The concept of ionospheric disturbance dynamo, as introduced by Blanc and Richmond [1980],, means that the main criterion set forth by Mayaud [1965, 1967] to discriminate the irregular variations from the regular ones would be no longer valid. Indeed it asserted that the basic difference between these two species of the transient geomagnetic variations is as follows: the latter are synchronous in local time from one longitude to another, while the former are such in universal time. Now the flow of significant ionospheric current vortices driven by auroral heating during daytime after the occurrence of auroral disturbances for a few hours (see Figure # of their paper), or a few days, would induce magnetic perturbations that are synchronous in local time, and furthermore rather smooth (see the lower diagram of their Figure 9). Consequently, the discrimination between the regular and irregular variations would become more difficult. We wish to comment on this new concept, and on the consequences in the K indices scalings or in the derivation of the Dst index, all the more that we believe we can afford an experimental fact in fayour of the theoretical finding of these authors. Figure I is an example of the eastward electrojet effects discussed in Mayaud [1965]. It illustrates a case when this phenomenon is rather small in amplitude and induces an almost smooth variation at auroral latitudes.

[1] We analyze the radial distribution of electron populations inside 20 R s in Saturn's 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 Saturn's 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.

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The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn's magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn's magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn's ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed