Stable auroral red arcs

Abstract

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...

Is the red arc a good indicator of ionosphere-magnetosphere conditions?

“…M. C. Kelley (personal communication, 1974) has also suggested temperature-driven instabilities in the ionosphere as possible mechanisms. Thus these observations along with other recent data [e.g., Robie et at., 1971] are in reasonably good agreement with present day theory of red arc formation, which was reviewed most recently by Hoch [1973] and by Rees and Robie [1974]. However, one important issue that still remains and has been somewhat puzzling is the following: Why are red arcs observed on certain magnetically disturbed nights and not on other apparently similarly disturbed nights?…”

“…The collection of instruments on board the Ogo 6 satellite was well suited to carry out a comprehensive study of midlatitude red arcs (SAR arcs) [Roach and Roach, 1963;Hoch, 1973;Rees and Robie, 1974]. However, a failure in the solar panel caused the spacecraft potential to be driven in excess of 20 V negative shortly after launch, and thus the 'plasma experiments' could only operate properly during the satellite eclipse period.…”

A very bright SAR arc: implications for extreme magnetosphere-ionosphere coupling

“…Early insights to SAR arc generation mechanisms associated it spatially and temporally with the ring currentplasmasphere interaction, i.e., a SAR arc occurs "just inside the plasmapause" (Hoch, 1973). Near the footpoint of this boundary lies a region of stagnation in the ionospheric circulation, producing a characteristic depression in F-region plasma density known as the "mid-latitude ionospheric trough" (e.g., Moffett and Quegan, 1983).…”

“…Yet they point out that emissions in the 6-10 kR range could account for reports of visual low-latitude aurora in Korean historical records of the 16th-17th centuries (Zhang, 1985). Hoch (1973) quoted 1 kR as common for SAR arcs during the peak and declining years of the IGY, with "the most intense arc ever detected" as 10 kR in 1958. For the post-IGY solar cycle, Hoch reports 300 R as the typical value with the very brightest at 5 kR, and the faintest at 50 R. Perhaps the most severe challenge to present-day understanding (and calibration methods) comes from the discussion of spectral purity in Roach and Roach (1963) using Barbier's (1960) reported SAR arc with a brightness of 125 kR!…”

“…Shortly thereafter, Cole (1965) gave the now accepted explanation of heating by thermal conduction from the ring current. The review paper by Hoch (1973) then summarized, in a clear and insightful fashion, the status of a field barely 15 years old, linking optical observations to plasmapause/ring current characteristics in the inner magnetosphere. The final major contribution of the first two decades of SAR arc research came from Rees and Roble (1975), providing an overall summary of observational characteristics and occurrence patterns and, most importantly, they offered a comprehensive formalism for calculating SAR arc emission.…”