Introduction

The ACIS Ops team learned in the first weeks of the Chandra mission that the camera's front illuminated (FI) chips are highly vulnerable to damage from low energy protons at roughly 100 keV. Such protons cause displacement damage in the Si lattice producing charge traps which significantly increase the CTI. A single belt passage increased the FWHM by 20 eV in FI chips at -120C. As detailed in MIT's report, the expected damage from belt passages varies markedly as Chandra's orbit changes. 1999-2001, 2010-2015, and 2020-2026 are the worst periods.

1. Trapped radiation belts. The ACIS instrument must be protected through each perigee passage through the radiation belts:
   1. The SIM must be translated to the HRC-S position while the Chandra Radiation Model (CRM) predicts proton fluxes above threshold. The CRM determines the radiation belt entry and exit times, which often but not always align with the EEF1000 and XEF1000 orbital events, as determined by NSDDC's AE-8 electron model. Translations to and from HRC-S regularly occur at pad times before entry and after exit. CTI measurements are taken during these pad times. More recently, since 2020, pad times may be omitted in effort to extend the science orbit up to the modeled proton threshold in the orbit.
   2. The "backstop pad time" runs from Radmon disable to EEF 1000, and from XEF 1000 to Radmon enable. The "CRM pad time" runs from the time of soft proton entry of the belts according to the CRM model (lengthened up to a maximum of 10 ks if ACIS Ops consents) to the AE8 predicted time of electron radiation belt entry.
   3. At all points in the orbit, the radiation monitor must be enabled whenever ACIS is in the focal plane. During radiation belt crossings, the radiation monitor should be disabled once the SIM translation to HRC-S is complete.
   4. Video boards must be powered down during belt passages. There should be a minimum of around 2 hours between powerdown and perigee.

   This requirement is enforced by diligent review of perigee passages in weekly command loads.

2. High solar background. In order to limit increase in the full width half max at the top of an FI chip to an annual 0.1%, the ACIS operations team has established a radiation exposure budget for for ACIS: an annual accumulated fluence of 2e10 protons/cm2-ster-MeV.

   As a rule of thumb, expecting 8 to 10 major events a year, we allow ourselves to accumulate a fluence of about 2e+9 for any one radiation event.

When unsafe radiations are anticipated or detected, either on-board radiation monitors or ground-based commanding must issue an SCS-107 command. Crucially, this safing action translates the SIM to HRC-S position and shuts down the ACIS vid-boards, keeping ACIS as safe as possible from radiation damage.

Monitoring and Alerts

Sunspot tracking

Solar X-rays and CMEs

In the event of an X class solar flare, NOAA's Space Weather Prediction Center will send an email alert to acisdude with the subject line "SUMMARY: X-Ray Event exceeded X1". That's our cue to jump to Today's Space Weather, which displays the last three days of solar X-ray flux in a logarithmic graph. Each horizontal line represents a factor of 10 increase in energy. Flares below M-7 or so are unlikely to give ACIS any grief. If they're above that level, we need to start keeping an eye on ACE and GOES rates. The most energetic particles may arrive in 0.5 to 3 days, depending in part on where the flare and associated CME were directed.

In the past, Shanil received email notices of Coronal Mass Ejections (CMEs) from stenborg@kreutz.nascom.nasa.gov, which he forwarded to the rest of us. More recently, SpaceWeatherAlerts is used to provide helpful and rapid notifications. Jack is subscribed and automatically forwards their alerts to the acisdude email list. The velocity of CMEs is variable, but if one is Earth directed, it typically takes two to five days for the ejected mass to arrive, and several hours for it to sweep past Earth. Note that the arrival of the bulk CME ejecta is generally distinct and delayed from the most energetic particles described in the paragraph above.

ACE

1. ACIS flumon alerts, when fluence for the orbit exceeds 1.0e9 and 2.0e9. Notice had been to acisdude only, but recently (Jan 2021) has been expanded to sot_ace_alert.
2. MTA high-flux alerts when the fluence integrated over two-hours exceeds 3.6e8. This more urgent condition will send text and email alerts to the sot_ace_alert list.

GOES

The GOES-R alert condition is being investigated with values TBD. Previously, MTA sent a yellow alert when GOES-11 P2 > 30 and P5 > 0.25; and red alerts for P2 > 90.9 and P5 > 0.70. The red limits are designed to indicate a probable trip of the old P4GM (P2) and P41GM (P5) EPHIN channels. (NB: GOES definitions of P2 and P5 channels differ from the ACE definitions of P3, etc.)

Kp index

Kp is a measure of the magnetic field at the surface of the Earth, averaged from data at various ground stations around the globe. Increases in the Kp indicates that the particle density in the radiation belts is changing.

In the past, MTA sent notification through sot_yellow_alert when Earth's near-term Kp index as projected by the Kp index model was high.

EPHIN

HRC

ACIS TXINGs

SCS 107 notification

Events that had been scheduled in the daily load before the SCS-107 will continue to show up on the Replan Central page, but greyed out.

Response to SCS 107

If other checks fail, consult the anomaly pages. There is a known class of anomalies in which the 1STAT1ST bit fails to set. This is expected to have been fully fixed by the updated buscrash patch included with FSW Std G Opt I (v58). But for completeness, these are documented under ACIS Science Run Termination Failure Anomaly.

Check the Fluence monitors (on the RT pages) to determine how much ACIS has accumulated already. These are based on the ACE Flux.
Run history-files.pl -s107 with the SCS 107 time. (The memos webpage instructions for radiation replan reviews are less detailed but more up-to-date.)
Be ready for the replan. Review the CTI RTS and be ready to write one up if there is enough time.

When to Call a Radiation Telecon

Gregg has a GUI interface that can help with the computations: see this memo page.

1. ContFlu < 1.0e9 and the first contact occurs after 7 am or
2. NxtContFlu < 1.0e9 and the following contact occurs after 7 am

Finally, if ACE flux is clearly falling, and both RadZFlu and NxtContFlu are below 1.0e9, you may defer a decision on calling a telecon and continue to monitor, and should send a notification to this effect. A template for the radiation notice is available here.

How to Call a Radiation Telecon

The purpose of calling this telecon is for ACIS to brief the rest of operations and argue the necessity of a shutdown. If time allows, first notify acisdude-text to organize a quick cross-check with the team. Upon agreement, send an email to sot_red_alert. Radiation telecons will ordinarily be on the contingency number (844) 467-6272 (111165#) or at the corresponding Google-meet.

Required personnel to make a decision on whether to shut down are a flight director, and representatives from FOT engineering and FOT MP. Desirable are people from PCAD (either Eric or Tom), SOT MP, SOT lead, and Steve O'Dell from NASA.

SCS-107 EXCUTION AND NON-LOAD TRACKING

Linkfest

There are more examples of radiation response, and memos about the decision process to be found off the ACIS memos page, under "Radiation Shutdown Decisions," which leads you to this page.

Here is a link to the MTA archive of old radiation shutdown plots and information. It's also off the SOT Homepage under Radiation, and Radiation Event Archive. The archive of Radiation Events.

There's a tutorial on space weather on the ACIS twiki, here.

Document history:
Royce Buehler, last update Sep 2017
Richard Edgar, last update Apr 2019
Jack Steiner, last update Aug 2022