Ultra-relativistic electron flux enhancement under persistent high speed solar wind stream

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 hours by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2 to 4 hours, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high‐speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin‐Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics.

Ground‐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.

The dynamics of the outer zone radiation belt has received a lot of attention mainly due to the correlation between the occurrence of enhancing relativistic electron flux and spacecraft operation anomalies or even failures (e.g., Baker et al. 1994). Relativistic electron events are often observed during great storms associated with ultra low frequency (ULF) waves. For example, a large buildup of relativistic electrons was observed during the great storm of March 24, 1991 (e.g., Li et al. 1993; Hudson et al. 1995; Mann et al. 2013). However, the dominant processes which accelerate magnetospheric radiation belt electrons to MeV energies are not well understood. In this paper, we present observations of Pc5 ULF waves in the recovery phase of the Bastille day storm of July 16, 2000 and electron and proton flux simultaneously oscillating with the same frequencies as the waves. The mechanism for the observed electron and proton flux modulations is examined using groundbased and satellite observations. During this storm time, multiple packets of discrete frequency Pc5 ULF waves appeared associated with energetic particle flux oscillations. We model the drift paths of electrons and protons to determine if the particles drift through the ULF wave to understand why some particle fluxes are modulated by the ULF waves and others are not. We also analyze the flux oscillations of electrons and protons as a function of energy to determine if the particle modulations are caused by a ULF wave drift resonance or advection of a particle density gradient. We suggest that the energetic electron and proton modulations by Pc5 ULF waves provide further evidence in support of the important role that ULF waves play in outer radiation belt dyanamics during storm times.

Ultra-low frequency (ULF) waves permeate near-Earth space and play a key role in the transfer of electromagnetic energy from the solar wind to the magnetosphere. These waves contribute to magnetosphere-ionosphere coupling, and are central to the dynamics of the inner magnetosphere, in particular through particle acceleration and transport in the radiation belts. ULF waves in the Pc5 frequency band (2-7 mHz) are especially important for radiation belt dynamics, as they cause radial diffusion of electrons and, when their amplitude is large enough, nonlinear interactions. Extensive statistical surveys have shown that the ULF wave power in the Pc5 range tends to increase during geomagnetic storms. However, we note that widely-used geomagnetic indices such as Kp, AE and Dst do not intrinsically quantify the level of ULF wave activity inside the magnetosphere. The goals of this study are (1) to define wave storms in Earth's magnetosphere, that is, intervals of elevated ULF wave power, irrespective of the occurrence of geomagnetic storms (as defined by the Dst or SYM-H index), (2) to identify the drivers of the most intense wave storms and (3) to compare the occurrence of wave storms with that of geomagnetic storms. The ULF wave activity inside the magnetosphere is quantified based on the ground ULF wave power index described by Pilipenko et al. [2017]. Similarly as storms being classified as moderate or intense based on the minimum Dst value reached during the event, we define two levels for wave storms, moderate and intense wave storms. We analyse the occurrence of wave storms as a function of large-scale solar wind drivers (interplanetary coronal mass ejections – ICMEs – and high-speed solar wind streams – HSSs). We find that about half of the wave storms are driven by HSSs, possibly due to the long duration of these structures, while ICMEs tend to drive the most intense wave storms. We also compare electron fluxes at geostationary orbit during wave storms and quiet times, and find significantly enhanced fluxes during wave storms, as expected. Finally, we compare the occurrence of wave storms with that of geomagnetic storms, and discuss possible applications of this new wave storm definition.

The adiabatic drift‐resonant interaction between relativistic, equatorially mirroring electrons and narrowband, Pc 5 ultra low frequency (ULF) waves in the magnetosphere is investigated using a time‐dependent magnetohydrodynamic (MHD) wave model. Attention is focused on the effect of a ULF wave packet with finite duration on the equatorially mirroring, relativistic electron phase space density (PSD) profile. It is demonstrated that a burst of narrow band ULF waves can give rise to the growth of strong localized peaks in PSD with L‐shell by nondiffusive radial transport. This contrasts with the diffusive “external source acceleration mechanism” described by Green and Kivelson (2004), a radial transport mechanism often attributed to ULF waves, which cannot produce peaks in PSD that increase with time. On the basis of this paradigm, observations of locally growing PSD peaks are usually attributed to very low frequency (VLF) wave acceleration by resonant interactions with lower‐band chorus (e.g., Horne et al., 2005). However, we show that in situations where large amplitude, narrow bandwidth ULF waves are also observed, these time‐limited coherent ULF waves can also generate growing PSD peaks and under such circumstances may offer an alternative explanation.

