Search

Using two separate databases of in situ ionospheric observations, we present case studies and perform a statistical investigation of the link between energetic precipitating particles during the polar night and high-latitude F-region steepening density spectra. Our study covers approximately 3 years of data obtained near the peak of the 24th solar cycle from four Defense Meteorological Satellite Program satellites and from the European Space Agency's Swarm satellites. Focusing on the midnight sector of the auroral oval, we found that there is a near-perfect co-location between high-energy precipitating particles and occurrence of dissipating F-region plasma density spectra. This is because the precipitating energy flux strongly enhances the E-region Pedersen conductivity, allowing fast and efficient dissipation of kilometer-scale F-region irregularities. Spectra that are possibly non-dissipating are in turn co-located with the distribution of soft electron precipitation. Together, dissipating and non-dissipating density spectra constitute two distinct irregularity regimes. Surprisingly, we also found that efficient dissipation notwithstanding, high-energy precipitating particles cause a net increase in the F-region irregularity power, suggesting that growth and dissipation are interlinked and that some of the observed F-region irregularities may conceivably be generated in the E region. This work is expected to be beneficial for the classification of F-region in situ density spectra and suggests that such density spectra can be used to infer the presence of high-energy or low-energy precipitations based on spectral properties. [ABSTRACT FROM AUTHOR]

Southward component of Interplanetary Magnetic Field (IMF) leads to the entry of energetic particles of solar origin into the magnetosphere. A part of this energy is subsequently released from the night-side as a geomagnetic substorm via the loading and unloading process in the magnetotail. However, other than this classical understanding of substorm, there exists an alternate particle entry mechanism, which mainly triggers weak to moderate substorms. So far, there is no clear understanding of how an intense substorm can be triggered without appreciable southward turning of IMF. In this paper, we have carried out a detailed multi-instrument analysis of a very strong substorm associated with the impingement of solar wind pressure impulse that took place on 21st January 2005 when IMF was not southward. It is observed that during the initial ~10 minutes of the substorm onset, the AL index reached ~–2000 nT, with particle precipitation being centred near dawn and not midnight. The usual substorm expansion phase with typical signatures of Near-Earth-Neutral-Line (NENL) magnetic reconnection, dipolarization and plasmoid formation started when IMF turned southward after ~10 minutes of the first onset on this day. Using the present case study, we have tried to explain what are the characteristics of shock-triggered substorms and where the energy comes from to power such strong events. The dominance of dawn centered precipitation during the initial phase can be attributed to increased particle interaction with the ULF waves enhanced due to prevalent K–H instability caused by strong velocity shear in the dawn sector. [ABSTRACT FROM AUTHOR]

The diffuse aurora is a natural phenomenon observed over the Earth's polar region. Compared with the nightside diffuse aurora, the brightness of the dayside diffuse aurora (0600–1800 magnetic local time (MLT)) is relatively weak, thus requiring more stringent observation conditions. Therefore, the current understanding of what causes the dayside diffuse aurora is still quite limited. Here, we present an intense morningside diffuse aurora (0600–1000 MLT) event observed on 1 January 2016 during the recovery phase of the substorm, using conjugate observations of wave and particle spectrum from the Radiation Belt Storm Probes and auroral emission from the Special Sensor Ultraviolet Spectrographic Imagers on the Air Force Defense Meteorological Satellite Program (DMSP/SSUSI). We perform calculations of diffusion coefficients and simulations of the electron fluxes for this event. Our results show that the chorus waves are the primary contributors to the formation of the morningside diffuse aurora, with precipitated electron energies ranging from a few keV to tens of keV. The lower band chorus shows significant pitch angle scattering efficiency for electrons with energies from 5 to 20 keV. The upper band chorus waves induce acceleration effects on 1–20 keV electrons. We suggest that the upper band chorus waves accelerate low‐energy electrons to higher energies, enabling them to engage in the scattering process of the lower band chorus waves. Our study makes a contribution to recent research on the formation mechanisms of diffuse aurora and deepens our understanding of wave‐particle interactions leading to dayside electron precipitation. Key Points: We present observations of an intense morningside diffuse aurora accompanied by chorus wave activity during disturbed geomagnetic conditionsLower band chorus waves scatter electrons with energies >5 keV during this event, causing morningside diffuse auroraUpper‐band chorus waves accelerate 1–20 keV electrons, and therefore also participate in the scattering process [ABSTRACT FROM AUTHOR]

The dayside particle precipitation during the passage of a solar wind rotational discontinuity (RD) event on 10 April 2015 is examined and reviewed. The RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. Particle precipitation data from DMSP F17 reveal the formation of an unusual boundary layer where the low energy (cold) ions exhibit energy-latitude dispersion that is usually associated with mantle while the high energy (hot) ions look like typical magnetospheric ions. DMSP F17 and F19 observe a double cusp that is a signature of magnetic reconnection occurring at both high- and low-latitudes due to the dominant IMF By. A global MHD simulation of the event supports the existence of the simultaneous reconnections at high- and low-latitude magnetopause that are consistent with the anti-parallel and component merging models, respectively. Finally, Cluster C2, located at high-latitude and high-altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak-to-peak amplitudes >800 km s-1 as it crosses the magnetopause. Guided by the MHD simulation, the Cluster observation can be interpreted as the spacecraft crossing reconnection outflows while moving from one side of the X-line to the other. [ABSTRACT FROM AUTHOR]

The Earth's atmosphere is influenced by energetic electrons coming from the magnetosphere. This energetic electron precipitation (EEP) is energized by the solar wind and directly affects in the high‐latitude mesosphere and lower thermosphere (MLT). EEP forms odd nitrogen (NOx) and hydrogen oxides (HOx) which destroy ozone. During winter EEP‐NOx descends to the stratosphere, establishing the indirect EEP effect. Several studies have found that EEP is related to changes in temperature and winds in the northern winter stratosphere. One of the most prominent effects of EEP is the influence on the northern polar vortex, a westerly wind system surrounding the winter pole in the middle atmosphere. Most studies of the EEP effect on dynamical features of the middle atmosphere have relied on either model simulations or reanalysis datasets which are mainly limited to stratospheric heights. We study here EEP effects on chemical and dynamical properties of the stratosphere and mesosphere in the northern hemisphere by using EOS Aura satellite's measurements of atmospheric properties and POES satellites' measurements of precipitating electrons. We confirm earlier results showing that EEP decreases ozone and affects the temperature in the polar middle atmosphere and strengthens the stratospheric polar vortex. We show that EEP weakens the mesospheric polar vortex in late winter. This effect on polar vortex is partly due to changes in propagation and convergence of planetary waves. Accordingly, the EEP effect on the northern polar vortex depends on planetary waves not only in the stratosphere, as found in earlier studies, but also in the mesosphere. Plain Language Summary: Electrons precipitate from space to the Earth's atmosphere when a plasma flow from the Sun, the solar wind, interacts with the Earth's magnetosphere. Energetic electron precipitation (EEP) affects the high‐latitude atmosphere by producing odd nitrogen and hydrogen oxides that destroy ozone and, thereby, affect the temperature. Several studies have shown that EEP strengthens the northern polar vortex, a westerly wind system surrounding the winter pole, and that this EEP effect depends on atmospheric disturbances called the planetary waves. Earlier observational studies of the EEP effect on the dynamical properties have relied on reanalysis datasets which are limited to stratospheric altitudes. We study here the EEP effect on chemical and dynamical properties of the northern winter atmosphere with satellite measurements. We confirm that the EEP strengthens the polar vortex in the stratosphere, but find that EEP weakens the polar vortex in the mesosphere in late winter. The effect of EEP on both mesospheric and stratospheric polar vortex is shown to depend on distribution of planetary waves. Since variations of the polar vortex also extend to surface level, an accurate information of the EEP effect can benefit long‐term predictions of wintertime weather and climate in the northern hemisphere. Key Points: We use data of Aura and POES satellites to study atmospheric variability related to electron precipitation in the northern hemisphereEnergetic electron precipitation enhances the stratospheric polar vortex and this effect depends on planetary wave distributionMesospheric part of the polar vortex is weakened by electron precipitation and this effect is also modulated by planetary waves [ABSTRACT FROM AUTHOR]

Energetic particle precipitation is the major source of electron production that controls the ionospheric Pedersen and Hall conductances at high latitudes. Many studies use empirical formulas to estimate conductances. The particle precipitation spectra measured by the Defense Meteorological Satellite Program (DMSP) Special Sensor J are often used as the input to the empirical formulas. In this study, we evaluate the empirical formulas of ionospheric conductances during four different types of auroral precipitation conditions based on 63 conjugate events observed by DMSP and EISCAT. The conductances calculated from the DMSP data with the empirical formulas are compared with those based on EISCAT measurements with the standard equations. The best correlation between these two is found when the empirical Robinson formulas (Robinson et al., 1987, https://doi.org/10.1029/ja092ia03p02565) are used in the presence of diffuse electron precipitation without ions. In the presence of ion precipitation, the correlation coefficients are smaller, but the correlation improves when the Galand formulas (Galand & Richmond, 2001, https://doi.org/10.1029/1999ja002001) are used to estimate the contribution of ion precipitation to the conductances. We also found that pure ion precipitation can cause the increase of conductances up to 2–7 S for Pedersen and 2.5–10 S for Hall conductances, which is positively correlated with the auroral electrojet index. Overall, the empirical formulas applied to the DMSP particle spectra underestimate the ionospheric conductances. Key Points: Performance of the Robinson and Galand formulas with the input of Defense Meteorological Satellite Program (DMSP) measurements is evaluated during different auroral conditionsRobinson formulas with the DMSP data underestimate the conductances for electron precipitationUsing Galand formulas for ions together with Robinson formulas for electrons improves the conductance estimates for dusk diffuse precipitation [ABSTRACT FROM AUTHOR]

Abstract Energetic electron precipitation (EEP) from the radiation belts into Earth's atmosphere leads to several profound effects (e.g., enhancement of ionospheric conductivity, possible acceleration of ozone destruction processes). An accurate quantification of the energy input and ionization due to EEP is still lacking due to instrument limitations of low‐Earth‐orbit satellites capable of detecting EEP. The deployment of the Electron Losses and Fields InvestigatioN (ELFIN) CubeSats marks a new era of observations of EEP with an improved pitch‐angle (0°–180°) and energy (50 keV–6 MeV) resolution. Here, we focus on the EEP recorded by ELFIN coincident with electromagnetic ion cyclotron (EMIC) waves, which play a major role in radiation belt electron losses. The EMIC‐driven EEP (∼200 keV–∼2 MeV) exhibits a pitch‐angle distribution (PAD) that flattens with increasing energy, indicating more efficient high‐energy precipitation. Leveraging the combination of unique electron measurements from ELFIN and a comprehensive ionization model known as Boulder Electron Radiation to Ionization (BERI), we quantify the energy input of EMIC‐driven precipitation (on average, ∼3.3 × 10−2 erg/cm2/s), identify its location (any longitude, 50°–70° latitude), and provide the expected range of ion‐electron production rate (on average, 100–200 pairs/cm3/s), peaking in the mesosphere—a region often overlooked. Our findings are crucial for improving our understanding of the magnetosphere‐ionosphere‐atmosphere system as they accurately specify the contribution of EMIC‐driven EEP, which serves as a crucial input to state‐of‐the‐art atmospheric models (e.g., WACCM) to quantify the accurate impact of EMIC waves on both the atmospheric chemistry and dynamics.

