11. Tracking CME effects from “the Sun to the mud”

Authors:  Peter Gallagher, Pietro Zucca, Eoin Carley and Joe McCauley are members of the Astrophysics Research Group at Trinity College Dublin, Ireland.

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Figure 1: The CME-shock relationship in the low corona. It is still unclear at what height the shock forms and what its stand-off distance is forward of the CME leading edge.

Introduction

Coronal mass ejections (CMEs) are spectacular eruptions of plasma and magnetic field from the surface of the Sun into the heliosphere. Traveling at speeds of up to ~2,500 km/s and with masses ~1016 g, they often form shocks in the solar corona, and accelerate particles into interplanetary space.  Radio waves at MHz frequencies provide the first signature of CME-driven shocks (Type IIs) and electron beams escaping along open magnetic field lines (Type III bursts). Type II bursts are generated by Langmuir waves excited in CME-driven shocks (Figure 1). The shocks form in the low corona at heights of  < 3 solar radii, when the CME velocity exceeds the characteristic speed of the plasma through which it is moving – the coronal Alfven speed [1].

The Rosse Solar-Terrestrial Observatory

In order to study Type II radio bursts and their relationship with CMEs, in September 2010 we set up an autonomous solar radio observing station, the Rosse Solar-Terrestrial Observatory (Figure 2). RSTO is located in the grounds of Birr Castle, Co. Offaly, Ireland and was named for William Parsons, the 3rd Earl of Rosse, who constructed the 6-foot diameter Leviathan of Parsonstown in the 1840s. The 3rd Earl, who was also Chancellor of Trinity College Dublin, is regarded as one of the greatest astronomers of the 19th century, using the Leviathan to resolve the spiral structure of what would become known as the Whirlpool Galaxy (M51). He is also credited with the naming of the Crab Nebula (M1). The Leviathan was the largest telescope in the World between 1845 and 1917, when the Hooker Telescope was constructed on Mount Wilson in California, USA.

Two radio instruments have now been installed at RSTO [2], namely CALLISTO, a solar radio burst monitor with two receivers in the range of 20-90 MHz and 45-870 MHz, and AWESOME, an ionospheric disturbance monitor operating in the range of 0.3-50 kHz.

Figure 2: At top is shown a panoramic view of the RSTO site. The RSTO control room (aka the “Sheep Shed Control Room”) is located at the centre of the image. The RSTO CALLISTO antenna is shown at bottom right.

I’ve got a herringbone to pick with you …

On 12 March 2011 at 15:26:20 UT, a Type II radio burst was first detected by both receivers of the RSTO CALLISTO (Figure 3). Assuming that the radio emission is due to plasma oscillations, the electron density of the emitting region can be estimated from

fp = 9000 √ne Hz

where fp is the observed radio frequency and ne is the density in electrons per cubic centimetre. The height of the emission can then be estimated by choosing from one of a number of theoretical or semi-empirical models to map the electron density onto a height in the corona. These models can produce heights that differ by more than a factor of 2 or 3, making it challenging to estimate the actual height that a Type II is generated at and hence making in near-impossible to say anything about their spatial relationship with the associated CME. In the case of the Type II in Figure 3, all that we can say with some certainty is that its velocity (~711 km/s) is comparable to the associated CME observed by SOHO/LASCO (~672 km/s).

Figure 3: A Type II radio burst with both fundamental emission from ~125-50MHz and harmonic emission from ~250-50 MHz observed by the RSTO CALLISTO.

Conclusions

Solar radio astronomy offers an important diagnostic of plasma processes related to solar flares and CMEs. Key to untangling the complex spatial relationship between CMEs and Type IIs are radio interferometers such as LOFAR and FASR. When combined with images from space-based observatories, such as NASA’s STEREO spacecrafts [3], radio spectra provide an additional method for understanding CME shocks from the Sun to the mud, and in our case, the mud in Co. Offaly!

References

  • [1] Mann, G., Klassen, A., Aurass, H., Classen, H.-T., “Formation and development of shock waves in the solar corona and the near-Sun interplanetary space”, Astronomy & Astrophysics, 400, p.329-336 (2003)
  • [2] Zucca, P., Carley, E., McCauley, J., Gallagher, P. T., Monstein, C, McAteer, R. T. J., “Space Weather Monitoring at the Rosse Solar-Terrestrial Observatory, Birr Castle, Ireland”, submitted to the Journal of Space Weather and Space Climate, 2011.
  • [3] Maloney, S. A., Gallagher, P. T., “STEREO Direct Imaging of an Interplanetary Shock Wave to 0.5 AU”, Astrophysical Journal Letters, in review, 2011.