Abstract
Microbursts are impulsive (<1s) injections of very energetic to relativistic electrons (energies from a few keV to MeV) into Earth’s atmosphere. They are important because they may represent a major loss process for the outer radiation belt (Ripoll and Claudepierre and Ukhorskiy and Colpitts and Li and Fennell and Crabtree, J. Geophys. Res. Space Physics, 2020, 125–e2019JA026735). Understanding and quantifying the underlying causes and consequences plus relative importance of microburst precipitation represent outstanding questions in radiation belt physics and may have significant implications ranging from space weather to atmospheric chemistry. Chorus waves are the likely dominant cause of microburst precipitation, but important questions remain regarding the exact nature of the resonance generating the microbursts and the overall importance of the precipitation. These important questions are limited by lack of systematic coordination of simultaneous observations of causative waves in the magnetosphere and resulting precipitating particles at low altitudes. Multi-spacecraft missions dedicated to answering these questions, themselves required to make progress in radiation belt physics, are critical.
Introduction and background
Many competing processes contribute to the formation and depletion of Earth’s radiation belts (see reviews by ; ; ). The outer radiation belt is highly dynamic, as many competing energization, loss and transport processes occur simultaneously and are energy dependent. Understanding and quantifying the importance of each process is fundamental to radiation belt physics and has significant implications ranging from human space flight () to space weather forecasting and atmospheric chemistry (; ; ; ) and even climatology (e.g., ; ).
Microbursts are impulsive (<1s) injections of energetic (few keV to MeV) electrons into the atmosphere. They are important because they may represent a major loss mechanism from the outer radiation belt during storm main and recovery phases (; ; ). Low energy (10s of keV) microburst precipitation has been observed since the 1960s via balloon measurements of bremsstrahlung X-rays produced by precipitating electrons when they enter Earth’s atmosphere (; ; ). Microbursts at higher energies (>100 s keV) have also been observed on satellites (; ). Figure 1 (from ) shows observations of >1 MeV electron flux taken by the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite, demonstrating that although the microbursts are short-lived, they can have fluxes more than an order of magnitude higher than the background precipitation. Therefore, microburst precipitation has the potential to be a significant loss mechanism for outer radiation belt energetic electrons.
FIGURE 1
Recent studies have also suggested that relativistic electron microbursts are the high-energy tail of pulsating aurora (), and a high correlation between patchy aurora (a type of pulsating aurora) and >1 MeV microbursts was found by . Figure 2 shows an example of concurrent pulsating aurora and relativistic electron microbursts. Microbursts are clearly observed at times when SAMPEX passes through pulsating aurora patches, however, the correlation is imperfect in part due to the uncertainty in the mapping. Physically, the imperfect correlation can also arise if the high-energy tail of a pulsating aurora patch does not exceed 1 MeV. analyzed a pulsating auroral event with a duration of >15 h with an extent larger than 10 h in MLT, indicating that these aurora events are more significant than previously predicted. These studies suggest electron microbursts have important implications for magnetosphere-ionosphere coupling and, in turn atmospheric composition. modeled a 6-h microburst ‘storm’ with typical characteristics and found that this precipitation significantly altered NOx and HOx, resulting in a >10% decrease in both middle mesospheric and upper mesospheric ozone, which has implications for regional weather. Further understanding of energetic particle precipitation at Earth will aid in interpretation of similar processes in different plasma regimes. Despite a long history of observations, the detailed physics of the underlying scattering mechanism and the relative importance of microburst precipitation as a loss mechanism for outer belt energetic electrons is poorly understood (see review by ; , ).
FIGURE 2
Current understanding and critical knowledge gaps
To date, strong evidence suggests that the dominant cause of microburst precipitation is through resonant wave-particle interactions with whistler mode chorus waves (; ; ; ). Electromagnetic ion cyclotron (EMIC) waves are another type of plasma wave that can scatter electrons (typically in the MeV energies) through anomalous cyclotron resonant interactions (). Recent observations have shown evidence that EMIC wave precipitation may extend to lower energies (e.g., ) in the hundreds of keV range, supported by simulation (), and only recently has evidence of EMIC-driven microburst precipitation been discovered (). Therefore, investigating resonant interactions with EMIC waves is important in the overall understanding of microburst precipitation. Although chorus is the likely dominant driver of microbursts, many details regarding the scattering process, including the latitude where the interaction takes place and the exact nature of the resonance, remain unverified or unknown. Additionally, direct causation has only been shown in limited cases (e.g., ). Furthermore, basic details of microbursts that are necessary to ascertain their significance as a loss mechanism including duration, isotropy, repetition, and temporal versus spatial variability are still largely unknown (e.g., ). The reason why these unanswered questions exist is largely due to lack of simultaneous observations of microbursts and waves over a large region and extended period of time.
