Solar Wind

The corona is the outermost layer of the Sun’s atmosphere. Charged particles constantly stream from it, extending the Sun’s atmosphere into what is known as the heliosphere. These charged particles form a plasma called the solar wind. The solar wind travels at supersonic and superalfvénic velocities through the interplanetary medium, interacting with everything in its path. As the solar wind travels away from the Sun it expands and cools down, however, this cooling occurs more slowly than expected for an adiabatic expansion. Besides being heated, the solar wind accelerates as it leaves the corona.

Studying the solar wind's evolution in interplanetary space helps us understand how it is heated, accelerated, and interacts with Earth and other celestial bodies. This can be done by analyzing its electromagnetic fluctuations and their effects on particle velocity. The solar wind is rich in turbulence and various types of plasma waves, including whistlers, ion-acoustic, ion-cyclotron, and Langmuir waves.

We investigate the solar wind using data from ESA:s Solar Orbiter mission, launched in 2020, which provides measurements of electromagnetic field and particle distributions in the solar wind between the orbits of Earth and Mercury. The Swedish Institute of Space Physics contributes to this mission through the Radio and Plasma Waves (RPW) instrument.

Besides the constant flow of plasma, the solar wind also experiences occasional transient events such as coronal mass ejections (CMEs), interplanetary shocks, and radio bursts. These events are typically highly energetic and can result in strong interactions with the objects in the heliosphere. Studying these transient phenomena is another way to gain insight into the solar wind and its impact on Earth.

Coronal mass ejections (CMEs) are powerful explosions of plasma and magnetic fields from the solar corona, often linked to solar flares. They appear as bright loops in coronagraph images from ground and space-based instruments. A typical CME has a three-part structure: a bright leading edge (the shock front), a darker cavity filled with twisted magnetic field lines, and a bright core of dense, cool plasma, often from a solar prominence or filament.

As a CME moves outward from the Sun, it expands and interacts with everything in its path, including the solar wind and planetary environments. Upon reaching Earth, it compresses the magnetosphere, disrupting the geomagnetic field. This can trigger physical processes throughout Earth's plasma environment, leading to geomagnetic storms. These storms not only produce auroras but can also harm human activities and technology. Predicting CME arrival and impact is crucial, but difficult due to their constant evolution and limited data from space.

Interplanetary shocks in the solar wind are mainly formed by CMEs and interactions between fast and slow solar wind streams. Though they have lower Mach numbers than planetary shocks, they cover larger areas and stay connected to the upstream longer. These shocks are key in forming energetic particles, making their dynamics and particle acceleration important scientific topics.

Team at IRF: Andrew Dimmock, Emiliya Yordanova, Daniel Graham, Yuri Khotyaintsev