Langmuir waves associated with magnetic holes in the solar wind

(Solar Orbiter nugget #3 by J.J. Boldú1,2, D. B. Graham1, M. Morooka1, M. André1 Yu. V. Khotyaintsev1, T. Karlsson3, J. Souček4, D. Píša4, and M. Maksimovic5)



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 Eperp (blue) is perpendicular to the magnetic field and Epara (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 Epara>>Eperp, 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.



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



[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




Nuggets archive



14/12/2023: Understanding STIX hard X-ray source motions using field extrapolations

07/12/2023: Multi-Spacecraft Observations of the 2022 March 25 CME and EUV Wave: An Analysis of their Propagation and Interrelation

16/11/2023: EUI data reveal a "steady" mode of coronal heating

09/11/2023: A new solution to the ambiguity problem

02/11/2023: Solar Orbiter and Parker Solar Probe jointly take a step forward in understanding coronal heating

25/10/2023: Observations of mini coronal dimmings caused by small-scale eruptions in the quiet Sun

18/10/2023: Fleeting small-scale surface magnetic fields build the quiet-Sun corona

11/10/2023: Unusually long path length for a nearly scatter free solar particle event observed by Solar Orbiter at 0.43 au

27/09/2023: Solar Orbiter reveals non-field-aligned solar wind proton beams and its role in wave growth activities

20/09/2023: Polarisation of decayless kink oscillations of solar coronal loops

23/08/2023: A sharp EUI and SPICE look into the EUV variability and fine-scale structure associated with coronal rain

02/08/2023: Solar Flare Hard Xrays from the anchor points of an eruptive filament

28/06/2023: 3He-rich solar energetic particle events observed close to the Sun on Solar Orbiter

14/06/2023: Observational Evidence of S-web Source of Slow Solar Wind

31/05/2023: An interesting interplanetary shock

24/05/2023: High-resolution imaging of coronal mass ejections from SoloHI

17/05/2023: Direct assessment of far-side helioseismology using SO/PHI magnetograms

10/05/2023: Measuring the nascent solar wind outflow velocities via the doppler dimming technique

26/04/2023: Imaging and spectroscopic observations of EUV brightenings using SPICE and EUI on board Solar Orbiter

19/04/2023: Hot X-ray onset observations in solar flares with Solar Orbiter/STIX

12/04/2023: Multi-scale structure and composition of ICME prominence material from the Solar Wind Analyser suite

22/03/2023: Langmuir waves associated with magnetic holes in the solar wind

15/03/2023: Radial dependence of the peak intensity of solar energetic electron events in the inner heliosphere

08/03/2023: New insights about EUV brightenings in the quiet sun corona from the Extreme Ultraviolet Imager