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Diamond (SWx◊) A 10x improvement in real-time forecasting

Submitted by: O. C. St. Cyr, J.M. Davila, & Y. Zheng, NASA-GSFC N. Zapp & D. Fry, NASA-JSC D. Cooke, AFRL W. Murtagh, NOAA-SWPC N. Murphy, JPL

Introduction

This white paper promotes an applied mission concept named “Space Weather Diamond” (SWx◊). Such a mission would facilitate the connection between science and societal needs ( e.g. , improvements in space weather prediction) by providing an order of magnitude improvement over present-day L1 libration point monitors that measure the input to ’s . Such concepts were explicitly called for in the Decadal Panel’s Request for Information to the community. Additionally, the mission concept provides for significant science return, which is outlined in this paper.

Since the 1995 National Space Weather Plan was published, the heliophysics community has done an excellent job identifying and promoting the “applied” side of our science and the relevance to national (and international) infrastructure (e.g. , the recent NRC report: Severe Space Weather Events— Understanding Societal and Economic Impacts Workshop Report, 2008). The next logical step in acquiring measurements to improve space weather forecasting is a sub-L1 platform . The SWx◊ concept offers an opportunity to make that significant advancement without the use of solar sails or other exotic methods of in-space propulsion.

Mission Concept

Space Weather Diamond is based on a constellation of four platforms that are phased into eccentric heliocentric but, from the perspective of a fixed -Earth line, the appear to Earth. It was described in a St. Cyr et al . (2000) report, which was based on a study done internally at NASA-GSFC. The approach is based on a concept called “distant retrograde orbits" outlined by Ocampo (Ph.D., University of Colorado, 1996) describing a that remained in the vicinity of Earth. It is similar to a mission concept called “elliptical string of pearls" studied by NASA's Jet Propulsion Laboratory (Harris, JPL/Caltech D12611, 1995). The mission orbit is readily achievable using present day launch capabilities and lunar gravity assists.

The figure below shows the locations of four Space Weather Diamond spacecraft (A, B, C, D) at a given point in time. In this , spacecraft “B” would provide the 10x early warning (compared to L1 platforms) of CMEs and interplanetary shocks. The eccentricity of this orbit is 0.10, which is about six times that of Earth. Lindsay et al. (1999) demonstrated the predictability of the Dst index using a variety of platforms inside Earth's orbit, and they concluded that a monitor located as distant as ~0:3 A.U. from Earth could provide reliable predictive capability. Based on recent STEREO results ( e.g. , Simunac et al. , 2009), spacecraft “C” in this figure would provide early warning (~12 hours) of corotating solar wind features (CIRs) that can also be drivers of geomagnetic activity.

Figure 1: Four platforms of SWx◊ mission appear to circle Earth at ~0.1 A.U.

From the applied science perspective, SWx◊ would provide continuous transmission of solar wind conditions 0.1 A.U. upstream from Earth. This is important for forecasting the arrival of coronal mass ejections (CMEs), which are the primary cause of the severe geomagnetic storms. CMEs, energetic particles, and other interplanetary disturbances can disrupt communications, navigation, and power infrastructure, as well as endanger human and robotic space explorers. The 2000 SWx◊ study assumed a 500 bit-per-second downlink (similar to the low rate telemetry provided by ACE (Zwickl et al. , 1998)), a 6W X-band transmitter, and a 0.5m dish . Data acquisition at the maximum range is then possible with a 10m ground station antenna, which is available commonly through commercial vendors. The use of “beacons” on heliophysics spacecraft ( e.g. , Zwickl et al. , 1998; St. Cyr & Davila, 2001) to telemeter relevant environmental parameters has led to a cottage-industry of space weather predictions.

The space weather monitoring platforms in our conceptual SWx◊ would be equipped with primarily in situ instrumentation to monitor solar wind , and interplanetary characteristics. Energetic particle instrumentation and a low-frequency radio instrument add the possibility of predicting the arrival time of interplanetary shocks inward of 0.1 A.U. The table below shows a straw payload and includes references for hardware and prediction heritage. An enhanced payload including remote sensing instruments would significantly increase the scientific return of the mission, and these potential science goals are considered in the next section.

