Showing papers by "Richard B. Horne published in 2022"

Ducted Chorus Waves Cause Sub‐Relativistic and Relativistic Electron Microbursts

Abstract: During magnetospheric storms, radiation belt electrons are produced and then removed by collisions with the lower atmosphere on varying timescales. An efficient loss process is microbursts, strong, transient precipitation of electrons over a wide energy range, from tens of keV to sub‐relativistic and relativistic energies (100s keV and above). However, the detailed generation mechanism of microbursts, especially over sub‐relativistic and relativistic energies, remains unknown. Here, we show that these energetic electron microbursts may be caused by ducted whistler‐mode lower‐band chorus waves. Using observations of equatorial chorus waves nearby low‐altitude precipitation as well as data‐driven simulations, we demonstrate that the observed microbursts are the result of resonant interaction of electrons with ducted chorus waves rather than nonducted ones. Revealing the physical mechanism behind the microbursts advances our understanding of radiation belt dynamics and its impact on the lower atmosphere and space weather.

Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

Abstract: Plasmaspheric hiss waves at the Earth are well known for causing losses of electrons from the radiation belts through wave particle interactions. At Saturn, however, we show that the different plasma density environment leads to acceleration of the electrons rather than loss. The ratio of plasma frequency to electron gyrofrequency frequently falls below one creating conditions for hiss to accelerate electrons. The location of hiss at high latitudes (>25°) coincides very well with this region of very low density. The interaction between electrons and hiss only occurs at these higher latitudes, therefore the acceleration is limited to mid to low pitch angles leading to butterfly pitch angle distributions. The hiss is typically an order of magnitude stronger than chorus at Saturn and the resulting acceleration is rapid, approaching steady state in one day at 0.4 MeV at L = 7 and the effect is stronger with increasing L‐shell.

Space Plasma Physics: A Review

Abstract: Owing to the ever-present solar wind, our vast solar system is full of plasmas. The turbulent solar wind, together with sporadic solar eruptions, introduces various space plasma processes and phenomena in the solar atmosphere all the way to the Earth's ionosphere and atmosphere and outward to interact with the interstellar media to form the heliopause and termination shock. Remarkable progress has been made in space plasma physics in the last 65 years, mainly due to sophisticated in-situ measurements of plasmas, plasma waves, neutral particles, energetic particles, and dust via space-borne satellite instrumentation. Additionally high technology ground-based instrumentation has led to new and greater knowledge of solar and auroral features. As a result, a new branch of space physics, i.e., space weather, has emerged since many of the space physics processes have a direct or indirect influence on humankind. After briefly reviewing the major space physics discoveries before rockets and satellites, we aim to review all our updated understanding on coronal holes, solar flares and coronal mass ejections, which are central to space weather events at Earth, solar wind, storms and substorms, magnetotail and substorms, emphasizing the role of the magnetotail in substorm dynamics, radiation belts/energetic magnetospheric particles, structures and space weather dynamics in the ionosphere, plasma waves, instabilities, and wave-particle interactions, long-period geomagnetic pulsations, auroras, geomagnetically induced currents (GICs), planetary magnetospheres and solar/stellar wind interactions with comets, moons and asteroids, interplanetary discontinuities, shocks and waves, interplanetary dust, space dusty plasmas and solar energetic particles and shocks, including the heliospheric termination shock. This paper is aimed to provide a panoramic view of space physics and space weather.

The Importance of Ion Composition for Radiation Belt Modeling

Abstract: The banded structure of electromagnetic ion cyclotron (EMIC) wave spectra and their resonant interactions with radiation belt electrons depend on the cold ion composition. However, there is a great deal of uncertainty in the composition in the inner magnetosphere due to difficulties in direct flux measurements. Here, we show that the hydrogen and helium band wave spectra are most consistent with a helium and oxygen composition of a few percent. Less than 10% of hydrogen band wave intensity is consistent with a high helium fraction of ∼20%. Similarly, only ∼20% helium band wave intensity is consistent with an oxygen torus ion composition. Furthermore, we find that the decay of the ultra‐relativistic electrons in the radiation belts by EMIC waves depends on the ion composition. The decay is most sensitive to the helium fraction, and the strongest agreement with Van Allen Probes data is found when the helium fraction is a few percent. We suggest that more observations of the cold ion composition would significantly help understand and set constraints on the decay of ultrarelativistic electrons in the radiation belts.

