A hybrid heliospheric modeling system: Background solar wind

Abstract

[1] We describe a Sun-to-Earth system of coupled models. Our main goal is to create a real-time, three-dimensional (3-D), MHD-based system to aid in the operational forecasting of geomagnetic activity, but we expect the system to have other uses. We give here our initial survey of the system's characteristics. The Hybrid Heliospheric Modeling System (HHMS) is composed of two physics-based models, combined with two simple empirical models. The physics-based models are a source surface (potential field) current sheet model for the corona and a time-dependent 3-D MHD solar wind model. The system is driven by a sequence of photospheric magnetic maps composed from daily magnetograms. An empirical relationship between magnetic flux tube expansion factor and solar wind speed at 0.1 AU is a key element of the system. The solar wind model gives a predicted time series of MHD parameters at the location of Earth in the model grid; this is verified against Omni, Wind, or ACE satellite data, depending on the time period. The predicted solar wind at Earth is used as input to the second, data-based, empirical model to predict the geomagnetic Ap index. We compare test results for simulated 1 day ahead Ap forecasts for the years 1993 through 2002 with forecast skill of the official Ap forecasts that were issued by the NOAA Space Environment Center in that time interval. Results show the HHMS would have been useful to forecasters in some years. Simulations of transient events such as coronal mass ejections and interplanetary shocks with the HHMS will be reported on later.

MHD simulation of the inner‐heliospheric magnetic field

“…The respective set of equations was taken from Detman et al [2006Detman et al [ , 2011, who work on a very similar goal, i.e., space weather forecasting, for which they use input data similar to those used here. These kinds of interfaces are constantly augmented as testing goes on.…”

“…Equations (25) and (26) model the Parker spiral in the corotating frame, whereas in Detman et al [2006] the rest frame is implemented. The equation for the azimuthal velocity mimics the behavior of the low corona rotating approximately like a rigid body, thus using the solar rotation frequency Ω multiplied with the radius of corotation, which Detman et al [2006] put at R ' = 1.5R . This approach is questionable in view of the behavior of the azimuthal velocity derived by Weber and Davis [1967] shown in Figure 1, because from the radius of corotation outward an approximate r À 1 behavior is evident that would yield considerably smaller values at R gb .…”

“…The tunable parameter p tot is the total pressure, but its optimal values derived by Detman et al [2006] change over the solar cycle, so that a rough tuning has been performed on our part to get realistic results and we chose p tot,min = 3.8p 0 and p tot,max = 4.2p 0 for solar minimum and maximum, respectively.…”

“…Although this is the key input to our code, it is however necessary to provide the code with initial conditions for all MHD variables. For this purpose the approach by Detman et al [2006Detman et al [ , 2011 is followed, which uses the empirical inverse relation between flux tube expansion factor and solar wind speed to determine the latter.…”

“…This approach allows for higher resolution than that of comparable observational data from synoptic magnetograms. The heliobase conditions for the other MHD quantities are obtained following the empirical approach by Detman et al [2006Detman et al [ , 2011, who employed the Wang-Sheeley-Arge approach.…”