C. K. Goertz

Numerical simulation of the plasma double layer

A one-dimensional particle-in-cell computer simulation is used to model the formation of an electrostatic double layer. The conditions for the onset of the layer formation are explored and a relation between the length of the layer and the electrostatic potential difference across is found.

Existence and stability of strong potential double layers

An elementary integral equation technique is used to construct strong and weak stationary shock solutions from the one-dimensional Vlasov equation. It is shown that the plasma is Penrose stable in all points in space under certain conditions.

Chaotic appearance of the AE index

A typical time sequence of AE shows no obvious regularities which suggests either that the magnetosphere is a random (stochastic) system or displays deterministic chaos. To find out whether the magnetosphere is random or chaotic, AE data have been studied by the embedding dimension method to find the correlation dimension of the magnetosphere. The autocorrelation (AC) time scale τ[AC(τ) = ½ AC(0)] is about 50 minutes for all the cases we have investigated. This time scale may represent the time over which the magnetosphere adjusts to sudden changes. The frequency spectrum of the AE data is broadband with a universal form P(ω) ∼ ω−β, where β is about 2 for the frequencies, f = ω/2π between 0.04 mHz and 4 mHz. The correlation dimension for the AE data using a 1 minute delay to construct a time series of m dimensional vectors is between 3 and 4. With the proper time delay τ ⋍ 50min, which is the autocorrelation time, the correlation dimension is around 2.4. The variation of the correlation dimension with different time delays is shown. But because colored noise with β = 2 has a correlation dimension near 2.1, it is not yet possible to determine whether AE index is a signal resulting from colored noise or from deterministic chaos.

Physics of the Jovian Magnetosphere: Theories of radio emissions and plasma waves

The complex region of Jupiters radio emissions at decameter wavelengths, the so-called DAM, is considered, taking into account the basic theoretical ideas which underly both the older and newer theories and models. Linear theories are examined, giving attention to direct emission mechanisms, parallel propagation, perpendicular propagation, and indirect emission mechanisms. An investigation of nonlinear theories is also conducted. Three-wave interactions are discussed along with decay instabilities, and three-wave up-conversio. Aspects of the Io and plasma torus interaction are studied, and a mechanism by which Io can accelerate electrons is reviewed.

Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation: 2. Time variability

We used a radial diffusion code for energetic electrons in Jupiters magnetosphere to investigate variations in Jupiters radio emission due to changes in the electron phase space density at L shells between 6 and 50, and due to changes in the radial diffusion parameters. We suggest that the observed variations in Jupiters radio emission are likely caused by changes in the electron phase space density at some boundary L1 > 6, if the primary mode of transport of energetic electrons is radial diffusion driven by fluctuating electric and/or magnetic fields induced by upper atmospheric turbulence. We noticed an excellent empirical correlation, both in phase and relative amplitude, between changes in the solar wind ram pressure and Jupiters synchrotron radiation if the electron phase space density at the boundary L1 (L1 ≃ 20-50) varies linearly with the square root of the solar wind ram pressure, fnof; ∼ (Ns υs2)1/2. The calculations were carried out with a diffusion coefficient DLL = DnLn with n = 3. The diffusion coefficient which best fit the observed variations in Jupiter’s synchrotron radiation D3 = 1.3 ± 0.2 × 10−9 s−1 ≃ 0.041 yr−1, which corresponds to a lagtime of approximately 2 years. A comparison with previous estimates on diffusion coefficients in Jupiters inner magnetosphere (L ≲ 10-20) suggests that all estimates can be described by n = 2.5 and D2.5 = 4.5 × 10−9 s−1 ≃ 0.142 year−1. We further show that the observed short term (days-weeks) variations in Jupiter’s radio emission cannot be explained adequately when radial diffusion is taken into account.

Io-Accelerated Electrons and Ions

Earth-based measurements of the Io-modulation effect on Jovian decametric radio noise emission (Bigg, 1964) and the recently announced observation of intense Sodium-D optical emissions from the vicinity of Io (Brown, 1974) as well as various results from the Pioneer 10 Jupiter flyby (see Science,1974) indicate that the Jovian moon Io (and possibly Europa and Ganymede) play an active role in the Jovian magnetosphere.

A new instability of Saturn's ring

Perturbations in the Saturn rings mass density are noted to be prone to instabilities through the sporadic elevation of submicron-size dust particles above the rings, which furnishes an effective angular momentum exchange between the rings and Saturn. The dust thus elevated from the ring settles back onto it at a different radial distance. The range of wavelength instability is determinable in light of the dust charge, the average radial displacement of the dust, and the fluctuation of these quantities. It is suggested that at least some of the B-rings ringlets may arise from the instability.

Secondary electron yields of solar system ices

The secondary electron yields of H2O, CO2, NH3 (ammonia), and CH3OH (methanol) ices have been measured as a function of electron beam energy in the 2- to 30-keV energy range. The ices were produced on a liquid-nitrogen-cooled cold finger and transferred under vacuum to a scanning electron microscope where the yield measurements were made. The imaging capabilities of the scanning electron microscope provide a means of correlating the yield measurements with the morphology of the ices and are also used to monitor charging effects. The yields were determined by measuring the amplified current from a secondary electron detector and calibrating this current signal with the amplified current signal from samples of metals with known secondary electron yields. Each of the measured yields is found to decrease with an increase in energy in the 2- to 30-keV range. Estimates are given for the maximum secondary electron yield Ymax of each ice and the energy at which this maximum yield occurs. Implications for the charging of solar system ice grains are discussed.

On the embedding‐dimension analysis of AE and AL time series

Several authors have employed the embedding-dimension method to analyze time series of geomagnetic indices, with differing results for the value of the correlation dimension ν. It is argued that these differences may arise from corresponding differences in the length and construction of the various data sets used. Practical application of the method to sets of discretized data requires use of a delay time scale set by the autocorrelation time of the data set. It is found that a particular data set containing 35 days of AE exhibits an autocorrelation time τc longer by an order of magnitude than that of a short-duration (<5 days) set, raising the possibility that extant analyses of long-duration sets may have employed delay times shorter than τc. In addition, the power spectrum of AE reveals modulation at a period of 24 hr. A numerical experiment on the logistic map shows that such modulation introduces an extra degree of freedom in the data, resulting in an augmented correlation dimension.

Origin and maintenance of the oxygen torus in Saturn's magnetosphere

Observations of thermal ions in Saturns inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by “self-sputtering” of E ring dust. Sputtered particles are “picked up” by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is ∼5, which implies that an avalanche of self-sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planets ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.