Ultralow Frequency Wave Power Research Articles

ULF Wave Transport of Relativistic Electrons in the Van Allen Belts: Criteria for Transition to Radial Diffusion

AbstractRelativistic electrons in the radiation belts can be transported as a result of wave‐particle interactions (WPI) with ultra‐low frequency (ULF) waves. Such WPI are often assumed to be diffusive, parametric models for the radial diffusion coefficient often being used to assess the rates of radial transport. However, these WPI transition from initially coherent interactions to the diffusive regime over a finite time, this time depending on the ULF wave power spectral density, and local resonance conditions. Further, in the real system on the timescales of a single storm, interactions with finite discrete modes may be more realistic. Here, we use a particle‐tracing model to simulate the dynamics of outer radiation belt electrons in the presence of a finite number of discrete frequency modes. We characterize the point of the onset of diffusion as a transition from separate discrete interactions in terms of wave parameters by using the “two‐thirds” overlap criterion (Lichtenberg & Lieberman, 1992, https://doi.org/10.1007/978‐1‐4757‐2184‐3), a comparison between the distance between, and the widths of, the electron's primary resonant islands in phase space. Further, we find the particle decorrelation time in our model system with typical parameters to be on the timescale of hours, which only afterward can the system be modeled by one‐dimensional radial diffusion. Direct comparison of particle transport rates in our model with previous analytic diffusion coefficient formulations show good agreement at times beyond the decorrelation time. These results are critical for determining the time periods and conditions under which ULF wave radial diffusion theory can be applied.

The Distribution and Evolution of Storm Time Pc3‐5 ULF Wave Power Based on Satellite and Ground Observations

AbstractThe geomagnetic storm is an intense geomagnetic activity, and one important research aspect is the storm time ultralow frequency (ULF) waves, which can resonate with injected particles and impact particle dynamics in the inner magnetosphere. This study focuses on analyzing ULF wave power during geomagnetic storms on the ground, in the magnetosphere and in the solar wind. During the Van Allen Probe era from year 2012–2019, we identified 94 geomagnetic storm events with 25 as strong storms and 69 as moderate storms based on the variation of SYM‐H index. The analysis revealed that for both satellite and ground‐based measurements during storms with different intensity, the ULF wave power was dramatically enhanced in the main phase. In addition, the ULF wave power of strong storms was significantly stronger than that of moderate storms within the magnetosphere and on the ground. While the finding is opposite in the solar wind. Furthermore, the ULF wave power of strong storms exhibited a higher intensity at higher L shell values and in some cases, it can last up to ∼2 days. Besides, the ULF wave power displayed a linear correlation with the absolute value of the SYM‐H index during the storm main phase while it exhibited an exponential association with the absolute value of the SYM‐H index during the recovery phase. This comprehensive analysis offers valuable insights into the evolution of ULF waves in the inner magnetosphere during geomagnetic storm events, shedding light on plasma waves and wave‐particle interactions in this region.

Global ULF Waves Excited by Solar Wind Dynamic Pressure Impulses: 1. Timescales and Geomagnetic Activity Dependence

AbstractSolar wind dynamic pressure pulse is one of the sources of ultra‐low frequency (ULF) waves. Based on 1‐second resolution magnetic field observations from SuperMAG from 2012 to 2019, we conducted a statistical study on temporal and spatial variations of global ULF power after the arrival of positive dynamic pressure pulses. The magnetosphere responds quickly to the dynamic pressure pulse within 1 min. At low L‐shells, it takes 4–6 min for the ULF waves to reach a power maximum, then ULF waves decay on a timescale of about 10 min. The spatial distribution of ULF wave power is closely related to the direction of the interplanetary magnetic field (IMF) when the dynamic pressure pulse arrives. The southward IMF leads to the reconnection on the dayside and more intense substorm activities on the nightside, accompanied by stronger ULF wave power globally. The power enhancement in the Pc2 band exhibits a notable fluctuation concentrated primarily in the range of L = 7 to 9, compared with the quiet period. In contrast, the amplification of Pc3 wave power occurs predominantly at higher L‐shells. However, the fluctuation of Pc5 power exhibits an opposite pattern, indicating disparate excitation and propagation mechanisms. In addition, seasonal effects affect the ULF power in different bands, which can be explained by the Russell‐McPherron (R‐M) effect. Our statistical results on long‐term, global‐scale geomagnetic observations illustrate that pressure pulses are important sources of ULF waves, and the spatial distribution of wave power varies greatly under different solar wind conditions.