Recently, studies utilizing a conjunction dataset of high-altitude chorus wave observations from Van Allen Probes (RBSP) and microburst observations on the low-altitude Focused Investigations of Relativistic Electron Burst: Intensity, Range, and Dynamics (FIREBIRD) CubeSats (; ) have unveiled details of the connection between chorus and microbursts. used a conjunction event to prove the chorus/microburst connection, but this event only led to a very uncertain estimate of the importance of microbursts as a loss mechanism. The relative importance of microbursts as a loss process cannot be fully constrained without accurate measurements of the flux of microbursts through a given area over a given amount of time. Obtaining accurate measurements requires very good temporal resolution, energy resolution, and angular resolution, the combination of which is extremely challenging for existing instrument technology. Future missions focused on addressing this question are needed to enable an accurate determination of the loss due to microbursts in a way that it can be compared with the overall loss from the radiation belts. Furthermore, missions focused on the properties and extents of various plasma waves and their effects on energetic electrons in the inner magnetosphere are necessary to progress in our global understanding of microburst precipitation.
Another area that needs further investigation is how chorus wave properties, which change with geomagnetic activity and location, affect wave-induced precipitation. These properties are important for the computation of diffusion coefficients, which would be valuable for modeling long-term dynamics of radiation belts due to wave-particle interaction. The morphology of the wave regions is also important for understanding the significance of microburst precipitation. A recent study by combined observations of chorus and microburst precipitation from nearly all available satellite-borne and ground-based data to estimate the typical size and duration of a microburst precipitation region. Even in this best-case scenario of available observations, significant assumptions were required regarding the continuity of chorus between two observational points. In addition, substantial gaps existed in regions of the magnetosphere due to lack of spacecraft observations. Figure 3 (from ) shows the microburst precipitation and chorus extent for three intervals. Regions of overlap between the chorus wave and microburst precipitation observations were used for the lower estimates. There were regions where microbursts were observed, but there was no spacecraft coverage to detect chorus waves. This study highlights the critical need for constellation missions studying the wave properties along with particle measurements in order to determine where wave-particle interactions can occur, which particle energies resonate with the waves, and the dependence of these regions on geomagnetic activity. Additionally, there exists a latitude gap of missing data between the equatorial source region of chorus waves and the regions where the scattering occurs.
FIGURE 3
Conclusion and science priorities
Important questions remain on microburst precipitation by waves in the inner magnetosphere, including, but not limited to, the exact nature of the resonance generating the microbursts, including the extent of the resonance region off the equator, and the overall importance microburst precipitation has on radiation belt electron loss. These gaps in our understanding largely exist due to the sparseness of simultaneous observations of waves and particles. Equatorial satellites are ideal for measuring in situ wave populations, such as chorus, generated near the equator. However, these satellites are typically unable to resolve electrons in the loss cone to quantify the amount of loss. In a similar vein, low Earth orbiting satellites, like FIREBIRD, are able to directly observe microburst precipitation, but are far away from the wave source region. This makes direct testing of microburst generation very difficult due to the large separation over which the waves and microbursts are observed. The current state of knowledge is therefore limited to multipoint observations and magnetic conjunctions between satellites.
It is important to improve our understanding of how and where plasma waves generate energetic electron microburst precipitation. Continued support of projects that investigate multipoint observations of waves ands microburst precipitation is necessary to make incremental, important steps towards our understanding of the generation mechanisms for microbursts. Modeling, including quasi-linear methods, ray tracing, and test particle simulations, will help us understand the mechanisms of the scattering process. Recent studies (e.g., ; ; ; ) used ray tracing simulations and satellite observations to show direct evidence of chorus elements propagating from the equator to higher latitudes where microbursts can be produced. This was a critical step towards understanding such a fundamental radiation belt physics question. However, the flux of microbursts through a given area in a given amount of time remains unknown and is necessary to quantify the contribution of this process to radiation belt loss. A mission with both an imager and particle detector, which could directly measure the particles while imaging a significant sector of the precipitation region in order to quantify the flux and spectrum of microburst precipitation throughout the region, is desirable. In addition, constellation missions dedicated towards answering these questions are essential. Measurements of both waves and particles along magnetic field lines from where the waves are generated near the magnetic equator, up to higher latitudes would provide insight into the structure of the waves and the impact they have on the particle populations. Additionally, the generation regions of microbursts and wave properties responsible could be identified, which would significantly constrain the spatial and temporal scale of microburst precipitation.
