J. M. Schmidt

The solar type II radio bursts of 7 March 2012: Detailed simulation analyses

Type II solar radio bursts are often indicators for impending space weather events at Earth. They are consequences of shock waves driven by coronal mass ejections (CMEs) that move outward from the Sun. We simulate such type II radio bursts by combining elaborate three-dimensional (3-D) magnetohydrodynamic (MHD) predictions of realistic CMEs near the Sun with an analytic kinetic radiation theory developed recently. The simulation approach includes the reconstruction of initial solar magnetic fields, the dimensioning of the initial flux rope of the CME with STEREO spacecraft data, and the launch of the CME into an empirical data-driven corona and solar wind. In this paper, we simulate a complicated double CME event (a very fast CME followed by a slower CME without interaction) and the related coronal and interplanetary type II radio bursts that occurred on 7 March 2012. We extend our previous work to show harmonic and interplanetary emission as well as the simulations surprising ability (for these events at least) for predicting emission for two closely spaced CMEs leaving the same active region. We demonstrate that the theory predicts well the observed fundamental and harmonic emission from ∼20 MHz to 50 kHz or from the high corona to near 1 AU. Specifically, the theory predicts flux, frequency, and time variations that are consistent with the presence or absence of observed type II emissions when interfering emissions are absent and are not inconsistent with observations when interfering type III bursts are present. The predicted and observed type II emission is predominantly fundamental for these two events. Harmonic emission occurs for the second CME only for a short time interval, when an extended shock has developed that can drive flank emission. The coronal and interplanetary emission follow closely hyperbolic lines in frequency-time space, consisting of a succession of islands of emission with varying intensity. The islands develop due to competition between the shock moving through varying coronal and solar wind magnetic field structures (e.g., loops and streamers), growth of the driven radio source due to the spherical expansion of the shock, and movement of the active radio sources from the shocks nose to its flanks.

Type II solar radio bursts predicted by 3‐D MHD CME and kinetic radio emission simulations

Impending space weather events at Earth are often signaled by type II solar radio bursts. These bursts are generated upstream of shock waves driven by coronal mass ejections (CMEs) that move away from the Sun. We combine elaborate three-dimensional (3-D) magnetohydrodynamic predictions of realistic CMEs near the Sun with a recent analytic kinetic radiation theory in order to simulate two type II bursts. Magnetograms of the Sun are used to reconstruct initial solar magnetic and active region fields for the modeling. STEREO spacecraft data are used to dimension the flux rope of the initial CME, launched into an empirical data-driven corona and solar wind. We demonstrate impressive accuracy in time, frequency, and intensity for the two type II bursts observed by the Wind spacecraft on 15 February 2011 and 7 March 2012. Propagation of the simulated CME-driven shocks through coronal plasmas containing preexisting density and magnetic field structures that stem from the coronal setup and CME initiation closely reproduce the isolated islands of type II emission observed. These islands form because of a competition between the growth of the radio source due to spherical expansion and a fragmentation of the radio source due to increasingly radial fields in the nose region of the shock and interactions with streamers in the flank regions of the shock. Our study provides strong support for this theory for type II bursts and implies that the physical processes involved are understood. It also supports a near-term capability to predict and track these events for space weather predictions.

Prediction of Type II Solar Radio Bursts by Three-dimensional MHD Coronal Mass Ejection and Kinetic Radio Emission Simulations

Type II solar radio bursts are the primary radio emissions generated by shocks and they are linked with impending space weather events at Earth. We simulate type II bursts by combining elaborate three-dimensional MHD simulations of realistic coronal mass ejections (CMEs) at the Sun with an analytic kinetic radiation theory developed recently. The modeling includes initialization with solar magnetic and active region fields reconstructed from magnetograms of the Sun, a flux rope of the initial CME dimensioned with STEREO spacecraft observations, and a solar wind driven with averaged empirical data. We demonstrate impressive accuracy in time, frequency, and intensity for the CME and type II burst observed on 2011 February 15. This implies real understanding of the physical processes involved regarding the radio emission excitation by shocks and supports the near-term development of a capability to predict and track these events for space weather prediction.

SLOW MAGNETOACOUSTIC WAVE OSCILLATION OF AN EXPANDING CORONAL LOOP

We simulated an expanding loop or slow coronal mass ejection (CME) in the solar corona dimensioned with size parameters taken from real coronal expanding loops observed with the STEREO spacecraft. We find that the loop expands to Suns size within about one hour, consistent with slow CME observations. At the top of the loop, plasma is being blown off the loop, enabled with the reconnection between the loops flux rope magnetic field and the radial magnetic field of the Sun, thus yielding feeding material for the formation of the slow solar wind. This mechanism is in accordance with the observed blob formation of the slow solar wind. We find wave packets traveling with local sound speed downward toward the footpoints of the loop, already seen in coronal seismology observations and simulations of stationary coronal loops. Here, we generalize these results for an expanding medium. We also find a reflection of the wave packets, identified as slow magnetoacoustic waves, at the footpoints of the loop. This confirms the formation of standing waves within the coronal loop. In particular, the reflected waves can partly escape the loop top and contribute to the heating of the solar wind. The present study improves our understanding on how loop material can emerge to form blobs, major ingredients of slow CMEs, and how the release of the wave energy stored in slow magnetoacoustic waves, and transported away from the Sun within expanding loops, contributes to the acceleration and formation of the slow solar wind.

