Interplanetary Field Orientation Research Articles

Solar control of the Martian magnetic topology: Implications from model-data comparisons

One of the goals of the upcoming MAVEN mission to Mars is to investigate the effects of the crustal remanent fields on the solar wind plasma interaction and the upper atmosphere. The MGS Electron Reflectometer and magnetometer observations can be used to test the idea that, if the future data from the electron spectrometer (SWEA) are separated for the two prevalent interplanetary field orientations (Parker spirals ׳toward׳ and ׳away׳ from the Sun), one may be able to detect specific differences in the pattern of locations of open magnetic fields (where photoelectrons can escape from Mars׳ ionosphere into space), as well as patterns of photoelectrons in the Martian magnetotail. We use a pair of BATS-R-US MHD model results of the Mars–solar wind interaction, in a manner similar to that tested by Liemohn et al. in 2006 on Mars Express ELS electron data, to define these patterns of expected photo-electron detections on a global scale. Those cases have the strongest southern hemisphere crustal fields at noon or midnight, a matter of importance in such investigations because these patterns will be sensitive to the local time of those fields. We compare some MGS data-based maps of the time periods selected for their open field signatures in the pitch angle distributions and energy spectra, and separated by interplanetary field orientation inferred from Mars magnetosheath observations. This exercise illustrates the power (and necessity) of the global model comparisons as a means of interpreting the very complex Mars–solar wind interaction and its effects.

Solar wind interaction effects on the magnetic fields around Mars: Consequences for interplanetary and crustal field measurements

The first unambiguous detections of the crustal remanent magnetic fields of Mars were obtained by Mars Global Surveyor (MGS) during its initial orbits around Mars, which probed altitudes to within ∼110km of the surface. However, the majority of its measurements were carried out around 400km altitude, fixed 2a.m. to 2p.m. local time, mapping orbit. While the general character and planetary origins of the localized crustal fields were clearly revealed by the mapping survey data, their effects on the solar wind interaction could not be investigated in much detail because of the limited mapping orbit sampling. Previous analyses (Brain et al., 2006) of the field measurements on the dayside nevertheless provided an idea of the extent to which the interaction of the solar wind and planetary fields leads to non-ideal field draping at the mapping altitude. In this study we use numerical simulations of the global solar wind interaction with Mars as an aid to interpreting that observed non-ideal behavior. In addition, motivated by models for different interplanetary field orientations, we investigate the effects of induced and reconnected (planetary and external) fields on the Martian field's properties derived at the MGS mapping orbit altitude. The results suggest that inference of the planetary low order moments is compromised by their influence. In particular, the intrinsic dipole contribution may differ from that in the current models because the induced component is so dominant.

Hemispheric asymmetry of the magnetic field wrapping pattern in the Venusian magnetotail

We examine statistically the magnetic field in the Venusian magnetotail which is formed by the draping of interplanetary magnetic field lines. Although the near‐planet and distant magnetotail regions have been sampled by the various missions to Venus and the general magnetic features of the distant magnetotail are well established, the near wake region from about 1.3 to 3 Venusian radii downstream of the planet remained unexplored until the Venus Express mission. Here we report the unanticipated finding of a draped field reversal in one hemisphere of the near Venus tail. When ordered by the interplanetary electric field orientation, the magnetic field lines in the hemisphere with inward motional electric field apparently are wrapped more tightly around Venus than in the other hemisphere, thus forming a field reversal region in the this portion of the near tail. A global hybrid simulation produces what we see and provides a three‐dimensional view of the observed hemispherical asymmetry.

Response of the magnetic field in the geosynchronous orbit to solar wind dynamic pressure pulses

We do a statistical survey of solar wind dynamic pressure (Pd) pulses and geosynchronous magnetic fields observed between 1998 and 2005. In geomagnetic quiet times with Dst > −50 nT, we find 111 solar wind dynamic pressure pulses which produce geosynchronous magnetic field responses. These responses are often observed by two or three GOES spacecraft at different local times in geosynchronous orbit. The magnitudes of the geosynchronous magnetic field changes (dBz) have a peak near the noon meridian, similar to the results obtained in the study of the response of the geosynchronous field to the large and sharp solar wind dynamic pressure variations. However, the relative change of the geosynchronous magnetic field dBz/AV‐Bz (where AV‐Bz is the average of the geosynchronous magnetic field Bz observed during the response to the pressure pulse) depends weakly on the local time; thus the change of Bz(dBz) is proportional to the average field (AV‐Bz). As the magnitude of the relative change of solar wind dynamic pressure (dPd/Pd) increases, the rate of geosynchronous magnetic field variation increases correspondingly. These results imply that the magnitude of the geosynchronous magnetic field response could be determined by AV‐Bz. In addition, the interplanetary field orientation does not affect the response significantly. Using an MHD code which models the global behavior of the solar wind‐magnetosphere‐ionosphere system, we reproduce the main characteristics of the observations.

