PS8.1

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.

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.

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.

The solar coronal heating problem has been around for several decades. While it has been well established that magnetic fields are responsible for transporting the energy from the photosphere to the corona, it has been a challenge to understand the details of the energy dissipation processes. One such process was proposed by Parker, who hypothesized that this dissipation occurs through small scale magnetic reconnections happening throughout the corona. While there are several indications that this mechanism may be active, till date direct observation of these small reconnections have not been possible. Hence searching for indirect signatures of these events is very important. One such probable signature was discovered by Mondal et al. (2020), where they demonstrated the presence of ubiquitous impulsive radio emissions in the solar corona during a very quiet time. These emissions are now named Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). Due to the potential importance of this discovery and its implications, it is crucial that the detection techniques are improved and that we search for these transients in more datasets to confirm/reject their ubiquitous nature. In this work, we have developed a machine learning (ML) algorithm suitable for identifying and characterising the spatial distribution and morphology  of WINQSEs. For morphological characterisation we use 2D Gaussians which are found to  describe the brightness distribution of the majority of these transients very well. Since WINQSEs are expected to be the radio counterparts of the weak reconnections, we expected them to  be essentially unresolved at an angular resolution of 3.5 arcminutes. We find, however, that most of the WINQSEs are resolved by the instrument, though the distribution of their area is very steep. We hypothesise that while intrinsically unresolved, the area of WINQSEs becomes large due to coronal scattering effects. This then also presents the exciting possibility of using WINQSEs to regularly study the nature of scattering close to the Sun, which currently is only possible during solar radio bursts. Here we present a quick overview of our ML algorithm, along with a summary of our results about the morphological properties of WINQSEs, and explore the possibility of using them to study coronal scattering. 

How to cite: Bawaji, S., Alam, U., Mondal, S., and Oberoi, D.: Understanding the morphology of Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11460, https://doi.org/10.5194/egusphere-egu22-11460, 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.

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.

Occultations of the Jovian low frequency radio emissions by the Galilean moons have been observed by the PWS (Plasma Wave Science, Gurnett et al. 1992) instrument of the Galileo spacecraft (Kurth et al. 1997). We use the ExPRES (Exoplanetary and Planetary Radio Emission Simulator) modelling code (Louis et al., 2019), which computes the location of the visible Jovian radio sources depending on the observers location. We show that this code accurately models the temporal occurrence of the occultations in the whole spectral range observed by Galileo/PWS. This validates of the ExPRES code on a new use case. In addition to supporting the analysis of the science observations, the method can be applied for preparing the JUICE moon flyby science operation planning (Cecconi et al. 2021).

Réferences

• Cecconi, Baptiste, Corentin K Louis, Claudio Muñoz Crego, and Claire Vallat. 2021. Jovian Auroral Radio Source Occultation Modelling and Application to the JUICE Science Mission Planning. PSS 209 (105344): 1–34. https://doi.org/10.1016/j.pss.2021.105344.

• Gurnett, D. A., W. S. Kurth, R. R. Shaw, A. Roux, R. Gendrin, C. F. Kennel, F. L. Scarf, & S. D. Shawhan (1992). The Galileo Plasma wave investigation. SSRv, 60(1-4), 341-355. https://doi.org/10.1007/BF00216861

• Kurth, W. S., S. J. Bolton, D. A. Gurnett, & S. Levin (1997). A determination of the source of Jovian hectometric radiation via occultation by Ganymede. GeoRL, 24(10), 1171-1174. https://doi.org/10.1029/97GL00988

• Louis, C. K., S. L. G. Hess, B. Cecconi, P. Zarka, L. Lamy, S. Aicardi, & A. Loh (2019). ExPRES: an Exoplanetary and Planetary Radio Emissions Simulator. A&A, 627 A30. https://doi.org/10.1051/0004-6361/201935161

   

How to cite: Cecconi, B., Louis, C. K., Muñoz Crego, C., and Vallat, C.: Jovian auroral radio source occultation modelling and application to the JUICE science mission planning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2270, https://doi.org/10.5194/egusphere-egu22-2270, 2022.