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Estimation of a Coronal Mass Ejection Magnetic Field Strength using Radio Observations of Gyrosynchrotron Radiation  

Eoin Carley   Submitted: 2017-09-18 04:01

Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the low solar corona into interplanetary space. These eruptions are often associated with the acceleration of energetic electrons which produce various sources of high intensity plasma emission. In relatively rare cases, the energetic electrons may also produce gyrosynchrotron emission from within the CME itself, allowing for a diagnostic of the CME magnetic field strength. Such a magnetic field diagnostic is important for evaluating the total magnetic energy content of the CME, which is ultimately what drives the eruption. Here we report on an unusually large source of gyrosynchrotron radiation in the form of a type IV radio burst associated with a CME occurring on 2014-September-01, observed using instrumentation from the Nançay Radio Astronomy Facility. A combination of spectral flux density measurements from the Nançay instruments and the Radio Solar Telescope Network (RSTN) from 300 MHz to 5 GHz reveals a gyrosynchrotron spectrum with a peak flux density at >1 GHz. Using this radio analysis, a model for gyrosynchrotron radiation, a non-thermal electron density diagnostic using the Fermi Gamma Ray Burst Monitor (GBM) and images of the eruption from the GOES Soft X-ray Imager (SXI), we are able to calculate both the magnetic field strength and the properties of the X-ray and radio emitting energetic electrons within the CME. We find the radio emission is produced by non-thermal electrons of energies >1MeV with a spectral index of δ∼3 in a CME magnetic field of 4.4 G at a height of 1.3 R⊙, while the X-ray emission is produced from a similar distribution of electrons but with much lower energies on the order of 10 keV. We conclude by comparing the electron distribution characteristics derived from both X-ray and radio and show how such an analysis can be used to define the plasma and bulk properties of a CME.

Authors: Eoin P. Carley, Nicole Vilmer, Paulo J. A. Simões, Brían Ó Fearraigh
Projects: None

Publication Status: In press
Last Modified: 2017-09-20 09:04
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Radio Diagnostics of Electron Acceleration Sites During the Eruption of a Flux Rope in the Solar Corona  

Eoin Carley   Submitted: 2016-09-07 01:46

Electron acceleration in the solar corona is often associated with flares and the eruption of twisted magnetic structures known as flux ropes. However, the locations and mechanisms of such particle acceleration during the flare and eruption are still subject to much investigation. Observing the exact sites of particle acceleration can help confirm how the flare and eruption are initiated and how they evolve. Here we use the Atmospheric Imaging Assembly to analyse a flare and erupting flux rope on 2014-April-18, while observations from the Nancay Radio Astronomy Facility allows us to diagnose the sites of electron acceleration during the eruption. Our analysis shows evidence for a pre-formed flux rope which slowly rises and becomes destabilised at the time of a C-class flare, plasma jet and the escape of >75 keV electrons from rope center into the corona. As the eruption proceeds, continued acceleration of electrons with energies of ~5 keV occurs above the flux rope for a period over 5 minutes. At flare peak, one site of electron acceleration is located close to the flare site while another is driven by the erupting flux rope into the corona at speeds of up to 400 km s-1. Energetic electrons then fill the erupting volume, eventually allowing the flux rope legs to be clearly imaged from radio sources at 150-445MHz. Following the analysis of Joshi et al. (2015), we conclude that the sites of energetic electrons are consistent with flux rope eruption via a tether-cutting or flux cancellation scenario inside a magnetic fan-spine structure. In total, our radio observations allow us to better understand the evolution of a flux rope eruption and its associated electron acceleration sites, from eruption initiation to propagation into the corona.

Authors: Eoin Carley, Nicole Vilmer, and Peter Gallagher
Projects: None

Publication Status: Accepted
Last Modified: 2016-09-07 12:11
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Low frequency radio observations of bi-directional electron beams in the solar corona  

