Langmuir waves associated with magnetic holes in the solar wind

(Solar Orbiter nugget #3 by J.J. Boldú^1,2, D. B. Graham¹, M. Morooka¹, M. André¹ Yu. V. Khotyaintsev¹, T. Karlsson³, J. Souček⁴, D. Píša⁴, and M. Maksimovic⁵)

Introduction

Langmuir waves are electrostatic waves near the electron plasma frequency, often observed in the solar wind.  In a weakly collisional environment, such as the solar wind, wave-particle interactions play a fundamental role in the energy dissipation mechanisms. The study of Langmuir waves is of particular interest, as they could play an important role in solar wind electron thermodynamics. They also provide a reliable way to diagnose the electron density, and the knowledge of Langmuir wave properties can be used for calibration purposes of particle detectors.

Statistical characteristics of Langmuir waves

The largest amplitude Langmuir waves in the solar wind are typically associated with type II and type III radio bursts. However, Langmuir waves unrelated to radio bursts also occur in the solar wind, but their source is still not well understood. Langmuir waves have been previously observed inside magnetic field depressions at 1 au, specifically inside magnetic holes [1,2]. A magnetic hole is defined as a region with a magnetic field depression of 50% or more from the background magnetic field level [3]. Magnetic holes are commonly found in the solar wind. These structures have magnetic pressure, typically, balanced by the plasma pressure and their presence can affect the energy transport in the solar wind, having an impact on its large-scale structure [4].

Thanks to the Radio and Plasma Waves (RPW) instrument onboard the Solar Orbiter spacecraft, we have identified Langmuir waves at different heliocentric distances between 0.5 and 1 au [5].  Langmuir waves are detected in the form of high-resolution snapshots, and we have associated these waves with different source regions, such as radio bursts and magnetic holes. In Figure 1 we show the frequency of all Langmuir waves detected and at which distance from the sun they occurred. When looking at the RPW-triggered waveforms snapshots, we found that ~27% of the Langmuir waves are related to radio bursts (green).

Of the remaining Langmuir waves (black), 78% occurred in magnetic field depressions, which include the ones inside magnetic holes (red).

Fig. 1: Radial distribution of Langmuir waves throughout different heliocentric distances as seen by Solar Orbiter. Langmuir waves are related to different source regions:  radio bursts (green), magnetic holes (red), and solar wind Langmuir waves not related to radio bursts or magnetic holes (black). The radial distance is plotted for reference (blue). The frequency of Langmuir waves is inversely proportional to the electron density.

An example of a magnetic hole containing several Langmuir waves seen by Solar Orbiter is shown in the interval highlighted in yellow in Fig. 2a.  The magnetic hole is characterized by a localized decrease in the magnetic field magnitude and an increase in density. Each red vertical line indicates the time of a Langmuir wave. The snapshot in Fig. 1b shows the two components of the measured electric field. Here E_perp (blue) is perpendicular to the magnetic field and E_para (black) is the component aligned with the projection of the magnetic field in the Solar Orbiter’s antenna plane. For this snapshot, we find that E_para>>E_perp, which is expected for Langmuir waves.

Fig. 2: Example of a magnetic hole containing several Langmuir waves. (a) Magnetic field magnitude (green) and electron density (black). The yellow background indicates the interval considered as a magnetic hole. Five Langmuir waves were spotted inside this magnetic hole (red vertical lines). (b) Parallel (black) and perpendicular (blue) components of the electric field high-resolution snapshot of the second Langmuir wave observed in the presented magnetic hole.

Plasma conditions near solar wind Langmuir waves

With our data set of Langmuir waves, we analyzed the solar wind conditions in the regions of Langmuir wave activity. Then, we characterized the importance of magnetic field depressions as Langmuir waves source regions in the solar wind. We found that most of the Langmuir waves that are not related to radio bursts occurred in magnetic field depressions (Fig 3a).

We compared the local conditions of all the solar wind regions monitored by Solar Orbiter to rule out that Langmuir waves are more common in magnetic field depressions simply because magnetic field depressions occur more often in the solar wind than magnetic field enhancements. The red distribution in Fig. 3a shows that magnetic field depressions are equally as common as magnetic field enhancements in the solar wind. Nevertheless, we observe higher counts of Langmuir waves from the black distribution when the local magnetic field is below the magnetic field background level B₀.

Fig. 3: Magnetic field and density solar wind conditions. (a) Local magnetic field conditions with respect to the background level at the time of the Langmuir waves (black) and in all the solar wind monitored by Solar Orbiter (red). (b) Local electron density conditions with respect to the background density at the time of the Langmuir waves (black) and in all the solar wind monitored by Solar Orbiter (red).

We also found that Langmuir waves tend to occur in density enhancements, as seen in Fig. 3b.  The increase of Langmuir wave counts at density enhancements and magnetic field depressions suggest that these waves tend to form in pressured balance structures, such as magnetic holes.

Conclusion

We performed a statistical study of Langmuir waves at heliocentric distances between 0.5 and 1 AU using Solar Orbiter/RPW data. We found that solar wind Langmuir waves tend to occur in magnetic field depressions. Furthermore, 8% of these solar wind Langmuir waves were found inside the deepest magnetic field depressions, defined as magnetic holes.

¹Swedish Institute of Space Physics (IRF), Uppsala 75121, Sweden.

²Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.

³Division of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm 11428, Sweden.

⁴Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czechia.

⁵LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

References

[1] Lin, N., Kellogg, P. J., MacDowall, R. J., et al. 1995, Geophysical Research Letters, 22, 3417

[2] MacDowall, R. J., Lin, N., Kellogg, P. J., et al. 1996 (AIP Publishing), 301–304

[3] Karlsson, T., Heyner, D., Volwerk, M., et al. 2021, Journal of Geophysical Research: Space Physics, 126

[4] Briand, C., Soucek, J., Henri, P., & Mangeney, A. 2010, Journal of Geophysical Research: Space Physics, 115

[5] Boldú, J.J., Graham, D.B., Morooka, M., et al. 2023, A&A, 674, 220