Solar Observing

Observations of the Sun can currently be performed in five frequency bands: 1-2 GHz (L band), 2-4 GHz (S band), 4-8 GHz (C band), 8-12 GHz (X band), and 12-18 GHz (Ku band). To observe the Sun, any 8-bit resource using these bands can be used in Solar mode (see below). As the Sun is moving across the sky, the source position should be defined in the source catalog.

If the center of the solar disk needs to be tracked, select the Sun using the drop-down for "Solar System Body with Internal Ephemeris". Otherwise, to allow for specifying the location of interest on or near the solar disk and the differential rotation model to be used for tracking the source, solar observations require selection of a source position type of "Solar System Body with Uploaded Ephemeris". An ephemeris file can be generated using the guidelines given in the Moving Object section or, for example, by using the ALMA Solar Ephemeris Generator. 

The user must select the “Solar” scan mode when specifying scan details under Observation Preparation in the OPT. This scan mode ensures that the necessary attenuators are switched into the signal path when observing the Sun and that special noise cal sources are employed in the switched power system; this allows users to flux calibrate their data. Two observing modes are currently supported:

• For quiet Sun observations, when no strong active regions or flares expected, users should select a scan mode of “Solar Attenuators with Low Noise Internal Cal”.

• If the science objective involves observing a strong active region and/or flare activity, users should select a scan mode of “Solar Attenuators with High Noise Internal Cal”.

These selections ensure that cal noise levels of these scans are of order a few percent of the anticipated system temperature. (A third scan mode, “Noise Reverse Coupler Setup (L Band only)”, is not yet supported.)

Solar observations rely on the switched power system to flux calibrate the data. This, in turn, requires the use of 8-bit sampling modes in the frequency bands that support solar observing. Solar observing is such that input power levels may change from scan to scan, particularly for active Sun targets. This being the case each source scan requires a setup scan to trigger the system to reset the requantizer gain.

Considerations for the calibration of solar observations are otherwise quite similar to standard gain calibration. Phase calibration proceeds in the same manner for solar observations as it does for general observing. However, it is important to be aware of two factors: First, when observing a calibrator source, the attenuation used when observing the Sun is removed from the signal path; the RFI that is suppressed, to a large degree, when observing the Sun is fully present in observations of calibrators. Second, when the attenuators are removed, solar radiation may be present in the sidelobes of the primary beam if the phase calibrator is too close to the Sun, particularly if the Sun is in an active phase. Therefore, it is advisable to observe phase calibrators that are further in angular distance from the source than would normally be used.