Investigation of electron radiation belt dropouts has revealed the importance of a number of loss processes, yet there remains a lack of quantitative detail as to how these processes wax and wane ...between events. The overarching aim of this study is to address the issue of electron radiation belt dropouts. This is achieved using in situ observations at geostationary orbit from GOES‐13 (pitch angle‐resolved electron data and magnetic field measurements) to examine the outer electron radiation belt during three high‐speed stream‐driven storms. Analysis and interpretation are aided by calculation of the phase space density (PSD) as a function of the three adiabatic invariants. Our results confirm the importance of outward adiabatic transport as a mechanism for causing electron dropouts at geosynchronous orbit; however, study of the pitch angle distributions indicates that other loss mechanisms are also likely to be occurring during these high‐speed solar wind stream (HSS)‐driven storms. Two of the studied events exhibit similar evolutionary structure in their pitch angle distributions: (i) highly peaked distributions immediately prior to the dropout (ii) sharp transitions between peaked and isotropic and then subsequent butterfly distributions, and (iii) isotropic distributions at minimum flux shortly afterwards (dusk). We also address the difficulty in interpreting PSD calculations by comparing the T96 model magnetic field with that measured by GOES‐13. Our results are intended as a first step in quantifying the timeline of events that occur in the radiation belts following the arrival of a HSS—particularly timely given the increase in HSS occurrence expected in the declining phase of the current solar cycle.
Key Points
Radial transport plays a key role in causing electron dropouts at GEO
Pitch‐angle‐distributions for 2 events display similar evolutionary structure
First step in quantifying the timeline of events that occur following a HSS
Space weather instrumentation on board the National Oceanic and Atmospheric Administration's (NOAA's) newest Geostationary Operational Environmental Satellite (GOES)‐R series includes the Solar and ...Galactic Proton Sensor (SGPS), which has been collecting data since January 8, 2017. SGPS supports real‐time alerts of solar energetic particle events at the NOAA Space Weather Prediction Center (SWPC) and provides data to the space science community, advancing basic space science research and understanding of space weather effects on satellite systems. The first GOES‐R series spacecraft, GOES‐16, was launched on November 19, 2016. A series of solar particle events in September 2017 enabled cross‐calibration of GOES‐16 SGPS with the Energetic Particle Sensors on GOES‐13 and ‐15. This paper is intended as a resource for SGPS data users, including comparisons with legacy GOES energetic particle measurements, corrections applied to SGPS Level‐2 data, important caveats, background level fluxes, and measurements of trapped magnetospheric protons.
Plain Language Summary
Instrumentation on board the National Oceanic and Atmospheric Administration's (NOAA's) weather satellites includes the Solar and Galactic Proton Sensor (SGPS), which is used by NOAA Weather Prediction Center to monitor the radiation hazard to spacecraft systems and humans in space from energetic solar protons. A series of solar particle events in September 2017 enabled comparisons of SGPS measurements with the Energetic Particle Sensors on NOAA's older Geostationary Operational Environmental Satellite (GOES)‐13 and ‐15 spacecraft. Comparisons with legacy GOES energetic particle measurements are critical for establishing consistent long‐term data sets and understanding changes in long‐term trends in solar energetic particle event fluxes.
Key Points
This paper includes a comprehensive description of Solar and Galactic Proton Sensor observations
An overview of the September 2017 solar particle event observations is presented
Results from cross‐calibrations with Geostationary Operational Environmental Satellite‐13 and 15 Energetic Particle Sensors are also included
Magnetic reconnection has been established as the dominant mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum ...to flow into the magnetosphere. One of the persistent problems of magnetic reconnection is the question of whether the process is continuous or intermittent and what input condition(s) might favor one type of reconnection over the other. Observations from imagers that record FUV emissions caused by precipitating cusp ions demonstrate the global nature of magnetic reconnection. Those images show continuous ionospheric emissions even during changing interplanetary magnetic field conditions. On the other hand, in situ observations from polar‐orbiting satellites show distinctive cusp structures in flux distributions of precipitating ions, which are interpreted as the telltale signature of intermittent reconnection. This study uses a modification of the low‐velocity cutoff method, which was previously successfully used to determine the location of the reconnection site, to calculate for the cusp ion distributions the “time since reconnection occurred.” The “time since reconnection” is used to determine the “reconnection time” for the cusp magnetic field lines where these distributions have been observed. The profile of the reconnection time, either continuous or stepped, is a direct measurement of the nature of magnetic reconnection at the reconnection site. This paper will discuss a continuous and pulsed reconnection event from the Polar spacecraft to illustrate the methodology.