The ultra‐low frequency (ULF) waves can stochastically accelerate radiation belt electrons. Radial diffusion is a well‐established mechanism that can enhance or reduce the electron population in combination with other processes. Using data from the Van Allen Probes, we investigated the response of the 2.10 MeV energy electrons and ULF waves to two types of solar wind structures interacting with Earth's magnetosphere, namely, interplanetary coronal mass ejections (ICMEs) and High‐Speed solar wind streams (HSS). We use measured electron differential flux and ULF waves in the Pc4–Pc5 frequency range from October 2012 to May 2019. We examine 155 events with changes in the outer radiation belt electron differential flux. Results considering all ICMEs and HSSs during the Van Allen Probes era show that for both solar wind structures, solar wind interplanetary magnetic field Bz, solar wind proton density, and speed are related to the outer radiation belt relativistic electrons' response. The persistent ULF power is present during enhancement cases, while for reduction, the ULF waves power is concentrated at the initial reduction on the outer radiation belt electron flux.

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high‐speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground‐ and space‐based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L‐shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Ultra Low Frequency (ULF) waves play important roles in magnetosphere-ionosphere coupling, ring current and radiation belt dynamics, and modulation of higher frequency wave modes and energetic particle precipitation. The “ULF wave modeling, effects, and applications” (UMEA) focus group - part of the Geospace Environment Modeling effort from 2016 to 2021 - sought to improve understanding of the physics of ULF waves and their specification in geospace models. Through a series of in person and virtual meetings the UMEA focus group brought modelers and experimentalists together to compare ULF wave outputs in different models, plan observation campaigns focused on ULF waves, discuss recent advances in ULF wave research, and identify unresolved ULF wave science questions. This article summarizes major discussion points and accomplishments in the UMEA focus group over the last 6 years, recent advances and their connection to Richard Thorne and Peter Gary’s significant contributions to ULF wave research, and the future of ULF wave research.

Earth's inner magnetosphere is a zoo of plasma waves where electromagnetic and electrostatic emissions with distinct frequencies coexist and interact. Spacecraft observations have shown that whistler‐mode chorus waves, one of the key components in the magnetospheric dynamics, are often modulated by ultralow frequency (ULF) waves. Here, we investigate the effects of two typical ULF wave modes (i.e., field line resonance and mirror mode) on the nonlinear generation process of chorus waves. We report for the first time periodic excitations of lower‐ and upper‐band chorus waves near ULF wave crests and troughs, respectively. Their anticorrelated occurrence is explained by the nonlinear theory, in which the threshold amplitude of nonlinear wave growth is modified by the ULF wave field configuration and the modulated electron distributions. In this framework, the newly observed feature of chorus wave occurrence near the ULF wave crests is attributed to the antisymmetric field profiles of mirror‐mode ULF waves, which periodically modulate the threshold amplitude by modifying the second‐order derivative of the background dipole field. In addition to the second‐order derivative, the first‐order derivative of the background magnetic field is also modulated by the ULF waves to regulate the size of the chorus wave source region.

Abstract. We examine data from a topside ionosphere and two magnetospheric missions (CHAMP, Cluster and Geotail) for signatures of ultra low frequency (ULF) waves during the exceptional 2003 Halloween geospace magnetic storm, when Dst reached ~−380 nT. We use a suite of wavelet-based algorithms, which are a subset of a tool that is being developed for the analysis of multi-instrument multi-satellite and ground-based observations to identify ULF waves and investigate their properties. Starting from the region of topside ionosphere, we first present three clear and strong signatures of Pc3 ULF wave activity (frequency 15–100 mHz) in CHAMP tracks. We then expand these three time intervals for purposes of comparison between CHAMP, Cluster and Geotail Pc3 observations but also to be able to search for Pc4–5 wave signatures (frequency 1–10 mHz) into Cluster and Geotail measurements in order to have a more complete picture of the ULF wave occurrence during the storm. Due to the fast motion through field lines in a low Earth orbit (LEO) we are able to reliably detect Pc3 (but not Pc4–5) waves from CHAMP. This is the first time, to our knowledge, that ULF wave observations from a topside ionosphere mission are compared to ULF wave observations from magnetospheric missions. Our study provides evidence for the occurrence of a number of prominent ULF wave events in the Pc3 and Pc4–5 bands during the storm and offers a platform to study the wave evolution from high altitudes to LEO. The ULF wave analysis methods presented here can be applied to observations from the upcoming Swarm multi-satellite mission of ESA, which is anticipated to enable joint studies with the Cluster mission.

Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI‐induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East‐west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest‐amplitude ULF responses to SIs in the nightside plasma sheet are tailward‐moving vortices, which have previously been reported, and the dynamic response of cross‐tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfvén waves (toroidal modes) observed via the field‐line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.