The dependence of radial diffusion coefficients on solar/interplanetary drivers

Radial diffusion driven by Ultra Low Frequency (ULF) waves is very important for magnetospheric dynamics, as it contributes to both acceleration and loss of relativistic electrons in the outer Van Allen radiation belt, as well as to other planetary magnetospheres. The dependence of ULF wave power spectral density and radial diffusion coefficients (DLL) on solar wind parameters has already been investigated but their dependence on the various solar and interplanetary drivers is poorly studied. In this study, we conduct a statistical analysis of radial diffusion coefficients DLL to look into their dependence on different interplanetary drivers, i.e. Interplanetary Coronal Mass Ejections (ICMEs) and Stream Interaction Regions (SIRs), which originate from different solar drivers: CMEs and coronal holes. We use the ”SafeSpace” database (https://synergasia.uoa.gr/modules/document/?course=PHYS120), which includes radial diffusion coefficients DLL and ULF wave power spectral density, created using magnetic and electric field measurements by the THEMIS satellites in the 2011–2019 time period. We study how the properties of these solar wind drivers influence the behavior of radial diffusion coefficients. The results indicate significant differences between drivers of different solar origin at the ratio of the electric over the magnetic DLL component (DLLE over DLLB), especially when the solar wind dynamic pressure is maximized due to shock existence. Furthermore, differences are observed between SIRs with shock and SIRs without shock. This feature introduces a significant energy dependence to the radial diffusion coefficients, which is further depending on the radial distance and the different first adiabatic invariant (μ) values. These results emphasize the importance of the energy dependence of the magnetic component of the DLL, especially during the occurrence of interplanetary shocks, which is neglected by most semi-empirical models used in radiation belt simulations.

Statistical Characterization of the Dynamic Near‐Earth Plasma Sheet Relative to Ultra‐Low Frequency (ULF) Wave Growth at Substorm Onset

AbstractMagnetospheric substorms are a complex phenomenon. During the initial stages of a substorm a variety of important processes occur in near‐Earth space within a span of several minutes. The relative timing and links between these processes are critical to understanding how, where and when substorms may occur. One of the first observed signatures at substorm onset is the exponential increase in ULF (Ultra‐Low Frequency) wave power in the near‐Earth magnetotail (e.g., −7.5 ≤ XGSM ≤ −12.5 RE). We use the Time History of Events and Macroscale Interactions during Substorms spacecraft to examine the conditions in the magnetotail plasma sheet before, during and after local ULF wave growth. Prior to the ULF wave growth, the magnetotail stretches with convectional flows dominating. We then find strong earthward and azimuthal flows that peak at a similar time to the peak ULF wave power. These flows are found to be faster in the mid‐tail (−10 ≤ XGSM ≤ −12.5 RE) than the near‐tail (−7.5 ≤ XGSM ≤ −10 RE). Examining the local plasma energy density (magnetic, thermal and kinetic), we find no statistical decrease that could explain the exponentially growing ULF waves, in fact the local energy density is found to increase. This suggests that there may be an injection of energy from elsewhere in the magnetotail. Following the peak ULF wave power the tail is seen to dipolarize, and the local energy density is enhanced.

