An operational space weather forecast system requires improved understanding in three broad areas of research: (1) the Sun and solar wind, (2) the magnetosphere, and (3) the ionosphere/thermosphere system. In June 1995, a working group for each of these research areas was assembled to formulate plans for addressing its space weather goals. Each of the three groups was given the goals listed in Table 2-2 and asked to identify what was needed in terms of physical understanding, models , and observations. The fifteen items in Table 2-2 were divided among the three groups, as indicated in Figure 3-1.
|· Coronal mass ejections|
|· Solar activity/flares|
|· Solar and galactic energetic particles|
|· Solar UV/EUV/soft x-rays|
|· Solar radio noise|
|· Solar wind|
|· Magnetospheric particles and fields|
|· Geomagnetic disturbances|
|· Radiation belts|
|· Ionospheric properties|
|· Ionospheric electric fields|
|· Ionospheric disturbances|
|· Ionospheric scintillations|
|· Neutral atmosphere|
Figure 3-1. Domains for Space Weather Research
A detailed description of the plan developed by these working groups is contained in Appendix A. Here we provide a summary of the overall plan, including a description of the physical understanding, model development, and observations required in each area.
3.1 Physical Understanding
As outlined in Chapter 2, the National Space Weather Program (NSWP) goals involve the ability to predict the state of the space environment. Hence, basic scientific research must be conducted to improve our fundamental understanding of the physical processes involved.
Beginning with the Sun, a critically important basic research objective is to understand the processes by which coronal mass ejections (CMEs) occur, including the factors that influence their sizes, shapes, masses, speeds,and magnetic field configurations. Equally important is an understanding of solar activity in general. This requires studying how the solar dynamo works and the identification of precursors to solar activity, such as short-term development of active regions and long-term buildup of polar magnetic fields. This involves studying the dynamics of magnetic energy in the solar corona and the role of magnetic fields in the occurrence of flares. It is also important to understand the origins of high-energy solar particles and how they propagate through the interplanetary medium. Similar processes play a role in modulating the fluxes of cosmic rays originating in galactic space. Solar radiation at ultraviolet (UV), extreme ultraviolet (EUV), and soft x-ray wavelengths has a direct effect on Earth's atmosphere. Research in this area is aimed at understanding the variability of the Sun at these wavelengths and how this variability influences the state of the ionosphere and thermosphere. The origin of solar radio noise, which affects communication systems, must also be understood. Finally, the solar wind has a direct influence on the state of Earth's magnetosphere, so it is vital that we understand the processes by which the solar wind is heated and accelerated in the solar corona, as well as the transient perturbations and shocks created by flares and CMEs.
Many space weather applications require knowledge of the particle populations and electromagnetic fields throughout the magnetosphere. This dynamic environment can only be understood by studying the coupling processes between the solar wind and magnetosphere; the transport and energization of plasma in the magnetosphere; and the onset, expansion, and recovery phases of storms and substorms. The strong magnetic coupling between the magnetosphere and Earth results in geomagnetic disturbances. The ability to predict geomagnetic disturbances depends on our understanding of the role played by the magnetosphere, ionosphere, and neutral atmosphere (the thermosphere and mesosphere) in modulating the strength of electric currents in space. It is also important to quantify the electrical currents induced in the ground by dynamic currents in the magnetosphere. The magnetospheric radiation belts represent a serious hazard to space systems. Because parts of the radiation belts have been observed to vary significantly, research must be conducted to understand the transport, production, and loss processes that determine the particle flux levels in both quiet and storm times.
Ionospheric properties, including electron density, electron and ion temperature, and composition, are determined by solar radiation, auroral particle impact, and Joule heating. Advances in predictive capabilities in this area depend on our understanding of the formation mechanisms responsible for large-scale and medium-scale electron density structures, and the production, transport, and loss mechanisms associated with these electron density structures. These mechanisms are dynamic in nature and respond to both geomagnetic storms and substorms. The day-to-day variability of large-scale ionospheric features and small-scale plasma density irregularities must be understood to determine their effects on radio wave propagation during quiet and disturbed times. It is also necessary to study the relation between ionospheric irregularities and radio wave scintillation, in particular, the interactions that control the formation and evolution of 10-kilometer to 50-meter electron density irregularities that produce scintillations. Understanding of auroral energy input requires knowledge of the processes that guide, accelerate, and otherwise control particle precipitation, both in quiet times and in times of magnetic storms or substorms. Ionospheric electric fields that drive currents and produce Joule heating must be accurately specified for accurate prediction of ionospheric properties. In particular, it is important to study the small-scale electric field (E-field) structures and the large-scale electrostatic fields and identify the ways in which they couple to the magnetosphere and respond to changes in the interplanetary magnetic field. Further research must be conducted on the process by which high-latitude E fields penetrate to low latitudes. Ionospheric and thermospheric research is strongly coupled, and advancements in the two areas must proceed in parallel. For the neutral atmosphere, basic research needs to be conducted to understand the chemical, radiative, and dynamical processes that act to modify and redistribute energy and constituents throughout the upper atmosphere.
