The Next Decade of Discovery in Solar and Space Physics Exploring and Safeguarding Humanity's Home in Space (2025) / Chapter Skim
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Appendix D: Report of the Panel on the Physics of Ionospheres, Thermospheres, and Mesospheres
Pages 381-452

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From page 381...
... Beginning nearly 100 years ago with the discovery of the Appleton Anomaly and extending to the dramatic ITM response to the 2022 Hunga Tonga–Hunga Ha‘apai volcanic eruption, the community has mostly focused on individual phenomena or state parameter observations to elucidate the underlying ITM processes. Furthermore, the limitations in the technological capabilities, budget resources, and interagency coordination of the past 2 decades drove previous decadal surveys to construct phenomenologically focused goals within individual regions.
From page 382...
... It is imperative that the ITM community make significant progress in the fundamental understanding of the ITM as an interconnected system if we are to meet society's space weather forecasting needs and fulfill obligations defined by the PROSWIFT Act.
From page 383...
... In summary, the ITM is a unique natural laboratory that is invaluable for advancing interdisciplinary science. The priority science goals and objectives defined in Sections D.3 and D.4, along with the implementation strategies defined in Sections D.5 and D.6, reflect the tremendous benefit of transforming the ITM community's focus toward system science investigations that advance both fundamental and applied knowledge.
From page 384...
... The ITM response also included large-scale ionospheric electron density depletions extending well beyond equatorial latitudes. This unprecedented forcing event has accelerated efforts to use the ITM system-wide Tonga response to better understand whole atmosphere coupling through studies that emphasize the importance of gravity wave forcing from the lower atmosphere, filtering effects of wave breaking and regeneration in the mesosphere and lower thermosphere, subsequent driving of pronounced ionospheric features, and the role of background thermospheric winds in mediating transient behavior.
From page 385...
... mission provided the first widescale observational quantification of the significance of atmosphere–ionosphere coupling mechanisms (e.g., dynamo electric fields, ion drag, and composition carried by tides and planetary waves) using coordinated measurements of low-latitude neutral winds, plasma flows, composition, and densities.
From page 386...
... . In the 2013 solar and space physics decadal survey (NRC 2013; hereafter the "2013 decadal survey")
From page 387...
... Through multiple mechanisms including electrodynamic, composition, and kinetic pathways, geomagnetic storms trigger large and complex perturbations in all ITM state variables on multiple temporal and spatial scales. Understanding this storm-time forcing and its impacts on system response is by no means a closed topic and remains a vital part of community research.
From page 388...
... Priority Science Goal 1, Objective 1.3: Determine How the ITM Responds to Lower Atmosphere Forcing on Local and Global Scales The ITM system has been shown to be extremely sensitive to lower atmosphere forcing, especially during periods of minimal solar activity. The 2013 decadal survey recognized the importance of lower atmosphere forcing on the ITM.
From page 389...
... The TIMED mission is an example of a current extended mission that provides a more than 20-year observational baseline for studying the energy transfer into and out of the mesosphere and lower thermosphere. In the next decade, it will be important to make continuity of measurements a priority for the successful closure of this science objective.
From page 390...
... 390 THE NEXT DECADE OF DISCOVERY IN SOLAR AND SPACE PHYSICS Priority Science Goal 1, Objective 1.4: Elucidate the Mechanisms That Govern the Transition in Chemical, Dynamical, and Thermal Drivers Across the ITM from ~100–200 Kilometers Within the lower thermosphere and ionosphere, many state parameters within the region between 100–200 km exhibit complex transitions as a function of altitude. For example, one set of tides and waves dominates at the lower end of this altitude range, but a pronounced transition to a different set of tides and waves occurs at higher altitudes, as shown in Figure D-4.
From page 391...
... D.3.2 Priority Science Goal 2: How Do Internal ITM Processes Transform Energy and Momentum Across a Continuum of Spatial and Temporal Scales? The key state parameters of the ITM system exhibit structure over spatial and temporal scales spanning several orders of magnitude (see Table D-1)
From page 392...
