Radio emission of the quiet Sun is generally believed to be generated from thermal bremsstrahlung emission of the hot solar atmosphere. The imaging properties of the quiet Sun in the microwave band have been well studied, and they fit well to the spectrum of bremsstrahlung emission. In the meter-wave and decameter-wave bands, imaging properties of the quiet Sun have rarely been studied due to the instrumental limitations. In this work, we use the LOw Frequency ARray (LOFAR) telescope to perform high-quality interferometric imaging spectroscopy observations of quiet Sun coronal emission at frequencies below 90~MHz. In these observations of the coronal emission, we achieved unprecedented imaging quality, spatial structures are well resolved. For the first time, we find dark regions with low brightness temperatures. The brightness temperature spectrum of the quiet Sun is obtained and compared with the bremsstrahlung emission of the corona model. 

How to cite: Zhang, P., Zucca, P., Kozarev, K., Wang, C., and Team, L. S. K.: Imaging of the Quiet Sun in the Frequency Range of 20-80MHz, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1253, https://doi.org/10.5194/egusphere-egu22-1253, 2022.

NenuFAR is the tied-array radio instrument recently deployed in France. It is the low-frequency extension of LOFAR. It covers frequencies between 10 and 85 MHz. Its large collecting surface (53000m² at 25MHz) makes it very sensitive. Spectral and temporal resolution can be very high, respectively, at <5kHz and < 3ms. Such resolution, associated with high sensitivity, is unique at low frequency. Each antenna is composed of two perpendicularly orientated antennas allowing polarization measurements in the four Stokes parameters. Observations were performed between December 16 and 25, 2021, for two hours around the maximum of Sun elevation. Several sunspot groups were present on the solar surface. In terms of flares, the activity was low during the observing time. Still, many Type III bursts were recorded, some with exceptional fine structures as stria or slowly drifting emission, others with a very weak signal. The capabilities of NenuFAR observations with such high resolution and polarimetric modes are presented. At the beginning of the solar cycle 25, the instrument provides unprecedented possibilities to study the solar corona.

How to cite: Briand, C., Carley, E., Cecconi, B., Reid, H., and Zarka, P.: NenuFAR performances for solar radio observations at high spectral and temporal resolutions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2920, https://doi.org/10.5194/egusphere-egu22-2920, 2022.

The radio emission of the quiet Sun in the metric and decametric bands has not been well studied historically due to limitations of existing instruments. It is nominally dominated by thermal brehmsstrahlung of the solar corona, but may also include significant gyrosynchrotron emission, usually assumed to be weak under quiet conditions. In this work, we investigate the expected gyrosynchrotron contribution to solar radio emission in the lowest radio frequencies observable by ground instruments, for different regions of the low and middle corona. We approximate the coronal conditions by a synoptic magnetohydrodynamic (MHD) model. The thermal emission is estimated from a forward model based on the simulated corona. We calculate the expected gyrosynchrotron emission with the Fast Gyrosynchrotron Codes framework by Fleishman and Kuznetsov (2010). The model emissions of different coronal regions are compared with quiet-time imaging observations between 20-90 MHz by the LOw Frequency ARray (LOFAR) radio telescope. The contribution of gyrosynchrotron radiation to low frequency solar radio emission may shed light on effects such as the hitherto unexplained brightness variation observed in decametric coronal hole emission, and help constrain measurements of the coronal magnetic fields. It can also improve our understanding of electron populations in the middle corona and their relation to the formation of the solar wind.

How to cite: Kozarev, K., Zhang, P., and Zucca, P.: Metric-Decametric Gyrosynchrotron Radio Emission From the Quiet Solar Corona, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5067, https://doi.org/10.5194/egusphere-egu22-5067, 2022.

