The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) studies the loss of energetic electrons from Earth's radiation belts. BARREL's array of slowly drifting balloon payloads was ...designed to capitalize on magnetic conjunctions with NASA's Van Allen Probes. Two campaigns were conducted from Antarctica in 2013 and 2014. During the first campaign in January and February of 2013, there were three moderate geomagnetic storms with SYM‐Hmin < −40 nT. Similarly, two minor geomagnetic storms occurred during the second campaign, starting in December of 2013 and continuing on into February of 2014. Throughout the two campaigns, BARREL observed electron precipitation over a wide range of energies and exhibiting temporal structure from hundreds of milliseconds to hours. Relativistic electron precipitation was observed in the dusk to midnight sector, and microburst precipitation was primarily observed near dawn. In this paper we review the two BARREL science campaigns and discuss the data products and analysis techniques as applied to relativistic electron precipitation observed on 19 January 2013.
Key Points
BARREL observed electron precipitation over wide range of energy and timescales
Precipitating electron distribution is determined using spectroscopy for 19 January 2013 event
BARREL timing data has accuracy within sampling interval of 0.05 s
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
BARREL is a multiple-balloon investigation designed to study electron losses from Earth’s Radiation Belts. Selected as a NASA Living with a Star Mission of Opportunity, BARREL augments the Radiation ...Belt Storm Probes mission by providing measurements of relativistic electron precipitation with a pair of Antarctic balloon campaigns that will be conducted during the Austral summers (January-February) of 2013 and 2014. During each campaign, a total of 20 small (∼20 kg) stratospheric balloons will be successively launched to maintain an array of ∼5 payloads spread across ∼6 hours of magnetic local time in the region that magnetically maps to the radiation belts. Each balloon carries an X-ray spectrometer to measure the bremsstrahlung X-rays produced by precipitating relativistic electrons as they collide with neutrals in the atmosphere, and a DC magnetometer to measure ULF-timescale variations of the magnetic field. BARREL will provide the first balloon measurements of relativistic electron precipitation while comprehensive in situ measurements of both plasma waves and energetic particles are available, and will characterize the spatial scale of precipitation at relativistic energies. All data and analysis software will be made freely available to the scientific community.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Wave‐particle interaction between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves is a highly debated loss process contributing to the dynamics of Earth's radiation belts. ...Theoretical studies show that EMIC waves can result in strong loss of relativistic electrons in the radiation belts (Summers & Thorne, 2003, https://doi.org/10.1029/2002JA009489). However, many of these studies have not been validated by observations. Li et al. (2014, https://doi.org/10.1002/2014GL062273) modeled the relativistic electron precipitation observed by Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) in a single‐event case study based on a quasi‐linear diffusion model and observations by Van Allen Probes and GOES 13. We expand upon that study to investigate the localization of the precipitation region and the effectiveness of EMIC waves as an electron loss mechanism.The model results of BARREL 1I observations on 17 January 2013 show that as the BARREL balloon drifts in local time to regions that map to lower equatorial magnetic field strength, the flux of precipitating electrons increases and peaks at lower energy. The hypothesis that the energy of the precipitating electrons is controlled by background magnetic field strength is further tested by considering observations from balloon campaigns conducted from 2000 to 2014, including BARREL. Consistent with theory for wave‐particle interaction between relativistic electrons and EMIC waves, we find observationally that stronger equatorial magnetic field strength generally correlates with more energetic electron precipitation and further conclude that magnetic field strength can drive the localization and distribution of precipitating electrons.
