ACIS Storms Analysis

protecting ACIS from space weather

Overview

The Advanced CCD Imaging Spectrometer (ACIS) camera on the Chandra spacecraft is sensitive to damage from protons in the solar wind with kinetic energies of ~100 keV, which can be reflected by the Chandra mirrors onto the CCDs and increase charge transfer inefficiency (CTI), degrading the spectral resolution of the CCDs. When solar storms occur, it is sometimes necessary to safe ACIS (shutting it down and moving it out of the focal plane) to prevent this radiation damage.

Shutdowns can be manually executed if the Chandra project judges the radiation environment to be too dangerous. There is also a mechanism for an autonomous shutdown. Earlier in the mission, Chandra used the Electron Proton Helium INstrument (EPHIN) on board to detect significant radiation levels and trigger autonomous instrument safing. EPHIN became inoperable in 2013; at this point the High Resolution Camera (HRC) anti-coincidence shield rates were used to detect solar storms. Beginning in 2020, HRC operations became reduced due to difficulties running the instrument at high temperatures, so the shield rate could no longer be used as an “always-on” radiation monitor.

The current radiation monitor aboard Chandra is the ACIS threshold crossings rate, or “txings” rate. The ACIS flight computer and software detects and recognizes X-ray events during the length of an observation. The computer examines each CCD frame to find pixels with detections above a pre-determined threshold value. Not all such events are X-rays; this is determined by further processing. However, the total txings rate is calculated and telemetered to the ground. This value is sensitive both to X-rays and to particles; because of the latter it can be used as a radiation monitor to detect solar storms and trigger autonomous safing of the science instruments.

ACIS Storm Pages

Various diagnostics of the solar wind environment are used in Chandra operations. These include:

  • “Soft” protons measured by the ACE satellite; the ACE “P3” channel is the one that records the radiation that can cause significant damage to ACIS
  • “Hard” protons measured by the GOES satellites
  • The HRC GOES Proxy, which is a linear combination of GOES proton channels
  • The ACIS txings rate, which appears to be related to the hard protons measured by GOES

For a number of solar storms, I have cataloged and plotted the behavior of these various diagnostics over time for further analysis; these plots are hosted at the ACIS Storm Pages. An example of such plots for a particular storm is shown in Figure 1 below.

Figure 1: Example of time-series plots of various radiation diagnostics during a solar storm. From top to bottom, the panels show the ACE P3 flux, the ACIS threshold crossings rate, and the HRC Proxy. The pink shaded region marks the time the instruments were safed; the purple shaded regions mark normal radiation zone passages for each orbit of Chandra around the earth.

Any time there is a significant solar storm that causes actions to be taken by the ACIS Ops team, a memo is written. The following are memos I have written concerning some of these events.