SPACE WEATHER AND THE STEREO MISSION Introduction to and Definition of Space Weather As modern society becomes increasingly reliant on technologically advanced systems for many of its day-to-day functions, our ability to predict and respond to the impacts of space weather gains greater importance. The systems most susceptible to geomagnetic disturbances include the power grid and satellites, our reliance on which increases dramatically every year. The use of pagers and mobile phones has become almost ubiquitous. The global positioning system (GPS) is used heavily by the military, commercial airlines, and the shipping and boating industry, and is being introduced into automobiles. As the use of these systems become more widespread, space weather disturbances impact a wider array of people and human activities An example of the apparent loss of a communications satellite and associated widespread loss of services due to a space weather disturbances was described by Baker et al. (1998). They found that the combination of coronal mass ejections (CMEs), solar flares, and high speed solar wind streams led to a prolonged period of geomagnetically disturbed conditions during which the Galaxy 4 communications satellite was subjected to an intense population of highly energetic, relativistic electrons. Such problems may in fact be increasing. Since our society is becoming more dependent on advanced technological systems, we are increasingly more vulnerable to malfunctions in those systems. Considering just civilian communication satellites, a single satellite can cost several hundred million dollars. There are now over 100 such spacecraft in orbit and entire new constellations of satellites are being placed into a variety of Earth orbits. If even a small percentage of these satellites exhibit severe problems due to the space environment, the costs will be significant. The need to study, forecast and mitigate such effects is gaining increasing attention at the national level. Specifically, the need to develop a coordinated plan to improve present capabilities in specifying and forecasting conditions in the space environment has led to the formation of national programs such as the U.S. National Space Weather Program and NASA's Living With a Star program, and ESA's Space Weather program. As a scientific pursuit, space weather is considered analogous to atmospheric weather, having both research and forecasting elements. In some sense our knowledge and ability to build predictive models of space weather is equivalent to the early stages of atmospheric weather studies 20-30 years ago. The ultimate source for most space weather effects is the sun and its activity. The time scales over which effects from a solar event or other activity can be felt at Earth vary from a few minutes to 27 days, the rotation rate of the sun. Radiation from solar flare photons can arrive at Earth at the speed of light, in 8 min., and continue for hours. This X-ray, EUV and radio emission ionizes the sunlight ionosphere causing sudden ionospheric disturbances (SIDs) and communication problems. Some eruptive solar events can emit high energy solar energetic particles (SEPs), the most energetic (>1 GeV) of which arrive at Earth within tens of min. Most SEPs have lower energies (10-100 MeV) and begin to arrive tens of min. or hours after solar onset. These particles also cause ionospheric-related communications and navigation problems. Most SEP protons are accelerated in an interplanetary shock wave driven ahead of a CME. The proton flux can continue to increase for hours to days and can cause state changes or latch ups in electronic devices on satellites and hazardous conditions for astronauts and even airline crew and passengers. CMEs and Space Weather Most of the energy imparted to the solar wind by a solar event is associated with the ejected plasma and magnetic fields which collectively are called a coronal mass ejection and can take several days to reach Earth. CMEs cause space weather disturbances, including the largest geomagnetic storms, in a number of ways. First, strong southward fields in the CME can reconnect with the predominately northward fields of the sun-facing magnetosphere, producing strong induced fields and currents in the magnetosphere, the ionosphere, and at the Earth's surface. The transient solar wind energy is transferred to the magnetosphere through. Thus, strong, sustained southward fields as well as high velocity are important drivers of geoactivity. These enhanced regions tend to occur both in the compressed plasma between the shock and CME and in the CME itself, which often contains magnetic field structures carried from the sun called magnetic clouds. Second, the impact of the CME compresses the Earth's day side magnetosphere down to lower altitudes, which can leave high altitude geostationary satellites directly exposed to the solar wind and highly energetic particles. Third, if the CME front is sufficiently faster than the ambient solar wind flow, the CME will drive a shock wave that accelerates particles that can penetrate the Earth's magnetically shielded environment and occasionally reach the Earth's surface. SOHO has observed many excellent examples of this wherein a high speed CME is ejected and energetic particles arrived at the orbit of SOHO and are detected as a 'snow storm' of cosmic ray hits on the LASCO CCD detector. The occurrence rate of CMEs follows the sunspot cycle of the sun in phase and amplitude. Therefore, the strongest storms tend to occur during the years around sunspot maximum. The Combinations of successive CMEs pushing into each other and/or