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Section 4.1.2. CHEOPS in Six transiting planets and a chain of Laplace resonances in TOI-178 says

Due to the low-Earth orbit of CHEOPS, the spacecraft-target line of sight was interrupted by Earth occultations and passages through the South Atlantic Anomaly (SAA), where no data were downlinked. This resulted in gaps in the photometry on CHEOPS orbit timescales (around 100 min). For our observations of TOI-178, this resulted in light curve efficiencies of 51%, 54%, 65%, and 86%. For all four visits, we used an exposure time of 60 s.

CHEOPS is in a ~700 km dawn-dusk sun-synchronous 92.8° inclination orbit aligned with local sunset/sunrise (06:00, 18:00) and an image of the South Atlantic Anomaly at 560 km in the 1990's is shown below. It is also discussed in Space SE's

We can probably assume that the enhanced energetic charged particle flux due to Earth's lower magnetic field in that region of space has something to do with it, but what exactly?

Questions:

  1. Why wasn't CHEOPS data taken during passage through the South Atlantic Anomaly downlinked in this case, resulting in gaps in photometry? Is this out of an abundance of caution at each investigator's discretion, or is it standard, or at least recommended to do so?
  2. Is the problem enhanced noise in the focal plane due to charged particle tracks in the CCD itself, or the potential for digital disruptions like flipped bits in registers or solid state memories? In other words, is the problem mostly analog or digital?

note: CHEOPS observations are already constrained by needing to shield its cryogenic focal plane from radiative heating from both the Sun and the Earth in low Earth orbit:

he required photometric precision will be achieved using a single frame-transfer, back-illuminated CCD detector from Teledyne e2v with 1024 × 1024 pixels and a pixel pitch of 13 µm. The CCD is mounted in the focal plane of the telescope, and will be passively cooled to 233 K (−40 °C), with a thermal stability of 10 mK.

South Atlantic Anomaly at 560 km in the 1990's

South Atlantic Anomaly at 560 km in the 1990's from source

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The high density of energetic charged particles causes problems for any instrument system transiting through the South Atlantic Anomaly. Here, for example, is a description from the Hubble Space Telescope WFC3 Instrument Science Report 2009-40 (Barker et al. 2010, "WFC3 IR SAA Passage Behavior") of its effects on HST:

The South Atlantic Anomaly (SAA) is the region where the Van Allen belts are closest to the Earth and is distinguished by elevated levels of high-energy particle radiation (protons and electrons) which adversely affects the scientific instruments of low-earth orbit satellites such as HST. Short and long term effects may include: high cosmic ray (CR) hit rates, high dark and background levels, residual glow and count rates, damage to the detector lattice, reduced charge transfer efficiency, flipped memory bits, and Single Event Upsets that can suspend the operations of HST instruments.

Below is a figure from that report, showing three 159-second exposures taken by the infrared channel of WFC3 with the shutter closed. The left and right exposures were taken before and after transiting the SAA (the left image's brighter background is Earth light leaking into the camera). The middle image was taken deep inside the SAA. They note that the cosmic-ray incidence is about 50 times higher in the middle image.

enter image description here

It's standard to not do observations when passing through the South Atlantic Anomaly. From the HST Cycle 29 Primer's "Orbital Constraints" section:

No astronomical or calibration observations are possible (except for some specialized and infrequently executed WFC3 observations) during spacecraft passage through the SAA because of high background induced in the scientific instruments and FGSs.

(While certain types of observations with the WFC3 instrument may be permitted, the document also notes that some instruments can't be used on an orbit that passes even through the edge of the SAA, so different instruments can evidently be affected more or less strongly.)

My guess (knowing very little about the details of how CHEOPS observes) is that "no data were downlinked" means either:

  1. The telescope kept taking data during the time it passed through the SAA, but the data were not downlinked because it was known they would be too noisy to use;
  2. "No data were downlinked" is a compact (and maybe slightly awkward) way of saying, "No data were downlinked because the telescope was not taking data during that time".

Edited to add: A quick look at the most recent CHEOPS Observers Manual reveals this discussion of the SAA

... a target is typically observable whilst flying through the SAA, however the data taken will show an increased number of glitches and noise spikes due to the impact of high energy particles. Data taken during IOC showed that images acquired during SAA passage are not amenable to photometric reduction due to the huge number of cosmic ray hits. Therefore, the images taken during SAA passage are not downlinked to ground in order to save bandwidth.

(So it's #1 of the two possibilities I suggested.)

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  • $\begingroup$ It seems that we are lucky to have a cozy shell of LEO sufficiently above the atmosphere to not require constant propulsive boosts and at the same time sufficiently below the Van Allen belt that heavy shielding is not necessary for CCDs, and the SAA is there to remind us of this :-) Thanks for the great answer! $\endgroup$ – uhoh Jan 27 at 19:29

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