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The sequence of events associated with the triggering of energy release during substorm expansion phase onset is still not well‐understood. Oberhagemann and Mann (2020b, https://doi.org/10.1029/2019gl085271) proposed a new substorm onset mechanism, where the transition toward parallel proton pressure anisotropy during tail stretching in the late growth phase could trigger a pressure anisotropic ballooning instability. Here we examine the evolution of energetic proton parallel pressure anisotropy at geosynchronous altitudes, seeking evidence in support of the proposed substorm onset mechanism. We use the Geostationary Operational Environment Satellite (GOES) proton flux and magnetometer data combined with substorm onset indicators derived from ground‐based magnetometers. Superposed epoch analysis of substorm onset times for 2014 using the isolated substorm list (Ohtani & Gjerloev, 2020, https://doi.org/10.1029/2020ja027902) clearly shows signatures of energetic proton parallel pressure anisotropy immediately before substorm onset, potentially supportive of the Oberhagemann and Mann theory. Plain Language Summary: Substorms are disturbances in the nightside region of the geospace associated with the rapid release of stored magnetic energy. In the ionosphere, the signatures of this energy release are the spectacular dancing lights known as aurorae (northern and southern lights). The processes that lead to energy storage are well‐known. However, there are competing theories on what triggers the release of this significant amount of energy at substorm onset. According to a new substorm onset theory proposed by Oberhagemann and Mann, when the magnetic field stretches in the nightside during the energy storage, the pressure becomes more parallel to the magnetic field, leading to a ballooning instability at substorm onset. Here, we look for observational support for the association of such pressure profile at geosynchronous altitudes with substorm onset to examine the proposed model. Superposed epoch analysis of isolated substorms in 2014 shows increasing energetic proton parallel pressure anisotropy at the onset, providing evidence to support the Oberhagemann and Mann theory. Key Points: Oberhagemann and Mann theory proposes that proton parallel temperature anisotropy triggers ballooning instability leading to substorm onsetWe use pitch angle resolved energetic proton fluxes at geosynchronous altitudes seeking observational evidence in support of the modelSuperposed epoch analysis of isolated substorms shows signatures of increasing energetic proton parallel anisotropy which peaks near onset [ABSTRACT FROM AUTHOR]

Understanding intense electron precipitation is crucial for characterizing radiation belt loss and assessing related impacts on the atmosphere. We investigate the evolution of electron flux during an ensemble of 70 geomagnetic storms, focusing on equatorial and low‐Earth orbit observations of trapped and precipitating ∼30–100 keV energy electrons. We reveal that the most intense electron precipitation is associated with equatorial flux capping through self‐limiting processes, for example, as described theoretically by Kennel and Petschek (1966, https://doi.org/10.1029/jz071i001p00001). Our results indicate that the most intense electron precipitation is caused by electron injections associated with self‐limiting processes. Dawn side injections are observed to have fluxes that exceed the Kennel‐Petschek limit, consistent with the excitation of strong chorus waves and resulting in intense precipitation and return of the trapped flux to the Kennel‐Petschek limit. Our results clearly demonstrate the important role of self‐limiting processes in affecting the dynamics of newly injected electrons and driving intense electron precipitation. Plain Language Summary: In this study, we investigate the behavior of electrons in Earth's magnetosphere and how they impact the upper atmosphere during geomagnetic storms. We focus on electron precipitation events with energies that are often associated with pulsating aurora. Our findings show that during the most intense periods of electron precipitation, the electron flux in the equatorial region is associated with a natural process that caps it at an upper limit. We also reveal that electron injections occurring on the dawn side of the magnetosphere trigger these processes, leading to the most significant electron precipitation events occurring in this region. Our results highlight the importance of these natural self‐limiting processes in shaping the behavior of the radiation belts, and the potential impact of the related precipitation of these particles into the upper atmosphere. Understanding these processes is crucial for studying space weather and its potential effects on our planet's atmosphere and climate. Key Points: We investigate the evolution of electron flux and pitch angle distributions (PADs) during intense precipitation eventsLow Earth orbit PADs show strong pitch angle diffusion associated with intense precipitation at dawn side magnetic local timesBoth are associated with eventual flux capping at the Kennel‐Petschek limit after electron injections [ABSTRACT FROM AUTHOR]

Chorus waves play a key role in outer Van Allen electron belt dynamics through cyclotron resonance. Here, we use Van Allen Probes data to reveal a new and distinct population of intense chorus waves excited in the heart of the radiation belt during the main phase of geomagnetic storms. The power of the waves is typically ~ 2–3 orders of magnitude greater than pre-storm levels, and are generated when fluxes of ~ 10–100 keV electrons approach or exceed the Kennel–Petschek limit. These intense chorus waves rapidly scatter electrons into the loss cone, capping the electron flux to a value close to the limit predicted by Kennel and Petschek over 50 years ago. Our results are crucial for understanding the limits to radiation belt fluxes, with accurate models likely requiring the inclusion of this chorus wave-driven flux-limiting process, that is independent of the acceleration mechanism or source responsible for enhancing the flux. [ABSTRACT FROM AUTHOR]