Energetic Electron Flux Predictions in the Near‐Earth Plasma Sheet From Solar Wind Driving

AbstractSuprathermal electrons in the near‐Earth plasma sheet are important for inner magnetosphere considerations. They are the source population for outer radiation belt electrons and they pose risks to geosynchronous satellites through their contribution to surface charging. We use empirical modeling to address relationships between solar driving parameters and plasma sheet electron flux. Using Time History of Events and Macroscale Interactions during Substorms, OMNI, and Flare Irradiance Spectral Model Version 2 data, we develop a neural network model to predict differential electron flux from 0.08 to 93 keV in the plasma sheet, at distances from 6 to 12 RE. Driving parameters include solar wind (SW) density and speed, interplanetary magnetic field (IMF) BZ and BY, solar extreme ultraviolet flux, IMF BZ ultra‐low frequency (ULF) wave power, SW‐magnetosphere coupling functions Pα1 and NXCF, and the 4‐hr time history of these parameters. Our model predicts overall plasma sheet electron flux variations with correlation coefficients of between 0.59 and 0.77, and median symmetric accuracy in the 41%–140% range (depending on energy). We find that short time‐scale electron flux variations are not reproduced using short time‐scale inputs. Using a recently published technique to extract information from neural networks, we determine the most important drivers impacting model prediction are VSW, VBS, and IMF BZ. SW‐magnetosphere coupling functions that include IMF clock angle, IMF BZ ULF wave power, and IMF BY have little impact in our model of electron flux in the near‐Earth plasma sheet. The new model, built directly on differential flux, outperforms an existing model that derives fluxes from plasma moments, with the performance improvement increasing with increasing energy.

Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Power Spectra During High Speed Streams and Quiet Solar Wind and Seeding of EMIC Waves.

Here, we extend the scope of the Gamayunov and Engebretson (2021, hereinafter Paper 1), https://doi.org/10.1029/2021JA029247 work by analyzing the low frequency ultra-low-frequency (ULF) wave power spectra in the Earth's inner magnetosphere during high speed stream (HSS) and quiet solar wind (QSW) driving conditions in the upstream solar wind (SW) and comparing our results to the results of Paper 1, where the statistics of ULF wave power spectra during coronal mass ejections (CMEs) are presented. The most important results of our statistical and comparative analyses are as follows. (a) During CMEs, HSSs, and QSW, the magnetic field power spectra of the transverse and compressional fluctuations are well approximated by power laws in the ∼mHz-Hz frequency range, where on average the parameters of power law fits during CMEs and HSSs are close, and those during QSW differ considerably from the respective parameters during CMEs and HSSs. (b) The dominance of the average compressional power over the average transverse power for the low frequency ULF waves during the 0<SYM/H≲25nT geomagnetic conditions may serve as a proxy of HSSs in the upstream SW, whereas the opposite relation between the average powers is an indication of CMEs. (c) Independently of the SW driving conditions, a turbulent energy cascade from low frequencies in the ULF wave frequency range into the higher frequency range exists in the Earth's inner magnetosphere, supplying the nonthermal electromagnetic seed fluctuations needed for the growth of electromagnetic ion cyclotron waves (∼Hz) due to relaxation of unstable distributions of energetic magnetospheric ions.

Quantifying the non-linear dependence of energetic electron fluxes in the Earth's radiation belts with radial diffusion drivers

Abstract. In this study, we use mutual information to characterise statistical dependencies of seed and relativistic electron fluxes in the Earth's radiation belts on ultra-low-frequency (ULF) wave power measured on the ground and at geostationary orbit. The benefit of mutual information, in comparison to measures such as the Pearson correlation, lies in the capacity to distinguish non-linear dependencies from linear ones. After reviewing the property of mutual information and its relationship with the Pearson correlation for Gaussian bivariates, we present a methodology to quantify and distinguish linear and non-linear statistical dependencies that can be generalised to a wide range of solar wind drivers and magnetospheric responses. We present an application of the methodology by revisiting the case events studied by Rostoker et al. (1998). Our results corroborate the conclusions of Rostoker et al. (1998) that ULF wave power and relativistic electron fluxes are statistically dependent upon one another. We also estimate that the Pearson correlation is missing between 20 % and 30 % of the statistical dependency between ULF wave power and relativistic electron fluxes. Thus, the Pearson correlation underestimates the impact of ULF waves on energetic electron fluxes. However, we find that observed enhancements in relativistic electron fluxes correlate modestly, both linearly and non-linearly, with the ULF power spectrum when compared with values found in previous studies (Simms et al., 2014) and with correlational values found between seed electrons and ULF wave power for the same case events. Our results are indicative of the importance of incorporating data analysis tools that can quantify linear and non-linear interdependencies of various solar wind drivers.

