Solar-Induced Chlorophyll Fluorescence (SIF) is a powerful proxy for gross primary productivity (GPP) in Boreal ecosystems. However, SIF and GPP are fundamentally different quantities that describe distinct, but related, physiological processes. Recent work has highlighted non-linearities between SIF and GPP at finer spatial (leaf- to canopy- level) and temporal (half-hourly) scales. Therefore, questions have arisen about when, where, and why SIF is a good proxy for GPP and what the potential sources for divergence between the two are. The goal of this study is to answer two specific questions: 1) At what temporal scale is SIF a good proxy for GPP and 2) What are the predominant physical and ecophysiological drivers of nonlinearity between SIF and GPP in boreal ecosystems? We collected tower-based measurements of SIF (and other common vegetation indices) with PhotoSpec (a custom spectrometer system) and eddy-covariance GPP data at a 30-minute resolution at the Southern Old Black Spruce Site (SOBS) in Saskatchewan, CA. We applied a combination of statistical and machine learning approaches to disentangle the influence of structural/illumination effects and ecophysiological variations on the SIF signal. Our results show that at a high temporal resolution (half-hourly), SIF and GPP are predominantly dependent on photosynthetically active radiation (PAR). Therefore, the non-linear light response of GPP drives non-linearity between SIF and GPP. Additionally, canopy structure and illumination effects become important to the SIF signal at high temporal resolutions. At the seasonal timescale, SIF and GPP exhibit co-varying responses to PAR, even when accounting for changes in canopy structure. We attribute changes in the light responses of SIF and GPP to sustained photoprotection over winter which co-varies with changes in temperature. Finally, we show that the relationship between SIF and GPP has a seasonal dependence caused by small differences between the light use efficiencies of fluorescence and photosynthesis. Accounting for this seasonally variable relationship will improve the use of SIF as a proxy for GPP.

The substorm current wedge (SCW) is believed to be driven by pressure gradients and vortices associated with fast flows. Therefore, it is expected that relevant observations are organized by the SCW’s central meridian, which cannot be determined using in-situ observations. This study takes advantage of the SCW inversion technique, which provides essential information about an SCW (e.g., location and strengths of field-aligned currents (FACs) and investigates the generation mechanisms of the SCW. First, we have found good temporal and spatial correlations between earthward flows and substorm onsets identified using the midlatitude positive bay (MPB) index. Over half of the flows are observed within 10 minutes of substorm onsets. Most flows (85%) were located inside the SCW between its upward and downward FACs. Second, superposed epoch analysis (SPEA) shows that the onset-associated flow velocity has a flow-scale (3-min) peak, while the equatorial thermal pressure has a substorm-scale (>30 min) enhancement and a trend similar to the westward electrojet and FACs in the SCW. Third, the pressure gradient calculated using in-situ observations is well organized in the SCW frame and points toward the SCW’s central meridian. These facts suggest that the SCW is likely sustained by substorm-scale pressure gradient rather than flow-scale flow vortices. The nonalignment between the pressure gradient and flux tube volume could generate an SCW with a quadrupole FAC pattern, similar to that seen in global MHD and RCM-E simulations. Their magnetic effects on the ground and geosynchronous orbit resemble a classic one-loop SCW.

Quantification of energetic electron precipitation caused by wave-particle interactions is fundamentally important to understand the cycle of particle energization and loss of the radiation belts. One important way to determine how well the wave-particle interaction models predict losses through pitch-angle scattering into the atmospheric loss cone is the direct comparison between the ionization altitude profiles expected in the atmosphere due to the precipitating fluxes and the ionization profiles actually measured with incoherent scatter radars. This paper reports such a comparison using a forward propagation of loss-cone electron fluxes, calculated with the electron pitch angle diffusion model applied to Van Allen Probes measurements, coupled with the Boulder Electron Radiation to Ionization (BERI) model, which propagates the fluxes into the atmosphere. The density profiles measured with the Poker Flat Incoherent Scatter Radar operating in modes especially designed to optimize measurements in the D-region, show multiple instances of quantitative agreement with predicted density profiles from precipitation of electrons caused by wave-particle interactions in the inner magnetosphere. There are two several-minute long intervals of close prediction-observation approximation in the 65-93 km altitude range. These results indicate that the whistler wave-electron interactions models are realistic and produce precipitation fluxes of electrons with energies between 10 keV to >100 keV that are consistent with observations.

Forecasting relativistic electron fluxes at geostationary Earth orbit (GEO) has been a long term goal of the scientific community, and significant advances have been made in the past, but the relation to the interior of the radiation belts, that is, to lower $L-$shells is still not clear. In this work we have identified 60 relativistic electron enhancement events at GEO to study the radial response of outer belt fluxes and the correlation between the fluxes at GEO and those at lower $L-$shells. The enhancement events occurred between 1 October 2012 and 31 December 2017 and were identified using GOES 15 $>$2 MeV fluxes at GEO, which we have used to characterize the radial response of the radiation belt, by comparing to fluxes measured by the Van Allen probes ECT-REPT between $2.55.0$, and generally similar for $L>4.5$. Post enhancement maximum fluxes show a remarkable correlation for all $L > 4.0$ although the magnitude of the pre-existing fluxes on the outer belt plays a significant role and makes the ratio of pre-to-post enhancement fluxes less predictable in the region $4.0

We use measurements from NASA’s Van Allen Probes to calculate the decay time constants for electrons over a wide range of energies (30 keV - 4 MeV) and values ( = 1.3 - 6.0) in the Earth’s radiation belts. Using an automated routine to identify flux decay events, we construct a large database of lifetimes for near-equatorially-mirroring electrons over a 5-year interval. We find long lifetimes (~100 days) in the inner zone that are largely independent of energy, contrasted with shorter, energy-dependent lifetimes (~1-20 days) in the slot region and outer zone. We compare our lifetime calculations with prior empirical estimates and find good quantitative agreement. The comparisons suggest that some prior estimates may overestimate electron lifetimes between ≈ 2.5-4.5 due to instrumental effects and/or background contamination. Previously reported two-stage decays are explicitly demonstrated to be a consequence of using integral fluxes.