In situ conjugate electromagnetic ion cyclotron (EMIC) waves observed by the Swarm mission in both hemispheres are presented. A complex and unusual pattern of Alfvénic EMIC wave energy is observed, with a mid‐latitude peak close to the source at L =3.3, as well as a secondary lower L‐peak. A wave propagation model reveals that the secondary peak at L =1.7 may be explained by wave power being redirected equatorward due to the Buchsbaum resonance, crossing and interfering with the same EMIC wave power propagating equatorwards from the opposite hemisphere. This interference creates a coherent equatorial driver for a low‐L field line resonance at the secondary peak, and which is associated with strong shear‐to‐fast mode coupling in the ionosphere. This behavior complicates the interpretation of low‐Earth orbit EMIC data for applications assessing radiation belt loss. Combined low Earth orbit observations and modeling enable these novel and localized magnetosphere‐ionosphere EMIC wave propagation pathways to be identified. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are important in near‐Earth space due to their role in reducing the amount of radiation in the Earth's radiation belts following geomagnetic storms. They are also often observed during non‐storm times. They are studied using satellites and ground observatories. Our paper reveals how these waves can follow complicated and previously unknown pathways to reach the upper atmosphere where they can be detected on the ground. This study shows a new and unusual effect where some EMIC wave energy is reflected and diverted toward the equator, where it meets its opposite‐hemisphere counterpart, interferes with it and sets up a resonance. This resonance then creates a new signal peak in the upper atmosphere at lower latitudes, far away from the location of the initial source. This presents a new and hitherto unseen pathway for wave energy to travel from their generation region in near‐Earth space down to the ionosphere. Understanding such pathways is very important for correctly diagnosing the location of these wave populations in space, and assessing their role in causing reductions in the levels of space radiation. Key Points: Electromagnetic ion cyclotron wave propagation from the magnetosphere to the ionosphere is complicated by reflection from the Buchsbaum resonance and interferenceWaves reflected from the Buchsbaum resonance interfere to generate a coherent driver for a secondary lower latitude field line resonanceThis generates a field‐guided secondary lower‐latitude peak associated with strong shear‐to‐fast mode energy conversion in the ionosphere [ABSTRACT FROM AUTHOR]

The flux of energetic electrons in the terrestrial Van Allen Belts varies by orders of magnitude during a magnetic storm. Here, we show how the dynamics of these electrons are clearly separated by energy into two distinct populations which are governed by different storm time behavior. We reveal how self‐limiting processes, described theoretically by Kennel and Petschek (1966), https://doi.org/10.1029/JZ071i001p00001, govern the energy‐dependent dynamics of the electrons. For terrestrial magnetic storms, accelerated electrons with energies below ∼850 keV quickly reach a maximum energy‐dependent differential flux level which is almost the same in every storm. Higher energy electrons typically do not reach a limiting flux and instead are governed by the differential impacts of competing acceleration and loss mechanisms. In general, electron fluxes saturate at the Kennel‐Petschek limit first at lower energies, impacting higher energies later in the storm. For the most intense storms, the maximum energy at which the fluxes are capped can also increase. Plain Language Summary: Very high energy electrons in near‐Earth space constitute a space radiation threat to spacecraft. All of this space radiation is traveling close to the speed of light. However, as we show here, nature remarkably imposes a limit to the number of these space radiation particles which can be generated once the acceleration is strong enough. At lower energies, where the numbers of particles are typically larger, we show that there is an absolute limit to the severity of the space radiation. That is, there is a natural limit to how bad this radiation can get. At higher energies, however, while there is still a theoretical limit it is almost never reached. Our results are based on the interpretation of a 50‐year‐old theory developed by Kennel and Petschek (1966), https://doi.org/10.1029/JZ071i001p00001 but which has been largely untested until now. None of the modern radiation belt models incorporate these effects, and our results provide an important input for both improved radiation specification and for assessing worst case space radiation threats. As we show here, depending on energy, it can be the best of times, and the worst of times, for the extremes of the level of space radiation reached during a space storm. Key Points: Superposed epoch analysis of 70 geomagnetic storms reveals drastically different behavior of radiation belt electrons depending on energyElectrons with energies below ∼850 keV are revealed to be strongly affected by self‐limiting Kennel‐Petschek processes in almost every stormGenerally, the flux of higher energy particles is not capped and only becomes self‐limited for the most intense and long‐lasting storms [ABSTRACT FROM AUTHOR]

