Discoveries of exo-auroral radio emission in the last two decades have led to an ongoing paradigm shift––many highly circularly polarized intense radio bursts detected in a variety of low-mass stars are likely signatures of auroral activities rather than flare-driven magnetic activities. Such discoveries have opened a new window in probing the magnetic field in stellar/substellar/exoplanetary systems. One of the outstanding challenges in discerning the two scenarios is characterizing the aurora-generating magnetic topologies of the stellar/substellar objects despite their large distances. Thanks to its proximity, the Sun provides much of the detailed context to study radio bursts similar to those in the stellar/substellar regime. A recent imaging spectroscopy observation with the Jansky VLA reveals a new type of radio bursts near a sunspot, which resembles exo-auroral radio emission in the literature both temporally and spectrally. Unlike the planetary aurora scenario, the detected radio signature is identified as electron cyclotron maser (ECM) emission from a sunspot driven by energetic electrons accelerated in flare activities. Comprehensive observations of sunspot auroral radio emissions will not only advance our understanding of the fundamental physical processes of ECM emissions on the Sun but also impose broad implications on stellar/substellar physics and exo-space weather sciences. These efforts will require long-term monitoring by a solar-dedicated, broad bandwidth radio telescope capable of imaging the Sun in dual circular polarization with a high image dynamic range and subsecond time resolution, which is still lacking. In this talk, after a brief introduction to ECM emissions from stars and the Sun, I will discuss the technical requirements in order to make a leap forward in observations of aurora-type ECM emissions from the Sun, and the expected science returns from a superior broadband radio imaging spectropolarimetry capabilities of a next-generation radio heliograph, such as the Frequency Agile Solar Radiotelescope (FASR) concept. 

How to cite: Yu, S., Chen, B., Sharma, R., Bastian, T., Mondal, S., Gary, D., Luo, Y., and Battaglia, M.: Long-Lasting Solar Coherent Radio Bursts and Implications for Solar–Stellar Connection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2887, https://doi.org/10.5194/egusphere-egu23-2887, 2023.

Type II and III radio bursts are associated with solar eruptive events–CMEs and solar flares. Since radio wave propagation in the interplanetary medium is strongly affected by random electron density fluctuations, radio bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements.  Radio wAve Propagation In thE solaR wind (RAPIER) is a proposal submitted to the Heliophysics Theory, Modeling, Simulations (H-TMS) program, which is a component of the Heliophysics Research Program (NASA). Within this project, we intend to analyze spacecraft data and computer simulations to improve our knowledge of the generation and propagation of type II and III radio bursts and density fluctuations in the inner heliosphere. We will achieve this goal by answering the following science question: “What is the role of solar wind structures on radio burst propagation?” We will study the role of small and large scale density structures on the propagation of radio waves in the solar wind using computer simulations. Specifically, we will focus on disentangling the intrinsic variations in solar radio emissions from propagation effects. We will study the role of scattering by plasma density inhomogeneities on the propagation of radio waves using computer simulations. It allows us to remotely investigate density fluctuations near the Sun, where plasma turbulence evolves and dissipates to heat and accelerate solar wind plasma. Recent solar radio dedicated instruments in space (Parker and Solar Orbiter) allow us for the first time to accurately track radio bursts from the photosphere to the inner heliosphere, and to quantitatively test our radio wave propagation model.

How to cite: Krupar, V., Kruparova, O., Merka, J., and Pasanen, J.: Radio wAve Propagation In thE solaR wind (RAPIER), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4361, https://doi.org/10.5194/egusphere-egu23-4361, 2023.

