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  • 1
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    Evolution of relativistic outer belt electrons during an extended quiescent period (2014)
    Wiley
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    Publication Date: 2014-10-21
    Description: To effectively study steady loss due to hiss-driven precipitation of relativistic electrons in the outer radiation belt, it is useful to isolate this loss by studying a time of relatively quiet geomagnetic activity. We present a case of initial enhancement and slow, steady decay of 700 keV - 2 MeV electron populations in the outer radiation belt during an extended quiescent period from ~15 December 2012 - 13 January 2013. We incorporate particle measurements from a constellation of satellites, including the Colorado Student Space Weather Experiment (CSSWE) CubeSat, the Van Allen Probes twin spacecraft, and THEMIS, to understand the evolution of the electron populations across pitch angle and energy. Additional data from calculated phase space density (PSD), as well as hiss and chorus wave data from Van Allen Probes, helps complete the picture of the slow precipitation loss of relativistic electrons during a quiet time. Electron loss to the atmosphere during this event is quantified through use of the Loss Index Method, utilizing CSSWE measurements at LEO. By comparing these results against equatorial Van Allen Probes electron flux data, we conclude the net precipitation loss of the outer radiation belt content to be greater than 92%, suggesting no significant acceleration during this period, and resulting in faster electron loss rates than have previously been reported.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    EMIC wave spatial and coherence scales as determined from multipoint Van Allen Probe measurements (2016)
    Wiley
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    Publication Date: 2016-05-12
    Description: Electromagnetic ion cyclotron (EMIC) waves can provide a strong source of energetic electron pitch angle scattering. These waves are often quite localized, however, thus their spatial extent can have a large effect on their overall scattering efficiency. Using measurements from the dual Van Allen Probes, we examine four EMIC wave events observed simultaneously on the two probes at varying spacecraft separations. Correlation of both the wave amplitude and phase observed at both spacecraft is examined to estimate the active region and coherence scales of the waves. We find well-correlated wave amplitude and amplitude modulation across distances spanning hundreds to thousands of km. Phase coherence persisting 30-60 s is observable during close conjunction events but is lost as spacecraft separations exceed ~1 Earth Radii.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Observations of coincident EMIC wave activity and dusk-side energetic electron precipitation on 18–19 January 2013 (2015)
    Wiley
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    Publication Date: 2015-07-15
    Description: Electromagnetic ion cyclotron (EMIC) waves have been suggested to be a cause of radiation belt electron loss to the atmosphere. Here simultaneous, magnetically conjugate measurements are presented of EMIC wave activity, measured at geosynchronous orbit and on the ground, and energetic electron precipitation, seen by the BARREL balloon campaign, on two consecutive days in January 2013. Multiple bursts of precipitation were observed on the dusk-side of the magnetosphere at the end of 18 Jan and again late on 19 Jan, concurrent with particle injections, substorm activity, and enhanced magnetospheric convection. The structure, timing, and spatial extent of the waves are compared to those of the precipitation during both days to determine when and where EMIC waves cause radiation belt electron precipitation. The conjugate measurements presented here provide observational support of the theoretical picture of dusk-side interaction of EMIC waves and MeV electrons leading to radiation belt loss.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 4
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    A comparison of magnetic field measurements and a plasma-based proxy to infer EMIC wave distributions at geosynchronous orbit (2012)
    Wiley
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    Publication Date: 2012-05-11
    Description: Wave-particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earth's atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma-based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single-wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Characteristics of Relativistic Microburst Intensity From SAMPEX Observations (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Relativistic electron microbursts are an important electron loss process from the radiation belts into the atmosphere. These precipitation events have been shown to significantly impact the radiation belt fluxes and atmospheric chemistry. In this study we address a lack of knowledge about the relativistic microburst intensity using measurements of 21,746 microbursts from the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX). We find that the relativistic microburst intensity increases as we move inward in L, with a higher proportion of low‐intensity microbursts (〈2,250 [MeV cm2 sr s]−1) in the 03–11 magnetic local time region. The mean microburst intensity increases by a factor of 1.7 as the geomagnetic activity level increases and the proportion of high‐intensity relativistic microbursts (〉2,250 [MeV cm2 sr s]−1) in the 03–11 magnetic local time region increases as geomagnetic activity increases, consistent with changes in the whistler mode chorus wave activity. Comparisons between relativistic microburst properties and trapped fluxes suggest that the microburst intensities are not limited by the trapped flux present alongside the scattering processes. However, microburst activity appears to correspond to the changing trapped flux; more microbursts occur when the trapped fluxes are enhancing, suggesting that microbursts are linked to processes causing the increased trapped fluxes. Finally, modeling of the impact of a published microburst spectra on a flux tube shows that microbursts are capable of depleting 〈500‐keV electrons within 1 hr and depleting higher‐energy electrons in 1–23 hr.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Correspondence between a plasma-based EMIC wave proxy and subauroral proton precipitation (2011)
    Wiley
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    Publication Date: 2011-12-13
