Jason Derr, UT-Austin

"Stability Analyses of Auroral Substorm Onset and Solar Wind"

Abstract: Pertaining to the stability analysis of auroral substorm onset, a geometric wedge model of the near-earth nightside plasma sheet is used to derive a wave equation for low frequency shear flow-interchange waves which transmit ExB sheared zonal flows along magnetic flux tubes towards the ionosphere. Discrepancies with the wave equation result used in Kalmoni (2015) for shear flow-ballooning instability are discussed. The wedge wave equation is used to compute rough expressions for dispersion relations and local growth rates in the midnight region of the nightside magnetotail where the instability develops, forming the auroral beads characteristic of geomagnetic substorm onset. Stability analysis for the shear flow-interchange modes demonstrates that nonlinear analysis is necessary for quantitatively accurate results and determines the spatial scale on which the instability varies. The Rice Convection Model-Equilibrium (RCM-E) is used to provide background fields for a global magnetospheric wedge wave equation, from which the growth rates and dispersion relations can be calculated for the shear flow-interchange instability. Mapping of this growing traveling wave back to the magnetosphere yields the auroral bead projections of the instability. The cause of magnetic substorm onset by comparison with the beads, and its location in the magnetotail, is determined. The linear stage of the instability is discussed in detail. Subsequent nonlinear relaxation properties of the auroral arc, including saturation value of instability amplitudes, are determined. Shear flow-interchange instability appears to cause magnetic substorm onset, insofar as auroral beads are its signature.

Pertaining to the stability analysis of jet microstreams, fast solar wind streams are known to be dominated by Alfvenic turbulence, i.e. large amplitude magnetic field and quasi-incompressible velocity fluctuations with a correlation corresponding to waves propagating away from the Sun. At the same time the Ulysses spacecraft showed that microstreams, persistent long period (1/2-2 days) fluctuations in the radial velocity field are ubiquitous in the fast wind. This contribution explores the possible causal relation between microstreams and Alfv{'e}nic turbulence. We carry out a parametric study of the stability of the microstream jets to Kelvin-Helmholtz (KH) instabilities: starting from the profiles of density, radial speed and magnetic field observed in the solar wind, we investigate both at what distance from the Sun KH instabilities may be triggered and the ensuing nonlinear dynamics.

Pertaining to the stability analysis of jet microstreams, fast solar wind streams are known to be dominated by Alfvenic turbulence, i.e. large amplitude magnetic field and quasi-incompressible velocity fluctuations with a correlation corresponding to waves propagating away from the Sun. At the same time the Ulysses spacecraft showed that microstreams, persistent long period (1/2-2 days) fluctuations in the radial velocity field are ubiquitous in the fast wind. This contribution explores the possible causal relation between microstreams and Alfv{'e}nic turbulence. We carry out a parametric study of the stability of the microstream jets to Kelvin-Helmholtz (KH) instabilities: starting from the profiles of density, radial speed and magnetic field observed in the solar wind, we investigate both at what distance from the Sun KH instabilities may be triggered and the ensuing nonlinear dynamics.