The low-electron flux variability (increase/decrease) in the Earth's radiation belts could cause low-energy Electron Precipitation (EP) to the atmosphere over auroral and South American Magnetic Anomaly (SAMA) regions. This EP into the atmosphere can cause an extra upper atmosphere's ionization, forming the auroral-type sporadic E layers (Esa) over these regions. The dynamic mechanisms responsible for developing this Esa layer over the auroral region have been established in the literature since the 1960s. In contrast, there are several open questions over the SAMA region, principally due to the absence (or contamination) of the inner radiation belt and EP parameter measurements over this region. Generally, the Esa layer is detected under the influence of geomagnetic storms during the recovery phase, associated with solar wind structures, in which the time duration over the auroral region is considerably greater than the time duration over the SAMA region. The inner radiation belt's dynamic is investigated during a High-speed Solar wind Stream (September 24-25, 2017), and the hiss wave-particle interactions are the main dynamic mechanism able to trigger the Esa layer's generation outside the auroral oval. This result is compared with the dynamic mechanisms that can cause particle precipitation in the auroral region, showing that each region presents different physical mechanisms. Additionally, the difference between the time duration of the hiss wave activities and the Esa layers is discussed, highlighting other ingredients mandatory to generate the Esa layer in the SAMA region. [ABSTRACT FROM AUTHOR]

Simultaneous observations from Defense Meteorological Satellite Program, Swarm, Resolute Bay all‐sky imagers, GPS Total Electron Content and Super Dual Auroral Radar Network, are used to investigate the evolution and key characteristics of the Tongue of Ionization (TOI) being restructured into a polar cap patch. Six satellites crossed the TOI of patch as it moved from the dayside to the nightside. It was initially hot, then a mix of both cold and hot, and finally it became a cold patch. This suggests that cold patch is not only a result of solar extreme ultraviolet radiation, but may also develop when a hot patch cools down. Soft‐electron precipitation and flow shears both contribute to the TOI restructuring and the appearance of polar cap patch. The plasma density of patch at ∼500 km was at least 4 times higher than at ∼800 km. The plasma density enhancement gradually decreased as the patch evolved due to decreased production and transport of cold nightside low‐density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern. Plain Language Summary: The appearance of high‐density plasma is a common phenomenon in the polar ionosphere. High‐density polar cap patches usually form near the dayside sunlit region, and then move from the dayside to the nightside with the ionospheric flow. In the paper, we use multiple instruments including six satellites and ground‐based observations to carefully investigate an event that started out on the dayside. It was initially a hot tongue of ionization (TOI) of high density and high electron temperature, followed by a mix of hot and cold (high vs. low electron temperature), before it finally became a cold patch. Soft‐electron precipitation and flow shears both influenced the dayside high‐density TOI plasma restructuring and the appearance of polar cap patch. The enhanced plasma density gradually weakened as the plasma migrated toward the nightside due to decreased production and transport of cold nightside low‐density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern. Key Points: Cold dense plasma in the polar cap can appear either via transport of solar extreme ultraviolet plasma or after a hot patch cools downSoft‐electron precipitation and flow shears both influence the Tongue of Ionization restructuring and the appearance of polar cap patchesThe enhanced plasma density is reduced due to decreased production and transport of cold nightside low‐density plasma [ABSTRACT FROM AUTHOR]

The dayside particle precipitation during the passage of a solar wind rotational discontinuity (RD) event on 10 April 2015 is examined and reviewed. The RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. Particle precipitation data from DMSP F17 reveal the formation of an unusual boundary layer where the low energy (cold) ions exhibit energy-latitude dispersion that is usually associated with mantle while the high energy (hot) ions look like typical magnetospheric ions. DMSP F17 and F19 observe a double cusp that is a signature of magnetic reconnection occurring at both high- and low-latitudes due to the dominant IMF By. A global MHD simulation of the event supports the existence of the simultaneous reconnections at high- and low-latitude magnetopause that are consistent with the anti-parallel and component merging models, respectively. Finally, Cluster C2, located at high-latitude and high-altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak-to-peak amplitudes >800 km·s–1 as it crosses the magnetopause. Guided by the MHD simulation, the Cluster observation can be interpreted as the spacecraft crossing reconnection outflows while moving from one side of the X-line to the other.

In this study, we provide the first detailed analysis of variations in the spacecraft potential (Vs) of the three Swarm satellites, which are flying at about 400–500 km. Unlike previous studies that have investigated extreme charging events, usually with spacecraft potentials as negative as −100 V, this study is focused on variations of Swarm Vs readings, which fall within a few negative volts. The Swarm observations show that spacecraft at low Earth orbital (LEO) altitudes are charged only slightly negatively, varying between −7 V and 0 V, with the majority of recorded potentials at these altitudes clustering close to −2 V. However, a second peak of Vs data is found at −5.5 V, though the event numbers for these more-negative observations are less, by an order of magnitude, than for incidents near the −2 V peak. These two distinct Vs peaks suggest two different causes. We have thus divided the Swarm spacecraft Vs data into two categories: less-negatively charged (−5 < Vs < 0 V) and more-negatively-charged (−6.5 < Vs < −5 V). These two Vs categories exhibit different spatial and temporal distributions. The Vs observations in the first category remain relatively closer to 0 V above the magnetic equator, but become much more negative at low and middle latitudes on the day side; at high latitudes, these first-category Vs readings are relatively more-negative during local summer. Second-category Vs events cluster into two bands at the middle latitudes (between ±20°–50° magnetic latitude), but with slightly more negative readings at the South Atlantic Anomaly (SAA) region; at high latitudes, these rarer but more-negative second-category Vs events exhibit relatively more-negative values during local winter, which is opposite to the seasonal pattern seen in the first category. By comparing Vs data to the distributions of background plasma density at Swarm altitudes, we find for the first category that more-negative Vs readings are recorded at regions with higher background plasma density, while for the second category the more-negative Vs data are observed at regions with lower background plasma density. This can be explained as follows: the electron and ion fluxes incident on Swarm surface, whose differences determine the potential of Swarm, are dominated by the background “cold” plasma (due to ionization) and “hot” plasma (due to precipitated particles from magnetosphere) for the two Vs categories, respectively.

The low-electron flux variability (increase/decrease) in the Earth’s radiation belts could cause low-energy Electron Precipitation (EP) to the atmosphere over auroral and South American Magnetic Anomaly (SAMA) regions. This EP into the atmosphere can cause an extra upper atmosphere’s ionization, forming the auroral-type sporadic E layers (Esa) over these regions. The dynamic mechanisms responsible for developing this Esa layer over the auroral region have been established in the literature since the 1960s. In contrast, there are several open questions over the SAMA region, principally due to the absence (or contamination) of the inner radiation belt and EP parameter measurements over this region. Generally, the Esa layer is detected under the influence of geomagnetic storms during the recovery phase, associated with solar wind structures, in which the time duration over the auroral region is considerably greater than the time duration over the SAMA region. The inner radiation belt’s dynamic is investigated during a High-speed Solar wind Stream (September 24-25, 2017), and the hiss wave-particle interactions are the main dynamic mechanism able to trigger the Esa layer’s generation outside the auroral oval. This result is compared with the dynamic mechanisms that can cause particle precipitation in the auroral region, showing that each region presents different physical mechanisms. Additionally, the difference between the time duration of the hiss wave activities and the Esa layers is discussed, highlighting other ingredients mandatory to generate the Esa layer in the SAMA region.

