Plasma shock waves are common in the universe and our solar system. Interplanetary (IP) shocks form in the stream of plasma that constantly flows from the Sun - the solar wind. These IP shocks are of great interest since they can accelerate particles and even disturb the space envirionment here on Earth. Solar Orbiter is a spacecraft launched in 2020 which will study the Sun up close. One of the scientific objectives is to study IP shocks at different distances from the Sun. A key parameter of plasma shock waves is the angle between the shock normal and the upstream magnetic field. It is therefore important to accurately determine the shock normal vector. The goal of this project is to investigate the reliability and accuracy of different methods to determine the shock normal vector of IP shocks observed by Solar Orbiter.
At the center of our solar system a main sequence star called the Sun constantly blows charged particles into the surrounding space, filling the solar system. These charged particles are plasma known as the solar wind. The activity of the Sun causes the solar wind to have varying flow speeds. When the solar wind encounters an obstacle such as Earth's magnetic field or overtakes slower moving solar wind, a shock wave may form, where the density, pressure and temperature of the plasma are suddenly increased. This work is about interplanetary shocks, which form for example after solar eruptions. Interplanetary shocks can accelerate particles to high enough velocities that they became dangerous for humans and spacecraft outside Earth's protective bubble, the magnetosphere. How effectively particles are accelerated in interplanetary shocks depends on the shock geometry, which we will study in this work.
An important parameter of the shock geometry is the shock normal vector, which is a vector perpendicular to the shock surface. Measuring the normal vector is a difficult task that relies on accurate spacecraft measurements of the shock. In this work we determine the shock normal vector from multiple shock events using Solar Orbiter, a newly launched European spacecraft investigating the Sun and the solar wind. We use multiple methods, some of which use measurements of solar wind properties and some that only require magnetic field data.
We find that the magnetic field methods produce normal vectors that deviate significantly, up to 30-50 degrees, from those determined by methods using solar wind properties in addition. We validate our results using Magnetospheric Multiscale mission (MMS) which is a NASA multi-spacecraft mission able to determine the shock normal with greater accuracy with multiple shock crossing measurements. We find that the methods using solar wind properties are about 5 degrees off of the normal vector measured by MMS while the magnetic field methods again produce normal vectors that deviate more. We conclude that it is impossible to determine the shock angle for an interplanetary shock using only magnetic field measurements.
Coronal mass ejections are large eruptions on the Sun able to create shock waves in the solar wind. [Image credit: SoHO/LASCO/ESA/NASA]