Low-energy Charged Particles Research Articles

Force balance in the magnetospheres of Jupiter and Saturn

Spacecraft measurements of the plasma populations and magnetic fields near Jupiter and Saturn have revealed that large magnetospheres surround both planets. Magnetic field measurements have indicated closed field line topologies in the dayside magnetospheres of both planets while plasma instruments have shown these regions to be populated by both hot and cold plasma components convected azimuthally in the sense of planetary rotation. By using published data from the Voyager Plasma Science (PLS), Low Energy Charged Particle (LECP), and Magnetometer (MAG) instruments, it is possible to investigate the validity of the time stationary MHD momentum equation in the middle magnetospheres of Jupiter and Saturn. At Saturn, the hot plasma population is negligible in the dynamic sense and the centrifugal force of the cold rotating plasma appears to balance the Lorentz force. At Jupiter, the centrifugal force balances ∼25% of the Lorentz force. The remaining inward Lorentz force is balanced by pressure gradients in the hot, high-β plasma of the Jovian magnetodisk.

Two‐component proton spectra in the inner Saturnian magnetosphere

The Low Energy Charged Particle (LECP) experiment onboard the Voyager‐2 spacecraft performed extensive measurements of energetic (∼28 keV ≲ Ep ≲ 150 MeV; ∼ 22 keV ≲ Ee ≲ 20 MeV) protons and electrons during spacecraft passage through the inner magnetosphere of Saturn in August 1981. Analysis of the measurements inside the orbits of Enceladus and Mimas reveals the presence of a two‐component proton spectrum consisting of a low‐energy population at ≲ 500 keV identified for the first time and described by dj/dE = KE−2.5, and a high‐energy part at E ≳ 16 MeV whose spectral form is similar to that expected from cosmic ray neutron albedo decay (CRAND), as previously inferred by the Pioneer‐11 investigators from integral measurements. In addition, there exist large fluxes of high (≳ 1.5 MeV) and low (22 to 35 keV) energy electrons (∼3.7 × 105 and ∼1.1 × 105 cm−2sec−1sr−1, respectively) throughout this region. The pitch angle distribution of low energy protons changes from a simple sin2/3 α dependence to one where intensities near 90° become depleted as the spacecraft approached the planet’s ring plane at ∼ 2.88 RS. It is likely that the pitch angle depletion is due to proton interactions with the tenuous G‐ring. These observations also resolve a number of uncertainties regarding the presence of energetic protons and electrons inside the orbits of Enceladus and Mimas, arising from the analysis of Pioneer‐11 measurements.

Approximate relativistic correction factors in L-shell ionisation

Simple approximate correction factors for electronic relativistic effects in L-shell ionisation cross sections by low-energy charged-particle impact have been derived. The factors obtained are compared with the relativistic plane-wave Born approximation calculations using the Dirac wavefunctions and with other approximate correction methods. In the energy region of interest, the present method is found to represent well the relativistic effects.

Charged particle periodicity in the Saturnian magnetosphere

The Low Energy Charged Particle (LECP) experiments on the Voyager 1 and 2 spacecraft performed measurements of electrons (∼22 keV to ∼20 MeV) and ions (∼28 keV to ∼150 MeV) during the Saturn encounters in 1980 and 1981. Count rate ratios of two of the low energy electron (22 to 35 keV and 183 to 500 keV) and ion (43 to 80 keV and 137 to 215 keV) channels exhibit an approximate 10 hour periodicity in the outer Saturnian magnetosphere beyond the orbit of Titan. Electron ratios vary from ∼50 to ∼300; ion ratios vary from ∼3 to ∼20. Similar but less pronounced periodicities are observed for higher and lower energy electron and ion spectral indices. Three complete cycles were observed during the Voyager 2 outbound portion of the encounter from which were determined an electron ratio period of 10h21m±48m and an ion ratio period of 9h49m±59m. Using Saturn Kilometric Radiation (SKR) and Saturn Electrostatic Discharge (SED) periods, extrapolation backward from Voyager 2 to Voyager 1 suggests that the periodicities are Saturnian rather than Jovian in nature, and that they persist in phase for time intervals at least as long as 287 days. Ratio minima, or spectral hardenings, occur in the same hemisphere as do auroral brightenings, SKR activity, and spoke enhancement. We interpret the observations as prima facie evidence of an asymmetry in the Saturnian magnetic field and the root cause of the observed SKR periodicity.

