Forecasting GOES 15 >2 MeV electron fluxes from solar wind data and geomagnetic indices

The flux of > 2 MeV electrons at geosynchronous orbit is used by space weather forecasters as a key indicator of enhanced risk of damage to spacecraft in low, medium or geosynchronous Earth orbits. We present a methodology that uses the amount of time a single input dataset (solar wind data or geomagnetic indices) exceeds a given threshold to produce deterministic and probabilistic forecasts of the > 2 MeV flux at GEO exceeding 1000 or 10000 cm‐2 s‐1 sr‐1 within up to 10 days. By comparing our forecasts with measured fluxes from GOES 15 between 2014 and 2016, we determine the optimum forecast thresholds for deterministic and probabilistic forecasts by maximising the ROC and Brier Skill Scores respectively. The training dataset gives peak ROC scores of 0.71 to 0.87 and peak Brier Skill Scores of ‐0.03 to 0.32. Forecasts from AL give the highest skill scores for forecasts of up to 6‐days. AL, solar wind pressure or SYM‐H give the highest skill scores over 7‐10 days. Hit rates range over 56‐89% with false alarm rates of 11‐53%. Applied to 2012, 2013 and 2017, our best forecasts have hit rates of 56‐83% and false alarm rates of 10‐20%. Further tuning of the forecasts may improve these. Our hit rates are comparable to those from operational fluence forecasts, that incorporate fluence measurements, but our false alarm rates are higher. This proof‐of‐concept shows that the geosynchronous electron flux can be forecast with a degree of success without incorporating a persistence element into the forecasts

Cross-L* coherence of the outer radiation belt during 2 storms and the role of the plasmapause

The high energy electron population in Earth’s outer radiation belt is extremely variable, changing by multiple orders of magnitude on timescales that vary from under an hour to several weeks. These changes are typically linked to geomagnetic activity such as storms and substorms. In this study, we seek to understand how coherent changes in the radiation belt are across all radial distances, in order to provide a spatial insight into apparent global variations. We do this by calculating the correlation between fluxes on different L* measured by the PET instrument aboard the SAMPEX spacecraft for times associated with 15 large storms. Our results show that during these times, variations in the 0.63 MeV electron flux are coherent outside the minimum plasmapause location and also coherent inside the minimum plasmapause location, when flux is present. However, variations in the electron fluxes inside the plasmapause show little correlation with those outside the plasmapause. During storm recovery and possibly main phases, flux variations are coherent across all L* regardless of plasmapause location, due to a rapid decrease, followed by an increase in radiation belt fluxes across all L*

Anomalous resistivity in non-Maxwellian plasmas

Vlasov simulations of the current-driven ion-acoustic instability produced in Maxwellian and non-Maxwellian (Lorentzian, kappa = 2) electron-ion plasma with number density 7 x 10(6) cm(-3), reduced mass ratio m(i)/m(e) = 25, and electron to ion temperature ratio T-e/T-i = 1 are presented and compared. A concise stability analysis of current-driven ion-acoustic waves in Maxwellian and non-Maxwellian plasmas modeled by generalized Lorentzian distribution function with index 2 less than or equal to kappa less than or equal to 7 and electron to ion temperature ratio 1 less than or equal to T-e/T-i less than or equal to 100 is also presented. The ion-acoustic instability is excited in low temperature ratio Lorentzian (kappa = 2) plasma for lower absolute electron drift velocity (up to half the critical electron drift velocity of a Maxwellian). The anomalous resistivity resulting from ion acoustic waves in a Lorentzian plasma is a strong function of the electron drift velocity and in the work presented here varies by a factor of similar to100 for a 1.5 increase in the electron drift velocity. Furthermore, ion-acoustic anomalous resistivity is excited for electron drift velocities that would be stable for Maxwellian plasmas. The magnitude of resistivity which can be generated by unstable ion-acoustic waves may be important for magnetic reconnection at the magnetopause

On the variability of EMIC waves and the consequences for the relativistic electron radiation belt population

The interactions between electromagnetic ion cyclotron (EMIC) waves and relativistic electrons are influential in diffusing radiation belt electrons into the loss code from which the electrons are lost into the atmosphere. These wave-particle interactions between EMIC waves and electrons with energies of a few MeV or more, depend strongly on wave spectra and plasma properties. Here we study the variability in wave spectra and plasma properties as a function of L* found during Van Allen Probe EMIC observations. These results are used to calculate statistical bounce and drift average diffusion coefficients that include the variation in wave spectra and plasma density as a function of L* and activity by averaging observation specific diffusion coefficients. The diffusion coefficients are included into global radiation belt simulations and the effect of the EMIC waves is explored. The distribution in the plasma frequency to electron gyrofrequency ratio decreases to lower values as L* decreases. As a result, few EMIC waves are able to resonate with 2-3MeV electrons at L* ≤ 3.75 while electrons of the same energy at larger L* are diffused by EMIC waves in high density regions. In comparison, a sufficient number of EMIC waves are able to resonate with higher energy electrons, urn:x-wiley:21699380:media:jgra56873:jgra56873-math-0001MeV, at L* ≥ 3.25 to significantly effect the decay in electron flux. EMIC wave parametrisations of electron diffusion by EMIC waves are compared and solar wind dynamic pressure is found to give the best agreement with Van Allen Probe observations

ULF Wave Driven Radial Diffusion During Geomagnetic Storms: A Statistical Analysis of Van Allen Probes Observations

The impact of radial diffusion in storm time radiation belt dynamics is well-debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012–2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm-time radial diffusion coefficients are provided as a function of L* and storm phase. © 2021. The Authors

Drift orbit bifurcations and cross-field transport in the outer radiation belt: Global MHD and integrated test-particle simulations

Energetic particle fluxes in the outer magnetosphere present a significant challenge to modeling efforts as they can vary by orders of magnitude in response to solar wind driving conditions. In this study, we demonstrate the ability to propagate test particles through global magnetohydrodynamic (MHD) simulations to a high level of precision and use this to map the cross-field radial transport associated with relativistic electrons undergoing drift orbit bifurcations (DOBs). The simulations predict DOBs primarily occur within an Earth radius of the magnetopause loss cone and appear significantly different for southward and northward interplanetary magnetic field orientations. The changes to the second invariant are shown to manifest as a dropout in particle fluxes with pitch angles close to 90° and indicate DOBs are a cause of butterfly pitch angle distributions within the night-time sector. The convective electric field, not included in previous DOB studies, is found to have a significant effect on the resultant long-term transport, and losses to the magnetopause and atmosphere are identified as a potential method for incorporating DOBs within Fokker-Planck transport models