LOFAR low band interferometric images of type III solar radio bursts during an M class flare on 7 September 2017 show distinct sources with variations in their positions and intermittent dual source structures. We identify these as fundamental and harmonic emission, with the one or other being dominant at times. The data show that transport effects due to refraction and scattering play a significant role, both in source separation and drift of their apparent positions. We present a method of automatically separating fundamental and harmonic contributions that allows for obtaining separate lightcurves. Comparing the lightcurves of fundamental and harmonic pairs, e.g. 35 MHz and 70 MHz, enables studies of radio wave propagation in the solar corona. Harmonic sources at the lowest observable frequencies are relevant for the transition into the solar wind, and for joint observing campaigns with Parker Solar Probe and Solar Orbiter that are currently investigating the inner heliosphere.

How to cite: Vocks, C., Zucca, P., Bisi, M., Dabrowski, B., Morosan, D., Gallagher, P., Krankowski, A., Magdalenic, J., Mann, G., Marque, C., Rothkaehl, H., and Matyjasiak, B.: Separating fundamental and harmonic sources in LOFAR solar type III radio burst images, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12283, https://doi.org/10.5194/egusphere-egu23-12283, 2023.

Large-scale solar coronal structures may have very different signatures in low-frequency metric-decametric interferometric images than their optical/EUV counterparts, or even at higher frequencies. Notable examples are coronal holes and streamers. This may be due to scattering effects of the thermal emission in the corona, or to unexpected mechanisms contributing to the overall emission at these frequencies, such as gyrosynchrotron emission. In this work, we explore the effects of frequency and emission mechanisms (thermal and gyrosynchrotron) on large-scale coronal structures, comparing data with synthetic observations based on global magnetohydrodynamic modeling and forward modeling. We analyze observations by the LOw Frequency ARray (LOFAR) and Murchison Widefield Array (MWA) radio telescopes in a frequency range between 20-250 MHz. We address the unanswered question of why coronal holes often appear bright in the lowest frequencies observable on the ground, and whether this changes with the observer’s viewpoint. We attempt to segment and classify large-scale coronal structures based on their multiwavelength appearance and emission mechanism.

How to cite: Kozarev, K., Zucca, P., Zhang, P., Stepanyuk, O., and Nedal, M.: Exploring Coronal Structures in Metric-Decametric Radio Imaging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13512, https://doi.org/10.5194/egusphere-egu23-13512, 2023.

INSPIRE-SAT 7 is a French 2U CubeSat very similar to the satellite UVSQ-SAT which was launched on 24 January 2021. The main purpose of INSPIRE-SAT 7 is the measurement of the Earth’s radiation budget at the top of the atmosphere. Its total mass is ~3.0 kg and its averaged power consumption 3 W. It will orbit at a maximum altitude of 600 km on a Sun-synchronous orbit with a descending node at ~0930 LT. The IONO experiment embarked on INSPIRE-SAT 7 is dedicated to the sounding of the Earth’s ionosphere which results from the ionization of the upper atmosphere due to UV radiations and X-rays coming from the Sun. The electron density in the ionosphere depends on the local time, the season, and the solar activity. The propagation of the radio waves is affected by the electron density and also by refraction and reflection phenomena. We consider the following goals for the IONO instrument: improving ionosphere models, in particular the IRI (International Reference Ionosphere); study of the propagation of electromagnetic waves in the ionosphere and the factors which can disturb it (e.g., thunderstorms); analysis of temporal and spatial variability at different scales; study of the coupling between ionosphere and magnetosphere, and the electrical circuit between ionosphere and lithosphere. The observations collected by IONO will be compared to those produced by a VLF-LF antenna network designed for investigating the perturbations of the ionosphere, and the wave propagation, by seismic phenomena.

How to cite: Galopeau, P., Meftah, M., Keckhut, P., Grossel, K., Boust, F., Boudjada, M., and Eichelberger, H.: Observation of the Earth’s ionosphere variability by IONO/INSPIRE-SAT 7 experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8808, https://doi.org/10.5194/egusphere-egu23-8808, 2023.