Key Points
Pitch angle diffusion model of electron scattering by EMIC waves agrees well with BARREL 1I observations
Magnetic field strength is an important parameter in determining the effectiveness of pitch angle scattering electrons by EMIC waves
Balloon observations show correlation between equatorial magnetic field strength and energetic precipitation consistent with EMIC waves
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Interactions between whistler mode chorus waves and electrons are a dominant mechanism for particle acceleration and loss in the outer radiation belt. One form of this loss is electron microburst ...precipitation: a sub‐second intense burst of electrons. Despite previous investigations, details regarding the microburst‐chorus scattering mechanism—such as dominant resonance harmonic—are largely unconstrained. One way to observationally probe this is via the time‐of‐flight energy dispersion. If a single cyclotron resonance is dominant, then higher energy electrons will resonate at higher magnetic latitudes: sometimes resulting in an inverse time‐of‐flight dispersion with lower‐energy electrons leading. Here we present a clear example of this phenomena, observed by a FIREBIRD‐II CubeSat on 27 August 2015, that shows good agreement with the Miyoshi‐Saito time‐of‐flight model. When constrained by this observation, the Miyoshi‐Saito model predicts that a relatively narrowband chorus wave with a ∼0.2 of the equatorial electron gyrofrequency scattered the microburst.
Plain Language Summary
Wave‐particle interactions are a ubiquitous phenomenon in plasmas. Around Earth, interactions between electrons and a plasma wave termed whistler mode chorus leads to both the acceleration of the outer Van Allen radiation belt electrons, and rapid precipitation of electrons into Earth's atmosphere. One form of this precipitation is called electron microbursts: a sub‐second and intense bursts of electrons most often observed by high altitude balloons and low Earth orbiting satellites. While microbursts have been studied since the dawn of the Space Age, fundamental details regarding how they are generated are largely unknown. One clue to the properties of the scattering mechanism comes from energy‐dependent time‐of‐flight dispersion signatures. Electrons with a larger kinetic energy move faster, and will therefore precipitate before the electrons with lower kinetic energy. However, in this paper we show observations made by the FIREBIRD‐II CubeSat mission of the opposite: lower‐energy electrons arriving first. This counter‐intuitive phenomena, termed inverse time‐of‐flight energy dispersion, together with models, is a powerful tool to sense the detailed nature of how plasma waves scatter electrons in Earth's near space environment.
Key Points
FIREBIRD‐II observed a microburst whose 250 keV electrons arrived before the 650 keV electrons
We estimate that the observed inverse energy dispersion of 0.1 ms/keV is statistically significant
Our observations are consistent with the inverse time‐of‐flight model of chorus waves resonating with 100s keV electrons
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
Microbursts are an impulsive increase of electrons from the radiation belts into the atmosphere and have been directly observed in low Earth orbit and the upper atmosphere. Prior work has ...estimated that microbursts are capable of rapidly depleting the radiation belt electrons on the order of a day; hence, their role to radiation belt electron losses must be considered. Losses due to microbursts are not well constrained, and more work is necessary to accurately quantify their contribution as a loss process. To address this question, we present a statistical study of
35 keV microburst sizes using the pair of AeroCube‐6 CubeSats. The microburst size distribution in low Earth orbit and the magnetic equator was derived using both spacecraft. In low Earth orbit, the majority of microbursts were observed, while the AeroCube‐6 separation was less than a few tens of kilometers, mostly in latitude. To account for the statistical effects of random microburst locations and sizes, Monte Carlo and analytic models were developed to test hypothesized microburst size distributions. A family of microburst size distributions were tested, and a Markov chain Monte Carlo sampler was used to estimate the optimal distribution of model parameters. Finally, a majority of observed microbursts map to sizes less than 200 km at the magnetic equator. Since microbursts are widely believed to be generated by scattering of radiation belt electrons by whistler mode waves, the observed microburst size distribution was compared to whistler mode chorus size distributions derived in prior literature.
Plain Language Summary
Electron microbursts are a subsecond, impulsive form of electron precipitation from the radiation environment right above Earth's atmosphere. Microbursts are believed to cause significant loss of electrons on the order of a day from the near‐Earth radiation belt environment. To make these estimates, researchers need to make simplifying assumptions that reduce the accuracy of loss estimates by an unknown amount, and it is necessary to understand these assumptions. This paper focuses on one assumption needed to calculate how many electrons are lost per microburst—the physical size of microbursts. This study is achieved by using a pair of AeroCube‐6 CubeSats that are orbiting a few hundred kilometers above Earth's surface. We find that most microbursts have a size less than a few tens of kilometers and some are as large as 100 km at AeroCube‐6's altitude. Furthermore, we found that small microbursts also correspond to a very small region where microbursts are believed to be generated in the heart of the radiation belts.
