model is 10−14. In addition, it is argued that some magnetosheath FTEs should, for the pressure pulse model, have been observed for northward IMF: the probability that the number is as low as actually observed is estimated to be 10−10. It is concluded that although the pressure model can be invoked to qualitatively explain a large number of individual FTE observations, the observed occurrence statistics are in gross disagreement with this model.

Quasi‐perpendicular/quasi‐parallel divisions of Earth's bow shock

Computer‐drawn diagrams of the boundaries between quasi‐perpendicular and quasi‐parallel areas of Earth's bow shock are displayed for a few selected cone angles of static interplanetary magnetic field (IMF). The effect on the boundary of variable IMF in the foreshock is also discussed and shown for one nominal case. The boundaries demand caution in applying them to the realistic, dynamic conditions of the solar wind and in interpreting the effects of small cone angles on the distributions of structures at the shock. However, the calculated, first‐order boundaries are helpful in defining areas of the shock where contributions from active structures inherent in quasi‐parallel geometry may be distinguishable from those derived secondarily from upstream reflected ion dynamics. The boundaries are also compatible with known behavior of daytime ULF geomagnetic waves and pulsations according to models postulating that cone angle‐controlled, time‐dependent ULF activity around the subsolar point of the bow shock provides the source of geomagnetic excitation.

Comment on inferred and measured interplanetary sector structure

Journal of Geophysical Research: Space PhysicsVolume 94, Issue A6 p. 7015-7016 CommentariesFree Access Comment on inferred and measured interplanetary sector structure Paul F. Fougere, Paul F. FougereSearch for more papers by this author Paul F. Fougere, Paul F. FougereSearch for more papers by this author First published: 1 June 1989 https://doi.org/10.1029/JA094iA06p07015Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Fougere, P. F., Dependence of inferred magnetic sector structure upon geomagnetic and solar activity, Planet. Space Sci., 22, 1173– 1184, 1974. Francia, P., U. Villante, Spectral analysis of the geomagnetic activity index Ap during different IMF conditions (1947–1978), Nuovo Cimento Soc. Ital. Fis. C., 9, 1085– 1091, 1986. Friis-Christensen, E., K. Lassen, J. M. Wilcox, W. Gonzalez, D. S. Colburn, Interplanetary magnetic sector polarity from polar geomagnetic field observations, Nature, 233, 48– 50, 1971. Friis-Christensen, E., K. Lassen, J. Wilhjelm, J. M. Wilcox, W. D. Gonzalez, D. S. Colburn, Critical component of the interplanetary magnetic field responsible for large geomagnetic effects in the polar cap, J. Geophys. Res., 77, 3371– 3376, 1972. Gonzalez, A. L. C., W. D. Gonzalez, Periodicities in the interplanetary magnetic field polarity, J. Geophys. Res., 92, 4357– 4375, 1987. Mansurov, S. M., New evidence of a relationship between magnetic fields in space and on earth, Engl. Transl., J. Geomagn. Aeron., 9, 622– 623, 1969. Mansurov, S. M., L. G. Mansurova, The relationship between the magnetic fields in space and at the Earth's surface, Ann. Geophys., 26, 397, 1970. Svalgaard, L., Sector structure of the interplanetary magnetic field and daily variation of the geomagnetic field at high latitudes, Geophys. Pap. R-6, Dan. Meteorol. Inst., Charlottenlund, Denmark, 1968. Svalgaard, L., Interplanetary magnetic sector structure, 1926–1971, J. Geophys. Res., 77, 4027– 4034, 1972. Villante, U., P. Francia, Geomagnetic activity and current sheet, Ann. Geophys., Ser. A, 4, 75, 1986. Villante, U., P. Francia, Comment on “Periodicities in the interplanetary magnetic field polarity” by A. L. C. Gonzalez and W. D. Gonzalez, J. Geophys. Res., 93, 4141, 1988. Wilcox, J. M., N. F. Ness, Quasi-stationary corotating structure in the interplanetary medium, J. Geophys. Res., 70, 5793– 5805, 1965. Citing Literature Volume94, IssueA61 June 1989Pages 7015-7016 ReferencesRelatedInformation

Time scales of expansion and contraction of the auroral oval

The auroral boundary index calculated by Gussenhoven et al. [1983] on the basis of Defense Meteorological Satellite Program precipitating electron data is used to examine the dynamics of the auroral oval in the midnight sector. The equatorward boundary of the oval expands equatorward and contracts poleward corresponding to southward and northward turnings, respectively, of the interplanetary magnetic field (IMF). The time scales are found to be, on the average, 45 min for the expansion and 8 hours for the contraction. A general empirical formula to predict the oval location for any IMF variation is devised. Physical processes in the magnetosphere consistent with the “rapid” expansion and “slow” contraction of the auroral oval are discussed.