Using MEPED observations to infer plasma density and chorus intensity in the radiation belts

Abstract: Efforts to model and predict energetic electron fluxes in the radiation belts are highly sensitive to local wave-particle interactions. In this study, we use multi-point measurements of precipitating and trapped electron fluxes to investigate the dynamic variation of chorus wave-particle interactions during the 17 March 2013 storm. Quasilinear theory characterizes the chorus wave-particle interaction as a diffusive process, with the diffusion coefficients depending on the particle energy and pitch angle, as well as the background plasma parameters such as the wave intensity and plasma density. These plasma parameters in the radiation belts are spatially localized and time-varying, so we construct event-specific diffusion coefficients using MEPED (onboard POES/MetOp) measurements of electron fluxes at low Earth orbit. This new method provides realistic diffusion coefficients for chorus waves that account for changes in the wave intensity, the plasma density, and the magnetic field strength in the outer radiation belt. We show that the inferred chorus intensity is significantly lower than previous estimates that use MEPED observations since the same amount of increased precipitation by 30–300 keV electrons can be explained by a change in the plasma density. This technique therefore allows for us to create time varying, global maps of the plasma-gyrofrequency ratio (fpe/fce), and therefore plasma density, in the outer radiation belts using the MEPED measurements. The global density estimates compare reasonably well to in situ density measurements from RBSP-B.

Attention‐Based Machine Vision Models and Techniques for Solar Wind Speed Forecasting Using Solar EUV Images

Abstract: Extreme ultraviolet images taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory make it possible to use deep vision techniques to forecast solar wind speed—a difficult, high‐impact, and unsolved problem. At a 4 day time horizon, this study uses attention‐based models and a set of methodological improvements to deliver an 11.1% lower RMSE and a 17.4% higher prediction correlation compared to the previous work testing on the period from 2010 to 2018. Our analysis shows that attention‐based models combined with our pipeline consistently outperform convolutional alternatives. Our study shows a large performance improvement by using a 30 min as opposed to a daily sampling frequency. Our model has learned relationships between coronal holes' characteristics and the speed of their associated high‐speed streams, agreeing with empirical results. Our study finds a strong dependence of our best model on the phase of the solar cycle, with the best performance occurring in the declining phase.

Electron Diffusion by Magnetosonic Waves in the Earth’s Radiation Belts

Abstract: We conduct a global survey of magnetosonic waves and compute the associated bounce‐ and drift‐averaged diffusion coefficients, taking into account colocated measurements of fpe/fce, to assess the role of magnetosonic waves in radiation belt dynamics. The average magnetosonic wave intensities increase with increasing geomagnetic activity and decreasing relative frequency with the majority of the wave power in the range fcp < f < 0.3fLHR during active conditions. In the region 4.0 ≤ L* ≤ 5.0, the bounce‐ and drift‐averaged energy diffusion rates due to magnetosonic waves never exceed those due to whistler mode chorus, suggesting that whistler mode chorus is the dominant mode for electron energization to relativistic energies in this region. Further in, in the region 2.0 ≤ L* ≤ 3.5, the bounce‐ and drift‐averaged pitch angle diffusion rates due to magnetosonic waves can exceed those due to plasmaspheric hiss and very low frequency (VLF) transmitters over energy‐dependent ranges of intermediate pitch angles. We compute electron lifetimes by solving the 1D pitch angle diffusion equation including the effects of plasmaspheric hiss, VLF transmitters, and magnetosonic waves. We find that magnetosonic waves can have a significant effect on electron loss timescales in the slot region reducing the loss timescales during active times from 5.6 to 1.5 days for 500 keV electrons at L* = 2.5 and from 140.4 to 35.7 days for 1 MeV electrons at L* = 2.0.

Modeling the Effects of Drift Shell Splitting in Two Case Studies of Simultaneous Observations of Substorm‐Driven Pi1B and IPDP‐Type EMIC Waves

Abstract: Intervals of pulsations of diminishing periods (IPDPs) are a subtype of electromagnetic ion cyclotron (EMIC) waves that can be triggered by substorm onset. Pi1B waves are ultralow frequency (ULF) broadband bursts that are well correlated with substorm onset. IPDPs are associated with increased fluxes of 40–60 keV substorm‐injected protons which undergo gradient‐curvature drifting and interact with the cold plasmasphere population. While particle trajectories and the generation of IPDPs have been modeled in the past, those models neglect the role that drift shell splitting plays in the process. This research investigates the different pathways that Pi1B and IPDPs take from their shared origin in substorm onset to their distinct observations on the ground, including the effects of drift shell splitting en route. This paper presents two case studies using data from an array of four ground‐based Antarctic magnetometers that cover the evening sector, as well as in situ magnetometer data, proton fluxes, and proton pitch angles from the Van Allen Probes spacecraft. These observations identify a separation in geomagnetic latitude between Pi1Bs and IPDPs, and pinpoint a separation in magnetic local time (MLT). From these observations we model the drift shell splitting which injected particles undergo post‐onset. This study shows that simulations that incorporate drift shell splitting across a full injection front are dominated by injection boundary effects, and that the inclusion of drift shell splitting introduces a slight horizontal component to the time axis of the time–frequency dependence of the IPDPs.

Variations in Observations of Geosynchronous Magnetopause and Last Closed Drift Shell Crossings With Magnetic Local Time

Abstract: We analyze a set of events in which both electron flux dropouts caused by magnetopause shadowing and geosynchronous magnetopause crossings (GMCs) are observed. These observations are compared to event‐specific last closed drift shell (LCDS) models derived from the TS05 and TS07 external field models and magnetopause standoff distance. The LCDS models show good association with losses due to magnetopause shadowing but fail to reproduce observations of GMCs on the timescale of minutes. We show that different satellites in geostationary orbit observe different trends in electron flux during storm events on timescales of less than a day due to their separation in longitude. These differences demonstrate that both satellite L* and magnetic local time must be taken into account when modeling rapid variations in the outer radiation belt, and at least three satellites in geostationary orbit, ideally more, may be required for accurate forecasting and reconstruction of these events on timescales shorter than days.

Temporal variability of quasi-linear pitch-angle diffusion

Abstract: Kinetic wave-particle interactions in Earth’s outer radiation belt energize and scatter high-energy electrons, playing an important role in the dynamic variation of the extent and intensity of the outer belt. It is possible to model the effects of wave-particle interactions across long length and time scales using quasi-linear theory, leading to a Fokker-Planck equation to describe the effects of the waves on the high energy electrons. This powerful theory renders the efficacy of the wave-particle interaction in a diffusion coefficient that varies with energy or momentum and pitch angle. In this article we determine how the Fokker-Planck equation responds to the temporal variation of the quasi-linear diffusion coefficient in the case of pitch-angle diffusion due to plasmaspheric hiss. Guided by in-situ observations of how hiss wave activity and local number density change in time, we use stochastic parameterisation to describe the temporal evolution of hiss diffusion coefficients in ensemble numerical experiments. These experiments are informed by observations from three different example locations in near-Earth space, and a comparison of the results indicates that local differences in the distribution of diffusion coefficients can result in material differences to the ensemble solutions. We demonstrate that ensemble solutions of the Fokker-Planck equation depend both upon the timescale of variability (varied between minutes and hours), and the shape of the distribution of diffusion coefficients. Based upon theoretical construction of the diffusion coefficients and the results presented here, we argue that there is a useful maximum averaging timescale that should be used to construct a diffusion coefficient from observations, and that this timescale is likely less than the orbital period of most inner magnetospheric missions. We discuss time and length scales of wave-particle interactions relative to the drift velocity of high-energy electrons and confirm that arithmetic drift-averaging is can be appropriate in some cases. We show that in some locations, rare but large values of the diffusion coefficient occur during periods of relatively low number density. Ensemble solutions are sensitive to the presence of these rare values, supporting the need for accurate cold plasma density models in radiation belt descriptions.

Whistler Waves Above the Lower Hybrid Frequency in the Ionosphere and Their Counterparts in the Magnetosphere

Abstract: In this study, we report the statistical properties of whistler mode lower hybrid (LH) emissions in the ionosphere, which have structureless spectra with a lower frequency boundary that matches the variation of the local lower hybrid resonance frequency fLHR. A potential source for the LH emissions is identified as the high‐frequency plasmaspheric hiss (HFPH) in the magnetosphere. We use Detection of Electromagnetic Emissions Transmitted from Earthquake Regions and Van Allen Probes data to perform a statistical study of the wave power distribution of the LH emissions and HFPH. Both LH and HFPH emissions show a similar frequency range, a similar invariant magnetic latitude range, and have similar trends in magnetic local time (stronger wave intensity on the dayside) and in the auroral electrojet (AE) index (stronger wave intensity for higher AE condition). A ray tracing simulation is also performed to demonstrate the propagation of HFPH waves from the magnetosphere into the ionosphere as LH waves.