Classification of fine structures of Saturn kilometric radiation

<p>Saturn kilometric radiation (SKR) is thought to be created by the cyclotron maser instability along auroral magnetic field lines, where radio emissions are generated near the electron cyclotron frequency at the source. Using the Wideband Receiver (WBR) of the Cassini Radio and Plasma Wave Science (RPWS) instrument it was possible to create dynamic spectra of high temporal and spectral resolution. They display the radio wave intensity as a function of time and frequency with a temporal resolution of a fraction of a second and a spectral resolution of ~0.1 kHz. </p> <p>In these dynamic spectra one can find a plethora of various fine structures which we classify in the following way: We first simply distinguish between 0-dimensional structures (dots), 1-dimensional structures (lines), and 2-dimensional structures (areas). For areas we require a minimum extension of 5 seconds in time and 5 kHz in frequency. Our main focus is on the lines, where we again distinguish between the four classes of horizontal lines, vertical lines, and lines with a positive or a negative slope (going to higher or lower frequencies). For the latter two it is thought that they are related to downward or upward moving radiation sources, i.e. bunches of energetic electrons moving down or up along the magnetic field lines. Using these simple 6 classes (DOTS, HORZ, VERT, POSS, NEGS, AREA) it is already possible to classify about 80% of all spectra showing SKR. Unclassified spectra contain no clear linear elements and mostly consist of patchy structures with holes that do not fulfill the minimum size requirement (5 s, 5 kHz) to be classified as areas. Linear elements appear in about one third of the spectra in the frequency range from 100 kHz to 1 MHz. Some spectra can of course be mixed and show dots, lines and areas, but in our classification we prioritize lines over areas and dots. </p>

Electron acceleration and radio emission following the early interaction of two coronal mass ejections

Context. Coronal mass ejections (CMEs) are large eruptions of magnetised plasma from the Sun that are often accompanied by solar radio bursts produced by accelerated electrons. Aims. A powerful source for accelerating electron beams are CME-driven shocks, however, there are other mechanisms capable of accelerating electrons during a CME eruption. So far, studies have relied on the traditional classification of solar radio bursts into five groups (Type I–V) based mainly on their shapes and characteristics in dynamic spectra. Here, we aim to determine the origin of moving radio bursts associated with a CME that do not fit into the present classification of the solar radio emission. Methods. By using radio imaging from the Nançay Radioheliograph, combined with observations from the Solar Dynamics Observatory, Solar and Heliospheric Observatory, and Solar Terrestrial Relations Observatory spacecraft, we investigate the moving radio bursts accompanying two subsequent CMEs on 22 May 2013. We use three-dimensional reconstructions of the two associated CME eruptions to show the possible origin of the observed radio emission. Results. We identified three moving radio bursts at unusually high altitudes in the corona that are located at the northern CME flank and move outwards synchronously with the CME. The radio bursts correspond to fine-structured emission in dynamic spectra with durations of ∼1 s, and they may show forward or reverse frequency drifts. Since the CME expands closely following an earlier CME, a low coronal CME–CME interaction is likely responsible for the observed radio emission. Conclusions. For the first time, we report the existence of new types of short duration bursts, which are signatures of electron beams accelerated at the CME flank. Two subsequent CMEs originating from the same region and propagating in similar directions provide a complex configuration of the ambient magnetic field and favourable conditions for the creation of collapsing magnetic traps. These traps are formed if a CME-driven wave, such as a shock wave, is likely to intersect surrounding magnetic field lines twice. Electrons will thus be further accelerated at the mirror points created at these intersections and eventually escape to produce bursts of plasma emission with forward and reverse drifts.

Dynamic Spectra of Small-Mass Meteors

We present dynamic (22 frames per second) observations of optical spectra of small-mass (2–200 mg) meteors observed with an EMCCD imager equipped with a diffraction grating. This observational campaign occurred at Arecibo, Puerto Rico during May 2012, resulting in eight hours of clear data over four nights. We detected 22 meteors with this setup and their spectra showed varying compositions, including evidence of Na, Mg, Fe, and Ca. Spectral lines persisting over multiple frames (up to 23 frames) with sufficient signal, showed evidence for differential ablation. Brighter, more massive meteors had stronger and varied spectral signals, which showed that the temporal and spectral resolution of the faintest meteors approached the noise level of the camera system. Optical and spectral detections of these small-mass meteors provide a greater understanding of the composition of the milligram-sized population of meteors.

