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3 Science Enabled by an Agile Rideshare Program Statement of task item 1: Kinds of solar and space physics science that would be enabled by an agile response to rideshare opportunities, at locations that would provide global perspectives, unique views, or continuous coverage, among others. Rideshare opportunities enable a vast variety of science topics, all of which cannot be listed or even envisaged currently. The ideal candidates are the subset of decadal survey science topics that would benefit from an agile program approach. In the discussion below, three categories of science are identified that can particularly benefit from an agile rideshare programânamely, space weather and space climatology, system or contextual science, and exploratory science. There is some overlap among these scientific regimes, but they nevertheless represent a useful grouping. For convenience, the solar and space physics system is divided in this document into three main subsystems: (1) the solar and heliospheric domains, (2) the magnetospheric domain, and (3) the ionospheric and upper atmospheric domains. A limited number of specific examples of science enabled by an agile rideshare program are provided in the following section. It is important to note that the orbit, host platform, and launch vehicle type will guide and constrain the range of science topics that can be addressed for a given rideshare opportunity. SPACE WEATHER AND SPACE CLIMATOLOGY Long-term temporally continuous observations (ideally obtained 24 hours a day, 7 days a week, with minimal or no temporal gaps) facilitate the collection of sufficient data to build a deeper understanding of space climatology as well as to monitor the Sun and the space environment for space weather purposes.1 Modern machine learning tools are available to make the most of such observations. In addition to enabling fundamental advances in the science underlying space weather, these measurements can be used by operational and research models as initial and boundary conditions, for data assimilation, and as a database for further development, testing, and validation of models. Examples of solar and heliospheric science relevant to space weather and space climatology include (but are not limited to) characterizing solar activity through remote sensing observations of solar irradiance, magnetism, and eruptions from multiple vantages, characterizing the state and variability of the solar wind through measurements of plasma and fields throughout the heliosphere, and characterizing the evolution of energetic particle populations with respect to their composition and energy distribution. Examples of magnetospheric science relevant to space weather and space climatology include (but are not limited to) understanding the dynamics of magnetospheric boundaries and the magnetotail with plasma, field and energetic particle measurements, analyzing dynamics and processes of the inner 1 See National Research Council (NRC), Chapter 7 in Solar and Space Physics: A Science for a Technological Society, The National Academies Press, Washington, D.C., 2013, https://doi.org/10.17226/13060. 7
magnetosphere ring current and radiation belts using both in situ and neutral-particle imaging, and characterizing the magnetospheric nonlinear response to solar-wind forcing from above and ionospheric- thermospheric forcing from below. Examples of ionospheric and upper atmospheric science relevant to space weather and space climatology include (but are not limited to) improved understanding of the underlying processes governing the formation of ionospheric density gradients, neutral density variations, the electrodynamic coupling within the ionosphere-thermosphere system in response to variable solar and geomagnetic energy inputs, and the feedback of this system on the magnetosphere. These topics include morphological studies of the ionosphere and thermosphere in both quiescent and geomagnetically disturbed times, along with climatological studies of energy inputs from the Sun and the magnetosphere, including solar EUV radiation, Poynting flux, particle precipitation, and the coupled nonlinear ionospheric-thermospheric response to these forcings. The committee refers to these types of observations as âcontinuous,â because continuity in time, at a cadence appropriate to a particular measurement, is a critical element for enabling a better understanding of the phenomena in question. An agile rideshare program would enable continuous observations by building up a large fleet or network over time. Finding: An agile rideshare program that exploits increased launch opportunities would augment existing temporally continuous observations, which are central to space weather monitoring and space climatology. SYSTEM OR CONTEXTUAL SCIENCE A spatially distributed network of measurements strengthens the understanding of the coupled heliophysical system and provides contextual information for other missions beyond their primary mission goals. This is referred to the Heliophysics Systems Observatory (HSO) in the decadal survey. Examples of solar and heliospheric science relevant to system or contextual science include (but are not limited to) determining the global photospheric magnetic field boundary on the heliosphere as viewed from all longitudes and latitudes, and resolving the mesoscale structures in coronal mass ejections and the solar wind between the Sun and 1 AU. Examples of magnetospheric science relevant to system or contextual science include (but are not limited to) measuring energetic particle acceleration and loss processes across the domains of ring current and radiation belts. Such measurements may elucidate wave-particle interactions, resolution of mesoscale structures in the magnetotail, and magnetospheric boundaries, including flow bursts and reconnection. Examples of ionospheric and upper atmospheric science relevant to system or contextual science include (but are not limited to) measurements that shed light on the ionosphere-thermosphere system, such as longitude-latitude specifications of geomagnetic energy inputs at high latitudes, including those associated with aurorae, Joule heating, and high-latitude convection. Upper atmospheric measurements of critical interest include neutral densities and winds, heating, and upwelling at a range of altitudes, with particular attention to the undersampled lower thermosphere. The committee refers to these types of observations as âdistributed,â because the use of multiple vantage points and spatial sampling are key to increasing understanding of these processes. An agile rideshare program would enable distributed observations by providing opportunities to send instrumentation to crucial points throughout geospace and the heliosphere. Finding: An agile rideshare program that enhances spatially distributed observations would extend the capabilities of the HSO and augment system science perspectives. 8
EXPLORATORY SCIENCE Unsurprisingly, some unsolved heliophysics science problems require measurements that do not currently exist. These include truly new observations using new types of instrumentation or technologies, as well as standard observations but from radically different viewpoints than those currently available. Another aspect of this category is the concept of a pathfinder instrument or mission where the scientific value is demonstrated as a precursor to a larger element in the NASA portfolio. For example, the STP has flown proof-of-concept missions to demonstrate new science and operational capabilities. Examples of exploratory solar and heliospheric science include (but are not limited to) analysis of explosive energy storage and release using observations obtained from precision formation-flying constellations at multiple wavelengths, and analysis of solar polar magnetic fields and flows using observations from high solar latitudes. Examples of exploratory magnetospheric science include (but are not limited to) magnetotail dynamics on scales between the kinetic and global, spanning a fraction of an Earth radius to several Earth radii to understand the dynamics and physics of flow channels, particle injection, and reconnection effects from the midtail to inner magnetosphere, using observations from expanded magnetospheric constellations. Examples of exploratory ionospheric and upper atmospheric science include (but are not limited to) measurements that lead to increased understanding of ion-neutral coupling, ion and neutral circulation on multiple scales, as well as establishing the mechanisms governing heavy ion heating and outflow at auroral latitudes in response to extreme solar geomagnetic driving. Measurements made by constellations of satellites would facilitate insight into these foundational ionospheric-upper atmospheric research topics. The committee refers to these types of observations as ânovel,â because they provide unprecedented information. An agile rideshare program would enable novel observations and concept development by increasing the number of launch opportunities. Finding: An agile rideshare program enables novel observations, which are necessary to fill gaps in the fundamental understanding of heliophysics. Taken together, these findings support the committeeâs conclusion, as follows: Conclusion: Science suited to an agile rideshare program can be categorized into three groups: space weather and space climatology enabled by temporally continuous observations; systems and contextual science enabled by spatially distributed observations; and exploratory science enabled by novel observations (e.g., technology demonstrators and new vantage points). 9