C. Polanskey

Io's Interaction with the Plasma Torus: Galileo Magnetometer Report

Galileo magnetometer data at 0.22-second resolution reveal a complex interaction between Io and the flowing plasma of the Io torus. The highly structured magnetic field depression across the downstream wake, although consistent with a magnetized Io, is modified by sources of currents within the plasma that introduce ambiguity into the interpretation of the signature. Highly monochromatic ion cyclotron waves appear to be correlated with the local neutral particle density. The power peaks in the range of molecular ion gyrofrequencies, suggesting that molecules from Io can remain undissociated over a region of more than 15 Io radii around Io.

The magnetic field and magnetosphere of Ganymede

Within Jupiters magnetosphere, Ganymedes magnetic field creates a mini-magnetosphere. We show that the magnetic field measured during Galileo‧s second pass by Ganymede, with closest approach at low altitude almost directly over the moons polar cap, can be understood to a large measure in terms of the structure of a vacuum superposition model of a uniform field and a Ganymede-centered dipole field. Departures from the simple model can be attributed principally to magnetopause currents. We show that the orientation of the observed magnetopause normal is qualitatively consistent with expectations from the vacuum superposition model. The magnetopause currents inferred from the inbound boundary crossing are closely related to expected values, and the magnetic structure of the boundary is similar to that observed at the magnetopause of Earth. We use the vacuum magnetic field model to infer the magnetic field near Ganymedes surface, and thereby predict the particle loss cones that should be present along the spacecraft trajectory. By mapping a fraction of the corotation electric field into the polar cap, we determine expected flow velocities near closest approach to Ganymede as a function of reconnection efficiency. We conclude by discussing prospects for measurements on Galileos remaining passes by Ganymede.

A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer

During the inbound pass of the Galileo spacecraft, the magnetometer acquired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by Io. A field decrease, of nearly 40 percent of the background jovian field at closest approach to Io, was recorded. Plasma sources alone appear incapable of generating perturbations as large as those observed and an induced source for the observed moment implies an amount of free iron in the mantle much greater than expected. On the other hand, an intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations. It seems plausible that Io, like Earth and Mercury, is a magnetized solid planet.

Three years of Galileo dust data

Abstract From its launch in October 1989 until the end of 1992, the Galileo spacecraft traversed interplanetary space from Venus to the asteroid belt and successfully executed close flybys of Venus, the Earth, and the asteroid Gaspra. The dust instrument has been operating most of the time since it was switched on in December 1989. Except for short time intervals near Earth, data from the instrument were received via occasional (once per week to once per month) memory read outs containing 282–818 bytes of data. All events (impacts or noise events) were classified by an onboard program into 24 categories. Over the three-year time span, the dust detector recorded 469 “big” dust impacts. These were counted in 21 of the 24 event categories. The three remaining categories of very low amplitude events contain mostly noise events. The impact rate varied from 0.2 to 2 impacts per day depending on heliocentric distance and direction of spacecraft motion with respect to the interplanetary dust cloud. Because the average data transmission rate was very low, some data were not received on the ground. Complete data sets for 358 “big” impacts were received, but the other 111 “big” impacts were only counted. The observed impact rates are compared with a model of the meteoroid complex.

Two years of Ulysses dust data

Abstract From October 18, 1990 to February 8, 1992 the Ulysses spacecraft traversed interplanetary space between the Earth and Jupiter; at Jupiter the spacecraft was deflected below the ecliptic onto a highly-inclined orbit ( i ∼80°). Here, we report on dust impact data obtained from launch until the end of 1992, nearly a year after the Jupiter flyby. During that time (792 days), the Ulysses dust detector recorded 968 impacts of dust particles with masses ranging from 10 −16 g to 10 −18 g. The impact rate varied from as low as one impact per week during quiet times to more than one per minute during the dust stream of March 10–11, 1992. In this paper, we present and describe the complete data set including both raw and reduced data. The performance of the sensor, which has been very satisfactory so far, is discussed in detail together with the noise discrimination scheme employed. The instruments detection threshold is given as a function of both the particles mass and its speed relative to Ulysses . The derived impact rates and the distribution of particle masses, speeds and impact directions are compared to a model of the meteoroid complex.

Three years of Galileo dust data. II. 1993-1995

Abstract Between January 1993–December 1995, the Galileo spacecraft traversed interplanetaryspace between Earth and Jupiter and arrived at Jupiter on 7 December 1995. The dust instrumentonboard the spacecraft was operating during most of the time and data from the instrument wereobtained via memory readouts which occurred at rates between twice per day and once per week.All events were classified by an onboard program into 24 categories. Noise events were usuallyrestricted to the lowest categories (class 0). During Galileos passage through Jupiters radiationbelts on 7 December 1995, several of the higher categories (classes 1 and 2) also show evidencefor contamination by noise. The highest categories (class 3) were noise-free all the time. Arelatively constant impact rate of interplanetary and interstellar (big) particles of 0.4 impacts perday was detected over the whole three-year time span. In the outer solar system (outside about2.6 AU) they are mostly of interstellar origin, whereas in the inner solar system they are mostlyinterplanetary particles. Within about 1.7 AU from Jupiter intense streams of small dust particleswere detected with impact rates of up to 20,000 per day whose impact directions are compatiblewith a Jovian origin. Two different populations of dust particles were detected in Jovianmagnetosphere: small stream particles during Galileos approach to the planet and big particlesconcentrated closer to Jupiter between the Galilean satellites. There is strong evidence that thedust stream particles are orders of magnitude smaller in mass and faster than the instrumentscalibration, whereas the calibration is valid for the big particles. Because the data transmissionrate was very low, the complete data set for only a small fraction (2525) of all detected particlescould be transmitted to Earth; the other particles were only counted. Together with the 358particles published earlier, information about 2883 particles detected by the dust instrumentduring Galileos six years journey to Jupiter is now available.