The polar cap hot patch is an enhanced density structure which is associated with particle precipitation, ion upflow and flow shears. Based on combined observations from DMSP satellites, EISCAT radar, and all‐sky imager at the Chinese Yellow River Station, the plasma characteristics and evolution of hot dense plasma and classical polar cap patch are investigated. Both of them show enhanced F region density, but the hot dense plasma is associated with higher electron temperature and enhanced E region electron density caused by particle precipitation. Compared to the cold patch, the hot dense plasma is closer to the cusp region and is associated with strong auroral emissions. Based on the joint observations, the evolution of the hot/cold dense plasma is discussed. In the initial phase the dense plasma comes from the sunlit lower latitude region, or from particle precipitation in the duskside auroral oval, and is heated locally by poleward moving auroral structures in the dayside auroral oval. Next the patches move poleward into the polar cap where a decaying electron temperature (i.e., less precipitation) gradually transform them into a cold patch. Plain Language Summary: The polar cap hot patch is a high‐density ionospheric irregularity with enhanced electron temperature and particle precipitation. The density source and associated auroral emission of the hot patch are not clear. The relationship between the hot dense plasma (hot patch) and classical polar cap patch (cold patch) is a lingering question. In this paper, we show two hot dense plasma and cold patch events using multi‐instrument observations. Two density sources are suggested: dense plasma in the lower latitude sunlit region, and plasma ionized by particle precipitation in the duskside auroral oval. Our study shows the hot dense plasma is associated with stronger aurora emissions and higher E region density in the cusp region, followed by no auroral emission (colder electron temperature) as the hot dense plasma drifts poleward and turns into a cold patch in the polar cap. It suggests that the hot dense plasma and cold patch may be different states of the same irregularity structure. Key Points: The different plasma characteristics of hot dense plasma and cold patch in the polar cap are investigated using multi‐instrument observationsBoth hot dense plasma and cold patches are associated with auroral forms, which are located further equatorward in the cusp regionThe hot dense plasma and cold patch may represent different energy states of the same plasma structure as it traverses the polar ionosphere [ABSTRACT FROM AUTHOR]

The auroral oval morphology has been investigated in previous studies presenting maps of average auroral precipitation. However, such distributions tend to emphasize auroral intensity rather than the actual extent of the auroral oval. We develop a statistical method to characterize the auroral oval morphology by using 20 years of electron energy flux measurements from the Defense Meteorological Satellite Program/Special Sensor J (DMSP/SSJ); instead of relying on auroral oval boundaries, we derive the probability of observing aurora from a threshold of 2.109 eV/cm2/s/sr above which the total energy flux of electrons (in the energy range 1–30 keV) is defined as aurora. We then investigate the auroral occurrence probability (AOP) in the magnetic latitude‐magnetic local time (MLat‐MLT) sectors covered by DMSP for various conditions related to geomagnetic activity. Regardless of those conditions, the AOP distributions reveal a width asymmetry with a wider dawn‐to‐noon sector (06–12 MLT) compared to the dusk‐to‐midnight sector (18–24 MLT), the dawn preference getting even more pronounced as the geomagnetic activity decreases. In the context of an open magnetosphere, we investigate the relation between the observed extent asymmetry in the auroral oval and the magnetospheric plasma convection. Representing the plasma sheet magnetic flux as a one‐dimensional fluid subject to production on the nightside (closing of flux via reconnection) and loss on the dayside (opening of flux), we highlight similarities with the AOP in terms of MLT asymmetries. Finally, making use of this fluid model, we demonstrate that the corotation influence on the plasma convection pattern is consistent with the dawn‐dusk asymmetry observed in the AOP distributions. Plain Language Summary: As an indirect consequence of the magnetic interaction between solar wind and Earth, protons and electrons precipitate into the ionized upper atmosphere (ionosphere) at high latitudes. These charged particles can excite atmospheric atoms at approximately 100–200 km altitude, which in turn potentially emit photons in the visible light spectrum during relaxation; this is what we call aurora. However, instruments aboard satellites crossing the polar regions, where aurora take place, can measure particles that will not necessarily result in sharp, visible aurora from the ground. This diffuse aurora represents the largest part of the total auroral energy deposited into the ionosphere. We show that the probability to measure this type of aurora is close to one within an oval‐shaped region (auroral oval) around each pole, at all local times (including daytime), independently of geomagnetic conditions. We find a systematic pattern in the statistical extent of the diffuse auroral oval depending on local time, with a larger oval in the dawn region, compared to dusk. We aim at understanding the auroral oval morphology in terms of large‐scale magnetospheric plasma dynamics, also including the Earth's rotation influence. Key Points: The auroral occurrence probability (AOP) is derived from Defense Meteorological Satellite Program/Special Sensor J measurements for investigating the auroral oval shape and extentThe distribution of AOP is persistently larger at dawn than at dusk, regardless of external conditionsA simple model of plasma sheet magnetic flux indicates that Earth's rotation may contribute to the observed dawn‐dusk asymmetry [ABSTRACT FROM AUTHOR]

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (Torbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with 1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.

Two important phenomena of the solar wind–magnetosphere–ionosphere coupling are auroral particle precipitation and the formation of ions flowing upward from the ionosphere. They have opposite transport directions of energy and substance. Based on the observations of particle precipitation and ion drift from the DMSP F13 satellite in January and July 2005, the ionospheric ion upflows in dayside auroral oval (0600–1800 MLT) can be divided into five types according to the velocity of ion upflows and the spectrum characteristics of auroral particle precipitation, and the distribution for different types of ion upflows is studied. The results show that the ion upflows mainly occur in the geomagnetic latitude (MLAT) range of 70–80°.The main magnetospheric source region of ion upflows (type A and D) caused by the accelerated electron (mainly the soft electron) corresponds to Low Latitude Boundary Layer (LLBL) and Cusp, and ion upflows of type B and C (related to the process of ambipolar diffusion caused by electron acceleration) mainly occur in LLBL and Boundary Plasma Sheet (BPS), while ion upflows of type E without electron acceleration mainly occur in the central plasma sheet (CPS).The dawn–dusk asymmetry is obvious in the winter season, with the ion upflows mainly occurring on the dawn/dusk side ionosphere. However, the ion upflows in summer mainly occur at the magnetic noon, with a symmetric distribution centered at the magnetic noon. The occurrence of ion upflow in winter is significantly higher than that in summer, and it is significantly enhanced during the period of moderate geomagnetic activity. The upward region expands to the lower latitude when the geomagnetic activity is enhanced. The effect of interplanetary magnetic field (IMF) components has also been studied in this paper. When IMF Bx is negative, the upflow occurrence increases in the region of 1500–1800 MLT and 0600–0900 MLT, with the MLAT range below 70°. The direction of IMF By may lead to the high-incidence area reverse at the prenoon or postnoon region. The occurrence of ion upflows with the MLAT range below 75° increases significantly when IMF is southward. Type A ion upflow has the highest velocity of ion upflows, followed by type E, and type D has the lowest. The average velocity of ion upflows in winter is significantly higher than that in summer. [ABSTRACT FROM AUTHOR]

Medium Energy Electron (MEE) precipitation (≳30 keV) ionizes the mesosphere and initiates chemical reactions, which ultimately can reduce mesospheric and stratospheric ozone. Currently, there are considerable differences in how existing parameterizations represent flux response, timing, and duration of MEE precipitation, especially considering its high‐energy tail (≳300 keV). This study compares the nature of ≳300 to ≳30 keV electron fluxes to better understand differences within MEE precipitation. The MEE fluxes are estimated from measurements by the Medium Energy Proton and Electron Detector (MEPED) onboard the Polar Orbiting Environmental Satellite (POES) from 2004 to 2014. The fluxes are explored in the context of solar wind drivers: corotating high‐speed solar wind streams (HSSs) and coronal mass ejections (CMEs) alongside their associated solar wind properties. Three key aspects of ≳300 keV electron fluxes are investigated: maximum response, peak timing, and duration. The results reveal a structure‐dependent correlation (0.89) between the peak fluxes of ≳30 and ≳300 keV electrons. The epsilon coupling function correlates well (0.84) with the ≳300 keV peak flux, independent of solar wind structure. The ≳300 keV flux peaks 0–3 days after the ≳30 keV flux peaks. The highest probability (∼42%) occurs for a 1‐day delay, while predictive capabilities increase when accounting for solar wind speed. The ≳300 keV flux response has the highest probability of lasting 4 days for both CMEs and HSSs. The results form a base for a stochastic MEE parameterization that goes beyond the average picture, enabling realistic flux variability on both daily and decadal scales. Plain Language Summary: Electrons with energies >30 keV precipitating into the Earth's atmosphere is known as medium energy electron (MEE) precipitation. Solar wind properties drive the rate and energy of MEE. MEE precipitation is a relevant solar forcing as it produces ozone‐depleting substances. Variations in ozone concentration can modify the atmospheric temperature profile and lead to changes in atmospheric circulation that can map down onto surface climate. The community's capability of parameterizing MEE precipitation is an active field of research. This study aims to build a foundation for an MEE parameterization that represents realistic variability on daily and decadal scales by exploring the variability in the context of different solar wind properties. The study focuses on three key aspects regarding the differences within the energy spectrum of MEE precipitation: maximum response, timing, and duration. The key aspects are evaluated regarding the probability of a specific response. This study looks at an entire solar cycle (11 yr). It is found that solar wind properties, such as solar wind speed, can help determine the probability of a particular MEE response. The results from this paper will be key elements for constructing an MEE precipitation parameterization to be applied in a chemistry‐climate model. Key Points: The solar wind parameter ε best predicts the peak flux of the >292 keV electron precipitation independent of solar wind structureThe delay in the peak flux of >292 keV electron precipitation relative to that of >43 keV electrons is influenced by solar wind speedOn average, electron precipitation >292 keV persists 2–3 days longer than electron precipitation >43 keV [ABSTRACT FROM AUTHOR]

We present a study about the atypical and spreading Sporadic E-layers (Es) observed in Digisonde data. We analyzed a set of days around space weather events from 2016 to 2018 over Cachoeira Paulista (CXP, 22.41°S, 45°W, dip ∼35°), a low-latitude Brazilian station. The inhomogeneous Es layer is associated with the auroral-type Es layer (Esa) occurrence in this region due to the presence of South American Magnetic Anomaly (SAMA). However, we also observe that the spreading Es layers occurred days before the magnetic storms or quiet times. Also, this specific type of Es layer has some different characteristics concerning the Esa layer. We used data from the imager, satellite, and meteor radar to understand the dynamic processes acting in this Es layer formation. Our results lead us to believe that other mechanisms affect the Es layer development. We show evidence that the instabilities added to the wind shear mechanism can cause the atypical Es layers, such as Kelvin-Helmholtz instability (KHI). Finally, an important discovery of this work is that the spreading Es layer, mainly during quiet times, is not necessarily due to the particle precipitation due to the SAMA. We found that the wind shear can be turbulent, influencing the Es layer development. Lastly, our analysis better understood the Es layer behavior during quiet and disturbed times.

