N. Krupp

Multi-instrument analysis of electron populations in Saturn's magnetosphere

We analyze the radial distribution of electron populations inside 20 R-s in Saturns magnetosphere, and we calculate moments for these populations by a forward modeling method using composite spectra produced by the CAPS/ELS (0.6 eV to 26 keV) and the MIMI/LEMMS (15 keV to 10 MeV) instruments on board Cassini. We first calculate and harmonize both data sets in physical units and apply corrections taking into account biases introduced by spacecraft interaction with the magnetospheric environment. We then test different bimodal isotropic electron distribution models, deciding on a model with two kappa distributions. We adjust our isotropic model to the flux composite spectra with a least square method to produce three sets of fluid parameters (density, temperature, spectral index) per electron population. The radial profiles are then analyzed, revealing a relevant boundary at 9 R-s in both thermal and suprathermal electron populations. Observed discontinuities in the moment profiles (sudden drop-off in cold density profile outside 9 R-s, hot electrons drop-off inside 9 R-s) coincide with the known outer edge of Saturns neutral OH cloud. Farther out, thermal electrons disappear completely beyond 15 R-s while suprathermal electrons are still observed in the middle and outer magnetosphere.

A new form of Saturn's magnetopause using a dynamic pressure balance model, based on in situ, multi-instrument Cassini measurements

The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturns magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low-energy particle pressures from the Cassini plasma spectrometers electron spectrometer and high-energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size-pressure dependence described by a power law D-P(-1/5.0 +/- 0.8). This exponent is consistent with that derived from numerical magnetohydrodynamic simulations.

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Three-dimensional magnetic field topology in a region of solar coronal heating

Flares and X-ray jets on the Sun arise in active regions where magnetic flux emerges from the solar interior amd interacts with the ambient magnetic field. The interactions are believed to occur in electric current sheets separating regions of opposite magnetic polarity. The current sheets located in the corona or upper chromosphere have long been thought to act as an important source of coronal heating, requiring their location in the corona or upper chromosphere. The dynamics and energetics of these sheets are governed by a complex magnetic field structure that, until now, has been difficult to measure. Here we report the determination of the full magnetic vector in an interaction region near the base of the solar corona. The observations reveal two magnetic features that characterize young active regions on the Sun: a set of rising magnetic loops and a tangential discontinuity of the magnetic field direction, the latter being the observational signature of an electric current sheet. This provides strong support for coronal heating models based on the dissipation of magnetic energy at current sheets.

Global Flows of Energetic Ions in Jupiter's Equatorial Plane: First-Order Approximation

Galileo, as the first orbiting spacecraft in an outer planets magnetosphere, provides the opportunity to study global energetic ion distributions in Jupiters magnetosphere. We present directional anisotropies of energetic ion distributions measured by the Galileo Energetic Particles Detector (EPD). The EPD measurements of proton (80–1050 keV), oxygen (26–562 keV/nucleon), and sulfur (16–310 keV/nucleon) distributions cover a wide energy range. Spatially, the data set includes measurements from 6 to 142 Jovian radii (RJ) and covers all local times inside the Jovian magnetosphere. For each species a single detector head scans almost the entire sky (≈ 4π sr), producing the three-dimensional angular distributions from which the anisotropies are derived. Consequently, the resulting anisotropy estimates are both global and robust. Such anisotropies, generally produced by convective flow, ion intensity gradients, and field-aligned components, have long been used to estimate flow velocities and to locate spatial boundaries within magnetospheres. They can therefore provide vital information on magnetospheric circulation and dynamics. We find that the EPD measured anisotropies in the Jovian magnetosphere are dominated by a component in the corotational direction punctuated by episodic radial components, both inward and outward. Under the assumption that anisotropies are produced predominantly by convective flow, we derive flow velocities of protons, oxygen ions, and sulfur ions. The validity of that approach is supported by the fact that these three independently derived flow velocities agree, to a large extent, in this approximation. Thus, for the first time, we are able to derive the global flow pattern in a magnetosphere of an outer planet. In a comparison between the first-order EPD flow velocities and those predicted by a magnetohydrodynamic (MHD) simulation of the Jovian magnetosphere, we find that qualitatively the directions appear similar, although no firm evidence of steady outflow of ions has been observed at distances covered by Galileo. A first rough comparison indicates that the measured first-order flow velocities are higher by at least a factor of 1.5 than the MHD simulation results.

