Rethinking the polar cap: Eccentric dipole structuring of ULF power at the highest corrected geomagnetic latitudes

Abstract

The day‐to‐day evolution and statistical features of Pc3‐Pc7 band ultralow frequency (ULF) power throughout the southern polar cap suggest that the corrected geomagnetic (CGM) coordinates do not adequately organize the observed hydromagnetic spatial structure. It is shown that that the local‐time distribution of ULF power at sites along CGM latitudinal parallels exhibit fundamental differences and that the CGM latitude of a site in general is not indicative of the site's projection into the magnetosphere. Thus, ULF characteristics observed at a single site in the polar cap cannot be freely generalized to other sites of similar CGM latitude but separated in magnetic local time, and the inadequacy of CGM coordinates in the polar cap has implications for conjugacy/mapping studies in general. In seeking alternative, observationally motivated systems of “polar cap latitudes,” it is found that eccentric dipole (ED) coordinates have several strengths in organizing the hydromagnetic spatial structure in the polar cap region. ED latitudes appear to better classify the local‐time ULF power in both magnitude and morphology and better differentiate the “deep polar cap” (where the ULF power is largely UT dependent and nearly free of local‐time structure) from the “peripheral polar cap” (where near‐magnetic noon pulsations dominate at lower and lower frequencies as one increases in ED latitude). Eccentric local time is shown to better align the local‐time profiles in the magnetic east component over several PcX bands but worsen in the magnetic north component. It is suggested that a hybrid ED‐CGM coordinate system might capture the strengths of both CGM and ED coordinates. It is shown that the local‐time morphology of median ULF power at high‐latitude sites is dominantly driven by where they project into the magnetosphere, which is best quantified by their proximity to the low‐altitude cusp on the dayside (which is not necessarily quantified by a site's CGM latitude), and that variations in the local‐time morphology at sites similar in ED latitude are due to both geographic local‐time control (relative amplification or dampening by the diurnal variation in the local ionospheric conductivity) and geomagnetic coastal effects (enhanced power in a coastally mediated direction). Regardless of cause, it is emphasized that the application of CGM latitudes in the polar cap region is not entirely meaningful and likely should be dispensed with in favor of a scheme that is in better accord with the observed hydromagnetic spatial structure.

Magnetopause ripples going against the flow form azimuthally stationary surface waves

“…MSE constitute the lowest possible frequency normal mode of the magnetospheric system and its fundamental frequencies can often be fractions of milliHertz 33 . Such long-period narrowband waves are challenging to identify observationally in general, either by orbiting spacecraft or ground-based measurements, due to potential spatio-temporal mixing 54 and the difficultly in distinguishing from turbulence/noise 55 , 56 . For these reasons global magnetospheric dynamics and their associated ULF waves have often concentrated on the continuous pulsation (Pc) bands above 2 mHz 57 , which would not typically incorporate the effects presented here.…”

Magnetopause ripples going against the flow form azimuthally stationary surface waves

“…MSE constitute the lowest possible frequency normal mode of the magnetospheric system and its fundamental frequencies can often be fractions of milliHertz [33]. Such long period narrowband waves are challenging to identify observationally in general, either by orbiting spacecraft or ground-based measurements, due to potential spatio-temporal mixing [54] and the difficultly in distinguishing from turbulence/noise [55,56]. For these reasons global magnetospheric dynamics and their associated ULF waves have often concentrated on the continuous pulsation (Pc) bands above 2 mHz [57], which would not typically incorporate the effects presented here.…”

Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex events

“…This region is typically thought to be located under open field lines mapping to the lobes of the tail and often directly connected to solar wind field lines. The lack of wave signatures could be because AGO5 is located far away from the wave source or its location in the region of open field lines is more susceptible to solar wind conditions and geomagnetic activities [e.g., Engebretson et al , ], prohibiting efficient propagation in the waveguide (see Urban et al [] for discussions of the station's location in the open field lines). We emphasize, however, that the data show very efficient ionospheric waveguide propagation over a large latitudinal extent from −62° MLAT (HBA) to −80° MLAT (AGO1).…”