Magnetorotational instability (MRI) is the most promising mechanism behind accretion in low-mass protostellar disks. Here we present the first analysis of the global structure and evolution of non-ideal MRI-driven T-Tauri disks on million-year timescales. We accomplish this in a 1+1D simulation by calculating magnetic diffusivities and utilizing turbulence activity criteria to determine thermal structure and accretion rate without resorting to a 3-D magnetohydrodynamical (MHD) simulation. Our major findings are as follows. First, even for modest surface densities of just a few times the minimum-mass solar nebula, the dead zone encompasses the giant planet-forming region, preserving any compositional gradients. Second, the surface density of the active layer is nearly constant in time at roughly 10 g cm −2 , which we use to derive a simple prescription for viscous heating in MRI-active disks for those who wish to avoid detailed MHD computations. Furthermore, unlike a standard disk with constant-α viscosity, the disk midplane does not cool off over time, though the surface cools as the star evolves along the Hayashi track. Instead, the MRI may pile material in the dead zone, causing it to heat up over time. The ice line is firmly in the terrestrial planet-forming region throughout disk evolution and can move either inward or outward with time, depending on whether pileups form near the star. Finally, steady-state mass transport is an extremely poor description of flow through an MRI-active disk, as we see both the turnaround in the accretion flow required by conservation of angular momentum and peaks inṀ (R) bracketing each side of the dead zone. We caution that MRI activity is sensitive to many parameters, including stellar X-ray flux, grain size, gas/small grain mass ratio and magnetic field strength, and we have not performed an exhaustive parameter study here. Our 1+1D model also does not include azimuthal information, which prevents us from modeling the effects of Rossby waves.

An empirical model of subauroral polarization stream (SAPS) electric fields has been developed using measurements of ion drifts and particle precipitation made by the Defense Meteorological Satellite Program from 1987 to 2012 and Dynamics Explorer 2 as functions of magnetic local time (MLT), magnetic latitude, the auroral electrojet index (AE), hemisphere, and day of year. Over 500,000 subauroral passes are used. This model is oriented in degree magnetic latitude equatorward of the aurora and takes median values instead of the mean to avoid the contribution of low occurrence frequency subauroral ion drifts so that the model is representative of the much more common, latitudinally broad, low‐amplitude SAPS field. The SAPS model is in broad agreement with previous statistical efforts in the variation of the SAPS field with MLT and magnetic activity level, although the median field is weaker. Furthermore, we find that the median SAPS field is roughly conjugate in both hemispheres for all seasons, with a maximum in SAPS amplitude and width found for 1800–2000 MLT. The SAPS amplitude is found to vary seasonally only from about 1800–2000 MLT, maximizing in both hemispheres during equinox months. Because this feature exists despite controlling for the AE index, it is suggested that this is due to a seasonal variation in the flux tube averaged ionospheric conductance at MLT sectors where it is more likely that one flux tube footprint is in darkness while the other is in daylight.

A new model of the equatorward boundary of the diffuse aurora has been developed using observations of precipitating particles made by the Defense Meteorological Satellite Program (DMSP) from 1987 to 2012 as well as Dynamics Explorer 2 (DE 2), which operated from August 1981 to February 1983. Using a local multilinear regression algorithm, we investigated the use of different combinations of magnetometer indices and solar wind coupling functions with different averaging periods and weights to find the best parameterization for a model of the equatorward boundary of the aurora. We find that weighted averages of the AE index and the solar wind coupling function dΦ MP /dt both outperform the often used Kp index. Using conjunction events where two DMSP satellites cross the auroral boundary at nearly the same time, we show that these models are better at extrapolating the auroral boundary to different local times than previous models.This function is derived such that it is proportional to the rate that magnetic flux is opened at the magnetopause. The equatorward boundary of the diffuse aurora is of particular interest when characterizing subauroral electric fields, such as subauroral ion drifts (Yeh et al., 1991) or subauroral polarization streams (Foster & LANDRY AND ANDERSON 2072 Key Points: • Models of the equatorward auroral boundary parameterized by AE and the solar wind coupling function dΦ MP /dt outperform a Kp-based model • Both the AE and dΦ MP /dt models show a seasonal variation not seen when parameterizing by Kp • The two new boundary models outperform previous models at extrapolating the auroral boundary latitude from a single determination