This study examines the rapid losses and acceleration of trapped relativistic and ultrarelativistic electron populations in the Van Allen radiation belt during the September 7-9, 2017 geomagnetic storm. By analyzing the dynamics of the last closed drift shell (LCDS), and the electron flux and phase space density (PSD), we show that the electron dropouts are consistent with magnetopause shadowing and outward radial diffusion to the compressed LCDS. During the recovery phase, an in-bound pass of Van Allen Probe A shows an apparent local peak in PSD, which does not exist in reality. A careful analysis of the multipoint measurements by the Van Allen Probes reveals instead how the apparent PSD peak arises from aliasing monotonic PSD profiles which are rapidly increasing due to acceleration from very fast inwards radial diffusion. In the absence of such multisatellite conjunctions during fast acceleration events, such peaks might otherwise be associated with local acceleration processes. Plain Language Summary This study presents a thorough analysis of terrestrially trapped electron space radiation during the September 2017 geomagnetic storm. By analyzing the measurements of the trapped electron population, we show that the predominant loss of the relativistic and ultrarelativistic electrons depleted from the radiation belt at the beginning of the storm arises from outwards loss into the solar wind and not downwards loss into the atmosphere. We also reveal, for the first time, that the signatures of the acceleration processes which refill the belts, after such losses can occur on much faster timescales than previously thought. Moreover, signatures attributed to the actions of high-frequency plasma waves are actually caused by a different physical phenomenon known as the radial diffusion. The new knowledge of the very fast rate of change of the amount of electron space radiation points to an urgent need to evaluate the processes, which control belt dynamics. As we show here, this can be faster than the orbital period of monitoring satellites. Overall, we show how the limited satellite spatiotemporal coverage may mask and confuse the signatures of the physical processes responsible. [ABSTRACT FROM AUTHOR]

The Experimental Albertan satellite #1 (Ex-Alta 1) was a three-unit cube satellite (CubeSat) developed at the University of Alberta (UAlberta). As the first 'made in Alberta' spacecraft, the Ex-Alta 1 CubeSat was the Canadian contribution to the international QB50 CubeSat constellation mission (www.qb50.eu). Ex-Alta 1's mission incorporated four science objectives, two technical objectives, and one educational outreach objective. These are reviewed in the context of the Ex-Alta 1 mission design, as well as future CubeSat constellation missions that may follow demonstration missions such as Ex-Alta 1 and the experiences of the QB50 project. In support of its objectives, Ex-Alta 1 flew the multi-needle Langmuir probe (m-NLP) QB50 standardized payload, a UAlberta developed miniaturized digital fluxgate magnetometer (DFGM) mounted on a two-element articulated deployable boom, a radiation dosimeter from Teledyne, and an open source onboard computer (OBC) developed at UAlberta. Ex-Alta 1 was released by a NanoRacks deployer from the International Space Station on May 26, 2017, and commanded and controlled from a ground-station developed and operated at UAlberta. Ex-Alta 1 operated continuously throughout its mission until it burned up in the atmosphere on November 14, 2018. The last two-way contact occurred on that day at 07:49:29 UTC, when the spacecraft reported an onboard temperature of 47 °C in eclipse at an altitude of 152 km and which marked the effective end of the mission. This paper provides an overview of the Ex-Alta 1 mission, its development, the spacecraft and mission design, and reports preliminary results from the payload focusing on the DFGM instrument. We also report briefly on related education and outreach activities, including the development and delivery of curriculum-targeted and CubeSat focused classroom sessions inspired by the Ex-Alta 1 mission to school pupils in Alberta. [ABSTRACT FROM AUTHOR]

Recent observations demonstrating that auroral beads are observed prior to the majority of substorm onsets (Kalmoni et al., 2017, https://doi.org/10.1002/2016GL071826) have reinforced the potential importance of ballooning instabilities for near‐Earth magnetospheric substorm onset. Here we examine pressure anisotropic ballooning instabilities in stretched magnetotail geometries. Our results show that transition from an initially perpendicular anisotropy toward parallel anisotropy reduces the plasma β threshold for triggering a ballooning instability. Such increasingly parallel anisotropies can form as a direct consequence of tail stretching that occurs during the late substorm growth phase through the well‐known effects of drift shell splitting and the competition between betatron and Fermi processes in the tail. We propose such increasingly parallel ballooning triggers auroral substorm onset on field lines in the transition region between dipolar and tail‐like fields, consistent with observational constraints on the location of the onset arc with respect to the ion isotropy boundary in the magnetotail. Plain Language Summary: The aurorae, the northern and southern lights, occur when charged particles are accelerated along the magnetic field and impact the atmosphere. Here we provide a new potential explanation for a process that can trigger the rapid release of stored energy in the nightside of near‐Earth space. Such rapid releases of energy lead to the development of very spectacular, bright, and dancing displays of the aurorae in a process known as a substorm. While the processes responsible for powering this are well understood, why the lights begin to dance exactly when then do, a moment known as the onset of the substorm, is not. It is generally agreed that this onset occurs after a period of stretching of the magnetic field on the nightside of Earth. Our new model suggests that as this stretching occurs, there is more pressure parallel to the magnetic field than across it in a confined region of the field. This leads to an instability, called a ballooning instability, that explosively releases energy in the magnetic field, leading to a substorm. The confined region of the field that our work predicts should be associated with onset agrees well with known locations where these bright substorm aurorae begin. Key Points: We propose a new paradigm for substorm onset, where a ballooning instability is triggered by pressure anisotropy becoming more parallelThis increasingly parallel pressure anisotropy occurs as a direct consequence of tail stretching during the substorm growth phaseOur paradigm predicts increasingly parallel temperature anisotropies pre‐onset, and onset locations in the transition region, as observed [ABSTRACT FROM AUTHOR]