3.2 Model Development
Many different types of models are important for achieving the goals of the NSWP. The ultimate goal is to develop an operational model that incorporates basic physical understanding to enable specification and forecasting of the space environment by following the flow of energy from the Sun to Earth. This coupled system of models is to be constructed by merging parallel models for the solar/solar wind, the magnetosphere, and the ionosphere/thermosphere. In addition to this, several other types of models will be necessary. Any forecast model must begin with a detailed specification of the current state of the system, which is provided either by empirical models or by assimilative models that take in available observations and fill in gaps. Also, during the course of development of the full operational model, other approaches including empirical predictive methods will be developed and tested to ensure that the most efficient and accurate method is used in the final system.
The ability to predict coronal mass ejections and their subsequent effects requires models of the initiation process and three-dimensional magnetohydrodynamic (3D MHD) simulations of the resulting disturbances in the solar wind. Models of particle acceleration in the CME-driven interplanetary shocks are also necessary for predicting the intensity and time of arrival of particle events at Earth's orbit. Models of radio emissions from CMEs are important for optimizing the use of radio noise as a remote sensing tool.
In modeling solar flares, it is necessary to know the magnetic field in the corona. The only method for determining this field is to observe it at the photosphere and use numerical modeling to extrapolate it into the corona. Simulating the flare itself requires 3D models of magnetic reconnection in active regions, including consideration of the processes that determine the distribution and magnitude of resistivity. Models relating to the processes by which solar flares accelerate particles and generate UV, EUV, and x-ray bursts are also necessary for accurate prediction of flare effects.
Models for the solar wind include 3D MHD simulations of the coronal acceleration region and the solar wind extension into interplanetary space. A coupled version of these two models can be used as a proxy for specifying solar wind velocity prior to its expansion into interplanetary space. Because it is likely that interplanetary magnetic field (IMF) data will come from a satellite at the Lagrangian (L1) point, a model is also necessary to predict solar wind and IMF conditions at the magnetosphere as extrapolated from the available information.
To predict solar UV, EUV, and x-ray emissions, it is necessary to develop 3D models of the solar atmosphere and improve the solar spectrum calculations covering all significant lines and bands from atoms, ions, and molecules.
Although there are several models in existence that specify and predict the particles and fields in the magnetosphere, one in particular, the Magnetospheric Specification Model (MSM), has just been implemented at the space forecast centers. Currently, a slightly more advanced model, the Magnetospheric Specification and Forecast Model (MSFM), is being prepared for operational use. Both of these models depend on accurate specifications of magnetic and electric fields, and ionospheric conductances, including the effects of auroral precipitation. Continuing development of these models will improve and extend the capabilities of the MSM and MSFM. The MSFM represents only one approach to numerical magnetospheric prediction codes. Another approach incorporates global MHD simulations that self-consistently solve for the plasma distributions and electric and magnetic field configuration. Because MHD simulations do not account for thermal drifts where spatial gradients are strong, a merger of an MSFM-like code and a global magnetospheric MHD code may represent an important step toward developing a physics-based predictive magnetospheric model. Approaches that utilize adaptive grids show new promise to resolve features over many scale lengths. A desired byproduct of the magnetospheric model is the specification of currents throughout the magnetosphere and ionosphere system. From this information, other codes can be developed that predict geomagnetically induced currents and the magnetic disturbance indexes derived from them. The MSM and MHD models do not specify the energetic particle populations in the radiation belts. Static models for the radiation belts already exist, but there is an urgent need to develop dynamic models to account for the variations in energetic particle fluxes that are observed during storm conditions.
