J. F. Carbary

Low-energy charged particle environment at Jupiter: A first look

The low-energy charged particle instrument on Voyager was designed to measure the hot plasma (electron and ion energies ≳ 15 and ≳ 30 kiloelectron volts, respectively) component of the Jovian magnetosphere. Protons, heavier ions, and electrons at these energies were detected nearly a third of an astronomical unit before encounter with the planet. The hot plasma near the magnetosphere boundary is predominantly composed of protons, oxygen, and sulfur in comparable proportions and a nonthermal power-law tail; its temperature is about 3 x 108 K, density about 5 x 10–3 per cubic centimeter, and energy density comparable to that of the magnetic field. The plasma appears to be corotating throughout the magnetosphere; no hot plasma outflow, as suggested by planetary wind theories, is observed. The main constituents of the energetic particle population (≳200 kiloelectron volts per nucleon) are protons, helium, oxygen, sulfur, and some sodium observed throughout the outer magnetosphere; it is probable that the sulfur, sodium, and possibly oxygen originate at 1o. Fluxes in the outbound trajectory appear to be enhancedfrom ∼90� to ∼130� longitude (System III). Consistent low-energy particle flux periodicities were not observed on the inbound trajectory; both 5-and 10-hour periodicities were observed on the outbound trajectory. Partial absorption of > 10 million electron volts electrons is observed in the vicinity of the Io flux tube.

Charged particle periodicities in Saturn's outer magnetosphere

[1] A Lomb periodogram analysis is applied to charged particle data from the LEMMS/CHEMS instruments on the Cassini spacecraft. The data represent count rates, averaged within 30 min bins, from electrons (28-330 keV) and protons and oxygen ions (2.8-236 keV) during 350 days in 2005 and all 365 days in 2006. Sun effects, spacecraft maneuvers, and measurements within 20 R S of Saturn were removed from the data prior to analysis. The main peaks in the frequency periodograms (or power spectra) were found within a frequency window from 9.5 hours to 12.5 hours. For signal-to-noise ratios exceeding 8, the periodograms within this window reveal a consistent peak near 10.80 hours (10 hours 48 min 36 sec) for all the charged particles regardless of energy or species. Even for lower signal-to-noise ratios, a peak near this period is generally present. The Lomb analyses are consistent with an azimuthal anomaly that rotates with a period of 10.80 hours.

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 ≳ 22 and ≳ 28 kiloelectron volts, respectively) in Saturns 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 ( ∼ 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 ∼ 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( ∼ 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies ≳ 200 kiloelectron volts per nucleon, H1, H2, and H3 (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H2 and H3 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 ( ∼ 22 to ∼ 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 ∼ 1.1� 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 ∼ 1.5 million electron volts and smaller fluxes of electrons of ∼ 10 million electron volts and of protons ≳ 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.

Ultraviolet and visible imaging and spectrographic imaging instrument

The Ultraviolet and Visible Imaging and Spectrographic Imaging experiment consists of five spectrographic imagers and four imagers. These nine sensors provide spectrographic and imaging capabilities from 110 to 900 nm. The spectrographic imagers share an off-axis design in which selectable slits alternate fields of view (1.00° × 0.10° or 1.00° × 0.05°) and spectral resolutions between 0.5 and 4 nm. Image planes of the spectrographic imager have a programmable spectral dimension with 68, 136, or 272 pixels across each individual spectral band, and a programmable spatial dimension with 5, 10, 20, or 40 pixels across the 1° slit length. A scan mirror sweeps the slit through a second spatial dimension to generate a 1° × 1° spectrographic image once every 5, 10, or 20 s, depending on the scan rate. The four imagers provide narrow-field (1.28° × 1.59°) and wide-field (10.5° × 13.1°) viewing. Each imager has a six-position filter wheel that selects various spectral regimes and neutral densities. The nine sensors ut lize intensified CCD detectors that have an intrascene dynamic range of ~ 10(3) and an interscene dynamic range of ~ 10(5); neutral-density filters provide an additional dynamic range of ~ 10(2-3). The detector uses an automatic gain control that permits the sensors to adjust to scenes of varying intensity. The sensors have common boresights and can operate separately, simultaneously, or synchronously. To be launched aboard the Midcourse Space Experiment spacecraft in the mid-1990s, the ultraviolet and visible imaging and spectrographic imaging instrument will investigate a multitude of celestial, atmospheric, and point sources during its planned 4-yr life.

The Dynamics of Saturn's Magnetosphere

The dynamics of Saturns magnetosphere differs considerably from that at the Earth. Saturns magnetosphere responds to both external and internal drivers. The solar wind ram pressure, rather than the solar wind speed and interplanetary field orientation, provides the primary external driver at Saturn, while the planets rotation provides the main internal driver. Saturns magnetosphere generally moves in the corotation sense all the way to the magnetopause, although at speeds less than rigid corotation. Little evidence for classic substorm phenomena exists, although substorm-like processes such as plasmoid formation have been detected. Brief, narrow injections of hot plasma from the outer to inner magnetosphere play an important role in the dynamics at Saturn, as do energetic ion acceleration events in the outer magnetosphere as revealed by energetic neutral atom bursts resulting from charge exchange. Internal variations of the magnetosphere exhibit strong modulations at ~10.8 hours and ~10.6 hours: this periodicity is manifest in Saturn kilometric radiation, energetic ions and electrons, low energy plasma, magnetic fields, energetic neutral atoms, and the motions of the plasma sheet and magnetopause. Slower, long term variations (~year) in the periodicities occur, and faster (~weeks) variations are linked to changes in the solar wind speed. The mechanisms driving the periodicities are an active subject of inquiry at this writing.