Interferometric Observations of the Active Regions in Radio Domain Before and After the Total Solar Eclipse on 21 August 2017

<p>We hereby present the interferometric LOFAR observations made before and after the total solar eclipse on 21 August 2017, during which the type III radio bursts have been detected.</p><p>The LOw-Frequency ARray (LOFAR) is a large radio interferometer operating in the frequency range of 10–240 MHz, designed and constructed by ASTRON (the Netherlands Institute for Radio Astronomy). The LOFAR telescope is an array of stations distributed throughout the Netherlands and other parts of Europe. Currently the system consist of 52 LOFAR stations located in Europe. Apart from the high time and frequency resolution of the dynamic spectra, LOFAR allows also a 2D imaging of the radio sources and tracking of their positions through the solar corona.</p><p>In this work we present a preliminary analysis of the dynamic spectra of type III radio bursts with radio images.</p>

Propagation of a Solar Moving Type IV Radio Burst Using LOFAR

<p>Type IV radio burst is the long-lasting broadband continuum emission in metric wave-length. In addition to the continuum emission Type IV radio bursts may show fine structure with high brightness temperature. The physical emission responsible for both continuum and fine structures is still under debate. In this study, we present a moving type IV radio burst observed by LOFAR. We performed a detailed comparison of NRH and LOFAR imaging. Using the full stokes parameterss from the LOFAR dynamic spectra, we have also calculated the degree of circular polarisation during the propagation of the moving type IV. Finally, we combined LOFAR interferometric data with SDO-AIA and LASCO-C2 to track the evolution of this type IV and relate it with the CME.</p>

Using supervised machine learning to automatically detect type II and III solar radio bursts

<p>Solar flares are often associated with high-intensity radio emission known as `solar radio bursts' (SRBs). SRBs are generally observed in dynamic spectra and have five major spectral classes, labelled type I to type V depending on their shape and extent in frequency and time. Due to their morphological complexity, a challenge in solar radio physics is the automatic detection and classification of such radio bursts. Classification of SRBs has become necessary in recent years due to large data rates (3 Gb/s) generated by advanced radio telescopes such as the Low Frequency Array (LOFAR). Here we test the ability of several supervised machine learning algorithms to automatically classify type II and type III solar radio bursts. We test the detection accuracy of support vector machines (SVM), random forest (RF), as well as an implementation of transfer learning of the Inception and YOLO convolutional neural networks (CNNs). The training data was assembled from type II and III bursts observed by the Radio Solar Telescope Network (RSTN) from 1996 to 2018, supplemented by type II and III radio burst simulations. The CNNs were the best performers, often exceeding >90% accuracy on the validation set, with YOLO having the ability to perform radio burst burst localisation in dynamic spectra. This shows that machine learning algorithms (in particular CNNs) are capable of SRB classification, and we conclude by discussing future plans for the implementation of a CNN in the LOFAR for Space Weather (LOFAR4SW) data-stream pipelines.</p>

Three-dimensional reconstruction of multiple particle acceleration regions during a coronal mass ejection

Context. Some of the most prominent sources for particle acceleration in our Solar System are large eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs). These accelerated particles can generate radio emission through various mechanisms. Aims. CMEs are often accompanied by a variety of solar radio bursts with different shapes and characteristics in dynamic spectra. Radio bursts directly associated with CMEs often show movement in the direction of CME expansion. Here, we aim to determine the emission mechanism of multiple moving radio bursts that accompanied a flare and CME that took place on 14 June 2012. Methods. We used radio imaging from the Nançay Radioheliograph, combined with observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, to analyse these moving radio bursts in order to determine their emission mechanism and three-dimensional (3D) location with respect to the expanding CME. Results. In using a 3D representation of the particle acceleration locations in relation to the overlying coronal magnetic field and the CME propagation, for the first time, we provide evidence that these moving radio bursts originate near the CME flanks and that some are possible signatures of shock-accelerated electrons following the fast CME expansion in the low corona. Conclusions. The moving radio bursts, as well as other stationary bursts observed during the eruption, occur simultaneously with a type IV continuum in dynamic spectra, which is not usually associated with emission at the CME flanks. Our results show that moving radio bursts that could traditionally be classified as moving type IVs can represent shock signatures associated with CME flanks or plasma emission inside the CME behind its flanks, which are closely related to the lateral expansion of the CME in the low corona. In addition, the acceleration of electrons generating this radio emission appears to be favoured at the CME flanks, where the CME encounters coronal streamers and open field regions.