Charged particle sputtering of ice surfaces in Saturn's magnetosphere

Following recent investigations of the Saturn system in situ by Pioneer 11 and Voyager 1 and 2 and remotely by the International Ultraviolet Explorer, we have reconsidered the sputtering of water ice surfaces by magnetospheric ions as a possible source of hydrogen atoms and heavy ions. We use results from laboratory measurements of water ice sputtering and dissociation rates as a function of ice temperature. We also use Voyager charged particle data from the low energy charged particle experiment to obtain more accurate assessments of energetic ion fluxes and loss rates. Ion phase space densities show that, if charged particle sputtering is an important physical process, it will occur predominantly in the vicinity of Saturn's E ring and the moons Dione and Tethys, not at the outer edge of the A ring as previously suggested prior to the availability of in situ data. Charged particle sputtering of ice surfaces between 4.5 and 8 RS, followed by the ionization of H2O and dissociation fragments, may be an important local source of the heavy ion plasma reported in the magnetosphere by Pioneer 11. We review several existing theories for the ring atmosphere and conclude that difficulties remain with all of them.

Effects of Titan on trapped particles in Saturn's magnetosphere

The close fly‐by of the Voyager 1 spacecraft to the Saturnian satellite Titan provided an opportunity for the low energy charged particle (LECP) experiment to investigate the influences of Titan on the magnetosphere energetic particle distributions. Magnetic field data from the magnetometer (MAG) experiment are used with angular distribution data from LECP to study the changes in the pitch angle distributions of ions (40 < Ei < 220 keV), and electrons (26 < Ee < 61 ReV). Before and after the Titan encounter the ions are observed to exhibit a bulk motion in the direction expected of corotation of the particles with the planetary magnetic field. The magnetosphere corotation velocity outside the influence of Titan is deduced to be ∼100 to 150 km/s, less than the ∼200 km/s expected for rigid corotation. Titan appears to disrupt the corotation motion of the ions: the fluxes of ions in the corotation direction are appreciably diminished in intensity, while those observed in the equatorial plane perpendicular to this direction are relatively unaffected. The resulting distribution and trajectory modeling conclusions suggest that the convection electric field is effectively absent in the wake of Titan. Before and after the Titan encounter the electrons are observed to exhibit pitch angle distributions peaked at ∼90°, representative of trapped particles. The presence of Titan produces a reduction in the fluxes of the electrons and flatter distributions with pitch angle.

Low-Energy Hot Plasma and Particles in Saturn's Magnetosphere

The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies greater, similar 22 and greater, similar 28 kiloelectron volts, respectively) in Saturn's magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( approximately 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend approximately 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( approximately 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies greater, similar 200 kiloelectron volts per nucleon, H(1), H(2), and H(3) (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H(2) and H(3) suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( approximately 22 to approximately 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within approximately 1.1 degrees in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of approximately 1.5 million electron volts and smaller fluxes of electrons of approximately 10 million electron volts and of protons greater, similar 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models.

Hot plasma zone near Saturn

Investigators using data collected by Voyager 2 during its flyby of Saturn this past August have found a place in the solar system containing the hottest gas yet observed. Temperatures in a region of space around Saturn range from 300 million to nearly 1 billion° C. The hot gas is an enormous doughnut‐shaped region encircling Saturn at an altitude ranging from 273,600 km above the planet's cloud top to as high as 724,000 km.The discovery was announced at a colloquium at the Applied Physics Laboratory of The Johns Hopkins University, Baltimore, by S. M. Krimigis, chief scientist of the Applied Physics Laboratory Space Department, who is principal investigator of the Voyager Low‐Energy Charged‐Particle Experiment, which made the observations. The measurements were analyzed by a team that includes investigators from The Johns Hopkins University, the universities of Maryland and Kansas, Bell Telephone Laboratories, and the Max Planck Institute in Germany.