L Shell Distribution of Energetic Electrons at Saturn

[1] Energetic electron fluxes (110-485 keV) observed by the MIMI/LEMMS instrument on the Cassini mission to Saturn were averaged into azimuthal bins and L shell bins for the period from day 150 2004 to day 366 2008. In local time, the electrons fluxes maximize on the night side between the Mimas and Rhea L shells and have a minimum near noon. This local time behavior may be a result of nightside injection combined with subcorotational drift in a nondipolar field. In SLS longitude, the electrons are smoothly distributed, showing no signs of spiral patterns on this multiyear timescale. The lower energy electrons (110-365 keV) form an inner belt near the Mimas L shell and an outer belt between the Dione and Rhea L shells.

“Blob” analysis of auroral substorm dynamics

One months worth of Polar ultraviolet imager (UVI) data were subjected to a “blob” analysis to determine the statistical dynamics of substorm features observed in the Lyman-Birge-Hopfield long (LBHL) band (152–188 nm). Adapted from similar DoD analyses of target images, the analysis consists of finding, on a frame-by-frame basis beginning at substorm onset, the following aspects of an individual auroral feature: peak power (i.e., power of precipitating electrons), total power, centroid location (magnetic local time (MLT) and magnetic latitude (MLAT), and speed of centroid. Over 120 individual auroral features were successfully acquired at onset and tracked until dissipation during January 1997. The power in the peak pixel and total power were random in time but displayed transient spikes that lasted 5–10 min. Over the course of a substorm, the total energy of blobs averaged ∼2.0 − 104 GJ. A histogram of these energies suggests no preferred energy but that lower energies were more common than higher energies. Analysis of the blob positional dynamics generally supports a poleward and westward movement. During the course of a substorm, 90% of the blobs moved poleward, while over 60% moved westward. However, these movements were not steady and displayed random components. Furthermore, a sizable minority (∼35%) of the blobs moved eastward, which does not agree with the conventional picture of auroral surges. Blob speeds varied from essentially zero up to several kilometers per second. However, during the January substorms the blobs did appear to have a preferred speed of 0.84 ± 0.34 km s−1.

Altitudes of polar mesospheric clouds observed by a middle ultraviolet imager

The first images of Earth limb in the middle ultraviolet (235–263 nm) have revealed in detail the altitude structures of polar mesospheric clouds (PMCs). The images were obtained from the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) spacecraft during the austral summer of 1997–1998. The satellite made multiple passes over the Antarctic and obtained over 750 images of PMCs at latitudes poleward of 70°S. Even without correction for scene backgrounds, the imager easily observed PMCs distinct from the atmospheric backgrounds. The clouds appeared as discrete, filamentary structures having altitudes between 80 and 85 km, although most PMCs appeared at altitudes between 82.0 and 83.0 km with a mean of 82.3 ± 0.8 km. The clouds were randomly distributed on a trans-polar scale of ∼1000 km, although in some instances the clouds clustered for distances of 200–300 km across the polar mesosphere. In other instances, PMCs were wholly absent on the mesospheric horizon. The imager also noted enhanced radiances on the topsides of the PMC altitude profiles; this excess radiance may be caused by “subvisible” particles not apparent at visible wavelengths. The PMC altitudes do not appear correlated with latitude or local time on the scale of the observations discussed here.

Hemispheric comparison of PMC altitudes

A middle ultraviolet imager observed the vertical structure of polar mesospheric clouds (PMCs) in the northern and southern polar regions during the austral summer of 1997–1998 and the arctic summer of 1999. During 23 transpolar passes, the imager obtained over 15,000 images at latitudes poleward of 55°. The accuracy and stability of the satellite platform and the large database permit the first statistical investigation of the small scale (∼1 km) altitude structure of PMCs on a transpolar scale. During a satellite pass over either polar region, PMCs clustered at different altitudes between 80 and 85 km. Southern clouds had a mean altitude of 83.2±1.4 km above the Earth ellipsoid, while northern clouds had a mean altitude of 82.6d±1.3 km, so southern and northern clouds had the same altitudes to within statistical variations. The mean PMC altitude was 82.7±1.3 km for both hemispheres. In neither hemisphere did cloud altitudes exhibit systematic variation with latitude.

Transpolar structure of polar mesospheric clouds

Middle-ultraviolet (210- to 252-nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid-UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83-km altitude reveals periodic structures with wavelengths ranging from ∼100 to ∼3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ∼100 km, and the longer wavelengths generally have more spectral “power” than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs.