Sun-aligned arcs are auroral features found within the polar cap, as opposed to the auroral oval where the more typical arcs reside. First global image of this phenomenon was reported by Frank et al. (1982); it consists of luminous belt reaching across the polar cap from noon to midnigh (that is why it is also called theta aurora; terms transpolar or polar cap arc are also known). However, first visual observations of such features were by the British Antarctic Expedition already during the austral winter of 1908 (Mawson, 1916). Similarly, first ground-based all-sky-camera observations originate from the International Geophysical Year (IGY) of 1957-1958 (Davis, 1962; Feldstein, 1963).
The arc is about a hundred kilometers wide or more, and its luminosity may be comparable to the average emissions within the oval (typically, however, it is less than that). The feature can last for several hours, and it moves slowly across the polar cap in the direction of the IMF (see solar wind) By component in the northern hemisphere (Frank et al., 1986). This motion is in the opposite direction in the southern hemisphere for the same sign of By (Craven et al., 1991).
Although the sun-aligned arcs are of much larger scale (and weaker) than the typical arcs found in the auroral oval, the word 'arc' is appropriate in one sense: at least sometimes they show signatures of inverted-V type electron spectra, indicating similar field-aligned acceleration than seen in other discrete aurora (Frank et al., 1986). It has been argued that the arc is most likely located on closed field lines, and connected to distant plasma sheet or its boundary layer (PSBL; see, e.g., Menietti and Burch, 1987). Note that if polar cap is defined as a region of open field lines, sun-aligned arcs are not anymore 'polar cap' events!
The formation of a sun-aligned arc requires a prolonged period of non-positive IMF Bz, and a subsequent change in either Bz or By component (Newell and Meng, 1995; Chang et al., 1998). They are most likely caused by changes in the merging rates in the dayside magnetopause affecting the global plasma convection (see magnetospheric electric field). See Rodriguez et al. (1997) for discussion of arc decay after southward turning of IMF, delay being 15-30 minutes.
- Chang, S.-W., J. D. Scudder, J. B. Sigwarth, L. A. Frank, N. C. Maynard, W. J. Burke, W. K. Peterson, E. G. Shelley, R. Friedel, J. B. Blake, R. A. Greenwald, R. P. Lepping, G. J. Sofko, J.-P. Villain, and M. Lester, A comparison of a model for the theta aurora with observations from Polar, Wind, and SuperDARN, J. Geophys. Res., 103, 17367-17390, 1998.
- Craven, J. D., J. S. Murphree, L. A. Frank, and L. L. Cogger, Simultaneous optical observations of transpolar arcs in the two polar caps, Geophys. Res. Lett., 18, 2297, 1991.
- Davis, T. N., The morphology of the auroral displays of 1957-1958, 2, Detail analyses of Alaska data and analyses of high-latitude data, J. Geophys. Res., 67, 75-110, 1962.
- Feldstein, Y. I., Some problems concerning the morphology of auroras and magnetic disturbances at high latitudes, Geomagn. Aeron., Engl. Transl., 3, 183-192, 1963.
- Frank, L. A., J. D. Craven, J. L. Burch, and J. D. Winningham, Polar views of the Earth's aurora with Dynamics Explorer, Geophys. Res. Lett., 9, 1101, 1982.
- Frank, L. A., J. D. Craven, D. A. Gurnett, S. D. Shawhan, D. R. Weimer, J. L. Burch, J. D. Winningham, C. R. Chappell, J. H. Waite, R. A. Heelis, N. C. Maynard, M. Sugiura, W. K. Peterson, and E. G. Shelley, The theta aurora, J. Geophys. Res., 91, 3177-3224, 1986.
- Mawson, D., Auroral observations at the Cape Royds station, Antarctica, Trans. R. Soc. S. Aust., 40, 151-212, 1916.
- Menietti, J. D. and J. L. Burch, DE 1 observations of theta aurora plasma source regions and Birkeland current charge carriers, J. Geophys. Res., 92, 7503-7518, 1987.
- Newell, P. T., and C.-I. Meng, Creation of theta-auroras: The isolation of plasma sheet fragments in the polar cap, Science, 270, 1338, 1995.
- Rodriguez, J. V., C. E. Valladares, K. Fukui, and H. A. Gallagher Jr., Antisunward decay of polar cap arcs, J. Geophys. Res., 102, 27227-27247, 1997.
- Shiokawa, K., T. Ogino, K. Hayashi, D. J. McEwen, Quasi-periodic poleward motions of morningside sun-aligned arcs: A multi-event study, J. Geophys. Res., 102, 24325-24332, 1997.