... The following sections describe specific scientific objectives identified by the panel within PSG 2 for which significant progress can be achieved in the next decade. Priority Science Goal 2, Objective 2.1: Determine How the Gravity Wave Spectrum Cascades Throughout the Thermosphere and Impacts the ITM System Gravity waves from the lower atmosphere significantly modulate the mean state and variability of the ITM.
From page 393...
... There are promising avenues emerging to address the situation, in the form of networked multistatic meteor radar sites, which are beginning to yield gravity wave characterizations in a limited altitude range (80–100 km)
From page 394...
... Recently launched mission Atmospheric Waves Experiment (AWE) and upcoming GDC will address gravity waves and their effects, but only FIGURE D-7 Perturbation vertical, zonal, and meridional winds modeled by the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X)
From page 395...
... For these reasons, the panel finds that the vertical life cycle of gravity wave momentum and energy is a key knowledge gap to be addressed in the next decade. Priority Science Goal 2, Objective 2.2: Determine How Small-Scale Structuring in High-Latitude ITM State Parameters Leads to Mesoscale Conductivity Enhancements That Have Aggregate Global Significance Small-scale filamentary current structures at high latitudes produce significant local heating and ionization.
From page 396...
... Such models can self-consistently account for cross-scale, magnetosphere– ionosphere–thermosphere coupling processes. GDC will provide critical surveys of the spatial scales and persistence times of various phenomena associated with the ITM's response to solar and magnetospheric energy inputs.
From page 397...
... Priority Science Goal 2, Objective 2.3: Determine How Nonlinear Coupling Between Mean Neutral Atmosphere Circulation, Tides, and Planetary Waves Drives ITM Variability A significant amount of observational and theoretical work over the past decade has improved the understanding of how global-scale waves (i.e., tides, planetary waves, and tropical waves) from the lower atmosphere force the ionosphere–thermosphere system.
From page 398...
... 2. Determine how small-scale structuring in high-latitude ITM state parameters leads to mesoscale conductivity enhancements, which have aggregate global significance.
From page 399...
... introduced system science, particularly system identification, as a valuable framework for ITM science. The 2013 decadal survey recognized the strong role that modeling and theoretical analysis play in advancing a system science approach, specifically noting that comprehensive models of the AIM system would benefit from "developing assimilative capabilities" and "the development of embedded grid and/or nested model capabilities, which could be used to understand the interactions between local- and regional-scale phenomena within the context of global AIM system evolution." It also noted that "Complementary theoretical work would enhance understanding of the physics of various-scale structures and the self-consistent interactions between them." The 2013 decadal survey recommended the implementation of a multiagency initiative, DRIVE, to more fully develop and effectively employ the many experimental and theoretical assets at NASA, NSF, and other agencies to address the need for multidisciplinary data and model integrated investigations of fundamental physical processes.
From page 400...
... There are climatological controlling factors, such as orientation of the terminator with respect to magnetic meridian, and variable factors, such as neutral winds and global electric fields. On many nights, low-density plasma bubbles emerge from the lower F-region ionosphere and rise quickly to altitudes of 1,000 km, often becoming very turbulent and leading to dramatic height versus time intensity variations.
From page 401...
... APPENDIX D 401 FIGURE D-11 The global distribution of Global Positioning System (GPS) signal total interruption for all channels observed by the Swarm A satellite.
From page 402...
... Priority Science Goal 3, Objective 3.2: Quantify the Relative Significance of Competing Drivers of Day-to-Day Variability in the ITM System Day-to-day variability in the ITM system can arise from changes in several factors. Many competing drivers of the observed variability in the neutral wind field in particular have been identified, including auroral energy deposition, electric fields, gravity wave breaking, nonlinear interactions among various wave types, and ion-neutral coupling.
From page 403...
... , but UQ is just as important for scientific understanding of ITM system variability. Priority Science Goal 3, Objective 3.3: Quantify the Most Salient Factors Governing the Global ITM Response During and Following Geomagnetic Activity The ITM response to geomagnetic storms has been a topic of study for several decades, and substantial recent progress in understanding individual mechanisms has been made by missions such as TIMED, GOLD, and ICON,
From page 404...
