Visual Light Flash Observations on Skylab 4

RUDOLF A. HOFFMAN, LAWRENCE S. PINSKY, W. ZACH OSBORNE, AND J. VERNON BAILEY

THE OBSERVATION OF LIGHT FLASHES was first reported by the Apollo 11 Lunar Module Pilot, Edwin Aldrin, with subsequent observations made on all Apollo missions (refs. 1,2). Professor C. A. Tobias predicted as early as 1952 (ref. 3) that this type of visual phenomenon would be experienced by humans when exposed to heavily ionizing cosmic particles. Although it has been quite generally accepted that the light flashes observed were caused by passage of cosmic particles through the visual apparatus, the exact mechanism of particle interaction is still uncertain. Some investigations (refs. 1,4,5,6, 7,10) support the premise that the visual flashes are caused by direct particle/retina interaction while others (refs. 8, 9) tend to favor Cherenkov radiation from relativistic particles as their etiology. While both mechanisms probably contribute, the current consensus seems to be that most of the flashes result from direct ionization energy loss as the particle traverses retinal cells. In either case, if cosmic particles are the cause, a strong latitude effect of the light flash rate would exist for an observer in Earth orbit. This effect is a consequence of the geomagnetic cutoff and the steep energy spectrum of cosmic ray fluxes. In other words, near the equator only cosmic particles with very high energy can reach orbital altitudes, while near the magnetic poles particles of much lower energies can reach comparable altitudes.

The primary objective of the study reported here was to investigate the frequency and character of visual light flashes in near Earth orbit as the Skylab trajectory passes from northern to southern latitudes. Because the trajectory periodically passed through the South Atlantic Anomaly, another study objective was the investigation of possible visual flashes during passage through this region.

Procedure

Two periods of observation by the Pilot were planned. These observation sessions were accomplished on orbits selected to provide data on both latitude and South Atlantic Anomaly effects. Unfortunately, no single orbit possessed ideal geomagnetic latitude and anomaly conditions. Hence the first session provided the best latitude conditions but passed only through the edge of the South Atlantic Anomaly region. The second session passed through the center of the South Atlantic Anomaly but did not achieve as high geomagnetic latitudes. The first observation session occurred on mission day 74 and was 70 minutes in duration, while the second occurred on mission day 81 and was 55 minutes long. The second period was shorter because of very critical time limitations during the last few days of the mission.

At the start of each session the Pilot got into his sleep restraint, set a timer for the prescribed period (either 70 or 55 minutes), donned a blindfold, and began observing for light flashes. Approximately the first 10 minutes of each session was allocated for dark adaptation by the subject. During the first session no particular position in the sleep restraint was specified. The Orbital Workshop was in a Solar Inertial Mode during both periods and local noon occurred very close to equator passage in both cases. For the second session, directions were given for head positioning which placed the anterior-posterior axis of head parallel to the Earth’s magnetic field lines in the anomaly.

The occurrence of each light flash event along with its description was voice recorded on the onboard tape recorder and a transcript of the recording for each of the two periods was obtained for analysis.

Results

A total of 168 flashes was reported: 24 during the first session and 144 during the second. Figure 14-1 shows a plot of the trajectory ground tracks for both observation sessions with each light flash occurrence marked. The numbers shown in the South Atlantic Anomaly on the ground track for session number two indicate the number of flashes observed during 1 minute intervals. Because the frequency of flashes in the South Atlantic Anomaly was much less in session number one, event marks instead of numbers were used.

It is almost impossible, because of the relatively few flashes observed and because of varying lengths of time spent at different latitudes, to show in a simple way the relationship between flash occurrence and geomagnetic latitude or HZE flux. However, figures 14-2 and 14-3 at-tempt to demonstrate this for the two observation sessions. As can be seen by referring to figure 14-1, time from equator passage is directly related to latitude. The calculated cosmic ray flux for latitude positions of the spacecraft corresponding to the times from equator passage is shown on both figures 14-2 and 14-3. Although there is evidence for correlation of flash occurrence with cosmic ray flux (or geomagnetic latitude) in figure 14-2, figure 14-3 more clearly demonstrates this relationship. The Van Allen Belt Dosimeter data for the observation periods are also shown on figures 14-2 and 14-3. The units shown on the ordinate of figures 14-2 and 14-3 do not apply to those curves; instead only relative units need to be visualized. It is apparent that the flash rate in the South Atlantic Anomaly coincides remarkably well with the increased radiation levels detected by the Van Allen Belt Dosimeter.

