The STEREO Space Weather Broadcast

Transcription

1 The STEREO Space Weather Broadcast O.C. St.Cyr 1 and J.M. Davila Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center Greenbelt, Maryland The NASA STEREO mission offers exciting possibilities for near-real-time transmission of important measurements for space weather. The STEREO payload will provide solar wind plasma, magnetic field, and energetic particle parameters, as well as optical and radio views of the Sun that cannot be obtained from groundbased observers or spacecraft near Earth. This space weather data will be transmitted continuously from each spacecraft over the X-band frequency (8.4 GHz) at a data rate of about 500 bps. Processing of the space weather broadcast data into useful online displays will be performed at the STEREO Science Center located at Goddard Space Flight Center. NASA will provide for partial coverage from each spacecraft through the Deep Space Network, and we are looking for partners who have ground stations to provide complementary coverage. We anticipate that these data will be very useful to forecasters of space environment conditions. INTRODUCTION The primary scientific objectives of NASA's STEREO (Solar TErrestrial RElations Observatory) are to advance the understanding of the origins of coronal mass ejections (CMEs) at the Sun; to track the evolution of CMEs through the interplanetary medium; and to study the dynamic coupling between CMEs and Earth's environment. As of this writing the STEREO mission is completing Phase A development and is scheduled for a mid-2004 launch. The mission includes two essentially identical three-axis stabilized spacecraft, each equipped with a payload of both remote sensing and in situ instrumentation. The current baseline calls for both spacecraft to be launched on a single Delta II expendable launch vehicle 1 Also at Computational Physics, Inc., Fairfax, Virginia, and The Catholic University of America, Washington, B.C. Space Weather Geophysical Monograph 125 Copyright 2001 by the American Geophysical Union from Kennedy Space Center and, following a series of Earth-Moon phasing orbits, to be injected into heliocentric orbits using gravitational assists from lunar fly-bys. The STEREO spacecraft will then drift away from the Sun- Earth line symmetrically, with one spacecraft "leading" Earth and the other "trailing." The planned drift rate is 22 per year for each spacecraft, so at the end of the nominal two year mission the spacecraft will be -90 apart (Figure 1). The engineering design goal for the mission is a fiveyear lifetime. The idea to include a "space weather beacon" capability on each STEREO spacecraft was noted in the NASA Science Definition Team Report [Rust et al., 1997]. That document described a plan to alert Earth via a radio signal if pre-defined thresholds of solar wind parameters were exceeded. However, the current concept is that the broadcast will operate continuously, sending to Earth a highly compressed stream of solar and heliospheric images and in situ measurements of the solar wind and energetic particle environment at each spacecraft. This manuscript describes the current conceptual design for the STEREO space weather broadcast. The second section provides a brief discussion of the scientific and applied (e.g., space environment forecasting) rationale for the beacon; and the third section describes the current imple-

2 206 STEREO SPACE WEATHER BROADCAST STEREO lagging - \ Sun _STEREO x leading Figure 1. STEREO mission configuration after two years. mentation concepts. Table 1 shows the payload, instrument teams, measurements, and proposed data content for the space weather broadcast. RATIONALE A limitation of space weather forecasting using groundbased and Earth-orbiting (or even L-l) platforms has been the difficulty in remotely sensing coronal mass ejections that might be headed toward Earth [e.g., Gosling, 1993]. Recent observations by the SOHO LASCO coronagraphs [Brueckner et al., 1995] have been routinely detecting halo CMEs, and in many cases these have been Earth-directed [e.g., Webb et al., 2000]. While these observations represent a significant step toward producing more accurate 2-5 day forecasts of geomagnetic activity, the timing of the arrival of any particular CME at Earth remains ambiguous [e.g., St. Cyr et al, 2000]. One goal in obtaining the STEREO space weather data in near-real-time is to assist NOAA's Space Environment Center and other international space environment forecasters. The baseline concept of the STEREO space weather broadcast has been to build on the highly successful ACE real-time solar wind monitoring capability [Zwickl et al, 1998] and to expand those in situ measurements to include remotely sensed images. In fact, assuming ACE or a follow-on mission is operating near L-l, then the addition of two STEREO spacecraft means there will be three-point in situ measurements available. This should provide significantly better understanding of the extent and uniformity of large scale structures at 1 A.U. At different times during the STEREO mission the space environmental forecasting utility may emphasize different portions of the payload. Early in the mission the in situ instruments (IMPACT and PLASTIC) will likely be considered more