Recent observations demonstrating that auroral beads are observed prior to the majority of substorm onsets (Kalmoni et al., 2017, https://doi.org/10.1002/2016GL071826) have reinforced the potential importance of ballooning instabilities for near‐Earth magnetospheric substorm onset. Here we examine pressure anisotropic ballooning instabilities in stretched magnetotail geometries. Our results show that transition from an initially perpendicular anisotropy toward parallel anisotropy reduces the plasma β threshold for triggering a ballooning instability. Such increasingly parallel anisotropies can form as a direct consequence of tail stretching that occurs during the late substorm growth phase through the well‐known effects of drift shell splitting and the competition between betatron and Fermi processes in the tail. We propose such increasingly parallel ballooning triggers auroral substorm onset on field lines in the transition region between dipolar and tail‐like fields, consistent with observational constraints on the location of the onset arc with respect to the ion isotropy boundary in the magnetotail. Plain Language Summary: The aurorae, the northern and southern lights, occur when charged particles are accelerated along the magnetic field and impact the atmosphere. Here we provide a new potential explanation for a process that can trigger the rapid release of stored energy in the nightside of near‐Earth space. Such rapid releases of energy lead to the development of very spectacular, bright, and dancing displays of the aurorae in a process known as a substorm. While the processes responsible for powering this are well understood, why the lights begin to dance exactly when then do, a moment known as the onset of the substorm, is not. It is generally agreed that this onset occurs after a period of stretching of the magnetic field on the nightside of Earth. Our new model suggests that as this stretching occurs, there is more pressure parallel to the magnetic field than across it in a confined region of the field. This leads to an instability, called a ballooning instability, that explosively releases energy in the magnetic field, leading to a substorm. The confined region of the field that our work predicts should be associated with onset agrees well with known locations where these bright substorm aurorae begin. Key Points: We propose a new paradigm for substorm onset, where a ballooning instability is triggered by pressure anisotropy becoming more parallelThis increasingly parallel pressure anisotropy occurs as a direct consequence of tail stretching during the substorm growth phaseOur paradigm predicts increasingly parallel temperature anisotropies pre‐onset, and onset locations in the transition region, as observed [ABSTRACT FROM AUTHOR]

Abstract: The electrodynamics associated with dual discrete arc aurora with antiparallel flow along the arcs were observed nearly simultaneously by the enhanced Polar Outflow Probe (e‐POP) and the Swarm A and C spacecraft. Auroral imaging from e‐POP reveals 1–10 km structuring of the arcs, which move and evolve on second timescales and confound the traditional single‐spacecraft field‐aligned current algorithms. High‐cadence magnetic data from e‐POP show 1–10 Hz, inferred Alfvénic, perturbations coincident with and at the same scale size as the observed dynamic auroral fine structures. High‐cadence electric and magnetic field data from Swarm A reveal nonstationary electrodynamics involving reflected and interfering Alfvén waves and modulation consistent with trapping in the ionospheric Alfvén resonator (IAR). These observations suggest a role for Alfvén waves, perhaps also the IAR, in discrete arc dynamics on 0.2–10 s timescales and ~1–10 km spatial scales and reinforce the importance of considering Alfvén waves in magnetosphere‐ionosphere coupling. [ABSTRACT FROM AUTHOR]

On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

This review describes the infrastructure and capabilities of the expanded and upgraded Canadian Array for Realtime InvestigationS of Magnetic Activity (CARISMA) magnetometer array in the era of the THEMIS mission. Formerly operated as the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer array until 2003, CARISMA capabilities have been extended with the deployment of additional fluxgate magnetometer stations (to a total of 28), the upgrading of the fluxgate magnetometer cadence to a standard data product of 1 sample/s (raw sampled 8 samples/s data stream available on request), and the deployment of a new network of 8 pairs of induction coils (100 samples per second). CARISMA data, GPS-timed and backed up at remote field stations, is collected using Very Small Aperture Terminal (VSAT) satellite internet in real-time providing a real-time monitor for magnetic activity on a continent-wide scale. Operating under the magnetic footprint of the THEMIS probes, data from 5 CARISMA stations at 29–30 samples/s also forms part of the formal THEMIS ground-based observatory (GBO) data-stream. In addition to technical details, in this review we also outline some of the scientific capabilities of the CARISMA array for addressing all three of the scientific objectives of the THEMIS mission, namely: 1. Onset and evolution of the macroscale substorm instability, 2. Production of storm-time MeV electrons, and 3. Control of the solar wind-magnetosphere coupling by the bow shock, magnetosheath, and magnetopause. We further discuss some of the compelling questions related to these three THEMIS mission science objectives which can be addressed with CARISMA. [ABSTRACT FROM AUTHOR]

