On the role of suprathermal electrons on the characteristics of electrostatic solitary waves in Saturn’s magnetosphere

Space plasmas are often characterized by non-thermal particle distributions that are generally characterized by a high-energy tail that follows a power law for large velocity arguments. For modelling purposes, these are often described by kappa-type distributions (Livadiotis, 2017). Over the past few decades, the kappa distribution has been adopted in interpretations of observations in various space plasma contexts including the solar wind (Chotoo et al., 2000), planetary magnetospheres (Collier and Hamilton, 1995), the outer heliosphere (Decker and Krimigis, 2003) and the inner heliosheath (Livadiotis and McComas, 2012) and also in theoretical models (Hellberg et al., 2009). An abundance of data from the Cassini and Voyager missions has established in Saturn's magnetosphere the coexistence of non-thermal electron populations (of different characteristics). Schippers et al. (2008) analysed the radial distribution of electron populations in Saturn's magnetosphere by using an ad hoc two-kappa model, thus establishing the relevance of multi-kappa models with respect to electron populations in Saturn's magnetosphere. This coexistence of electron clouds (at distinct temperatures) is a key element in our work.

Electrostatic Solitary Waves (ESWs), generally associated with bipolar electric field waveforms observed alongside propagating density disturbances, are known to occur in Saturn's magnetosphere (Pickett et al., 2015). In this study, we have relied on a multi-fluid plasma model to investigate the significance of suprathermal electron populations in determining the characteristics of different types of solitary wave solutions. Our investigation reveals that the spectral index (i.e. the  parameter value related to the cold electron population mainly) is crucial in explaining the difference among different types of nonlinear structures. A comparison with spacecraft observations suggests that our theoretical estimations may be relevant in the interpretation of ESW observations in Saturn's magnetosphere.

References

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