The importance of microburst precipitation relative to other radiation belt loss processes has yet to be established. estimated from FIREBIRD microburst data the differential flux loss rate to the atmosphere due to microbursts. This was then used to estimate the amount of time it would take for this mechanism to deplete the entire outer radiation belt. This loss rate was then compared to other loss rates in order to determine the relative importance of microburst precipitation. However, the differential flux loss rate is poorly constrained due to lack of accurate measurements of the microburst flux within a given area over a given amount of time. Measuring the spatial distribution and energy spectrum of precipitating electrons, including microbursts, is therefore required to accurately determine the loss timescales. Further complications arise from other competing mechanisms for radiation belt electron loss, including magnetopause shadowing (e.g. ; ), therefore making it difficult to separate and quantify the relative importance of each. Improved measurement capabilities, including good temporal resolution, energy resolution, and angular resolution are needed. We also recommend constellation missions, including multipoint observations at low Earth orbit and wave and particle measurements along field lines at varying latitudes. This would provide wave and particle measurements near where the scattering is occurring in an effort to fully understand the overall importance of microburst precipitation.
Statements
Author contributions
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
Funding
Work at the University of Minnesota and NASA Goddard was supported by the NASA Heliophysics Supporting Research program NNH18ZDA001N-HSR award 80NSSC19K0842. AH and LB were in part supported by the Space Precipitation Impacts project at Goddard Space Flight Center through the Heliophysics Internal Science Funding Model.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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References
1
AndersonK. A.MiltonD. W. (1964). Balloon observations of X-rays in the auroral zone: 3. High time resolution studies. J. Geophys. Res.69 (21), 4457–4479. 10.1029/JZ069i021p04457
2
AnderssonM. E.VerronenP. T.RodgerC. J.CliverdM. A.SeppäläA. (2014). Missing driver in the Sun-Earth connection from energetic electron precipitation impacts mesospheric ozone. Nat. Commun.5, 5197. 10.1038/ncomms6197
3
BlakeJ. B.LooperM. D.BakerD. N.NakamuraR.KleckerB.HovestadtD. (1996). New high temporal and spatial resolution measurements by SAMPEX of the precipitation of relativistic electrons. Adv. Space Res.18 (8), 171–186. 10.1016/0273-1177(95)00969-8
4
BlumL. W.BrenemanA. W. (2020). “Chapter 3—observations of radiation belt losses due to cyclotron wave-particle interactions ’’in The dynamic loss of Earth’s radiation belts. Tharamani, Chennai: Elsevier. 10.1016/B978-0-12-813371-2.00003-2
5
BrenemanA. W.CrewA.SampleJ.KlumparD.JohnsonA.AgapitovO.et al (2017). Observations directly linking relativistic electron microbursts to whistler mode chorus: Van Allen Probes and FIREBIRD II. Geophys. Res. Lett.44 (22), 11265. 10.1002/2017GL075001
6
CapannoloL.LiW.SpenceH.JohnsonA. T.ShumkoM.SampleJ.et al (2021). Energetic electron precipitation observed by FIREBIRD-II potentially driven by EMIC waves: Location, extent, and energy range from a multievent analysis. Geophys. Res. Lett.48 (5), e2020GL091564. 10.1029/2020GL091564
7
ChenL.BrenemanA. W.XiaZ.ZhangX.-J. (2020). Modeling of bouncing electron microbursts induced by ducted chorus waves. Geophys. Res. Lett.47, e2020GL089400. 10.1029/2020GL089400
8
ChenL.ZhangX.-J.ArtemyevA.AngelopoulosV.TsaiE.WilkinsC.et al (2022). Ducted chorus waves cause sub-relativistic and relativistic electron microbursts. Geophys. Res. Lett.49, e2021GL097559. 10.1029/2021GL097559