An investigation into how discrete frequency fast mode waves propagating sunward from the magnetotail may drive field line resonant Alfvén waves on closed field lines on the afternoon flank of the Earth's magnetosphere is presented. Due to the morphology of the magnetic field lines on the flank, higher latitude poleward boundary auroral field lines map deeper into the magnetotail, implying that phase lags from mode coupling to sunward propagating fast mode waves in this stretched field geometry can overcome the standard poleward phase propagation of typical field line resonances, producing resonant Alfvén wave-driven discrete auroral arcs that propagate equatorward on closed field lines. A comparison is made between the predicted results and magnetometer, all sky camera and meridian scanning photometer observations of multiple east-west aligned discrete auroral arcs that propagate equatorward and sunward from the poleward boundary of the dayside auroral oval between 1400 and 1500 UT (approximately 1600 to 1700 MLT) on 3 January 1998. From this comparison, the conclusion is reached that the observations are consistent with the theoretical picture of discrete auroral arcs being driven by Alfvénic waves on closed field lines, the Alfvén waves being excited by discrete frequency fast mode waves propagating sunward from the magnetotail in the flank waveguide of the outer magnetosphere. [ABSTRACT FROM AUTHOR]

Using the total radiation belt electron content calculated from Van Allen Probe phase space density, the time-dependent and global response of the outer radiation belt during storms is statistically studied. Using phase space density reduces the impacts of adiabatic changes in the main phase, allowing a separation of adiabatic and nonadiabatic effects and revealing a clear modality and repeatable sequence of events in storm time radiation belt electron dynamics. This sequence exhibits an important first adiabatic invariant (μ)-dependent behavior in the seed (150 MeV/G), relativistic (1,000 MeV/G), and ultrarelativistic (4,000 MeV/G) populations. The outer radiation belt statistically shows an initial phase dominated by loss followed by a second phase of rapid acceleration, while the seed population shows little loss and immediate enhancement. The time sequence of the transition to the acceleration is also strongly μ dependent and occurs at low μ first, appearing to be repeatable from storm to storm. [ABSTRACT FROM AUTHOR]

We use multi-satellite and ground-based magnetic data to investigate the concurrent characteristics of Pc3 (22- 100 mHz) and Pc4-5 (1-22 mHz) ultra-low-frequency (ULF) waves on the 31 October 2003 during the Halloween magnetic superstorm. ULF waves are seen in the Earth's magnetosphere, topside ionosphere, and Earth's surface, enabling an examination of their propagation characteristics. We employ a time-frequency analysis technique and examine data from when the Cluster and CHAMP spacecraft were in good local time (LT) conjunction near the dayside noon-midnight meridian. We find clear evidence of the excitation of both Pc3 and Pc4-5 waves, but more significantly we find a clear separation in the L shell of occurrence of the Pc4-5 and Pc3 waves in the equatorial inner magnetosphere, separated by the density gradients at the plasmapause boundary layer. A key finding of the wavelet spectral analysis of data collected from the Geotail, Cluster, and CHAMP spacecraft and the CARISMA and GIMA magnetometer networks was a remarkably clear transition of the waves' frequency into dominance in a higher-frequency regime within the Pc3 range. Analysis of the local field line resonance frequency suggests that the separation of the Pc4-5 and Pc3 emissions across the plasmapause is consistent with the structure of the inhomogeneous field line resonance Alfvén continuum. The Pc4-5 waves are consistent with direct excitation by the solar wind in the plasma trough, as well as Pc3 wave absorption in the plasmasphere following excitation by upstream waves originating at the bow shock in the local noon sector. However, despite good solar wind coverage, our study was not able to unambiguously identify a clear explanation for the sharp universal time (UT) onset of the discrete frequency and largeamplitude Pc3 wave power. [ABSTRACT FROM AUTHOR]

