HAO 2012 Profiles In Science: Dr. Michael Wiltberger

Contact:

303-497-1532
wiltbemj@ucar.edu

Dr. Michael Wiltberger is a Scientist III in the High Altitude Observatory of the National Center for Atmospheric Research. His main research interest is modeling and analysis of magnetospheric processes using global scale magnetohydrodynamic simulations. Specific areas of interest include coupling between the magnetosphere and ionosphere, structure and dynamics of ultra-low frequency waves and their impacts of radiation belt particles, and coupling between the magnetotail and the inner magnetosphere during storms and substorms.

Papers

  1. Cnossen, I., A. D. Richmond, M. Wiltberger, W. Wang, and P. Schmitt, 2011: The response of the coupled magnetosphere-ionosphere-thermosphere system to a 25% reduction in the dipole moment of the Earth's magnetic field, J. Geophys. Res, 116(A), 12304, doi:10.1029/2011JA017063.
  2. Wang, W., J. Lei, A. G. Burns, L. Qian, S. C. Solomon, M. Wiltberger, and J. Xu, 2011: Ionospheric Day-to-Day Variability Around the Whole Heliosphere Interval in 2008, Solar Physics, 76, doi:10.1007/s11207-011-9747-0.
  3. Bhattari, S. K., R. E. Lopez, R. Bruntz, J. G. Lyon, and M. Wiltberger, 2012: Simualtion of the Polar Cap Potential during Peroids with Northward Interplanetary Magnetic Field, in J. Geophys. Res., doi:10.1029/2011JA017143.
  4. Bruntz, R., R. E. Lopez, M. Wiltberger, and J. G. Lyon, 2012: Investigation of the viscous potential using an MHD simulation, J. Geophys. Res., doi:10.1029/2011JA017022.
  5. Cnossen, I., A. D. Richmond, and M. Wilberger, 2012: The dependence of the coupled magnetosphere-ionosphere-thermosphere system on the Earth's magnetic dipole moment, J. Geophys. Res., 116(A), A05302, doi:10.1029/2012JA017555.
  6. Cnossen, I., M. Wiltberger, J. E. Ouellette, The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind- maagnetosphere- ionosphere coupling, J. Geophys. Res., doi:10.1029/2012JA017825.
  7. Lopez, R. E., S. Bhattarai, R. Bruntz, K. Pham, M. Wiltberger, J. G. Lyon, Y. Deng, and Y. Huang, 2012: The Role of Dayside Merging in Generating the Ionospheric Potential During the Whole Heliosphere Interval, in J. Atmos. Solar Terr. Phys., 10.1016/j.jastp.2012.03.001.
  8. Pembroke, A., F. Toffoletto, S. Sazykin, M. Wiltberger, J. G. Lyon, V. Merkin, and P. Schmitt, 2012: Initial results from a Dynamic Coupled Magnetosphere-Ionosphere-Ring Current model, in J. Geophys. Res., doi:10.1029/2011JA016979.
  9. Wiltberger, M., L. Qain, C. –L., Huang, W. Wang, R. E. Lopez, A. G. Burns, S. C. Solomon, Y. Deng, and Y. Huang, 2011: CMIT study of CR2060 and 2068 comparing L1 and MAS solar wind drivers, in J. Atmos. Solar Terr. Phys., doi:10.1016/j.jastp.2012.01.005.
  10. Mary Hudson, Thiago Brito, Scot Elkington, Brian Kress, Zhao Li and Mike Wiltberger, 2012: Radiation belt 2D and 3D simulations for CIR-driven storms during Carrington Rotation 2068, J. Atmos. Solar Terres. Phys., 83, p. 51-62.
  11. Zhang, B., W. Lotko, O. Brambles, P. Damiano, M. Wiltberger and J. Lyon, 2012: Magnetotail Origins of Auroral Alfvenic Power, J. Geophys. Res., doi:10.1029/2012JA017680.
  12. Solomon, S. C., A. G. Burns, B. A. Emery, M. G. Mlynczak, L. Qian, W. Wang, D. R. Weimer, and M. Wiltberger, 2012: Modeling studies of the impact of high-speed streams and co-rotating interaction regions on the thermosphere-ionosphere, J. Geophys. Res., submitted, doi:10.1029/2011JA017417.