We report effects in the upper high-latitude atmosphere related to the interaction of fast magnetosheath plasma streams, so-called jets, with the dayside magnetopause. The jets were observed by THEMIS mission in the dayside magnetosphere during a quiet day on 12 July 2009. It was found that the jet interaction was accompanied by strong localized compression and penetration of suprathermal magnetosheath plasma inside the dayside magnetosphere. The compression caused prominent magnetic variations with amplitudes up to 100 nT observed by ground-based magnetic networks SuperMAG and CARISMA. The magnetic variations were also visible in the geomagnetic Dst and AE indices. The jets also resulted in intense precipitation of the suprathermal ions with energies < 10 keV and energetic electrons with energies > 30 keV observed by low-altitude NOAA/POES satellites in a wide longitudinal range. The precipitations produced enhancements of ionization with an amplitude of ~1 TECU (~30% in relative units) and intensification of the ionospheric E and F1 layers as observed in the FORMOSAT-3/COSMIC misson. The enhanced ionization in the upper atmosphere might affect radio communication and navigation in the high-latitude regions. These results also provide new insight into the contribution of magnetospheric forcing to day-to-day ionospheric variability. [ABSTRACT FROM AUTHOR]

Context: The main challenge in atmospheric ionisation modelling is that sparse measurements are used to derive a global precipitation pattern. Typically this requires intense interpolation or scaling of long-term average maps. In some regions however, the particle flux might be similar and a combination of these regions would not limit the results even though it would dramatically improve the spatial and temporal data coverage. Aims: The paper intends to statistically analyse the particle flux distribution close to the geomagnetic poles labelled as Polar Particle Flux Distribution (PPFD) and identify similar distributions in neighbouring bins. Those bins are grouped and the size of the PPFD area is estimated. The benefit is that single measurements within the PPFD area should be able to represent the particle flux for the whole area at a given time. Methods: We use spatially binned energetic particle flux distributions measured by POES and Metop spacecraft during 2001–2018 to identify a Kp-dependent area with a similar flux distribution as the one found close to the geomagnetic poles (|magn.lat| > 86°). First, the particle flux is mapped on a magnetic local time (MLT) vs. magnetic latitude grid. In the second step, the gridded data is split up according to Kp levels (forming the final bins). Third, the particle flux in every bin has been recalculated in order to replace zero-count rates with rates based on longer measurement periods which results in a more realistic low flux end of the particle distribution. Then the binned flux distributions are compared to the PPFD. A "Δ-test" indicates the similarity. A threshold for the Δ-test is defined using the standard deviation of Δ-test values inside the (|magn.lat| > 86°) area. Bins that meet the threshold are attributed as PPFD area. Results: PPFDs and the corresponding PPFD areas have been determined for all investigated particle channels, covering an energy range of 154 eV–300 keV for electrons and 154 eV–2.5 MeV for protons. Concerning low energy channels a gradual flux increase with rising Kp has been identified. High energy channels show a combination of background population and solar particle event (SPE) population that adds up with increasing Kp. The size of the PPFD area depends on particle species, energy and geomagnetic disturbance, as well as MLT. The main findings are: a) There are small but characteristic hemispheric differences. b) Only above a certain energy threshold do the PPFD areas increase with particle energy. c) A clear enlargement with rising Kp is identified – with exceptions for very low Kp. d) The centre of the PPFD area is shifted towards midnight and moves with Kp. Asymmetries of the boundaries could be explained by auroral intensity. e) For low-energy particles the main restriction of the PPFD area seems to be the auroral precipitation. [ABSTRACT FROM AUTHOR]

Auroral particle precipitation potentially plays the main role in ionospheric plasma structuring. The impact of auroral particle precipitation on plasma structuring is investigated using multi-point measurements from scintillation receivers and all-sky cameras from Longyearbyen, Ny-Ålesund, and Hornsund on Svalbard. This provides us with the unique possibility of studying the spatial and temporal dynamics of the aurora. Here we consider three case studies to investigate how plasma structuring is related to different auroral forms. We demonstrate that plasma structuring impacting the GNSS signals is largest at the edges of auroral forms. Here we studied two stable arcs, two dynamic auroral bands, and a spiral. Specifically for arcs, we find elevated phase scintillation index values at the poleward edge of the aurora. This is observed for auroral oxygen emissions (557.7 nm) at 150 km in the ionospheric E-region. This altitude is also used as the ionospheric piercing point for the GNSS signals as the observations remain the same regardless of different satellite elevations and azimuths. Further, there may be a time delay between the temporal evolution of aurora (e.g., commencement and fading of auroral activity) and observations of elevated phase scintillation index values. The time delay could be explained by the intense influx of particles, which increases the plasma density and causes recombination to carry on longer, which may lead to a persistence of structures – a "memory effect". High values of phase scintillation index values can be observed even shortly after strong visible aurora and can then remain significant at low intensities of the aurora. [ABSTRACT FROM AUTHOR]

Within the fully integrated magnetosphere-ionosphere system, many electrodynamic processes interact with each other. We review recent advances in understanding three major meso-scale coupling processes within the system: the transient field-aligned currents (FACs), mid-latitude plasma convection, and auroral particle precipitation. (1) Transient FACs arise due to disturbances from either dayside or nightside magnetosphere. As the interplanetary shocks suddenly compress the dayside magnetosphere, short-lived FACs are induced at high latitudes with their polarity successively changing. Magnetotail dynamics, such as substorm injections, can also disturb the current structures, leading to the formation of substorm current wedges and ring current disruption. (2) The mid-latitude plasma convection is closely associated with electric fields in the system. Recent studies have unraveled some important features and mechanisms of subauroral fast flows. (3) Charged particles, while drifting around the Earth, often experience precipitating loss down to the upper atmosphere, enhancing the auroral conductivity. Recent studies have been devoted to developing more self-consistent geospace circulation models by including a better representation of the auroral conductance. It is expected that including these new advances in geospace circulation models could promisingly strengthen their forecasting capability in space weather applications. The remaining challenges especially in the global modeling of the circulation system are also discussed. [ABSTRACT FROM AUTHOR]

We present a Python implementation of a D- and E-region chemistry and ionization code called pyGPI5. Particle precipitation that penetrates into the E- and D-region of the ionosphere-thermosphere causes significant enhancements of the electron density. Dissociative recombination of molecular ions with electrons is the primary electron loss mechanism in the E-region, down to approximately 85 km. However, below 85 km, chemical processes become significantly more complicated with positive and negative ions being generated in addition to electrons. The complex D-region ion chemistry has been known for many decades. We present a formulation to quantify the concentrations of four ion species composed of positive and negative, light and heavy ions, and the electrons. The implementation we describe in this investigation solves five ordinary stiff differential equations simultaneously. We present an overview of the code, along with discussions of the reaction rates, and assumptions used in the model. We describe an implementation of the electron transport model to quantify the altitude ionization profile caused by energetic particle precipitation. We show how to instantiate the model, generate the ion and electron profiles as a function of altitude for background conditions, how to generate altitude ionization profiles, and running the code to produce ion and electron profiles caused by energetic particle precipitation. Recent investigations that have used a D-region chemistry model are discussed, along with some applications.

At mid‐ and high‐latitudes, energetic protons of magnetospheric origin precipitate down to the upper atmosphere, manifested as one of the most important coupling processes between the ionosphere and magnetosphere. The equatorward boundary of the energetic proton precipitation, or called isotropic boundary (IB), is usually believed to coincide with the boundary in the magnetosphere where field line curvature scattering of protons transits from strong to weak. In this study, we statistically examine the global distribution of the IB for 39 and 115 keV ring current protons as detected by the NOAA/Polar Operational Environmental Satellites in 2013–2018 and propose an empirical model of the IB as a function of solar wind conditions, magnetic local time, and geomagnetic activity index. The IB is well ordered by SYM‐H index and interplanetary magnetic field Bz, moving to lower latitudes with more negative SYM‐H index and Bz values, and with larger AE index and solar wind pressure. Energy dispersions of the IB are also explored, consistent with the scenario of field curvature scattering in the magnetosphere. This empirical model can be used as an indirect measure of the strength of the magnetospheric stretching and may be applied in magnetosphere‐ionosphere mapping studies. Plain Language Summary: At mid‐ and high‐latitudes, considerable energetic particles can be deposited onto the Earth's upper atmosphere, reflecting the coupling process between the ionosphere and magnetosphere. This study statistically examined a low‐latitude boundary where proton precipitation flux suddenly decreases along with the drop of trapped flux. Such a boundary is called isotropic boundary (IB). It often corresponds to the magnetospheric boundary where the curvature radius of a geomagnetic field line becomes comparable to the proton's gyro‐radius. This paper proposes an empirical model of IB in response to interplanetary and geomagnetic conditions. Such a model can be of great significance in magnetosphere‐ionosphere mapping studies and can be used to estimate the stretching of the global magnetospheric configuration. Key Points: An empirical model of isotropic boundary (IB) is proposed as a function of geomagnetic activity and solar wind conditionsIB latitude is positively correlated with the SYM‐H index and interplanetary magnetic field Bz, but negatively with AE index and solar wind pressureIB is located more equatorward with higher‐energies, consistent with field line curvature scattering [ABSTRACT FROM AUTHOR]

