Temporal Evolution of Substorm‐Driven Global Alfvén Wave Power Above the Auroral Acceleration Region

Abstract

The onset of substorms is associated with bursty enhancements of Alfvén wave power throughout the magnetotail. While impossible to assess the total Alfvén wave power in the entire magnetotail, we have instead monitored waves that are funneled into the auroral acceleration region, in order to assess the temporal evolution of Alfvén wave power above the nightside auroral zone in relation to substorm phases. The substorms were grouped by three conditions: nonstorm periods, storm periods, and all (unconditioned) periods. Using superposed epoch analysis, we found that the global magnetohydrodynamic Alfvén wave power increased significantly at onset for all three conditions, while a power decrease to preonset values occurred within 2 hr. Specifically, the peak inflowing power during the expansion phase was 5.7 GW for unconditioned substorms, 5.6 GW for nonstorm-time substorms, and 7.8 GW for storm-time substorms. These results correspond to power increases with respect to preonset values of 138%, 366%, and 200%, respectively. Additional analysis in relation to the aurora was performed for nonstorm-time substorms only. During the expansion phase, about 50% of the Alfvén wave power over the entire nightside auroral zone is collocated with the auroral bulge region. Furthermore, the total inflowing Alfvén wave power over the entire nightside auroral zone is 17% of the conjugate auroral power, while the inflowing power over the auroral bulge region is 32% of the conjugate aurora. However, allowing for a finite absorption efficiency inside the auroral acceleration region, the likely average Alfvénic contributions to the aurora are approximately 10% and 18%, respectively.

Observational Evidence for the Role of Hall Conductance in Alfvén Wave Reflection

“…However, the highly positive Alfvén wave reflection coefficient here yields an increase in local FAC through positive wave interference. This increase in FAC comes in addition to electron precipitation in the diffuse aurora, highlighting the duality between electromagnetic energy flux and auroral precipitation in the magnetosphere‐ionosphere energy transfer, and should be seen in context with reports that electromagnetic Poynting flux powers auroral precipitation (Keiling et al, 2003, 2020). Furthermore, the co‐location of areas of enhanced α obs and of pulsating aurora (Grono & Donovan, 2020) should likewise be seen in context with reports that pulsating aurora significantly modulates the FAC system (Hosokawa et al, 2010).…”

Validation of Simulated Statistical Characteristics of Magnetosphere‐Ionosphere Coupling in Global Geospace Simulations Over an Entire Carrington Rotation

“…(2017). This simulated dawn‐dusk asymmetry can be supported by many observations, such as the dawn‐dusk asymmetry in the post‐substorm period driven by southward IMF observed by the Polar satellite and SuperMAG ground magnetometer (Keiling et al., 2020).…”

“…These statistical characteristics are consistent with the distributions of inertial Alfvén wave (IAW) Poynting flux observed by FAST and shown in Figure 2a of Hatch et al (2017). This simulated dawn-dusk asymmetry can be supported by many observations, such as the dawn-dusk asymmetry in the post-substorm period driven by southward IMF observed by the Polar satellite and SuperMAG ground magnetometer (Keiling et al, 2020).…”

Observational evidence for the role of Hall conductance in Alfvén wave reflection

“…However, the highly positive Alfvén wave reflection coefficient here yields an increase in local FAC through positive wave interference. This increase in FAC comes in addition to electron precipitation in the diffuse aurora, highlighting the duality between electromagnetic energy flux and auroral precipitation in the magnetosphere-ionosphere energy transfer, and should be seen in context with reports that electromagnetic Poynting flux powers auroral precipitation (Keiling et al, 2003(Keiling et al, , 2020. Furthermore, the co-location of areas of enhanced obs and of pulsating aurora Journal of Geophysical Research: Space Physics 10.1029/2020JA028119 (Grono & Donovan, 2020) should likewise be seen in context with reports that pulsating aurora significantly modulates the FAC system (Hosokawa et al, 2010).…”