The authors has revisited the Anik E1 and Anik E2 failures with the details of the events and the scrutiny of space weather conditions at that time. This communication failure is often used as the example to illustrate the potential damages from the space environment on our satellites and communications systems. The events begins on 20 January 1994 when Anik E1 began spinning uncontrollably; this disabled 40 northern Canadian communities’ telephone service and the radio stations. Luckily, by activating the redundant momentum wheel system (which is used for altitude determination and control), Telesat Canada was able to restored Anik E1 service.
However, at 0214UT, January 21, 1994, Anik-E2 faced the similar issue except this time the backup system also failed. Due to the Anik-E2 outrage, more than 1600 northern communities lost their data and almost all their television services. To reestablish control of Anik-E2, Telessat Canada developed an innovative Ground Loop Atittude Control System (GLACS). Finally, on June 21, 1994 (six months after), they successfully regained control of Anik-E2 and restored full commercial service on August 1, 1994. But, this event has costed Telesat Canada at least $13.3 million or as high as $50-70 million.
Based on the investigation, the electrostatic discharge (ESD) is suspected to be the main cause of the failure. The ESD caused damages to a specific part in the speed control loop of the momentum wheel assemblies and this resulted in a false ‘full speed’ signal; eventually, it slowed the reaction wheel down to zero. The high fluxes of energetic electrons are responsible for the internal charging of the satellite which leads to ESD. To provide the overall picture of how the solar weather conditions lead to the failure, the authors traced the solar weather conditions, including the solar, interplanetary medium, particle and magnetic environment in geosynchronous orbit (L ~ 6.6).
During the year 1994, the sun is in the declining phase of a solar cycle (approaching solar minimum). There are no coronal mass ejections or solar flares event during that period. However, from the soft X-ray images taken by the Yohkoh Soft X-ray Telescope (SXT), it indicates a large southern coronal hole from January 8 to January 13, which is the source of high-speed solar wind streams.
Corresponding to the arrival of the high-speed solar wind streams, the solar wind speed started to increase from ~300 km/s to 700 km/s on January 11 and the solar wind density dropped to ~2.5 particles/cm^3. Kim et al. [2016] has previously pointed out that quiet solar wind conditions -the prolonged period of low particle density and of small amplitude Bz fluctuation- are prerequisites for the occurrence of long-duration intervals of enhanced energetic electron fluxes.
From the X-Ray and particle observations by GOES-7, which was in close proximities around the times of Anik-E failure, Figure 1 showed that the proton fluxes were at low levels but >2MeV energetic electron fluxes are at constant high flux levels from January 12 to January 21. Pc5 magnetic observations at YKN proved that the frequent occurrence of substorms provided the seed electrons to be accelerated and later become energetic electrons by Pc5 waves. Besides, the Hp (north-south mangetic omponent) at GOES orbit changed sharply during the occurrences of the Anik-E failure. Therefore, it might suggest that the natural electromagnetic transients could trigger a discharge in the satellite.
Figure 1 illustrates the low proton flux level as contrast to the high >2MeV electron flux level for a prolonged period.
The authors also compared the flux levels to the existing flux thresholds. With the flux constantly above the thresholds before and after the failure events, it indicated an enhanced radiation environment during the event period. To prevent undesired situations like the Anik failures from occurring, the authors suggest the considerations of radiation shielding methods in the satellites and close monitoring of the space weather particularly the flux levels. However, these flux thresholds can only be used as a reference because the impacts of the internal charging varies with satellites’ size, shape, age, etc. Therefore, it is up to the satellite operators to decide how the flux levels impact their satellites and the satellite operators should monitor the flux level closely to perform necessary maneuvers and mitigate any undesired outcome.
Question:
1. What domain of the space environment is the manuscript about (i..e., solar wind, radiation belts, ionosphere…)? The manuscript went over a wider range of space environment conditions as an attempt to find out the cause of the Anik E1 and E2 failures. It includes the solar condition, interplanetary medium, particle and magnetic environment in geosynchronous orbit (L ~ 6.6), before, during and after the failure events.
2. What problem(s) are the authors are addressing/trying to solve? The authors revisited the Anik-E1 and E2 failure by providing more information on the space weather, the impact and the cause of this failure. They summarized the space conditions during the event periods to explain what leads to the Anik E1 and E2 failures. Particularly, what components in the space environment that causes the electrostatic discharge in the satellites.
3. What method(s)/data do the authors employ? They used Yohkoh soft X-ray to study the solar condition during the event period, GOES particle observations and X-ray data to study the flux levels, FCC and YKC flux levels to infer the Pc5 power which gave us the magnitude of ULF wave and Ap and Dst index to learn about the magnitude of the solar storms.
4. What are the primary new result(s) of the paper? The authors provide the evidence that the Anik E failures is due to the coronal holes. Using the observational data, the authors also showed that the quiet solar wind conditions and prolonged period of high energetic electron fluxes contributed to the damaging ESD. The authors also identified that Pc5 ULF is the accelerating mechanism for electrons.
5. Do the authors identify remaining issues that need further work? Yes. Since Hp changes sharply around the times of occurrences of the Anik-E failures, the author suggests that it is worth to study if the change in Jp (natural electromagnetic transient) could trigger a discharge. Also, he pointed out the importance of understanding the space weather impacts on the satellites and close monitoring of the electron fluxes.
6. Did the authors clearing indicate how the manuscript addressed a problem in forecasting or an impact on a technical system or society?
No. Although they did mentioned that the existing flux thresholds varies in the definition of the harmful flux levels, they then urges the satellite operators to understand their instruments well and determine which thresholds to follow. Besides, the authors emphasized the importance of understanding and monitoring the fluxes level because in both the Anik failures events, it showed that the flux levels are above the threshold levels. It indicated an enhanced radiation environment and the operators need to aware of situations like this and perform the necessary measures.
7. How did this paper extend your knowledge of the space environment?
It gave me a really good idea on how the space environment can affect our communication system. We understand that space environment will cause damages to the satellites and this paper demonstrates the conditions that might lead to the similar kind of events and the magnitude of damages it can cause. It also showed how different data can used to describe the overall picture of the space environment; particularly, how they used the use of Pc5 data to provide evidence of the ULF mechanism in accelerating the electrons.
8. What do you wish the authors had done differently in explaining their science results? The authors has explained the events and space environment very well. One thing that I wish to be included in the paper is a plot of the relative location of the spacecrafts like Anik E1, Anik E2,YKC and GOES during the events so that we can visualize the situations better and understand how well these values represent the actual environment Anik E1 and E2 experienced.