Solar ALMA Frequently Asked Questions
- Can I select the observing frequency for solar observations?
- Can I use an FDM mode for (spectral) solar observations?
- Which polarization products are available for solar observing?
- Do I select active Sun or quiet Sun for e.g. prominences?
- How long can I observe the Sun continuously (without interruption)?
- What is the highest time cadence of solar images?
- What happens if there is no target visible on the Sun, of the type I want to observe?
- How do I determine the total observing time for my solar proposal?
- Is the feathering process for combining total power and interferometric data for solar observations different from other observations?
Can I select the observing frequency for solar observations?
No. The LO1 frequency for solar observations is fixed to 100 GHz for Band3 and 239 GHz for Band6 in Cycle4.
Can I use an FDM mode for (spectral) solar observations?
No. In Cycle 4, only continuum observing in TDM mode is offered.
Which polarization products are available for solar observing?
In Cycle 4, only linear XX and YY polarization products will be delivered. Cross-correlations (circular polarization) will not be available for solar observations in Cycle 4.
Do I select active Sun or quiet Sun for e.g. prominences?
Quiet Sun mode is designed for the normal quiet Sun brightness level, while active Sun mode is for phenomena with higher brightness like active regions and flares. Current models predict that prominences and filaments should have brightness roughly equal to the quiet Sun level at ALMA frequencies so quiet Sun would be a good choice in that case.
How long can I observe the Sun continuously (without interruption)?
To calibrate the phase, a quasar has to be observed every 10 minutes in Band3 and Band6. Therefore, the duration of a continuous solar observation is 10 minutes. The calibration takes around 3 minutes and then another continuous observation can take place.
What is the highest time cadence of solar images?
When you request an observation with single-pointing, the time cadence of images is limited by the integration time of the 64-bit correlator. Since the integration time is set to 2 seconds in Cycle4, the highest time cadence is 2 seconds. When you request a mosaic observation, the time cadence of mosaic images depends on the size of the field, i. e. number of points in the field of view. Since the mosaic observation for one point takes 7.6 seconds (integration time + overhead) for mosaic observations, the time cadence is 7.6 seconds times the number of points. Also note that phase calibration scans are regularly inserted during the scanning of a mosaic image and take ~3 minutes.
Should I submit the ephemeris file in the proposal?
Yes, you should submit a dummy ephemeris file in your proposal. If your proposal is accepted, you will be contacted before the actual observation to provide the ephemeris file for the target you wish to observe. The target should correspond to the type you specified in your proposal.
How do I generate the ephemeris file for my target?
Use the ALMA Solar Ephemeris Generator Tool located at Czech ARC Node web pages: http://www.asu.cas.cz/alma.
What happens if there is no target visible on the Sun, of the type I want to observe?
For short-lived structures and unpredictable dynamic phenomena like flares, your observation will be carried according to the schedule (coordinated by ALMA staff based on PI requests). If the data passes quality check it will be provided to you even if the phenomenon did not occur or disappeared before/during observation. For longer living structures like sunspots, if there is none visible at the time of observation, the observation will not be carried out. If your proposal is ranked as Grade A and your target is not visible during the whole period of the solar campaigns in the cycle, your observation will be carried over to the next cycle.
How do I determine the total observing time for my solar proposal?
First, we mention the limitations of ALMA observations; 1) The longest duration of one scheduling block (SB) is 2 hours. 2) Every SB includes the calibrating session before starting scientific observation. The duration of the session is 25~30 minutes. Therefore, the total duration of on-source (solar observation) in a SB is about 1.5 hours. 3) The total observing time for one proposal must be less than 50 hours, because a solar proposal is categorized to ‘Regular Proposal’ of ALMA. When you determine the total observing time for your project, you have to consider these limitations.
The total observing time of a solar project have to be determined from the scientific point of view. For example, when your target can be found on the Sun at any time, the total observing time might be based on its lifetime. On the other hand, when your target occurs only occasionally, the total observing time might be based on its occurrence frequency. As an example, we consider the proposal to detect the oscillation on an umbra with ALMA. Since umbra oscillations always exist in a sunspot, the total observing time might be based on the period of the oscillation. Reznikova et al. (2012) reported the frequency of the umbra oscillation at 17GHz is around 6-7 mHz (~3 minutes). If we assume that we need to observe 100 periods of the oscillation to achieve our science goal, the required on-source time becomes 5 hours. As mention above, the duration of one SB is limited to 2 hours and the on-source time in one SB is 1.5 hours. Therefore, we need to carry out three 2-hours SBs and one 1-hour SB. Finally, the required total observing time for the proposal is estimated to 7 hours.
Is the feathering process for combining total power and interferometric data for solar observations different from other observations?
There is no difference in the feathering process between the Sun and other observations, except in the case when the field of view is very near the solar limb (< the diameter of the primary beam). For limb observations, you might need to apply the primary beam deconvolution to the total power data. The matter depends on your scientific goal.