Stable auroral red (SAR) arcs are diffuse, persistent, practically monochromatic (/•/•6300-6364 A) auroral forms peculiar to mid-latitude regions of earth. Measurements of such parameters as spectral composition, geomagnetic location, range of intensity, and frequency of occurrence lead to the conclusion that the phenomenon is due to the excitation of atomic oxygen by hot electrons in the plasmapause region. However, on the basis of the data available, it is not clear whether fresh electrons are precipitating into the region of the arc or whether ambient electrons are being heated in place. There is strong evidence that the source of energy in either case is the ring current. Suggested mechanisms by which energy is transferred from the ring current to the electrons in the SAR arc region are (1) heat flow, that is, transfer of kinetic energy to the SAR arc region by Coulomb collisions, (2) transfer of ring current proton kinetic energy to hydromagnetic waves, which are in turn damped by the electrons in the SAR arc region, and (3) direct influx of energetic electrons into the SAR arc region. Which of these mechanisms predominates is still not resolved at this time. There are other open questions as well, including questions dealing with the motion of the arcs and the spectral composition. A few hours after the earth's magnetic field has been disturbed by a strong increase in the solar plasma flux, two glowing red zones are often detected, occurring approximately along lines of constant geomagnetic latitude in the mid-latitude regions of the earth. These glowing zones are called stable auroral red arcs (SAR arcs) and are caused by emission,from the neutral atomic oxygen atom. It appears that SAR arcs occur simultaneously, one in each hemisphere, at geomagnetically conjugate positions typically 500-55 ø north and south of the geomagnetic equator. The arcs are subvisual and have been detected only at night with photometric and photographic equipment. It is not known with certainty whether or not they occur on the dayside of earth and thus encircle the earth completely.Undoubtedly, the appearance of such phenomenal geophysical features, occurring as they do over populated regions of earth, would have been the subject of much investigation throughout scientific history if they had been bright enough to be visible. As it is, however, the faint emission of the arcs was not detected until 1958, and only about 100 such events have ever been measured. They are thus considered rare events compared, for example, with visible polar auroras. But even now only a few stations are set up to observe SAR arcs systematically, and some of the instrumentation used has not been very sensitive. Therefore it is not clear whether the SAR arcs are really rare or are just inconspicuous.The fact that the SAR arcs are subvisual does not make the physical processes involved in their occurrence any less interesting, and in the 14 years that the SAR arcs have been studied a good deal has been learned about them. To give a brief preview, 935 936 R...

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Stable auroral red (SAR) arcs were definitely recorded on nine occasions and possibly recorded on six other occasions at Richland, Washington, between September 1967 and mid‐May 1969. A systematic program of photometric sky mapping and meridian spectrograms was used in collecting the data. The SAR arcs recorded were relatively faint, and all were accompanied by poleward auroral displays. It is clear that the SAR arcs are separate in latitude from the poleward auroras. New spectrographic evidence again verifies that [O I] 6300–6364 A is the predominant radiation of the SAR arc.

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A stable auroral red (SAR) arc was recorded the night of September 28/29 from stations at Fritz Peak, Colorado, and Richland, Washington. The first direct determination of position and altitude of an SAR arc was accomplished by triangulation from these two stations. The appearance of this arc marks the definite recurrence of this phenomenon after an apparent absence of four years. The arc occurred southward of an intense auroral display, and it was aligned parallel to lines of constant L. Average intensity of [O I] 6300 A emission in the brightest part of the arc was approximately 150 R. Altitude of the brightest region was determined from angular coordinates at the two stations to be 400 km at 0745 UT. A typical width across the narrow dimension of the arc was 160 km between half‐maximum intensity values. A sector of the arc almost 4200 km long was simultaneously within the combined range of the two stations. The arc occurred during a period of high magnetic activity, and though a comparably intense aurora borealis occurred the night before (September 27/28), no SAR arc was observed from either station.