Science Highlight




Solar Full Polarization in Band 3

Science Target Overview

The target of the test observation is an active region with the NOAA number 13119. The active region passed the central meridian on October 10, 2022, and its heliographic latitude is 28 degrees north. It had a large following spot and several small preceding spots, which is unique from usual active regions. The magnetic polarity of the following sunspot is negative. In the center of the synthesized image with FieldID #0 there is a following sunspot of NOAA13119, and the helioocentric coordinate of the center of FieldI #0 is XCEN:486", YCEN:398" at 17:11UT, 13 October 2022.


ALMA Data Overview

The observation with Band 3 (LO1: 100 GHz) was executed on 13 October 2022. The antenna configuration was C-3, and the correlator was used in the Time Division Mode (TDM) with full-polarization measurments. The total observing duration of the observation is 4.3 hours, including the three Execution Blocks (EBs). However, we were unable to use the third EB because the elevation of the Sun was too low, causing antennas to significantly shadow one another. Hence, the released dataset includes only 1st and 2nd EBs. The observing period of each EB was 15:11:48 – 17:19:47 UT for 1st EB and 17:21:14 – 18:47:37 UT for 2nd EB. While these interferometric observations were carried out with the heterogeneous array constructed with 7 m and 12 m antennas, the dataset includes data only from the 12-m array.

To evaluate the the effects of off-axis instrumental polarization, we carried out a dense 45-pointing observation using the MOSAIC function with pointings spaced 7” apart.

During the observation, we observed 3C279 as the bandpass and polarization calibrators and J1256-0547 as the phase calibrator. The observing sequence for solar polarization calibration is the same as that used for non-solar celestial objects, except for the treatment for measuring the antenna temperature of the Sun.

The calibration of the dataset are performed in two steps. The first step is to calibrate the parallel-hand data (XX and YY data) using the same solar calibration procedures that have been employed since Cycle4. The second step is to calibrate both the cross-hand (XY and YX) and parallel-hand data. Polarization calibration is exactly the same as that for other celestial objects.

We synthesized 3 mm maps in all four Stokes parameters I, Q, U, and V, for each pointing from the calibrated solar data. For the image synthesis, we use all scans of the sunspot for which the total integration time for each pointing is 67 seconds on average. The synthesized beam size is 2.06 arc-seconds for the major axis and 1.44 arc-seconds for the minor axis. No self-calibration was performed.

Dring the interferometric observations, we also obtained several full disk maps of the Sun using fast-scanning techniques. The dataset includes the full-disk map of the Sun obtained with the TP-array from 17:08:32 – 17:23:15 UT.

Using the full-Sum map and synthesized Stokes-I images, we combined the low-resolution TP data with the high-resolution interferometric data using techniques such as feathering. This has been done for each field of the dense mosaic. The polarization observations with the TP-array were not established at the observation. Hence, we do not apply the feathering process to the Stokes-V maps, and the resolve-out occurs in the maps.

The FieldID#0 map is shifted from the coordinate of the neighborhood maps because of the time gaps between the integration of FieldID#0 and the neighborhood fields. The values of the shift are about -1.52 arcsec in the RA direction and -0.45 arcsec in the DEC direction, estimated from the sunspot structures.


Using the data for publication

The following statement should be included in the acknowledgement of papers using the dataset listed above:
"This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00011.E. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.”


Obtaining the Data

The data products are contained in the following directory:

Parent directory for download

The above directory contains README, uncalibrated data, calibrated data, data reduction scripts, and reference images.