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Nick Arge 1 , Dusan Odstrcil 1 ,  Vic Pizzo 2 , & Leslie Mayer 1

A Simple Modular Sun-to-Earth Propagation Model. & An Improved Method for Specifying Solar Wind Speed Near the Sun. Nick Arge 1 , Dusan Odstrcil 1 ,  Vic Pizzo 2 , & Leslie Mayer 1. 1 University of Colorado/CIRES & NOAA/Space Environment Center 2 NOAA/Space Environment Center.

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Nick Arge 1 , Dusan Odstrcil 1 ,  Vic Pizzo 2 , & Leslie Mayer 1

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  1. A Simple Modular Sun-to-Earth Propagation Model & An Improved Method for Specifying Solar Wind Speed Near the Sun Nick Arge1, Dusan Odstrcil1,  Vic Pizzo2, & Leslie Mayer1 1University of Colorado/CIRES & NOAA/Space Environment Center 2NOAA/Space Environment Center This work is supported by NSF grant No. ATM-0001851 and by ONR grant N00014-01-F-0026.

  2. ABSTRACT We are developing a simplified model to explore issues relating to operational aspects of Sun-Earth space weather models. Because quantitative solar inputs are currently quite limited, the four modular components of the prototype Sun-Earth model consist of simple physics-based and semi-empirical models as well as numerical MHD models that can run on workstations. An advantage of the modular architecture is that it allows for extensive experimentation and permits easy replacement of individual models by improved ones as they (and any required data streams) become available. In this study, we use three of the four inter-linked modules in an effort to find an improved technique for empirically predicting solar wind flow speed near (~1/10 AU) the Sun. The coronal field is determined out to 2.5 Rs using a potential field source surface (PFSS) model, with photospheric field synoptic maps as input. The output from the PFSS model is then used as input to a version of Schatten’s current sheet (SCS) model. This model determines the field out to 21.5 Rs. Finally, the SCS model’s output is fed into a simple 1-D kinematic propagation code, which transports the wind out to Earth. In this study, the 1-D code is preferred over a more advanced 3-D MHD code, as it significantly reduces the time required for each trial run. We conduct our study using polar field corrected Mount Wilson Solar Observatory Carrington maps from 1995. During this period, the Sun was in the declining phase of the solar cycle and the solar wind had relatively simple structure.

  3. OBJECTIVE To develop a simple model to explore issues relating to operational aspects of Sun-Earth space weather models. APPROACH Construct a modular Sun-to-Earth model that permits easy replacement of individual modules by improved ones as they (and any required data streams) become available. MODEL 1) Consists of:  Simple physics-based and semi-empirical models.    Numerical MHD models that can run on workstations.

  4. 2) Allows extensive experimentation. • Eventually controlled by a Java-Based Modular Control Software originally developed by Lance Williams at TRW. •  Presently being modified to better suit our purposes. Control Panel for Sun-to-Earth Model

  5. MODEL STRUCTURE The Simple Sun-to-Earth model consists of 4 modules: Module 1:Solar Boundary Conditions (Photosphere: R = 1 R) • The surface magnetic field is the sole quantitative large-scale physical observable. • The details in the processing of raw individual magnetograms into a global, synoptic characterization are crucial for model accuracy (e.g., [Arge and Pizzo, 2000]). Module 2:Lower corona (1 R< R < 2.5 R) • The NOAA/SEC operational version of the Wang and Sheeley potential-field source surface model (PFSS) is used in this module. Module 3:Upper corona/trans-alfvenic region (2.5 R< R < 20-30 R) • Schatten’scurrent sheet model [1971], which provides a more realistic magnetic field topology in this region, is used in this module. Module 4:Interplanetary propagation (20-30 R< R < 1AU) • A3-D time dependent MHD model such as the Odstrcil [1999] or the Han & Detman [1991]code can be used in this module to establish the slowly varying ambient flow out past 1 AU.

  6. Search for an Improved Method for Specifing Solar Wind Speed Near the Sun Objective: Search for an improved empirical relationship between solar wind speed and coronal field parameters in order to better predict flow speed close (~1/10 AU) to the Sun. Approach: • Radially map WIND solar wind speed measurements (taken at L1) back to 21.5 R • • Assume constant flow speed. • • Neglect stream interactions. • • Account for solar rotation (i.e., Parker Spiral).

  7. Trace the field lines down to the photosphere using • • Potential Field Source Surface (PFSS) model (1.0 R < R < 2.5 R) • • Schatten Current Sheet (SCS)model (2.5 R < R < 21.5 R) • • Polar field corrected, daily updated MWO synoptic maps from 1995 as input. • Compare solar wind speed with • i)Magnetic field expansion factor (fs). • ii)Footpointfield strength. • iii)Footpoint-subearth point angular separation. • iv) Footpoint-current sheet angular separation. • v) Footpoint-coronal hole boundary angular separation. • 4) Search (via trial and error) for an empirical relationship between the above parameters (items i-iv in 3) and solar wind speed. • • Test by propagating the solar wind out to L1 and comparing with WIND data. • • Use a simple 1-D propagation code to minimize the time required for each trial run.

  8. Old: V(fs) = 285 + 650/(fs)5/9 km s-1 New: km s-1 • Compare new empirical relationship with one derived previously, which is a function only of fs. • 6)Compare results using new and old empirical relations and • i) PFSS model alone. • ii) PFSS + Current Sheet model combination. Empirical Relationships Where: fs = Magnetic field expansion factor. θb = Minimum angular distance from photospheric field footpoint to coronal hole boundary.

  9. SUMMARY • When the solar wind source region (i.e., open field footpoints) is located at higher latitudes, the SCS + PFSS model combination typically produce better predictions than those made using the PFSS model in isolation. However, when the open field footpoints are located near the equator the reverse appears to be true. (This may be an artifact of our field line tracing routine). • 2) In addition to magnetic field expansion factor(fs), solar wind speed also appears to be influenced by the (minimum) angular distance(θb)open field footpoints are from coronal hole boundaries. • We have found a new empirical relationship for specifying solar wind speed near the Sun that is a function both of fs and θb andthat works much better than an earlier relationship, which is a function only of fs. • 4) To establish its robustness, the new empirical relationship needs to be tested during different periods of the solar cycle. • 5) We plan to test the new empirical relationship using a 3D MHD solar wind model (i.e., in Module 4 of our simple Sun-to-Earth Propagation Model).

  10. PFSS MODEL (R = 2.5 R) PFSS+SCS MODEL (R = 21.5 R) Top – Inferred distribution ofsolar wind speed(based only on expansion factor) at the source surface (R = 2.5 R). Bottom – Source Surface field Br (R = 2.5 R) Same as Figure 1a except now theSchatten Current Sheetmodel is used in combination with the PFSS model (R = 21.5 R).

  11. Solar Wind Speed Predictions & WIND Satellite Observations PFSS+SCS MODEL (R = 21.5 R) PFSS MODEL (R = 2.5 R)

  12. Solar Wind Speed Predictions & WIND Satellite Observations PFSS MODEL (R = 2.5 R) PFSS+SCS MODEL (R = 21.5 R)

  13. Solar Wind Speed Predictions & WIND Satellite Observations PFSS+SCS MODEL (R = 21.5 R) PFSS MODEL (R = 2.5 R)

  14. Solar Wind Speed Predictions & WIND Satellite Observations PFSS MODEL (R = 2.5 R) PFSS+SCS MODEL (R = 21.5 R)

  15. Solar Wind Speed Predictions & WIND Satellite Observations PFSS+SCS MODEL (R = 21.5 R) PFSS MODEL (R = 2.5 R)

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