On the Interplay Between Solar Wind Parameters and ULF Wave Power as a Function of Geomagnetic Activity at High‐ and Mid‐latitudes

AbstractUsing ground magnetometer measurements from the IMAGE array over the timespan of one solar cycle, we apply wavelets to statistically assess the power of ultra‐low frequency (ULF) waves from L = 3.34 to L = 13.6 as a function of Kp, solar wind speed (), solar wind dynamic pressure (), Dst, and . We find that although geomagnetic storms account for the overwhelming majority of ULF wave power at mid latitudes, this effect decreases for higher L‐shells. We present analysis of the effect of interplanetary magnetic field (IMF) on ULF wave power for different values of Kp and , revealing a strong inter‐dependency especially in relation to . A parameterization additionally incorporating appears to be more important at large‐L. Further, analysis for positive reveals a perhaps unexpected additional dependence on which points to effects of the interplanetary magnetic field cone angle and Kelvin‐Helmholtz instabilities. Finally, we derive ULF wave radial diffusion coefficients from the measured ULF wave power spectral densities and briefly discuss their importance for radiation belt dynamics. Interestingly, in part because of the correlation between Dst‐defined storm times and high Kp, we find that the low‐Kp radial diffusion coefficients during storms are higher than previously assumed almost certainly because of the dominance of nonstorm epochs in the statistics of median ULF wave power under conditions of low Kp. These inter‐dependencies should be taken into account in future specification models for ULF wave radial diffusion coefficients when applied in storm time simulations.

Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Statistics During Coronal Mass Ejections and Seeding of EMIC Waves

AbstractHere we perform a statistical analysis of low frequency ultra‐low‐frequency (ULF) waves ( mHz ‐ Hz) in the Earth's inner magnetosphere excluding electromagnetic ion cyclotron waves concurrently observed. We use the magnetic field data from the two Van Allen Probes during their first magnetic local time (MLT) revolution that cover the periods of coronal mass ejections. The major results of our analysis are as follows. (a) Spectra of both the transverse and compressional ULF waves are well approximated by the power‐laws in the mHz ‐ Hz frequency range. (b) There are two sources of the low frequency ULF waves: an internal magnetospheric source and an external source outside of the magnetosphere. (c) The average transverse power in the 6–24 h MLT sector dominates that in the 0–6 h sector, whereas the compressional power in the 12–24 h sector dominates that in the 0–12 h sector. (d) The average powers of transverse and compressional ULF waves in the plasmasphere dominate the average powers in the high L shell region of , and there is a deep power minimum in the intermediary region of . (e) The compressional ULF wave power has a maximum in the near equatorial region, whereas the transverse power has a minimum there. (f) A wave energy cascade from low frequency ULF waves into the higher frequency range of EMIC waves ( Hz) supplies the nonthermal seed fluctuations from which EMIC waves can then grow due to instabilities of the energetic magnetospheric ions.

Random Forest Model of Ultralow‐Frequency Magnetospheric Wave Power

AbstractModels of magnetospheric ultralow‐frequency (ULF) waves can be used to study wave phenomena and to calculate the effect of these waves on the energization and transport of radiation belt electrons. We present a decision tree ensemble (i.e., a random forest) model of ground‐based ULF wave power spectral density driven by solar wind speed vsw, north‐south component of the interplanetary magnetic field Bz, and variance of proton number density var(Np). This model corresponds to four radial locations in the magnetosphere (roughly L ∼ 4.21 to 7.94) and spans 1–15 mHz, with hourly magnetic local time resolution. The decision tree ensembles are easier to use than the previous model generation; they have better coverage, perform better at predicting power, and have reduced error due to intelligently chosen bins in parameter space. We outline the difficulties in extracting physics from parameterized models and demonstrate a hypothesis testing framework to iteratively explore finer driving processes. We confirm a regime change for ULF driving about Bz = 0. We posit that ULF wave power directly driven by magnetopause perturbations corresponds to a latitude‐dependent dawn‐dusk asymmetry, which we see with increasing speed. Model uncertainty identifies where the underlying physics is not fully captured; we find that power due to substorms is less well characterized by Bz &gt; 0, with an effect that is seen globally and not just near midnight. The largest uncertainty is seen for the smallest amounts of solar wind driving, suggesting that internal magnetospheric properties may play a significant role in ULF wave occurrence.