Major flattening of the distant geomagnetic tail

We quantify the Michel‐Dessler magnetotail cross section model, which predicts that the magnetotail flattens due to the anisotropic pressure of draped magnetosheath field lines. We assume steady, typical, solar wind and magnetotail conditions and allow flux to pass through both the current sheet and magnetopause. With these assumptions, our numerical model predicts that the semimajor axis of the elliptical magnetotail cross section at x = −200 RE is 37.8 RE long and lies in the solar ecliptic plane. The semiminor axis (perpendicular to the ecliptic plane) should be no greater than 18.5 RE at this distance. By x = −500 RE, the semimajor and minor axes should be 60 and 11.7 RE long, respectively. The ratio of the major to minor axis lengths depends on the strength of the interplanetary magnetic field (IMF) in the plane transverse to the solar wind flow and is greater for a strong IMF. The reaction time of the distant magnetotail to an impressed IMF variation is on the order of 20 minutes for typical solar wind conditions. The locations of ISEE 3 magnetppause crossings and magnetotail (including boundary layers) observations beyond x = −180 RE are consistent with the model cross section. Our definition of the magnetotail requires or Te &gt; 5×105 K. Furthermore, magnetotail magnetopause boundary normals point nearly parallel to the semiminor axis of the ellipse and therefore indicate strong flattening. We look for, but do not find, evidence of flattening in prior observations of the near‐earth magnetotail cross sectional dimensions. Some Pioneer 7 and 8 observations of the deep tail (x &lt; −500 RE) are consistent with magnetotail flattening.

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field ( B z ). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the Δ H = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.

Plasma wave levels and IMF orientations preceding observations of interplanetary shocks By Isee‐3

Some interplanetary shocks detected by ISEE‐3 are preceded by many hours of strongly‐enhanced plasma wave noise at a few kHz, while others have essentially no wave precursors above background. It has been shown that these extremes correspond to quasi‐parallel and quasi‐perpendicular shocks, respectively, based on the instantaneous orientation angle θBn of the interplanetary magnetic field (IMF) to the shock normal at the time the shocks cross the spacecraft. We show that precursor wave noise level is correlated with field orientation and an extrapolated θBn throughout the preshock observation interval for two contrasting active and quiet cases, and that intermediate, variable noise levels correspond to intermediate, variable IMF orientations. We infer that foreshocks are an intrinsic part of the structure of quasi‐parallel interplantary shocks.

An electrodynamic model of electric currents and magnetic fields in the dayside ionosphere of Venus

The electric current configuration induced in the ionosphere of Venus by the interaction of the solar wind has been calculated in previous papers (Cloutier and Daniell, Planet. Space Sci. 21, 463, 1973; Daniell and Cloutier. Planet. Space Sci. 25, 621, 1977; Cloutier and Daniell, Planet. Space Sci. 27, 1111, 1979) for average steady-state solar wind conditions and interplanetary magnetic field. This model is generalized to include the effects of (a) plasma depletion and magnetic field enhancement near the ionopause, (b) velocity-shear-induced MHD instabilities of the Kelvin-Helmholtz type within the ionosphere, and (c) variations in solar wind parameters and interplanetary magnetic field. It is shown that the magnetic field configuration resulting from the model varies in response to changes in solar wind and interplanetary field conditions, and that these variations produce magnetic field profiles in excellent agreement with those seen by the PIONEER-VENUS Orbiter. The formation of “flux-ropes” by the Kelvin-Helmholtz instability is shown to be a natural consequence of the model, with the spatial distribution and size of the flux-ropes determined by the magnetic Reynolds number.