important as they detect large scale structures in the solar wind while the spacecraft are still relatively close to Earth. An example of this is that the spacecraft trailing Earth may encounter interplanetary CME (ICME) shocks prior to their arrival at Earth. Also, solar energetic particles from activity east of about 45 west heliographic longitude will travel along interplanetary magnetic field lines and be detected by the trailing spacecraft minutes to hours before their arrival at Earth [e.g., Cane et al, 1988]. Further, corotating interaction regions can be geoeffective [e.g., McAllister and Crooker, 1997], and these will be detected by the trailing spacecraft several days prior to their passage by Earth. Identification of potentially geoeffective features in the plasma electron, magnetic field, and energetic particle data from IMPACT should be straightforward [e.g., Neugebauer and Goldstein, 1997]; and the ion compositional content provided by PLASTIC will be useful for identifying ICMEs and other structures in the solar wind [e.g., Galvin, 1997]. SWAVES will be used as a remote sensing instrument, producing low frequency radio dynamic spectra for the space weather broadcast. These spectra will be used to track the heliospheric propagation of shocks associated with Type II interplanetary radio bursts [e.g., Kaiser et al, 1998]. The comparison of dynamic spectra from the two spacecraft will give an estimate of the true location of the emission, and hence of the shock, by calculating the time delay between the observations. SWAVES will also measure in situ plasma waves, but these data are not part of the baseline space weather broadcast. As the STEREO spacecraft drift farther away from Earth, the SECCHI remote sensing instrument suite will become more important for space weather forecasting when they provide observations that are not available from the Sun-Earth line. Using images from the EUVI telescope, one will be able to locate CMEs low in the corona [e.g., Thompson et al, 1999] to identify the timing and direction of the launch more precisely than from coronagraphic data alone. Also, images from EUVI on the trailing spacecraft will show newly formed active regions prior to their appearance at the Sun's east limb (as seen from Earth). EUVI will also image other potentially geoeffective structures such as coronal holes and filament channels. The near-sun fields of view from the SECCHI coronagraphs (COR1 and COR2) and the wide field views from the heliospheric imagers (HI1 and HI2) will provide detection of and directional information about CMEs. This, along with more precise speed determination as the ICME propagates through the interplanetary medium, will provide significantly better predictions for Earth-arrival times than has been possible in the past, at least for those ICMEs detected by HI. Several studies combining coronagraphic and in situ data have demonstrated the difficulty in com-

3 ST. CYR AND DA VILA 207 Table 1. The STEREO mission instrument complement and proposed space weather broadcast content. Instrument Name and Collaborating Institutions IMPACT (In situ Measurement of Particles and CME Transients) Principal Investigator: Dr. J. G. Luhmann, University of California, Berkeley, NASA-GSFC, Caltech, U. Md, U. Kiel, CESR, MPAe, JPL, ESTEC, UCLA, NOAA, LANL, KFKI, et al. PLASTIC (PLAsma and SupraThermal Ion and Composition) Principal Investigator: Dr. A. B. Galvin University of New Hampshire University of Bern, MPE-Garching, et al. SECCHI (Sun-Earth Connection Coronal and Heliospheric Investigation) Principal Investigator: Dr. R. A. Howard Naval Research Laboratory, Washington, D. C. Lockheed-Martin Solar and Astrophysics Lab, NASA- GSFC, University of Birmingham (U.K.), IAS, RAL, MPAe, U. Kiel, CSL, et al. SWAVES (STEREO/WAVES) Principal Investigator: Dr. J.-L. Bougeret CNRS, Observatoire de Paris, University of Minnesota, UC Berkeley, NASA-GSFC, U. Colorado Measurement and Proposed Space Weather Broadcast Content Solar wind plasma characteristics; magnetic field parameters; solar energetic particles One minute average solar wind electron fluxes (6 energy bands); magnetic field strength and direction; energetic electron, proton, ion (He,CNO,Fe) fluxes (multiple bands) Ions in the energy-per-charge range of 0.2 to 100 kev/e One minute average solar wind proton density, bulk speed, thermal speed, and direction; alpha density; representative charge (or abundance) state distributions; suprathermal rates EUV imager, two coronagraphs with overlapping fields of view; two heliospheric imagers with overlapping fields of view 256x256 pixel highly compressed images from EUVI, COR1, COR2, HI1, HI2 Interplanetary radio bursts from 40 khz to 16 MHz One minute average radio dynamic spectrum (Intensity, frequency, time) paring CME speed measurements made near the Sun with ICME speed measurements in the solar wind [e.g., Lindsay et al, 1999; Gopalswamy et al, 2000]. Of course, models will still be necessary to predict ICME properties such as magnetic field strength and direction, and density enhancements due to compression. Another beneficial aspect to space weather forecasting and to public outreach will be the combination of STEREO in situ measurements and images with other available groundbased