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Charged particles constantly stream outward from the Sun to fill the solar system. These particles, consisting mainly of protons and electrons, form a plasma called the solar wind. The solar wind interacts with every celestial body in the solar system, giving rise to different phenomena, such as the auro- ras observed at high latitudes on Earth or disruption of the systems onboard artificial satellites. 

The general structure of the solar wind has been established several decades ago, however we still do not fully understand how the solar wind properties, like temperature and velocity distribution, evolve as it propagates outward in the solar system. Observations of these properties cannot be explained from a conventional fluid description. In a system approximated as a fluid, particle collisions dictate its thermodynamic state. However, the solar wind is a weakly collisional plasma that deviates from thermodynamic equilibrium. Therefore, the radial evolution of the solar wind properties must be driven by different processes. In particular, wave-particle interactions are an important regulator of the solar wind properties, because of the strong connection between the electromagnetic fields and the charged particles. 

In this thesis, we probe how the velocity distribution of solar wind par- ticles evolves as it travels from the Sun to the Earth. Specifically, we study the contribution of waves on the observed solar wind properties at different distances and how these waves can affect the interplanetary environment. We focus on two types of plasma waves frequently observed in the solar wind, Langmuir and ion-acoustic waves. We present their occurrence rates at differ- ent heliocentric distances and suggest wave generation mechanisms based on Solar Orbiter observations. We show that Langmuir waves in the unperturbed solar wind are more commonly observed in regions where the magnetic field magnitude is lower than the background value. Furthermore, we also find that the occurrence rate of ion-acoustic waves is increased in the ramp regions of interplanetary shocks observed at different heliocentric distances, compared to the ion-acoustic wave occurrence rate in the unperturbed solar wind. 

The solar wind is a continuous and turbulent stream of charged particles and magnetic field originating from the solar corona. As it expands it fills the solar system and creates the heliosphere. The global dynamics of the heliosphere are typically described by magnetohydrodynamic (MHD) models. However, many of the observed properties of the solar wind, such as temperature and bulk velocity, cannot be entirely described within the MHD framework. In the low-density, high-temperature environment of the solar wind, particle collisons are infrequent. Consequently, the solar wind is a weakly collisional plasma that readily deviates from local thermodynamic equilibrium (LTE)-a characteristic not accounted for by ideal MHD models. These non-equilibrium conditions manifest as non-Maxwellian features in the ion and electron velocity distribution functions (VDFs).

However, deviations from LTE are constrained by plasma instabilities, which prevent the indefinite growth of non-Maxwellian features, such as beam components or temperature anisotrop\-ies. By exciting electrostatic and electromagnetic waves, these instabilities redistribute energy and drive the plasma toward more stable configurations. Because these processes occur at scales where MHD equations are no longer valid, a kinetic theory is required. This thesis investigates the role of kinetic waves arising from unstable VDF configurations using data from the Solar Orbiter mission. By probing the inner heliosphere at heliocentrirc distances between 0.28 and 1.1~au, Solar Orbiter provides continuous measurements that allow for the study of the radial evolution of solar wind properties. 

We identify and characterize several kinetic wave modes critical to solar wind evolution, establishing their occurrence rates and dependence on heliocentric distance. We further associate these waves with transient solar wind phenomena that facilitate their emission, such as magnetic holes, interplanetary (IP) shocks, and radio burst source regions.  Our results show that Langmuir waves are preferentially excited within localized magnetic field depressions, while ion-acoustic wave activity is significantly enhanced in the vicinity of IP shocks. Furthermore, a detailed analysis of an individual IP shock links its macroscopic structure to the kinetic behavior of ions, which is, in turn coupled to electron-scale processes through the excitation of ion-acoustic waves. In general, these results demonstrate that kinetic waves are a recurring feature of the solar wind, typically associated with larger-scale structures where the development of unstable VDF configurations readily occurs. This cross-scale coupling reflects the necessity of studying the solar wind across multiple scales, and in particular kinetic scales, to fully understand its evolution. 

Finally, we refine existing electron density calibration methods based on Solar Orbiter spacecraft potential measurements. This refinement enables the retrieval of electron density at the high temporal resolution necessary to investigate kinetic-scale processes in the solar wind.