We explore the hypothesis that electron precipitation curtains such as those observed by the AeroCube‐6 satellite pair can be produced by electron microbursts. Precipitation curtains are latitudinal structures of stable precipitation that persist for timescales of 10s of seconds or longer. The electrons involved have energies of 10–100s of keV. The microburst formation hypothesis states that a source region in the equatorial region produces a series of very low frequency chorus wave emissions. Each of these emissions in turn produces a microburst of electron precipitation, filling the drift and bounce loss cone on the local field line. Electrons in the drift loss cone remain on the field line and bounce‐phase mix over subsequent bounces while also drifting in azimuth. When observed at downstream azimuths by a satellite equipped with an integral energy sensor, no bounce phase structure remains, or, equivalently, the same time profile is present when two such satellites pass by many seconds apart. The spatial structure that remains reflects the pattern of microburst sources. Statistical studies of where and when curtains occur have indicated that some, but not all, curtains could be caused by microbursts. We use test particle tracing in a dipole magnetic field to show that spatially stationary source regions generating periodic microbursts can produce curtain signatures azimuthally downstream. We conclude that one viable explanation for many of the curtains observed by the AeroCube‐6 pair is the accumulation of drift‐dispersed microburst electron byproducts in the drift loss cone. Plain Language Summary: The pair of low altitude, polar AeroCube‐6 satellites observed stable small‐scale structure in the electrons present in low Earth orbit (LEO). Even when the two vehicles are separated in time by over a minute, both measure roughly the same structured time profile of radiation intensity, offset by the time separation between vehicles. Individual features in this stable structure are known as curtains. We test whether the curtains could be formed by accumulation of electrons from short‐lived microbursts of radiation intensity, which individually last less than a second. Accordingly, each microburst adds electrons to the population that reaches LEO but does not enter the atmosphere before drifting into the atmosphere in the South Atlantic Anomaly. Because microbursts contain many energies, over time the sub‐second temporal structure will spread out during the bounce and drift motion of the electrons. Further, if the microburst source repeatedly produces bursts in the same location, fluxes from new and old bursts will eventually overlap. Thus, it is possible that satellites in LEO with wide‐energy sensors to see a stable temporal profile reflective of the spatial structure of the microburst source locations. We demonstrate this hypothesized mechanism by tracing electrons in a dipole magnetic field. Key Points: Curtains are small‐scale latitude structures observed by a pair of low altitude satellitesWe use test particle tracing to investigate the origin of curtainsCurtains can be caused by microbursts, which are transient, smaller‐scale structures [ABSTRACT FROM AUTHOR]

The spatial coverage of the energetic electron precipitation (EEP) region during three substorms has been determined using a combination of ground-based instruments in North America. The primary datasets used to determine the presence or absence of EEP are riometer measurements of cosmic noise absorption (CNA); attenuation of the 10–11 MHz atmospheric noise in the D-region ionosphere determined using the Super Dual Auroral Radar Network (SuperDARN); amplitude variations in subionospheric very low frequency (VLF) propagation observed using the Antarctic-Arctic Radiation-belt (Dynamic) Deposition VLF Atmospheric Research Konsortium (AARDDVARK) VLF receivers, and total electron content measurements derived from global navigation satellite systems. Our results show that substorm EEP during moderate geomagnetic conditions can extend considerably equatorward or poleward of the average latitude limits reported in a previous statistical study of substorm EEP and an EEP model. This evidence is provided by the SuperDARN radars and the radar signature is not always accompanied by cosmic noise absorption measured by the riometers. Further work is required to determine which EEP energies and fluxes the SuperDARN radars are sensitive to, but we can conclude that EEP with sufficient flux to attenuate 10–11 MHz radiowaves by ∼5 dB reaches latitudes well equatorward of the statistical EEP latitude limits previously modelled for >30 keV electrons.

Characteristics of pulsating aurora (PsA) at the equatorward part of the auroral oval have been well described in the literature by previous studies. We extend our knowledge on high-latitude PsA observations by analysing 68 PsA events from the optical observatory on Svalbard, at 75° magnetic latitude. We found that the pulsating emission structures are particularly large and transient, they do not experience drift motion, or their drift motion cannot be traced. Our results show that the high-latitude PsA events relate to lower geomagnetic activity and weaker solar wind driving than the lower latitude PsA. The high-latitude PsA events also occur less frequently, which is in agreement with their association to lower-than-average geomagnetic activity. We further show that the ionospheric electron density values during high-latitude PsA events are low compared to the lower latitude PsA. This, together with the non-traceable nature of the pulsating emission structures, suggests that these events are strongly dominated by a sub-type called Amorphous Pulsating Aurora (APA). We therefore conclude that, unlike the lower latitude PsA events, the high- latitude PsA events are not likely to cause direct changes in the chemical composition of the mesosphere. [ABSTRACT FROM AUTHOR]

The accurate determination of auroral precipitation in global models has remained a daunting and rather inexplicable obstacle. Understanding the calculation and balance of multiple sources that constitute the aurora, and their eventual conversion into ionospheric electrical conductance, is critical for improved prediction of space weather events. In this study, we present a semi‐physical global modeling approach that characterizes contributions by four types of precipitation—monoenergetic, broadband, electron, and ion diffuse—to ionospheric electrodynamics. The model uses a combination of adiabatic kinetic theory and loss parameters derived from historical energy flux patterns to estimate auroral precipitation from magnetohydrodynamic (MHD) quantities. It then converts them into ionospheric conductance that is used to compute the ionospheric feedback to the magnetosphere. The model has been employed to simulate the 5–7 April 2010 Galaxy15 space weather event. Comparison of auroral fluxes show good agreement with observational data sets like NOAA‐DMSP and OVATION Prime. The study shows a dominant contribution by electron diffuse precipitation, accounting for ∼74% of the auroral energy flux. However, contributions by monoenergetic and broadband sources dominate during times of active upstream solar conditions, providing for up to 61% of the total hemispheric power. The study also finds a greater role played by broadband precipitation in ionospheric electrodynamics which accounts for ∼31% of the Pedersen conductance. Plain Language Summary: The aurora is comprised of electrically charged particles that enter the upper atmosphere from outer space. The entry is driven by diverse processes at different locations of the high‐latitude atmosphere; these helps define the different sources that constitute the bulk of the aurora. Since the aurora is an important phenomenon in the study of near‐Earth space physics and space weather, it is important to account for the contribution and balance of each individual source and deduce their impact. In this study, we have introduced a novel modeling approach that is capable of estimating contributions from four diverse sources of aurora, and used this approach to study auroral dynamics during a famous space weather event. Our results indicate that the proportion and strength of each source varies over time, location, and activity. Additionally, we identify which sources have a pronounced contribution to the ionosphere's electrical conductance. Key Points: A semi‐physical global modeling approach is used to estimate diffuse and discrete sources of auroral precipitation during the Galaxy15 eventDiffuse sources contribute 74% of the total auroral power. Discrete sources are strongly driven by activity and can contribute up to 61%Broadband precipitation contributes 31% of the auroral Pedersen conductance playing a significant role in ionospheric electrodynamics [ABSTRACT FROM AUTHOR]

By using a database of 4,634 cold patches (high density and low electron temperature) and 4,700 hot patches (high density and high electron temperature) from Defense Meteorological Satellite Program F16 in 2005–2018 winter months (October–March), we present a statistical survey of the distributions of polar cap patches for different interplanetary magnetic field (IMF) orientations and ionospheric convection geometries. We investigate the dependence of cold and hot patches on local plasma transport and soft‐electron precipitation. Our results indicate that: in winter, (a) more cold and hot patches occur in the stronger anti‐sunward flow organized by different IMF orientations. (b) cold patches are frequent near the central polar cap, while hot patches are closer to the auroral oval. (c) enhanced anti‐sunward flow (E × B drift) mainly contributes to cold patch occurrence under Bz < 0, and soft‐electron precipitation contributes to hot patch occurrence both under southward and northward IMF. Plain Language Summary: Polar cap patches are common phenomena in the polar ionosphere and consist of high‐density plasma. We use 4,634 cold patches and 4,700 hot patches observed by the Defense Meteorological Satellite Program to investigate the distributions of polar cap patches for different interplanetary magnetic field (IMF) orientations and ionospheric convection geometries. We find that more cold and hot patches are distributed in areas of stronger anti‐sunward flow in winter. The hot patches are more frequent at lower magnetic latitudes, and located closer to the auroral oval than cold patches. Furthermore, in winter months, higher anti‐sunward flow velocity mainly contributes to a higher occurrence of cold patches under southward IMF, and stronger soft‐electron precipitation mainly contributes to a higher occurrence of hot patches both under southward and northward IMF. Key Points: The spatial distributions of cold and hot patches respond well to the convection pattern for different interplanetary magnetic field orientationsThe hot patches are more frequent at lower magnetic latitudes than cold patches, and located closer to the auroral ovalEnhanced anti‐sunward flow promotes cold patch occurrence under Bz < 0, and soft‐electron precipitation promotes hot patch occurrence [ABSTRACT FROM AUTHOR]

We present the results from investigation of spectra of polar mesospheric summer echoes (PMSE) observed with EISCAT VHF 224 MHz radar during energetic particle precipitation events in the summer of 2019. We used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We find that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. On some occasions, PMSE suddenly appeared during particle precipitation events. After the particle precipitation subsided, the PMSE intensities continued to be stronger for a few minutes and then decreased. At the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding broadening in the spectral widths. Interestingly, the most intense PMSE echoes almost always coincided with the lower values of spectral widths both during particle precipitations and other times. In some cases, the spectral widths show well separated regimes between the upper and lower parts of the PMSE layers and this also happened irrespective of particle precipitation. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. Based on these observations, we conclude that the sudden enhancement in the electron densities during particle precipitation intensifies PMSE structures that match the Bragg scale of the incident radio wave thereby enabling an enhancement in the backscatter strength. Similarly, PMSE backscattering may get initiated in the altitudes in which the structures were already present but where electron density was not sufficient before the onset of precipitation. Spectral widths and Doppler velocities does not show sudden variations with particle precipitation because they depend on the neutral dynamics. [ABSTRACT FROM AUTHOR]

Characteristics of pulsating aurora (PsA) at the equatorward part of the auroral oval have been well described in the literature by previous studies. We extend our knowledge on high-latitude PsA observations by analysing 68 PsA events from the optical observatory on Svalbard, at 75° magnetic latitude. We found that the pulsating emission structures are particularly large and transient, they do not experience drift motion, or their drift motion cannot be traced. Our results show that the high-latitude PsA events relate to lower geomagnetic activity and weaker solar wind driving than the lower latitude PsA. The high-latitude PsA events also occur less frequently, which is in agreement with their association to lower-than-average geomagnetic activity. We further show that the ionospheric electron density values during high-latitude PsA events are low compared to the lower latitude PsA. This, together with the non-traceable nature of the pulsating emission structures, suggests that these events are strongly dominated by a sub-type called Amorphous Pulsating Aurora (APA). We therefore conclude that, unlike the lower latitude PsA events, the high-latitude PsA events are not likely to cause direct changes in the chemical composition of the mesosphere.

