Kind III photo voltaic radio bursts are generated by beams of energetic electrons propagating by way of interplanetary area. These electron beams, launched from the Solar, journey alongside the Parker spiral and develop Langmuir waves by way of a kinetic instability within the electron velocity distribution operate. The generated Langmuir waves subsequently endure mode conversion, producing a radio emission on the plasma frequency or its harmonic.
Langmuir waves are electrostatic waves producing the electrical subject fluctuations alongside the native magnetic subject line. As reported by Malaspina et al., (2011), and others, transverse electrical fields have additionally been noticed throughout in situ sort IIIs, suggesting that the noticed waves should not all the time purely electrostatic in nature. As recommended by Krauss-Varban, (1989), the polarization of such waves may very well be defined by the generalized Langmuir/Z-mode with indirect wave vectors. These waves even have a magnetic part, which was first reported by Scarf et al., (1970) from OGO 5 observations and extra not too long ago confirmed by PSP (Larosa et al., 2022). In our paper (Formanek et al., 2025), we current the primary polarization evaluation that features a magnetic part of Langmuir/Z-mode waves as noticed by Photo voltaic Orbiter throughout an in situ sort III burst.
Determine 1. In situ observations in the course of the analyzed sort III occasion. Prime: spectrogram of the radio emission and Langmuir waves, highlighting three chosen time intervals. Backside: measurement of the electron beam by the EPD SupraThermal Energetic Particles (STEP) sensor.
Polarization evaluation
We give attention to the in situ sort III radio burst noticed on September 22, 2022, as depicted in Determine 1. Utilizing the Radio and Plasma Waves (RPW) Time Area Sampler (TDS), we analyze waveforms of two electrical subject elements and one magnetic subject part. Moreover, the electron velocity distribution was measured by the Photo voltaic Wind Analyser’s (SWA) Electron Analyser System (EAS) and the Energetic Particle Detector’s (EPD) SupraThermal Electron and Proton sensor. These observations are additionally essential for our polarization evaluation as a result of they supply the mandatory enter to a dispersion relation solver, which we used to acquire a theoretical prediction for the wave polarization.
In Determine 2, we present the dispersion relation and spotlight two wave quantity intervals (OR1 and OR2), the place the amplitude ratio of the noticed electrical and magnetic elements matched the theoretical answer. By inspecting the relative phases between elements (proven in Determine 3), solely considered one of them (OR2) matches the noticed polarization.
Determine 2. Dispersion relation of the Langmuir/Z-mode and the free area L-O mode with wave vectors parallel (dashed traces) and at an angle θ=43° (full traces) from the magnetic subject. Areas highlighted in grey correspond to the noticed ratio E/cB of the electrical and magnetic fields.
Conclusions
The noticed waveform confirmed a transverse electrical part with an elliptical polarization and a coherent magnetic subject part. Our outcomes verify that the noticed wave polarization is in step with an obliquely propagating Langmuir/Z-mode at low wave numbers. The dispersion relation answer signifies {that a} beam-driven instability can immediately excite wave progress at indirect wave vector angles. Whereas a number of mechanisms may be accountable for the following discount in wave vector magnitude, a doable mechanism to achieve the noticed polarization is propagation by way of density gradients.
Determine 3. Comparability of the noticed and predicted relative phases between elements. The dashed traces present the noticed relative phases between the Ey, Ez, and BMF elements. The complete traces present the theoretical prediction for a wave vector within the OR2 interval as a operate of the azimuthal angle $alpha$ of the wave vector.
More information
Based mostly on the latest paper by Formánek, T. et al., 2025, Polarization Evaluation of Kind III Langmuir/Z-mode Waves with Coherent Magnetic Element Observations by Photo voltaic Orbiter, ApJL, Quantity 985, Challenge 2 DOI: 10.3847/2041-8213/add687
Writer contact: formanek@ufa.cas.cz
References
Formánek, T.; Santolík, O.; Souček, J. et al.: 2025, ApJL, Quantity 985, Challenge 2
Krauss-Varban, D.: 1989, JGR, Vol. 94, Challenge A4
Larosa, A.; Dudok de Wit, T.; Krasnoselskikh, V. et al.: 2022, ApJ, Quantity 927, Challenge 1
Malaspina, D. M., I. H. Cairns, and R. E. Ergun: 2011, Geoph. Res. Lett., 38, L13101
Scarf, F. L. ; Fredricks, R. W. ; Inexperienced, I. M. et al.: 1970, JGR, Vol. 75, Challenge 19