Eoin Carley   Submitted: 2015-08-06 02:57

The radio signature of a shock travelling through the solar corona is known as a type II solar radio burst. In rare cases these bursts can exhibit a fine structure known as 'herringbones', which are a direct indicator of particle acceleration occurring at the shock front. However, few studies have been performed on herringbones and the details of the underlying particle acceleration processes are unknown. Here, we use an image processing technique known as the Hough transform to statistically analyse the herringbone fine structure in a radio burst at ~20-90 MHz observed from the Rosse Solar-Terrestrial Observatory on 2011 September 22. We identify 188 individual bursts which are signatures of bi-directional electron beams continuously accelerated to speeds of 0.16 [-0.10, +0.11] c. This occurs at a shock acceleration site initially at a constant altitude of ~0.6 Rsun in the corona, followed by a shift to ~0.5 Rsun. The anti-sunward beams travel a distance of 170 [-97, +174] Mm (and possibly further) away from the acceleration site, while those travelling toward the sun come to a stop sooner, reaching a smaller distance of 112 [-76, +84] Mm. We show that the stopping distance for the sunward beams may depend on the total number density and the velocity of the beam. Our study concludes that a detailed statistical analysis of herringbone fine structure can provide information on the physical properties of the corona which lead to these relatively rare radio bursts.

Authors: Eoin P. Carley, Hamish Reid, Nicole Vilmer, Peter T. Gallagher
Projects: None

Publication Status: Accepted
Last Modified: 2015-08-06 08:46
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Quasiperiodic acceleration of electrons by a plasmoid-driven shock in the solar atmosphere  

Eoin Carley   Submitted: 2013-10-10 02:34

Cosmic rays and solar energetic particles may be accelerated to relativistic energies by shock waves in astrophysical plasmas. On the Sun, shocks and particle acceleration are often associated with the eruption of magnetized plasmoids, called coronal mass ejections (CMEs). However, the physical relationship between CMEs and shock particle acceleration is not well understood. Here, we use extreme ultraviolet, radio and white-light imaging of a solar eruptive event on 22 September 2011 to show that a CME-induced shock (Alfvén Mach number 2.4[+0.7,-0.8]) was coincident with a coronal wave and an intense metric radio burst generated by intermittent acceleration of electrons to kinetic energies of 2?46 keV (0.1?0.4 c). Our observations show that plasmoid-driven quasiperpendicular shocks are capable of producing quasiperiodic acceleration of electrons, an effect consistent with a turbulent or rippled plasma shock surface.

Authors: Eoin P. Carley, David M. Long, Jason P. Byrne, Pietro Zucca, D. Shaun Bloomfield, Joseph McCauley and Peter T. Gallagher
Projects: SDO-AIA

Publication Status: Published
Last Modified: 2013-10-11 12:22
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Coronal Mass Ejection Mass, Energy, and Force Estimates Using STEREO  

Eoin Carley   Submitted: 2012-04-23 04:53

Understanding coronal mass ejection (CME) energetics and dynamics has been a long-standing problem, and although previous observational estimates have been made, such studies have been hindered by large uncertainties in CME mass. Here, the two vantage points of the Solar Terrestrial Relations Observatory (STEREO) COR1 and COR2 coronagraphs were used to accurately estimate the mass of the 2008 December 12 CME. Acceleration estimates derived from the position of the CME front in 3-D were combined with the mass estimates to calculate the magnitude of the kinetic energy and driving force at different stages of the CME evolution. The CME asymptotically approaches a mass of 3.4pm1.0x1015 g beyond ~10 R_sun. The kinetic energy shows an initial rise towards 6.3pm3.7x1029 erg at ~3 R_sun, beyond which it rises steadily to 4.2pm2.5x1030 erg at ~18 R_sun. The dynamics are described by an early phase of strong acceleration, dominated by a force of peak magnitude of 3.4pm2.2x1014N at ~3 R_sun, after which a force of 3.8pm5.4x1013 N takes affect between ~7-18 R_sun. These results are consistent with magnetic (Lorentz) forces acting at heliocentric distances of <7 R_sun, while solar wind drag forces dominate at larger distances (>7 R_sun).

Authors: Eoin P. Carley, R.T. James McAteer, Peter T. Gallagher
Projects: STEREO

Publication Status: Accepted
Last Modified: 2012-04-23 09:20
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Abstracts by Author
Estimation of a Coronal Mass Ejection Magnetic Field Strength using Radio Observations of Gyrosynchrotron Radiation
Radio Diagnostics of Electron Acceleration Sites During the Eruption of a Flux Rope in the Solar Corona
Low frequency radio observations of bi-directional electron beams in the solar corona
Quasiperiodic acceleration of electrons by a plasmoid-driven shock in the solar atmosphere
Coronal Mass Ejection Mass, Energy, and Force Estimates Using STEREO

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