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Detecting and Tracking Solar Ejecta with Next-Generation Heliospheric Imaging Systems Janet C. Johnston AFRL/RVBXS, Space Weather Center for Excellence, Air Force Research Laboratory, Hanscom AFB, MA 01731-3010 David F. Webb ISR, Boston College, Chestnut Hill, MA 02467 USA 1. Background and Need Most significant space weather is initiated at the Sun. Coronal mass ejections (CMEs), huge blobs of plasma with embedded magnetic structure, are the primary cause of severe space weather at Earth because they drive shocks and trigger geomagnetic storms that can damage spacecraft and ground-based systems. Magnetic storms can damage both military and civilian, space and ground assets. These hazardous storms are difficult to forecast and there are many false alarms. What makes solar and heliospheric disturbances geoeffective, in terms of causing storms, is primarily southward interplanetary magnetic field (IMF) and compression. Southward IMF is important because it allows merging of the IMF and Earth’s magnetic field and transfer of solar wind energy and mass into the magnetosphere. Compression is important because it strengthens existing southward IMF and, to a lesser extent, increases density. CMEs usually contain longduration flows of southward IMF and fast CMEs compress any southward field ahead. Sometimes the geoeffective compressed, southward field can be entirely within the shock “sheath”. In addition, CMEs themselves can carry high-density structures, such as solar filaments. High-speed streams are geoeffective when they compress any southward IMF in corotating interactive regions (CIRs), and also because they are associated with the acceleration of electrons in the magnetosphere to hazardous levels. While coronagraphs observe the launch of a CME close to the Sun, the wide variety of CME speeds and morphologies make it necessary to image and track them through interplanetary space to predict their arrival, duration, strength and effectiveness at Earth. Space imaging is required because of the difficulty to detect the dim CME light against brighter background sources such as the zodiacal light and starlight. Tracking and specification of significant solar ejecta in the next decade will rely on a data assimilative approach, incorporating imagers, interplanetary scintillation, wide field radar arrays, and other assets not yet in operation. As DoD systems (communications, navigation, surveillance) become more complex, space weather effects will play a more significant role in the fidelity of Earth and space situational awareness. Anomaly assessments are needed both reliably and within a useful timeframe. Forecasting can also be useful both to postpone critical space operations and take mitigating actions to protect sensors or to take advantage of adverse conditions. “All clear” forecasts are also valuable to the operational community. Heliospheric imagery can lead to improvements to solar wind forecast models. The accuracy of these models is crucial should a failure of surveillance assets occur. 2. Prior Remote Sensing Missions Since the Helios mission it has been known that electrons from CMEs can be viewed in white light from space through Thomson-scattering (Richter et al., 1982). Beginning early 2003, a new era in heliospheric imaging started with the operation of the Air Force Space Test Program demonstration experiment, the Solar Mass Ejection Imager (SMEI – Eyles et al., 2003). SMEI, launched into a circular, Sun-synchronous orbit, captures a full sky image (minus a 20° exclusion zone around the Sun and a small patch at the anti-solar direction) every orbit (102 minutes). A photometric limit of at least 0.01% of the background sky must be attained to detect and track CMEs (Howard et al., 2006; Webb et al., 2006; 2009). Since late 2006 SMEI’s data have been augmented by imagery from the SECCHI suite of staring instruments (Howard et al., 2009) on the twin STEREO spacecraft in ~1 AU orbits around the Sun. Each STEREO has two heliospheric imagers (HI-1 and HI-2; Harrison et al., 2008; Eyles et al., 2009) which together have 70o x 90° views of the heliosphere centered east and west of the ecliptic. STEREO-A is ahead of the Earth and B lags behind, and thus provides an even longer lead time forecast for some phenomena. The STEREO craft diverge from each other at the rate of 22° each per year and will soon be 180° apart. The SMEI and STEREO HI data are complementary. HI-1 fills the gap between a coronagraph and the SMEI inner camera and HI-2 overlaps with SMEI over tens of degrees and has better spatial resolution. SMEI suffers from area obscurations due to particles in Earth orbit, but despite these gaps, SMEI observes nearly the entire sky. Both HI fields have about the same temporal resolution, about 2 hours. The improved spatial resolution of the HIs allows detection of plasma, or density waves that may be associated with stream interaction regions. Although their fields of view are more limited than SMEI’s, HI data are free of the contamination suffered by SMEI from particle hits from traversing the