Observations indicate that the extended auroral arc is the basic form of the discrete aurora, the brightest and most obvious kind of aurora. Both motions of auroral arcs and their distortions into convoluted forms indicate the presence of shear processes involving substantial charge excesses and magnetic field-aligned currents. Consequently, strong electric fields, both horizontal and vertical, characterize the discrete aurora. The observations of auroral arcs and observations of associated charged-particle fluxes, electric fields and currents fit together into a relatively cohesive description of the auroral arc which is compatible with at least one proposed model of the causative processes. On the other hand, an equally important type of aurora — pulsating aurora — exhibits quite different characteristics which distinguish it from the discrete aurora and which are difficult to interpret satisfactorily in terms of existing proposed models of particle precipitation and excitation of auroral emission. The lack of shearing behavior in the pulsating aurora indicates that substantial electric fields are not associated with it. Transitional forms of auroras exhibit an intermediate degree of shear motion.

Observed characteristics of auroral forms

[1] During 2001 and 2002, when the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE-FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (AMLT) is strongly correlated with the IMF clock angle (θ C ) and a linear fit, ΔMLT = -0.017c C + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

http://folk.uib.no/nfyno/papers/Conj_Subst2003JA010370.pdf

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

The solar-wind magnetosphere interaction primarily occurs at altitudes where the dipole component of Earth’s magnetic field is dominating. The disturbances that are created in this interaction propagate along magnetic field lines and interact with the ionosphere–thermosphere system. At ionospheric altitudes, the Earth’s field deviates significantly from a dipole. North–South asymmetries in the magnetic field imply that the magnetosphere–ionosphere–thermosphere (M–I–T) coupling is different in the two hemispheres. In this paper we review the primary differences in the magnetic field at polar latitudes, and the consequences that these have for the M–I–T coupling. We focus on two interhemispheric differences which are thought to have the strongest effects: 1) A difference in the offset between magnetic and geographic poles in the Northern and Southern Hemispheres, and 2) differences in the magnetic field strength at magnetically conjugate regions. These asymmetries lead to differences in plasma convection, neutral winds, total electron content, ion outflow, ionospheric currents and auroral precipitation.

/pdf/north-south-asymmetries-in-earth-s-magnetic-field-4gnoqqtijc.pdf

North–South Asymmetries in Earth’s Magnetic Field

New evidence suggests that the commonly held idea that the aurora borealis (in the Northern Hemisphere) and aurora australis (in the south) are mirror images of each other is not always the case. Aurora do tend to occur at the same time in northern and southern polar regions at locations connected by geomagnetic field lines, a connection that might be expected to link patterns, positions and timing of the two aurora. On 12 May 2001 two Earth observation spacecraft — IMAGE and Polar — were well placed to observe the two poles simultaneously. The resulting images have now been analysed and they provide clear evidence that the auroras can be asymmetric. The asymmetry may be caused by inter-hemispheric currents predicted to arise from conductivity differences, but not previously demonstrated. It is commonly assumed that the aurora borealis (Northern Hemisphere) and the aurora australis (Southern Hemisphere) are mirror images of each other. Here, observations are reported that clearly contradict this common assumption: intense spots are seen at dawn in the Northern summer Hemisphere, and at dusk in the Southern winter Hemisphere. It is commonly assumed that the aurora borealis (Northern Hemisphere) and aurora australis (Southern Hemisphere) are mirror images of each other because the charged particles causing the aurora follow the magnetic field lines connecting the two hemispheres. The particles are believed to be evenly distributed between the two hemispheres, from the source region in the equatorial plane of the magnetosphere. Although it has been shown that similar auroral features in the opposite hemispheres can be displaced tens of degree in longitude1,2 and that seasonal effects can cause differences in global intensity3,4, the overall auroral patterns were still similar. Here we report observations that clearly contradict the common assumption about symmetric aurora: intense spots are seen at dawn in the Northern summer Hemisphere, and at dusk in the Southern winter Hemisphere. The asymmetry is interpreted in terms of inter-hemispheric currents related to seasons, which have been predicted5,6 but hitherto had not been seen.

Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres

Earlier results from analyses of auroral all-sky data obtained on several series of conjugate flights along the College, Alaska, magnetic meridian are reevaluated, and it is suggested that while the diffuse aurora is conjugate, the discrete aurora is nonconjugate but still on closed field lines. The suggestion that the discrete aurora is nonconjugate is contrary to an earlier analysis of the same data set. The old analysis ascribed observed shifts in conjugate points within the discrete auroras to auroral-activity-related perturbations in the geomagnetic field, but it has been impossible to find a physical mechanism for the perturbations. The nonconjugacy likely reflects that the processes creating the visual discrete auroral forms operate independently in each hemisphere. Assuming that the discrete aurora is a consequence of field-aligned currents, we suggest that the observed hemispherical differences in the discrete auroras may be caused by an interhemispherical field-aligned current component. Such a current component will result from the penetration of the interplanetary magnetic field into the magnetosphere and should primarily depend on the sign of By. The conjugate flight data previously analyzed and published appear to qualitatively support this conclusion.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JA03563

Conjugate auroras and the interplanetary magnetic field

>Conjugate auroral all-sky camera data obtained in 18 flights through the auroral zone near the College, Alaska, magnetic meridian show hemispherical differences in auroral frequency and intensity. Conjugate auroras on ihe equatorward boundary of the display (the equatorward are system) are found to be conisistently brighter in the northern hemisphere by a sector of approximately 1.3. At higher latitudes (in the poleward arc system) the auroras exhibit varying degrees of conjugacy, but the auroras in the north tend to occur more frequently and to be brighter. The hemispherical difference is attributed to the 800- gamma difference in magnetic field strength between the conjugate areas. Simplified theoretical calculations of the difference in precipitation at conjugate points from idealized cases of pitch angle diffusion and from a small hemispherical potential difference show that pitch angle diffusion may explain observations in the equatorward arc system but not in the poleward arc sygtem. Thc evidence for hemispheric differences in auroral particle precipitation adds substantially to carrier indications of variation with longitude of auroral precipitation patterns. (auth)

E. M. Wescott

Papers

Differences in auroral intensity at conjugate points

Airborne conjugate auroral studies.