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

Abstract

Citation:Schmidt, J. M., and I. H. Cairns (2014), Type II solar radio bursts predicted by 3-D MHD CME and kinetic radio emission simulations, J. Geophys. Res. Space Physics, 119, 69-87, doi:10.1002 Abstract 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.

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

“…In Figure a, we display our simulation results for the fundamental and harmonic emission caused by the second CME's shock for times 40 to 160 min on 7 March 2012. These predictions are identical to those of Schmidt and Cairns [] for the fundamental emission but show the harmonic emission for the first time. Here, we extend the investigation of the second CME to include harmonic and interplanetary emission.…”

“…The second island has an intensity of ∼10 −20 W m −2 Hz −1 . As our simulations show (see below and [ Schmidt and Cairns , ]), the growth of an island is related to regions with large radio volume emissivity increasing in volume due to variations in the shock's ability to accelerate electrons and in the conversion of electron energy into Langmuir waves and radio emission. These variations are due to the shock moving through the spatially varying corona and solar wind.…”

“…These processes are described quantitatively in a theory developed mostly at the University of Sydney [ Knock et al , , , ; Knock and Cairns , ; Florens et al , ; Schmidt and Gopalswamy , ; Hillan et al , , ; Schmidt and Cairns , , , ; Schmidt et al , ]. Schmidt and Cairns [, ] made the theory fully analytic.…”

“…We combine (or bolt on) this radio radiation theory with the BATS‐R‐US 3‐D magnetohydrodynamic (MHD) simulation code [ Powell et al , ; Tôth et al , ; Schmidt and Cairns , , , ; Schmidt et al , ] in order to simulate a type II radio burst. BATS‐R‐US can simulate realistic three‐dimensional CMEs and associated shock waves launched from solar active regions into an empirical solar wind.…”

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

“…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 , ].…”