... This can lead to overestimation of the F-region electron density by a factor of 2. Resolving these difficulties to further advance modeling capability needs to incorporate simultaneous images of both north and south auroral regions, coupled with in situ measurements and comprehensive ground-based measurements in both hemispheres.
From page 405...
... The magnetosphere–ionosphere–thermosphere–mesosphere system regulates Earth's global current circuit, highlighting the critical importance of ionospheric conductivity. A number of ITM processes that govern structure in conductivity (Joule heating, transport, precipitation, composition)
From page 406...
... Over the past few decades, it has become clear that the ITM is also undergoing secular evolution on much longer timescales. Several key ITM state parameters, such as total atmospheric mass density and temperature, have been observed to be slowly but significantly deviating from their historical ranges associated with sporadic and climatological variability.
From page 407...
... The following sections describe specific scientific objectives identified by the panel within PSG 4 for which significant progress can be achieved in the next decade. Priority Science Goal 4, Objective 4.1: Determine How the ITM Is Influenced by Slowly Evolving Trends in Its Natural System Drivers The ITM system is driven predominantly by the conditions of the natural environment in which it is embedded -- namely, solar radiation controls ionization and heating; the interaction of the solar wind and interplanetary magnetic field with the magnetosphere generates high-latitude ionospheric current systems and induces plasma transport; and these, together with lower atmospheric conditions, influence atmospheric chemistry and dynamics.
From page 408...
... , and expanded in size (by 5 percent) , such that energetic particles from Earth's radiation belt can penetrate farther into the upper atmosphere, posing increased risk of damage to satellites in low Earth orbit.
From page 409...
... Furthermore, ongoing and expected changes in lower atmosphere temperature, circulation, and weather patterns, including increased tropospheric storm frequency and intensity, are expected to alter the generation of gravity waves and wave-driven circulation in the ITM relative to present-day conditions. Intentional geoengineering efforts, with the aim of mitigating some of the harmful effects of anthropogenic increases in greenhouse gas concentrations, may themselves induce persistent changes in the ITM.
From page 410...
... Priority Science Goal 4, Objective 4.3: Determine Which Aspects of the ITM State Are Most Sensitive to Persistent Changes in Its System Drivers As described earlier, several key ITM system drivers have sustained significant secular evolution over the past several decades, and these trends are widely expected to continue or even accelerate in the decades to come.
From page 411...
... A particularly important open question is whether the ITM could undergo a transition to a new and potentially radically different dynamical state. For example, it is established that the mesosphere and lower thermosphere region departs from radiative equilibrium owing to dissipation of atmospheric gravity waves, but how will this region be impacted if the sources of these waves change following the troposphere, potentially crossing a climatic tipping point?
From page 412...
... D.4 LONG-TERM SCIENCE GOAL: HOW DO ITM SYSTEMS OPERATE ON OTHER WORLDS WITH VERY DIFFERENT CHARACTERISTICS AND PHYSICAL DRIVERS? Because of its accessibility and societal importance, ITM science has concentrated on the study of Earth's upper atmosphere.
From page 413...
... Although the Parker Solar Probe has conducted 7 flybys of Venus to date, with another 22 planned in the next decade, no previous investigation has obtained key measurements of the time series of solar inputs and planetary thermosphere/ionosphere reactions coincidentally with observations of the ITM response to those inputs. Important questions remain about what processes control atmospheric escape and how small-scale waves contribute to the super-rotation of the upper atmosphere.
From page 414...
... While ground-based assets provide measurements at many of the key altitudes in the mesosphere and lower thermosphere, there remain many ionosphere and thermosphere state parameters that can be observed only from space-based platforms. Ultimately, multipoint global and multiple altitude observations of ionospheric and thermospheric state variables are required to investigate the cross-scale processes within the regions and those across regional boundaries.
From page 415...
... In a number of areas, ITM science has used these tools by leveraging informal data sets of opportunity using a few monolithic instrument sites and multiple smaller networks that are sometimes anchored by a primary facility. While this architecture has made the ITM discipline the most data-rich subdiscipline of solar and space physics, ITM data are sparse in terms of data parameter types, the associated spatial coverage, and temporal continuity, which limits the communities' ability to address the PSGs of the coming decade.
