The Broken Lane of a Type II Radio Burst Caused by Collision of a Coronal Shock with a Flare Current Sheet

Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20–90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs. Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon. Methods. Thetype II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory(STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary. Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with θBn ~ 70°. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3–1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4–1.6.

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A BROKEN SOLAR TYPE II RADIO BURST INDUCED BY A CORONAL SHOCK PROPAGATING ACROSS THE STREAMER BOUNDARY

The Astrophysical Journal ◽

10.1088/0004-637x/750/2/158 ◽

2012 ◽

Vol 750 (2) ◽

pp. 158 ◽

Cited By ~ 31

Author(s):

X. L. Kong ◽

Y. Chen ◽

G. Li ◽

S. W. Feng ◽

H. Q. Song ◽

...

Keyword(s):

Radio Burst ◽

Type Ii ◽

Coronal Shock ◽

Solar Type

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Real-time forecasting of ICME shock arrivals at L1 during the "April Fool’s Day" epoch: 28 March – 21 April 2001

Annales Geophysicae ◽

10.5194/angeo-20-937-2002 ◽

2002 ◽

Vol 20 (7) ◽

pp. 937-945 ◽

Cited By ~ 26

Author(s):

W. Sun ◽

M. Dryer ◽

C. D. Fry ◽

C. S. Deehr ◽

Z. Smith ◽

...

Keyword(s):

Real Time ◽

Radio Burst ◽

Type Ii ◽

Interplanetary Shocks ◽

Arrival Times ◽

Interplanetary Coronal Mass Ejection ◽

Coronal Shock ◽

Ex Post ◽

Interplanetary Physics ◽

Ex Post Facto

Abstract. The Sun was extremely active during the "April Fool’s Day" epoch of 2001. We chose this period between a solar flare on 28 March 2001 to a final shock arrival at Earth on 21 April 2001. The activity consisted of two presumed helmet-streamer blowouts, seven M-class flares, and nine X-class flares, the last of which was behind the west limb. We have been experimenting since February 1997 with real-time, end-to-end forecasting of interplanetary coronal mass ejection (ICME) shock arrival times. Since August 1998, these forecasts have been distributed in real-time by e-mail to a list of interested scientists and operational USAF and NOAA forecasters. They are made using three different solar wind models. We describe here the solar events observed during the April Fool’s 2001 epoch, along with the predicted and actual shock arrival times, and the ex post facto correction to the real-time coronal shock speed observations. It appears that the initial estimates of coronal shock speeds from Type II radio burst observations and coronal mass ejections were too high by as much as 30%. We conclude that a 3-dimensional coronal density model should be developed for application to observations of solar flares and their Type II radio burst observations.Key words. Interplanetary physics (flare and stream dynamics; interplanetary shocks) – Magnetosheric physics (storms and substorms)

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Analyzing the propagation of EUV waves and their connection with type II radio bursts by combining numerical simulations and multi-instrument observations

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038699 ◽

2020 ◽

Vol 644 ◽

pp. A90

Author(s):

A. Koukras ◽

C. Marqué ◽

C. Downs ◽

L. Dolla

Keyword(s):

Magnetic Field ◽

Wave Front ◽

Ray Tracing ◽

Radio Burst ◽

Radio Bursts ◽

Type Ii ◽

Fast Mode ◽

Type Ii Radio Bursts ◽

The Magnetic Field ◽

Burst Emission

Context. EUV (EIT) waves are wavelike disturbances of enhanced extreme ultraviolet (EUV) emission that propagate away from an eruptive active region across the solar disk. Recent years have seen much debate over their nature, with three main interpretations: the fast-mode magneto-hydrodynamic (MHD) wave, the apparent wave (reconfiguration of the magnetic field), and the hybrid wave (combination of the previous two). Aims. By studying the kinematics of EUV waves and their connection with type II radio bursts, we aim to examine the capability of the fast-mode interpretation to explain the observations, and to constrain the source locations of the type II radio burst emission. Methods. We propagate a fast-mode MHD wave numerically using a ray-tracing method and the WKB (Wentzel-Kramers-Brillouin) approximation. The wave is propagated in a static corona output by a global 3D MHD Coronal Model, which provides density, temperature, and Alfvén speed in the undisturbed coronal medium (before the eruption). We then compare the propagation of the computed wave front with the observed wave in EUV images (PROBA2/SWAP, SDO/AIA). Lastly, we use the frequency drift of the type II radio bursts to track the propagating shock wave, compare it with the simulated wave front at the same instant, and identify the wave vectors that best match the plasma density deduced from the radio emission. We apply this methodology for two EUV waves observed during SOL2017-04-03T14:20:00 and SOL2017-09-12T07:25:00. Results. The simulated wave front displays a good qualitative match with the observations for both events. Type II radio burst emission sources are tracked on the wave front all along its propagation. The wave vectors at the ray-path points that are characterized as sources of the type II radio burst emission are quasi-perpendicular to the magnetic field. Conclusions. We show that a simple ray-tracing model of the EUV wave is able to reproduce the observations and to provide insight into the physics of such waves. We provide supporting evidence that they are likely fast-mode MHD waves. We also narrow down the source region of the radio burst emission and show that different parts of the wave front are responsible for the type II radio burst emission at different times of the eruptive event.

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