Semiannual variation of Pc5 ultra-low frequency (ULF) waves and relativistic electrons over two solar cycles of observations: comparison with predictions of the classical hypotheses

Abstract. Pc5 ULF (ultra-low frequency) waves can energize electrons to relativistic energies of &gt;2 MeV in geostationary orbits. Enhanced fluxes of such electrons can induce operational anomalies in geostationary satellites. The variations of the two quantities in timescales ranging from days to solar cycles are thus of interest in gauging their space weather effects over different time frames. In this study, we present a statistical analysis of two 11-year solar cycles (cycles 22 and 23) of data comprising the daily relativistic electron fluence observed by Geostationary Environment Satellites (GOESs) and daily Pc5 ULF wave power derived from auroral zone magnetic observatories in Canada. First, an autocorrelation analysis is carried out, which indicates a 27 d periodicity in both parameters for all solar phases, and such a periodicity is most pronounced in the declining and late declining phase. Also, a 9 and 13 d periodicity are seen in some years. Then, a superposed epoch analysis is performed to scrutinize semiannual variation (SAV), which shows that fluence near the equinoxes is 1 order of magnitude higher than near solstices, and Pc5 ULF wave power is 0.5 orders of magnitude higher near the equinoxes than near the solstices. We then evaluate three possible SAV mechanisms (which are based on the axial, equinoctial, and Russell and McPherron effect) to determine which one can best explain the observations. Correlation of the profiles of the observational curves with those of the angles that control each of the SAV mechanisms suggests that the equinoctial mechanism may be responsible for the SAV of electron fluence, while both the equinoctial and the Russell and McPherron mechanisms are important for the SAV of Pc5 ULF wave power. Comparable results are obtained when using functional dependencies of the main angles instead of the angles mentioned above. Lastly, superposed curves of fluence and Pc5 ULF wave power were used to calculate least-square fits with a fixed semiannual period. Comparison of the maxima and minima of the fits with those predicted by the three mechanisms shows that the equinoctial effect better estimates the maxima and minima of the SAV in fluence while for the SAV in Pc5 ULF wave power the equinoctial and Russell and McPherron mechanisms predict one maximum and one minimum each.

Accounting for Variability in ULF Wave Radial Diffusion Models

AbstractMany modern outer radiation belt models simulate the long‐time behavior of high‐energy electrons by solving a three‐dimensional Fokker‐Planck equation for the drift‐ and bounce‐averaged electron phase space density that includes radial, pitch‐angle, and energy diffusion. Radial diffusion is an important process, often characterized by a deterministic diffusion coefficient. One widely used parameterization is based on the median of statistical ultralow frequency (ULF) wave power for a particular geomagnetic index Kp. We perform idealized numerical ensemble experiments on radial diffusion, introducing temporal and spatial variability to the diffusion coefficient through stochastic parameterization, constrained by statistical properties of its underlying observations. Our results demonstrate the sensitivity of radial diffusion over a long time period to the full distribution of the radial diffusion coefficient, highlighting that information is lost when only using median ULF wave power. When temporal variability is included, ensembles exhibit greater diffusion with more rapidly varying diffusion coefficients, larger variance of the diffusion coefficients and for distributions with heavier tails. When we introduce spatial variability, the variance in the set of all ensemble solutions increases with larger spatial scales of variability. Our results demonstrate that the variability of diffusion affects the temporal evolution of phase space density in the outer radiation belt. We discuss the need to identify important temporal and length scales to constrain variability in diffusion models. We suggest that the application of stochastic parameterization techniques in the diffusion equation may allow the inclusion of natural variability and uncertainty in modeling of wave‐particle interactions in the inner magnetosphere.

