Diana E. Morosan

Context. Electrons accelerated by coronal shock waves can be remotely detected as type II solar radio bursts. Radio imaging of these bursts can be used to study the presence of shocks in the solar corona especially when other clear observations of the shock are missing at other wavelengths. Aims. Our aim is to investigate the origin and source locations of two type II solar radio bursts associated with a faint and slow coronal mass ejection (CME) using imaging and spectroscopic observations from the LOw Frequency ARray (LOFAR) together with a magnetohydrodynamic (MHD) model of the solar corona. Methods. We use dynamic spectra of the observed type II bursts to track them in frequency and time. We then use the imaging observations at the corresponding frequencies and times to see where the type II sources are located relative to the Sun. Finally, we combine the imaging observations with a global MHD model of coronal densities and magnetic field to de-project the radio source locations and investigate the properties of the emitting regions. Results. Imaging of the type II bursts shows that they originate from different regions in the corona. Combining LOFAR imaging with the MHD model shows that despite their separate source locations, both type II bursts originate along a coronal streamer characterised by high density and low Alfvèn speed. Conclusions. The different source locations of the type II bursts suggest that a single coronal shock driven by a weak CME can accelerate electrons at multiple separate locations along the shock front. The results indicate that a CME interacting with a streamer can create favourable conditions for the generation of type II bursts. The high density leads to low values of Alfvèn speed, and thus, even a slow CME can drive a shock capable of accelerating electrons in these regions.

Context. Type II radio bursts are solar radio emissions generated by electrons accelerated by coronal shocks. These bursts are typically found close to expanding coronal mass ejections (CMEs), making them valuable for studying the properties and dynamics of CME-driven shocks in the solar corona. Aims. Here, we aim to determine the regions in the solar corona where shock waves accelerate electrons and determine their characteristic properties. To do this, we combine radio observations of type II solar radio bursts with magneto-hydrodynamic (MHD) simulations of the solar corona. Methods. We analyse ten type II radio bursts from Solar Cycle 25 exhibiting emissions in the 150–300 MHz frequency range. The novelty of this study lies in using radio imaging data for all type II bursts to examine the positions of the radio sources. The radio source positions, combined with a geometrical fitting of the CME shock and the MHD simulations, are used to determine essential shock parameters at the acceleration region, such as the Alfvén Mach number and shock normal angle relative to the magnetic field. The shock parameters are then combined with the properties of the radio emission and the associated eruption in a comprehensive study. Results. We found that for all events, the type II bursts are located near or inside coronal streamers. These streamer regions have a low Alfvén speed while the estimated shock speeds are high, resulting in the formation of supercritical shocks, with Alfvén Mach numbers in the range 3.8 to 7.7. However, in most events, type II bursts are located at oblique shocks rather than near-perpendicular geometries, suggesting that the shock structure is more complex at local scales than the simple spherical shock models usually applied to CME shocks. Conclusions. Our results suggest that CME-streamer interaction regions are necessary for the generation of type II bursts, as they provide ideal plasma conditions for the formation of supercritical shocks and the subsequent acceleration of electrons.

Context. Shocks in the solar corona are capable of accelerating electrons that in turn generate radio emission known as type II radio bursts. The characteristics and morphology of these radio bursts in the dynamic spectrum reflect the evolution of the shock itself, together with the properties of the local corona where the shock propagates. Aims. In this work we study the evolution of a complex type II radio burst with a multilane structure to find the locations where the radio emission is produced and relate them to the properties of the local environment where the shock propagates. Methods. Using radio imaging, we were able to separately track each lane composing the type II burst and relate the position of the emission to the properties of the ambient medium, such as density, Alfvén speed, and magnetic field. Results. We show that the radio burst morphology in the dynamic spectrum changes with time and is related to the complexity of the local environment. The initial stage of the radio emission is characterized by a single broad lane in the spectrum, while the later stages of the radio signature evolve in a multilane scenario. The radio imaging reveals how the initial stage of the radio emission separates with time into different locations along the shock front as the density and orientation of the magnetic field change along the shock propagation. At the time when the spectrum shows a multilane shape, we find a clear separation of the imaged radio sources propagating in regions with different densities. Conclusions. By combining radio imaging with the properties of the local corona, we describe the evolution of a type II radio burst and, for the first time, identify three distinct radio emission regions above the coronal mass ejection front. Two regions were located at the flanks, producing earlier radio emission than the central position, in accordance with the complexity of density and Alfvén speed values in the regions where radio emission is generated. This unprecedented observation of a triple-source structure provides new insights into the nature of multilane type II bursts.

