Geopace Climate Present and Future

Geospace Climate Present and Future A White Paper for the Decadal Survey of Solar and Space Physics

John T. Emmert U.S. Naval Research Laboratory, Washington, DC [email protected] 202-767-0467

Contributors and Signatories Hugh G. Lewis University of Southampton, UK David E. Siskind Naval Research Laboratory Lucilla Alfonsi Istituto Nazionale di Geofisica e Vulcanologia, Italy Douglas P. Drob Naval Research Laboratory Ana G. Elias Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Argentina John M. Holt Massachusetts Institute of Technology Jan Laštovička Czech Academy of Sciences Tao Li University of Science and Technology of China Richard J. Niciejewski University of Michigan Susan M. Nossal University of Wisconsin-Madison Craig A. Tepley Arecibo Observatory

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Figure 1. Qualitative summary of long-term changes that have been detected in ionospheric electron density (left) and atmospheric temperature (right). The directions of the observed changes, indicated by the arrows, are consistent with those predicted to occur as a result of increased CO2 levels. 1. The Importance of Geospace Climate It has long been known that Earth’s thermosphere, ionosphere, and mesosphere respond strongly to variations in the Sun’s photon, particle, and magnetic field output. More recently, it has become increasingly apparent that Earth’s upper atmosphere is also intricately coupled to its underlying layers, all the way down to the Earth’s surface. These interactions occur via a number of mechanisms, including radiative transport, chemical transport, propagation and selective filtering of waves of all types, and electrical fields and currents. It is now clear that a meaningful understanding of upper atmospheric behavior – including both climate and weather – cannot be attained except by considering the entire atmosphere as a whole system that responds not only to direct solar influences, but also to internal variations and changes of both natural and anthropogenic origin. From a societal perspective, the climate of the upper atmosphere (here defined as the mesosphere, thermosphere, ionosphere, exosphere, and geocorona) is important for several applications, including satellite drag and radio signal propagation. Satellite drag is relevant to orbit and reentry prediction and to long-term mitigation of orbital debris. Societal use of the radio wave spectrum is fundamentally affected by the ionosphere, which alters the paths and properties of radio waves of all frequencies, including GPS signals. The same gases that have been implicated in lower atmospheric climate change also play important roles in the energy balance of the upper atmosphere. CO2 is the primary cooling agent of the upper mesosphere and thermosphere. Anthropogenic increases in CO2 propagate into the upper atmosphere, and there are strong theoretical grounds and experimental evidence1,2 that these increases are causing the upper atmosphere to cool and contract. Changes in CH4, H2O, and 3 O3 may also be affecting upper atmospheric energy balance and structure . Anthropogenic changes in CO2, CH4, and O3 originate primarily from ground-based emissions (of CFCs in the case of O3 changes), but upper atmospheric H2O concentrations may be influenced by direct injection of rocket exhaust4 as well as by the background climate change5. NO is another major cooling agent; it is primarily produced by the interaction of the magnetosphere with the lower thermosphere and is transported down into the lower mesosphere and stratosphere, where it 6 catalytically destroys O3 . Although NO is produced naturally, theoretical simulations suggest 7 that it will modulate the climate change induced by CO2 increases . Some of the changes predicted to occur as a result of greenhouse gas increases have been detected in historical upper atmospheric data, and a coherent picture is beginning to emerge2,8 (Figure 1). There are also natural sources of geospace climate change. Long-term variations in the Sun’s photon, particle, and magnetic field output will directly affect the structure and climatology of the upper atmosphere. Secular changes in the geomagnetic field will futher alter the distribution of plasma and the location and intensity of ionospheric currents9,10. Because the state of the upper atmosphere responds strongly to solar and heliospheric drivers, both anthropogenic and natural climate changes of terrestrial origin will, to varying degrees, depend on the phase of the solar cycle. Conversely, any terrestrial climate change is expected to alter how the upper atmosphere responds to solar variations. The anthropogenic and natural evolution of geospace climate will have a profound effect on the low-Earth orbit (LEO) debris environment (Figure 2), because atmospheric drag is currently the only effective mechanism by which debris are removed from orbit. Of the 16,000 objects in the active satellite catalog (which only includes objects larger than 10 cm), 12,000 are debris or expended rocket bodies. The greatest density of debris is found between 500 and 1100 km altitude, where removal by satellite drag can take from 25 years to centuries11. Recent studies