A Framework for Understanding and Quantifying the Loss and Acceleration of Relativistic Electrons in the Outer Radiation Belt During Geomagnetic Storms

AbstractWe present detailed analysis of the global relativistic electron dynamics as measured by total radiation belt content (RBC) during coronal mass ejection (CME) and corotating interaction region (CIR)‐driven geomagnetic storms. Recent work has demonstrated that the response of the outer radiation belt is consistent and repeatable during geomagnetic storms. Here we build on this work to show that radiation belt dynamics can be divided into two sequential phases, which have different solar wind dependencies and which when analyzed separately reveal that the radiation belt responds more predictably than if the overall storm response is analyzed as a whole. In terms of RBC, in every storm we analyzed, a phase dominated by loss is followed by a phase dominated by acceleration. Analysis of the RBC during each of these phases demonstrates that they both respond coherently to solar wind and magnetospheric driving. However, the response is independent of whether the storm response is associated with either a CME or CIR. Our analysis shows that during the initial phase, radiation belt loss is organized by the location of the magnetopause and the strength of Dst and ultralow frequency wave power. During the second phase, radiation belt enhancements are well organized by the amplitude of ultralow frequency waves, the auroral electroject index, and solar wind energy input. Overall, our results demonstrate that storm time dynamics of the RBC is repeatable and well characterized by solar wind and geomagnetic driving, albeit with different dependencies during the two phases of a storm.

On the Magnetospheric ULF Wave Counterpart of Substorm Onset

AbstractOne near‐ubiquitous signature of substorms observed on the ground is the azimuthal structuring of the onset auroral arc in the minutes prior to onset. Termed auroral beads, these optical signatures correspond to concurrent exponential increases in ground ultralow frequency (ULF) wave power and are likely the result of a plasma instability in the magnetosphere. Here, we present a case study showing the development of auroral beads from a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky camera with near simultaneous exponential increases in auroral brightness, ionospheric and conjugate magnetotail ULF wave power, evidencing their intrinsic link. We further present a survey of magnetic field fluctuations in the magnetotail around substorm onset. We find remarkably similar superposed epoch analyses of ULF power around substorm onset from space and conjugate ionospheric observations. Examining periods of exponential wave growth, we find the ground‐ and space‐based observations to be consistent, with average growth rates of ∼0.01 s−1, lasting for ∼4 min. Cross‐correlation suggests that the space‐based observations lead those on the ground by approximately 1–1.5 min. Meanwhile, spacecraft located premidnight and ∼10 RE downtail are more likely to observe enhanced wave power. These combined observations lead us to conclude that there is a magnetospheric counterpart of auroral beads and exponentially increasing ground ULF wave power. This is likely the result of the linear phase of a magnetospheric instability, active in the magnetotail for several minutes prior to auroral breakup.

On the Confinement of Ultrarelativistic Electron Remnant Belts to Low Shells

AbstractUltrarelativistic electron remnant belts are frequently observed at low shells between the inner belt and a re‐forming outer belt following geomagnetic disturbances that led to a dropout of electron fluxes at higher radial distances from the Earth. Using wave, particle, and plasma measurements from the Van Allen Probes and Pc3‐Pc5 ultra low frequency (ULF) wave data from ground magnetometers from September 2012 to November 2017, we find significant correlations between the upper edge of the remnant belts and the minimum plasmapause and last closed drift shell locations. The maximum 2‐hr‐averaged radial diffusion rate based on ULF wave power recorded during the dropouts is correlated with the upper edge of the remnant belts and last closed drift shell position. Frequently, ULF wave power is sufficiently strong down to the upper edge of the remnant belts to allow a fast outward radial diffusion of electrons up to the last closed drift shell and to account for the observed confinement of remnant belts to low shells. The electron phase space density often exhibits the needed negative or oscillating outward gradients in the region of flux loss. Accordingly, fast outward radial diffusion by ULF waves turns out to be a crucial contributor to the depletion of the outer belt that leads to the formation of remnant belts of ultrarelativistic electrons, although we show that multi‐MeV electron precipitation through combined scattering by contemporaneous electromagnetic ion cyclotron (EMIC) and lower‐band chorus waves probably contributes in a limited number of cases.

