Global Distribution / Structure of Aurora Photograph by Jan Curtis Synthetic Aurora pre- midnight,multi-banded Resonant ULF waves produce pre- midnight, multi-banded aurora Ground Observations Multi-band Multi-band arc structure is typical Satellite Observations pre- midnight Intense aurora occur statistically in pre- midnight sector [ Newell et al., 1996] D. Pokhotelov, W. Lotko, A. Streltsov— Dartmouth College, 2000

PI: W. Lotko/Dartmouth Are Alfvénic arcs the most common type of discrete aurora? Photograph by Jan Curtis Alfvénic Arcs pre-midnight, multi-banded, N-S drifting Resonant ULF waves produce pre-midnight, multi-banded, N-S drifting auroral arcs Ground Observations Multi-band drifting Multi-band, N-S drifting discrete arcs are common Satellite Observations pre-midnight Bright arcs occur statistically in the pre-midnight sector Research by D. Pokhotelov, W. Lotko, A. Streltsov 2000 P.T. Newell et al Discrete auroras intensify, drift and fade, form multi- banded structure, and occur statistically in pre-midnight and low-conductivity regions of the ionosphere. Simulated Alfvénic arcs behave similarly. Latent energy in magnetospheric convection is radiated as resonant Alfvén waves where the ionospheric conductivity is low (nightside and winter) and the N-S Pedersen and Hall currents maximize (typically pre- midnight). “Atkinson-Sato” feedback between the magnetosphere and ionosphere ensues when the Doppler frequency of N-S drifting, ionospheric density fluctuations matches the natural frequency of participating, standing Alfvén waves. The aurora ignites as the wave field-aligned current develops microturbulence near 1 R E altitude, producing a parallel potential drop and a kilovolt energy boost to precipitating plasma sheet electrons. Computer Simulation

PI: W. Lotko/Dartmouth Are Alfvénic arcs the most common type of discrete aurora? Photograph by Jan Curtis Alfvénic Arcs pre-midnight, multi-banded drifting Resonant ULF waves produce pre-midnight, multi-banded, N-S drifting auroral arcs Ground Observations Multi-banded, N-S drifting Multi-banded, N-S drifting discrete arcs are common Satellite Observations pre-midnight Bright arcs occur statistically in the pre-midnight sector Research by D. Pokhotelov, W. Lotko, A. Streltsov 2000 P.T. Newell et al Discrete auroras intensify, drift and fade, form multi-banded structure, and occur statistically in pre-midnight and low-conductivity regions of the ionosphere. Simulated Alfvénic arcs behave similarly. Latent energy in magnetospheric convection is radiated as transverse oscillations of the local magnetic field—Alfvén waves—where the ionospheric conductivity is low (nightside and winter) and the N-S Pedersen and Hall currents maximize (typically pre-midnight). “Atkinson-Sato” feedback between the magnetosphere and ionosphere ensues when the Doppler frequency of N-S drifting, ionospheric density fluctuations matches the Alfvén wave frequency. The aurora ignites as the wave field-aligned current develops microturbulence near 1 R E altitude, producing a parallel potential drop and a kilovolt energy boost to precipitating plasma sheet electrons. Computer Simulation

Are Alfvénic arcs the most common type of discrete aurora? Photograph by Jan Curtis Alfvénic Arcs pre-midnight, multi-banded drifting Resonant ULF waves produce pre-midnight, multi-banded, N-S drifting auroral arcs Ground Observations Multi-banded, N-S drifting Multi-banded, N-S drifting discrete arcs are common Satellite Observations pre-midnight Bright arcs occur statistically in the pre-midnight sector Research by D. Pokhotelov, W. Lotko, A. Streltsov 2000 P.T. Newell et al Discrete auroras intensify, drift and fade, form multi-banded structure, and occur statistically in pre-midnight and low-conductivity regions of the ionosphere. Simulated Alfvénic arcs behave similarly. Latent energy in magnetospheric convection is radiated as Alfvén waves— transverse oscillations of the geomagnetic field— where the ionospheric conductivity is low (nightside and winter) and the N-S Pedersen and Hall currents maximize (typically pre- midnight). “Atkinson-Sato” feedback between the magnetosphere and ionosphere ensues when the Doppler frequency of N-S drifting, ionospheric density fluctuations matches the Alfvén wave frequency. The aurora ignites as the wave field-aligned current develops microturbulence near 1 R E altitude, producing a parallel potential drop and a kilovolt energy boost to precipitating plasma sheet electrons. Computer Simulation

Physical models of the interaction between the ionosphere, magnetosphere and thermosphere (ITM) are essential for characterizing energy flow through the sun-earth connection and for interpreting magnetospheric dynamics through the lenses of satellite and ground-based auroral images. While significant progress has been made in modeling large-scale features of this interaction, the prediction of intermediate- and small-scale features has progressed far more slowly. Detailed physical models exist for many key elements of the ITM interaction at km spatial scales, but linking these models causally and dynamically to each other, and to large-scale processes where most of the energy resides, remains a major challenge for SEC theory. Structure at scales of km in auroral images projects along field lines to magnetospheric dimensions representative of the ion gyroscale at the small end and the thickness of magnetic and velocity shear layers, at the higher end, where enhanced transport takes place. Recent satellite data studies have shown that discrete auroras, the main visible feature of the ITM interaction with arc-like structure at km scales, are statistically more prevalent in the pre-midnight, winter ionosphere. It is also well-known that, when viewed from the ground, these discrete arcs often appear as multi-banded structures that intensify, drift north or south, and then fade. By coupling a model for active ionospheric dynamics with a model for the ultra-low-frequency electrodynamics of a flux tube connecting the northern and southern ionosphere, Dartmouth SECTP researchers have captured the above features of discrete aurora, for the first time, in a computer model based on a physical representation of the small-scale ITM interaction. The following picture emerges from the model. Latent energy stored in large-scale magnetospheric convection is effectively radiated as Alfvén waves—transverse electromagnetic oscillations of the geomagnetic field—in regions where the ionospheric conductivity is low (nightside and winter) and where the combined north-south components of the Hall and Pedersen currents maximize (typically pre-midnight). Conversion from convective to radiative energy forms is enabled by a feedback instability (Atkinson-Sato) between the magnetosphere and ionosphere that ensues when the Doppler frequency of north-south drifting, ionospheric density fluctuations matches the natural (shear Alfvén wave) frequency of the field line. The intensifying, drifting fluctuation ignites as a discrete arc when the field-aligned current carried by the Alfvén wave develops microturbulence near 1 RE altitude, producing a parallel potential drop and a kilovolt energy boost to precipitating plasma sheet electrons. Computed energy fluxes of precipitating electrons in narrow arc structures are sufficient to produce observed auroral luminosities. The model explains both the observed seasonal and local-time dependence of discrete aurora by causally linking the ambient state of the ionosphere with feedback instability of large-scale magnetospheric convection. Dynamical arc features—intensification, drift motion and fading—are associated with inherent time delays in ITM feedback involving Alfvén wave propagation along geomagnetic field lines between the northern and southern ionosphere.