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

Abstract

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 simulation's 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 shock's nose to its flanks.

The Langmuir waves associated with the 1 December 2013 type II burst

“…Evidence for nonlinear currents at 2 f p is found in all 11 events, so antenna mechanisms may generate some of the observed radio emission. However, the recent simulation analyses of Schmidt and Cairns [] and Schmidt et al [], which include electrostatic decay, electromagnetic decay, and coalesence but not antenna mechanisms, predict well the dynamic spectra of some specific type II bursts in the range 1–14 MHz. This suggests that antenna mechanisms are less likely to be important for the present type II burst.…”

“…Finally, we calculate the conversion efficiencies for fundamental and harmonic radio emission, φ F and φ H , respectively, for electromagnetic decay and Langmuir coalescence. We calculate φ F and φ H using equations (6)–(9) of Robinson and Cairns [], as in current theories for type II bursts [ Knock et al , ; Schmidt and Cairns , , , ; Schmidt et al , ], for the observed solar wind conditions, and v b =0.025 c . For the parameters that we cannot measure we assume typical values for the solar wind at 1 AU.…”

“…They are interpreted as radio emissions produced near the electron plasma frequency f p and/or 2 f p via the so‐called “plasma emission” mechanism. The emission physics involves several steps [ Nelson and Melrose , ; Cairns , , ; Knock et al , ; Schmidt and Cairns , , , ; Schmidt et al , ]: acceleration of electrons at the shock front, development of electron beams in the foreshock regions upstream of the shock, generation of Langmuir waves via the bump‐on‐tail (or beam) instability, and conversion of Langmuir wave energy into radiation via various processes. The shock waves are usually interpreted in terms of bow shocks in front of coronal mass ejections (CMEs), although blast wave shocks are sometimes considered relevant for coronal type IIs [ Nelson and Melrose , ; Bastian et al , ; Cliver et al , ; Cane and Erickson , ; Gopalswamy , ; Cairns , ].…”

“…However, type II source regions distributed along the macroscopic shock front are sporadic in both time and space [ Hoang et al , ]. This is expected theoretically because the plasma is strongly inhomogenous, and angles θ b n between the upstream magnetic field B and the local shock normal n must be in the range θ b n ≈80°–90° for the electron acceleration process (shock‐drift acceleration) to produce enough fast electrons for production of high levels of Langmuir waves and radio emission [ Cairns et al , ; Knock and Cairns , ; Schmidt and Gopalswamy , ; Schmidt and Cairns , , , ; Schmidt et al , ].…”

The Solar Energetic Particle Event of 2010 August 14: Connectivity with the Solar Source Inferred from Multiple Spacecraft Observations and Modeling

“…A possible scenario to account for the metric type II event and the EUV and WL observations of the CME shock is that, if both were due to the same shock, radio emissions were produced at the portions of the shock moving at low altitudes. Intense radio emission from a shock is enhanced when the shock is strong, fast, perpendicular, with large magnetic field compression ratios, and propagates into dense regions, favoring then the mirroring and acceleration of the lowenergy electrons that eventually generate the radio emission (e.g., Knock et al 2003;Feng et al 2013;Schmidt et al 2014). Quasi-perpendicular shock configurations were more likely to be found on the portions of the shock propagating at low altitude, and when the shock is traveling through high-density regions, such as coronal streamers, resulting in high-frequency metric type II radio emission.…”