From page 416...
... Over the past decade, the NSF, NOAA, and NASA portfolios have maintained the traditional combination of a few large-scale ground facilities and space missions, along with more numerous, lower-cost, focused experimental platforms, such as Fabry-Pérot interferometers, all-sky imagers, ionosondes, suborbital launches, and CubeSats, which provide observational "snapshots" of ITM state parameters in terms of either location or time. In addition to these vital measurements,
From page 417...
... Ground-based observations have always played a key role in ITM studies, and current larger-scale ITM facility programs remain important for implementing each of the coming decade's science goals. Such programs include multi-instrument IS radar facilities, which provide comprehensive measurements of multiple state parameters at fixed locations.
From page 418...
... The ITM community will be able to address this decade's four priority system science goals only through a paradigm shift enabled through active and frequent coordination among agencies, facilities, and programs. D.6.2 Spaceflight Missions To advance the ITM PSGs defined in Section D.3, the panel identified five spaceflight mission concepts that each provide critical measurements of key state parameters needed to understand the ITM as a holistic system.
From page 419...
... The BRAVO science goal is to explain the ITM effects of momentum and energy transport and deposition through direct uninterrupted observations of their propagation from the lower atmosphere to their breaking in the ITM. The specific science objectives are to 1.
From page 420...
... sensors on satellites M1, M2, M3, and S1, which measure electron density from 90–400 km. • A network of ground-based meteor radars, which measure the neutral wind at high resolution from ~80 to ~100 km altitude from observations of sporadic meteor trail scatter.
From page 421...
... . Resolve Implementation strategy: Deploy a dense constellation of space-borne sensors monitoring the neutral wind, temperature, and density in the lower thermosphere, in order to resolve the pathways for global-scale energy and momentum transport from the lower atmosphere to the middle and upper atmosphere.
From page 422...
... . TABLE D-3 Science Traceability for the Resolve Mission Science Objectives Measurement Objectives Required Observables 1.
From page 423...
... A comprehensive OSSE would be useful for further optimization of the distribution of orbital planes, spacecraft, and the look directions within each plane. The notional spacecraft is a 3-axis-stabilized 6U CubeSat bus with a single instrument to observe vertical profiles of line-of-sight neutral wind, temperature, and atomic oxygen density, at all latitudes, longitudes, and local times.
From page 424...
... I-Circuit constitutes a major advance for solar and space physics science by addressing the causal relationships between fundamental physical processes that are distributed in space, in scale, and in parameter space. A new level of understanding of geospace will be achieved through the simultaneous global measurements of interconnected state variables.
From page 425...
... With this configuration, as depicted in Figure D-21, I-Circuit delivers the necessary global simultaneous observations covering all latitudes and local times that will enable significant advances toward PSG 3.4. The I-Circuit measurement objectives address the preceding science objectives as follows: • Science Objective 1 requires the determination of the relationship between large-scale and mesoscale currents and relates solar wind and geospace conditions to measurements of all three components of the current connecting the magnetosphere–ionosphere system.
From page 426...
... CC BY 4.0. Mission Configuration, Deployment, and Operations I-Circuit addresses the measurement objectives using global, common volume sensing by heterogeneous platforms in both high (HEO)
From page 427...
... The I-Circuit mission directly addresses • PSG 1.2: Transport processes across the high-altitude/latitude ITM boundary, both through the coupling among the solar wind, magnetosphere, and ITM through field-aligned electric currents and energetic particle precipitation and the horizontal coupling through ionospheric closure currents and vertical coupling through the ionospheric conductivity volume. • PSG 2.2: Cross-scale coupling at high latitudes is addressed with observations spanning local auroral arc scales, conductivity pattern scales, and global scales.
From page 428...
... The science objectives of LAITIR are to 1. Determine how ionospheric Joule heating is influenced by and influences neutral wind, composition, temperature, and density.
From page 429...
... Gaining a better understanding of Joule heating and the densities of the lower thermosphere would enable significant advances in space weather forecasting, especially during geomagnetic events. Expected Outcomes The notional LAITIR mission described here will produce several compelling science results.
