G. Moreno

Evolution of mirror structures in the magnetosheath of Saturn from the bow shock to the magnetopause

Mirror modes have been systematically observed by Voyagers 1 and 2 in wide portions of Jupiters and Saturns magnetosheaths. In particular, in one crossing of Saturns subsolar magnetosheath, mirror waves are present almost continuously from the bow shock to the magnetopause. Therefore in this crossing, taking advantage also of relatively steady interplanetary conditions, we can track the evolution of mirror structures from a quasi-perpendicular bow shock to a low-shear magnetopause. We find that these structures evolve from quasi-sinusoidal waves to nonperiodic structures, consisting of both magnetic field enhancements and wells, and, finally, to dips in the plasma depletion layer (PDL) close to the magnetopause. Both the amplitude and wavelength of the fluctuations tend to increase with increasing distance from the bow shock, except in the PDL, where they decrease toward the magnetopause. The waves are always compressional, and the direction of maximum variance forms an angle of ∼30° with B in the outer magnetosheath and a smaller angle in the inner magnetosheath. A comparison with the predictions of a nonlinear theory of the mirror instability shows some discrepancies, indicating that further theoretical studies are necessary.

Observations of mirror waves and plasma depletion layer upstream of Saturn's magnetopause

The two inbound traversals of the Saturns magnetosheath by Voyagers 1 and 2 have been studied using plasma and magnetic field data. In a great portion of the subsolar magnetosheath, large-amplitude compressional waves are observed at low frequency (∼0.1 fp) in a high-β plasma regime. The fluctuations of the magnetic field magnitude and ion density are anticorrelated, as are those of the magnetic and thermal pressures. The normals to the structures are almost orthogonal to the background field, and the Doppler ratio is on the average small. Even though the data do not allow the determination of the ion thermal anisotropy, the observations are consistent with values of T⊥/T∥ > 1, producing the onset of the mirror instability. All the above features indicate that the waves should be most probably identified with mirror modes. One of the two magnetopause crossings is of the high-shear type and the above described waves are seen until the magnetopause. The other crossing is of the low-shear type and, similarly to what has been observed at Earth, a plasma depletion occurs close to the magnetopause. In this layer, waves with smaller amplitude, presumably of the mirror mode, are present together with higher-frequency waves showing a transverse component.

Semiannual variation of the geomagnetic activity and solar wind parameters

The semiannual variation of the geomagnetic activity is investigated in connection with a large set of solar wind and interplanetary magnetic field data (4494 daily averages from 1965 to 1987). Our analysis confirms that the geomagnetic activity (described by the aa index), is mainly modulated by the southward component of the magnetic field (Bs), as suggested by Russell and McPherron. On the other hand, it is also found that the solar wind velocity (V) has a relevant role in this phenomenon. In fact, the amplitude of the aa modulation is best correlated with the function BsV². We also explore the linkage between the annual trend of aa and the sunspot activity (1868–1989), showing that the modulation of the geomagnetic activity follows a more regular pattern during the descending phase of the solar cycle than during the rising and maximum parts.

Search for periodicities in the IMP 8 Charged Particle Measurement Experiment proton fluxes for the energy bands 0.50–0.96 MeV and 190–440 MeV

[1] Past studies revealed that the photospheric magnetic field, as well as many solar activity phenomena, undergoes both periodic and quasiperiodic variations on different time scales. Nevertheless, only a few attempts have been made so far to detect corresponding variations in the occurrence frequency of solar energetic particle events. Here we search for periodicities in the proton fluxes, measured in the interplanetary space, on time scales ranging from a few (>6) Bartels rotations (27 days) up to the Schwabe (~11 years) period. We apply the wavelet technique to the proton fluxes recorded by the Charged Particle Measurement Experiment (CPME) instrument aboard IMP 8 spacecraft, from 1974 to 2001, in the energy bands 0.50-0.96 MeV and 190-440 MeV. The reliability of the obtained results is tested by analyzing the wavelet response to suitable artificial functions. The ~9.8, ~3.8, and -1.7-2.2 year periods are the most significant found in the interplanetary proton flux. Shorter periods (such as ~1 year) are detected in some time intervals, but they are not significant in the whole sequence of data.

Diurnal modulation of the geomagnetic activity induced by the southward component of the interplanetary magnetic field

The diurnal (universal time) modulation of the geomagnetic activity is investigated, using the am index along with interplanetary plasma and magnetic field data collected by a variety of spacecraft from 1965 to 1987. A direct comparison with satellite measurements of the Bs component of the interplanetary magnetic field (IMF) is made. Our results confirm the Russell-McPherron (R-M) model, which attributes the above phenomenon to the varying southward component (Bs) of the IMF in the geocentric solar magnetospheric system. Moreover, we compared the contributions of the equinoctial and R-M mechanisms to the observed am modulation, showing that the latter effect is dominant.

Upstream waves in Saturn's foreshock

Waves in the foreshock of Saturn have been studied using data from the plasma and magnetic field experiments on board Voyager 1. The appearance of the waves, in the plasma as well as in the magnetic field data, is related to magnetic connection of the spacecraft to the bow shock. Their relative amplitude {Delta}B/B lies between 0.3 and 0.8 and their period in the spacecraft frame is {approximately}500 s. The waves are elliptically polarized and propagate at {approximately}30{degree} to the average magnetic field direction.

MHD turbulence in Saturn's magnetosheath downstream of a quasi‐parallel bow shock

The origin of the MHD turbulence downstream of a quasi-parallel shock is investigated using plasma and magnetic field data taken by Voyagers 1 and 2 during their outbound traversals of the Saturns magnetosheath. The characteristics of the waves observed in that region are compared with those of the waves present (1) downstream of a quasi-perpendicular shock and (2) in the foreshock. The last comparison supplies direct evidence that the upstream waves, filling the foreshock, are the main source of the magnetosheath turbulence, immediately downstream of a quasi-parallel shock.