Context. Solar radio bursts can provide important insights into the underlying physical mechanisms that drive the small and large-scale eruptions on the Sun. Since metric radio observations can give us direct observational access to the inner and middle corona, they are often used as an important tool to monitor and understand the coronal dynamics. Aims. While the sizes of the radio sources that can be observed in the solar corona are essential for understanding the nature of density turbulence within the solar corona and its subsequent influence on the angular broadening observed in radio source measurements, the smallest radio sources associated with solar radio bursts have so far been limited by observational techniques and the radio instrument’s baselines. Methods. We selected three type II bursts that were observed with the LOFAR core and remote stations in the Solar Cycle 24. We estimated the sizes and shapes (ellipticity) of the radio sources from 20–200 MHz using a two-dimensional (2D) Gaussian approximation. Results. Our analysis shows that the smallest radio source size for type II bursts in the solar corona that can be observed in the solar atmosphere at low frequencies is 1.5′±0.5′ at 150 MHz. However, even though the observations were taken with remote baselines (with a maximum distance of ∼85 km), the effective baselines were much shorter (∼15 km), likely due to snapshot imaging of the Sun. Conclusions. Our results show that the radio source sizes are less affected by scattering than suggested in previous studies. Our measurements indicate smaller source sizes at frequencies below 95 MHz compared to previous reports, though some overlap exists with measurements at higher frequencies when using smaller baselines.

Context. Type II solar radio bursts are generated by electrons accelerated by coronal shock waves. They appear in dynamic spectra as lanes drifting from higher to lower frequencies at the plasma frequency and its harmonic. These lanes can often be split into two or more sub-bands that have similar drift rates. This phenomenon is called band-splitting, and its physical origins are still under debate. Aims. Our aim is to investigate the origin of band-splitting using novel imaging and spectropolarimetric observations of a type II solar radio burst from the Low Frequency Array (LOFAR). Methods. We used LOFAR imaging at multiple frequencies and time steps to track the locations of the radio sources corresponding to the two components of the band-split emission lane. In addition, we estimated the degree of circular polarisation (dcp) for both components using LOFAR's full Stokes dynamic spectra. Results. From the imaging of the type II burst, we found two close but clearly separated emission regions clustered over several frequencies spanning each split band. One emission region corresponds to the lower frequency band and the other to the higher frequency band of the split lane. Using the full Stokes dynamic spectra, we also found the dcp to be very similar for both bands. Conclusions. The two distinct emission regions suggest that the split bands originate from two separate regions at the shock. The similar values of dcp for both sub-bands correspond to similar values of magnetic field strength in the two regions and indicate little to no change in the emission region plasma. Thus, our findings are in contradiction with previous theories, which have suggested that split bands originate in the same region but upstream and downstream of the shock. Instead, our results suggest that both bands originate in two separate upstream regions since we find a clear separation in locations and no magnetic compression.

The Energetic Solar Eruptions: Data and Analysis Tools (SOLER) project (2024–2027) will investigate the most energetic phenomena occurring at the Sun using the newly expanded unprecedented heliospheric spacecraft fleet including Solar Orbiter, Parker Solar Probe, BepiColombo, STEREO A, and near-Earth spacecraft. We investigate energetic solar eruptions starting from three perspectives: fast (speed > 1000 km/s) coronal mass ejections (CMEs), strong (GOES class > M5) X-ray flares, and large (detected at >25 MeV proton energies) solar energetic particle events. Key parameters of the various eruption phenomena will be determined and their interrelations examined to make significant leaps in our understanding on how the eruptive phenomena are linked to each other, how they interact with each other, and how they result in acceleration and release of high energy particles from the solar corona into interplanetary space. Because of their direct link to particle energisation, large-amplitude coronal waves and shocks related to these events will be in focus as well. Magnetic connections of sources with each other and with the in-situ observers will be determined. In addition to producing significant amounts of new scientific knowledge in the field, SOLER will provide the wider scientific community a wide array of advanced data products, and novel data analysis and visualisation tools that will be openly distributed.This poster will present an overview of SOLER, its objectives and plans, and the first results of the project.SOLER has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No 101134999.

Energetic particles in the heliosphere are produced by flaring processes on the Sun or shocks driven by coronal mass ejections. These particles can be detected remotely through the electromagnetic radiation they generate (X-rays or radio emission) or in situ by spacecraft monitoring the Sun and the heliosphere. Here, we investigate the acceleration location, escape, and propagation directions of electron beams producing radio bursts observed with the Low Frequency Array (LOFAR), Parker Solar Probe (PSP) and Solar Orbiter (SolO) and compare it to hard X-ray (HXR) emission and in situ electrons observed at SolO. These observations are combined with a three-dimensional (3D) representation of the electron acceleration locations and results from a magneto-hydrodynamic (MHD) model of the solar corona in order to determine the connectivity to Solar Orbiter and relate the electrons observed remotely to in situ electrons. We observed a long-duration metric-decametric type II radio burst with good connectivity to Solar Orbiter, which also observed a significant in situ electron event. The injections times of the in situ electrons are simultaneous with the onset of the type II radio burst. The properties of the SolO electrons also indicate that shock acceleration is likely the main contributor to the observed fluxes, with a possible smaller contribution coming from the flare, given the presence of HXRs and type III radio bursts.