CME Flux Rope and Shock Identifications and Locations: Comparison of White Light Data, Graduated Cylindrical Shell Model, and MHD Simulations

Coronal mass ejections (CMEs) are major transient phenomena in the solar corona that are observed with ground-based and spacecraft-based coronagraphs in white light or with in situ measurements by spacecraft. CMEs transport mass and momentum and often drive shocks. In order to derive the CME and shock trajectories with high precision, we apply the graduated cylindrical shell (GCS) model to fit a flux rope to the CME directed toward STEREO A after about 19:00 UT on 29 November 2013 and check the quality of the heliocentric distance-time evaluations by carrying out a three-dimensional magnetohydrodynamic (MHD) simulation of the same CME with the Block Adaptive Tree Solar-Wind Roe Upwind Scheme (BATS-R-US) code. Heliocentric distances of the CME and shock leading edges are determined from the simulated white light images and magnetic field strength data. We find very good agreement between the predicted and observed heliocentric distances, showing that the GCS model and the BATS-R-US simulation approach work very well and are consistent. In order to assess the validity of CME and shock identification criteria in coronagraph images, we also compute synthetic white light images of the CME and shock. We find that the outer edge of a cloud-like illuminated area in the observed and predicted images in fact coincides with the leading edge of the CME flux rope and that the outer edge of a faint illuminated band in front of the CME leading edge coincides with the CME-driven shock front.

Importance of Kappa Distributions to Solar Radio Bursts

Abstract Non-Maxwellian electron distributions are observed widely throughout the solar system and are often well described as kappa distributions. This chapter addresses the importance of kappa distributions to the growth and damping of plasma waves (specifically Langmuir waves), the reflection of electrons from shocks, the evolution of electron beams, and the generation of type II and III solar radio bursts. Kappa distributions extend to much higher speeds and have many more fast particles than Maxwellians. Accordingly, kappa distributions lead to a much larger spontaneous emission of plasma waves and radio emissions produced by incoherent processes. Similarly, shocks that accelerate the background electrons have much higher numbers of fast electrons upstream for kappa-distributed electrons than for Maxwellian electrons, leading to stronger electron beams and stronger Langmuir waves and radio emissions. This is demonstrated theoretically for type II solar radio bursts. The extension of kappa distributions to larger speeds than for Maxwellian distributions leads to larger damping rates for electron beam-generated Langmuir waves. It also limits the relaxation of the beam to higher speeds and reduces energy transfers to the Langmuir waves and any radio emissions generated therefrom. These effects are demonstrated theoretically for type III solar radio bursts, allowing us to understand why kappa distributions for the background electrons lead naturally to faster beams with speeds of ≳0.5c and to the type III bursts being weaker, having faster frequency drift rates, and starting at lower frequencies for smaller values of the kappa index, κ.

Testing a theory for type II radio bursts from the Sun to near 0.5 AU

Type II solar radio bursts have resisted detailed explanation for over 60 years despite being the archetype for collective radio emission associated with shocks. Type II bursts are important because they involve fundamental physics and because most large space weather events at Earth are associated with large, fast, coronal mass ejections (CMEs) and so with type II bursts. Here we present strong evidence for the accurate and quantitative simulation of a type II burst from the deep corona (near 3 solar radii) to near 0.5 AU. The event was observed by the widely separated STEREO A and B spacecraft between 29 November and 1 December 2013. To do so we combine data-driven three-dimensional magnetohydrodynamic simulations (the BATS-R-US code) for the CME and plasma background with an analytic quantitative kinetic model for electron reflection at the shock, transfer of electron energy into Langmuir waves and radio emission, and propagation of radiation to an arbitrary observer. The intensities and frequencies of the radio emissions vary by factors ≈ 104 and ≈ 102, respectively. The theory predicts the intensities, frequencies, and timings of the multiple islands of type II emission very well, with the theory typically in error by less than a factor of 10, 20%, and less than an hour, respectively, for both STEREO A and B. This agreement is strong evidence for the type II theory itself and for accurate prediction by BATS-R-US, when carefully initialised with available data, of the background plasma and magnetic field configurations and of the CMEs properties and motion.

Demonstration of a viable quantitative theory for interplanetary type II radio bursts

Between 29 November and 1 December 2013 the two widely separated spacecraft STEREO A and B observed a long lasting, intermittent, type II radio burst for the extended frequency range ≈ 4 MHz to 30 kHz, including an intensification when the shock wave of the associated coronal mass ejection (CME) reached STEREO A. We demonstrate for the first time our ability to quantitatively and accurately simulate the fundamental (F) and harmonic (H) emission of type II bursts from the higher corona (near 11 solar radii) to 1 AU. Our modeling requires the combination of data-driven three-dimensional magnetohydrodynamic simulations for the CME and plasma background, carried out with the BATS-R-US code, with an analytic quantitative kinetic model for both F and H radio emission, including the electron reflection at the shock, growth of Langmuir waves and radio waves, and the radiations propagation to an arbitrary observer. The intensities and frequencies of the observed radio emissions vary hugely by factors ≈ 106 and ≈ 1...