Device for precise measurement of charged particle beam intensity

A charged particle beam intensity monitor is described. This beam monitor is useful for measurement of number of incident particles entering an experimental chamber which is operated at pressures high compared with those needed for satisfactory performance of a Faraday cup. The monitor can also be used for low-energy ions or charged particles. The device is designed to minimize the escape of secondary electrons. A description of the device and its principle of operation are given.

Shock‐associated low‐energy ion enhancements observed by Voyagers 1 and 2

Observations of shock‐associated ≥30 keV ion enhancements are presented using data from the Low Energy Charged Particle (LECP) experiment on Voyagers 1 and 2, launched on days 248 and 232, 1977, respectively. The observations include examples of energetic storm particle (ESP) events associated with flare‐produced shocks and examples of corotating particle events (CPE) associated with forward and reverse shocks that bound corotating interaction regions in the outer heliosphere. The first well‐defined CPE are not seen at Voyagers 1 and 2 until late in 1978 when each spacecraft was at a heliocentric radial distance of ∼4 AU. Thus far, seven CPE have been identified in the Voyager 1 LECP data from launch through day 170, 1979. Most of these CPE show features similar to many of those observed at Pioneers 10 and 11, e.g., they have recurrent double‐peaked intensity enhancements showing little or no velocity dispersion at peak intensities, time durations of several days, soft (γ ≥4 for protons ∼1 MeV) energy spectra extending up to ∼5 MeV/nucleon and p/α ratios that are lowest at the reverse peaks. The new LECP measurements also show for the first time that the CPE ion spectra extend, with no sign of a low‐energy turnover, to energies as low as 30 keV/ion. If the CPE are produced by shock acceleration of an ambient particle source, the fact that the observed CPE exhibit well‐formed high intensity peaks at 30 keV/ion means that the pre‐acceleration energy of the source particles was &lt;30 keV/ion. In several of the shock events, the lowest energy ion enhancements are confined mainly downstream of the CIR shocks, with the magnitude and duration of the upstream enhancements increasing with increasing ion energy. The similarities in the low energy ion morphologies observed near the shock during an ESP event and a CPE suggest that the same shock acceleration mechanism and propagation processes were operative in each case. These observations are consistent with the dynamical and kinematical effects expected when low energy ions are accelerated at quasi‐perpendicular shocks.

Energetic particle events (≥30 keV) of Jovian origin observed by Voyager 1 and 2 in interplanetary space

Short‐lived and long‐lived ion flux increases (E ≥30 keV) of Jovian origin have been observed by the low energy charged particle (LECP) instrument on the Voyager 1 and 2 spacecraft. The short‐lived events are observed more than 860 RJ upstream and more than 1500 RJ downstream of Jupiter. Observations of long‐lived events appear to be confined to ≲200 RJ upstream of Jupiter. The short‐lived events last from a few minutes to a couple of hours, while the long‐lived events last from 8 to 21 hours. Both types of events have sharp onsets and decays, are usually confined to energies below 1 MeV total energy, and show a large general enrichment of Z ≥6 particles relative to proton and helium particles when compared with energetic particle events of solar or interplanetary origin. Many of the events have a noticeable peak in the energy spectrum above 100 keV after the main portion of the event. In addition, the short‐lived events (upstream and downstream) are (1) extremely anisotropic with a flow direction consistent with flow away from Jupiter, (2) display no noticeable velocity dispersion, and (3) display initially steep energy spectra that flatten with time. The peak flux level at the lowest energies in the magnetosheath is similar to that observed during long‐lived events. We conclude that a significant fraction of the particles observed during Jovian ion events originate from within the magnetosphere of Jupiter and simply leak out into the magnetosheath. If the interplanetary magnetic field favorably connects to the bow shock, particles can leak out into the interplanetary medium. Together with the necessary leakage model, the observations presented here cannot rule out the existence of a wave particle acceleration region located immediately upstream of the bow shock.