2 have found that even without any new launches, the amount of debris in LEO will increase via collisional fragmentation faster than it is removed from orbit by atmospheric drag12. Active debris removal (ADR) strategies are therefore being explored, but little attention has yet been paid to how geospace climate change may affect these strategies. An initial study13 suggests that when long-term contraction of the thermosphere is taken into account, the effectiveness of ADR is dramatically reduced, thereby increasing the costs of achieving a Figure 2. Snapshot of debris > 10 cm in low-Earth orbit on 1 stable debris population within the 41 May 2001. From Lewis et al., 2005 . next hundred years (Figure 3). In addition to long-term contraction, anomalous reductions in thermospheric density, such as occurred unexpectedly during the recent solar minimum14, may also adversely affect efforts to control debris growth.

16000 MITIGATION No Contraction 15000 MITIGATION Contraction ADR No Contraction 14000 ADR Contraction ADR10 Contraction 13000

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9000 Effective number of objects (LEO,10> cm) 8000 2010 2020 2030 2040 2050 2060 2070 2080 Year Figure 3. Simulation of orbital debris evolution under different scenarios of mitigation measures and upper atmospheric climate change. The red lines show the predicted number of objects assuming no climate change, and the blue lines apply a scenario of upper atmospheric contraction. The solid lines denote passive debris mitigation measures only, and the dashed lines denote active removal of 5 large objects per year. The black dashed line shows the predicted debris evolution when 10 objects per year are actively de-orbited, under the assumption of upper atmospheric contraction.

3 2. Advancing Geospace Climate Science The climate questions facing the space science community are quite analogous to those that Earth scientists have been intensively investigating for the past few decades:  What is the typical systematic response of the upper atmosphere (which includes the ionosphere) to variations in solar and lower atmospheric drivers?  How is the climate of the upper atmosphere evolving in response to anthropogenic changes in the lower atmosphere, and to long-term changes in natural drivers?  How is upper atmospheric climate likely to change over the next 100 years?  What are the implications of predicted changes for orbital debris management and the utilization of the radio wave spectrum? From a measurement perspective, investigation of geospace climate is likely to be more challenging than the detection and attribution of terrestrial climate, because of the generally greater difficulty of probing the vastly more extensive space environment and the lack of a long historical measurement record for most parameters. However, we believe that pivotal advances in the field can be affordably attained by adopting the following strategies:

A) Continuously monitor key upper atmospheric climate parameters. The dominant rhythm of geospace climate is the 11-year solar cycle. Historically, space science has focused on the dramatic short-term perturbations forced by solar and geomagnetic activity, which are most prevalent during and immediately after the period of the sunspot maximum. However, it has long been recognized that solar maxima differ from one cycle to the next, and the 2008 solar minimum has illustrated that even the ‘ground state’ of the solar- terrestrial system is not reliably constant14,15. Furthermore, theoretical and empirical evidence16,17,18,19 suggests that climatic changes of terrestrial origin become most apparent during solar minimum. Consequently, because the system cannot be regarded as stationary even over century time scales, it is essential that it be continuously measured. Unfortunately, space-based measurements to date have generally been short-lived (less than a solar cycle), sporadic, and sparse. Unlike the Earth science community, which two decades ago implemented dedicated monitoring of 24 crucial observables, geospace remains poorly defined observationally (and theoretically, lacking sufficient model validation). Large temporal gaps between missions and the scarcity of data from previous solar cycles impose serious obstacles to the understanding and attribution of geospace climate. For example, the lack of lower thermospheric temperature and composition data during previous solar minima is a severe impediment to interpretation of the anomalous behavior of the entire thermosphere and ionosphere during the 2008 minimum. Ground-based instruments have historically provided much better temporal continuity than space-based measurements, but they are spatially very sparse for most types of instruments. To advance space climate science, it is essential to improve the temporal coverage of space- based measurements and the spatial coverage of ground-based measurements. A mixture of space-based and ground-based assets is needed: Space-based measurements provide the coverage