a CME interacting with a high speed stream can be especially geoeffective and occur more frequently around maximum. The STEREO mission will extend from early 2006 to 2008, during the late declining phase of this cycle through minimum to the early rise of the next cycle. Thus, the average CME rate during the mission may be as low as 0.5-1 CME/day, with maybe 15% of these aimed Earthward. However, important geomagnetic storms still occur during the declining phase of a cycle. Also, the global solar magnetic field and its extension into space simplifies during the approach to minimum so that identification and tracking of individual CME events from the new STEREO viewpoint should be straightforward. This, in turn, should enhance our understanding of the origins of CMEs, their propagation through the solar wind, and their coupling to the Earth's environment. CMEs are also probably an important, but not the sole cause of small and moderate level storms. There are sufficient resources on each spacecraft to extend to a 5 year mission, which would continue well into the rise phase of the next cycle.. Although recent strides have been made in our understanding of the relationship between CMEs and space weather, our current ability to forecast space weather disturbances caused by CMEs is still relatively poor. This applies to our ability to predict if and when a CME will impact the Earth as well as the magnitude of the anticipated impact. The LASCO and EIT observations from SOHO have demonstrated a first step in this prediction capability by reliably detecting halo CMEs - events that are directed along the Earth-Sun line and are visible as an expanding 'halo' around the coronagraph occulting disk. Halo CMEs were first identified with the NRL SOLWIND coronagraph (Howard et al., 1982) but only a handful were seen. With the increased sensitivity of the LASCO coronagraphs, however, it is now possible to detect almost all of these events, while observations of the origin of the CME on the disk by EIT, as well as with the Yohkoh SXT, H-alpha, radio, etc., can be used to determine whether the CME is headed toward or away from Earth. The associated surface activity of "frontside" halo CMEs can be well observed and, in addition, the physical structure of CMEs along their central axes can be analyzed from in-situ data from near-Earth spacecraft a few days later. Initial studies using the LASCO data showed that frontside halo CMEs were associated with flaring active regions and filament eruptions within half a solar radius of Sun center and often accompanied at coronal wavelengths by circular, expanding fronts, or waves, and dimming of the emission, also known as transient coronal holes. The transit time of the CMEs from Sun to Earth often is not well correlated with the measured CME veloicty. One clue may be the acceleration of slow CMEs and the deceleration of fast CMEs fairly close to the Sun. Sheeley et al. (1999) found that fast halo CMEs tended to decelerate down to the solar wind speed, as seen in projection within the LASCO C3 field of view (FOV). The geoeffectiveness of halo CMEs has been examined in several studies by comparing the timings of the CME onsets with WIND and ACE spacecraft data at 1 AU and with geoactivity. Brueckner et al. [1998], St. Cyr et al. [2000] and Webb et al. [2000] found good correlations between frontside halo CMEs and moderate geomagnetic storms with a lag time of 2-5 days dependent on the phase of the cycle. However, St. Cyr et al. also found a high level of false alarms using the halo data. Magnetic clouds preceded by interplanetary shocks usually signified the arrival at Earth of the CME structure. The lesson from the SOHO data that pertains to the STEREO mission is that even moderately energetic CMEs, such as observed by LASCO near solar minimum, can be very geoeffective because they often contain coherent magnetic structures and move faster than the ambient solar wind. From the space weather viewpoint, the key is to determine whether the CME is aimed toward Earth or not. With STEREO we anticipate significant improvements in our ability to predict the timing and impact of CME-induced geomagnetic disturbances. Use of the STEREO Instruments for Space Weather For the STEREO mission, in general, and for space weather purposes, in particular, the two most important parameters to determine for a CME aimed in Earth's direction will be the trajectory of the central axis of the CME and the timing of its arrival at Earth. In terms of its path we need to be able to answer the questions: 1) Will it hit Earth directly along its central axis or will it graze or miss Earth completely? 2) What is the size of the CME, especially its longitudinal extent? Obviously the larger a CME is the more likely it will be to pass over the Earth and be geoeffective. 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 [http://sprg.ssl.berkeley.edu/impact/] and PLASTIC [http://stereo.sr.unh.edu/]) 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 deg. 