The concept of the auroral oval was proposed independently by Feldstein and by Khorosheva in the early 1960's, and it has since been widely used as a reference frame for the organization of high-latitude optical, particle, and ionospheric data. More recently, different structural regions associated with the oval have been distinguished, and these have been identified with different magnetospheric regions by different workers. The net result is an inconsistent set of nomenclature and relationships. The literature of this development is reviewed in detail, and an interpretation and terminology that is consistent, in the opinion of the authors at least, are proposed. It is hoped that this proposal will serve as a useful basis for further discussion on the ordering of auroral phenomena in relation to the magnetosphere.

The auroral luminosity structure in the high-latitude upper atmosphere: Its dynamics and relationship to the large-scale structure of the Earth's magnetosphere

Over 1600 Defense Meteorological Satellite Program north polar images from the winters 1972–1973, 1974–1975 were studied for the occurrence of auroral arcs at corrected geomagnetic latitudes greater than 80°. Auroras at these latitudes are designated extremely high latitude auroras and are categorized by the local time sectors in which they occur and their relationship to discrete oval arcs. A data base of ∼300 cases of cap auroras was formed for correlation studies with geomagnetic activity indices and solar wind parameters. By far the greatest number of extremely high latitude auroras occur in the morning sector and appear to be expansions of the system of morning oval arcs poleward. The remaining categories include evening arcs expanding into the cap; single, sun-aligned arcs; and midnight sector oval auroras expanding into the cap. Except for the last category the auroras occur for similar magnetospheric conditions: moderate values of Kp and Dst, but comparatively low values of AE. They also occur preferentially for high solar wind speed and for positive Bz. While far less clear, their location in the cap shows a dependence on By. Morning (evening) arcs occur with greater frequency for negative (positive) By.

Extremely high latitude auroras

Quiet time arcs observed from the Greenland all-sky camera network have been ordered in corrected geomagnetic latitude/time mass plots according to the values of the Y and Z components of the IMF. Two different patterns of discrete quiet arcs occur. In one system the arcs are ordered along the statistical auroral oval; in the other, which we have called the polar cap pattern of discrete arcs, the arcs are ordered partly in the sun-earth direction over the polar cap, especially on the dawnside of the pole, partly along the 78°–80° corrected geomagnetic latitude curve in the morning to noon and noon to evening hours. The oval arc pattern is most prominent when Bz is negative. It gradually contracts and finally practically vanishes as the steady IMF is shifted toward a northward direction. The contraction takes place differently, depending on the sign of By. The polar cap pattern is dominant when Bz is positive. The sun-aligned arcs of the polar cap pattern gradually disappear as Bz goes toward negative values. The high-latitude arcs in the forenoon sector seem to be present for all values of Bz considered. No dependence on By has been found in the pure polar cap pattern. Apart from periods of extremely great numerical values of Bz, the oval and the polar cap discrete arc patterns are found to coexist, at least in our statistical presentation.

Quiet time pattern of auroral arcs for different directions of the interplanetary magnetic field in the Y‐Z plane

This letter suggests that most of the so-called 'polar cap arcs' with the sun-aligned orientation at the high geomagnetic latitude region occurring during the quiescent magnetosphere are actually a part of discrete arcs in the auroral oval as a consequence of the poleward widening of the quiet auroral oval. This suggestion is based on (1) a detailed examination of two months of DMSP electron precipitation data and simultaneous auroral observations, (2) the observed widening in the latitudinal extent of the auroral oval precipitation to the very high geomagnetic latitude (> or approx. =80 /sup 0/) and the simultaneous occurrence of the sun-aligned arcs, and (3) the conjugate widening of the quiet auroral oval over both the northern and southern polar regions. The new interpretation of the 'polar cap arcs' is discussed in terms of the present understanding of the solar wind-magnetospheric interaction.

Polar cap arcs and the plasma sheet

This is the first analysis, using a statistically significant data set, of the morphological dependence of the presence, orientation, and motion of stable sun-aligned polar cap arcs upon the vector interplanetary magnetic field (IMF). For the one winter season analyzed we had 1392 all-sky 630.0-nm images of 2-min resolution containing a total of 150 polar cap arcs, all with corresponding values of the IMF as measured by International Monitoring Platform (IMP) 8 or International Sun Earth Explorer (ISEE) 2. After demonstrating an unbiased data set with smooth normal distributions of events versus the dimensions of time, space, and IMF component, we examine IMF dependencies of the properties of the optical arcs. A well-defined dependence for B(sub z) is found for the presence/absence of stable Sun-aligned polar cap arcs. Consistent with previous statistical studies, the probability of observing polar cap aurora steadily increases for larger positive values of B(sub z), and linearly decreases when B(sub z) becomes more negative. The probability of observing Sun-aligned arcs within the polar cap is determined to vary sharply as a function of the arc location; arcs were observed 40% of the time on the dawnside and only 10% on the duskside. This implies an overall probability of at least 40% for the whole polar cap. 20% of the arcs were observed during 'southward IMF conditions,' but in fact under closer inspection were found to have been formed under northward IMF conditions; these 'residual' positive B(sub z) arcs ha d a delayed residence time in the polar cap of about what would be expected after a north to south transition of B(sub z). A firm dependence on B(sub y) is also found for both the orientation and the dawn-dusk direction of motion of the arcs. All the arcs are Sun-aligned to a first approximation, but present deviations from this orientation, depending primarily upon the location of the arc in corrected geomagnetic (CG) coordinates. The arcs populating the 06-12 and the 12-18 quadrants of the CG coordinate system point toward the cusp. The B(sub y) dependency of the arc alignment is consistent with a cusp displacement in local time according to the sign of B(sub y). We found that the arc direction of motion depended both on B(sub y) and the arc location within the polar cap. For a given value of B(sub y) two well-defined regions (or cells) exist. Within each cell the arcs move in the same direction toward the boundary between the cells. The arcs located in the duskside move dawnward; those in the dawnside move duskward. The relative size of these dusk and dawn regions (or cells) are controlled by the magnitude of B(sub y). This persistent dusk-dawn motion fo the polar cap arcs is interpreted in terms of newly open flux tubes entering the polar cap and exerting a displacement of the convective cells and the polar cap arcs that are embedded within them.

Interplanetary magnetic field dependency of stable Sun-aligned polar cap arcs

Sun-aligned arcs are long and sometimes narrow optical structures which are oriented in the sun-earth direction and which occur in the polar cap, generally during intervals of low magnetic activity. Their appearance, as seen by the DMSP and ISIS-2 satellite photometers, is examined with respect to the north-south orientation of the interplanetary magnetic field. In the DMSP pictures it is found that they appear when the IMF is directed northward. In 16 out of 18 unambiguous observations of sun-aligned arcs by ISIS-2, the IMF was directed northward; for the other two no IMF data was available. These results provide evidence for a strong correlation between the northward direction of the IMF and the occurrence of sun-aligned arcs in the northern polar cap.

Y. Kamide

Papers

Evidence for a correlation between Sun-aligned arcs and the interplanetary magnetic field direction