February 19, 2010, 12:00 pm - 1:00 pm
February 19, 2010, 12:00 pm - 1:00 pm
Recent Results from the Wind Mission
Adam Szabo (Heliospheric Physics Laboratory)
After over 15 years of successful operation, the Wind spacecraft, orbiting the L1 Lagrange point in the upstream solar wind, is still returning significant new observations. Recently, solar wind thermal plasma measurements revealed 1 AU signatures of the physical processes accelerating the supersonic solar wind near the Sun. Enhanced perpendicular heating of alpha particles point to wave dissipation through resonant scattering. Furthermore, combined Wind, ACE and STEREO observations of interplanetary coronal mass ejections(ICMEs) established clear departures from the commonly assumed force-free magnetic flux rope geometry. The implication is that ICMEs loose considerable amount of their magnetic flux through reconnection in the inner heliosphere.
Magnetopause boundary region vortices observed during fixed solar wind conditions
Yaireska M. Collado-Vega (Heliospheric Physics Laboratory)
With MHD simulations using fixed solar wind speeds and a step function in IMF direction we have found vortices on the magnetopause flanks near the ecliptic plane. We visualized the 2D and 3D nature of the vortices using a tool called “MHD Explorer”. We compare and contrast a case of nominal solar wind speed, around 360 km/s, and a higher solar wind speed, 700 km/s, both starting with southward IMF, -5 nT, for 15 minutes, and then turning northward, +5 nT for two hours. A total of 11 vortices were found for the case with nominal solar wind speed. For the case with higher solar wind speed, 700 km/s, around 25 vortices were found. In a previous study we presented a total of 304 vortices found near the ecliptic plane on the magnetopause flanks using simulated MHD data driven by real solar wind conditions. Comparisons of these results to our recent simulations with fixed conditions will be presented.
Modeling Spectral Turnovers in Interplanetary Shocks Observed by Ulysses
Errol Summerlin (Heliospheric Physics Laboratory)
Interplanetary shocks in the heliosphere provide excellent test cases for the simulation and theory of particle acceleration at shocks thanks to the presence of in-situ measurements and a relatively well understood initial particle distribution. The Monte-Carlo test particle simulation employed in this work has been previously used to study injection and acceleration from thermal energies into the high energy power-law tail at co-rotating interaction regions (CIRs) in the heliosphere presuming a steady state planar shock (Summerlin & Baring, 2006, Baring and Summerlin, 2008). These simulated power-spectra compare favorably with in-situ measurements from the ULYSSES spacecraft below 60 keV. However, to effectively model the high energy exponential cutoff at energies above 60 keV observed in these distributions, simulations must apply spatial or temporal constraints to the acceleration process. This work studies the effects of a variety of temporal and spatial constraints (including spatial constraints on the turbulent region around the shock as determined by magnetometer data, spatial constraints related to the scale size of the shock and constraints on the acceleration time based on the known limits for the shock's lifetime) on the high energy cut-off and compares simulated particle spectra to those observed by the ULYSSES HI-SCALE instrument in an effort to determine which constraint is creating the cut-off and using that constraining parameter to determine additional information about the shock that can not, normally, be determined by a single data point, such as the spatial extent of the shock or how long the shock has been propagating through the heliosphere before it encounters the spacecraft. Shocks observed by multiple spacecraft will be of particular interest as their parameters will be better constrained than shocks observed by only one spacecraft. To achieve these goals, the simulation will be modified to include the retrodictive approach of Jones (1978) to accurately track time spent downstream while maintaining, to large degree, the large dynamic range and short run times that make this type of simulation so attractive. This work is inspired by examinations of acceleration cutoffs in SEP events performed by various authors (see Li et al., 2009, and references therein), and it is hoped that this work will pave the way for a multi-species analysis similar to theirs that should greatly enhance the information one can derive about shocks based on individual observations. This talk presents preliminary results of this study.