Often invoked to explain solar radio bursts is the standard model of plasma systems with electron beams and their Langmuir wave excitations, although the parameterization favorable to these excitations is very narrow. Here we use first-principle kinetic theory and numerical simulations to prove a direct as well as an indirect involvement of electrostatic electron-beam waves in the generation of radio emissions. At first sight, these primary excitations with frequency below the plasma frequency do not conform to the nonlinear wave decays in the standard model. However, at their origin are denser or cooler electron beams than in the standard model, which mostly fall within the typical parameterization of plasma sources of type II and type III solar radio bursts. These radio bursts are associated with energetic solar events, such as coronal mass ejections and coronal eruptions, and can be exploited in forecasting these events, provided we understand their origin and propagation. Moreover, broadbands of downshifted excitations are confirmed by in situ observations in association with interplanetary shocks and electron beams, and by contrast with narrowband Langmuir waves.

How to cite: Lazar, M., Lopez, R. A., Shaaban Hamd, S. M., Poedts, S., and Fichtner, H.: Extended regimes of energetic (superthermal) electron beams at the origin of solar radio bursts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13304, https://doi.org/10.5194/egusphere-egu25-13304, 2025.

Magnetospheric substorms are among the most dynamic phenomena in Earth’s magnetosphere, yet their triggering mechanisms remain unclear. Ground-based observations have identified auroral beads as precursors to substorms. Here, we report a new precursor feature in space-based auroral kilometric radiation (AKR), marked by the appearance of emissions with slowly frequency-drifting tones (<2 kHz/s) above 100 kHz. Simultaneous observations and statistical analysis show that both AKR precursors and auroral beads occur simultaneously, ~10 minutes before substorm onset, indicating a shared physical process. Analysis of the emissions with frequency-drifting tones suggests they are linked to moving double-layers driven by dispersive Alfvén waves, consistent with the Alfvénic acceleration mechanism for auroral beads. These findings highlight the importance of Alfvénic activity in substorms and suggest that Alfvénic acceleration is not only responsible for optical auroral features but also for radio emissions, potentially explaining the ubiquitous frequency-drifting emission features observed at other magnetized planets like Saturn and Jupiter.

How to cite: Wu, S., Whiter, D., Lamy, L., Wang, M., Zarka, P., Jackman, C., Ye, S., Waters, J., Fogg, A., Mende, S., Kaweeyanun, N., Kasaba, Y., Kurita, S., and Kojima, H.: Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4235, https://doi.org/10.5194/egusphere-egu25-4235, 2025.

In the vast array of Jovian auroral radio emissions, the broadband kilometric (bKOM) component (10-300 kHz) has received comparatively less research attention. Utilizing Juno in situ measurements within the auroral regions, a survey of Juno/Waves radio observations over the first 60 orbits was conducted to identify seven bKOM source candidates. These candidates were predominantly detected during dawn storm auroral episodes (four out of seven) and three were found to be colocated with auroral cavities. A subsequent growth rate analysis, employing JADE-E electron measurements, revealed that the observed waves were driven by the Cyclotron Maser Instability from two free energy sources. The primary emission manifested above the electron gyrofrequency ($f_{ce}$) and was amplified by conics-type electron distribution functions (EDF) with 2 to 30~keV electron characteristic energies. Sporadic bursts, produced slightly below $f_{ce}$, are driven by shell-type EDF of 0.1 to 10~keV.A comparative analysis of these results with those obtained previously for Jovian hectometric and decametric emissions is also presented.

How to cite: Collet, B., Lamy, L., Louis, C., Hue, V., and Kim, T.: Jovian Broadband Kilometric auroral radio emissions with in situ Juno measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2775, https://doi.org/10.5194/egusphere-egu25-2775, 2025.

We have investigated the angular distribution of the Jovian decameter radiation occurrence probability, relatively to the local magnetic field B and its gradient ∇B in the source region, using the magnetic field model for Jupiter, based on Juno’s first nine orbits observations, JRM09, was recently proposed by Connerney et al. [Geophys. Res. Lett., 45, 2590-2596, 2018]. The results were compared to those obtained earlier using older models (O6, VIP4, VIT4 and VIPAL). The JRM09 model confirms the former results: the radio emission is beamed in a hollow cone presenting a flattening in a specific direction. The Jovian decameter radiation is supposed to be produced by the cyclotron maser instability (CMI). We interpret this flattening by the fact that the magnetic field in the radio source does not have any axial symmetry because B and ∇B are not parallel. This hypothesis is confirmed by the amplitude of the flattening of the emission cone which appears to be more important for the northern emission (34.8%) than for the southern emission (12.5%) probably due to the fact that the angle between the directions of B and ∇B is greater in the North (~10°) than in the South (~4°). A theory of CMI is being developed in this context of a magnetized plasma not exhibiting axial symmetry.

How to cite: Galopeau, P. and Boudjada, M.: Study of emission cone of Io-controlled Jovian decameter radiation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17408, https://doi.org/10.5194/egusphere-egu25-17408, 2025.