Venus ionospheric “clouds”: relationship to the magnetosheath field geometry

Venus plasma “clouds,” apparently detached regions of ionospheric plasma above the main ionosphere of Venus observed by the Langmuir probe on the Pioneer Venus orbiter, are thought to be evidence for bulk removal of ionospheric ions by the solar wind interaction. In addition to the primary feature of an electron density enhancement above the ionopause, the characteristic signature of a cloud may include a change in the sign of the Bx (sunward/antisunward) component of the surrounding magnetosheath magnetic field. The plasma wave instrument also sometimes observed enhanced electric field noise in the clouds. In an effort to learn more about clouds and their cause(s), we analyzed the magnetic field control of cloud positions relative to the planet by rotating their observed locations into a coordinate system in which all of the transverse upstream (interplanetary) magnetic fields were aligned. The results indicate that clouds are scattered around the periphery of the planet in the terminator plane. There is no evidence of a concentration of clouds where the magnetosheath magnetic field is most strongly “draped,” as suggested by some earlier studies of selected occurrences. On the other hand, statistics show that the change in the orientation of the transverse upstream magnetic field between the inbound and outbound bow shock crossings, for the orbits where clouds are seen, is approximately 30° greater than the average upstream change at Venus over the same time intervals. The Spreiter and Stahara gasdynamic model of the magnetosheath provides further support for the idea that a rotation in the upstream transverse magnetic field accompanies cloud formation. Modeling of the magnetic field time series in the magnetosheath for a “cloud” orbit is possible only when a sequence of upstream fields is used to reconstruct the observed time series. Thus, clouds are associated not simply with normal magnetosheath field draping but with interplanetary field orientation changes.

A gas dynamic magnetosheath field model for unsteady interplanetary fields: Application to the solar wind interaction with Venus

The steady state gas dynamic model of magnetosheath magnetic fields previously developed by Spreiter and Stahara (1980) is generalized for the common situation of a temporally varying interplanetary field orientation. Examples for the particular case of Venus in the solar wind illustrate the application of the model to the passage of rotational discontinuities and MHD waves through planetary magnetosheaths. The results of this model illustrate how the field structure near a planetary magnetopause or ionopause can be affected by interplanetary field variations rather than by local processes because of the “pile up” of magnetosheath fields from a sequence of upstream fields. Changes in the spectrum of interplanetary waves on their transmission to the magnetopause are also indicated.

Spatial distributions of magnetic field fluctuations in the dayside magnetosheath

In a study that tests the hypothesis that magnetosheath magnetic fields are disturbed on plasma streamlines which are connected to the quasi‐parallel bow shock, magnetometer observations from the ISEE 2 and IMP 8 spacecraft are used to investigate the dayside spatial distributions of fluctuating magnetosheath fields for different interplanetary field orientations. The results suggest that although other sources such as Kelvin‐Helmholtz instabilities and flux transfer processes probably contribute to the fluctuations in the magnetosheath field, the quasi‐parallel shock source is an important contributor in the dayside region.

Patterns of potential magnetic field merging sites on the dayside magnetopause

Models of the magnetospheric and magnetosheath magnetic fields are used to determine the relative orientations of these fields at the dayside magnetopause in order to locate potential merging sites. Areas on the magnetopause with different fractional antiparallel components are displayed by contour diagrams for a variety of interplanetary field orientations. For interplanetary fields oriented perpendicular to the solar wind velocity the areas of nearly antiparallel field agree with those obtained by Crooker using simplified representations for the magnetic field geometry. Here, the application of more realistic models gives the locations of areas where any antiparallel component occurs. Potential merging sites for interplanetary fields with radial components are also illustrated. The results suggest that the topology of the magnetosheath and magnetospheric fields provides antiparallel components over a substantial fraction of the magnetopause for most interplanetary field orientations.

Solar Wind Control of Daytime, Midperiod Geomagnetic Pulsations

Numerous studies have established that various properties of geomagnetic pulsations are linked to various properties of the solar wind. The linkage in most cases is rather loose, suggesting that combinations of factors must be involved in generation and control of pulsation activity. We review briefly the most significant observational results and we describe and discuss critically the two most prominent models for external generation of magnetospheric waves. We present arguments favoring joint application of the models, wherein perturbations in the magnetosheath resulting from favorable interplanetary field orientation are delivered to the magnetopause, transferred directly into the subsolar magnetosphere, and amplified into surface waves on the flank of the magnetosphere by Kelvin-Helmholtz instability at high solar wind speed. This combination of circumstances can account for experimental correlations of pulsation occurrence with interplanetary field orientation, periods with interplanetary field strength, and amplitudes with solar wind velocity.