Solar cycle evolution of ULF wave power in solar wind and on ground

The solar cycle evolution of the ultra-low frequency (ULF) power was studied in solar wind and on ground. We aim finding out how the ULF power in interplanetary and on ground magnetic field evolves over the solar cycle 23 (SC23) and how well do they follow each other in monthly time scales. The hourly power of the ULF waves was computed in the Pc5 frequency range 2–7 mHz for years 1998–2008. The highest wave power in SC23 is found to occur in late 2003 and the lowest at the solar minimum. Ground ULF power follows the IMF power and solar wind speed, particularly well during declining phase. The ULF power in winter exceeds the ULF power in other seasons during the declining phase of SC23, while equinoxes dominate in the ascending phase and the solar maximum. The ground ULF power was found to rise with magnetic latitude from 54° to 73°, after which Pc5 power decreases towards the polar cap. The Pc5 power in the auroral zone is larger in the nightside than the dayside due to substorm activity implying that magnetotail processes are an important contributor to the nightside ULF power.

How Do Ultra‐Low Frequency Waves Access the Inner Magnetosphere During Geomagnetic Storms?

AbstractWave‐particle interactions play a key role in radiation belt dynamics. Traditionally, ultra‐low frequency (ULF) wave‐particle interaction is parameterized statistically by a small number of controlling factors for given solar wind driving conditions or geomagnetic activity levels. Here we investigate solar wind driving of ULF wave power and the role of the magnetosphere in screening that power from penetrating deep into the inner magnetosphere. We demonstrate that during enhanced ring current intensity, the Alfvén continuum plummets, allowing lower frequency waves to penetrate deeper into the magnetosphere than during quiet periods. With this penetration, ULF wave power is able to accumulate closer to the Earth than characterized by statistical models. During periods of enhanced solar wind driving such as coronal mass ejection driven storms, where ring current intensities maximize, the observed penetration provides a simple physics‐based reason for why storm time ULF wave power is different compared to nonstorm time waves.

Estimating the Azimuthal Mode Structure of ULF Waves Based on Multiple GOES Satellite Observations

AbstractCharacterizing the azimuthal mode number, m , of ultralow‐frequency (ULF) waves is necessary for calculating radial diffusion of radiation belt electrons. A cross‐spectral technique is applied to the compressional Pc5 ULF waves observed by multiple pairs of GOES satellites to estimate the azimuthal mode structure during the 28‐31 May 2010 storm. We find that allowing for both positive and negative m is important to achieve a more realistic distribution of mode numbers and to resolve wave propagation direction. During the storm commencement when the solar wind dynamic pressure is high, ULF wave power is found to dominate at low‐mode numbers. An interesting change of sign in m occurred around noon, which is consistent with the driving of ULF waves by solar wind buffeting around noon, creating antisunward wave propagation. The low‐mode ULF waves are also found to have a less global coverage in magnetic local time than previously assumed. In contrast, during the storm main phase and early recovery phase when the solar wind dynamic pressure is low and the auroral electrojet index is high, wave power is shown to be distributed over all modes from low to high. The high‐mode waves are found to cover a wider range of magnetic local time than what was previously assumed. Furthermore, to reduce the 2nπ ambiguity in resolving m , a cross‐pair analysis is performed on satellite field measurements for the first time, which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high auroral electrojet periods.

On the Close Correspondence Between Storm Time ULF Wave Power and the POES VLF Chorus Wave Amplitude Proxy

AbstractGround‐based Pc5 ultralow frequency (ULF) wave power in multiple ground‐based meridians is compared to the very low frequency (VLF) wave amplitude proxy, derived from Polar Operational Environmental Satellite (POES) precipitation, for the 33 storms studied by Li et al. (2015, https://doi.org/10.1002/2015GL065342). The results reveal common L‐shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at L ≤ 6, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation, which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L‐shell and time dependence. Global ground‐based ULF wave power coherence implies that a small number of meridians can be used to estimate storm time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration.