This work resolves a 30-year controversy regarding the use of impulsive spikes in nitrate ion measurements from ice cores as indicators of historical solar proton events, calling into question statistical and predictive space weather studies based on nitrate records from ice cores. This topic is of importance to both the heliospheric space science community, seeking to understand historical solar activity, as well as the glaciology community, seeking to understand solar and upper atmospheric contributions to paleoclimate records.  Space weather poses an enormous natural threat to our 21st century technological society, particularly the largest space weather events that the Sun can produce.  In the past, nitrate spikes in ice have been contentiously used to estimate properties of large pre-space-age space weather events.  The Sun-to-Ice project has conclusively shown that nitrate spikes in the ice core record are not good indicators of historial solar proton events.  

In addition to meeting the initial goals, the Sun-to-Ice project team also addressed new objectives revealed by our initial findings.  During the ensuing years of the project, the focus of our analysis shifted to the impact of energetic particles on the middle atmosphere. Previously in the project, we had used ice core analysis and numerical models to refute the utility of nitrates recorded in ice cores as a proxy for historic solar events. This allowed us to move forward with studies of what proxies should be sought in the geologic record, as opposed to using an imperfect proxy that have been studied historically mostly because they existed for other reasons (such as nitrates). The remaining major activities thus focused on studies using numerical models and analysis of existing data sets to set the input parameters to the upper atmosphere WACCM model. The WACCM model has been extended to include radionuclide production (to search for new proxies of interactions of solar protons) and also extended ton include the physics of other sources of energetic particle precipitation (including medium energy electrons from the radiation belts).

The objective of these parallel studies using WACCM are twofold: (1) To develop new proxies (such as radionuclides) as tracers of energetic particle interactions in the middle atmosphere that have been recorded robustly in the geologic record (whether in the living record, such as tree ring studies) or in ice cores. and (2) To study more generally the impacts of energetic particle precipitation into the middle atmosphere chemistry and its effect both locally and potentially climatologically.  The first new objective remains very much a work in progress, but shows great promise.  Intital results of the second objective are extermely positive and are summarized next.

A final significant result during the project is the estimation of the impact of medium energy (100's of keV) into the middle atmosphere from the radiation belts. This involved the development and use of a module to incorporate this new ionization and energy source into the WACCM model and then the comparison of middle atmosphere results both with and without this source. The source input parameters for this heretofore missing ionization source was derived partly from the NSF CubeSat mission called Focussed Investigation of Relativistic Electon Bursts: Intensity, Range, and Dynamics (FIREBIRD). FIREBIRD measures the energy spectrum of electrons precipitating into the upper atmosphere from the radiation belts while in low Earth orbit. We also appeal to NASA's Van Allen Probes mission to provide global measures of electron loss to the atmosphere at these energies using a quantity developed by the Radiation Belt Storm Probes - Energetic particle, Composition, and Thermal plasma (RBSP-ECT) suite called the Total Radiation Belt Electron Content (TRBEC). We find that the inclusion of this previously missing source of ionization may be responsible for explaining differences between modeled and measured properties in the middle atmosphere, including both chemical differences and ionization differences.

The Sun-to-Ice project also generated many significant outcomes in terms of training and professional development.  These activities ran the gamut from high school student engagement, to graduate student training, to postdoctoral mentoring, and to professional development for both early career and senior researchers.  The Sun-to-Ice research experience was richly interdisciplinary, providing all cohorts an opportunity to learn and train in an environment that represents the sort of convergent science required to solve the world's greatest challenges.

One critically important broader impact that Sun-to-Ice has caused is a deliberate and focused dialog between the space science and glaciochemistry communities. Without that dialog, the use of nitrate spikes would continue to erroneously inform engineers and policy makers about the impact and range of extreme solar events.   Our work essentially rewrites the textbook on the use of nitrate spikes and reopens the question of the range of extreme events that the Sun is capable of producing as well as the response in the geospace system.

Last Modified: 01/08/2019
Modified by: Harlan Spence