The shape and dynamics of coronal mass ejections (CMEs) varies significantly based on the instrument and wavelength used. This has led to significant debate about the proper definitions of CME/shock fronts, pile-up/compression regions, and cores observed in projection in optically thin vs. optically thin emission. Here we present an observational analysis of the evolving shape and kinematics of a large-scale CME that occurred on May 7, 2021 on the eastern limb of the Sun as seen from 1 au. The eruption was observed continuously, consecutively by the Atmospheric Imaging Assembly (AIA) telescope suite on the Solar Dynamics Observatory (SDO), the ground-based COronal Solar Magnetism Observatory (COSMO) K-coronagraph (K-Cor) on Mauna Loa, and the C2 and C3 telescopes of the Large Angle Solar Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SoHO). We apply the recently developed Wavetrack Python suite for automated detection and tracking of coronal eruptive features to evaluate and compare the evolving shape of the CME front as it propagated from the solar surface out to 30 solar radii.

How to cite: Stepanyuk, O. and Kozarev, K.: Multi-source observations of a coronal mass ejection front from low to middle corona, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10739, https://doi.org/10.5194/egusphere-egu22-10739, 2022.

Magnetic field measurements at middle and higher coronal heights are challenging using conventional techniques with observations at visible or extreme ultra-violet wavelengths. Low radio frequencies are ideal for probing magnetic fields at higher coronal heights. Polarization properties of solar radio emissions are known to be a rich source of information about the emission mechanisms and magnetic fields of the corona. Nonetheless, largely due to technical challenges, precise polarimetric solar observations at low radio frequencies have remained challenging. The degree of polarization of solar radio emission varies dramatically over time, frequency, and also in morphology, depending on the emission mechanism. The radio bursts show a moderate to a high degree of circular polarization, while the quiet sun thermal emissions show a very low degree of circular polarization (<1%). Once feasible, detection of this very low circular polarisation from quiet Sun thermal emission will be an important tool to measure the quiet Sun coronal magnetic field. Simultaneous measurements of linear and circular polarisation from active emissions are important to understand the quasi-longitudinal and quasi-transverse propagation and will be a direct probe of the magnetic field geometry. According to the conventional views, linear polarization the low-frequency solar emission is expected to be wiped out due to large differential Faraday rotation. Hence, the few polarization studies of the low-frequency Sun in the past many decades have concentrated on measuring only the circular polarization. Nonetheless, we will show a few examples of convincing detections of linearly polarized emission from a variety of active solar emissions using observations from the Murchison Widefield Array (MWA). Perhaps the most rewarding, and also challenging, will be the polarimetric observations of faint gyrosynchrotron or thermal emission from the coronal mass ejection (CME) plasma, which will allow us to model the CME plasma parameters unambiguously at higher coronal heights. We have recently developed state-of-the-art polarization calibration and imaging pipeline for snapshot spectro-polarimetric solar imaging to enable the studies enumerated above and more. Here we summarise its current status and showcase some early science results which challenge the conventional wisdom and open a new window of the polarimetric study of the low-frequency radio Sun. While this pipeline is optimized for the MWA, a Square Kilometer Array (SKA) precursor, it can be adapted for the future SKA-Low and other future solar arrays in a straight forward manner.

How to cite: Kansabanik, D., Oberoi, D., and Mondal, S.: Probing coronal magnetic fields using high fidelity spectro-polarimetric low radio frequency observations of the Sun using the Murchison Widefield Array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10946, https://doi.org/10.5194/egusphere-egu22-10946, 2022.

Type III solar radio bursts form a well known class of active solar emissions and are associated with energetic electron beams propagation outwards through the coronal plasma, and in the process giving rise to their characteristic rapid spectral drifts. Though they have been the subject of a large number of studies since their discovery in the 1950s, the high fidelity and dynamic range spectroscopic snapshot images from the new technology instruments, like the Murchison Widefield Array (MWA) are enabling the exploration of a previously inaccessible part of phase space and leading to the discovery of previously unknown aspects of these well known bursts even in recent times (e.g. Mohan et al., 2019, ApJ, 875). We have now developed a robust and general full Stokes polarization calibration and imaging algorithm optimized for MWA solar observations.. Referred to as “Polarimetry using Automated Imaging Routine for Compact Arrays for the Radio Sun'' (P-AIRCARS; Kansabanik et al., 2022, in prep.), it can deliver full Stokes solar images with leakages on par with usual astronomical radio maps. Here we use this novel capability to carry out a detailed polarimetric study of a type III solar radio burst observed with the MWA. This is, once again, enabling an exploration of new phase space with an exciting discovery potential. Preliminary results show that these type III bursts show presence of linearly polarized emission. While conventional wisdom says that all traces of linear polarization should get washed out due to differential Faraday rotation in the corona, we have convincing reasons to believe that this emission is solar in origin. Here we present the current status of our first detailed polarimetric imaging study oa this  type III radio source. 

