A particle instrument on Aditya-L1, built by the Physical Research Laboratory, that samples the solar wind directly at the L1 point.
The solar wind is a stream of charged particles flowing out from the Sun. ASPEX measures it where Aditya-L1 sits, rather than from a distance. It has two parts. SWIS (Solar Wind Ion Spectrometer) counts protons and alpha particles. STEPS (Supra Thermal and Energetic Particle Spectrometer) measures faster ions, from 20 keV to 5 MeV per nucleon, and electrons above 1 MeV.
SWIS has two sensors, each with a 360° view, set at right angles to each other, so it can tell which way particles are moving. STEPS has six sensors looking in different directions. A change in the ratio of protons to alpha particles can hint that a coronal mass ejection is arriving.
STEPS was switched on on 10 September 2023, more than 50,000 km from Earth, and measured particles in Earth's magnetic environment during the cruise. SWIS started on 2 November 2023.