Key Points
Methodology to distinguish pulsed form continuous reconnection
Determine the reconnection time for cusp magnetic field lines
Determine the reconnection location simultaneously
The radiation belt electrons in Earth's magnetosphere exhibit substantial variability driven by changing solar wind conditions. The electron dynamics are due to a number of different adiabatic and ...nonadiabatic processes that can result in rapid increases and decreases in the particle flux levels. In this paper we present observations of abrupt flux decreases driven by a moderate geomagnetic storm. The particle dynamics are found to have significant local time and energy dependence that developed over roughly a 10‐hour period beginning with the onset of the storm. The electrons with energies greater than 2 MeV dropped fairly abruptly at various local times, but not simultaneously at different local times. It is shown that the initial flux dropout was due to the development of local taillike magnetic field stretching, rather than due to more global processes such as ring current buildup or large‐scale radial diffusion. It is also found that while the lower energy electrons (E < 300 keV) fully recovered by the end of the storm, the >2 MeV electrons were lost from the magnetosphere and did not recover. These results indicate that the initial dropout of the radiation belt electrons at geostationary orbit was controlled by the adiabatic response to localized changes in the geomagnetic field that develop over many hours, but that eventually nonadiabatic processes acted to cause the loss of electrons from the magnetosphere. It is also shown that during geomagnetically quiet conditions, the energetic electron flux can remain at nearly constant levels for as long as 1 week, suggesting that in the absence of geomagnetic activity either the outer radiation belt electron loss rate becomes quite small or the loss and growth rates are balanced.
The Polar spacecraft had a prolonged encounter with the high‐latitude dayside magnetopause on May 29, 1996. This encounter with the magnetopause occurred when the interplanetary magnetic field was ...directed northward. From the three‐dimensional electron and ion distribution functions measured by the Hydra instrument, it has been possible to identify nearly all of the distinct boundary layer regions associated with high‐latitude reconnection. The regions that have been identified are (1) the cusp; (2) the magnetopause current layer; (3) magnetosheath field lines that have interconnected in only the Northern Hemisphere; (4) magnetosheath field lines that have interconnected in only the Southern Hemisphere; (5) magnetosheath field lines that have interconnected in both the Northern and Southern Hemispheres; (6) magnetosheath that is disconnected from the terrestrial magnetic field; and (7) high‐latitude plasma sheet field lines that are participating in magnetosheath reconnection. Reconnection over this time period was occurring at high latitudes over a broad local‐time extent, interconnecting the magnetosheath and lobe and/or plasma sheet field lines in both the Northern and Southern Hemispheres. Newly closed boundary layer field lines were observed as reconnection occurred first at high latitudes in one hemisphere and then later in the other. These observations establish the location of magnetopause reconnection during these northward interplanetary magnetic field conditions as being at high latitudes, poleward of the cusp, and further reinforce the general interpretation of electron and ion phase space density signatures as indicators of magnetic reconnection and boundary layer formation.
We describe an energy spectrum method for scaling electron integral flux, which is measured at a constant energy, to phase space density at a constant value of the first adiabatic invariant which ...removes much of the variation due to reversible adiabatic effects. Applying this method to nearly a solar cycle (1995–2006) of geosynchronous electron integral flux (E > 2.0 MeV) from the GOES satellites, we see that much of the diurnal variation in electron phase space density at constant energy can be removed by the transformation to phase space density at constant μ (4000 MeV/G). This allows us a clearer picture of underlying nonadiabatic electron population changes due to geomagnetic activity. Using scaled phase space density, we calculate the percentage of geomagnetic storms resulting in an increase, decrease, or no change in geosynchronous electrons as 38%, 7%, and 55%, respectively. We also show examples of changes in the electron population that may be different from the unscaled fluxes alone suggest. These examples include sudden electron enhancements during storms which appear during the peak of negative Dst for μ‐scaled phase space density, contrary to the slow increase seen during the recovery phase for unscaled phase space density for the same event.
