Tracking the Source of Solar Type II Bursts through Comparisons of Simulations and Radio Data

The most intense solar energetic particle events are produced by coronal mass ejections (CMEs) accompanied by intense type II radio bursts below 15 MHz. Understanding where these type II bursts are generated relative to an erupting CME would reveal important details of particle acceleration near the Sun, but the emission cannot be imaged on Earth due to distortion from its ionosphere. Here, a technique is introduced to identify the likely source location of the emission by comparing the dynamic spectrum observed from a single spacecraft against synthetic spectra made from hypothesized emitting regions within a magnetohydrodynamic (MHD) numerical simulation of the recreated CME. The radio-loud 2005 May 13 CME was chosen as a test case, with Wind/WAVES radio data being used to frame the inverse problem of finding the most likely progression of burst locations. An MHD recreation is used to create synthetic spectra for various hypothesized burst locations. A framework is developed to score these synthetic spectra by their similarity to the type II frequency profile derived from the Wind/WAVES data. Simulated areas with 4× enhanced entropy and elevated de Hoffmann–Teller velocities are found to produce synthetic spectra similar to spacecraft observations. A geometrical analysis suggests the eastern edge of the entropy-derived shock around (−30°, 0°) was emitting in the first hour of the event before falling off, and the western/southwestern edge of the shock centered around (6°, −12°) was a dominant area of radio emission for the 2 hr of simulation data out to 20 solar radii.

On the Issue of the Origin of Type II Solar Radio Bursts

Type II solar radio bursts are among the most powerful events in the solar radio emission in the meter wavelength range. It is generally accepted that the agents generating type II radio bursts are magnetohydrodynamic shock waves. But the relationship between the shock waves and the other manifestations of the large-scale disturbances in the solar atmosphere (coronal mass ejections, Morton waves, EUW waves) remains unclear. To clarify a problem, it is important to determine the conditions of generation of type II radio bursts. Here, the model of the radio source is based on the generation of radio emission within the front of the collisionless shock wave where the Buneman instability of plasma waves is developed. In the frame of this model, the Alfvén magnetic Mach number must exceed the critical value, and there is a strict restriction on the perpendicularity of the front. The model allows us to obtain the information about the parameters of the shock waves and the parameters of the medium by the parameters of type II bursts. The estimates, obtained in this paper for several events with the band splitting of the fundamental and harmonic emission bands of the type II bursts, confirm the necessary conditions of the model. In this case the registration of type II radio bursts is an indication of the propagation of shock waves in the solar atmosphere, and the absence of type II radio bursts is not an indication of the absence of shock waves. Such a situation should be taken into account when investigating the relationship between type II radio bursts and other manifestations of solar activity.

Analysis of Type II radio burst relationship with CME driven shocks

Type II radio bursts are known to be the signatures of coronal shocks. In this paper we examine the relationship between 129 type II bursts in the frequency range 35 – 450 MHz observed at Culgooora observatory during May 2002 – October 2015 and the associated CMEs. We apply Newkirk (1961) density model to determine the formation height of type IIs. We find that in 109/129 cases, type II bursts were preceded/ succeeded by CMEs. The CME associated type II events in which the CME height is above the type II burst source are categorized as group I events (91/129 cases). 91% of the bursts in this group are also associated with flares and 58% of these bursts originate during decaying phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.33.The CME associated type IIs in which the CME height is below the type II source are categorized as group II (18/129 cases). CME driven shock could have been the exciter of these type II bursts.88% of this group events are associated with flares and 62% of these bursts originate during the rising phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.96. In 20/129 cases of our data set, type II bursts are not associated with CME and are categorized as group III. 90% of the bursts in this group are associated with flares. 77% of the bursts in the group are originating in the decaying phase of flares. Poor temporal association (9/69 cases) between type IIs and flares of X class during this period. Our results suggest that inspite of temporal association with metric type II bursts, majority of the CME driven shocks (84%) are not successful in exciting type II bursts in 35-450 MHz domain. The type II bursts temporally correlated with CMEs and likely to have been excited by CMEs (type II height > CME height) are originating during the rising phase of the flares in majority of the events. In case of type II bursts temporally correlated with CMEs supposedly not excited by the CMEs (type II height < CME height) ,majority of them are originating in the decaying phase of flares.

Formation of Isolated Radio Type II Bursts at Low Frequencies

The first appearance of radio type II burst emission at decameter-hectometer (DH) waves typically occurs in connection, and often simultaneously, with other types of radio emissions. As type II bursts are signatures of propagating shock waves that are associated with flares and coronal mass ejections (CMEs), a rich variety of radio emissions can be expected. However, sometimes DH type II bursts appear in the dynamic spectra without other or earlier radio signatures. One explanation for them could be that the flare-CME launch happens on the far side of the Sun, and the emission is observed only when the source gets high enough in the solar atmosphere. In this study we have analysed 26 radio type II bursts that started at DH waves and were well-separated (‘isolated’) from other radio emission features. These bursts were identified from all DH type II bursts observed in 1998 – 2016, and for 12 events we had observations from at least two different viewing angles with the instruments on board Wind and the Solar Terrestrial Relations Observatory (STEREO) satellites. We found that only 30% of the type II bursts had their source origin on the far side of the Sun, but also that no bursts originated from the central region of the Sun (longitudes E30 – W40). Almost all of the isolated DH type II bursts could be associated with a shock near the CME leading front, and only few were determined to be shocks near the CME flank regions. In this respect our result differs from earlier findings. Our analysis, which included inspection of various CME and radio emission characteristics, suggests that the isolated DH type II bursts could be a special subgroup within DH type II bursts, where the radio emission requires particular coronal conditions to form and to die out.