and spacebased observations. Based on these, one can imagine producing near-real-time visualizations of inner heliospheric conditions [e.g., Jackson et al, 1998]. The STEREO space weather broadcast can also be considered a "test bed" for future monitors of conditions affecting humans in both near-earth and interplanetary space. As humans venture into space to establish a more permanent presence with the International Space Station, knowledge of environmental conditions will be crucial to their protection. Future human voyages to Mars or other destinations will require communications from remote monitoring platforms because warnings from Earth of environmental hazards would be too late. IMPLEMENTATION Onboard each STEREO spacecraft the space weather broadcast data will be encoded into one of four types of telemetry packets ~ one each for the instruments described in Table 1. The IMPACT, PLASTIC, and SWAVES packets will contain the most recent one-minute average for the parameters listed in Table 1. These packets will be "stand alone" in that receipt of any single packet will provide a snapshot of the previous minute's values for the solar wind, energetic particle, and low frequency radio environment at

4 208 STEREO SPACE WEATHER BROADCAST that spacecraft. In contrast, the individual SECCHI packets will contain only a part of a single highly compressed image. Assuming a packet size of 500 bits and excluding overhead, then a compressed 256x256 pixel image will require about 200 packets (and an equivalent number of seconds) to acquire an entire frame. Since transmission of a SECCHI image would delay receipt of data from the other instruments, the in situ packets will be placed in the telemetry downlink at least once every minute. Communications with the STEREO spacecraft will be through the Deep Space Network (DSN) 34 meter antennas. The RF system on the spacecraft will transmit in the X-band frequency range (8.4 GHz), and the space weather broadcast packets will be downlinked at a rate of about 500 bits per second. The baseline operations concept is that the flight operations control center will be in contact with each spacecraft for four hours each day. It is likely that the DSN contacts will be at different times during the day for each spacecraft. During those DSN contacts commands will be uplinked to each spacecraft, and the solid state recorders containing the stored data will be transmitted to the ground. Some data will be available in real-time such as housekeeping parameters, the space weather packets, and some science data. This real-time telemetry stream will be transmitted to the STEREO Science Center (SSC), which will be located at NASA's Goddard Space Flight Center. At some later time, the recorder data (containing the full resolution data) will be transmitted to the SSC, reformatted at instrument team workstations, and put online for public access. The SSC, modeled after successful facilities for SOHO [St.Cyr et al, 1995] and ACE [Garrard et al, 1998], will also maintain an archive of the space weather broadcast data. Upon receipt in the SSC, the space weather broadcast packets will be reformatted with software provided by the instrument teams and made available for public access on an Internet server. Users of the STEREO broadcast data can expect to see plots of the most recent in situ data, as well as recent movies of direct and differenced images from SECCHI. An archive of space weather broadcast data from the previous solar rotation should be available online. We encourage modelers to incorporate predictions and novel visualization displays of these data, either on the STEREO site or via links to their own Internet sites. NASA plans to provide four hours of space weather broadcast coverage for each of the two STEREO spacecraft. Ground stations for the remaining 20 hours per day are not presently accounted for, but we have initiated discussions with potential partners to increase the space weather broadcast coverage. Ideally, complete coverage for each spacecraft will be attained through multiple private, university, or national facilities. Since the downlink rate is rather small (-500 bps), we believe that even a modest Internet connection from a remote antenna will be sufficient to transmit the raw packets to the SSC for reformatting and display. Of course, there will be light-travel time latency in the data, and that will increase as the spacecraft separate from Earth. But ground processing should be rapid after receipt of the data stream at the SSC. Although detailed engineering design work is in progress, our present estimate is that a groundbased antenna and receiver with a gain/temperature (G/T) ratio of about 26.6 db/k would be sufficient to acquire the space weather broadcast packets through the entire two-year nominal mission. At that time, the range to each spacecraft will be more than 0.7 A.U. A three year extension to the nominal mission is the engineering design goal for major spacecraft subsystems. A groundbased system with G/T-33.2 db/k would be sufficient to obtain the space weather broadcast packets five years after launch. At that time the distance to each spacecraft will be slightly greater than 1.5 A.U. Given these values and an assumed ground system noise value of 440 