Glasgow Callisto and CMEless type II bursts

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Number: 246
1st Author: Peter Wakeford
2nd Author: Hugh Hudson
Published: February 16, 2015
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The techniques of radio astronomy let us study many aspects of solar flares and coronal mass ejections (CMEs). Generally these observations reflect dramatic plasma effects in the solar atmosphere, such as particle acceleration.

In the conventional picture, a CME traveling out of the lower solar atmosphere and into the solar wind drives a coronal shock wave that produces a type II ("slow drift") radio burst, which then accelerates solar energetic particles (SEPs) that can fill the heliosphere and often cause trouble for spacecraft. In an earlier Nugget we noted the development of NOAA active region 2192, in October 2014; this region produced a remarkable series of major flares but few CMEs and no SEPs. We were thus quite surprised to find that our Glasgow Callisto radio observatory had actually detected type II bursts from some of these flares.

e-Callisto and Glasgow Callisto

The e-Callisto project is a global development of over 70 solar radio spectrometers scattered widely around the world, organized brilliantly from Switzerland by Christian Monstein. By providing an inexpensive and easily maintained receiver, this project aims to provide full-time broad-band monitoring of solar radio bursts. The Glasgow CALLISTO site, initiated in 2012, uses a log-periodic antenna on an tracking mount, allowing automated observation throughout the day at frequencies between 47 and 80 MHz. Few sites within the larger e-Callisto network observe at such long wavelengths, but they turn out to be crucial for the detection of the type II solar radio bursts.

Glasgow Callisto
Figure 1: A type II solar radio burst, SOL2014-10-21T12, observed at the Glasgow Callisto site. This is a spectrogram showing the solar flux density as a function of frequency and time. The near-vertical features are type III bursts, produced by streams of fast electrons; the faint diagonal line drifting slowly to lower frequency is the type II burst. Standard interpretation would associate this with a global MHD wave driven by a CME; in this case it's probably a blast wave instead.

Figure 1 shows the event that caught our attention: SOL2014-10-21T12 (C4.4) (see our explanatory Nugget for this nomenclature). Such events rarely happen with such feeble flares, and in this case there were two events in quick succession from the same active region, but minimal CME development. EUV movies from the SDO AIA telescope, and other sources, showed these flare disturbances to have narrowly focused ejecta, suggestive of plasma flow along existing field large-scale (open) magnetic fields, rather than eruption of the field itself as in a classical CME. Normally we would identify these disturbances as jets and appeal to a different kind of physics for the explanation. Nevertheless, well-developed type II bursts resulted.

Light curves
Figure 2: CALLISTO light curve at 55MHz. The sharp spikes and main peak are from type III emission, and bumps peaking at 12:31 and 12:41 are harmonics of type II emission.

Figure 2 shows the the radio emission at 55 MHz from this event as observed by the Glasgow CALLISTO. Strong type III bursts can be seen peaking at approximately 12:21, 12:25 and 12:27 UT. These are identifiable with outward-moving streams of near-relativistic electrons. Using plasma density and magnetic field estimates, we calculate that the electrons were travelling at 10% of the speed of light. Type II emission is also observed, which is typically associated with slower-traveling shock fronts. The harmonics of type II emission peak at about 12:31 and 12:41 UT. The 12:41 peak corresponds to the trace of the diagonal band seen in the spectrogram of Figure 3. This harmonic splitting is a common property, interpreted as the ability of the traveling shock wave to emit radiation both at the plasma frequency ωpe and at its first harmonic 2 ωpe. The Type II (or II-like) spectral signatures appear very well in the the data from the decametric array at Nançay.

Light curves
Figure 3: Glasgow CALLISTO spectrogram, stretched to enhance type II emission, including the harmonic.

Were there really no CMEs associated with these two events? A light overview of the LASCO data is here at the CACTus site, and it indeed shows coronal disturbances (as alerted by the Callisto data). They are clearly not CMEs in the classical sense in either case, but these multi-wavelength data cry out for a careful analysis.


Live data from the Glasgow CALLISO can be seen here, and back data can be downloaded by following the links. CALLISTO data can be analysed using IDL routines, or using the nice new Python package SunPy. The Glasgow data are also available automatically on the e-Callisto home page, which allows one to scroll through the many sites with different glimpses of the same events. In this case another Callisto site on a nearby longitude, near the historically famous Birr Castle in Ireland, also observed the type III bursts; the same site has a LOFAR test array, which overlaps the Glasgow Callisto frequencies and also showed the type II burst. This e-Callisto redundancy, resulting from its global nature, is a great advantage.


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