Approaches to modeling the ionosphere include empirical models based on worldwide data sets, assimilative models that incorporate real-time observations, and 3D time-dependent physical models. The Parameterized Realtime Ionospheric Specification Model (PRISM) is an operational system being used at the 50th Weather Squadron to ingest real-time ionospheric data from ground- and space-based sensors and produce electron density profiles. To achieve predictive capabilities, it is important to focus future work on dealing with large-scale and medium-scale structures in a self-consistent manner, and to incorporate the effects of storms and substorms. This may require the development of nested-grid and adaptive-grid models. More realistic boundary conditions must be applied with the eventual goal of developing a fully coupled model that encompasses the mesosphere, thermosphere, and ionosphere using computationally fast, empirical-numerical hybrid models. As with magnetospheric models, the E-field configuration is an important element in accurate predictions of ionospheric behavior. The E-field can be specified, analytically, semianalytically, or empirically, but in all cases is driven by interplanetary parameters and magnetospheric processes. These models must be able to account for the penetration of high-latitude E-fields to low latitudes, and the coupling to neutral atmosphere winds. Ionospheric structures, such as sporadic E, descending layers, equatorial plasma bubbles, auroral blobs, and polar cap patches, must be accounted for in specifying the state of the ionosphere. From the standpoint of satellite-based communication and navigation systems, it is most important to also include the effects of small-scale irregularities associated with these structures that cause ionospheric scintillations. To be operationally useful, the currently available climatological model that specifies scintillation for any radiowave propagation path at any frequency needs to be driven by real-time data from a network of stations. The ultimate goal is to develop a physics-based model incorporating the processes that lead to structuring at all scale sizes.
Neutral atmosphere modeling efforts focus on numerical Thermosphere-Ionosphere-Electrodynamics General Circulation Models (TIEGCMs) that can self-consistently calculate density perturbations and neutral wind systems on a global, 3D, time-dependent basis from physical principles. These models must continue to be upgraded, validated, and tested. Empirical, semiempirical, and assimilative models of the neutral atmosphere are also important to specify the starting point for physics-based models.
Observations in support of the NSWP include both operational and research-oriented data. Ground-based operational sensors include magnetometers and ionosondes, and sensors for ground-based solar observations at both radiowave and optical wavelengths. Future enhancements in ground-based operational sensors include extending the current networks, adding scintillation monitoring systems, upgrading solar observations, and adding a network of solar coronagraphs and interplanetary scintillation monitors. Ground-based sensors for research purposes include the array of HF and incoherent scatter radars, riometers, and optical instrumentation. Future enhancements include the Polar Cap Observatory (PCO) which extends the existing chains of incoherent scatter radars into the geophysically important region poleward of the auroral zone.
Space-based sensors for the NSWP must be deployed in many different orbital configurations. Low-Earth orbiting satellites, such as those of the Defense Meteorological Satellite Program (DMSP), are measuring properties of the ionosphere and thermosphere, as well as the plasma processes at low altitudes along auroral field lines. Highly elliptic, polar-orbiting spacecraft are needed to study ionosphere-magnetosphere coupling. Ideally, they should also carry optical and x-ray imagers for determining the instantaneous distribution of auroral precipitation. Sensors on geosynchronous satellites must continue to monitor the energetic particle populations in the magnetosphere and the solar x-ray emission. The particle monitors on the Global Positioning System (GPS) satellites can also be used to specify radiation belt flux levels. Extremely critical to the success of the NSWP is a satellite at the L1 point between the Earth and Sun to monitor the solar wind. This requirement is being filled currently by the WIND satellite and will be in the near future by the Advanced Composition Explorer (ACE) satellite. Plans should be developed for a follow-on to these satellites, ideally incorporating solar imaging capability in addition to the plasma monitors. The satellites described here will all provide operational data for space weather forecasters. Also important to meeting NSWP objectives is the existing and planned satellites that are part of the Department of Defense and National Aeronautics and Space Administration space missions. These satellites are identified individually in Appendix A.
One concern is that many of the existing research satellites are approaching the end of their funded mission lifetimes, and they will effectively be turned off just as we are starting solar maximum. These satellites could provide invaluable research data for understanding the basic science of the Sun, and they could also provide critical operational observations such as solar wind speed, interplanetary magnetic field orientation, etc.
Progress in pursuit of NSWP goals will be enabled by synergistic advances in physical understanding, model development, and observations. After developing plans to reach these research goals, members of the three working groups were asked to develop timelines for the accomplishment of the objectives. These timelines are presented in the next chapter, along with a list of items for near-term emphasis. A critical challenge for the space weather research effort will be to provide the linkage between the broad regions (solar/solar wind, magnetosphere, and ionosphere/thermosphere) of the Sun-Earth system.
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