How to cite: Dey, S., Kansabanik, D., and Oberoi, D.: First detailed polarimetric study of type III solar radio bursts with the Murchison Widefield Array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10960, https://doi.org/10.5194/egusphere-egu22-10960, 2022.

The confluence of the data from the Murchison Widefield Array (MWA) and an imaging pipeline tailored for spectroscopic snapshot images of the Sun at low radio frequencies have led to enormous improvements in the imaging quality of the Sun. Among other science advances, these developments have lowered the detection threshold for weak nonthermal emissions by up to two orders of magnitude as compared to earlier studies, and have enabled our discovery of Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). Their typical flux densities lie in the range of a few mSFU (1 SFU = 10,000 Jy) and they are found to occur in large numbers all over the quiet Sun regions. In the solar radio images, they appear as compact sources and our estimate of their median duration is limited by the instrumental resolution of 0.5 s. Their spatial distribution and various other properties are consistent with being the radio signatures of coronal nanoflares hypothesized by Parker (1988) to explain coronal heating in the quiet Sun emissions. As steps towards exploring this tantalising possibility of making progress on the coronal heating problem, we have been pursuing multiple projects to improve our ability to detect and characterise WINQSEs. These include attempts to look for WINQSEs in multiple independent datasets; using different independent detection techniques; attempting to characterise their morphologies in radio maps using Artificial Intelligence/Machine Learning based approaches; looking for their counter parts in EUV wavelengths; estimating the energy associated with groups of WINQSEs; and investigation of the spectro-temporal structure of WINQSEs. Here we present the current status of these projects and summarise our learnings from them.

How to cite: Oberoi, D., Mondal, S., Sharma, R., Biswas, A., Bawaji, S., and Alam, U.: Exploring Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs): Clues to coronal heating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11001, https://doi.org/10.5194/egusphere-egu22-11001, 2022.

Solar radio bursts have been studied for over 60 years, however some aspects of their generation and propagation remain to be open questions to the present day. It is generally accepted that majority of solar radio bursts observed in the corona is via the coherent plasma emission mechanism, and a substantial amount of work has been done to support this idea. Fine structures in solar radio bursts can therefore provide important input for understanding the background plasma characteristics.  The presently available advanced ground-based radio imaging spectroscopic techniques (using e.g. LOFAR, MWA, etc.,) and space-based observations (Wind, STEREO A & B, Parker solar probe, Solar Orbiter) provide a unique opportunity to identify, and study fine structures observed in the low corona and interplanetary space.

In this study, we focus on the radio fine structures observed in range of the hecto-kilometric wavelengths that were much less studied than the one in the metric-decametric range. We present for the first time three different types of fine structures observed in interplanetary type III radio bursts (radio signatures of fast electron beams propagating via open and quasi-open magnetic field lines). The presented fine structures show spectral characteristics similar to the striae-like fine structures observed within the type IIIb radio bursts at decametric wavelengths. We employ the probabilistic model for beam-plasma interaction to investigate the role of density inhomogeneities on the generation of the striae elements. The results suggest that there is a good correlation between the width of the striae elements and the scale of density inhomogeneities found in interplanetary space.

How to cite: Jebaraj, I. C., Magdalenic, J., Krasnoselskikh, V., and Poedts, S.: Generation of fine structures in interplanetary type III radio bursts induced by density inhomogeneities in the ambient plasma, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12087, https://doi.org/10.5194/egusphere-egu22-12087, 2022.