South Atlantic Anomaly, the horns of the auroral zones and the noise introduced by high altitude auroras. Radio techniques have also been used to remotely detect and track and coarsely image heliospheric disturbances related to CMEs. These use km-wavelength radio observations from space and interplanetary scintillation (IPS) observations from the ground. The km-wavelength observations can track the type II emission typically from strong shocks traveling ahead of fast CMEs. Such instruments have been flown on the ISEE-3, Wind, Ulysses and STEREO spacecraft. The IPS technique relies on measurements of the fluctuating intensity level of signals from strong, point-like distant radio sources from one or more ground arrays operating in the MHz range. IPS arrays detect changes in density inhomogeneities in the (local) IP medium moving across the line of sight to the source. Disturbances are detected by either an enhancement of the scintillation level and/or an increase in velocity. 3. Space Weather Requirements and Tradeoffs Extensive studies have shown that a SMEI–like instrument can detect and track most earthbound CMEs capable of generating moderate to severe geomagnetic storms (e.g., Webb et al., 2009). However, “halo” CMEs, those CMEs which have an earthward component when viewed from a detector along the Sun-Earth line, require aggressive processing for determining the distance to Earth of the incoming material. With such detectors, only the angular distance (elongation angle) from the Sun is measured and for halo events, there can be an ambiguity between a wide CME near the Sun and a narrow CME near the Earth. And the geometric shape of the leading edge is in question. White light imagers detect light produced by Thomson scattering, so the brightest part of an image is a convolution of the amount of material and its angle from the illumination source. We are currently testing several methods and models that use HI data for improving forecasts. One method is to assimilate SMEI data into a forecast model. Currently the HAFv2 model is AFWA’s operational solar wind model. SMEI data can enable a mid-course correction to the HAF-only forecast up to about a day before CME arrival. AFRL developed and transitioned a simple point-&-click tool/operator interface that permits a forecaster to fit the leading edge of a CME using SMEI data to estimate the arrival time at 1 AU. This tool also yields estimates of the CME speed and its direction angle with respect to the Sun-Earth line. The Tappin-Howard (2009) model is a 3-D interplanetary CME reconstruction model based on leading edge measurements from heliospheric images and deriving the CME geometry using Thomson-scattering physics. As input, the model currently uses SMEI and/or STEREO HI data. It outputs (1) the basic structure, speed, direction, size, height- and speed-time profile, and (2) an estimation of time of arrival and likelihood of impact with the Earth. The systems impact for the AF/DoD is that it has the potential to provide a fast and accurate prediction of the arrival time of any observed CME at the Earth, or any other point in space. In the near future we should be able to incorporate not only enhanced plasma density data but also velocity and IMF information in assimilative 3-D interplanetary reconstruction models. Such assimilative processes are increasingly being used operationally by AFWA and other government agencies in geospace weather models such as the Global Assimilation of Ionospheric Measurements (GAIM) model (e.g., Schunk et al., 2004). The velocity data are available now from the ground-based IPS arrays, but these are scattered in time and space coverage. International studies are underway to collect, intercalibrate and make accessible all the IPS data (Oberoi and Benkevitch, 2010). An important parameter of CMEs is the orientation of the embedded magnetic field. A structure having a southward-pointing magnetic field is more likely to cause significant geomagnetic storming at Earth because it reconnects with the predominantly north-pointing geomagnetic field allowing solar wind plasma and energy to flow into geospace. During the next decade radio facilities will be developed to image and track the magnetic structures within CMEs. The MWA sited in western Australia is currently being constructed and will be capable of obtaining both greatly enhanced IPS data and unique IMF imaging data via the Faraday rotation technique (Jensen et al., 2010). Relevant requirements for operational space weather support include: time of arrival (ToA) accuracy 1 -3 hours; lead or warning times > 12 hours; a 15 min. cadence, high precision measurements of CMEs to establish initial speed, geoeffectiveness degree; 30 arc-second sampling resolution; the ability to detect and track corotating interaction regions (CIRs); and an optimum CME viewing perspective. This latter involves the orbits into which an HI system could be placed. We discuss three possibilities (Table 1 and Figure 1): - Earth orbit: Any spacecraft/instrument in Earth orbit suffers from radiation damage and penetrating trapped particles. Also low earth orbit (LEO) and others that pass through the South Atlantic Anomaly (SAA) or auroral zones suffer from particle effects and/or bright auroral emission. In a LEO, near-equatorial orbit, the particle and auroral contamination is reduced, and other Earth orbits such as medium Earth orbit (MEO) are possible. - Along the Sun-Earth line: For deep space, an L1 halo orbit is probably the easiest and cheapest to get to and to maintain an orbit. However, coronagraphs and HIs cannot directly observe CMEs heading towards Earth, which appear as halos around the Sun, because of the necessity to block the bright sunlight around the Sun.. Thus, that part of the Earth-directed CME that is most geoeffective is not seen. Therefore, CMEs and CIRs need to be observed from a different vantage point that provides a full view of the CMEs, a profile en route, and the CIRs well before they arrive at Earth. - L5: Coronagraphs and an HI at the fifth Sun-Earth Lagrange point (L5) orbit position (60o trailing the Earth at 1 AU) will be able to detect CME speed and direction for a range of solar origin locations, especially those expected to be most geoeffective. In addition, L5 has the advantage that it views beyond the eastern limb of the Sun as viewed from Earth. Thus, active regions and coronal holes can be viewed before they arrive on the Earth-facing disk, and their location, size and activity assessed. Finally, the geoeffective space weather resulting from high speed solar wind streams, such as radiation belt relativistic electron enhancements, can be forecast days in advance. Observations at or near this location using STEREO-B as a pathfinder for such a mission are discussed by Webb et al. (2010). 4. Summary Interplanetary space is a dynamic environment, driven to first order by the Sun. The DoD has invested in the capability to specify conditions and phenomena in the heliosphere. An L5 imager, and/or an L1 or LEO Sun-Earth line imager can be accomplished with low risk, banking on knowledge gained from SMEI and STEREO. The ideal system would incorporate both and lead to a system producing stereo viewing of a CME, unambiguous profiling of its leading edge, advanced viewing of solar activity, and an inexpensive Sun/Earth line instrument giving full sky images. Interplanetary scintillation and wide-field radio arrays promise to add details of embedded magnetic fields to complete the CME specification for use in research and space weather forecasting. References Eyles, C.J., G.M. Simnett, M.P. Cooke, B.V. Jackson, A. Buffington, P.P. Hick, N.R. Waltham, J.M. King, P.A. Anderson, and P.E. Holladay, Solar Phys., 217, p. 319-347 (2003) Eyles, C.J. et al., Solar Phys., 254: 387– 445 (2009) Harrison, R.A. et al., Solar Phys., 247, 171 (2008) Howard, R.A. et al., Space Sci. Rev., 136, 67 (2008) Howard, T.A., D.F. Webb, S.J. Tappin, D.R. Mizuno and J.C. Johnston, J. Geophys. Res., 111 (2006) Jensen, E.A., Hick, P.P., Bisi, M.M., Jackson, B.V., Clover, J., Mulligan, T.: Solar Phys., 265: 31–48 (2010). Oberoi, D. and L. Benkevitch, Solar Phys., 265: 293–307 (2010) Richter, I., C. Leinert, and B. Planck, Astron. Astrophys., 110, 115 (1982) Schunk, R. W., et al., Radio Sci., 39, RS1S02 (2004) Tappin, S.J. and T.A. Howard, “Interplanetary Coronal Mass Ejections Observed in the Heliosphere: 2. Model and Data Comparison”, Space Sci. Rev., 147, 55 (2009) Webb, D.F. et al., J. Geophys. Res., 111 (2006) Webb, D.F., T. A. Howard, C. D. Fry, T. A. Kuchar, D. R. Mizuno, J. C. Johnston and B. V. Jackson, Space Weather, 7, S05002 (2009) Webb, D.F., D. A. Biesecker, N. Gopalswamy, O. C. St. Cyr, J. M. Davila, C. J. Eyles, B. J. Thompson, K. D. C. Simunac and J. C. Johnston, Space Research Today, 178, 10 (2010) Table 1. Options for HI Orbits/Locations Location 1. Earth orbit Polar LEO Near-equatorial LEO MEO, etc. 2. Sun-Earth Line L1 orbit Other 3. L5 orbit L5-L1 Constellation Advantages Lower cost; more launch options. Disadvantages Must be >1000 km. height; Bright auroral emission; Solar & auroral zone particles; SAA particles & radiation dose. Lower cost; more launch Must be >1000 km. height; options; Few solar particles; Still some auroral emission Reduced auroral emission and solar & auroral zone & radiation damage. particles. Minimize auroral emission. Still some auroral emission and solar & auroral zone particles. Lowest cost & ease of More expensive than LEO; maintaining orbit; avoids HIs cannot observe Earth orbit problems; nose/core of CME; Hard- & Detectors (CCDs) easily software: rad hard & deep cooled space conditions L2 has Earth in Sun view; no other desirable orbits. With a coronagraph can Higher cost with weight & view geo- eff. CMEs from power restrictions; Hard- & side & from Sun to Earth; software: rad hard & deep Early views of solar space conditions. features east of Earth view; Early detection of geoeff. high speed streams. Combines two views to Higher cost to launch & reconstruct 3D kinematics support two missions; Hard& geometry of CMEs & & software: rad hard & CIRs. deep space conditions. Figure1: Proposed orbits for a heliospheric imager with a CME (not to scale).