Augmenting the Wide Field Infrared Survey Telescope (WFIRST) microlensing campaigns with intensive observations from a ground-based network of wide-field survey telescopes would have several major advantages. First, it would enable full two-dimensional (2-D) vector microlens parallax measurements for a substantial fraction of low-mass lenses as well as planetary and binary events that show caustic crossing features. For a significant fraction of the free-floating planet (FFP) events and all caustic-crossing planetary/binary events, these 2-D parallax measurements directly lead to complete solutions (mass, distance, transverse velocity) of the lens object (or lens system). For even more events, the complementary ground-based observations will yield 1-D parallax measurements. Together with the 1-D parallaxes from WFIRST alone, they can probe the entire mass range M & M⊕. For luminous lenses, such 1-D parallax measurements can be promoted to complete solutions (mass, distance, transverse velocity) by high-resolution imaging. This would provide crucial information not only about the hosts of planets and other lenses, but also enable a much more precise Galactic model. Other benefits of such a survey include improved understanding of binaries (particularly with low mass primaries), and sensitivity to distant ice-giant and gas-giant companions of WFIRST lenses that cannot be detected by WFIRST itself due to its restricted observing windows. Existing ground-based microlensing surveys can be employed if WFIRST is pointed at lower-extinction fields than is currently envisaged. This would come at some cost to the event rate. Therefore the benefits of improved characterization of lenses must be weighed against these costs.

Abstract
1. INTRODUCTION
2. MICROLENSING PARALLAXES
3. WFIRST + GROUND PARALLAXES
4. ONE-DIMENSIONAL PARALLAXES
    4.1. WFIRST-only 1-D Parallaxes
    4.2. Combined Orbital and Two-Observatory1-D Parallaxes
    4.3. Xallarap and Lens Orbital Motion
    4.4. Value of πE,∥ Measurements
    4.5. Value of πE Measurements
5. EINSTEIN RADIUS MEASUREMENTS: θE
    5.1. WFIRST-only Einstein-Radius Measurements
    5.2. WFIRST +Ground Einstein-Radius Measurements
    5.3. Value of θE Measurements
6. COMPLETE SOLUTIONS FOR PLANETARY ANDBINARY EVENTS
7. AUXILIARY BENEFITS OF WFIRSTMICROLENSING IN OPTICAL FIELDS
8. DISCUSSION
REFERENCES

• Wei Zhu(Department of Astronomy, Ohio State University/Max-Planck-Institute for Astronomy) Corresponding author
• Andrew Gould(Department of Astronomy, Ohio State University/Max-Planck-Institute for Astronomy/Korea Astronomy and Space Science Institute)