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.

A dynamic, rotating ring current around Saturn

The concept of an electrical current encircling the Earth at high altitudes was first proposed in 1917 to explain the depression of the horizontal component of the Earth’s magnetic field during geomagnetic storms. In situ measurements of the extent and composition of this current were made some 50 years later and an image was obtained in 2001 (ref. 6). Ring currents of a different nature were observed at Jupiter and their presence inferred at Saturn. Here we report images of the ring current at Saturn, together with a day–night pressure asymmetry and tilt of the planet’s plasma sheet, based on measurements using the magnetospheric imaging instrument (MIMI) on board Cassini. The ring current can be highly variable with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with the Earth’s ring current, where there is no rotational modulation and initial asymmetries are organized by local time effects.

Retrieval of the full magnetic vector with the He I multiplet at 1083 nm. Maps of an emerging flux region

A technique is presented to invert Stokes profiles of the He I 1083 nm multiplet lines in order to obtain the full magnetic vector and the line-of-sight velocity. The technique makes use of spectropolarimetry connected with the Zeeman effect supplemented by a simple Hanle effect based diagnostic when appropriate. It takes into account effects like line saturation, magnetooptical effects, etc. and is coupled with a genetic algorithm, which ensures that the global minimum in a goodness of fit hypersurface is found. Tests using both artificial and real data demonstrated the robustness of the method. As an illustration maps of deduced parameters of an emerging flux region are shown and briefly discussed.

Particle pressure, inertial force, and ring current density profiles in the magnetosphere of Saturn, based on Cassini measurements

We report initial results on the particle pressure distribution and its contribution to ring current density in the equatorial magnetosphere of Saturn, as measured by the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft. Data were obtained from September 2005 to May 2006, within +/- 0.5 R-S from the nominal magnetic equator in the range 6 to 15 RS. The analysis of particle and magnetic field measurements, the latter provided by the Cassini magnetometer (MAG), allows the calculation of average radial profiles for various pressure components in Saturns magnetosphere. The radial gradient of the total particle pressure is compared to the inertial body force to determine their relative contribution to the Saturnian ring current, and an average radial profile of the azimuthal current intensity is deduced. The results show that: ( 1) Thermal pressure dominates from 6 to 9 RS, while thermal and suprathermal pressures are comparable outside 9 RS with the latter becoming larger outside 12 RS. ( 2) The plasma beta (particle/magnetic pressure) remains >= 1 outside 8 RS, maximizing (similar to 3 to similar to 10) between 11 and 14 RS. ( 3) The inertial body force and the pressure gradient are similar at 9-10 R-S, but the gradient becomes larger >= 11 R-S. ( 4) The azimuthal ring current intensity develops a maximum between approximately 8 and 12 RS, reaching values of 100-150 pA/m(2). Outside this region, it drops with radial distance faster than the 1/r rate assumed by typical disk current models even though the total current is not much different to the model results.

A multi-instrument view of tail reconnection at Saturn

Three instances of tail reconnection events at Saturn involving the ejection of plasmoids downtail have been reported by Jackman et al. (2007) using data from Cassini’s magnetometer (MAG). Here we show two newly discovered events, as identified in the MAG data by northward/southward turnings and intensifications of the field. We discuss these events along with the original three, with the added benefit of plasma and energetic particle data. The northward/southward turnings of the field elucidate the position of the spacecraft relative to the reconnection point and passing plasmoids, while the variability of the azimuthal and radial field components during these events indicates corresponding changes in the angular momentum of the magnetotail plasma following reconnection. Other observable effects include a reversal in flow direction of energetic particles, and the apparent evacuation of the plasma sheet following the passage of plasmoids.