Linear prediction studies for the solar wind and Saturn kilometric radiation

U Taubenschuss,P Zarka,M K Dougherty,B Cecconi,H O Rucker,J T Steinberg,W S Kurth

doi:10.5194/angeo-24-3139-2006

Abstract

Abstract. The external control of Saturn kilometric radiation (SKR) by the solar wind has been investigated in the frame of the Linear Prediction Theory (LPT). The LPT establishes a linear filter function on the basis of correlations between input signals, i.e. time profiles for solar wind parameters, and output signals, i.e. time profiles for SKR intensity. Three different experiments onboard the Cassini spacecraft (RPWS, MAG and CAPS) yield appropriate data sets for compiling the various input and output signals. The time period investigated ranges from DOY 202 to 326, 2004 and is only limited due to limited availability of CAPS plasma data for the solar wind. During this time Cassini was positioned mainly on the morning side on its orbit around Saturn at low southern latitudes. Four basic solar wind quantities have been found to exert a clear influence on the SKR intensity profile. These quantities are: the solar wind bulk velocity, the solar wind ram pressure, the magnetic field strength of the interplanetary magnetic field (IMF) and the y-component of the IMF. All four inputs exhibit nearly the same level of efficiency for the linear prediction indicating that all four inputs are possible drivers for triggering SKR. Furthermore, they act at completely different lag times ranging from ~13 h for the ram pressure to ~52 h for the bulk velocity. The lag time for the magnetic field strength is usually beyond ~40 h and the lag time for the y-component of the magnetic field is located around 30 h. Considering that all four solar wind quantities are interrelated in a corotating interaction region, only the influence of the ram pressure seems to be of reasonable relevance. An increase in ram pressure causes a substantial compression of Saturn's magnetosphere leading to tail collapse, injection of hot plasma from the tail into the outer magnetosphere and finally to an intensification of auroral dynamics and SKR emission. So, after the onset of magnetospheric compression at least ~1.2 rotations of the planet elapse until intensified SKR emission is visible in a Cassini-RPWS dynamic spectrum.

Highlights

Before the launch of the Cassini spacecraft, the knowledge of Saturn’s radio emitting properties was based on data gained by the Voyager 1 and 2 missions (Warwick et al, 1981, 1982) and by the Ulysses spacecraft (Lecacheux et al, 1997).The Saturn kilometric radiation (SKR) was detected for the first time when Voyager 1 was approaching Saturn in 1980 (Kaiser et al, 1980)
Radiation coming from the northern hemisphere is right-handed polarized (RH) and radiation coming from the southern hemisphere is left-handed polarized (LH)
The results of the Linear Prediction Theory (LPT) computations found for the time period DOY 224−240, 2004 will be presented showing an evident triggering effect of the solar wind on SKR

Summary

Introduction

Before the launch of the Cassini spacecraft, the knowledge of Saturn’s radio emitting properties was based on data gained by the Voyager 1 and 2 missions (Warwick et al, 1981, 1982) and by the Ulysses spacecraft (Lecacheux et al, 1997). Correlation studies between variations of the solar wind and SKR emission were published by Desch (1982) for the first time. He found clear correlations between the solar wind ram pressure ρ v2 (mass density ρ, bulk velocity v) and SKR intensity in Voyager 1 and 2 data sets. Later on, Desch and Rucker (1983, 1985) improved those correlation studies using the superposed epoch method and a compilation of various solar wind quantities They came to the conclusion that besides the ram pressure the solar wind momentum (ρ v) and the kinetic energy (ρ v3) are significant drivers for triggering SKR.

Basics of the Linear Prediction Theory

Preparation of input and output signals

Results

Summary and discussion