Quasiperiodic acceleration of electrons by a plasmoid-driven shock in the solar atmosphere

Journal name:
Nature Physics
Volume:
9,
Pages:
811–816
Year published:
DOI:
doi:10.1038/nphys2767
Received
Accepted
Published online

Abstract

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 ) was coincident with a coronal wave and an intense metric radio burst generated by intermittent acceleration of electrons to kinetic energies of 2–46keV (0.1–0.4c). 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.

At a glance

Figures

  1. AIA [thinsp]21.1[thinsp]nm images over-plotted with NRH 150.9[thinsp]MHz contours.
    Figure 1: AIA 21.1nm images over-plotted with NRH 150.9MHz contours.

    af, The 150MHz source follows closely the CBF as it propagates around the east limb, indicating that they belong to a common structure. The intensity of the radio source is indicated by the colour bar on the right, showing the brightness temperature (TB) of the source ranges between ~107 and 109K. Such high intensities are indicative of coherent plasma emission produced by high-energy electron beams. c, The role of the CME in the event, as observed by the LASCO C2 coronagraph. The combination of the white-light coronagraph (C2) and the EUV images (AIA) reveals the full spatial extent of the CME bubble; that is, the frontal structure in white light has clear extensions back towards the solar surface, imaged at EUV. The locations of the radio source and CBF show that they clearly have a relationship with the southward CME flank. The dashed pink lines indicate the predicted height range of the radio emissions observed in the Nançay Decametric Array dynamic spectrum (Fig. 3b). A movie of this figure is available in Supplementary Movie 1.

  2. Position angle (degrees anticlockwise from solar north) versus time for the 150[thinsp]MHz source, shown in plus signs, and CBF at 1[thinsp]Ro (circles) and 1.27[thinsp]Ro (diamonds).
    Figure 2: Position angle (degrees anticlockwise from solar north) versus time for the 150MHz source, shown in plus signs, and CBF at 1R (circles) and 1.27R (diamonds).

    The great circle along which the CBF was tracked at 1R is indicated by the dashed black line in the inset; the dashed pink line marks a height of 1.27R. Both the radio burst and CBF have a consistent propagation in the same direction and have similar speeds at a height of 1.27R, implying that they belong to a common propagating coronal structure. The uncertainty on radio source position angle is taken to be from 1σ uncertainties of the source width (~7°) plus the fluctuation of source position due to coronal and ionospheric scattering effects (3° at frequencies up to 160MHz; ref. 49). The CBF position uncertainty is from Gaussian centroid uncertainty from a tracking and fitting algorithm of the CBF pulse50.

  3. Radio dynamic spectra from STEREO-B/WAVES (0.01-16[thinsp]MHz), Nancay DA (20-90[thinsp]MHz) and RSTO eCallisto (10-400[thinsp]MHz).
    Figure 3: Radio dynamic spectra from STEREO-B/WAVES (0.01–16MHz), Nançay DA (20–90MHz) and RSTO eCallisto (10–400MHz).

    a, Composite dynamic spectrum showing interplanetary type III bursts. b, A zoom of the Nancay DA and eCallisto dynamic spectra, showing the type II radio burst, with both fundamental and harmonic emission. This shock signature is characterized by two emission bands drifting slowly (−0.2MHzs−1) towards lower frequency over time. Negative frequency drift in dynamic spectra is a result of plasma emission occurring at decreasing density with respect to time. This is due to the emission exciter travelling to larger heights (lower densities) in the solar atmosphere; as the density drops, so too does the frequency of plasma emission. The type III bursts observed by S/WAVES are also observed in Nancay DA, as indicated in b. c, A zoom of the the herringbone radio bursts, observed by RSTO eCallisto. Each herringbone or spike is indicative of an electron beam travelling away from the shock. Note that all of the radio activity from a to c is indicative of either particle acceleration or a plasma shock in the corona. The start and stop times of this radio activity in these dynamic spectra show good temporal correspondence with the start/stop times of the activity in Fig. 1. This is especially apparent for the features between 100 and 200MHz; the correspondence between dynamic spectral activity and imaging activity is most apparent in Supplementary Movie 1.

  4. White-light CME observations and 3D reconstruction of the CME front.
    Figure 4: White-light CME observations and 3D reconstruction of the CME front.

    a, Top-down view of the Heliocentric Earth Equatorial system, showing the separations and locations of STEREO-B and SOHO spacecraft with respect to the Sun. b, LASCO/C2 base-differenced image of the CME (logged intensity scale), with AIA 21.1nm image inset. White crosses indicate a point-and-click along the CME front with a corresponding ellipse fit in blue, where the solid lines indicate the major and minor axes, and dashed lines indicate the apex points back towards the Sun centre. The white circles indicate the white-light shock. The red asterisk points indicate the northern and southern flanks of the CME. c, Base difference image of the CME from the COR1-B coronagraph, with a corresponding ellipse fit and EUVI 19.5nm image inset. The red lines are the red asterisk points in b projected as lines-of-sight across the COR1 field of view. d, 3D reconstruction of the CME with the white-light shock indicated on the plane of sky (only 2D information is available for this feature). The red dotted lines are the projected points from the ellipse on the C2 image, and the blue dotted lines are the projected points from the ellipse on the COR1 image. The black ellipses are those inscribed in the resulting quadrilateral slices using the elliptical tie-pointing method for 3D CME reconstruction, as described in ref. 1.

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Author information

Affiliations

  1. Astrophysics Research Group, School of Physics, Trinity College Dublin, Dublin 2, Ireland

    • Eoin P. Carley,
    • Pietro Zucca,
    • D. Shaun Bloomfield,
    • Joseph McCauley &
    • Peter T. Gallagher
  2. Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking RH5 6NT, UK

    • David M. Long
  3. Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA

    • Jason P. Byrne

Contributions

E.P.C. performed the data analysis of the radio source kinematics, the radio burst analysis, the Alfvén Mach number calculations, and the in situ particle analysis. E.P.C. also wrote the article. D.M.L. performed the data analysis of the coronal bright front and gave constructive advice on the writing of the article. J.P.B. performed the 3D reconstruction of the CME and gave advice on the white-light shock analysis section. P.Z. provided the density maps, and D.S.B. provided the magnetic field maps that were used in the radio source and CBF Mach number calculations. J.M. installed the electronic systems at RSTO. P.T.G. conceived of the project and guided data analysis and writing of the article.

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