Two important phenomena of the solar wind–magnetosphere–ionosphere coupling are auroral particle precipitation and the formation of ions flowing upward from the ionosphere. They have opposite transport directions of energy and substance. Based on the observations of particle precipitation and ion drift from the DMSP F13 satellite in January and July 2005, the ionospheric ion upflows in dayside auroral oval (0600–1800 MLT) can be divided into five types according to the velocity of ion upflows and the spectrum characteristics of auroral particle precipitation, and the distribution for different types of ion upflows is studied. The results show that the ion upflows mainly occur in the geomagnetic latitude (MLAT) range of 70–80°.The main magnetospheric source region of ion upflows (type A and D) caused by the accelerated electron (mainly the soft electron) corresponds to Low Latitude Boundary Layer (LLBL) and Cusp, and ion upflows of type B and C (related to the process of ambipolar diffusion caused by electron acceleration) mainly occur in LLBL and Boundary Plasma Sheet (BPS), while ion upflows of type E without electron acceleration mainly occur in the central plasma sheet (CPS).The dawn–dusk asymmetry is obvious in the winter season, with the ion upflows mainly occurring on the dawn/dusk side ionosphere. However, the ion upflows in summer mainly occur at the magnetic noon, with a symmetric distribution centered at the magnetic noon. The occurrence of ion upflow in winter is significantly higher than that in summer, and it is significantly enhanced during the period of moderate geomagnetic activity. The upward region expands to the lower latitude when the geomagnetic activity is enhanced. The effect of interplanetary magnetic field (IMF) components has also been studied in this paper. When IMF Bx is negative, the upflow occurrence increases in the region of 1500–1800 MLT and 0600–0900 MLT, with the MLAT range below 70°. The direction of IMF By may lead to the high-incidence area reverse at the prenoon or postnoon region. The occurrence of ion upflows with the MLAT range below 75° increases significantly when IMF is southward. Type A ion upflow has the highest velocity of ion upflows, followed by type E, and type D has the lowest. The average velocity of ion upflows in winter is significantly higher than that in summer.

The work presented here introduces a new data set for inclusion of energetic electron precipitation (EEP) in climate model simulations. Measurements made by the medium energy proton and electron detector (MEPED) instruments onboard both the Polar Orbiting Environmental Satellites and the European Space Agency Meteorological Operational satellites are used to create global maps of precipitating electron fluxes. Unlike most previous data sets, the electron fluxes are computed using both the 0° and 90° MEPED detectors. Conversion of observed, broadband electron count rates to differential spectral fluxes uses a linear combination of analytical functions instead of a single function. Two dimensional maps of electron spectral flux are created using Delaunay triangulation to account for the relatively sparse nature of the MEPED sampling. This improves on previous studies that use a 1D interpolation over magnetic local time or L‐shell zonal averaging of the MEPED data. A Whole Atmosphere Community Climate Model (WACCM) simulation of the southern hemisphere 2003 winter using the new precipitating electron data set is shown to agree more closely with observations of odd nitrogen than WACCM simulations using other MEPED‐based electron data sets. Simulated EEP‐induced odd nitrogen increases led to ozone losses of more than 15% in the polar stratosphere near 10 hPa in September of 2003. Plain Language Summary: This study introduces an updated version of an energetic electron precipitation (EEP) data set for use in global model simulations and precipitating electron flux comparisons. EEP is important because it can cause significant chemistry changes in the middle atmosphere, especially during years of high geomagnetic activity. The EEP data set is based on the Medium Energy Proton and Electron Detector (MEPED) instruments aboard the Polar Orbiting Environmental Satellites and European Space Agency Meteorological Observational satellites. EEP fluxes are calculated using both the 0° and 90° MEPED telescopes, which results in better estimates of precipitating electron fluxes than methods based only on the 0° detector. The flux calculations also take advantage of a MEPED proton channel that indirectly measures relativistic electrons with energies >700 keV. Therefore, the new EEP data set has an upper level energy limit of nearly 10 MeV. The new EEP calculation uses an improved mapping method and removes data artifacts that were present in the previous version. This investigation shows that simulations using the new EEP data describe middle atmosphere chemistry more realistically than simulations that do not include any EEP or simulations that estimate EEP using only the 0° detector. Key Points: A new and improved medium energy electron (MEE) precipitation data set is introducedIonization rates based on the new data result in better agreement between observed and modeled reactive odd nitrogenWACCM simulations of the 2003 Antarctic winter show significant MEE‐induced stratospheric odd nitrogen increases and ozone decreases [ABSTRACT FROM AUTHOR]

Based on the Defense Meteorological Satellite Program (DMSP) observations during Solar Cycle 23, this paper examines solar activity dependence of ionospheric bulk ion upflow events (IUEs) in the Southern Hemisphere (SH). Much previous similar work was conducted over the Northern Hemisphere (NH) with measurements from European Incoherent Scatter (EISCAT). To eliminate the influence of geomagnetic disturbance on IUEs, we pick out observations during geomagnetic quiet periods (with Kp ≤ 2+). Results show that, ion upward densities and fluxes are dramatically elevated at times of high solar activity (HSA) but ion upward drifts and occurrences are increased at times of low solar activity (LSA) in the SH, which is consistent with the situation in the NH. The ratios between HSA and LSA for these four parameters (IUEs’ density, flux, upward drift and occurrence) are ~2.71, ~1.98, ~0.76 and ~0.57, respectively. Furthermore, lower flux event takes place frequently at LSA as the background ion density is low but the upward drift is large, while higher flux event happens commonly at times of HSA accompanied by high ion density but low upward velocity. Quantitatively, an increase in unit of solar activity (characterized by P index) causes a 4.2×108 m−3 increase in ion density and a 1.2×1011 m−2·s−1 enhancement in upward flux, together with a 0.6 m·s−1 and 0.02 % decrease in ion upward velocity and uprate, respectively. The acceleration from the ambipolar electric field is thought to be a possible mechanism affecting the dependence of IUEs on solar variations. For HSA, the acceleration from the ambipolar electric field weakens, but a large number of background ions provide abundant seeds for acceleration and upflow, which maintains a high IUE flux. It is inferred that upflow events and upward drifts are inhibited by the enhanced ionospheric background density.

Abstract In this study, we have derived field-aligned currents (FACs) from magnetometers onboard the Defense Meteorological Satellite Project (DMSP) satellites. The magnetic latitude versus local time distribution of FACs from DMSP shows comparable dependences with previous findings on the intensity and orientation of interplanetary magnetic field (IMF) B y and B z components, which confirms the reliability of DMSP FAC data set. With simultaneous measurements of precipitating particles from DMSP, we further investigate the relation between large-scale FACs and precipitating particles. Our result shows that precipitation electron and ion fluxes both increase in magnitude and extend to lower latitude for enhanced southward IMF B z , which is similar to the behavior of FACs. Under weak northward and southward B z conditions, the locations of the R2 current maxima, at both dusk and dawn sides and in both hemispheres, are found to be close to the maxima of the particle energy fluxes; while for the same IMF conditions, R1 currents are displaced further to the respective particle flux peaks. Largest displacement (about 3.5°) is found between the downward R1 current and ion flux peak at the dawn side. Our results suggest that there exists systematic differences in locations of electron/ion precipitation and large-scale upward/downward FACs. As outlined by the statistical mean of these two parameters, the FAC peaks enclose the particle energy flux peaks in an auroral band at both dusk and dawn sides. Our comparisons also found that particle precipitation at dawn and dusk and in both hemispheres maximizes near the mean R2 current peaks. The particle precipitation flux maxima closer to the R1 current peaks are lower in magnitude. This is opposite to the known feature that R1 currents are on average stronger than R2 currents.

We report effects in the upper high-latitude atmosphere related to the interaction of fast magnetosheath plasma streams, so-called jets, with the dayside magnetopause. The jets were observed by THEMIS mission in the dayside magnetosphere during a quiet day on 12 July 2009. It was found that the jet interaction was accompanied by strong localized compression and penetration of suprathermal magnetosheath plasma inside the dayside magnetosphere. The compression caused prominent magnetic variations with amplitudes up to 100 nT observed by ground-based magnetic networks SuperMAG and CARISMA. The magnetic variations were also visible in the geomagnetic Dst and AE indices. The jets also resulted in intense precipitation of the suprathermal ions with energies < 10 keV and energetic electrons with energies > 30 keV observed by low-altitude NOAA/POES satellites in a wide longitudinal range. The precipitations produced enhancements of ionization with an amplitude of ~1 TECU (~30% in relative units) and intensification of the ionospheric E and F1 layers as observed in the FORMOSAT-3/COSMIC misson. The enhanced ionization in the upper atmosphere might affect radio communication and navigation in the high-latitude regions. These results also provide new insight into the contribution of magnetospheric forcing to day-to-day ionospheric variability.