Radio Astronomical Tools for the Study of Solar Energetic Particles II.Time-Extended Acceleration at Subrelativistic and Relativistic Energies

Solar energetic particle (SEP) events are commonly separated in two categories: numerous “impulsive” events of relatively short duration, and a few “gradual” events, where SEP-intensities may stay enhanced over several days at energies up to several tens of MeV. In some gradual events the SEP spectrum extends to relativistic energies (>1 GeV), over shorter durations. The two categories are strongly related to an idea developed in the 1960s based on radio observations: Type III bursts, which were addressed in a companion chapter, outline impulsive acceleration of electrons to subrelativistic energies, while the large and the relativistic SEP events were ascribed to a second acceleration process. At radio wavelengths, typical counterparts were bursts emitted by electrons accelerated at coronal shock waves (type II bursts) and by electron populations in large-scale closed coronal structures (type IV bursts). Both burst types are related to coronal mass ejections (CMEs). Type II bursts from metric to kilometric wavelengths tend to accompany large SEP events, which is widely considered as a confirmation that CME-driven shocks accelerate the SEPs. But type II bursts, especially those related to SEP events, are most often accompanied by type IV bursts, where the electrons are rather accelerated in the wake of the CME. Individual event studies suggest that although the CME shock is the most plausible accelerator of SEPs up to some yet unknown limiting energy, the relativistic SEP events show time structure that rather points to coronal acceleration related to type IV bursts. This chapter addresses the question what type II bursts tell us about coronal shock waves and how type II and type IV radio bursts are related with relativistic proton signatures as seen by particle detectors on the Earth and by their gamma-ray emission in the solar atmosphere, focusing on two relativistic SEP events, on 2005 Jan 20 and 2017 Sep 10. The importance of radio emissions as a complement to the upcoming SEP observations from close to the Sun is underlined.

Localizing the Source of Type II Emission Around a CME with the Sun Radio Interferometer Space Experiment (SunRISE) and MHD Simulations

&lt;p&gt;The Earth&amp;#8217;s Ionosphere limits radio measurements on its surface, blocking out any radiation below 10 MHz. Valuable insight into many astrophysical processes could be gained by having a radio interferometer in space to image the low frequency window, which has never been achieved. One application for such a system is observing type II bursts that track solar energetic particle acceleration occurring at Coronal Mass Ejection (CME)-driven shocks. This is one of the primary science targets for SunRISE, a 6 CubeSat interferometer to circle the Earth in a GEO graveyard orbit. SunRISE is a NASA Heliophysics Mission of Opportunity that began Phase B (Formulation) in June 2020, and plans to launch for a 12-month mission in mid-2023. In this work we present an update to the data processing and science analysis pipeline for SunRISE and evaluate its performance in localizing type II bursts around a simulated CME.&lt;/p&gt;&lt;p&gt;To create realistic virtual type II input data, we employ a 2-temperature MHD simulation of the May 13th 2005 CME event, and superimpose realistic radio emission models on the CME-driven shock front, and propagate the signal through the simulated array. Data cuts based on different plasma parameter thresholds (e.g. de Hoffman-Teller velocity and angle between shock normal and the upstream magnetic field) are tested to get the best match to the true recorded emission. &amp;#160;This model type II emission is then fed to the SunRISE data processing pipeline to ensure that the array can localize the emission. We include realistic thermal noise dominated by the galactic background at these low frequencies, as well as new sources of phase noise from positional uncertainty of each spacecraft. We test simulated trajectories of SunRISE and image what the array recovers, comparing it to the virtual input, finding that SunRISE can resolve the source of type II emission to within its prescribed goal of 1/3 the CME width. This shows that SunRISE will significantly advance the scientific community&amp;#8217;s understanding of type II burst generation, and consequently, acceleration of solar energetic particles at CMEs.&amp;#160; This unique combination of SunRISE observations and MHD recreations of space weather events will allow an unprecedented look into the plasma parameters important for these processes.&amp;#160;&lt;/p&gt;