Space-based measurements of Earth's magnetic field are required to understand the plasma processes responsible for energising particles in the Van Allen radiation belts and influencing space weather. This paper describes a prototype fluxgate magnetometer instrument developed for the proposed Canadian Space Agency's (CSA) Outer Radiation Belt Injection, Transport, Acceleration and Loss Satellite (ORBITALS) mission and which has applications in other space and suborbital applications. The magnetometer is designed to survive and operate in the harsh environment of Earth's radiation belts and measure low-frequency magnetic waves, the magnetic signatures of current systems, and the static background magnetic field. The new instrument offers improved science data compared to its predecessors through two key design changes: direct digitisation of the sensor and digital feedback from two cascaded pulse-width modulators combined with analog temperature compensation. These provide an increase in measurement bandwidth up to 450Hz with the potential to extend to at least 1500Hz. The instrument can resolve 8 pT on a 65 000 nT field with a magnetic noise of less than 10 pT/ √Hz at 1Hz. This performance is comparable with other recent digital fluxgates for space applications, most of which use some form of sigma-delta (ΣΔ ) modulation for feedback and omit analog temperature compensation. The prototype instrument was successfully tested and calibrated at the Natural Resources Canada Geomagnetics Laboratory. [ABSTRACT FROM AUTHOR]

Space-based measurements of the Earth's magnetic field are required to understand the plasma processes responsible for energizing particles in the Van Allen radiation belts and influencing space weather. This paper describes a prototype fluxgate magnetometer instrument developed for the proposed Canadian Space Agency (CSA) Outer Radiation Belt Injection, Transport, Acceleration and Loss Satellite (ORBITALS) mission and which has applications in other space and suborbital applications. The magnetometer is designed to survive and operate in the harsh environment of the Earth's radiation belts and measure low-frequency magnetic waves, the magnetic signatures of current systems, and the static background magnetic field. The new instrument offers improved science data compared to its predecessors through two key design changes: direct digitisation of the sensor and digital feedback combined with analog temperature compensation. These provide an increase in measurement bandwidth up to 450 Hz with the potential to extend to at least 1500 Hz. The instrument can resolve 8 pT on a 65 000 nT field with a magnetic noise of less than 10 pT per square-root Hz at 1 Hz. The prototype instrument was successfully tested and calibrated at the Natural Resources Canada Geomagnetics Laboratory showing that the mostly-digital design matches or exceeds its radiation-soft analog predecessor in sensitivity, noise, frequency range, and RMS accuracy. [ABSTRACT FROM AUTHOR]

Hybrid Vlasov-Fourier modeling is used to investigate the nonlinear evolution of electromagnetic ion cyclotron (EMIC) waves driven by proton temperature anisotropy in plasmas with a population of He+ ions and a cold proton background. In the pure proton-electron plasma, most of the free energy is converted into high-amplitude waves and currents. In the nonlinear stage, within a few hundred proton gyroperiods after the saturation, the wave spectrum shifts toward lower wave numbers and frequencies, from ω∼0.6Ωp to below ω∼0.25Ωp. In the presence of even a small population of He+ ions almost all of the free energy is used in He+ heating. The wave activity in the saturated state moves from the linearly unstable upper branch to the linearly stable lower one. In the presence of a background of cold protons, the waves can propagate in the frequency stop-band. Our results demonstrate that linear stability theory cannot be used to estimate the characteristics of the expected saturated wave spectra in the terrestrial magnetosphere. Significantly, our nonlinear simulations produce wave spectra which are in close agreement with the EMIC waves observed in situ by satellites as well as by ground-based magnetometers positioned at the ends of the magnetic field lines. [ABSTRACT FROM AUTHOR]

Pi2s are a category of Ultra Low Frequency (ULF) waves associated with the onset of magnetic substorms. Recent work has suggested that the deceleration of bulk plasma flows in the central plasmasheet, known as bursty bulk flows (BBFs), are able to directly-drive Pi2 oscillations. Some of these studies have further shown evidence that there is a one-to-one correlation between Pi2 magnetic waveforms observed on the ground and periodic peaks in flow velocity within the BBF, known as flow bursts. Utilising a favourable conjunction between the Geotail spacecraft and the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) magnetometer array on 31 May 1998, we examine the causality of the link between BBF flow bursts and Pi2 waveforms. Using a series of analytical tests in both the time and frequency domains, we find that while the Pi2 and BBF waveforms are very similar, the ground response for this event occurs prior to the observed flow enhancements in the magnetotail. We conclude that during this specific case study the temporal variations of the flow bursts within the BBF are not directly-driving the observed ground-based Pi2 waveforms, despite the fact that a visual inspection of both time-series might initially suggest that there is a causal relationship. We postulate that rather than there being a direct causal relation, the similar waveforms observed in both Pi2s and BBFs may result from temporal variations in a common source for both the BBFs and the Pi2s, such as magnetic reconnection in the tail, this source modulating both the Pi2 and BBF at the same frequency. [ABSTRACT FROM AUTHOR]