From page 430...
... Ionosphere/thermosphere/mesosphere physical processes parameters between 350 and 1,500 km Determine the horizontal neutral mass flow of dominant of horizontal at 2 km spatial resolution: species. neutral mass flow • Electron density that regulate ion Determine the altitude variation in the plasma upward flow • Electron temperature upwelling and distribution and the vertical ion mass outflows.
From page 431...
... D.6.3 Ground-Based Facilities As with spaceflight mission concepts, the panel identified five specific ground-based facility implementation concepts that provide critical measurements of key state parameters needed to understand the ITM as a holistic system. Because the diversity and complexity in ITM science precludes the notion of any single all-encompassing concept or a strictly descending priority order of implementation, the ground-based facility concepts in this section are not presented in a ranked order.
From page 432...
... . Meanwhile, sensor heterogeneity at each DASHI network node is designed to provide height-resolved observations of coupled ITM state parameters, as these data are critical for understanding how forcing from the lower atmosphere, especially associated with gravity waves, is transmitted throughout the ITM (PSGs 1.3, 1.4, and 2.1)
From page 433...
... : ground-based cameras imaging at 630 nm and 558 nm; four red dots: narrow-field Fabry-Pérot interferometer sampling; lilac circles: Scanning Doppler Imager field of view; blue wedges: SuperDARN field of view; orange/green shading: multistatic meteor radar field of view; dashed contours: magnetic latitude. SOURCE: Conde et al.
From page 434...
... A notional list of state parameters and associated sensors, designed to enable simultaneous characterization of winds, temperatures, and electrodynamics in a region spanning altitudes ~80 km to ~400 km, is given in Table D-6. Note that this representative configuration includes a meteor radar and GNSS receiver at each DASHI station; a network of meteor radars, which is required for science closure on the BRAVO mission, is described as a standalone concept in the section "Meteor Radar Network" below, and an extension of the GNSS network beyond the American longitude sector is described as a standalone concept in the section "Extended GNSS Network" below.
From page 435...
... Selection, implementation, and operation of the DASHI science products is best evaluated jointly with current and future community needs to enhance the science return of space-based missions and other ground-based facility products. Meteor Radar Network Implementation strategy: Develop and provide sustained support for a meteor radar network with global coverage, including a continent-scale region with dense multistatic capability, to simultaneously measure parameters of gravity waves and meso- to global-scale atmospheric waves.
From page 436...
... Providing vital constraints on lower atmosphere drivers would support more accurate space weather forecasting by enabling near-real-time data assimilation, never before available in this configuration. An OSSE will need to be conducted to determine the optimal distribution, density, and design parameters for the sites.
From page 437...
... IS radars provide direct, height-resolved, measurements of electron density, ion and electron temperature, ion composition, and line-of-sight plasma drift velocities with quantified parameter uncertainties, and the use of multiple radar beams enables vector velocity measurements on regional scales. In conjunction with ancillary models, the IS technique also supports inferences regarding ionospheric conductance, Joule heating, electric current systems, electron energy distributions, and neutral winds.
From page 438...
... in addition to active radar sensing. While a new IS radar facility with these state-of-the-art capabilities would greatly advance ITM science at any latitude, its deployment in the subauroral region would make it particularly well suited for addressing numerous science objectives that are strategic priorities for the coming decade.
From page 439...
... While alternative techniques for sensing TEC in geographical gap areas exist -- for example, from VLF measurements of lightning -- these would require regular access to data derived from the VHF transient-sensing systems on board the Department of Defense GPS satellites. The proposed dedicated ITM science GNSS network, implemented as at least an NSF MSRI-2 class initiative, would provide surface-based TEC and scintillation measurements in large data gap regions.
From page 440...
... The LiDAR facility concept builds on the 2013 decadal survey AIMI panel suggestion, "Create and operate a LiDAR facility capable of measuring gravity waves, tides, wave–wave and wave–mean flow interactions, and wave dissipation and vertical coupling processes from the stratosphere to 200 km." In the context of the new PSGs, a modernized large LiDAR facility would address PSGs 1.3 and 1.4, which examine the ITM response to external forcing and the transition in governing physics within the mesosphere and lower thermosphere. Additionally, it would address PSGs 2.1 and 3.2, which investigate cross-scale coupling of waves and the origin of short-term (day-to-day)
From page 441...