EXPLORATION OF THE SOLAR DECAMETER RADIO EMISSION WITH THE UTR-2 RADIO TELESCOPE

Purpose: The overview of the scientifi c papers devoted to the study of the solar decameter radio emission with the world’s largest UTR-2 radio telescope (Ukraine) published for the last 50 years. Design/methodology/approach: The study and analysis of the scientifi c papers on both sporadic and quiet (thermal) radiation of the Sun recorded with the UTR-2 radio telescope at the decameter wavelength range. Findings: The most signifi cant observational and theoretical results of the solar radio emission studies obtained at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine for the last 50 years are given. Conclusions: For the fi rst time, at frequencies below 30 MHz, the Type II bursts, Type IV bursts, S-bursts, drift pairs and spikes have been recorded. The dependences of these bursts parameters on frequency within the frequency band of 9 to 30 MHz were obtained. The models of their generation and propagation were suggested. Moreover, for the fi rst time the fi ne time-frequency structures of the Type III bursts, Type II bursts, Type IV bursts, U- and J-bursts, S-bursts, and drift pairs have been observed due to the high sensitivity and high time-frequency resolutions of the UTR-2 radio telescope. The super-fi ne structure of Type II bursts with a “herringbone” structure was identifi ed, which has never been observed before. New types of bursts were discovered: “caterpillar” bursts, “dog-leg” bursts, Type III bursts with decay, Type III bursts with changing drift rate sign, Type III-like bursts, Jb- and Ub-bursts, etc. An interpretation of the unusually high drift rates and drift rates with alternating signs of the Type III-like bursts was suggested. Based on the dependence of spike durations on frequency, the coronal plasma temperature profi le at the heliocentric heights of 1.5–3RS was determined. Usage of the heliographic and interferometric methods gave the possibility to start studies of the spatial characteristics – sizes and locations of the bursts emission sources. Thus, it was shown that at the decameter band, the Type III burst durations were defi ned by the emission source linear sizes, whereas the spike durations were governed by the collision times in the source plasma. It was experimentally proved that the effective brightness temperatures of the sources of solar sporadic radio emission at the decameter band may reach values of 1014–1015 K. In addition, it was found that the radii of the quiet Sun at frequencies 20 and 25 MHz are close to the distances from the Sun at which the local plasma frequency is equal to the corresponding observed frequency of radio emission in the Baumbach–Allen model. Key words: UTR-2; Sun; decameter radio emission; radio bursts; corona

Using radio triangulation to understand the origin of two subsequent type II radio bursts

Context. Eruptive events such as coronal mass ejections (CMEs) and flares accelerate particles and generate shock waves which can arrive at Earth and can disturb the magnetosphere. Understanding the association between CMEs and CME-driven shocks is therefore highly important for space weather studies. Aims. We present a study of the CME/flare event associated with two type II bursts observed on September 27, 2012. The aim of the study is to understand the relationship between the observed CME and the two distinct shock wave signatures. Methods. The multiwavelength study of the eruptive event (CME/flare) was complemented with radio triangulation of the associated radio emission and modelling of the CME and the shock wave employing MHD simulations. Results. We found that, although temporal association between the type II bursts and the CME is good, the low-frequency type II (LF-type II) burst occurs significantly higher in the corona than the CME and its relationship to the CME is not straightforward. The analysis of the EIT wave (coronal bright front) shows the fastest wave component to be in the southeast quadrant of the Sun. This is also the quadrant in which the source positions of the LF-type II were found to be located, probably resulting from the interaction between the shock wave and a streamer. Conclusions. The relationship between the CME/flare event and the shock wave signatures is discussed using the temporal association, as well as the spatial information of the radio emission. Further, we discuss the importance and possible effects of the frequently non-radial propagation of the shock wave.

The Ep,i–Eiso correlation: type I gamma-ray bursts and the new classification method

ABSTRACT We present the most extensive sample of 45 type I (short) and 275 type II (long) gamma-ray bursts (GRBs) with known redshift to investigate the correlation between the rest-frame peak energy, Ep,i and the total isotropic equivalent energy, Eiso of the prompt emission (Amati relation). The Ep,i–Eiso correlation for type I bursts is found to be well distinguished from the one constructed for type II bursts and has a similar power-law index value, Ep,i$\propto E_\text{iso}^{~0.4}$, which possibly indicates the same emission mechanism of both GRB types. We show that the initial pulse complex (IPC) of type I bursts with an extended emission and regular type I bursts follow the same correlation. We obtain similar results for type II bursts associated with Ic supernovae and for regular type II bursts. Three possible outliers from the Ep,i–Eiso correlation for type II subsample are detected. Significant evolution of the Ep,i–Eiso correlation with redshift for type II bursts is not found. We suggest the new classification method, based on the Ep,i–Eiso correlation and introduce two parameters, $EH = E_\text{p,i,2} ~ E_\text{iso,51}^{~-0.4}$ and $EHD = E_\text{p,i,2} ~ E_\text{iso,51}^{~-0.4} ~ T_\text{90,i}^{~-0.5}$, where Ep,i,2 is the value of Ep,i parameter in units of 100 keV, Eiso,51 is the value of Eiso parameter in units of 1051 erg, and T90,i is the rest-frame duration in units of seconds. EHD is found to be the most reliable parameter for the blind type I/type II classification, which can be used to classify GRBs with no redshift.