Does the Shutdown of a Single Site Affect the GONG Network Duty Cycle?

The duty cycle measures how many observations are available over a given period. For GONG, it generally represents the fraction of observations obtained during a single day. The six GONG sites distributed across the northern and southern hemispheres were selected to ensure that at least two sites observe at any given time, with no interruption to the observations during the non-observing period of one site. Their locations were carefully chosen based on long‑term weather patterns and year‑round sunlight availability. A detailed study of the duty cycle for individual GONG sites and for the full network clearly demonstrates that the network duty cycle is adversely affected by the shutdown of a particular site only when adjacent sites also lack observations.

For example, the Udaipur (UD) site was turned off on July 8 due to the Indian monsoon. The gap in the network observations on this day (marked in the left figure) is not caused solely by the shutdown of UD - it occurred because the adjacent El Teide (TD) site during the overlapping periods with LE and UD on the same day was down due to a technical issue. Once that issue was resolved the following day, the network duty cycle returned to its usual high value and has remained stable. 

The figure below reinforces this point: on July 7 and 9, both TD (green oval) and Learmonth (LE marked by the red oval) provided observations, and no comparable gap appeared in the window function despite UD being offline. The absence of UD observations is indicated by the brown ovals for July 7 - 9, while the brief overlap between LE and TD on July 7 and 9 (shown by the gray shaded bars) ensued continuity to the observations. Provided TD and LE remain operational, no major duty‑cycle impact is expected during the UD monsoon shutdown.

It is important to note, as mentioned previously, that the Mauna Loa (ML) site in Hawaii has been offline since late November 2022 due to a volcanic eruption, and efforts are ongoing to restore operations. There have been periods when the network duty cycle has dipped for the reasons described above, including weather‑related or technical issues. Even when high duty cycle values can be achieved with fewer observing sites, full operation of all six sites remains essential for the precise determination of the p‑angle (the angle that defines the true solar north in the images) which is critical for measuring subsurface flows using helioseismic methods.  Moreover, a network with a reduced number of operational sites is inherently more vulnerable to lower duty cycle performance over the course of the year.