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. Further, corotating interaction regions can be geoeffective, 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, and the ion compositional content provided by PLASTIC will be useful for identifying ICMEs and other structures in the solar wind. SWAVES [http://www-lep.gsfc.nasa.gov/swaves/swaves.html] 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. As the STEREO spacecraft drift farther away from Earth, the SECCHI imaging instrument suite [http://stereo.nrl.navy.mil/] will become more important for space weather forecasting when they provide observations that are not available from the Sun-Earth line. The eruption of Earth- directed CMEs will be detected by the SECCHI EUVI, COR1, and COR2 experiments. Early in the mission, when the STEREO spacecraft are near the orbit of Earth, COR2 will record halo CMEs produced by front-side sources detected with EUVI. As the separation of the STEREO spacecraft increases, the different vantage points will allow us to determine the velocity vector of the CME, as well as the material that will directly impact Earth. The velocity vector will be determined by means of triangulation techniques involving simultaneous stereo image pairs. In cases where simultaneous image pairs will be obtained with EUVI, COR1, COR2, and HI, the velocity vector should reveal any acceleration or deceleration of CMEs. 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. A real advance in our ability to predict the arrival of CMEs at Earth will be provided by the Heliospheric Imagers (HIs) on both STEREO spacecraft, which will track CMEs throughout their trajectory to Earth. This is an important component of our space weather prediction capability. Near solar minimum, when STEREO will be in operation, even moderate strength CMEs can have significant geomagnetic effects. Consequently, just the simple ability to predict the arrival at Earth of a CME will be important. The HIs are designed to be able to reliably determine the 3D dimensions and trajectory of any Earthward CME. The HI field of view is up to 70 x 70 deg. with the vertical dimension centered on the Sun-Earth line and, in the horizontal dimension, extending from near the sun on one side to near or beyond the Earth on the other side, depending on the phase of the mission. Since the HIs will view any CME traveling along the Sun-Earth line in opposing off-center projections, image reconstruction will allow us to determine the direction of its central axis, its overall geometry and size, its mass and its velocity. In terms of timing we need to be able to determine the time of arrival at Earth of a CME from distance and time measurements of its frontal speed, or the speed of any interplanetary shock being driven ahead of the CME. We know that the leading edge of a CME is its fastest portion, with velocity dispersion through to the rear of the CME where the material is moving slowest. Thus, prominence material, if embedded within a CME, always moves outward more slowly than the leading edge. The coronagraphs and the HIs will be important for determination of the kinematics of a CME as it moves outward from the lower corona through the solar wind. The next step in advancing our understanding of space weather lies in predicting the geoeffectiveness of CMEs, which is strongly governed by the southward component of its magnetic field, Bz. Strong Bz can be created in several ways, from a strong internal Bz in the plasma configuration ejected from the Sun, by shock compression of the field in fast CMEs, and by the interaction of the CME with strong fields in the heliospheric plasma sheet. In all cases, the complete diagnostic capabilities of the STEREO investigation will be needed to disentangle the various components involved in the complex interaction of the CME with the magnetosphere. The twin STEREO spacecraft will provide valuable platforms for detecting and studying SEP events. SEPs are known to be well associated with CMEs and their associated interplanetary shocks (Reames, 1997). High energy (1 to hundreds of MeV) particles in a SEP event are thought to be accelerated in the shock near and ahead of a fast CME. When the particles gain access to open field lines ahead of the shock, they can propagate into the heliosphere far from the acceleration site. Since the open field lines emanating from the Sun form an Archimedean spiral, the particles are forced to travel along these curved paths. Thus, SEPs will have most direct access and arrive earliest at a point in space which is east of that point's central meridian on the solar disk. Thus, a particle detector on a spacecraft near Earth will be "well-connected" to SEP source regions at western solar longitudes (i.e., W60 deg.), and will see a prompt rise in particle flux (within tens of minutes) with a duration of days. In terms of the STEREO mission, the in-situ instruments on the STEREO spacecraft, IMPACT and PLASTIC, will provide valuable observations for detecting and studying SEP events. The STEREO spacecraft lagging the Earth (Behind) will be well connected to field lines which are near sun center as viewed from Earth. So its particle detectors can be used to predict the onset and size of a prompt SEP, as well as the onset of the CME associated with the SEP, at Earth. This is one of the most important reasons for the desire to have a near-realtime Beacon available for the STEREO mission. The Beacon mode of operation for STEREO (e.g., St. Cyr and Davila, 2001) is desired for use by both civilian and military agencies for detecting Earthward-directed eruptions at the Sun and tracking them from the Sun to 1 AU. STEREO will have a Beacon mode which will utilize low-rate X-band ground antennas at Earth to continually downlink pertinent STEREO data. NASA will provide coverage through the DSN for about 4 hours per day. Six other sites approximately evenly spaced in longitude around the Earth will provide worldwide coverage from each STEREO. The Beacon is currently configured to use about a 600 bps data rate. At this rate a compressed 256x256 pixel image will require about 200 packets (and an equivalent number of seconds) to acquire an entire frame. 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