Key Points
The dual AeroCube‐6 CubeSats simultaneously observed
35 keV microbursts at a variety of spatial separations ranging from 2 to 100 km
In low Earth orbit the majority of microbursts have a size on the order of a few tens of km
Mapped to the magnetic equator, the majority of microbursts are less than 200 km in size, corresponding to the size of chorus wave packets
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present the first in-flight results from a novel miniaturised anisotropic magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial College), aboard the first CINEMA (CubeSat for ...Ions, Neutrals, Electrons and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent calibration technique is detailed using the International Geomagnetic Reference Field (IGRF), which is temperature dependent in the case of the outboard sensor. We show that the sensors accurately measure the expected absolute field to within 2% in attitude mode and 1% in science mode. Using a simple method we are able to estimate the spacecraft's attitude using the magnetometer only, thus characterising CINEMA's spin, precession and nutation. Finally, we show that the outboard sensor is capable of detecting transient physical signals with amplitudes of ~ 20–60 nT. These include field-aligned currents at the auroral oval, qualitatively similar to previous observations, which agree in location with measurements from the DMSP (Defense Meteorological Satellite Program) and POES (Polar-orbiting Operational Environmental Satellites) spacecraft. Thus, we demonstrate and discuss the potential science capabilities of the MAGIC instrument onboard a CubeSat platform.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Curtain precipitation is a recently discovered stationary, persistent, and latitudinally narrow electron precipitation phenomenon in low Earth orbit. Curtains are observed over consecutive passes of ...the dual AeroCube‐6 CubeSats while their in‐track lag varied from a fraction of a second to 65 s, with dosimeters that are sensitive to >35‐keV electrons. This study uses the AeroCube‐6 mission to quantify the statistical properties of 1,634 curtains observed over 3 years. We found that many curtains are narrower than 10 km in the latitudinal direction with 90% narrower than 20 km. We examined the geographic, magnetic local time, and geomagnetic dependence of curtains. We found that curtains are observed in the late‐morning and premidnight magnetic local times, with a higher occurrence rate at premidnight, and curtains are observed more often during times of enhanced Auroral Electrojet. We found a few curtains in the bounce loss cone region above the North Atlantic, whose electrons were continuously scattered for at least 6 s. Such observations suggest that continuous curtain precipitation may be a significant loss of >35‐keV electrons from the magnetosphere into the atmosphere. We hypothesize that the curtains observed in the bounce loss cone were accelerated by parallel electric fields, and we show that this mechanism is consistent with the observations.
Plain Language Summary
Electron curtain precipitation from space into Earth's atmosphere is a recently discovered phenomenon observed by dual‐spacecraft missions such as the AeroCube‐6 CubeSats that are nearly in the same orbit, ≈700 km above Earth's surface. Curtains appear stationary between consecutive passes of the AeroCube‐6 CubeSats, while the leader CubeSat was ahead of the follower CubeSat by up to a minute in orbital time. Curtains are also very narrow along the satellite orbit that is mostly in the latitudinal direction. Besides these two properties, curtains and their impact on the magnetosphere and atmosphere are not well understood. Therefore, we used the AeroCube‐6 mission that took data together for 3 years to statistically quantify curtain properties and to better understand their origin. We found 1,634 curtains and found that 90% of curtains are narrower than 20 km in the latitudinal direction, curtains are observed on the outer radiation belt field lines predominately in the antisunward region, and curtains are observed when the magnetosphere is disturbed. Curtains observed in a special region above the North Atlantic shed light on their origin. A surprising result is that a few dozen curtains observed in this North Atlantic region were continuously precipitating into the atmosphere for multiple seconds. Therefore, curtains may be a significant source of atmospheric ionization responsible for the natural depletion of ozone.