Nuggets

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(1) Wiltberger, M., L. Qain, C. -L., Huang, W. Wang, R. E. Lopez, A. G. Burns, S. C. Solomon, Y. Deng, and Y. Huang, 2011: CMIT study of CR2060 and 2068 comparing L1 and MAS solar wind drivers, in J. Atmos. Solar Terr. Phys., doi:10.1016/j.jastp.2012.01.005.

For the first time CMIT has be run continuously for an entire solar rotation. The Whole Heliosphere Interval (March 20-April16, 2008) was selected by the IHY was period for concentrated study and analysis. Using the CMIT model we completed simulations of the response of geospace to the Corotating Interaction Regions (CIRs) present in this interval. The simulations driven by the solar wind observations show the ability of the model to capture the response throughout geospace. It highlights an issue with the hemispheric power, or energy input by the aurora being underestimated in the model results. It also shows the need for improved ring current modeling in the poor SYMH results an area of current research for the CMIT team. The differing responses of geospace the CIR intervals illustrates the importance of considering the entire solar wind not just the velocity or magnetic field in isolation. Another clear conclusion is that while the heliospheric simulations can reproduce some of the solar wind velocity features the inability of the model to accurately represent the magnetic field imposes serious limitations on the quality of CMIT results conducted using the heliospheric solar wind data as input.

Figure 1 caption: Comparison of geophysical response within the CMIT to driving by both solar wind and observations and modeling.

(2) Pembroke, A., F. Toffoletto, S. Sazykin, M. Wiltberger, J. G. Lyon, V. Merkin, and P. Schmitt, 2012: Initial results from a Dynamic Coupled Magnetosphere-Ionosphere-Ring Current model, in J. Geophys. Res., doi:10.1029/2011JA016979.

Global models of the magnetosphere have difficulties in modeling the inner magnetosphere. In order to address that issue we have completed a two-way coupling of the Rice Convection Model (RCM) of the inner magnetosphere to the LFM-MIX global model of the magnetosphere. This coupling is accomplished by passing magnetic field, plasma density and pressures to the RCM. The RCM computes the drifts of the plasma in the inner magnetosphere and returns the plasma information to the global model. A key component of this coupling is exclusion of regions of high-speed flows that violate RCM slow flow assumptions from the coupling region. Results of the coupling show improved representation of the ring current in the inner magnetosphere, including stronger Dst index. The coupling model also has stronger region 2 field align currents and better captures the dynamics of potential shielding seen when the IMF changes direction. The new coupled model represents a significant improvement in the capabilities available to community of scientists using the LFM-MIX model to simulate geospace.

Figure 2 caption: Comparison between the simulations results for the uncoupled version of the LFM-MIX model and the one coupled to the RCM. The top row shows the perturbation magnetic field in with a stronger ring current. The bottom row shows the plasma beta indicating how including the ring current inflates the magnetosphere.

(3) Zhang, B., W. Lotko, O. Brambles, P. Damiano, M. Wiltberger and J. Lyon, 2012: Magnetotail Origins of Auroral Alfvenic Power, J. Geophys. Res. doi:10.1029/2012JA017680.