GALILEO AT JUPITER: CHANGING STATES OF THE MAGNETOSPHERE AND FIRST LOOKS AT IO AND GANYMEDE*

Several investigations based on data from the magnetometer on the Galileo Orbiter are described. From Galileo’s initial inbound pass, we learn that the magnetosphere can experience large changes in its magnetic configuration. Between 50 and 30 R,, the radial magnetic forces on the plasma were larger in 1995 than in 1973 when Pioneer 10 passed through the same region of the magnetosphere, implying that either external or internal plasma forces were also larger. Although there are several ways to interpret the change of the magnetic configuration, we suggest that the variations are governed principally by the solar wind dynamic pressure and that the dayside magnetosphere as far in as -30 RJ may be more strongly affected by the solar wind than has previously been recognized. Minor effects of higher mass loading may also be present. Data from the flyby of 10 show a large magnetic perturbation; we argue that it is plausible that 10 has an intrinsic magnetic field and that also contributions from plasma perturbations are significant. We find unexpected evidence that molecular ions are being picked up over a large spatial region in the vicinity of the moon. During the pass by Ganymede we observed a large magnetic perturbation consistent with an intrinsic dipole field. The multiple flybys of Ganymede scheduled for later portions of Galileo’s mission will allow us to test our understanding of the magnetic signature.

Dust Measurements During Galileo's Approach to Jupiter and Io Encounter

About a hundred dust impacts per day were detected during the first week in December 1995 by Galileo during its approach to Jupiter. These impacts were caused by submicrometer-sized particles that were just above the detection limit. After the closest approach to Io on 7 December, impacts of these small particles ceased. This effect is expected for dust grains emitted from Io that exit the field of view of the instrument after the flyby. The impact rate of bigger micrometer-sized dust grains continued to increase toward Jupiter. These dust particles are in orbit about Jupiter or are interplanetary grains that are gravitationally concentrated near Jupiter.

Galileo and Ulysses dust measurements: Fz Venus to Jupiter

The Galileo and Ulysses spaceprobes carry two similar dust detectors through interplanetary space from Venus to Jupiter. We report here on impacts which correspond to dust particles above a mass threshold of about 10−13g for which we have complete records. Between December 1989 and January 1992 Galileo repeatedly traversed interplanetary space between 0.7 and 2.26 AU and recorded 374 impacts. The observed impact rates ranged from 0.1 to about 3 impacts per day strongly dependent on whether the spacecraft moved towards or away from the Sun. From October 1990 to January 1992 the Ulysses spacecraft had reached a distance of 5.17 AU from the Sun and had recorded 72 impacts at rates between 0.1 and 0.5 per day. Inside about 2 AU the observed fluxes are compatible with a population of interplanetary dust particles moving on low to moderately eccentric (e = 0.1 to 0.5) and low inclination (i = 0 deg. to 30 deg.) orbits. Outside this distance a dust particle population on different orbits is required in order to explain the Ulysses data.

Three years of Ulysses dust data: 2005 to 2007

Abstract The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse ( i = 79 ∘ , perihelion distance 1.3xa0AU, aphelion distance 5.4xa0AU) since it encountered Jupiter in February 1992. Since then it has made almost three revolutions about the Sun. Here we report on the final three years of data taken by the on-board dust detector. During this time, the dust detector recorded 609 dust impacts of particles with masses 10 − 16 g ⩽ m ⩽ 10 − 7 g , bringing the mission total to 6719 dust data sets. The impact rate varied from a low value of 0.3 per day at high ecliptic latitudes to 1.5 per day in the inner solar system. The impact direction of the majority of impacts between 2005 and 2007 is compatible with particles of interstellar origin; the rest are most likely interplanetary particles. We compare the interstellar dust measurements from 2005/2006 with the data obtained during earlier periods (1993/1994) and (1999/2000) when Ulysses was traversing the same spatial region at southern ecliptic latitudes but the solar cycle was at a different phase. During these three intervals the impact rate of interstellar grains varied by more than a factor of two. Furthermore, in the two earlier periods the grain impact direction was in agreement with the flow direction of the interstellar helium while in 2005/2006 we observed a shift in the approach direction of the grains by approximately 30 ∘ away from the ecliptic plane. The reason for this shift remains unclear but may be connected with the configuration of the interplanetary magnetic field during solar maximum. We also find that the dust measurements are in agreement with the interplanetary flux model of Staubach et al. (1997) which was developed to fit a 5-year span of Ulysses data.