In this article we explore theoretically the role of lightning and atmospherical discharges in destabilizing Ultra Low Frequency (ULF) waves in the Ionosphere. The idea, which could seem quite exotic, takes its origin from the experimental observation that the occurrence of Ultra Low Frequency ULF ~ 6–50 Hz) waves is strongly related to the occurrence of positive polarity cloud-to-ground flashes (+CG) The interest in the subject, on the other hand, comes from the fact that the occurrence of ULF waves is not only related to the occurrence of positive polarity flashes but also to the occurrence of newly discovered transient luminous phenomena as sprites, blue jets and gigantic jets. Sprites, as well as blue jets and gigantic jets, are luminous events which have been detected in discharges between thunderclouds (~ 10 km from Earth's surface) and the lower Ionosphere (mesosphere ~90 km) These transient luminous phenomena strongly affect the atmospheric electricity, due to transfer of large amounts of charge between different regions of the atmosphere (a gigantic jet can remove as much as 0.02% of the total atmospheric charge), and suggest that some important components of the global electric circuit have still to be identified and incorporated in the theoretical framework. Another important aspect is that, during the charge transfer process (electrical discharge), sprites, blue jets and gigantic jets modify the chemistry of a large portion of the stratosphere and mesosphere, with poorly understood influences on global climate changes. In order to survey the total rate of occurrence and the implications of such phenomena on continental and global scale one needs a signature that can be easily detected by remote measurements and independent from in situ video observations. To understand whether the ULF waves can be used as sprites signatures, we explore the relation between positive polarity lightning and ULF waves by presenting a theory which explains the origin and threshold of the observed ULF waves. As a result, we obtain that very large positive polarity flashes give rise to ULF Dust Acoustic waves in the Ionosphere. We also find that the ULF waves may be destabilized, in absence of positive polarity flashes, by mean of very large Ionosphere to thunderclouds discharges, such as gigantic blue jets, which create large dipole electric fields, comparable with the ones produced by +CG lightning.

Characterizing the azimuthal mode number 𝑚 of Ultra Low Frequency (ULF) waves is critical to quantifying the radial diffusion of radiation belt electrons. A Wavelet cross-spectral technique is applied to the compressional ULF waves observed by multiple pairs of GOES and MMS satellites to estimate the mode structure of ULF waves. A more realistic distribution of mode numbers is achieved by inclusion of the modes corresponding to different wave propagation directions as well as at 𝑚 higher than fundamental mode number. For the event study of a geomagnetic storm using GOES data, ULF wave power is found to dominate at low mode numbers during high solar wind dynamic pressure. The change of sign in 𝑚 around noon was observed to be consistent with anti-sunward wave propagation due to solar wind. To reduce the 2𝜋 ambiguity in resolving 𝑚, a cross-pair analysis is performed on GOES field measurements which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high Auroral Electrojet (AE) periods. During another event with two consecutive interplanetary shocks compressing the dayside magnetopause, contribution of low versus high modes in the power of ULF pulsations and their frequency signatures are resolved using high-fidelity multi-probe MMS magnetometer data. The analysis clearly shows that shock onset corresponds to more in-phase magnetic fluctuations in the Pc4-5 regimes than what follows, and smaller spatial scale fluctuations are implied by the dominant high mode numbers observed after both shocks hit and passed the magnetosphere. At the shock impacts, the contribution of higher frequencies (e.g., >7 mHz, corresponding to Pc-4) to the wave power is not negligible, while after the impacts, the power distributes significantly over lower frequencies (e.g.,pulsations). A threshold mode, 𝑚th, is introduced to give an approximate range of the largest resolvable 𝑚 using ideal-MHD models. In addition, a first-principle calculation is introduced to address the long-lasting debate on the contribution of higher ULF wave azimuthal mode number (e.g., |𝑚|>1) on the radial diffusion rates 𝐷𝐿𝐿 of energetic electrons. We showed that the simplified assumption of 𝑚=+1 in ULF waves would overestimate the 𝐷𝐿𝐿 by more than 300%. Therefore, contrary to the previous assumptions in earlier work, inclusion of the negative as well as higher mode values are both important and must be considered in the estimations of radial diffusion of radiation belt electrons.

The ultra low frequency (ULF) wave in magnetosphere can act as an important means for solar wind energy inward transmission. This paper quantitatively analyzes the propagation process of the ULF wave triggered by the interplanetary shock propagating from inner magnetosphere equatorial plane along magnetic field lines to the top of the ionosphere and below ionosphere propagating process and establishes a relatively complete magnetosphere-ionosphere-atmosphere propagation model which can be used to study the relationship between the amplitude of the ULF waves triggered by the interplanetary shock wave in magnetospheric space and the magnetic effect caused by the ULF waves. After a comparison with recent observations, we found that: in the event during November 7, 2004 that an interplanetary shock wave interacted with the magnetosphere, Cluster satellites observed that electric field fluctuations and the band-pass filtered result of ground stations meridional component had similar characteristics. Comparing with the geomagnetic measurement near the footprints, we found that the electric field disturbance in the magnetosphere spread along the ground magnetic field lines in the form of the ULF waves and changed into geomagnetic disturbance. The result reveals that the ULF wave is in contact with the ground geomagnetic observation. The ULF waves couple with ionized components in ionosphere and spread to the ground in the form of electromagnetic waves. In this research, we believe that the magnetosphere, ionosphere and ground magnetic effects caused by interplanetary shock wave are the same physical phenomena responding in different locations. Based on the overall consideration of entire electromagnetic response to the interplanetary shock wave, we found that the correlation between CLUSTER multi-satellite observation and geomagnetic station observation is due to the ULF wave propagated in magnetosphere-ionosphere-atmosphere system, and we quantitatively interpreted this response process.