Acknowledgments This material is based upon work supported by CISM, which is funded by the

1. Comparing the Ionospheric Cross-Polar Cap Potential from an Assimilative Model to a Semi-Empirical Equation Cezanne Narcisse, Joseph Schinco, Kyle Van Zuiden, Robert Bruntz, and Ramon E. Lopez Bruntz et al. ViscousPotential Bruntz et al. (2012) used the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamic (MHD) simulation of the magnetosphere to find a formula to predict the value of the ionospheric potential due to the viscous interaction - the viscous potential - based on input solar wind conditions. VP = (0.00431)*n^(0.439)*Vx^(1.33), where VP is the predicted viscous potential (in kV), n is the solar wind density (#/cc), Vx is the component of the solar wind velocity aligned directly from the Sun to the Earth (in km/s) Bhattarai et al. (2012) recently discovered that the viscous interaction is reduced when the IMF has a northward component to it. The Bruntz et al. (2012) formula works well for southward or zero IMF, but does less well as the IMF becomes more northward. Results So Far Introduction Though most people are unaware of it, space weather is constantly going on above our heads - and under our feet, and in every other direction around the Earth. Ionized gas, or plasma, flows out from the Sun in all directions and at all times. That "solar wind" blows past the Earth, whose magnetic field creates a bubble that protects Earth from the solar wind. Solar storms and other space weather can disturb that bubble, though, and threaten satellites, cell phones, power grids, and a variety of other technology that we depend on every day. In order to better predict and avoid the effects of space weather, we must understand the physical processes that are occurring. One important indicator of the interaction between the solar wind and Earth's magnetosphere is the cross-polar cap potential (CPCP), and one important method of studying the CPCP is through models and simulations. In order to trust the output of our models, though, we must understand how well their output describes physical processes. OMNI Data Omni is a database of collected information obtained by numerous satellites in the solar wind. These satellites measure data, such as the x, y, and z components of the solar wind velocity and interplanetary magnetic field (IMF), the plasma density and temperature, and other related data. That data is propagated from the satellite position to the Earth's bowshock, to predict what solar wind the magnetosphere will encounter. Plotting all of the AMIE data that we have, for multiple years, shows that the CPCP from AMIE never drops below ~9 kV. This is lower than the floor for the Weimer CPCP (~25 kV), but it isn't clear if the minimum value in AMIE is due to the minimum in Weimer or the fact that there is always interaction between the solar wind and Earth's magnetosphere. Our Work Purpose: To find out if the AMIE CPCP has a "floor" value, like the Weimer 05 model does. • Cross-Polar Cap Potential (CPCP) • Interaction between Earth's magnetic field produces electric fields in Earth's ionosphere, near the poles • The two main sources of those electric fields are: • magnetic reconnection between Earth's magnetic field and the interplanetary magnetic field (IMF) • the viscous interaction between the solar wind plasma and plasma at the boundary of Earth's magnetosphere • Electric fields in the polar cap can be used to find electric potentials over the polar cap (E = -(grad)phi, where phi is the electrostatic potential) • The cross-polar cap potential is the difference between maximum and minimum potentials over the whole polar ionosphere Comparing the AMIE CPCP and the Bruntz VP, we find that there are times when the AMIE CPCP drops below the Bruntz VP, indicating that AMIE is capturing the reduction of the viscous potential that was predicted by Bhattarai et al. (2012), but is not seen in the Weimer 05 model. (The data in this plot have been shifted by about 10 minutes, to line up solar wind conditions and corresponding ionospheric reactions better, but the exact amount of the shift has not yet been empirically determined.) • The LFM MHD model (black trace) shows a reduced viscous potential (VP) for northward Bz, whereas the Bruntz et al. VP formula (blue) does not - so LFM CPCP can drop below the Bruntz et al. VP for northward Bz (purple, at the bottom of the plot). • The Weimer model (red trace) does not account for the reduced viscous potential either, so it never drops below the Bruntz et al. VP. In fact, it has a minimum value of about 25 kV. • If the AMIE model includes the reduced viscous potential, it can drop below the Bruntz et al. VP value, like LFM; if AMIE does not include the reduced viscous potential, it will never drop below the Bruntz et al. VP value, in any conditions. AMIE AMIE is an algorithm that takes input from ionospheric electric fields, magnetic fields, and currents, collected from various data sources, used to calculate the cross polar cap potential (CPCP) and other values. Some of these sources are magnetometers on earth, radar systems, and low-altitude satellites. AMIE uses the Weimer 05 model, which predicts the CPCP, but has a minimum value that the CPCP can take. This minimum value might not be true for the CPCP calculated by AMIE due to it taking other factors into consideration. References Bhattarai, S. K., R. E. Lopez, R. Bruntz, J. G. Lyon, M. Wiltberger, Simulation of the Polar Cap Potential during Periods with Northward Interplanetary Magnetic Field (2012), J. Geophys. Res., doi:10.1029/2011JA017143, in press. Bruntz, R., R. E. Lopez, M. Wiltberger, and J. G. Lyon (2012), Investigation of the viscous potential using an MHD simulation, J. Geophys. Res., 117, A03214, doi:10.1029/2011JA017022. Bruntz, R., R. E. Lopez, S. K. Bhattarai, K. H. Pham, Y. Deng, Y. Huang, M. Wiltberger, and J. G. Lyon, Investigating the viscous interaction and its role in generating the ionospheric potential during the Whole Heliosphere Interval, J. Atmos. Sol. Terr. Phys., conditionally accepted, revision submitted (Feb. 2012), awaiting decision. Weimer, D. R. (2005), Improved ionospheric electrodynamic models and application to calculating Joule heating rates, J. Geophys. Res., 110, A05306, doi:10.1029/2004JA010884. Acknowledgments This material is based upon work supported by CISM, which is funded by the STC Program of the National Science Foundation under Agreement Number ATM-0120950. The Authors thank OMNIWeb for providing OMNI data and Gang Lu for providing AMIE data.