Pc 3, 4 activity and interplanetary field orientation

Analysis of Pc 3, 4 micropulsation wave forms recorded at Calgary in September 1969 shows a tendency toward signal enhancement in cases where the interplanetary magnetic field is at a small angle to the sun-earth line. Scatter plots of hourly Pc 3, 4 amplitudes exhibit a definite trend toward large signals when this angle is less than 50 or 60 degrees, and a corresponding disappearance of significant amplitudes when the angle is greater than 60 degrees. There is, however, an appreciable variability in individual cases. Power density spectrograms improved the correlation of pulsation strength with low angle in some cases.

Magnetic Field Observations near Venus: Preliminary Results from Mariner 10

The NASA-GSFC magnetic field experiment on Mariner 10 is the first flight of a dual magnetometer system conceived to permit accurate measurements of weak magnetic fields in space in the presence of a significant and variable spacecraft magnetic field. Results from a preliminary analysis of a limted data set are summarized in this report, which is restricted primarily to Venus encounter. A detached bow shock wave that develops as the super Alfvénic solar wind interacts with the Venusian atmosphere has been observed. However, the unique coincidence of trajectory position and interplanetary field orientation at the time of bow shock crossing led to a very disturbed shock profile with considerably enhanced upstream magnetic fluctuations. At present it is not possible to ascertain the nature and characteristics of the obstacle responsible for deflecting the solar wind flow. Far downstream disturbances associated with the solar wind wake have been observed.

Observation of nonuniform structure of the Earth's bow shock correlated with interplanetary field orientation

Explorer 33 and 35 magnetometers, on the western and eastern flanks of the earth's bow shock, respectively, observed the boundary concurrently between 0130 and 0430 UT, Oct. 30, 1968. Contrasting shock structures were recorded. Explorer 35 saw a quiet abrupt shock, whereas Explorer 33 saw an irregular noisy boundary with much upstream wave activity. The interplanetary field was roughly in the average archimedean spiral angle and was therefore approximately tangent to the shock at Explorer 35 and normal to the shock at Explorer 33. Gross motions and variable tilting of the aberrated shock probably contributed to the peculiar sequence of shock crossings at the two spacecraft. The observations support a model of the shock in which perpendicular and oblique collisionless structures coexist and form a nonuniform magnetosheath outer boundary.

The Dungey Model of the Magnetosphere, and Astro‐Geophysical Current Sheets

By the addition of an interplanetary magnetic field in various orientations to a closed magnetosphere model, the Dungey model was generated for all orientations other than a strictly northward interplanetary magnetic field. Particles can be accelerated in both the dayside magnetopause current sheet and in the tail current sheet, but there are inherent asymmetries. Particles may have asymmetric access to the north or the south polar caps, depending on the interplanetary field orientation. The breakdown of the usual adiabatic theory is discussed for a model current sheet.

Spatial variations of the magnetosheath magnetic field

Measurements by Explorers 28, 33, 34 and 35 have been used to study the spatial characteristics of the magnetic field in the magnetosheath to a distance of 70 R E behind the Earth. During 1966–1967 there were 1661 hr when one spacecraft of this multi-satellite system was monitoring the interplanetary medium while at least one other satellite was making magnetosheath measurements. This has made possible a separation of time and space variations and has permitted a study of the direction and magnitude of the magnetosheath field as a function of position and interplanetary field orientation. Results indicate that the mag-netosheath field is several times the strength of the simultaneously measured interplanetary field in the sunward magnetosheath. This magnetosheath to interplanetary magnitude ratio decreases with distance from the subsolar point to values which are frequently less than unity at distances beyond 30 R E and away from the bow shock. This ratio also displays a dawn-dusk asymmetry which is dependent on the interplanetary field orientation. Interplanetary field lines perpendicular to the Earth-Sun line are associated with symetrically distorted magnetosheath field lines in the dawn and dusk hemispheres and are consistent with the draping of field lines around the magnetosheath. When the interplanetary field is aligned near the spiral angle, fields measured in the dusk hemisphere are much more ordered than those measured in the dawn hemisphere behind the Earth. These experimental results are in general agreement with the theoretical predictions of the gasdynamic model.