Characteristics of hot plasma in the Jovian magnetosphere: Results from the Voyager spacecraft

The low‐energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, angular distributions, and composition of ions (30 keV ≲E ≲150 MeV) and electrons (14 keV ≲E ≲10 MeV) during encounters with the Jovian magnetosphere in 1979. Detailed analysis of the multicomponent (H, He, O, S) low‐energy (∼30 keV to ∼4 MeV) ion population reveals the Jovian environment to be dominated by magnetospheric ions to distances ≳200 RJ upstream and ≳350 RJ downstream from the planet. Inside the magnetosphere, ions move generally in the sense of corotation to the dayside magnetopause, and on the nightside to distances of ∼130–150 Rj, beyond this distance, but inside the magnetopause, ion flow abruptly changes to an antisunward, anti‐Jupiter direction and continues to large (&gt;350 RJ) radial distances outside the magnetosphere. The ion particle spectrum is characterized by a nonthermal power law (E−γ) component for E ≳200 keV, and a convected Maxwellian for E ≲200 with characteristic temperatures (kT) of ∼20‐45 keV. Temperature maxima generally coincide with crossings of the Jovian plasma sheet, while at higher energies spectra become softer at the equator. The ion spectra and composition are affected strongly by convective flows in all parts of the magnetosphere. By using the observed spectra and angular distributions, density and pressure profiles are produced for ions measured above the lowest LECP detector threshold (E ≳30 keV) and are compared with reported ambient total electron densities and magnetic field pressures. The particle pressures are found to be comparable to magnetic field pressures to at least ∼10 RJ, i.e., Jovian magnetosphere dynamics are determined by pressure variations in a high β plasma. Energetic ion densities are found to be comparable with the total electron densities in the outer (≳40 RJ) dayside magnetosphere but are generally lower at smaller radial distances and exhibit substantial variability. We interpret the hot plasma outflow on the nightside of Jupiter as a ‘magnetospheric wind' and estimate the mass and energy loss through this region at ∼2 × 1027 ions/s and ∼2 × 1020 ergs/s, respectively. We find the plasma source from the active volcanoes on Io to be adequate for supplying the mass outflow; only the rotational kinetic energy of Jupiter is sufficient to provide the energy in the flow, although energy from the solar wind interaction can perhaps contribute a significant fraction. A phenomenological model of Jupiter's magnetosphere is presented which accounts for the observations.

Composition of nonthermal ions in the Jovian magnetosphere

We present observations from Voyager 1 and 2 of nonthermal ions from H through Fe in the Jovian magnetosphere using the low‐energy particle telescope (LEPT), one of the two sensors of the low‐energy charged particle (LECP) experiment. At ∼1 MeV/nucleon the major constituents of the ion population were H, He, C, O, Na, S, and the hydrogen molecules H2 and H3. Relative to He, the abundance of H and H3 at equal energy/nucleon was highest in the outer magnetosphere, the abundance of O, Na, and S was highest in the inner magnetosphere, and the abundance of C was constant throughout the magnetosphere. He and C may be of largely solar origin, while H2, H3, O, Na, and S are largely of local origin. The variations in abundance ratios were accompanied by a general hardening of the energetic particle spectra near 1 MeV/nucleon with decreasing radial distance. We are able to find a parameter η, assumed to be a species‐dependent constant times energy/nucleon, in terms of which the flux ratios among most species do not change with radial distance. These ‘invariant’ ratios indicate that there are approximately equal numbers of nonthermal O, S, and He ions, with the abundance of H ions being a factor of ∼15 higher. The large ratio changes at equal energy/nucleon can then be ascribed to the changing spectral slopes. The parameter η may be reasonably identified with energy/charge, in which case we are able to deduce relative charge to mass ratios for H, He, C, O, and S. If He is doubly ionized, we find that the nonthermal carbon has an approximate charge state of +6, typical of solar wind particles, while oxygen and sulfur have low charge states (+2 to +4), typical of local plasma sources. Spectral changes over the range of ∼30 to ∼16 RJ, in this interpretation, are consistent with particle energization by inward radial diffusion conserving the first adiabatic invariant. In the Jovian magnetotail, at the time of the Voyager 2 outbound pass, the energetic particle population was a combination of Jovian particles with soft spectra and interplanetary particles (probably of solar flare origin) with much harder spectra. Above ∼1 MeV/nucleon the interplanetary particles were dominant. Beyond ∼160 RJ, in the ‘magnetospheric wind’ region, the nonthermal ion intensity increased and reflected an oxygen‐and sulfur‐rich composition typical of the middle magnetosphere. Two short‐lived intensity increases were observed at ∼290 RJ which had heavy ion abundance ratios and spectra similar to those found in the inner magnetosphere. The release of these particles was apparently associated with the arrival at Jupiter of a fast solar wind stream.