K, antenna dish diameters of 7.2 meters (for the two year nominal mission) and 15.3 meters (for the extended mission to five years) would be sufficient. SUMMARY The real-time broadcast of space weather data from the two STEREO spacecraft offers new possibilities for environmental forecasters. The STEREO payload will provide solar wind plasma, magnetic field, and energetic particle parameters, as well as optical and radio views of the Sun that cannot be obtained from groundbased observers or spacecraft near Earth. This space weather data will be transmitted continuously from each spacecraft over the X- band frequency range at a data rate of about 500 bps. The combination of STEREO in situ measurements with those obtained from a solar wind monitor at L-l offers threepoint "ground truth" for modeling geomagneticallyeffective heliospheric structures. This capability provides an opportunity to test one concept of an environmental monitoring system that will be necessary for long duration human space flight in the future. Acknowledgments. We wish to acknowledge the continuing contributions by the STEREO instrument teams and by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) engineers who are developing the spacecraft. We are particularly appreciative of the ongoing efforts of NASA Project Manager A. Harper and systems engineer H. Maldonado, JHU/APL Project Manager J.T. Mueller, systems engineer A. Driesman, and RF engineer J. von Mehlem. We thank J.G. Luhmann, ML. Kaiser, K. Goetz, D. Curtis, C.T. Russell, R. Mewald, and J. Wolfson for

5 ST. CYR AND DAVILA 209 insightful comments about the manuscript. We appreciate the efforts of S. St.Cyr in formatting the manuscript. One of us (OCS) received partial support from the National Space Weather Program under NSF grant ATM REFERENCES Brueckner, G.E., and 14 co-authors, The large angle spectroscopic coronagraph (LASCO), Solar Physics, 162, , Cane, H.V., D.V. Reames, and T.T. von Rosenvinge, The role of interplanetary shocks in the longitude distribution of solar energetic particles, Journal of Geophysical Research, 93, 9,555-9,567, Galvin, A.B., Minor ion composition in CME-related solar wind, in Coronal Mass Ejections, editors N. Crooker, J.A. Joselyn, and J. Feynman, AGU Monograph 99, American Geophysical Union, Washington, D.C., , Garrard, T.L., A.J. Davis, J.S. Hammond, and S.R. Sears, The ACE Science Center, Space Science Reviews, 86, , Gopalswamy, N., A. Lara, R.P. Lepping, ML. Kaiser, D. Berdichevsky, and O.C. St.Cyr, Interplanetary acceleration of coronal mass ejections, Geophysical Research Letters, 27, , Gosling, J.T., The solar flare myth, Journal of Geophysical Research, 98, 18,937-18,949, Jackson, B.V., PL. Hick, M.Kojima, and A. Yokobe, Heliospheric tomography using interplanetary scintillation observations: 1. Combined Nagoya and Cambridge data, Journal of Geophysical Research, 103, 12,049-12,067, Kaiser, ML., M.J. Reiner, N. Gopalswamy, R.A. Howard, O.C. St.Cyr, B.J. Thompson, and J.-L. Bougeret, Type II radio emission in the frequency range from 1-14 MHz associated with the April 7, 1997 solar event, Geophysical Research Letters, 25, , Lindsay, G.M., J.G. Luhmann, C.T. Russell, and J.T. Gosling, Relationships between coronal mass ejection speeds from coronagraph images and interplanetary characteristics of associated interplanetary coronal mass ejections, Journal of Geophysical Research, 104, 12,515-12,523, McAllister, A.H. and N.U. Crooker, Coronal mass ejections, corotating interaction regions, and geomagnetic storms, in Coronal Mass Ejections, editors N. Crooker, J.A. Joselyn, and J. Feynman, Geophysical Monograph 99, , American Geophysical Union, Washington, D.C., Neugebauer, M. and R. Goldstein, Particle and field signatures of coronal mass ejections in the solar wind, in Coronal Mass Ejections, editors N. Crooker, J.A. Joselyn, and J. Feynman, Geophysical Monograph 99, American Geophysical Union, Washington, D.C., , Rust, D., and 17 co-authors, The Sun and Heliosphere in Three Dimensions: Report of the NASA Science Definition Team for the STEREO Mission, The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, December St.Cyr, O.C, L. Sanchez-Duarte, P.C.H. Martens, J.B. Gurman, and E. Larduinat, SOHO ground segment, science operations, and data products, Solar Physics, 162, 39-59, St.Cyr, O.C, and 13 co-authors, Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998, Journal of Geophysical Research, 105, 18,169-18,185, Thompson, B.J., O.C. St.Cyr, S.P. Plunkett, J.B. Gurman, N. Gopalswamy, H.S. Hudson, R.A. Howard, D.J. Michels, and J.-P. Delaboudiniere, The correspondence of EUV and white light observations of coronal mass ejections with SOHO EIT and LASCO, in Sun-Earth Plasma Connections, Geophysical Monograph 109, American Geophysical Union, Washington, D.C., 31-46, Webb, D.F., E.W. Cliver, N.U. Crooker, O.C. St.Cyr, and B.J. Thompson, Relationship of halo coronal mass ejections, magnetic clouds, and magnetic storms, Journal of Geophysical Research, 705,.7,491-7,5O8, Zwickl, R.D., and 11 co-authors, The NOAA Real-Time Solar- Wind (RTSW) system using ACE data, Space Science Reviews, 86, , O. C. St.Cyr, Computational Physics, Inc., Code 682, NASA- Goddard, Greenbelt, Maryland J. M. Davila, Code 682, NASA-Goddard, Greenbelt, Maryland 20771