Measurements of Suprathermal Particles at 1 AU and in the inner Heliosphere

Yu, Jia

In this thesis, interplanetary suprathermal particles at 1 astronomical unit (AU) are studied with three time-of-flight mass spectrometers onboard three spacecraft. We study the variation and evolution of suprathermal particles during stream interaction regions (SIRs). As preparation work, the background of the Suprathermal Time-of-Flight spectrometer (STOF) of the Solar and Heliospheric Observatory (SOHO) is analyzed. We identify that the STOF background is mainly caused by energetic penetrating particles. Due to a possible leakage of photons at the entrance system of STOF, these particles are more easily recorded by STOF than originally anticipated. In addition, we propose a method for the background estimation. This part of work guides the event selection for the SIR analysis. In addition, based on this work, we further estimate the background for the Suprathermal Electrons and Protons sensor (STEP) which will be carried by the Solar Orbiter spacecraft and start its journey of exploration in 2020. Both STOF and STEP cover suprathermal energies and the main source of their background is energetic penetrating particles. For the SIR study, we have observed that the time profile of the suprathermal particles peaks inside the compressed fast wind (F') region, close to the trailing edge. When observers travel from the F' region via the trailing edge into the undisturbed fast wind (F) region, spectra harden with time, together with an increase of the He+/He++ abundance ratio. These observations are consistent with previous ones, but cover lower suprathermal energies than before. Moreover, we have identified turnover spectra at low suprathermal energies during some SIR events, excluding the instrumental influence, e.g., efficiency and background. The turnover spectral shape is predicted by the classical Fisk & Lee theory but has not been reported in previous observations so far.

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Yu, Jia: Measurements of Suprathermal Particles at 1 AU and in the inner Heliosphere. 2018.

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