9
ColpittsC.MiyoshiY.KasaharaY.DelzannoG. L.WygantJ. R.CattellC. A.et al (2020). First direct observations of propagation of discrete Chorus elements from the equatorial source to higher latitudes, using the Van Allen probes and Arase satellites. JGR. Space Phys.125 (10), e2020JA028315. 10.1029/2020JA028315
10
CrewA. B.SpenceH. E.BlakeJ. B.KlumparD. M.LarsenB. A.O'BrienT. P.et al (2016). First multipoint in situ observations of electron microbursts: Initial results from the NSF FIREBIRD II mission. J. Geophys. Res. Space Phys.121 (6), 5272–5283. 10.1002/2016ja022485
11
DentonR. E.OfmanL.ShpritsY. Y.BortnikJ.MillanR. M.RodgerC. J.et al (2019). Pitch angle scattering of sub-MeV relativistic electrons by electromagnetic ion cyclotron waves. JGR. Space Phys.124, 5610–5626. 10.1029/2018JA026384
12
DoumaE.RodgerC.BlumL.O’BrienT.ClilverdM.BlakeJ. (2019). Characteristics of relativistic microburst intensity from SAMPEX observations. JGR. Space Phys.124 (7), 5627–5640. 10.1029/2019JA026757
13
ElliottS. S.BrenemanA. W.ColpittsC.PettitJ. M.CattellC. A.HalfordA. J.et al (2022). Quantifying the size and duration of a microburst-producing chorus region on 5 December 2017. Geophys. Res. Lett.49–e2022GL099655. 10.1029/2022GL099655
14
ImhofW. L.NightingaleR. W. (1992). Relativistic electron enhancements observed over a range of L shells trapped at high altitudes and precipitating at low altitudes into the atmosphere. J. Geophys. Res.97 (A5), 6397–6403. 10.1029/92ja00229
15
JonesS. L.LessardM. R.RychertK.SpanswickE.DonovanE.JaynesA. N. (2013). Persistent, widespread pulsating aurora: A case study. JGR. Space Phys.118, 2998–3006. 10.1002/jgra.50301
16
LanzerottiL. J.BakerD. N. (2017). Space weather research: Earth’s radiation belts. Space weather.15, 742–745. 10.1002/2017SW001654
17
LorentzenK. R.LooperM. D.BlakeJ. B. (2001). Relativistic electron microbursts during the GEM storms. Geophys. Res. Lett.28 (13), 2573–2576. 10.1029/2001gl012926
18
MatthesK.FunkeB.AnderssonM. E.BarnardL.BeerJ.CharbonneauP.et al (2017). Solar forcing for CMIP6 (v3.2). Geosci. Model Dev.10, 2247–2302. 10.5194/gmd-10-2247-2017
19
MeredithN. P.HorneR. B.SandbergI.PapadimitriouC.EvansH. D. R. (2017). Extreme relativistic electron fluxes in the Earth’s outer radiation belt: Analysis of INTEGRAL IREM data. Space weather.15, 917–933. 10.1002/2017SW001651
20
MillanR. M.LinR. P.SmithD. M.LorentzenK. R.McCarthyM. P. (2002). X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer. Geophys. Res. Lett.29 (24), 47-1–47-44. 10.1029/2002GL015922
21
MillanR. M.ThorneR. M. (2007). Review of radiation belt relativistic electron losses. J. Atmos. Sol. Terr. Phys.69, 362–377. 10.1016/j.jastp.2006.06.019
22
MironovaI. A.AplinK. L.ArnoldF.BazilevskayaG. A.HarrisonR. G.KrivolutskyA. A.et al (2015). Energetic particle influence on the earth’s atmosphere. Space Sci. Rev.194 (1-4), 1–96. 10.1007/s11214-015-0185-4
23
MiyoshiY.SaitoS.KuritaS.AsamuraK.HosokawaK.SakanoiT.et al (2020). Relativistic electron microbursts as high energy tail of pulsating Aurora electrons. Geophys. Res. Lett.47 (21), e2020GL090360. 10.1029/2020GL090360
24
NakamuraR.IsowaM.KamideY.BakerD. N.BlakeJ. B.LooperM. (2000). SAMPEX observations of precipitation bursts in the outer radiation belt. J. Geophys. Res.105 (A7), 15875–15885. 10.1029/2000JA900018
25
O'BrienT. P.LooperM. D.BlakeJ. B. (2004). Quantification of relativistic electron microburst losses during the GEM storms. Geophys. Res. Lett.31, L04802. 10.1029/2003GL018621
26
RipollJ.-F.ClaudepierreS. G.UkhorskiyA. Y.ColpittsC.LiX.FennellJ.et al (2020). Particle dynamics in the earth's radiation belts: Review of current research and open questions. J. Geophys. Res. Space Phys.125, e2019JA026735. 10.1029/2019JA026735