We examined the growth and propagation of fine‐structured electromagnetic ion cyclotron (EMIC) waves related to time‐varying density irregularities using multipoint measurement data observed by Arase, Van Allen Probe A, and two ground‐based induction magnetometers (Gakona and Dawson) during a field line conjunction event on April 18, 2019. We analyzed the wave data obtained by the aforementioned spacecraft and stations, and found that the appearance of fine structures in the observed EMIC waves clearly coincided with the ambient electron density irregularities in the magnetosphere, which can cause periodic wave growth and waveguiding on their propagation. Furthermore, we found that the latitudinal widths of the EMIC wave activity region and the wave propagation duct were ∼185 km and less than 80 km at an auroral altitude of 100 km, respectively. We also found thermal ion heating (<200 $< 200$ eV/q) during the EMIC wave activity. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are an important plasma waves that control energetic ion and relativistic electron precipitations in the terrestrial inner magnetosphere. We investigated the growth and propagation of fine‐structured EMIC waves observed simultaneously by two spacecraft (Japanese Arase and U.S. Van Allen Probe A) and two ground stations (Gakona and Dawson). The two spacecraft orbited along the same field line, and we measured the same fine‐structured EMIC waves in different geomagnetic latitude regions. The same EMIC waves were observed at the two ground stations located near the magnetic footprints of the two spacecraft. We report that the periodic growth of the observed EMIC waves was associated with the time‐varying electron density irregularity in the midlatitude region. Additionally, we analyzed the spatial scale of the observed EMIC waves and their contribution to the thermal ion heating. Key Points: The simultaneous multipoint observation of fine‐structured electromagnetic ion cyclotron (EMIC) waves was achieved by Arase, Van Allen Probe, and two ground‐based stationsThe growth of the observed fine‐structured EMIC waves was associated with ambient electron density irregularities in the midlatitudesWe investigated the spatial scale of the observed EMIC waves and their contribution to ion heating by utilizing the multipoint measurement [ABSTRACT FROM AUTHOR]

We present new and previously unreported in situ observations of Hertz frequency multiharmonic mode field line resonances detected by the Electric Field and Waves instrument on‐board the NASA Van Allen probes during low‐L perigee passes. Spectral analysis of the spin‐plane electric field data reveals the waves in numerous perigee passes, in sequential passes of probes A and B, and with harmonic frequency structures from ∼0.5 to 3.5 Hz which vary with L‐shell, altitude, and from day‐to‐day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model, we conclude that the waves are standing Alfvén field line resonances and that only odd‐mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics. Key Points: We present previously unreported low‐L (L ∼ 1.2) Hertz frequency multiharmonic field line resonances observed in the electric field spectraThe field line resonance harmonics were regularly observed by the Van Allen probes and have frequencies consistent with odd harmonic modesThe observed harmonic frequencies are compared to models using empirical density profiles, offering a new diagnostic for density dynamics [ABSTRACT FROM AUTHOR]

Geospace magnetic storms, driven by the solar wind, are associated with increases or decreases in the fluxes of relativistic electrons in the outer radiation belt. We examine the response of relativistic electrons to four intense magnetic storms, during which the minimum of the Dst index ranged from -105 to -387 nT, and compare these with concurrent observations of ultra-low-frequency (ULF) waves from the trans-Scandinavian IMAGE magnetometer network and stations from multiple magnetometer arrays available through the worldwide SuperMAG collaboration. The latitudinal and global distribution of Pc5 wave power is examined to determine how deep into the magnetosphere these waves penetrate. We then investigate the role of Pc5 wave activity deep in the magnetosphere in enhancements of radiation belt electrons population observed in the recovery phase of the magnetic storms. We show that, during magnetic storms characterized by increased post-storm electron fluxes as compared to their pre-storm values, the earthward shift of peak and inner boundary of the outer electron radiation belt follows the Pc5 wave activity, reaching L shells as low as 3-4. In contrast, the one magnetic storm characterized by irreversible loss of electrons was related to limited Pc5 wave activity that was not intensified at low L shells. These observations demonstrate that enhanced Pc5 ULF wave activity penetrating deep into the magnetosphere during the main and recovery phase of magnetic storms can, for the cases examined, distinguish storms that resulted in increases in relativistic electron fluxes in the outer radiation belts from those that did not. [ABSTRACT FROM AUTHOR]

Magnetospheric ultra-low frequency (ULF) oscillations in the Pc 4-5 frequency range play an important role in the dynamics of Earth's radiation belts, both by enhancing the radial diffusion through incoherent interactions and through the coherent drift-resonant interactions with trapped radiation belt electrons. The statistical distributions of magnetospheric ULF wave power are known to be strongly dependent on solar wind parameters such as solar wind speed and interplanetary magnetic field (IMF) orientation. Statistical characterisation of ULF wave power in the magnetosphere traditionally relies on average solar wind-IMF conditions over a specific time period. In this brief report, we perform an alternative characterisation of the solar wind infiuence on magnetospheric ULF wave activity through the characterisation of the solar wind driver by its variability using the standard deviation of solar wind parameters rather than a simple time average. We present a statistical study of nearly one solar cycle (1996-2004) of geosynchronous observations of magnetic ULF wave power and find that there is significant variation in ULF wave powers as a function of the dynamic properties of the solar wind. In particular, we find that the variability in IMF vector, rather than variabilities in other parameters (solar wind density, bulk velocity and ion temperature), plays the strongest role in controlling geosynchronous ULF power. We conclude that, although time-averaged bulk properties of the solar wind are a key factor in driving ULF powers in the magnetosphere, the solar wind variability can be an important contributor as well. This highlights the potential importance of including solar wind variability especially in studies of ULF wave dynamics in order to assess the efficiency of solar wind-magnetosphere coupling. [ABSTRACT FROM AUTHOR]