The radio emissions nicknamed "caterpillars" are believed to be a special form of Saturn kilometric radiation (SKR) at low to very low frequencies. They are coarse spectral structures lasting for several hours mostly below 40 kHz, and their constant central frequency with a typical bandwidth of 10-15 kHz makes them look like caterpillars in a time-frequency spectrogram. We found almost 600 caterpillar emissions with the RPWS (Radio and Plasma Wave Science) instrument throughout Cassini's tour around Saturn. We present a statistical investigation of their occurrence with respect to the position of Cassini, their duration, central frequency, bandwidth, polarization, intensity, and connection to SKR at higher frequencies. We also compare their occurrence with the occurrence of SKR low frequency extensions (LFEs) as many of them are found during so-called long LFEs. We will discuss and investigate the reasons for the loss of total polarization of caterpillars, which could be due to wave reflections at the magnetosheath or due to an incoherent superposition of X-mode with O-mode SKR.

How to cite: Fischer, G., Taubenschuss, U., Pisa, D., Lamy, L., Wu, S., Ye, S., Jackman, C., and O'Dwyer, E.: Statistical investigation of SKR caterpillar emissions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2066, https://doi.org/10.5194/egusphere-egu23-2066, 2023.

As an alternative to the traditionally considered electron cyclotron maser (ECM) mechanism of exoplanetary radio emission (RE), we study plasma maser mechanism. The latter, contrary to ECM operates in dense and weakly magnetized plasmas, where electron cyclotron frequency fc is less than Langmuir frequency f_L [1]. Similar mechanism is known to contribute the generation of RE in solar corona, as well as in magnetospheres of the Solar System planets [2,3]. It is a two-step process. At first, the plasma waves are excited due to Cherenkov instability in a weakly anisotropic background plasma by a small admixture of hot electrons with a loss-cone type non-equilibrium distribution function. Then, the electromagnetic radiation at f_RE arises due to, e.g., plasma wave scattering on the background ions (Rayleigh scattering, f_RE = f_L), or nonlinear coupling of two plasma waves (Raman scattering, f_RE = 2f_L). In the first case, the maser effect at plasma frequency f_L takes place under certain conditions, leading to an exponential grow of the RE intensity with the growing energy of plasma waves. In the case of Raman scattering of two plasma waves, resulting in generation of the RE at doubled plasma frequency, the maser effect is absent, but the collisional dissipation of RE is significantly reduced at the same time. This improves the requirements regarding the brightness temperature of the RE source, to provide a detectable on Earth radiation flux. In both cases the frequency band of the exoplanetary RE is defined not by magnetic field, but by the structure of planetary plasmasphere and density distribution there.

We evaluate the efficiency of plasma mechanism of the RE generation and its detectability on Earth for the case of hot Jupiter HD189733b, for which the 3D structure of plasmasphere is simulated with the global multi-fluid self-consistent numerical model [4], taking into account the realistic stellar wind and radiation conditions. It is shown that the RE flux at doubled plasma frequency sharply increases from several mJy at 20MHz to several tens of Jy at 4 MHz. This means that the most favorable frequency range for detection of the RE from HD189733b falls into the decameter band in vicinity of the ionospheric cut-off.

1. Zaitsev, V.V., Shaposhnikov, V.E., MNRAS, 2022, 513, 4082 (DOI:10.1093/mnras/stac1140)

2. Zaitsev V. V., et al., A&A, 1986, 169, 345-354 (ISSN 0004-6361)

3. Zlotnik E. Y., et al., JGR Space Physics, 2016, 121, 5307-5318 (DOI: 10.1002/2016JA02265)

4. Rumenskikh, M. S., et al., ApJ, 2022, 927(2):238 (DOI: 10.3847/1538-4357/ac441d)

How to cite: Khodachenko, M. L., Zaitsev, V. V., Shaposhnikov, V. E., Rumenskikh, M. S., and Shaikhislamov, I. F.: Plasma mechanism of exoplanetary radio emissions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4514, https://doi.org/10.5194/egusphere-egu23-4514, 2023.