Comments And Conclusions

Although a few light flashes were reported as casual observations by the crews of Skylab 2 and Skylab 3, the events reported here represent the first observations made in Earth orbit. No flashes were observed during previous Mercury or Gemini flights or during Apollo missions prior to Apollo 11. Why no flashes were observed prior to Apollo 11 has been considered before (ref. 1) and even now no clear explanation exists. The most logical explanation appears to be that the eye must be dark adapted and the observer must be reasonably relaxed and free from most distracting activities to observe light flashes. This was not the case on earlier flights. Also without a precedent for their observation, there would probably be the tendency to discount minor flashes as nothing unusual and simply an innocuous event in a milieu of more important observations. It seems obvious now that with eyes trained for observing these events their occurrence will be noted whenever proper conditions exist.

The following conclusions can be drawn from the data presented:

        Dark adaptation of at least 10 minutes duration is required to begin observing the flashes.

        There is a strong correlation of very high flash rates with passage through the South Atlantic Anomaly,and, from            physical arguments and event descriptions, it appears certain that these flashes are due to the trapped radiation.

        There is evidence for the predicted latitude effect, although existing data are insufflcient for a thorough statistical          evaluation.

    A greater particle flux in the trajectory through the South Atlantic Anomaly during the second observation period probably explains the increased number of flashes observed at that time, but there were also more flashes observed outside the anomaly during this second period where the cosmic particle environment should have been comparable. This variation remains unexplained at this time.

There is an additional suggestion from the event rates and descriptions of flashes during the South Atlantic Anomaly passes, that there may be particles heavier than protons in the inner belt of trapped radiation. The current knowledge of the inner belt includes an upper limit of only approximately one heavy nucleus per 1000 protons. The Skylab 4 light flash data are compatible with this limit, but still suggest the existence of a significant flux of Z= or >2 particles. This provides strong motivation for making detailed and accurate measurements of the South Atlantic Anomaly (inner belt) heavy component.

The observation of flashes during space flight reported here and those reported previously represent very few events from a statistical standpoint. More such observations need to be made and are planned (1) for the Apollo-Soyuz Test Program mission. Although there is a basic interest in studying the visual flashes per se, the real importance to manned space flight is the question of their significance. Are they mere flashes similar to other visual observations we make continually and represent no danger? Does each flash signify the destruction of one or more retinal cells? Are the flashes observed and the resultant damage, although potentially serious in itself, indicative of even more damaging interaction of HZE particles with other tissues, e.g., the brain? The need for extensive ground investigations using accelerator-produced radiation is apparent. Space observations as reported here must serve as guidelines for ground studies currently underway and others yet to be conducted.

REFERENCES

1. CHAPMAN, P.L., L.S. PINSKY, R.E. BENSON, and T.F. BUDINGER. Observationsof Cosmic Ray Induced Phosphenes on Apollo 14. Proc. Nat. Symp. Natural and Manmade Radiation in Space, p. 1002. NASA TM X-2440, 1972.

2. PINBKY, L.S., W.Z. OSBORNE, J.V. BAILEY, R.E. BENSON, and L.F. THOMPSON.Light flashes observed by astronauts on Apollo 11 through Apollo 17. Science, 183:957-959, March 8, 1974.

3. TOBIAS, C.A.J. Aviation Medicine, 23:345, 1952.

4. BUDINGER, T.F., HANS BICHSEL, and C.A. TOBIAS. Visual phenomena noted byhuman subjects on exposure to neutrons of energies less than 25 MEV. Science, 172:868-870, May 21, 1971.

5. BUDINGER, T.F., JOHN T. LYMAN, and C.A. TOBIAS. Visual perception of accelerated nitrogen nuclei interacting with the human retina. Nature, 239(5369):209-211, September 22, 1972.

6. CHARMAN, W.N., and C.M. ROWLANDS. Visual sensations produced by cosmic raymuons. Nature, 232:574-575, August 20,1971.

7. FREMLIN, J.H. Cosmic ray flashes. New Scientist, 47:42, July 2, 1970.

8. FAZIO, G.G., J.V. JELLY, and W.N. CHARMAN. Generation of Cherenkov light flashes by cosmic radiation within the eyes of the Apollo astronauts. Nature, 228:260, 1970.

9. MCNULTY, P.J. Light flashes produced in the human eye by extremely relativisticmuons. Nature, 234:110, November 12, 1971.

10. TOBIAS, C.A., T.F. BUDINGER, and J.T. LYMAN. Radiation induced light flashes observed by human subjects in fast neutron, X-ray and positive ion beams. Nature, 230:596, 1971.

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