Low‐energy charged particle observations in the 5–20 RJ region of the Jovian magnetosphere

Ion (&gt;0.5 MeV) and electron (&gt;30 keV) measurements made by the low‐energy charged particle instrument during the Voyager 1 and 2 traversais of the 5–20 RJ region of the Jovian magnetosphere are presented. The spatial morphology of particle intensities, energy spectra, and composition is emphasized. Diffusive radial transport is also discussed. The Jovian magnetopshere seemed to be much more disturbed during the Voyager 2 passage than during that of Voyager 1. Significant inbound‐outbound asymmetries of the radial profiles of intensities are observed; an appropriate magnetic field model to provide closure has not been found. Low‐energy electrons are not enhanced or depleted in the Io torus region except at the inner edge, about 5.2 RJ, where they sharply decrease. This may be due to enhanced electron loss associated with a region of increased plasma density.

X-ray and energetic neutral particle emission from Saturn's magnetosphere

A description is presented of observations of the Voyager 1 spacecraft obtained a few days before encounter of the Saturn magnetosphere in November 1980. As Voyager 1 has no special experiment onboard for X-ray and neutral particle detection, the sensitivity and directional measurements of the low energy charged particle (LECP) analyzer is used to identify the neutral radiation above the charged particle background. The LECP telescope uses a silicon detector of 96.6 micrometers thickness and 0.08 sq cm area to accumulate counts in eight separate sectors near the ecliptic plane. The existence of radiation above the detector background emanating from the vicinity of Saturn, upstream from the bow shock is shown. The excess counts are most plausibly attributed to neutral radiation, that is, X rays and/or neutral energetic particles. However, in connection with quantitative considerations the observed excess counts are probably not due to X-rays. It is concluded that charge exchange of energetic ions with satellite tori is an important loss mechanism at Saturn as well as at Jupiter.

Erosion of Galilean Satellite Surfaces by Jovian Magnetosphere Particles

The Galilean satellites of Jupiter-Io (J1), Europa (J2), Ganymede (J3), and Callisto (J4)-are embedded in the intense ion and electron fluxes of the Jovian magnetosphere. The effect of these particles on the icy surfaces of the outer three satellites depends on the fluxes and the efficiency of the sputtering of water ice by such particles. Recent laboratory measurements provided data on the erosion of water ice by energetic particles and showed that it occurs much faster than would be expected from normal sputtering theory. The Voyager spacecraft encounters with Jupiter provided the first measurements of ion fluxes (energies greater, similar 30 kiloelectron volts) in the vicinity of the Galilean satellites. Using the laboratory sputtering data together with particle measurements from the Voyager 1 low-energy charged particle experiment, the effects of erosion on the surfaces of J2 to J4 are estimated. It is shown that the surface of Europa could be eroded by as much as 100 meters over an eon (10(9) years). Column densities of water vapor that could be produced around the three satellites from particle bombardment of their surfaces are also calculated, and the sources and losses of oxygen in the gravitationally bound gas produced by sputtering or sublimation are estimated.