Injections of non-thermal electrons into the heliosphere often manifest as intense radio emissions, the most common of which are known as Type III solar radio bursts.  The emission frequency of solar radio bursts is closely related to the local plasma frequency of the heliosphere, meaning that they can be used to probe the local conditions of the solar corona and interplanetary space.  However, observations of these radio emissions do not represent the true nature of the radio sources due to the scattering of radio photons.  Such radio-wave scattering is induced by anisotropic density fluctuations in the heliosphere and impacts both the imaging and spectroscopic properties of radio sources in a frequency-dependent manner, where lower frequencies are affected to a larger extent.  Using a significant number of multi-spacecraft observations, including from Solar Orbiter and Parker Solar Probe, we investigate the angular dependence of spectroscopic radio observations due to the presence of anisotropic scattering.  We present an improved estimation of the spectroscopic properties and probe whether the spacecraft position affects the recorded decay times.  Comparing observations and state-of-the-art anisotropic scattering simulations introduces new constraints on the models used to describe heliospheric radio-wave scattering.

How to cite: Chrysaphi, N. and Maksimovic, M.: The angular dependence of spectroscopic radio measurements using multi-spacecraft observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12239, https://doi.org/10.5194/egusphere-egu22-12239, 2022.

We present the imaging spectroscopy of C-class flare SOL2017-04-04 observed by Expanded Owens Valley Solar Array (EOVSA) to investigate the source morphology and the behavior of the accelerated particles through the low-frequency microwave emission. Unlike the usually observed flare emission that neatly fit the “standard solar model” from a simple, straightforward loop system/arcade, we report that the low-frequency sources have shown an extended emission over the flaring active region and are spatially almost ten times as large as the other associated observations. These sources cannot be entirely explained by a standard two-dimensional model but with a “three-dimensional loop-loop interaction” scenario as observed from the contributions of multiple loop systems with different sizes. This scenario leads to observational evidence for a more realistic flare model consisting of a multi-polar magnetic field configuration with the accelerated particles having large access to travel over the flaring region, where other wavelength emissions are almost invisible. Thus, the study highlights the diagnostic potential of the observed microwave frequencies through which the physical conditions of the secondary emission observed in the low-frequency sources are presented.

How to cite: Shaik, S. B., Gary, D. E., and White, S. M.: Large Microwave Flare Sources with multi-loop Magnetic Reconnection observed by EOVSA Imaging Spectroscopy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13050, https://doi.org/10.5194/egusphere-egu22-13050, 2022.

Auroral kilometric radiation (AKR) is a strong terrestrial radio emission at frequencies below 1 MHz from source regions at high latitudes along auroral magnetic field lines. Non-thermal electron distributions (e.g. loss-cone or shell distribution) provide the free energy that is converted into electromagnetic energy via the cyclotron maser instability. Improved instrumentation installed on modern spacecraft enabled observations of spectral fine structures in AKR which is composed of discrete emissions seen at narrow frequency bandwidths (<1 kHz) and short time scales below 1 second. We will present data from the Cluster mission, where each of the four satellites is equipped with a Wideband Receiver (WBD). The extensive Cluster-WBD dataset is mostly unexplored to date, despite that a few case studies already analyzed specific AKR fine structures like striations, narrowband emissions drifting up and down in frequency or so-called V- or U-events. We will provide an overview of the large variety of AKR fine structures from Cluster-WBD and introduce a classification scheme.

How to cite: Taubenschuss, U., Fischer, G., Pisa, D., Santolik, O., and Soucek, J.: Fine structure of Auroral Kilometric Radiation observed by the Cluster Wideband Receiver, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4041, https://doi.org/10.5194/egusphere-egu22-4041, 2022.

INSPIRE-SAT 7 is a French 2 Unit CubeSat weighting approximately 3 kg, very similar to the satellite UVSQ-SAT which was launched on 24 January 2021. Its main purpose is the measurement of the Earth’s radiation budget at the top of the atmosphere and the sounding of the ionosphere. 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 the CubeSat is dedicated to the sounding of the Earth’s ionosphere. The latter 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. H. M., Meftah, M., Keckhut, P., Grossel, K., Rannou, V., Boust, F., Phan, H. K., Turpaud, V., Boudjada, M. Y., and Eichelberger, H. U.: Ionospheric sounding experiment IONO onboard CubeSat INSPIRE-SAT 7, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10052, https://doi.org/10.5194/egusphere-egu22-10052, 2022.