Measurement Platform Instrument Reference Prediction Reference Magnetic field WIND, ACE, Lepping et al. (1995), Recognized in 1960’s (e.g. , see strength and STEREO Smith et al. (1998); review by Baker et al. , 1984) direction Acuna et al. (2008) Solar wind WIND, ACE, Ogilvie et al. (1995) ; Recognized in 1960’s ( e.g. , see distribution STEREO McComas et al. (1998); review by Burton et al. , 1975) function Galvin et al. (2008) Low frequency WIND, Bougeret et al. (1995); Cremades et al. (2007) radio emissions STEREO Bougeret et al. (2008) Relativistic SOHO Mueller -Mellin et al. Posner (2007) (1995) Energetic ACE , STEREO Stone et al. (1998) ; Cohen et al. (2001) Mewaldt et al. (2008)

Potential Science with SWx◊

Beyond the obvious utility as a monitor of upstream solar wind conditions, there are numerous scientific possibilities for the SWx◊ concept. Here we describe several that we have considered, but a formal airing in the scientific community would certainly uncover additional ideas. The first science goal involves resolving the internal structure of interplanetary coronal mass ejections (ICMEs). We know that at 1 A.U. ICMEs have typically a 0.1-0.3 A.U. diameter, but details of their cross-sectional geometry are unknown at this time. Coronal white observations would suggest near circular cross-sections (this is also the minimum energy configuration of the internal magnetic field); however, current state-of-the- art MHD heliospheric models predict significantly distorted cross-sections due to the interaction of the fast moving ICMEs and the ambient solar wind. Current 1 A.U. in situ observations can provide only a single track through the body of the ICME. These limited observations have been unable to distinguish between highly elliptical and circular cross sections. However, multiple measurements separated by ~0.1 A.U. would provide the necessary measurements to resolve this puzzle.

A second science goal would be to resolve the beam width of solar energetic particles (SEPs). The precise physical mechanism of solar energetic particle acceleration is still debated, and different proposed mechanisms have different predicted initial beam widths. Spacecraft separated by ~0.1 A.U. would provide the ideal platform to compare the SEP flux and energy profiles from the same event, thus providing valuable clues to the energization process.

A third science goal would employ triangulation studies with low frequency radio instruments. Single spacecraft observations heavily depend on the theoretical density profile of the solar wind in the inner . A separation of ~0.1 A.U. is sufficient to allow precise tracking of shocks in the inner heliosphere, and multi-spacecraft triangulation relaxes this critical assumption.

If an enhanced payload included remote sensing instrumentation, then significant additional science would be possible. The first additional scientific aspect that SWx◊ would address is the critical need to perform 3D imaging of the Sun--particularly active regions to obtain coronal magnetic structure using triangulation techniques. Magnetic field directions (but not magnitude) can be obtained for a large number of loops in the active region corona from stereo pairs of EUV images. These data can be compared with results from magnetic field extrapolation models. Initial comparisons using STEREO data and vector magnetic field measurements have shown that significant errors are found in the extrapolations of ALL models. The extrapolated field directions typically differ from the observed direction of the magnetic field by an average of 30 degrees. The STEREO spacecraft traversed these small elongations rapidly (in a few months) and very few active regions were available when these measurements were possible. Additional observations would provide unique checks on the validity of coronal magnetic field extrapolations which are used as the basis for most modeling and scientific research on the corona and heliosphere. Sandman et al. (, 259 , 2009) and others have concluded that there is a need for either more suitable (coronal rather than photospheric) magnetic field measurements or more realistic field extrapolation models. STEREO did not include magnetographs as part of the payload, so one approach to solving this problem would be to obtain vector magnetograph measurements from multiple vantage points such as would be offered by SWx◊.

A second scientific problem addressed by SWx◊ involves stereoscopic helioseismology, which has been widely discussed as a promising direction to extend that technique in the future. With stereoscopic helioseismology, new acoustic ray paths can be taken into account to probe deeper layers in the solar interior. The value of smaller separation angles for stereoscopic helioseismology is under study (Duvall, personal communication). Such measurements could help solve the puzzle of the solar cycle and advance our understanding of the operation of the solar dynamo.

What is Needed Next?

The objective of this white paper is to add a level of design maturity to the conceptual SWx◊. Heightened interest within NASA and other government agencies for sub-L1 solar wind measurements provides an applied science justification. SWx◊ would be “game-changing” for both heliophysics and for space weather prediction.

The instrumentation for SWx◊ is mature and at high technical readiness level, and the accommodation requirements for the space platforms have been accomplished numerous times successfully. The novel mission orbit can be achieved with existing launch capabilities. A rough cost estimate to station four platforms with this payload is that it should be similar to the THEMIS mission, a recent MIDEX (Angelopoulos, 2008), making SWx◊ low cost with high benefit to operational customers . Clearly the next step is to conduct a concurrent engineering exercise to validate the mission design, required power and communications, and to generate a cost estimate. This activity could be accomplished via NASA- GSFC Integrated Mission Design Center (IMDC) or JPL’s Team X.

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