Short baselines projected towards the Sun, particularly at sunrise and sunset, will detect solar fringes. This is seen in both UHF and L-band observations, but is generally more prominent at UHF. The worst affected baselines are

M000 - M001; M002 - M003; M003 - M000; M000 - M004; M000 - M017; M002 - M006; M003 - M029; M004 - M005; M011 - M020; M012 - M020; M018 - M020; M038 - M039

Continuum observations are generally minimally impacted by this effect. However, spectral line observers may be adversely affected, particularly if the science interest is in low surface brightness emission. In the case of relatively bright, unresolved sources, it is advised to simply flag these few baselines, the loss of which will have relatively low impact on overall sensitivity.

Figure 1 shows an example of the ripples in phase, which cease abruptly at sunset. Figure 2 shows the RMS amplitude across channels. More details can be found in the report on  Solar interference on short baselines in the UHF band.

Open

Figure 1: Visibility phases on the M000-M002 (29 m) baseline during a stability observation on J0408-6545, started two hours before sunset. The x-axis is the time since the start of the observation. This data are at 8 second integration, with 32k channels across the band.

Open

Figure 2: RMS of amplitudes computed over 50 channels at the lower end of the band plotted against time. Only the 25 baselines with the largest RMS are shown. The total fraction of baselines affected is less than 5%.

The affected baselines are those with the shortest projected lengths towards the Sun (Figure 3). These constitute less than 5% of the total number of baselines. These baselines can be flagged for the duration of the interference or, if they happen to be vital for a particular science case, observations should be scheduled after sunset.

Open

Figure 3: The inner uv coverage (corresponding to the mid point of the observation) as computed towards the Sun. The colours represent the RMS of the visibility phases for each baseline calculated over the duration when the Sun is above the horizon (redder implies a higher RMS). The circle represents the first null of the jinc function corresponding to 1.3 times the optical solar disc.

Figures 4 and 5 below shows an example at L-band. These plots come from an observation on the DEEP2 field on 14 September 2018. Note the ripple which appears on short baselines after SAST 06:00 (sunrise). This is due to solar fringes and is clearly visible on shorter baselines.

Interference from DME at ~1100 MHz is clearly visible from early morning through to the late evening, but absent once local flights are no longer in the air.

Open

Figure 4: Dynamic spectrum of amplitudes on a short baseline; M002-M003 (40 m).

Open

Figure 5: Dynamic spectrum of amplitudes on a long baseline; M058-M063 (7700 m).

Solar imaging

Studies have been performed with MeerKAT of the Sun to study Coronal mass ejections (CMEs) and polarised sources around the Sun. Early spectroscopic images of the Sun that were obtained with MeerKAT were presented in (Kansabanik et al. 2024).

The system temperature of the MeerKAT receivers increases as the angular separation degreases. The separations where there is a suitably small solar contribution to the system temperature: 4.5 degrees (L-band), 6.9 degrees (UHF-band), 5.2 (S0-band), 3.9 degrees (S4-band) from the sun. Observing closer than these specified separations, there is significant degradation to the data quality.

Open

There are calibration effects for observations taken close to the sun.  The SDP pipeline calibration and imaging that are used for observation QA do not function well at these separations. In Figure 2 the flagging/interference plot of an observation that was near the Sun shows the over flagging from the Pipeline due to the additional power of the sun in the side-lobes .

Open

Figure 2: Percentage of time data flagged for an observation where the target was close to the Sun in July 2024.

1. Kansabanik D., et al., 2024, ApJ, 961, 96