Low-Energy Charged Particles in Saturn's Magnetosphere: Results from Voyager 1

The low-energy charged particle instrument on Voyager 1 measured low-energy electrons and ions (energies >/= 26 and >/= 40 kiloelectron volts, respectively) in Saturn's magnetosphere. The first-order ion anisotropies on the dayside are generally in the corotation direction with the amplitude decreasing with decreasing distance to the planet. The ion pitch-angle distributions generally peak at 90 degrees , whereas the electron distributions tend to have field-aligned bidirectional maxima outside the L shell of Rhea. A large decrease in particle fluxes is seen near the L shell of Titan, while selective particle absorption (least affecting the lowest energy ions) is observed at the L shells of Rhea, Dione, and Tethys. The phase space density of ions with values of the first invariant in the range approximately 300 to 1000 million electron volts per gauss is consistent with a source in the outer magnetosphere. The ion population at higher energies (>/= 200 kiloelectron volts per nucleon) consists primarily of protons, molecular hydrogen, and helium. Spectra of all ion species exhibit an energy cutoff at energies >/= 2 million electron volts. The proton-to-helium ratio at equal energy per nucleon is larger (up to approximately 5 x 10(3)) than seen in other magnetospheres and is consistent with a local (nonsolar wind) proton source. In contrast to the magnetospheres of Jupiter and Earth, there are no lobe regions essentially devoid of particles in Saturn's nighttime magnetosphere. Electron pitch-angle distributions are generally bidirectional andfield-aligned, indicating closed field lines at high latitudes. Ions in this region are generally moving toward Saturn, while in the magnetosheath they exhibit strong antisunward streaming which is inconsistent with purely convective flows. Fluxes of magnetospheric ions downstream from the bow shock are present over distances >/= 200 Saturn radii from the planet. Novel features identified in the Saturnian magnetosphere include a mantle of low-energy particles extending inward from the dayside magnetopause to approximately 17 Saturn radii, at least two intensity dropouts occurring approximately 11 hours apart in the nighttime magnetosphere, and a pervasive population of energetic molecular hydrogen.

Methodes nucleaires d'analyse isotopique du carbone, de l'azote et de l'oxygene

The principal nuclear methods of the isotopic analysis of carbon, nitrogen and oxygen, involving activation by thermal or fast neutrons, charged particles and gamma photons, or direct observation of nuclear reactions produced by low-energy charged particles, are reviewed. It is shown that these techniques, although much less effective than mass spectrometry, have advantages for the treatment of certain very special cases. Numerous examples of applications, in particular to biological and metallurgical studies, are given.

Upper limits for X ‐ ray and energetic neutral particle emission from Jupiter: Voyager‐1 results

The x‐ray sensitivity of the low energy proton channels in the Low Energy Charged Particle (LECP) experiment and its directional measurements are used to determine whether x‐rays from Jupiter can be recognized as additional flux above background as Voyager 1 approached the planet. Between ∼230 and ∼100 RJ a statistically significant count rate was detected which we interpret as x‐rays and/or neutral particles. The upper limit for x‐rays, related to earth‐based measurements is ≤ 1.3 × 10−4 photons/cm² sec keV in the energy range 14 to 31 keV. Such an x‐ray flux leads to estimates of precipitating electrons over the Jovian polar caps which are 200 times larger than over observed electron fluxes in the Io plasma torus. The same upper limit corresponds to ≤ 4 × 10−2 neutral particles/cm² sec keV at 100 RJ distance from Jupiter. We favor the neutral hypothesis, and estimate that it represents a loss of ≤ 1025 atoms/sec, i.e. &lt; 0.05% of the atoms generated by Io’s volcanoes.

Io and its plasma environment

We describe and examine the interaction of Io with its plasma torus and the Jovian magnetic field in the context of several currently popular models. We address three specific matters. First, we discuss features implied by sub‐Alfvenic flow which must be common to all models. Next, we examine the magnetic signature observed near Io by the Goddard Space Flight Center Voyager 1 magnetometer and point out that the preliminary estimate of 5 × 106 A current may be an overestimate. We obtain good fits with alternative current distributions which yield currents as small as 7 × 105 A through Io. The best fits are obtained for an Alfven Mach number (MA) of 0.15, but good fits are also found for 0.1 &lt;MA &lt;0.25. Lastly, we point out the crucial role of charged particle data for probing the near Io interaction. We use published data from the low energy charged particle (LECP) detector to suggest that Io has an intrinsic magnetic field of magnitude comparable with earlier estimates. We make predictions which can test this picture as further data become available.