The Empirical Canadian High Artic Ionospheric Model (E‐CHAIM) provides the four‐dimensional ionosphere electron density at northern high latitudes (>50° geomagnetic latitude). Despite its emergence as the most reliable model for high‐latitude ionosphere density, there remain significant deficiencies in E‐CHAIM's representation of the lower ionosphere (below ∼200 km) due to a sparsity of reliable measurements at these altitudes, particularly during energetic particle precipitation events. To address this deficiency, we have developed a precipitation component for E‐CHAIM to be driven by satellite‐based far‐ultraviolet (FUV) imager data. Satellite observations of FUV emissions may be used to infer the characteristics of energetic particle precipitation and subsequently calculate the precipitation‐enhanced ionization rates and ionosphere densities. In order to demonstrate the improvement of E‐CHAIM's ionosphere density representation with the addition of a precipitation component, this paper presents comparisons of E‐CHAIM precipitation‐enhanced densities with ionosphere density measurements of three auroral region incoherent scatter radars (ISRs) and one polar cap ISR. Calculations for 29,038 satellite imager and ISR conjunctions during the years 2005–2019 revealed that the root‐mean‐square difference between E‐CHAIM and ISR measurements decreased by up to 2.9 × 1010 ele/m3 (altitude dependent) after inclusion of the precipitation component at auroral sites, and by 2.6 × 109 ele/m3 in the polar cap. Improvements were most substantial in the winter season and during active auroral conditions. The sensitivity of precipitation‐enhanced densities to uncertainties inherent to the calculation method was also examined, with the bulk of the errors due to uncertainties in FUV imager data and choice of distribution function for precipitation energy spectra. Plain Language Summary: The Empirical Canadian High Artic Ionospheric Model (E‐CHAIM) is a measurement‐based model which provides the electron density of the upper atmosphere (the ionosphere) for a user‐specified date, time, and location at northern high latitudes (>50° geomagnetic latitude). The Earth's ionosphere can aid or disrupt the operation of technologies such as global navigation and radio communication systems, and thus ionosphere models are a critical component for reliable operation of these systems. E‐CHAIM is currently the most reliable model for high‐latitude ionosphere densities, which are particularly dynamic and complex due to the vertical orientation of Earth's high latitude magnetic field. A lack of reliable, widespread observations of the lower ionosphere (at roughly 95–200 km altitude) has resulted in deficiencies in E‐CHAIM at these altitudes, mainly an inability to capture enhanced ionosphere densities associated with the precipitation of energetic particles that travel from near‐Earth space, along Earth's magnetic field, and down into the atmosphere. To address this model deficiency, we have used satellite ultraviolet imager data as an indirect means of estimating enhanced ionosphere densities. Comparison of E‐CHAIM densities with ground‐based ionosphere density measurements has revealed that integration of these satellite‐derived densities into E‐CHAIM significantly improves model performance. Key Points: Satellite ultraviolet imager (UVI) measurements are used to develop a precipitation component for the Empirical Canadian High Arctic Ionospheric Model for high‐latitude ionosphere densityComparisons with incoherent scatter radar measurements reveal significant improvement in density representation at 95–200 km altitudePrecipitation enhanced densities are most sensitive to errors in satellite UVI data and selected electron energy spectrum [ABSTRACT FROM AUTHOR]

Due to its importance for global energy dissipation in the ionosphere, the diffuse aurora has been intensively studied in the past 40 years. Its origin (precipitation of 0.5-10 keV electrons from the plasma sheet without potential acceleration) has been generally attributed to whistler-mode chorus wave scattering in the inner magnetosphere (R < ∼8 RE), while the scattering mechanism beyond that distance remains unresolved. By modeling the quasi-linear diffusion of electrons with realistic parameters for the magnetic field, loss cone size, and wave intensity (obtained from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations as a function of magnetospheric location), we estimate the loss cone filling ratio and electron cyclotron harmonic (ECH) wave-induced electron precipitation systematically throughout the entire data set, from 6 RE out to 31 RE (the THEMIS apogee). By comparing the wave-induced precipitation directly with the equatorially mapped energy flux distribution of the diffuse aurora from ionospheric observations (OVATION Prime model) at low altitudes, we quantify the contribution of auroral energy flux precipitated due to ECH wave scattering. Although the wave amplitudes decrease, as expected, with distance from the Earth, due to the smaller loss cone size and stretched magnetic field topology, ECH waves are still capable of causing sufficient scattering of plasma sheet electrons to account for the observed diffuse auroral dissipation. Our results demonstrate that ECH waves are the dominant driver of the diffuse aurora in the outer magnetosphere, beyond ∼8 RE.

Abstract The locations of region 1 and 2 field‐aligned current systems were determined using fluxgate magnetometer measurements from 875 dawn‐dusk passes of the Swarm A satellite. Within each field‐aligned current region, the ionospheric Pedersen conductance was derived from the newly corrected Swarm electric and magnetic field measurements. The Pedersen conductances are generally consistent with photoionization models. However, we show that the in situ method of determining Pedersen conductance allows for a more complete description of magnetosphere‐ionosphere coupling and should be used in future studies when possible. We show that, overall, the Pedersen conductance is larger in the upward current region than in the downward region by ∼0.6 S on the dawn side of the Earth. Meanwhile, the dusk side Pedersen conductance is equivalent in both current regions. We attribute this asymmetry to dawn side energetic electron precipitation, commonly associated with substorm electron injections from the magnetotail.

The locations of region 1 and 2 field‐aligned current systems were determined using fluxgate magnetometer measurements from 875 dawn‐dusk passes of the Swarm A satellite. Within each field‐aligned current region, the ionospheric Pedersen conductance was derived from the newly corrected Swarm electric and magnetic field measurements. The Pedersen conductances are generally consistent with photoionization models. However, we show that the in situ method of determining Pedersen conductance allows for a more complete description of magnetosphere‐ionosphere coupling and should be used in future studies when possible. We show that, overall, the Pedersen conductance is larger in the upward current region than in the downward region by ∼0.6 S on the dawn side of the Earth. Meanwhile, the dusk side Pedersen conductance is equivalent in both current regions. We attribute this asymmetry to dawn side energetic electron precipitation, commonly associated with substorm electron injections from the magnetotail. Key Points: In situ Pedersen conductance in field‐aligned current (FAC) regions are obtained from the Swarm measurements of the magnetic field and plasma flow velocityPedersen conductance is larger in the upward than downward FAC regions on the dawn side, the dusk side values are similar in both regionsAsymmetry between dawn and dusk current regions is attributed to intense electron precipitation commonly associated with substorm injections [ABSTRACT FROM AUTHOR]

We advance the modeling capability of electron particle precipitation from the magnetosphere to the ionosphere through a new database and use of machine learning (ML) tools to gain utility from those data. We have compiled, curated, analyzed, and made available a new and more capable database of particle precipitation data that includes 51 satellite years of Defense Meteorological Satellite Program (DMSP) observations temporally aligned with solar wind and geomagnetic activity data. The new total electron energy flux particle precipitation nowcast model, a neural network called PrecipNet, takes advantage of increased expressive power afforded by ML approaches to appropriately utilize diverse information from the solar wind and geomagnetic activity and, importantly, their time histories. With a more capable representation of the organizing parameters and the target electron energy flux observations, PrecipNet achieves a >50% reduction in errors from a current state‐of‐the‐art model oval variation, assessment, tracking, intensity, and online nowcasting (OVATION Prime), better captures the dynamic changes of the auroral flux, and provides evidence that it can capably reconstruct mesoscale phenomena. We create and apply a new framework for space weather model evaluation that culminates previous guidance from across the solar‐terrestrial research community. The research approach and results are representative of the "new frontier" of space weather research at the intersection of traditional and data science‐driven discovery and provides a foundation for future efforts. Plain Language Summary: Space weather is the impact of solar energy on society and a key to understanding it is the way that regions of space between the Sun and the Earth's surface are connected. One of the most important and most challenging to model are the way that energy is carried into the upper atmosphere (100–1,000 km altitude). Particles moving along magnetic field lines "precipitate" into this region, carrying energy and momentum which drive space weather. We have produced a new model, using machine learning (ML), that better captures the dynamics of this precipitation from a large volume of data. Machine learning models, carefully evaluated, are capable of better representing nonlinear relationships than simpler approaches. We reveal our approach to using ML for space weather and provide a new framework to understand these models. Key Points: We utilize a data‐driven organization of input parameters to produce a new total electron energy flux nowcast model (see Figure 4)Extended input features and higher expressive power provided by machine learning (ML) approach yields more capable mesoscale and peak flux specification (see Figure 7) and an overall reduction in specification errors compared with the state‐of‐the‐art (see Table 1)A framework for evaluation of ML (and any model) in geospace is suggested, building on momentum in the community (see Section 6) [ABSTRACT FROM AUTHOR]

The need to reparametrize the OVATION Prime (2010) empirical auroral precipitation model using the Russian polar cap index (PC index) is considered. For this purpose, the integrated auroral power of particle precipitation obtained from the Polar satellite data for the period from December 1996 to June 1998 is compared with the PC index and the Newell's coupling function. The analysis revealed that the PC index at the time delays up to 5–20 minutes correlates with the magnitude of auroral power much better (the correlation coefficient ) than the Newell's coupling function (). Thus, for the purpose of nowcasting the zone of active particle precipitation, the PC index showed much higher scores, although the predicting abilities of the Newell's coupling function for the time delays of more than 20 minutes remain the best. [ABSTRACT FROM AUTHOR]

We report on observations of electromagnetic ion cyclotron (EMIC) waves and their interactions with injected ring current particles and high energy radiation belt electrons. The magnetic field experiment aboard the twin Van Allen Probes spacecraft measured EMIC waves near L = 5.5–6. Particle data from the spacecraft show that the waves were associated with particle injections. The wave activity was also observed by a ground‐based magnetometer near the spacecraft geomagnetic footprint over a more extensive temporal range. Phase space density profiles, calculated from directional differential electron flux data from Van Allen Probes, show that there was a significant energy‐dependent relativistic electron dropout over a limited L‐shell range during and after the EMIC wave activity. In addition, the NOAA spacecraft observed relativistic electron precipitation associated with the EMIC waves near the footprint of the Van Allen Probes spacecraft. The observations suggest EMIC wave‐induced relativistic electron loss in the radiation belt.Plain Language Summary: Electromagnetic waves caused by plasma particles in the Earth's magnetic fields are known to interact with high energy particles in the Earth's radiation belts, leading to loss of the particles into the Earth's atmosphere. We report on observations of the wave phenomena and their interaction with radiation belt particles using data from NASA's twin spacecraft mission called Van Allen Probes. Particle detectors aboard the spacecraft measured the increased flux of particles, providing the source energy to generate electromagnetic waves. Such wave activity was also observed by instruments on the ground. A parameter calculated using data from various instruments aboard the spacecraft enabled us to quantitatively assess how the radiation belt electron population changes over time. This analysis reveals that the wave event caused the radiation belt electrons of certain energies to decay over a limited spatial extent. In addition, particle data from low earth orbiting spacecraft show an increase in particle flux, which appears to be associated with the wave activity. The observations suggest that the wave can contribute to loss of electrons in the radiation belts.Key Points: A radiation belt electron dropout was observed during injection‐associated electromagnetic ion cyclotron (EMIC) wavesL‐shell and energy‐dependent electron dropout indicates EMIC‐wave induced scattering of radiation belt electronsNOAA spacecraft data show relativistic electron precipitation during the dropout event [ABSTRACT FROM AUTHOR]

