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Mission to the Sun-Earth L 5 Lagrangian Point: An Optimal Platform for Space Weather Research

2015; American Geophysical Union; Volume: 13; Issue: 4 Linguagem: Inglês

10.1002/2015sw001173

1542-7390

A. Vourlidas,

Astro and Planetary Science

The Sun-Earth Lagrangian L 5 point is a uniquely advantageous location for space weather research and monitoring.It covers the "birth-to-impact" travel of solar transients; it enables imaging of solar activity at least 3 days prior to a terrestrial viewpoint and measures the solar wind conditions 4-5 days ahead of Earth impact.These observations, especially behind east limb magnetograms, will be a boon for background solar wind models, which are essential for coronal mass ejection (CME) and shock propagation forecasting.From an operational perspective, the L 5 orbit is the space weather equivalent to the geosynchronous orbit for weather satellites.Optimal for both research and monitoring, an L 5 mission is ideal for developing a Research-to-Operations capability in Heliophysics.The Problem: Incomplete Knowledge of CME Propagation Our society's growing dependence on Global Positioning System, telecommunications, interconnected power grids, global travel, and (in the not-so-distant future) space travel brings attention to the conditions in Earth's near space and increases the demand for accurate space weather predictions.Research from NASA heliophysics missions has established that eruptions of magnetic field and plasma from the Sun's corona, so-called coronal mass ejections (CMEs), are the main drivers of space weather.The reaction of the terrestrial space environment to a CME impact depends mainly on the CME's internal magnetic field configuration, time of arrival (ToA), and velocity on arrival (VoA).The objective of Space Weather (SpW) forecasting is to determine if a CME-Earth intercept is likely, and if so to obtain accurate values for all three parameters as far ahead of the CME impact as possible, mostly via remote sensing.Of the three, the entrained magnetic field of an Earth-directed CME is beyond the reach of current remote sensing capabilities.However, ToA and VoA are readily derived from coronagraphic and heliospheric observations of propagating transients although they suffer from serious projection biases when based on observations from the Sun-Earth line.The availability of imaging off the Sun-Earth line from the Solar Terrestrial Relations Observatory (STEREO) mission has improved ToA accuracy to ±6 h from > ±12 h pre- STEREO [e.g., Millward et al., 2013;Colaninno et al., 2013].The VoA accuracy is still rather low (within about ±30-40% of observed velocity).Figure 1 shows that from the L 5 vantage point, a CME can be easily followed all the way to Earth.Why are the predictions not better?How can we improve them?The answer lies in the complexity of the ambient solar wind (e.g., fast and slow streams and preceding events).The interactions may alter the CME structure or its path or both (see section 6.2 in Webb and Howard [2012], and references therein).The brute force approach is to place a series of in situ probes along the Sun-Earth line-akin to the NOAA National Data Buoy Center tsunami warning network of buoys dispersed in the Pacific.Orbital mechanics make this approach impossible.Only one such stable orbit exists at the L 1 Lagrangian point, 1 h upstream from Earth.The STEREO spacecraft have long since moved past their optimum locations for such monitoring to pursue the research objectives of their mission.The best solution is to survey the whole Sun-Earth space at once by placing a satellite off the Sun-Earth line.The L 4 /L 5 Lagrangian points located 60°on either side of the Sun-Earth line are perfect for this purpose (Figure 2). Why L 5 ?A satellite located at the L 5 point will be trailing Earth by 60°and, therefore, will image an additional 60°of the solar disk invisible from Earth.Since the solar rotation is about 13°/day, an L 5 satellite will be capable of a 4-5 day warning regarding the emergence, complexity, and eruptive potential of active regions and irradiance variations.Crucially, an L 5 spacecraft could provide real-time behind east limb magnetograms.VOURLIDAS