4 necessary for detecting global shifts in climate, while ground-based measurements can provide a stable baseline and a link to past measurements. For all the required measurements, it is vital that continuity be maintained from one solar cycle to the next. While a comprehensive, high- resolution specification of each and every upper atmospheric property is not required for geospace climate science, knowledge of ALL of the following key state parameters is essential:  Neutral temperature is perhaps the most important upper atmospheric climate parameter, since the vertical temperature profile and horizontal gradients fundamentally affect neutral and ion composition and dynamics.  Composition of major species O, O2, N2, He, H and their corresponding ions. O is a key minor species in the mesosphere and lower thermosphere, but is the dominant species throughout most of thermosphere, and O+ is the dominant species of the ionosphere20,21. The proportion of O near the base of the thermosphere, where O is strongly influenced by chemistry and dynamics, largely determines the composition of the upper thermosphere. He and H are minor species below about 400 km, but are the dominant species between 600 and 1000 km, where the greatest density of orbital debris resides. He and H exhibit large-scale spatial variations22, and anthropogenic increases in methane emissions may be affecting the H population in the geocorona1,23.  Concentration of minor species CO2, NO, O3, H2O, and CH4. CO2, NO, and O3 are (along 24 with O) the dominant agents of radiative balance in the upper atmosphere . CO2 is the primary cooling agent of the thermosphere. NO is also a major cooling agent, especially at high latitudes and during solar maximum; model simulations of CO2 increases suggest 7 that there may be significant feedbacks from NO . CH4 and H2O are also radiatively 25 1 active species, and CH4 is a source of H2O and H . Observed changes in polar mesospheric ice cloud characteristics may be caused in part by H2O trends, but hydrostatic effects associated with changes in the middle atmospheric temperature profile 5 near the stratopause also play a major role . At these altitudes, O3 is the key constituent controlling the radiative budget.  Neutral wind fundamentally affects, in a nonlocal manner, the electrodynamics and plasma distribution in the ionosphere26, and also drives chemical transport6,27. Currently, neutral winds are so poorly constrained that they are essentially free parameters in theoretical ionosphere models.  Wave activity on all scales can affect the vertical and horizontal distribution of both neutral and charged particles. Small-scale waves such as gravity waves play an important role in the energy balance of the upper atmosphere28.  Electric fields drive ionospheric plasma dynamics and structure. They are generated by neutral winds, but are also imposed by the magnetosphere29, and therefore must be measured independently of winds.  Electron density profile. Knowledge of how the electron density profile is evolving is necessary for predicting long-term impacts of space climate change on communication and navigation systems. Ground-based instruments are an essential component for this parameter, because of their good geographic coverage, long measurement histories, and ability to measure all or most of the vertical profile.  Total mass density. This parameter can be derived from the major species, but it is of major importance in its own right because of its direct relationship to satellite drag, which can be measured by accelerometers and derived from changes in satellite orbits. There is

5 an operational mandate to measure orbits of all objects larger than 10 cm, and it is expected that orbit data will be available indefinitely, so that funding for data processing is sufficient to produce orbit-derived mass density data. Accelerometer data provide much higher spatial and temporal resolution, and are therefore a valuable complement to orbit data.  Solar UV and EUV spectral irradiance. Ultraviolet irradiance is the dominant energy source of the upper atmosphere, so accurate long-term knowledge of this driver is crucial. This measurement must be made from space and is prone to instrumental drift, so ample overlap between instruments and a vigorous calibration program are essential requirements. The Mg II core-to-wing ratio has proven to be a reliable indicator of UV irradiance and of the upper atmospheric response30,31; this proxy overcomes some of the instrumental difficulties of UV measurements and is available beginning in 197832. Continued monitoring and improvement of Mg II would provide a highly valuable asset for upper atmospheric climate studies. Solar UV and EUV spectral irradiance is currently being monitored by TIMED, SORCE, and SDO. Otherwise, there are only two NASA satellites, C/NOFS and TIMED, dedicated to measuring critical state parameters of the upper atmosphere, and neither mission is expected to extend beyond 2014. C/NOFS was launched in 2008 and measures certain key ionospheric parameters via in-situ techniques at altitudes between 400 and 800 km. Since 2002, TIMED has remotely measured temperature, winds, and a few of the key minor species in the mesosphere and lower thermosphere (MLT); in the middle thermosphere, TIMED measured temperature and major species composition profiles until 2007. Thus, of the key parameters listed above, only temperature, winds, large-scale wave activity, and some minor species are currently being monitored in the MLT, and only sparsely. In the middle and upper thermosphere, the column O/N2 ratio is the only quantity currently being provided by NASA missions. There are no upper atmospheric missions planned for the next decade. The Ionosphere Thermosphere Storm Probes mission, which received a high priority in the previous Decadal Survey (as part of the Geospace Network program) 33, following an extended and thorough science definition team study, is absent in NASA’s plans34,35,36, and presumably cancelled.