The generation of Alfvénic Poynting flux in the central plasma sheet and its polar distribution at low altitude are studied using three dimensional global simulations of
the solar wind-magnetosphere-ionosphere interaction. A 24-hour event simulation
(4–5 Feb 2004) driven by solar wind and interplanetary magnetic field data reproduces the global morphology of Alfvénic Poynting flux measured by the Polar satellite, including its dawn-dusk asymmetry. Controlled simulations show that the dawn-dusk asymmetry is regulated by the spatial variation in ionospheric conductance. The asymmetry disappears when the conductance is taken to be spatially uniform. The simulated Alfvénic Poynting flux is generated in the magnetotail by time-variable, fast flows emerging from nightside reconnection. The simulated fast flows are more intense in the premidnight sector as observed; this asymmetry also disappears when the ionospheric conductance is spatially uniform. Analysis of the wave propagation in the plasma sheet source region, near xGSM ≈ 15 RE, shows that as the fast flow brakes, a portion of its kinetic energy is transformed into the electromagnetic energy of intermediate and fast magnetohydrodynamic waves. The wave power is dominantly compressional in the source region and becomes increasingly Alfvénic as it propagates along magnetic field lines toward the ionosphere.

Figure 3 caption: The wave power generated as the flows in the inner magnetosphere brake generates electromagnetic wave power. As this wave power propogates down to the ionosphere it becomes more Alfvenic.

(4) Cnossen, I., M. Wiltberger, J. E. Ouellette, The effects of seasonal and diurnal variations in the Earth’s magnetic dipole orientation on solar wind- maagnetosphere- ionosphere coupling, J. Geophys. Res., doi:10.1029/2012JA017825.

The angle m between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) z-axis, sometimes called the “dipole tilt”, varies as a function of UT and season. Observations have shown that the cross-polar cap potential tends to maximize near the equinoxes, when on average m = 0, with smaller values observed near the solstices. This is similar to the well-known semi-annual variation in geomagnetic activity. We used numerical model simulations to investigate how seasonal and diurnal variations in m influence the magnetosphere-ionosphere system. We found that variations in solar wind-magnetosphere coupling, largely associated with variations in the magnetic reconnection rate, are responsible for 70-90% of variations in cross-polar cap potential with m. This is illustrated in figure 1, which shows the mean strength of the electric field along the separator line for equinox and solstice for a 24-hour average. A larger electric field corresponds to a higher reconnection rate. At equinox the section of the separator line along which relatively strong reconnection occurs is much longer than at solstice, leading to a higher cross-polar cap potential. Variations in the high-latitude ionospheric conductance with m also contribute to seasonal and diurnal variations in cross-polar cap potential, but this accounts only for 10-30% of the variation. For other variables, such as field-aligned currents and geomagnetic activity, variations in ionospheric conductance may play a relatively larger role.

Figure 4 caption: The 24-hour mean position of the separator line for March equinox and June solstice for simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model for southward IMF, colour-coded with the mean strength of the electric field parallel to the separator line.

(5) Bhattari, S. K., R. E. Lopez, R. Bruntz, J. G. Lyon, and M. Wiltberger, 2012: Simualtion of the Polar Cap Potential during Peroids with Northward Interplanetary Magnetic Field, in J. Geophys. Res., doi:10.1029/2011JA017143.

In this paper we examine the response of the ionospheric cross-polar cap potential to steady, purely northward interplanetary magnetic field (IMF) using the Lyon-Fedder- Mobarry global magnetohydrodynamic simulation of the Earth’s magnetosphere. The simulation produces the typical, high-latitude “reversed cell” convection that is associated with northward IMF, along with a two cell convection pattern at lower latitude that we interpret as being driven by the viscous interaction. The behavior of the potential can be divided into two basic regions: the viscous dominated region and the reconnection dominated region. The viscous dominated region is characterized by decreasing viscous potential with increasing northward IMF. The reconnection dominated region may be further subdivided into a linear region, where reconnection potential increases with increasing magnitude of northward IMF, and the saturation region, where the value of the reconnection potential is relatively insensitive to the magnitude of the northward IMF. The saturation of the cross-polar cap potential for northward IMF has recently been documented using observations and is here established as a feature of a global MHD simulation as well. The region at which the response of the potential transitions from the linear region to the saturation region is also the region in parameter space at which the magnetosheath transitions from being dominated by the plasma pressure to being dominated by the magnetic energy density. This result is supportive of the recent magnetosheath force balance model for the modulation of the reconnection potential. Within that framework, and including our current understanding of the viscous potential, we present a conceptual model for understanding the full variation of the polar cap potential for northward IMF, including the simulated dependencies of the potential on solar wind speed and ionospheric conductivity.