Context. Collisionless shocks are one of the most powerful particle accelerators in the Universe. In the heliosphere, type II solar radio bursts are signatures of electrons accelerated by collisionless shocks launched at the Sun. Spectral observations of these bursts show a variety of fine structures often composing multiple type II lanes. The origin of these lanes and structures is not well understood and has been attributed to the inhomogeneous environment around the propagating shock. Aims. Here, we aim to determine the large-scale local structures near a coronal shock wave using high-resolution radio imaging observations of a complex type II radio burst observed on 3 October 2023. Methods. By using inteferometric imaging from the Low Frequency Array (LOFAR), combined with extreme ultraviolet observations, we investigate the origin of multiple type II lanes at low frequencies (30−80 MHz) relative to the propagating shock wave. Results. We identify at least three radio sources at metric wavelengths corresponding to a multi-lane type II burst. The type II burst sources propagate outwards with a shock driven by a coronal mass ejection. We find a double radio source that exhibits increasing separation over time, consistent with the expansion rate of the global coronal shock. This suggests that the overall shock expansion is nearly self-similar, with acceleration hotspots forming at various times and splitting at a rate proportional to the shock’s expansion. Conclusions. Our results show the importance of increased spatial resolution in determining either the small-scale spatial properties in coronal shocks or the structuring of the ambient medium. Possible shock corrugations or structuring of the upstream plasma at the scale of 10⁵ km can act as hotspots for the acceleration of suprathermal electrons. This can be observed as radiation that exhibits double sources with increasing separation at the same expansion rate as the global shock wave.

Collisionless shocks are one of the most powerful particle accelerators in the Universe. In the heliosphere, type II solar radio bursts are signatures of electrons accelerated by collisionless shocks launched at the Sun. Spectral observations of these bursts show a variety of fine structures often composing multiple type II lanes. The origin of these lanes and structures is not well understood and has been attributed to the inhomogeneous environment around the propagating shock. Here, we aim to determine the large-scale local structures near a coronal shock wave using high-resolution radio imaging observations of a complex type II radio burst observed on 3 October 2023. By using inteferometric imaging from the Low Frequency Array (LOFAR), combined with extreme ultraviolet observations, we investigate the origin of multiple type II lanes at low frequencies (30--80~MHz) relative to the propagating shock wave. We identify at least three radio sources at metric wavelengths corresponding to a multi-lane type II burst. The type II burst sources propagate outwards with a shock driven by a coronal mass ejection. We find a double radio source that exhibits increasing separation over time, consistent with the expansion rate of the global coronal shock. This suggests that the overall shock expansion is nearly self-similar, with acceleration hotspots forming at various times and splitting at a rate proportional to the shock's expansion. Our results show the importance of increased spatial resolution in determining either the small-scale spatial properties in coronal shocks or the structuring of the ambient medium. Possible shock corrugations or structuring of the upstream plasma at the scale of 10$^5$~km can act as hotspots for the acceleration of suprathermal electrons.

Context. Solar energetic particles in the heliosphere are produced by flaring processes on the Sun or by shocks driven by coronal mass ejections. These particles are regularly detected remotely as electromagnetic radiation (X-rays or radio emission), which they generate through various processes, or in situ by spacecraft monitoring the Sun and the heliosphere. Aims. Our aim is to combine remote-sensing and in situ observations of energetic electrons to determine the origin and acceleration mechanism of these particles. Methods. Here we investigate the acceleration location, escape, and propagation directions of electron beams producing radio bursts observed with the Low Frequency Array (LOFAR), hard X-ray (HXR) emission, and in situ electrons observed at Solar Orbiter on 3 October 2023. These observations are combined with a three-dimensional (3D) representation of the electron acceleration locations and results from a magnetohydrodynamic (MHD) model of the solar corona in order to investigate the origin and connectivity of electrons observed remotely at the Sun to in situ electrons. Results. We observed a type II radio burst with good connectivity to Solar Orbiter, where a significant electron event was detected. However, type III radio bursts and hard X-rays were also observed co-temporally with the electron event, but likely connected to Solar Orbiter by different far-side field lines. The injection times of the Solar Orbiter electrons are simultaneous with both the onset of the type II radio burst, the group of type III bursts, and the presence of a second HXR peak; however, the most direct connection to Solar Orbiter is that of the type II burst location. The in situ electron spectra point to shock acceleration of electrons with a short-term connection to the source region. Conclusions. We propose that there are two contributions to the Solar Orbiter electron fluxes based on the results and magnetic connectivity determined from remote-sensing data: a smaller flare contribution from the far-side of the Sun and a main shock contribution from the region close to the eastern limb as viewed from Earth. We note that these two electron acceleration regions are distinct and separated by a large distance and are connected via two separate field lines to Solar Orbiter.