Editing Cosmic ray visual phenomena (section)

==Experiments==
===ALFMED experiment===
[[File:Apollo 17 ALFMED AS17-162-24080HR-et-078HR.jpg|thumb|320px|Apollo 17 astronaut [[Ronald Evans (astronaut)|Ron Evans]] wearing the ALFMED light-flash detector during the outbound flight from Earth]]
During the [[Apollo 16]] and [[Apollo 17]] transits, astronauts conducted the [[Apollo Light Flash Moving Emulsion Detector]] (ALFMED) experiment where an astronaut wore a helmet designed to capture the tracks of cosmic ray particles to determine if they coincided with the visual observation. Examination of the results showed that two of fifteen tracks coincided with observation of the flashes. These results in combination with considerations for geometry and [[Monte Carlo method|Monte Carlo estimations]] led researchers to conclude that the visual phenomena were indeed caused by cosmic rays.<ref name="ALFMED">{{cite web|url=http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/docs/ApolloCat/Part2/ALFMED.htm |title=Experiment: Light Flashes Experiment Package (Apollo light flash moving emulsion detector) |series=Experiment Operation During Apollo IVA at 0-g |publisher=[[NASA]] |date=2003 |archive-url=https://web.archive.org/web/20140511163326/http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/docs/ApolloCat/Part2/ALFMED.htm |archive-date=11 May 2014 |url-status=dead}}</ref><ref name="Osborne1975">{{cite book |chapter-url=https://history.nasa.gov/SP-368/s4ch2.htm |chapter=Apollo Light Flash Investigations |title=Biomedical Results of Apollo |publisher=NASA |first1=W. Zachary |last1=Osborne |first2=Lawrence S. |last2=Pinsky |first3=J. Vernon |last3=Bailey |editor1-first=Richard S. |editor1-last=Johnston |editor2-first=Lawrence F. |editor2-last=Dietlein |editor3-first=Charles A. |editor3-last=Berry |date=1975 |volume=NASA-SP-368 |id=NASA SP-368}}</ref>

===SilEye-Alteino and ALTEA projects===
The [[SilEye-Alteino]] and [[Anomalous Long Term Effects in Astronauts' Central Nervous System]] (ALTEA) projects have investigated the phenomenon aboard the [[International Space Station]], using helmets similar in nature to those in the ALFMED experiment. The SilEye project has also examined the phenomenon on Mir.<ref name="Avdeev2002" /> The purpose of this study was to examine the particle tracks entering the eyes of the astronauts when the astronaut said they observed a LF. In examining the particles, the researchers hoped to gain a deeper understanding of what particles might be causing the LF. Astronauts wore the SilEye detector over numerous sessions while on Mir. During those sessions, when they detected a LF, they pressed a button on a joystick. After each session, they recorded down their comments about the experience. Particle tracks that hit the eye during the time when the astronauts indicated that they detected a LF would have had to pass through silicon layers, which were built to detect protons and nuclei and distinguish between them.

The findings show that "a continuous line" and "a line with gaps" was seen a majority of the time. With less frequency, a "shapeless spot", a "spot with a bright nucleus" and "concentric circles" were also reported.<ref name="Avdeev2002" />{{rp|518}} The data collected also suggested to the researchers that one's sensitivity to the LF tends to decrease during the first couple of weeks of a mission. With regards to the probable cause of the LF, the researchers concluded that nuclei are likely to be the main cause. They based this conclusion off of the finding that in comparison to an "All time" period, an "In LF time window" period saw the nucleus rate increase to about six to seven times larger, while the proton rate only increased by twice the amount when comparing the two time periods. Hence, the researchers ruled out the Cherenkov effect as a probable cause of the LF observed in space, at least in this case.

===Ground experiments in the 1970s===
Experiments conducted in the 1970s also studied the phenomenon. These experiments revealed that although several explanations for why the LF were observed by astronauts have been proposed, there may be other causes as well. Charman ''et al.'' (1971) asked whether the LF were the result of single cosmic-ray nuclei entering the eye and directly exciting the eyes of the astronauts, as opposed to the result of Cherenkov radiation within the retina. The researchers had observers view a neutron beam, composed of either 3 or 14 MeV monoenergetic neutrons, in several orientations, relative to their heads. The composition of these beams ensured that particles generated in the eye were below 500 MeV, which was considered the Cherenkov threshold, thereby allowing the researchers to separate one cause of the LF from the other. Observers viewed the neutron beam after being completely dark-adapted.<ref name="Charman1971" />

The 3 MeV neutron beam produced no reporting of LF whether it was exposed to the observers through the front exposure of one eye or through the back of the head. With the 14 MeV neutron beam, however, LF were reported. Lasting for short periods of time, "streaks" were reported when the beam entered one eye from the front. The "streaks" seen had varying lengths (a maximum of 2 degrees of visual angle), and were seen to either have a blueish-white color or be colorless. All but one observer reported seeing fainter but a higher number of "points" or short lines in the center of visual field. When the beam entered both eyes in a lateral orientation, the number of streaks reported increased. The orientation of the streaks corresponded to the orientation of the beam entering the eye. Unlike in the previous case, the streaks seen were more abundant in the periphery than the center of visual field. Lastly, when the beam entered the back of the head, only one person reported seeing the LF. From these results, the researchers concluded that at least for the LF seen in this case, the flashes could not be due to Cherenkov radiation effects in the eye itself (although they did not rule out the possibility that the Cherenkov radiation explanation was applicable to the case of the astronauts). They also suggested that because the number of LF observed decreased significantly when the beam entered the back of the head, the LF were likely not caused by the visual cortex being directly stimulated as this decrease suggested that the beam was weakened as it passed through the skull and brain before reaching the retina. The most probable explanation proposed was that the LF were a result of the receptors on the retina being directly stimulated and "turned on" by a particle in the beam.

In another experiment, Tobias ''et al.'' (1971) exposed two people to a beam composed of neutrons ranging from 20 to 640 MeV after they were fully dark-adapted. One observer, who was given four exposures ranging in duration from one to 3.5 seconds, observed "pinpoint" flashes. The observer described them as being similar to "luminous balls seen in fireworks, with initial tails fuzzy and heads like tiny stars". The other observer who was given one exposure lasting three seconds long, reported seeing 25 to 50 "bright discrete light, he described as stars, blue-white in color, coming towards him".<ref name="Tobias1971" />{{rp|596}}

Based on these results, the researchers, like in Charman ''et al.'' (1971), concluded that while the Cherenkov effect may be the plausible explanation for the LF experienced by astronauts, in this case, that effect cannot explain the LF seen by the observers. It is possible that the LF observed were the result of interaction of the retina with radiation. They also suggested that the tracks seen may point to tracks that are within the retina itself, with the earlier portions of the streak or track fading as it moves.

Considering the experiments conducted, at least in some cases the LF observed appear to be caused by activation of neurons along the visual pathway, resulting in phosphenes. However, because the researchers cannot definitively rule out the Cherenkov radiation effects as a probable cause of the LF experienced by astronauts, it seems likely that some LF may be the result of Cherenkov radiation effects in the eye itself, instead. The Cherenkov effect can cause Cherenkov light to be emitted in the vitreous body of the eye and thus allow the person to perceive the LF.<ref name="Fuglesand2006" /> Hence, it appears that the LF perceived by astronauts in space have different causes. Some may be the result of actual light stimulating the retina, while others may be the result of activity that occurs in neurons along the visual pathway, producing phosphenes.