On 16 July 2008, two pairs of consecutive bursts of Pi2 pulsations were recorded simultaneously across the THEMIS ground-based observatory system. Wavelet transformation reveals that for each high-latitude pair, the dominant frequency of the first burst is higher than that of the second. But at low latitudes, the dominant frequency does not change. It is suggested that both pairs result from fast magnetospheric cavity waves with the second burst also containing shear Alfvén waves. INTERMAGNET magnetograms at auroral latitudes showed magnetic variations affected by two recurrent electrojets for each pair. The ground-based magnetometers and those at geostationary orbit sensed magnetic perturbations consistent with the formation of the substorm current wedge. Four consecutive enhancements of energetic electron and ion fluxes detected by the THEMIS probes in the dayside magnetosphere appeared in the later afternoon and then in the early afternoon. The horizontal magnetic variation vectors had vortex patterns similar to those induced by the upward and downward field-aligned currents during substorm times. The hodogram at mid-L stations had a polarization pattern similar to the one induced by the substorm current wedge for each Pi2 burst. The mapping of ground Pi2 onset timing to the interplanetary magnetic field (IMF) observations shows that they appear under two cycles of north-to-south and then north variation. CLUSTER 4 in the south lobe observed wave-like magnetic fluctuations, probably driven by near-Earth reconnection, similar to those on the ground. These two observations are consistent with the link of double-onset substorms to magnetotail reconnection externally triggered by IMF variations. [ABSTRACT FROM AUTHOR]

The QB50 mission is a satellite constellation designed to carry out measurements at between 200–380 km altitude in the ionosphere. The multi-needle Langmuir probe (m-NLP) instrument has been mounted on board eleven QB50 satellites in order to characterize ambient plasma. The distinct feature of this instrument is its capability of measuring the plasma density at high spatial resolution without the need to know the electron temperature or the spacecraft potential. While the instrument has been deployed on many sounding rockets, the QB50 satellites offer the opportunity to demonstrate the operation of the instrument in low-earth orbit (LEO). This paper provides a brief review of the m-NLP instrument specifically designed for the QB50 mission and the case studies of the instrument's performance on board the Ex-Alta 1 and Hoopoe satellites. The system has also been functionally verified in a plasma chamber at the European Space Research and Technology Center (ESTEC). Although the QB50 mission's scientific goals have not been reached yet and some uncertainties still remain, there are some optimistic in-orbit preliminary results which could be helpful for the system improvement in future campaigns. Particularly, the electron emitter as part of the m-NLP science unit has demonstrated its capability in the plasma chamber and in orbit to mitigate spacecraft charging effects. [ABSTRACT FROM AUTHOR]

Electron flux measurements are an important diagnostic for interactions between ultralow‐frequency (ULF) waves and relativistic (∼1 MeV) electrons. Since measurements are collected by particle detectors with finite energy channel width, they are affected by a phase mixing process that can obscure these interactions. We demonstrate that ultrahigh‐resolution electron measurements from the Magnetic Electron Ion Spectrometer on the Van Allen Probes mission—obtained using a data product that improves the energy resolution by roughly an order of magnitude—are crucial for understanding ULF wave‐particle interactions. In particular, the ultrahigh‐resolution measurements reveal a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, the standard measurements provide estimates for the ULF flux modulation amplitude, period, and phase that may not be representative of true flux modulations, potentially leading to ambiguous conclusions concerning electron dynamics. Plain Language Summary: Plasma waves with periods of a few minutes interact with high‐energy electrons in the near‐Earth space environment. Understanding how the waves affect the electrons is important, as these electrons can potentially damage orbiting satellites. However, these interactions are obscured by a process known as phase mixing where electrons with different energies behave differently yet are mixed together in measurements that have finite energy resolution. We show how a unique data set obtained from National Aeronautics and Space Administration's Van Allen Probes mission can dramatically increase the energy resolution, revealing a range of interactions that are obscured by measurements with lower energy resolution. Key Points: Previous electron measurements could not resolve many energy‐dependent ULF wave interactions with >∼1 MeV electrons due to phase mixingMagEIS histogram channels resolve many interactions: dE/E of 2‐3% compared to standard 20‐30% that is already considered high resolutionWithout ultrahigh‐resolution measurements, ambiguous conclusions may be reached concerning >∼1 MeV electron dynamics (e.g., drift resonance) [ABSTRACT FROM AUTHOR]