... For example, this type of modern LiDAR facility data would provide a comprehensive ensemble of neutral ITM state parameters that are vital for validating physics-based models and informing their continued development, and characterization of the small-scale wave spectrum will help to define lower atmosphere drivers useful for space weather forecasting. In terms of notional missions, both BRAVO and LAITIR would benefit from conjunction measurements for additional science and calibration/validation studies.
From page 442...
... Implementation strategy: Prioritize coordinated deployment and operation of heterogeneous sensors and distributed ground- and space-based platforms to support ITM system science investigations. Implementation strategy: Prioritize the deployment of ground- and space-based sensors that extend the existing temporal baseline of ITM state parameter measurements, enabling detection of persistent changes in the geospace environment.
From page 443...
... Implementation strategy: Deploy a high-resolution solar irradiance monitor to measure the spectrum of solar soft X-ray fluxes that are a primary source of photoelectrons and ions in the lower thermosphere. Solar irradiance measurements are critical for continual improvement in the understanding of Earth's upper atmosphere, space weather, and fundamental solar physics.
From page 444...
... These include programmatic support for ITM strategic system science needs, efficient application of the new capabilities afforded by data science, key instrument development on strategically chosen timelines, advancement in access to space along multiple capability axes, laboratory investigations of key ITM parameters and pathways, and education and workforce development. This section details implementation strategies in these areas.
From page 445...
... As stated throughout this report, long-term observational continuity and open availability of ITM state variables form an important linchpin in a future ITM strategy for effective system science focused work as well as support of ITM forecasting and space weather research. Achieving both measurement continuity and opportunities for focused studies in these areas can be effectively done by extending the hosted payload concept beyond commercial and other types of satellite platforms through utilizing high-heritage instruments on a combination of balloons, suborbital sounding rockets, and CubeSats.
From page 446...
... The value of data science tools was recognized explicitly in the decadal midterm assessment report, Progress Toward Implementation of the 2013 Decadal Survey for Solar and Space Physics: A Midterm Assessment (NASEM 2020) that noted NASA and NSF should maximize the scientific return from large and complex data sets by supporting (1)
From page 447...
... CubeSat missions offer many benefits for ITM science: unique science returns from lower budgets, multipoint measurements from constellations in place of a single large satellite program, a platform for technology demonstration, and training for next-generation engineering and scientific workforce. The NSF CubeSat program has demonstrated great success over the past decade in terms of technology demonstration, training, and public engagement and fills an important role for the health of the solar and space physics community.
From page 448...
... Implementation strategy: Continue support for novel ground- and space-based instrument development to enable measurements of key ITM state parameters. One example of a key measurement gap, which was identified explicitly in the 2013 decadal survey, is neutral wind measurements over the altitude range from 90 km to 300 km, where the transition from a collision-dominated to a magnetized atmosphere occurs.
From page 449...
... The ongoing reduction in in situ sensor mass, power, and size in the past decade motivates investment in new approaches for multiple ITM sensor fabrication. Such efforts, which include the careful assessment and implementation of appropriate interfaces, create an important step toward improving the spatial coverage and resolution of key ITM state parameter measurements that are a vital component of comprehensive ITM system science investigations.
From page 450...
... Properly preparing ITM researchers for long and successful careers requires reducing the siloing of their training to develop a generation of scientists with the skills to nimbly cross disciplines, niches, and boundaries to effectively meet exciting ITM science challenges. Continued educational programs, inspiring students with ITM science, will help cohere and maintain continuity in ITM research spanning disparate fields.
From page 451...
... -- A Mission Concept." Community input paper submitted to the Decadal Survey for Solar and Space Physics (Helio physics)
From page 452...
... 2022. "Circuit: A Mission to Unlock the Secrets of Earth's Global Current System." Community input paper submitted to the Decadal Survey for Solar and Space Physics (Heliophysics)


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