Solar Orbiter in-situ observations of electron beam – Langmuir wave interactions and how they modify electron spectra

(Solar Orbiter Nugget #31 by C.Y. Lorfing¹, H.A.S. Reid¹, R. Gomez-Herrero², M. Maksimovic³, G. Nicolaou¹, C.J. Owen¹, J. Pacheco-Rodriguez², D. Trotta⁴, D. Verscharen¹)

Introduction

A long lasting science question in solar and space plasma physics included in the Solar Orbiter science activity plan is to understand the origin of multiple power-law features in solar electron spectra. Previous observation work up to 1AU [1, 2, 3, 4] have shown breaks in the electron spectra as a function of energy. These observations are supported by simulations [5] of solar electron beam – plasma wave interactions, where the resulting spectrum also displays similar features at energies corresponding to the interactions. We analyse energetic electron fluxes, spectra, pitch angle distributions, associated Langmuir waves, and type III solar radio bursts for 3 events between 0.5 and 1AU to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. This study uses for the first time in a solar study, all four in-situ instruments onboard Solar Orbiter, and makes the bridge between different electron populations measured by SWA and EPD. These observations wouldn’t be complete without remote sensing STIX and EUI measurements of the active region the events originate from.

Solar electron beams, associated Langmuir waves, and type III Solar Radio Bursts

We observe an electron beam measured by EPD’s STEP (Top of figure 1c) and the associated Langmuir waves and type III solar radio burst observed by RPW’s TNR (Middle of figure 1c) for the 9th of October 2021 (Figure 1a). We see the 40-60keV electrons co-temporal  with the Langmuir waves being grown 6 orders of magnitude above the background heliospheric plasma and assume this energy range of electrons is responsible for the locally generate Langmuir oscillations. Previous simulation work has shown that closer to the Sun, higher energy electrons grow Langmuir waves [6]. The 40-60keV energy range is also the one where we observe a break in the electron spectrum (Figure 1b). We highlight the importance of using overlapping fields of view when sewing electron distributions measured by different sensors (EAS1 and STEP in this case). 

Figure 1: a) STIX X-ray image contours overlaid on closest available EUV images. b) Combined peak electron flux from SWA and EPD (STEP and EPT sensors). c) Top: Electron flux (EPD), Middle: Type III Solar Radio Burst and overplotted the Langmuir wave flux (RPW), Bottom: Plasma frequency (RPW). 9th October 2021.

We observe an electron beam measured by EPD’s STEP (Top of figure 1c) and the associated Langmuir waves and type III solar radio burst observed by RPW’s TNR (Middle of figure 1c) for the 9th of October 2021 (Figure 1a). We see the 40-60keV electrons co-temporal  with the Langmuir waves being grown 6 orders of magnitude above the background heliospheric plasma and assume this energy range of electrons is responsible for the locally generate Langmuir oscillations. Previous simulation work has shown that closer to the Sun, higher energy electrons grow Langmuir waves [6]. The 40-60keV energy range is also the one where we observe a break in the electron spectrum (Figure 1b). We highlight the importance of using overlapping fields of view when sewing electron distributions measured by different sensors (EAS1 and STEP in this case). 

Electron spectra and modified electron flux

To investigate the origin of the spectral break at 40keV observed in Figure 1b), we look at the temporal evolution of the electron flux at different timestamps when we observe Langmuir waves. The purple lightcurve at 06:58UT displays a bump at higher energies resulting in a positive velocity gradient. This is a necessary condition for Langmuir waves to grow. As time passes and slower electrons arrive at the spacecraft, the bump moves down in velocity space as is seen on the blue lightcurve at 07:06UT. Evidence of quasilinear relaxation with a flattening and widening of the electron flux from the resonant wave-particle interactions is seen on the green and orange light curves at 07:15UT and 07:23UT respectively. This corresponds to times where we observe a significant growth of Langmuir waves on the middle panel of Figure 1 c). These phenomena happen around the break in the electron spectrum and seem to be the cause of this feature to appear. To validate this assumption we investigate the role of pitch angle scattering on the electron spectrum breaks. Figure 2 b) shows a highly anisotropic beam. As pitch angle scattering is energy dependent and does not affect non-thermal electrons, we do not see its effects on the 40-60keV electrons that are co-temporal with the observed Langmuir waves. Furthermore, previous observations [4] link the pitch angle scattering to the appearance of spectral breaks in the 100-120keV range, way above any breaks observed on figure 1 b).

Figure 2: a) Black dotted lines are the STEP (EPD) peak electron flux observed during each event at each energy [keV]. Coloured lines are the electron velocity distribution function at the given times. 2021 October 9 event, SO was at 0.679 au.  b) Top: Pitch angle (Θ) distribution (EPD/STEP). Bottom: Directional and total magnetic field (MAG). 

Conclusion

We present 3 energetic electron events, their associated Langmuir waves, and type III solar radio bursts observed by Solar Orbiter’s four in-situ instruments. We show that the spectral breaks observed in the electron spectra are caused by wave-particle interactions and Langmuir wave growth in the deca-keV range. We investigate the role of pitch angle scattering of the same energetic electrons and conclude they are not responsible for the features we see appearing in the electron spectrum below 100 keV.

Affiliations

1 Mullard Space Science Laboratory, University College London

2 Universidad de Alcala, Space Research Group, Spain

3 LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Univ. Paris Diderot, Sorbonne Paris Cite

4 The Blackett Laboratory, Department of Physics, Imperial College London

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. Solar Orbiter Solar Wind Analyser (SWA) data are derived from scientific sensors which have been designed and created, and are operated under funding provided in numerous contracts from the UK Space Agency (UKSA), the UK Science and Technology Facilities Council (STFC), the Agenzia Spaziale Italiana (ASI), the Centre National d’Etudes Spatiales (CNES, France), the Centre National de la Recherche Scientifique (CNRS, France), the Czech contribution to the ESA PRODEX programme and NASA. Solar Orbiter SWA work at UCL/MSSL was funded under STFC grants ST/T001356/1, ST/S000240/1, ST/X002152/1 and ST/W001004/1. Solar Orbiter EUI at UCL/MSSL was funded under STFC grants ST/P002463/1, ST/S00002X/1 and ST/T000317/1. Reid, C.J. Owen and D. Verscharen acknowledge funding from the STFC Consolidated Grant ST/W001004/1. C.Y. Lorfing and H. Reid acknowledge support from the Royal Society International Exchange Project IEC\R2\202175. The UAH team acknowledges the financial support by the Spanish Ministerio de Ciencia, Innovación y Universidades FEDER/MCIU/AEI Projects ESP2017- 88436-R and PID2019-104863RB-I00/AEI/10.13039/501100011033. This work has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement No.101004159 SERPENTINE. The RPW instrument has been designed and funded by CNES, CNRS, the Paris Observatory, The Swedish National Space Agency, ESA-PRODEX and all the participating institutes. D.F. Ryan thanks Paolo Massa (Western Kentucky University) and Ewan Dickson (University of Graz) for their helpful clarifications.

References

[1] Lin et al, 2009

[2] Krucker et al, 2007

[3] Krucker et al, 2009

[4] Dresing et al, 2021

[5] Kontar and Reid, 2009

[6] Lorfing and Reid, 2023