The arctic regions have long been a fascinating location to those interested in the folklore and science of aurora. The aurora borealis is a beautiful and clear manifestation of the causal relationship of the sun to earth. Some of the earliest references to and mythological explanations for the aurora can be found in the ancient Norwegian writings of The King’s Letters dating back to 1250 A.D. Today we understand that the optical luminescence in aurora is due to energetic charged particles streaming downward into the earth’s atmosphere along magnetic field lines and then colliding with atmospheric gases. Figure 1 is an auroral curtain of luminosity where the specific colors result from the de-excitation of mostly oxygen and nitrogen at specific wavelengths. Some of the precipitating particles which cause aurora are energized within the outer regions of the earth’s magnetosphere whereas others trace back to interplanetary space. The morphology of the geomagnetic field with the magnetic poles in close proximity to the geographic poles is such that these fields converge at high-latitudes. Thus, the skies above the arctic region contain the tell-tale signatures of phenomena occurring within the earth’s outer regions and boundary regions with interplanetary space.

The earth’s northern and southern auroral zones consist of annular rings of luminosity centered about their respective geomagnetic poles. Figure 2 shows a view of the entire auroral oval taken by the NASA Dynamics Explorer 1 satellite high above the northern polar cap. Dayside is on the upper left of the image whereas local midnight is on the lower right. Earth reference features are provided for convenience. The altitude of the auroral emissions is typically from 110 to 250 km. Notice that the polar cap above the auroral zone and the sub-auroral region are devoid of luminosity. While this is generally true there are many times when atmospheric emissions can be observed well outside the auroral zone. Also, while this auroral image gives the appearance of being generally stable it belies the dynamic nature of the aurora that is apparent in the ground-based photograph above.

The earth is constantly bombarded by the streaming charged particles of the solar wind. The earth’s magnetic field effectively blocks these particles and causes them to deflect, for the most part, around the earth. The pressure of the solar wind on the sunward side of the earth compresses the dayside magnetic field and elongates the field on the nightside. Embedded within the high-beta plasma of the solar wind is the highly dynamic interplanetary magnetic field (IMF). Depending on the orientation of the IMF with respect to the earth’s geomagnetic field there can be either strong coupling between the IMF and the terrestrial field or weaker coupling.

The cartoon in Figure 3 (not to scale) illustrates the configuration of the earth’s outer magnetic field under the influence of the solar wind. Prior to the mid 1960’s there was vigorous debate concerning whether the earth’s magnetic topology was open or closed; that is, if the geomagnetic field did or did not, respectively, merge with the IMF. The currently accepted notion is that the earth’s magnetic topology is open and that the IMF and geomagnetic fields merge on the dayside and reconnect within the tail region. The physics which govern these processes are complex and modeling their interaction is only possible using non-linear plasma theory. On the dayside there is anecdotal evidence for magnetic field merging from satellites near the space-earth interface typically referred to as the magnetopause. The evidence for magnetic merging within the lower atmosphere can be seen in the motion of discrete auroral structures that form within the auroral zone near noon and then move poleward. Similarly on the nightside the stretched magnetic field lines reconnect and tend to snap back to a more dipolar configuration which causes transient currents to flow within the auroral zones resulting in ground-based magnetic perturbations.

The topic of dayside magnetic merging is one in which I have been interested for many years. In both the northern and southern hemispheres (for the idealized illustration above) the dayside regions of open (merged) magnetic fields that trace down to the lower atmospheric are referred to as the magnetic cusps. The weakly ionized gas region in the lower atmosphere above about 90 km is the ionosphere and the electrodynamics of this region can be monitored by suitably instrumented polar-orbiting satellites. Data from these satellites can then be combined with auroral imagery obtained at high-latitude locations, such as Svalbard in northern Scandinavia, to better separate space and time. Figure 4 is how I see the arctic regions in satellite data. This composite plot includes measurements of particle precipitation, convection, and currents for a polar pass above Svalbard during observations of a poleward moving auroral form. While more detailed discussion of these measurements is beyond the scope of this generalized overview, it is worthwhile noting that the dispersive ion signature (decreasing particle energy with latitude) seen in the middle panel is a clear signature of dayside magnetic merging.