Key Points
Adiabatic variation obscures nonadiabatic changes in radiation belt electrons
Using a measured energy spectrum to scale PSD, mu variation can be removed
Storm time enhancement of electrons is more common than loss
The trapped radiation belt electron population is maintained through a competition between multiple source and loss processes occurring within the magnetosphere and driven by the solar wind. In this ...research we have concentrated on the solar wind and the magnetospheric conditions that lead to the loss of electrons through abrupt energetic electron flux dropouts. We have focused on times when there is only a moderate level of geomagnetic activity, since the magnetospheric response during these conditions is expected to be far less complex than during large geomagnetic storms. We have found that under certain circumstances the radiation belt electrons are remarkably sensitive to the onset of southward IMF and to solar wind dynamic pressure increases. The onset of southward IMF is found to be sufficient to cause the flux dropouts, while increases in solar wind pressure are not necessary but are likely to enhance the loss when they occur in conjunction with southward IMF, as is often the case. It is not clear if an increase in solar wind pressure in the absence of southward IMF is sufficient to cause a flux dropout. The radiation belt fluxes can decrease by more than an order of magnitude with the onset of only minor geomagnetic activity. The level of solar wind forcing (as estimated by the epsilon parameter) and of geomagnetic activity (as estimated by AE, Dst, and the local magnetic field inclination at geosynchronous orbit) responsible for the flux loss is intermediate between lower levels of activity that create localized, adiabatic variations in the flux and large geomagnetic storms that result in both loss and acceleration. The dropout events examined here occurred after one or more days of quiet geomagnetic conditions, which we suggest preconditioned the magnetosphere to be highly sensitive to the onset of new activity. Although it is not known which specific conditions within the magnetosphere lead to this extreme sensitivity of the relativistic electrons, the time periods identified here are ones where the electron loss processes appear to operate in relative isolation of the acceleration processes.
Stakeholders met at the recent European Space Weather Week to discuss the development of a satellite anomaly database. This summary reviews the presentations and suggestions given at the discussion ...session. Presenters reviewed ongoing international efforts to collect and distribute satellite anomaly information. Well‐defined suggestions were given for different possible frameworks. Group discussion after suggested that a way forward would be to first demonstrate the utility of an anomaly database with available public anomaly information. More discussion is needed to define the exact framework for future implementation.
Key Point
Both commercial and government have interest in curated and shared satellite anomaly information
International efforts through agencies such as the Coordination Group for Meteorological Satellites (CGMS) are underway to routinely collect satellite anomaly information
The global context of the 14 November 2012 storm event Hwang, K.‐J.; Sibeck, D. G.; Fok, M.‐C. H. ...
Journal of geophysical research. Space physics,
March 2015, 2015-03-00, 20150301, 2015-03-01, Letnik:
120, Številka:
3
Journal Article
Recenzirano
Odprti dostop
From 2 to 5 UT on 14 November 2012, the Van Allen Probes observed repeated particle flux dropouts during the main phase of a geomagnetic storm as the satellites traversed the post‐midnight to ...dawnside inner magnetosphere. Each flux dropout corresponded to an abrupt change in the magnetic topology, i.e., from a more dipolar configuration to a configuration with magnetic field lines stretched in the dawn‐dusk direction. Geosynchronous GOES spacecraft located in the dusk and near‐midnight sectors and the LANL constellation with wide local time coverage also observed repeated flux dropouts and stretched field lines with similar occurrence patterns to those of the Van Allen Probe events. THEMIS recorded multiple transient abrupt expansions of the evening‐side magnetopause ∼20–30 min prior to the sequential Van Allen Probes observations. Ground‐based magnetograms and all sky images demonstrate repeatable features in conjunction with the dropouts. We combine the various in situ and ground‐based measurements to define and understand the global spatiotemporal features associated with the dropouts observed by the Van Allen Probes. We discuss various proposed hypotheses for the mechanism that plausibly caused this storm‐time dropout event as well as formulate a new hypothesis that explains the combined in situ and ground‐based observations: the earthward motion of magnetic flux ropes containing lobe plasmas that form along an extended magnetotail reconnection line in the near‐Earth plasma sheet.
Key Points
The cause of multiple flux dropouts during the storm main phase
Comprehensive analyses using data from multispacecraft and ground monitors
Propagation from tail to inner magnetosphere of flux rope containing lobe plasma
Earth's outer electron radiation belt is a highly dynamic region, driven externally by the solar wind and controlled within the magnetosphere by numerous source and loss processes. Critical to ...assessing the importance of the different source and loss processes and to developing accurate physics‐based models of the radiation belts is knowing phase space density and its radial gradient in terms of adiabatic invariants. In this paper, we demonstrate a new technique for determining the radial gradient of relativistic electron phase space density at geosynchronous orbit. This technique utilizes the fact that the GOES geosynchronous satellites are located at different geomagnetic latitudes because of their separation in longitude, even though both are located on the geographic equator. From simultaneous measurements at different geomagnetic latitudes, and therefore in different L shells, we are able to obtain the local radial gradient of phase space density. We have restricted these initial calculations to a single value of magnetic moment (M = 6000 MeV/G) and to equatorially mirroring particles. In order to minimize the sensitivity of our results to the magnetic field model used, we have analyzed data from a time period with quiet geomagnetic conditions, when the relativistic electron flux was elevated and steady. We find that during this time period the radial gradient of phase space density was positive. This indicates that the direction of radial diffusion was inward, transporting electrons from outside to within geosynchronous orbit.
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