Krishan K. Khurana

A new functional form to study the solar wind control of the magnetopause size and shape

In this study a new functional form, r = r 0 [2/(1 + cos θ)] α , is used to fit. the size and shape of the magnetopause using crossings from ISEE 1 and 2, Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM), and IMP 8 satellites. This functional form has two parameters, τ 0 and α, representing the standoff distance and the level of tail flaring. The value r is the radial distance at an angle (θ) between the Sun-Earth line and the direction of τ. It is found that r 0 varies with the interplanetary magnetic field (IMF) B z component and has a break in the slope at B z = 0 nT. The best-fit value of τ 0 decreases with increasing southward IMF B z . For northward IMF B z , the best-fit value of τ 0 increases slightly with increasing B z . The best-fit value of α increases monotonically with decreasing IMF B z . The dynamic pressure (D p ) also changes τ 0 and α. The parameters D p and τ 0 are related by a power law of -1/(6.6±0.8). The best-fit value of α is slightly larger for larger dynamic pressure, which implies that D p also has a role in flux transfer from the dayside to the nightside, but the size of this effect is small. An explicit function for the size and shape of the magnetopause, in terms of D p and B z , is obtained by using multiple parameter fitting in a form that is useful for operational space applications such as predicting when satellites at geosynchronous orbit will be found in the magnetosheath.

Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment

Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moons surface or atmosphere. The plasma effects vary with Europas height above the center of Jupiters extended plasma disk. Europa is comet-like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiters ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europas radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time-varying part of Jupiters magnetospheric field at Europas orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiters magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of ∼10 km or more in thickness would explain the observations.

A model of the Earth's distant bow shock

We present a new global model of the Earths bow shock that is parametrized by the solar wind conditions. We begin with a conic section base model taken to be correct for average solar wind conditions. Then we apply modifications to the base model, based on physical arguments, to account for the changes in the size and shape of the bow shock caused by changes in the prevailing solar wind dynamic pressure, Alfven and sonic Mach numbers, and interplanetary magnetic field orientation. We show that our model matches the location and timing of shock crossings observed, at large downtail distances, by the Galileo spacecraft during December 1990 and December 1992 and by the Pioneer 7 spacecraft during September 1966. Magnetic field and plasma moments in a shock normal coordinate system change across the model shock surface as required by conservation laws.

Mirror‐mode structures at the Galileo‐Io flyby: Instability criterion and dispersion analysis

The mirror mode is typically excited in high-beta plasmas when there is a significant pressure anisotropy, with the greatest plasma pressure perpendicular to the magnetic field, B. Large-amplitude mirror-mode structures were identified in the Galileo magnetic field data on the edges of the cold Io wake. Here, despite the high ambient B field and low plasma beta, the enhanced perpendicular pressure due to the ring-type velocity distributions of heavy Iogenic pickup ions overcomes the instability threshold for the mirror mode and provides the free energy. In the center of the Io wake, the local pickup velocities are very low and hence the perpendicular pressure contribution is small, while farther from Io in the torus, the corotating isotropic plasma dominates and the mirror mode is not unstable but instead ion cyclotron waves grow. Thus the spatial region in which the mirror mode dominates is narrow. A warm plasma dispersion analysis is performed for the multispecies plasma conditions appropriate to the edges of the Io wake. In a multispecies plasma, multiple ion cyclotron modes are possible, each with a growth rate dependent upon the anisotropy and free energy provided by the specific gyroresonant ion species component. However, the mirror mode can dominate, since its growth rate depends on the combined anisotropic pressure of all ions present.