Figure 5 caption: Saturation of the polar cap potential as seen in the LFM-MIX simulation during intervals with northward IMF.

(6) Bruntz, R., R. E. Lopez, M. Wiltberger, and J. G. Lyon, 2012: Investigation of the viscous potential using an MHD simulation, J. Geophys. Res., doi:10.1029/2011JA017022.

The viscous interaction between the solar wind and Earth’s magnetosphere is extremely difficult to study through direct observations. The viscous contribution to the polar cap potential, the viscous potential, is typically swamped by the much larger reconnection potential or obscured by rapidly changing solar wind conditions. We used the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamic simulation to study the response of the viscous potential to a variety of ideal conditions both in the solar wind and the ionosphere. We found that the viscous potential in LFM increases with either increasing solar wind density or velocity, with a relation that is similar to some previous empirical results in form but different in detail. The density dependence scales as n0.439 (in cm3) and velocity scales as V1x.33 (in km s1). Combining these results with a reference value, the viscous potential in LFM can be predicted using the formula FV = (0.00431)n0.439Vx1.33 kV. We also found that the viscous potential changes inversely in relation to constant Pedersen conductivity in an idealized ionosphere, a result that was previously predicted for LFM but not explored until now.

Figure 6 caption: Simulation results from the LFM-MIX showing the development of vortices along the magnetopause that contributes to the viscous potential seen in the ionosphere.

(7) Lopez, R. E., S. Bhattarai, R. Bruntz, K. Pham, M. Wiltberger, J. G. Lyon, Y. Deng, and Y. Huang, 2012: The Role of Dayside Merging in Generating the Ionospheric Potential During the Whole Heliosphere Interval, in J. Atmos. Solar Terr. Phys 10.1016/j.jastp.2012.03.001.

In this paper we examine the role of dayside merging between the interplanetary magnetic field (IMF) and the geomagnetic field in the generation of the polar cap potential in the ionosphere during the Whole Heliospheric Interval using the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) and Lyon–Fedder–Mobarry (LFM) global simulations of the geospace system from the Center for Integrated Space Weather Modeling (CISM). We isolate the portion of the total ionospheric potential due to the viscous interaction by simulating the interval with a zero IMF, but with the same solar wind plasma conditions. For southward IMF, the cross polar cap potential is the sum of the merging potential and the viscous potential, so we can determine the merging potential by subtracting the viscous potential from the total potential. From the dependence of the merging potential on southward IMF we calculate a geoeffective length of 5 RE. For northward IMF the situation is more complicated since the cross polar cap potential, defined as the peak to peak potential, will be almost always either the value of the viscous potential or of the merging potential, whichever is larger. We find that during periods of northward IMF the cross polar cap potential can be less than what the viscous interaction would produce with no IMF present. This means that the viscous interaction is weakened by the cycle of merging and reconnection for northward IMF. Our results also indicate that current representations of merging rates or electric fields are flawed in the manner in which they describe northward IMF. Typical representations simply produce a weak reconnection rate when the IMF is northward that adds to the viscous potential to create a cross polar cap potential that is larger than the viscous potential, whereas the effect of merging for northward IMF reduces the viscous interaction so that the cross polar cap potential for moderate northward IMF values is smaller than the value that would be expected from solar wind plasma conditions of the viscous potential in isolation.

Figure 7 caption: Comparison for the WHI interval simulations of CPCP potential with and without the IMF. During regions with northward IMF the viscous interaction and the IMF driven convection pattern have opposite sense resulting in a weaker CPCP.