B) Support data analysis and modeling of upper atmospheric climate and upper atmospheric climate change. Climate science is data-intensive by nature, since it requires large amounts of data of diverse types and varying quality to accurately determine the mean state or to validate theoretical predictions of the mean state, and rigorous statistical analysis is necessary to determine the evolution of the system. Funding opportunities should therefore place special emphasis on data analysis activities such as:  Acquisition, preservation, digitization, and exploitation of existing data archives that can enhance the temporal and spatial extent of the upper atmospheric climate record. For example, unprocessed raw ionograms exist from several ionosonde stations, and standardized reprocessing of previously scaled ionograms can produce a uniform record of ionospheric parameters.  Improvement of existing data inversion methods. Climate science requires continual assessment of biases among different data sets and within individual data sets. In many cases, such biases can be resolved by changes in data processing techniques.

6  Core analysis of data for climate patterns and long-term change, including rigorous analysis of variances and uncertainties.  Development of empirical and semi-empirical climate models. Data-based models are indispensible for tying together different data sets and different types of measurements in a physically consistent way, for filtering data to isolate specific climatic processes, and as a benchmark for assessing the quality of new data sets and for validating physics-based models. As in lower atmospheric climate science, theoretical modeling should play a substantial complementary role. Important theoretical modeling activities include:  Validation of model output against available data.  Interpretation and attribution of climate patterns and trends.  Long-term forecasting of upper atmospheric climate under different scenarios.  Assessment of the impact of future changes on societal activities, and of the efficacy of possible mitigation strategies. Funding from NASA for Heliophysics Research and Analysis projects has been low in relation to the size of the U.S. Heliophysics community and the number of worthy proposals submitted. Since 2003, the Heliophysics R&A proposal selection rate has been about 29%, which is slightly lower than the other Science Mission Directorate divisions (each of which funds over twice the number of R&A projects as Heliophysics) 37. In SMD as a whole in 2007 and 2008, only 45% of proposals rated Very Good were selected, and only 65% of proposals rated Very Good or higher were selected37, suggesting that the community could be performing much more quality science than is currently funded. Significant additional investment in Heliophysics R&A is badly needed in order to make meaningful progress in geospace climate science.

C) Fill in critical gaps in the climate record. In situ measurement of thermospheric neutral temperature and composition has not occurred since 1983. Mass spectrometers provided the biggest share of information about the upper atmosphere during the Explorer era, and today these instruments still have enormous capability to produce new discoveries and resolve long-standing problems. Unlike previous-generation instruments, modern mass spectrometers can differentiate between atomic and molecular oxygen, so it is now possible to fully probe the poorly understood composition between about 80 km, where O-O2 chemistry and turbulent mixing are important, and 150 km, where diffusive separation is well established and O is free to become the dominate species of the thermosphere. Mass spectrometers can also provide needed validation of composition measured by the relatively new technique of UV remote sensing, as well as nightside composition and the density of species such as He and H, which cannot be inferred from UV emissions. Finally, mass spectrometers flown alongside accelerometers would provide badly needed closure on the problem of satellite drag coefficients and their dependence on object shape and ambient composition, which are the largest sources of uncertainty of mass density inferred from satellite drag. CO2 is the key species implicated in upper atmospheric climate change, but very few 38 measurements of the vertical CO2 profile have been made . CO2 is assumed to be well mixed up to about 85 km, but this assumption has never been rigorously tested. New measurements of the complete CO2 profile are needed to verify how quickly anthropogenic increases propagate into the upper atmosphere and to estimate how the influence of CO2 on the upper atmosphere is evolving.