The Galileo Earth encounter: Magnetometer and allied measurements

The Galileo spacecraft flew by Earth on December 8, 1990, at high speed along a trajectory that traversed the magnetotail and the near Earth magnetosphere. Galileos orbit through a region of the magnetotail from which limited data are available provided a unique opportunity to study a number of substorm-related phenomena. Several groups cooperated in collecting correlative data in order to take advantage of this special opportunity. Fortunately, geomagnetic conditions were rather disturbed during the entire day, and an interplanetary shock passed Earth when the spacecraft was in the magnetotail at about 30 RE geocentric distance. In this first report we provide an overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 RE through exit into the solar wind on the dayside. We link these measurements with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. Based on our analysis, we present a time line of the important geomagnetic events of the day that we believe provides a framework for the full multi-instrument analysis of the flyby data. In this paper we use the observations to investigate aspects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground-based data. The spacecraft spent 6 h downstream of lunar orbit, of which more than 4 h were spent outside of the plasma sheet in regions where traveling compressional regions (TCRs) should have been apparent. Although six substorm intensifications were recorded on the ground during this interval, we did not observe a detectable TCR or plasmoid for every intensification. Our interpretation has important implications for the description of substorm dynamics in the tail. We propose that the signatures associated with individual substorm intensifications are localized in dawn-to-dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank. This latter interpretation is appealing as it can explain the failure of Galileo to observe a signature associated with each intensification without invalidating the conclusion of ISEE 3 investigators that in the same region of the magnetotail at least one signature can be associated with each substorm viewed as a collection of individual intensifications. Plasmoidlike signatures with strong axial fields along the GSM y axis and parallel to the By of the interplanetary magnetic field (IMF) were present when the spacecraft was embedded close to the center of the plasma sheet. We interpret these signatures as flux ropes, that is, twisted magnetic structures with one end possibly tied to the ionosphere. The modeled structure yields j × B/jB ≪ 1 which suggests that the flux ropes are magnetically force free to within the limitations of the model. We point out that plasmoids and flux ropes form a continuum of structures distinguished by the magnitude of By. Our observations lend additional support to the view that bipolar Bz signatures in the magnetotail may often be better described as flux ropes than as disconnected plasmoids. Our other principal results are only summarized in this paper; they will be discussed in greater detail elsewhere. They include (1) additional evidence that the IMF By controls the lobe magnetic field only in the quadrants that are magnetically linked to the solar wind, and (2) evidence that the low-frequency response (the classic “sudden impulse” or SI signature) to a solar wind shock can be absent in the magnetic signature obtained within a high β plasma sheet. We believe that these observations will provide insight useful for improving phenomenological models of substorms.

Mirror-mode structures at the Galileo-Io flyby : Observations

As Galileo passed through the wake of Io it encountered a number of magnetic depressions that have been interpreted to be the result of the mirror-mode instability. Herein we examine the magnetic signatures of these structures and simultaneous measurements of the electron density and temperature. These structures have phase fronts that propagate at large angles to the magnetic field and scale sizes of several ion gyroradii. The inferred density enhancements that accompany the magnetic field depressions range up to 200% of the background density. The spread in normal directions suggests that the depressions are cylindrical and not sheet-like. A companion paper [Huddleston et al., this issue] discusses the theoretical aspects of these waves.

Mode conversion at the Jovian plasma sheet boundary

The plasma wave data obtained by Galileo in Jupiters magnetosphere often exhibit three distinct frequency bands in the frequency range between a few hertz and a few kilohertz. It is shown that these emissions are generally electromagnetic. They are identified by relating their characteristic frequencies to the solutions of the cold plasma dispersion relation. Four modes are possible: X, Z, O, and whistler. Knowing the electron gyrofrequency fce measured by the fluxgate magnetometer, we have considered two different hypotheses for the observed lower-frequency cutoff of the intermediate frequency emissions which occur below fce. Under these assumptions, characteristic frequencies have been computed from the cold plasma theory and compared with the set of cutoff frequencies derived from the observations. Consistency checks lead to the identification of the intermediate frequency band as being on O mode with a low-frequency cutoff at the electron plasma frequency fp. Below the O mode, Galileo detects whistler mode emissions (below fp). Above fce the observed emission is consistent with being X mode. An attempt is made to identify the source of the O mode radiation. Quasi-electrostatic waves are sometimes identified below the upper hybrid frequency when the plasma sheet boundary is crossed. We suggest that these electrostatic waves, which are presumably generated by field-aligned electron beams flowing along plasma sheet boundary, are successively mode converted into Z and later O mode. Thus the O mode observed mostly outside the plasma sheet is generated by mode conversion of primary electrostatic waves.