We analyze the electric field measurements recorded by the radio and plasma wave experiment (RPWS) onboard Cassini spacecraft. This mission has been designed to study mostly plasma waves and radio emissions in the environment of Saturn (Galopeau et al., 2007). RPWS instrument allowed to investigate Saturnian magnetosphere and its vicinity over a frequency range from 1 Hz to 16 MHz. RWPS dynamic spectra displayed the Type II radio intensity variation (in dB) versus the frequency (in kHz) and the observation time (in UT). The daily spectral features are principally linked to the periodic modulation of Saturnian Kilometric Radiation (SKR) emissions. Despite the huge distance (~ 1.5 10⁹ km) between the Sun and Saturn, this experiment detected Solar Type III radio bursts superposed to SKR planetary rotations (Boudjada et al., 2023).  In this work, we investigate Type III bursts recorded from the beginning of January 2004 to the end of August 2017. Three aspects are addressed and developed taking into consideration Type III spectral shapes: (a) the high level of radio intensity (saturated emission) despite the distance Sun-Saturn, (b) the presence of Faraday fringes over a bandwidth of few MHz, and (c) the particular features when the local time occurrence is close to midday or midnight. Those aspects allow us to characterize the physical processes which happen to the Solar Type III emission, propagating in different plasma environment, from the generation region (i.e., Solar corona) and up to the Saturn’s magnetosphere.

References:

Boudjada et al., Statistical analysis of Solar Type III radio bursts observed by RPWS experiment in 2004-2017 during the Solar cycles 23-24. In Proceedings Kleinheubach Conference, Ed. U.R.S.I. Landesausschuss in Deutschland e.V., IEEE, Miltenberg, 2023.

Galopeau et al., Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization. Journal of Geophysical Research, 112, A11, 2007.

How to cite: Boudjada, M. Y., Galopeau, P. H. M., Lammer, H., Eichelberger, H. U., Voller, W., and Stachel, M.: Case study of Solar Type III radio bursts recorded in the environment of Saturn’s magnetosphere , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9112, https://doi.org/10.5194/egusphere-egu25-9112, 2025.

The abrupt brightening of an Energetic Neutral Atom (ENA) blob or cloud has been interpreted as plasma injection in the Saturnian magnetosphere (termed ENA injection herein).  Morphologically, there appears to be two types of abrupt ENA cloud brightening: (1) the brightening of a large cloud usually seen at r > 10-12 Rs (Rs = 60,268 km) in the midnight or postmidnight region; (2) the brightening of a smaller cloud usually seen at r < 10-12 Rs around 21-03 magnetic local time (MLT).  Among many radio waves observed at Saturn, type 2 ENA injections correlate best with the 5 kHz narrowband waves.  Using Cassini INCA and RPWS data, we examine the periodicities of the type 2 ENA injections and the 5 kHz narrowband emissions as well as their cross-correlations, which have been previously used to measure the lag times or phase differences.  Because correlational analysis can only establish linear relationships, we also use mutual information to establish linear and nonlinear relationships.  On average, the peaks of the 5 kHz narrowband emission lag those of the type 2 ENA injection by a few minutes to 2 hr.  The injection of hot plasma to the inner magnetosphere can lead to temperature anisotropy, which can lead to the growth of the electrostatic upper hybrid waves, which upon encountering the density gradient at the outer edge of the Enceladus plasma torus, can mode convert to the Z mode and then to O mode.  The 5 kHz narrowband waves commonly propagate in the O mode. It is expected that the same processes can occur in Jovian magnetosphere and hence our study can provide insights into the upcoming observations by JUICE/PEP/JENI and RPWI. The results also have implications to more distant astrophysical objects such as brown dwarfs, which have been observed to emit periodic radio waves. 

How to cite: Wing, S., Johnson, J., Brandt, P., Ma, X., Mitchell, D., Kurth, B., Menietti, D., Delamere, P., and Caggiano, J.: Periodic narrowband radio wave emissions and inward plasma transport at Saturnian magnetosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3364, https://doi.org/10.5194/egusphere-egu25-3364, 2025.

The Cyclotron Maser Instability (CMI) is a well-known mechanism responsible for auroral radio emissions from the Earth, Jupiter, Saturn, Uranus, and Neptune. These emissions occur at frequencies near or equal to the local electron cyclotron frequency, which is directly related to the local magnetic field strength. Detecting CMI radio emissions from exoplanets would provide direct evidence of planetary magnetic fields, offering a unique method to identify such fields. This approach is particularly valuable since techniques like Zeeman Doppler Imaging are ineffective for exoplanets due to their weak magnetic fields, which are insufficient to produce a detectable Zeeman effect. Jupiter, often regarded as a miniature exoplanetary analog, serves as a valuable benchmark for testing detection methods. In this presentation, we will introduce two techniques for identifying these weak radio signals using observations from the Nançay Decameter Array and NenuFAR ground-based radio telescopes, employing both high and low time and frequency resolutions. Additionally, we will showcase an example of CMI stellar radio emission from the red dwarf AD Leonis, detected by the FAST Chinese radio telescope. This detection highlights the ability to constrain in situ parameters, such as source locations and the energy of the electrons responsible for these emissions.

How to cite: Louis, C., Zarka, P., Mauduit, E., Loh, A., Girard, J., Lamy, L., and Griessmeier, J.-M.: Methods for Detecting Cyclotron Maser Radio Emissions from Stars and Exoplanets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18696, https://doi.org/10.5194/egusphere-egu25-18696, 2025.