We investigate the beaming of 11 Io-Jupiter decametric (Io-DAM) emissions observed by Juno/Waves, the Nançay Decameter Array and NenuFAR. Using an up-to-date magnetic field model and three different methods to position the active Io Flux Tube (IFT), we accurately locate the radiosources and determined their emission angle theta from the local magnetic field vector. These methods rely on (i) updated models of the equatorial lead angle, (ii) ultraviolet (UV) images of Jupiter's aurorae from the Hubble Space Telescope simultaneous with radio data and (iii) multi-point radio measurements. The kinetic energy E(e-) of source electrons is then inferred from theta in the framework of the Cyclotron Maser Instability. The precise position of the active IFT obtained from methods (ii) or (iii), when compared to (i), can be used to test of the effective torus plasma density. Simultaneous radio and UV observations reveal that multiple Io-DAM arcs are associated with multiple UV spots and provide the first direct evidence of an Io-DAM arc associated with a trans-hemispheric beam UV spot. Multi-point radio observations alternately probe the Io-DAM sources at various altitudes, times and hemispheres. Overall, theta decreases from ~75-80° to ~70-75° over 10-40 MHz and varies both as a function of frequency (altitude) and time (longitude of Io). Its uncertainty of a few degrees is dominated by that on the longitude of the active IFT. The inferred values of E(e-), also depending on altitude and time, vary between 3 and 16 keV, in agreement with Juno in situ measurements.

How to cite: Lamy, L., Colomban, L., Zarka, P., Prangé, R., Marques, M., Louis, C., Kurth, W., Cecconi, B., Girard, J., Griessmeier, J. M., and Yerin, S.: Determining the beaming of Io decametric emissions, a remote diagnostic to probe the Io-Jupiter interaction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8637, https://doi.org/10.5194/egusphere-egu22-8637, 2022.

The Ganymede laser-altimeter (GALA) is one of 10 instruments on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. The scientific goals cover a wide range  from geology, geophysics to geodesy of the icy moons Ganymede, Europa and Callisto. JUICE will explore Jupiter, its magnetosphere and satellites first in orbit around Jupiter before going finally into polar orbit around Ganymede.  GALA is developed under responsibility of the DLR Institute of Planetary Research in collaboration with industry and institutes from Germany, Japan, Switzerland and Spain. GALA has two main objectives: (1) providing Ganymede’s topography from global to local scales (2) determination of Ganymede's tidal variations of surface elevations. GALA is a single-beam laseraltimeter: a laser pulse (1064 nm) is emitted by using a Nd:YAG laser firing at 30 Hz (nominal). After about 3 msec (500 km altitude) the reflection of the pulse from the surface of Ganymede is received by a telescope and transferred to the detector (Avalanche Photo Diode). The signal is digitized and transferred to the range finder module, which determines (a) time of flight (b) pulse shape, and (c) energy of the received pulse. Including information on the spacecraft position and attitude the height of the terrain above a reference surface is determined for each shot from time-of-flight measurements. The GALA flight model was delivered to ESA in August 2021. After several tests on instrument level the integration on the JUICE spacecraft started in September 2021 and first tests were performed successfully in October 2021. With the launch scheduled for 2023, GALA will go through several tests, among them an end-to-end test including laser-receiver measurements. Here we present the instrument's current status with respect hardware integration and regarding the verification of its performance.

How to cite: Hussmann, H., Lingenauber, K., Kallenbach, R., Lüdicke, F., Enya, K., Thomas, N., Luisa M., L., Touhara, K., Masanori, K., and Jun, K.: Status of the Ganymede Laser Altimeter (GALA) for ESA’s JUICE Mission, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12890, https://doi.org/10.5194/egusphere-egu22-12890, 2022.