7 Other key upper atmospheric constituents, such as NO, have been measured for over 11 years, but not for more than one solar minimum: The UARS/HALOE NO time series begins in late 1991 and encompasses the 1996 solar minimum, but ends before the most recent solar minimum. Although the radiative effects of NO are considered to be relatively unimportant at solar minimum39, it is a critical measure of the response of the lower thermosphere to both solar flux changes and temperature changes. Thermospheric neutral winds play a fundamental role in the structure and dynamics of the ionosphere, but their behavior is still poorly understood, especially on the nightside between 110 and 200 km altitude, where only a handful of in situ measurements exist. In the upper thermosphere, there is currently very little space-based monitoring of winds (TIMED/TIDI began measuring Doppler shifts of 630.0 nm airglow in late 2008), and only a very small number of ground-based datasets have sufficient temporal extent to attempt a determination of long-term changes. Neutral winds must be taken into account in the interpretation of ionospheric climate, so it is vital that a reliable long-term measurement record be established.

3. Benefits and Relevance to Science and Society The topic of upper atmospheric climate science and the strategies proposed above are highly relevant to the national and international space science enterprise, as follows. Climate change in the geospace system was identified as a major theme of the NSF/CEDAR strategic plan for the next decade. Geospace climate is also directly relevant to Challenge 3 of the previous Decadal Survey, “Understanding the space environments of Earth and other solar system bodies and their response to external and internal influences”, and this challenge was partially addressed in NASA’s 2009 Heliophysics Roadmap with priority science target #7, “Climate Impacts of Space Radiation”. In addition to being a priority of U.S. research plans, space climate is also a cornerstone of the ongoing international SCOSTEP program “Climate and Weather of the Sun-Earth System”, in which major themes are solar influences on terrestrial and geospace climate and the geospace response to altered terrestrial climate. Geospace climate science also contributes to the applications of orbit prediction, debris mitigation, and radio wave utilization. Understanding the climate of the upper atmosphere is prerequisite to fulfilling at least two directives of U.S. National Space Policy. The first is to preserve the space environment, in part by pursuing “research and development of technologies and techniques…to mitigate and remove on-orbit debris, reduce hazards, and increase understanding of the current and future debris environment” and leading “the continued development and adoption of international and industry standards to minimize debris.” 40 The second relevant space policy directive is to “take necessary measures to sustain the radiofrequency environment in which critical U.S. space systems operate.” 40 The strategies outlined in the previous section complement other observational systems and programs. Measurement requirements are very similar to those needed to advance space weather science; for climate science, the requirements are more basic and less stringent in terms of sampling density and short-term precision, but include a need for long-term monitoring. Most of the key climate parameters can be measured with well-developed, cost-efficient instrument technology. Geospace climate science contributes primarily to the Atmosphere-Ionosophere- Magnetosphere Interactions theme of the Decadal Survey, but there is also significant overlap with the Solar and Heliospheric Physics theme, because of the need for accurate specification of solar UV irradiance on time scales of days to decades.

8 A strong commitment to geospace climate research would contribute to two of the most important scientific questions facing the solar and space physics community. First, it would elucidate the intricate systematic interplay between solar and lower atmospheric influences on the near-Earth space environment. Second, it would address the natural upward extension of one of the most compelling science questions of our time: How are humans altering the climate of our planet? The U.S. and international aeronomy communities are well-positioned to make significant advances in geospace climate science. There is already a vibrant, cooperative enterprise of study that only lacks a stable supply of data and support for analysis to bring our level of understanding up to par with that of terrestrial climate. There is also a strong culture of student development in the aeronomy community, across the three major skill areas needed for geospace climate science: instrument development and construction, data analysis, and theoretical modeling. The future is therefore highly promising for the specification and understanding of geospace climate, which is the foundation that underlies and informs all planetary space environment research.

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