    Description: The loss of relativistic electrons from the Earth's radiation belts as a result of resonant interactions with electromagnetic ion cyclotron waves (EMIC) waves has yet to be fully quantified, in part, due to the lack of global measurements of the wave distribution during individual storm events. Recent work has focused on augmenting direct wave measurements with proxy wave indicators. Here we compare two different techniques for inferring the presence of EMIC waves: 1) a wave-growth proxy and amplitude estimate based on in situ plasma measurements of the cold and hot ion distributions, and 2) FUV observations of subauroral proton precipitation, which is thought to result from interactions with EMIC waves. For two event intervals, we show good correspondence between proxy predictions of wave growth, calculated using measurements from geostationary spacecraft, and precipitation observed at the northern hemisphere ionospheric footprint. Further, for times when the proxy is positive, we observe a moderate positive correlation (r = 0.56) between the predicted wave amplitude and the mean FUV brightness in a 300-km circle about the footprint. Further development and verification of these techniques will enhance our ability to infer the global distribution of EMIC waves when direct measurements are not available.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Increase in salicylic Acid at the onset of systemic acquired resistance in cucumber (1990)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 1990-11-16
    Description: In an effort to identify the signal compound that mediates systemic acquired resistance (SAR), changes in the content of phloem sap were monitored in cucumber plants inoculated with either tobacco necrosis virus or the fungal pathogen Colletotrichum lagenarium. The concentration of a fluorescent metabolite was observed to increase transiently after inoculation, with a peak reached before SAR was detected. The compound was purified and identified by gas chromatography-mass spectrometry as salicylic acid, a known exogenous inducer of resistance. The data suggest that salicylic acid could function as the endogenous signal in the transmission of SAR in cucumber.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Metraux, J P -- Signer, H -- Ryals, J -- Ward, E -- Wyss-Benz, M -- Gaudin, J -- Raschdorf, K -- Schmid, E -- Blum, W -- Inverardi, B -- New York, N.Y. -- Science. 1990 Nov 16;250(4983):1004-6.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17746926" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 8
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    Occurrence characteristics of relativistic electron microbursts from SAMPEX observations (2017)
    Wiley
    Details
    Publication Date: 2017-07-19
    Description: We study the occurrence of relativistic microbursts observed by the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) satellite. An algorithm is used to identify 193,694 relativistic microbursts in the 〉 1.05 MeV electron fluxes occurring across the time period 23 August 1996 to 11 August 2007, nearly a full solar cycle. Our observations are normalized to provide the change in absolute occurrence rates with various parameters. We find that relativistic microbursts are mostly confined to the outer radiation belt, from L = 3 – 8, occurring primarily on the morning side, between 0 and 13 Magnetic Local Time (MLT). This L and MLT distribution is consistent with the L and MLT distribution of whistler mode chorus amplitude. Thus our observations favor whistler mode chorus wave activity as a driver of relativistic microbursts. Relativistic microbursts become more frequent as the geomagnetic activity level increases and are more frequent during equinoxes than during the solstices. The peak occurrence frequency of the relativistic microbursts moves to lower L as the geomagnetic activity increases, reaching a peak occurrence rate of one microburst every 10.4 s (on average) at L = 4 for 6.6 ≤ Kp ≤ 8.7. Microbursts primarily occur outside of the plasmapause and track the inward movement of the plasmapause with increasing geomagnetic activity. The L and MLT distribution of the relativistic microbursts exhibit a peak occurrence of one microburst every 8.6 (98.0) s during active (disturbed) conditions, with the peak located at L = 5 ( L = 5.5) and 08 (08) MLT.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    New conjunctive CubeSat and balloon measurements to quantify rapid energetic electron precipitation (2013)
    Wiley
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    Publication Date: 2013-11-10
    Description: [1]  Relativistic electron precipitation into the atmosphere can contribute significant losses to the outer radiation belt. In particular, rapid narrow precipitation features termed precipitation bands have been hypothesized to be an integral contributor to relativistic electron precipitation loss but quantification of their net effect is still needed. Here we investigate precipitation bands as measured at low earth orbit by the CSSWE CubeSat. Two precipitation bands of MeV electrons were observed on Jan 18–19, 2013, concurrent with precipitation seen by the 2013 BARREL balloon campaign. The newly available conjugate measurements allow for a detailed estimate of the temporal and spatial features of precipitation bands for the first time. We estimate the net electron loss due to the precipitation bands and find that ~20 such events could empty the entire outer belt. This study suggests that precipitation bands play a critical role in radiation belt losses.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    Chorus whistler wave source scales as determined from multipoint Van Allen Probe measurements (2017)
    Wiley
    Details
    Publication Date: 2017-03-17
    Description: Whistler-mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The key parameters for both nonlinear and quasi-linear treatment of wave-particle interactions are the temporal and spatial scales of the wave source region and coherence of the wave field perturbations. Neither the source scale nor the coherence scale, are well-established experimentally, mostly because of a lack of multi-point VLF waveform measurements. We present an unprecedentedly long interval of coordinated VLF waveform measurements (sampled at 16384 s -1 ) aboard the two Van Allen Probes spacecraft - nine hours (0800-1200UT and 1700-2200UT) during two consecutive apogees on July 15, 2014.The spacecraft separations varied from about 100 to 5000 km (mostly radially); measurements covered an L -shell range from 3 to 6; MLT 0430-0900, and magnetic latitudes were ~15 and ~5 degrees during the two orbits. Using time-domain correlation techniques, the single chorus source spatial extent transverse to the background magnetic field has been determined to be about 550-650 km for upper band chorus waves with amplitudes less than 100 pT and up to 800 km for larger amplitude, lower band chorus waves. The ratio between wave amplitudes measured on the two spacecraft is also examined to reveal that the wave amplitude distribution within a single chorus element generation area can be well approximated by a Gaussian exp(–0.5* r 2 / r 0 2 ), with the characteristic scale r 0 around 300 km. Waves detected by the two spacecraft were found to be coherent in phase at distances up to 400 km.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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