Seasonal dependence of localized, high‐latitude dayside aurora (HiLDA)

Abstract

[1] The FUV instrument on the IMAGE spacecraft frequently observes intense ultraviolet (UV) emissions from a localized High Latitude Dayside Aurora (HiLDA) poleward of the general auroral oval location [Frey et al., 2003a]. It has been shown that this aurora is entirely created by high-energy precipitating electrons, which have probably been accelerated in a quasi-static field-aligned electric potential. Here we extend the previous case study to an investigation of the HiLDA occurrence in the Northern Hemisphere over more than 2 years and compare it with the averaged solar wind plasma and interplanetary magnetic field (IMF) properties. HiLDA occurrence is strongly biased toward low solar wind density and IMF with positive B z , strong positive B y (clock angles around 70°), and negative B x components. Such IMF conditions are favorable for lobe reconnection in the Northern Hemisphere, and the creation of a dominating dusk convection cell with an upward field-aligned current in its center. Additionally, we investigate the seasonal occurrence of this phenomenon, which shows a maximum in the Northern Hemisphere during sunlit summer months and an almost complete absence in the dark winter. In contrast to the daylight-suppressed aurora in the auroral oval, the HiLDA cannot be the result of ionospheric feedback due to the stronger ionospheric conductance in sunlight. Instead, in agreement with ionospheric convection models, it is caused by the asymmetry of field-aligned currents in different seasons, which result from the different dipole tilt angles in summer and winter. We further discuss two scenarios for how the low solar wind density can enhance the field-aligned parallel potential over the dusk convection cell.

The asymmetric geospace as displayed during the geomagnetic storm on August 17, 2001

“…In the southern hemisphere we do not see any features that could ) which makes the northern lobe more exposed to the IMF than the southern lobe, and can explain why we see lobe reconnection in one hemisphere and not in the other. This is in agreement with Crooker and Rich (1993) who suggested that lobe reconnection is a summer phenomenon that can occur in one hemisphere only and Frey et al (2003bFrey et al ( , 2004 who found the HiLDA also to be a summer phenomenon.…”

“…The combination of dayside reconnection and lobe reconnection was suggested by Reiff and Burch (1985) for weakly southward IMF with dominated B Y , which is exactly the IMF conditions we have. Compared to their Figure 1, our red line corresponds to their region L to M, and our green line (lobe reconnection) to their region between 10 M and H. The statistical studies by Frey et al (2003bFrey et al ( , 2004) also support our interpretation. In Figure 11C we have drawn (by hand) the same convection pattern and open-closed boundary onto the WIC color image from 17:47:00 UT.…”

“…Figure 13 shows IMAGE WIC images (northern hemisphere) from 16:19 UT to 16:49 UT, where this spot is seen clearly detached from the 15 auroral oval in all the ten images (see also the video, uploaded as auxiliary data). Frey et al (2003bFrey et al ( , 2004) explored statistically the characteristics of this high-latitude dayside aurora (HiLDA) and concluded that the major driving process was high-latitude (lobe) reconnection and that it occurred preferentially in the summer hemisphere.…”

Seasonal dependence and solar wind control of transpolar arc luminosity

“…The seasonal dependence of single auroral arcs appearing poleward of the main oval has been addressed by Frey et al [2004] for localized high‐latitude dayside aurora (HilDA) and by Cumnock [2005] for TPAs. Frey et al [2004] observed a maximum occurrence of HiLDAs during sunlit summer months and an almost complete absence in the dark winter. Cumnock [2005] found a similar seasonal dependence of the TPA luminosity.…”

Propagating plasma patch and associated neutral stream flows

“…Recently, it has been observed that intense, localized, auroral emissions occur in the dayside polar cap during northward IMF (Milan et al, 2000;Sandholt et al, 2001;Fuselier et al, 2002;Frey et al, 2002Frey et al, , 2003Frey et al, , 2004Østgaard et al, 2005). Particle precipitation associated with the intense aurora could then cause significant ionization, resulting in regions of high electron concentrations that may lead to F region patches that can then convect across the polar cap.…”