Key Points
The dual AeroCube‐6 CubeSats are used to identify stationary, narrow in latitude, and persistent >35‐keV electron curtain precipitation
Ninety percent of the observed curtains in low Earth orbit are narrower than 20 km in the latitudinal direction
Some curtains continuously precipitated into the atmosphere for multiple seconds
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present an analysis of lightning interflash intervals in 219 terrestrial gamma ray flash (TGF) producing thunderstorms. Clustering was used to identify groups of lightning sferics, interpreted as ...individual thunderstorms, in combined World Wide Lightning Location Network and Earth Networks Total Lightning Network data. In these individual groups of sferics, analysis was done on the lightning flash frequency within ±10 min of the Fermi recorded TGF. We find that typical interflash intervals immediately prior to TGFs are 24% longer than mean interflash intervals in their individual producing storms, while the interflash intervals immediately following the TGFs are typically 8% shorter than normal. The significance of these results, tested using a numerical bootstrap method, was found to be highly significant for the pre‐TGF interval. These results could imply that a stronger electric field is necessary for the production of TGFs and may help to explain why some lightning strikes produce TGFs while others do not.
Key Points
Interflash intervals in 219 terrestrial gamma ray flash (TGF)‐producing thunderstorms are analyzed
We typically find a 24% longer than normal interval immediately prior to the TGF
These results indicate that the electric field prior to the TGF is possibly stronger or more extended than for typical flashes
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Cyclic loading induces fatigue in bone and initiates a complex, functionally adaptive response. We investigated the effect of a single period of fatigue on the histologic structure and biomechanical ...properties of bone. The ulnae of 40 rats were subjected to cyclic fatigue (−6000 με) unilaterally until 40% loss of stiffness developed, followed by 14 days of adaptation. The contralateral ulna served as a treatment control (
n = 20 rats), and a baseline loaded/non-loaded group (
n = 20 rats/group) was included. Bones from 10 rats/group were examined histologically and the remaining bones (10 rats/group) were tested mechanically. The following measurements were collected: volumetric bone mineral density (vBMD); ultimate force (
F
u); stiffness (
S); energy-to-failure (
U); cortical area (Ct.Ar); microcrack density (Cr.Dn); microcrack mean length (Cr.Le); microcrack surface density (Cr.S.Dn); osteocyte density (Ot.N/T.Ar and Ot.N/TV); bone volume fraction (B.Ar/T.Ar); resorption space density (Rs.N/Ct.Ar); and maximum and minimum area moments of inertia (
I
MAX and
I
MIN). Using confocal microscopy, the bones were examined for diffuse matrix injury, canalicular disruption, and osteocyte disruption. The adapted bones had increased B.Ar,
I
MAX, and
I
MIN in the mid-diaphysis. Fatigue loading decreased structural properties and induced linear microcracking. At 14 days, adaptation restored structural properties and microcracking was partially repaired. There was a significant nonlinear relationship between Ot.N/T.Ar and B.Ar/T.Ar during adaptation. Disruption of osteocytes was observed adjacent to microcracks immediately after fatigue loading, and this did not change after the period of adaptation. In fatigue-loaded bone distant from microcracks, diffuse matrix injury and canalicular disruption were often co-localized and were increased in the lateral (tension) cortex. These changes were partially reversed after adaptation. Loss of canalicular staining and the presence of blind-ends in regions with matrix injury were suggestive of rupture of dendritic cell processes. Taken together, these data support the general hypothesis that the osteocyte syncytium can respond to cyclic loading and influence targeted remodeling during functional adaptation. Changes in the appearance of the osteocyte syncytium were found in fatigue-loaded bone with and without linear microcracks. We hypothesize that the number of dendritic cell processes that experience load-related disruption may determine osteocyte metabolic responses to loading and influence targeted remodeling.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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