27
SeppäläA.ClilverdM. A. (2014). Energetic particle forcing of the northern hemisphere winter stratosphere: Comparison to solar irradiance forcing. Front. Phys.2, 25. 10.3389/fphy.2014.00025
28
SeppäläA.DoumaE.RodgerC.VerronenP.ClilverdM. A.BortnikJ. (2018). Relativistic electron microburst events: Modeling the atmospheric impact. Geophys. Res. Lett.45, 1141–1147. 10.1002/2017GL075949
29
ShumkoM.BlumL. W.CrewA. B. (2021b). Duration of individual relativistic electron microbursts: A probe into their scattering mechanism. Geophys. Res. Lett.48, e2021GL093879. 10.1029/2021GL093879
30
ShumkoM.Gallardo-LacourtB.HalfordA. J.LiangJ.BlumL. W.DonovanE.et al (2021a). A strong correlation between relativistic electron microbursts and patchy aurora. Geophys. Res. Lett.48 (18), e2021GL094696. 10.1029/2021GL094696
31
ShumkoMGallardo-LacourtBHalfordABlumLLiangJMiyoshiYet al (2022). Proton aurora and relativistic electron microbursts scattered by electromagnetic ion cyclotron waves. Front. Astron. Space Sci.9, 975123. 10.3389/fspas.2022.975123
32
SpenceH. E.BlakeJ. B.CrewA. B.DriscollS.KlumparD. M.LarsenB. A.et al (2012). Focusing on size and energy dependence of electron microbursts from the van allen radiation belts. Space weather.10. 10.1029/2012SW000869
33
StaplesF. A.KellermanA.MurphyK. R.RaeI. J.SandhuJ. K.ForsythC. (2022). Resolving magnetopause shadowing using multimission measurements of phase space density. J. Geophys. Res. Space Phys.127, e2021JA029298. 10.1029/2021JA029298
34
ThorneR. M.KennelC. F. (1971). Relativistic electron precipitation during magnetic storm main phase. J. Geophys. Res.76 (19), 4446–4453. 10.1029/ja076i019p04446
35
ThorneR. M.O’BrienT. P.ShpritsY. Y.SummersD.HorneR. B. (2005). Timescale for MeV electron microburst loss during geomagnetic storms. J. Geophys. Res.110, A09202. 10.1029/2004JA010882
36
ThorneR. M. (2010). Radiation belt dynamics: The importance of wave particle interactions. Geophys. Res. Lett.37 (22), L22107. 10.1029/2010GL044990
37
TuW.XiangZ.MorleyS. K. (2019). Modeling the magnetopause shadowing loss during the June 2015 dropout event. Geophys. Res. Lett.46, 9388–9396. 10.1029/2019GL084419
38
van de KampM.SeppäläA.ClilverdM. A.RodgerC. J.VerronenP. T.WhittakerI. C. (2016). A model providing long‐term data sets of energetic electron precipitation during geomagnetic storms. J. Geophys. Res. Atmos.121 (520–12), 12, 520–12, 540. 10.1002/2015JD024212
39
WincklerJ. R.BhavsarP. D.AndersonK. A. (1962). A study of the precipitation of energetic electrons from the geomagnetic field during magnetic storms. J. Geophys. Res.67 (10), 3717–3736. 10.1029/jz067i010p03717
Summary
Keywords
microbursts, chorus, radiation belts, precipitation, wave-particle interactions
Citation
Elliott SS, Breneman A, Colpitts C, Bortnik J, Jaynes A, Halford A, Shumko M, Blum L, Chen L, Greeley A and Turner D (2022) Understanding the properties, wave drivers, and impacts of electron microburst precipitation: Current understanding and critical knowledge gaps. Front. Astron. Space Sci. 9:1062422. doi: 10.3389/fspas.2022.1062422
Received
05 October 2022
Accepted
25 October 2022
Published
03 November 2022
Volume
9 - 2022
Edited by
Michael G. Henderson, Los Alamos National Laboratory (DOE), United States
Reviewed by
Jean-Francois Ripoll, CEA DAM Île-de-France, France
Brian Larsen, Los Alamos National Laboratory (DOE), United States
Updates
Copyright
© 2022 Elliott, Breneman, Colpitts, Bortnik, Jaynes, Halford, Shumko, Blum, Chen, Greeley and Turner.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Sadie S. Elliott, tetri006@umn.edu
This article was submitted to Space Physics, a section of the journal Frontiers in Astronomy and Space Sciences
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.