Particle precipitation is a central aspect of space weather, as it strongly couples the magnetosphere and the ionosphere and can be responsible for radio signal disruption at high latitudes. We present the first hybrid-Vlasov simulations of proton precipitation in the polar cusps. We use two runs from the Vlasiator model to compare cusp proton precipitation fluxes during southward and northward interplanetary magnetic field (IMF) driving. The simulations reproduce well-known features of cusp precipitation, such as a reverse dispersion of precipitating proton energies, with proton energies increasing with increasing geomagnetic latitude under northward IMF driving, and a nonreversed dispersion under southward IMF driving. The cusp is also found more polewards in the northward IMF simulation than in the southward IMF simulation. In addition, we find that the bursty precipitation during southward IMF driving is associated with the transit of flux transfer events in the vicinity of the cusp. In the northward IMF simulation, dual lobe reconnection takes place. As a consequence, in addition to the high-latitude precipitation spot associated with the lobe reconnection from the same hemisphere, we observe lower-latitude precipitating protons which originate from the opposite hemisphere's lobe reconnection site. The proton velocity distribution functions along the newly closed dayside magnetic field lines exhibit multiple proton beams travelling parallel and antiparallel to the magnetic field direction, which is consistent with previously reported observations with the Cluster spacecraft. In both runs, clear electromagnetic ion cyclotron waves are generated in the cusps and might further increase the calculated precipitating fluxes by scattering protons to the loss cone in the low-altitude cusp. Global kinetic simulations can improve the understanding of space weather by providing a detailed physical description of the entire near-Earth space and its internal couplings. [ABSTRACT FROM AUTHOR]

This paper presents observations of polar cap arc substructure down to scale sizes of meters and temporal resolution of milliseconds. Two case studies containing polar cap arcs occurring over Svalbard are investigated. The first occurred on 4 February 2016 and is consistent with formation on closed field lines; the second occurred on 15 December 2015 and is consistent with formation on open field lines. These events were identified using global‐scale images from the Special Sensor Ultra‐violet Spectrographic Imager (SSUSI) instruments on board Defense Meteorological Satellite Program (DMSP) spacecraft. Intervals when the arcs passed through the small‐scale field of view of the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, were then found using all sky images from a camera also located on Svalbard. These observations give unprecedented insight into small‐scale polar cap arc structure. The energy and flux of the precipitating particles above these arcs are estimated using the ASK observations in conjunction with the Southampton Ionospheric model. These estimates are then compared to in situ DMSP particle measurements, as well as data from ground‐based instrumentation, to infer further information about their formation mechanisms. This paper finds that polar cap arcs formed on different magnetic field topologies exhibit different behavior at small‐scale sizes, consistent with their respective formation mechanisms. Key Points: We compare observations (at multiple scales) of two polar cap arcs that are consistent with different formation mechanismsSmall‐scale observations of polar cap arcs down to scale sizes of meters and temporal resolution of milliseconds are presentedWe find the small‐scale structures associated with the arcs in each event are very different, consistent with the formation mechanisms [ABSTRACT FROM AUTHOR]

• Multi-field modeling on simultaneously removing mercury and particulate matter. • The mass transfer module shows relatively higher accuracy than previous models. • Sorbent particle size predominates over simultaneous removal efficiency. • The influences of other typical operating parameters are carefully discussed. • EHD conditions have complex effects on capture of mercury and particulate matter. A computational fluid dynamics (CFD) model coupled with gas-particle mass transfer, electrohydrodynamic (EHD) effect, electric field, particle motion and particle charging is established to advance the understanding of combined particulate matter precipitation and mercury capture within industrial electrostatic precipitators (ESPs). The comparisons between experimental data and numerical results demonstrate that this model can reasonably predict the mercury removal efficiency by powdered sorbent injection (PSI). The mechanism of simultaneous removal of mercury and particulate matter is then discussed in detail by considering the complex interactions among multi-physics. The influences of particle size, mercury concentration, particle injection rate and the EHD effect are investigated. The simulation results indicate that the mercury removal process is primarily controlled by the sorbent particle residence time, surface area and mass transfer rate. Accordingly, reducing the size of sorbent particles (activated carbon) can promote mercury removal efficiency while decreasing the particle collection efficiency. Increasing the initial mercury concentration and adsorbent mass loading also benefit mercury adsorption by influencing the mass transfer rate and the surface area. The EHD effect plays important roles in mercury removal and particle collection by means of altering the flow patterns and particle migration. The two mechanisms of in-flight and wall-bounded mercury adsorption affected by ionic wind are also evaluated and some interesting phenomena are observed. [ABSTRACT FROM AUTHOR]

Electrostatic precipitators (ESPs) separate particles from flue gas by corona discharge and accompany by the generation of ionic wind. In this work, a Computational Fluid Dynamics model was presented to investigate the effects of ionic wind on flow field and particle precipitation in a honeycomb ESP in terms of the coupled processes among corona discharge, gas flow, particle charging and transport. Results indicated that the electric field and ion charge density around the needle tips were highest and respectively reach 2.5 × 106 V/m and 0.164 C/m3 in the gas without suspended particles, which played a key role in corona-electrostatic field. The maximum velocity of ionic wind could reach 13.81, 11.74 and 9.42 m/s under the applied voltage of 27, 21 and 15 kV, and ionic wind velocity sharply decreased away from the needle tip. Meanwhile, the flow field could be divided into three regions (ionic wind affected region, vortex and back flow region) according to the direction of gas flow velocity. When the electrically neutral particles with a spherical shape were injected into ESP at the entrance, the charged particles were transported by drag force and electric force, and inhomogeneous distribution of the deposited particle has occurred because of ionic wind. More intense vortices were formed with the increase in applied voltage, and promoting the particle precipitation process. Moreover, higher applied voltage enhanced the aggregation of particles in the upstream, and reduced the dead zone of deposited particles in the downstream. The ionic wind generated by corona discharge strongly disturbed the flow field and particle precipitation process. Image 1 • A 3D spike-honeycomb ESP model was established to study the role of ionic wind. • The distributed pattern of the electric field and ion charge were investigated. • High velocity ionic wind was generated near the tips and could reach 13.81 m/s. • The flow field of gas phase was divided into three regions because of ionic wind. • The correlation between the ionic wind and particle precipitation were analyzed. [ABSTRACT FROM AUTHOR]

We demonstrate that the Super Dual Auroral Radar Network (SuperDARN) radar at Syowa station, Antarctica, can be used to detect high frequency radio wave attenuation in the D region ionosphere during energetic electron precipitation (EEP) events. EEP‐related attenuation is identified in the radar data as a sudden reduction in the backscatter power and background noise parameters. We focus initially on EEP associated with pulsating aurora and use images from a colocated all‐sky camera as a validation data set for the radar‐based EEP event detection method. Our results show that high‐frequency attenuation that commences during periods of optical pulsating aurora typically continues for 2–4 hr after the camera stops imaging at dawn. We then use the radar data to determine EEP occurrence rates as a function of magnetic local time (MLT) using a database of 555 events detected in 2011. EEP occurrence rates are highest in the early morning sector and lowest at around 15:00–18:00 MLT. The postmidnight and morning sector occurrence rates exhibit significant seasonal variations, reaching approximately 50% in the winter and 15% in the summer, whereas no seasonal variations were observed in other MLT sectors. The mean event lifetime determined from the radar data was 2.25 hr, and 10% of events had lifetimes exceeding 5 hr. Key Points: D region ionization due to energetic electron precipitation can be detected using a high‐frequency radar at Syowa station, AntarcticaEvent detection method is validated using all‐sky camera observations of pulsating auroraeEnergetic electron precipitation occurrence rates peak in the morning sector and in the winter [ABSTRACT FROM AUTHOR]

Today, the near‐Earth space is facing a paradigm change as the number of new spacecraft is literally skyrocketing. Increasing numbers of small satellites threaten the sustainable use of space, as without removal, space debris will eventually make certain critical orbits unusable. A central factor affecting small spacecraft health and leading to debris is the radiation environment, which is unpredictable due to an incomplete understanding of the near‐Earth radiation environment itself and its variability driven by the solar wind and outer magnetosphere. This paper presents the FORESAIL‐1 nanosatellite mission, having two scientific and one technological objectives. The first scientific objective is to measure the energy and flux of energetic particle loss to the atmosphere with a representative energy and pitch angle resolution over a wide range of magnetic local times. To pave the way to novel model‐in situ data comparisons, we also show preliminary results on precipitating electron fluxes obtained with the new global hybrid‐Vlasov simulation Vlasiator. The second scientific objective of the FORESAIL‐1 mission is to measure energetic neutral atoms of solar origin. The solar energetic neutral atom flux has the potential to contribute importantly to the knowledge of solar eruption energy budget estimations. The technological objective is to demonstrate a satellite deorbiting technology, and for the first time, make an orbit maneuver with a propellantless nanosatellite. FORESAIL‐1 will demonstrate the potential for nanosatellites to make important scientific contributions as well as promote the sustainable utilization of space by using a cost‐efficient deorbiting technology. Key Points: FORESAIL‐1 mission measures energetic electron precipitation and solar energetic neutral atom fluxWe will demonstrate a cost‐efficient deorbiting and orbit maneuvering technology without propellantsThe goal of the mission is to contribute significantly to the sustainable utilization of space [ABSTRACT FROM AUTHOR]