Polarization analysis of Langmuir/Z mode of type III with observations of magnetic components coherent by solar orbiter by T. formánek et al. – European astronomer radio community

The bursts of Type III radio are generated by means of energy electrons that propagate through the interplanetary space. These electron beams, released from the sun, travel along the Parker spiral and Langmuir waves through a kinetic instability in the electron speed distribution function. Langmuir waves generated later suffer conversion, producing a radio emission to the plasma frequency or in their harmonic.

Langmuir waves are electrostatic waves that generate the fluctuations of the electric field along the local magnetic field line. As reported by MALESPINA et al., (2011)And others, transversal electric fields have also been observed during type III in situ, suggesting that observed waves are not always purely electrostatic. As suggests KRAUSS-VARBAN, (1989)The polarization of such waves could be explained by the generalized LANGMUU/Z mode with oblique wave vectors. These waves also have a magnetic component, which was first reported by Scarf et al., (1970) Ogo 5 observations and more recently confirmed by PSP (Larroso et al., 2022). In our article (Formedk et al., 2025)We present the first polarization analysis that includes a magnetic component of the LANGMUU/Z mode as observed by the solar orbiter during a type III explosion.

Figure 1. Observations in situ during the event analyzed type III. Above: radio emission spectrogram and Langmuu waves, highlighting three selected time intervals. Below: Measurement of the electron beam by the Suprateric Empratheric Empramal Sensor EPD (Step).

Polarization analysis

We focus on the Radio III Radio burst observed on September 22, 2022, as shown in Figure 1. using the time domain sample (TDS) of radio and plasma waves (RPW), we analyze the two -component of electric field components and a magnetic field component. In addition, the electron speed distribution was measured by the Electron Analyzer System of the Solar Wind Analyzer (SWA) (EAS) and the Electron and Protons sensor of the energy particle detector (EPD). These observations are also crucial for our polarization analysis because they provide the necessary entry to a dispersion relations solution, which we use to obtain a theoretical prediction for the polarization of the wave.

In Figure 2, we show the dispersion ratio and highlight two wave number intervals (OR1 and OR2), where the amplitude ratio of the electrical and magnetic components observed coincided with the theoretical solution. When inspecting the relative phases between the components (shown in Figure 3), only one of them (OR2) coincides with the polarization observed.

Figure 2. Dispersion ratio of the LANGMUU/z mode The free space mode with parallel wave vectors (discontinuous lines) and at an angle θ = 43 ° (complete lines) from the magnetic field. The regions highlighted in gray correspond to the relationship observed E/CB of the electric and magnetic fields.

Conclusions

The observed waveform showed a transverse electrical component with an elliptical polarization and a coherent magnetic field component. Our results confirm that the observed wave polarization is consistent with a LANGMUU/Z mode of propagation obliquely to low wave numbers. The dispersion ratio solution indicates that an instability driven by the beam can directly excite wave growth at oblique wave vector angles. While several mechanisms may be responsible for the subsequent reduction in the magnitude of the wave vector, a possible mechanism to achieve the polarization observed is the propagation through density gradients.

Figure 3. Comparison of the relative phases observed and predicted between components. The discontinuous lines show the relative phases observed between the E_ANDAND_zYB_MF Components. The complete lines show the theoretical prediction for a wave vector in the OR2 interval depending on the azimutal angle $ \ alpha $ of the wave vector.

Additional information

Based on the recent article by Formánek, T. et al., 2025, Wave polarization analysis of Langmuir mode/type III mode with observations of magnetic components consistent by solar orbiter, APJL, volume 985, number 2 doi: 10.3847/2041-8213/ADD687

Author’s contact: formnek@ufa.cas.cz

References

Formánek, T.; Santolík, O.; Souček, J. et al.: 2025, APJL, volume 985, number 2

KRAUSS-VARBAN, D.: 1989, JGR, vol. 94, problem A4

LAROSO, A.; Dudok of Wit, T.; KRASNOSSKIKH, V. et al.: 2022, APJ, volume 927, number 1

Malaspina, DM, IH Cairns and Re Ergun: 2011, Geoph. Res. Lett., 38, L13101

Scarf, fl; Fredricks, RW; Green, IM et al.: 1970, JGR, vol. 75, number 19