Triana, as a second spacecraft at L-1 in a halo orbit similar to ACE, but with its orbit phase-shifted by 90°, would significantly improve the accuracy of the National Space Weather predictions. The improvement will occur because one spacecraft will always be near the ecliptic plane and Sun-Earth line. This effort will not require the continuous transmission of the highest resolution measurements. On-board calculated averages and moments transmitted at significantly lower rates (e.g., 1/min) will be sufficient.
It has been well established that the magnetic field and plasma scale length in interplanetary space is significantly less than 80 Re (Earth-radius), the radius of the orbit of ACE around the L-1 point (Russell et al., 1980; Crooker et al., 1982; Kelly et al, 1986; Richardson and Paularena, 1998; Paularena et al., 1998; Collier et al., 1998). Therefore, when ACE is at a large distance from the Sun-Earth line, there is a significant reduction in the accuracy of space weather predictions based on the collected data. It has been argued that IMP-8 and WIND, when in the solar wind in front of the magnetosphere, are a reliable source of relevant space data. While this is often correct, the proximity of these spacecraft to Earth does not leave sufficient warning time for incoming events.
Triana would also provide a monitor of the solar wind in addition to any other spacecraft that may still be available at the time of its launch. This would allow the detailed study of the non-radial correlation with solar wind fluctuations. This study was begun with earlier spacecraft such as the Explorers, IMP, and ISEE, but new opportunities would now be available. For example, if WIND or a STEREO spacecraft were measuring the solar wind at a variety of positions away from L-1, this would provide correlation at multiple baselines. These measurements would help to determine the symmetry of the fluctuations in the wind that in turn determine the way in which energetic particles propagate in the heliosphere. This basic understanding is also central to determining how, for example, solar events affect the Earth and its near-space environment, and thus is important for determining the effects of solar activity on spacecraft and manned space flights.
The combination of ACE, Triana, WIND, and IMP-8, would provide the necessary four-point studies to investigate curvatures of shock and discontinuity surfaces. When the four spacecraft are separated by large distances (on the order of 200 Re), we can measure curvatures in relatively stable structures (e.g., magnetic clouds). We expect to observe curvatures in the heliospheric current sheet (Szabo et al., 1999). The four- spacecraft constellation would afford a unique opportunity to resolve the question of multi-layer heliospheric current sheets. Tracing the passage of a specific current sheet from one spacecraft to the next would allow discrimination between the general case of multiple current sheets or a single wavy sheet. A four spacecraft constellation would allow the determination of the size of typical magnetic holes.