Glasgow Callisto optimistic: first light comes in focus

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Hard X-Ray (HXR) bursts and Type III radio bursts are some of the commonest solar emissions caused by energetic electrons. In the well known solar flare cartoon, electron acceleration produces beams propagating both downwards and upwards along magnetic field lines. The downwards going beam propagates along closed loops and produces HXR emission by collisional bremsstrahlung. The signature of the upwards going beam is a Type III radio burst, which occurs at the local plasma frequency or its harmonics. As the beam propagates away from the sun along open magnetic field lines, it encounters plasma of rapidly decreasing density, so emission occurs at rapidly decreasing frequency, giving TIIIs a characteristic "hockey stick" shape.

A Callisto radio spectrometer was installed at Glasgow's Acre Road Observatory on October 2nd, as shown in the video, and began observing a few days later. Here we present the first light observations and a second burst observed simultaneously by Callisto and RHESSI.

Type III burst observations with Glasgow Callisto

Figure 1: The first radio event captured with Glasgow Callisto (with averaged background subtracted) and with Fermi

Fig 1 shows the first light observation, a probable Type III burst, identified by its frequency range and rapid frequency drift. Observations from other sites in the Callisto network help to confirm the identification. RHESSI did not observe this first burst, but data from the Fermi GBM are available, and are also shown in the figure. Simultaneous observations of HXR and Type III emission such as this provide evidence for a common acceleration mechanism for the beams producing the two types.


Slightly over an hour later Glasgow Callisto detected a second burst, also identified as a Type III, for which RHESSI data are available, shown in Fig. 2. This event is visible in the 12-25 keV channel, as well as the lower energies.

The e-Callisto network

The Glasgow Callisto antenna is one part of a larger network, named e-Callisto. The spectrometer is constructed from parts costing less than Euro 200. Data are available here. Other sites range across the globe, in countries including Switzerland, India, Alaska, Australia, Kenya, meaning the network as a whole offers nearly 24 hour observations of the sun across the entire year. All of the sites suffer some interference from nearby radio sources, the severity of which varies by location. The Glasgow site has a reasonable background level once the frequencies from e.g airport Air Traffic Control are excluded. Strong events are visible, such as the two discussed here, and in these cases data from the various locations complement each other.

A brief survey of the first 10 days of Glasgow Callisto data show a few TIII observations with simultaneous RHESSI detections, and several without. Conversely, several RHESSI events are not associated with detections at Glasgow. Comparison with other Callisto sites shows that this is partly due to the Glasgow site missing events which are visible elsewhere, and partly to the relatively low correlation between observed HXR sources, and Type IIIs.


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