Solar, Anomalous, And Magnetospheric Particle Explorer Research Articles

Pitch Angle Isotropy of Relativistic Electron Microbursts as Observed by SAMPEX/HILT: Statistical and Storm‐Time Properties

AbstractObservations of relativistic electron precipitation events from Earth's inner magnetosphere to the upper atmosphere provide essential information about the source/loss processes and dynamics of Earth's radiation belts. Equatorial electron pitch angle isotropy can be an indicator of electron precipitation into the atmosphere, with isotropic populations resulting in a full bounce loss cone (BLC) and anisotropic populations resulting in an emptier BLC. Here we study electron microbursts, which are ˜100 millisecond duration streams of relativistic electrons thought to be scattered into the BLC by particle‐wave scattering. By measuring the ratios of >1 MeV electron flux across different detector rows in the Heavy Ion Large Telescope (HILT) instrument onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, we quantify the pitch angle isotropy of an electron population. Applying this calculation to microburst events, we have determined the dependence of microburst pitch angle distributions on burst magnitude, magnetic local time (MLT), L‐Shell, and storm phase. We found that high flux magnitude microbursts are typically highly isotropic, while lower flux magnitude microbursts have varying degrees of anisotropy. Furthermore, microbursts are most isotropic as compared to the background population in the afternoon/evening sector as well as initial and main phases of geomagnetic storms. These results have important implications for microburst scattering mechanisms as well as quantification of precipitation driven by this process.

A Strong Correlation Between Relativistic Electron Microbursts and Patchy Aurora

AbstractIn this letter, we present the results of a conjunction between the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite and a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager in Gillam, Canada, showing a high correlation between relativistic, MeV, electron microbursts and a type of pulsating aurora called patchy aurora. The correlation was 0.8, and is not serendipitous. While the relationship between pulsating aurora and 10–100s keV microbursts has been previously predicted, here we show a strong association between keV and MeV electron dynamics, possibly spanning two orders of magnitude. Importantly, this result shows that the dynamics of relativistic radiation belt electrons are at times intimately tied to keV electron precipitation, and cannot be studied in isolation.

Semiannual variation in relativistic electron fluxes of the outer radiation belt: Phases comparison with classical hypotheses predictions

Abstract Relativistic electron fluxes of the outer Van Allen belt present a clear semiannual signature, with maxima fluxes near the equinoxes and minima near the solstices. We investigate the semiannual variation (SAV) of 3.75 MeV and 3.4 MeV electron fluxes from Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and Van Allen probe A (RBSP-A) data, respectively. The fluxes of both missions are binned in McIlwain’s L-shell values and in day of year (DOY). In order to obtain the SAV from the data, a superposed epoch analysis and then a 30-day smoothing is carried out for the entire L-shell range utilized. The results show clear semiannual patterns with maximum differences of ~ 1 and ~ 2 orders of magnitude between values near the equinoxes and solstices, for SAMPEX and RBSP-A fluxes, respectively. The comparison of the maxima-minima dates of the observed SAVs with corresponding dates predicted by the Axial, Equinoctial and Russell & McPherron (RM) theory shows that the latter gives the best estimations for both data sets. However, considerable delays from the theories’ predictions are found. The most important delay occurs in the Mar/Apr maximum and is found to be ~ 26.8 days on average for L ≳ 3.5 RBSP-A data. In the case of SAMPEX, this delay is found for L 3.8 with a maximum of 22.4 days. Accurately determining the Mar/Apr maximum and the Sep/Oct maximum has implications in space weather since it indicates when the SAV will affect the most particular flux enhancements capable of inducing satellite operational anomalies. .

New Insights From Long-Term Measurements of Inner Belt Protons (10s of MeV) by SAMPEX, POES, Van Allen Probes, and Simulation Results.

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission provided long‐term measurements of 10s of megaelectron volt (MeV) inner belt (L < 2) protons (1992–2009) as did the Polar‐orbiting Operational Environmental Satellite‐18 (POES‐18, 2005 to present). These long‐term measurements at low‐Earth orbit (LEO) showed clear solar cycle variations which anticorrelate with sunspot number. However, the magnitude of the variation is much greater than the solar cycle variation of galactic cosmic rays (>GeV) that are regarded as a source of these trapped protons. Furthermore, the proton fluxes and their variations sensitively depend on the altitude above the South Atlantic Anomaly (SAA) region. With respect to protons (>36 MeV) mirroring near the magnetic equator, both POES measurements and simulations show no obvious solar cycle variations at L > 1.2. This is also confirmed by recent measurements from the Van Allen Probes (2012–2019), but there are clear solar cycle variations and a strong spatial gradient of the proton flux below L = 1.2. A direct comparison between measurements and simulations leads to the conclusion that energy loss of trapped protons due to collisions with free and bound electrons in the ionosphere and atmosphere is the dominant mechanism for the strong spatial gradient and solar cycle variation of the inner belt protons. This fact is also key of importance for spacecraft and instrument design and operation in near‐Earth space.

The Secular Variation of the Center of Geomagnetic South Atlantic Anomaly and Its Effect on the Distribution of Inner Radiation Belt Particles

AbstractIn this study, we carry out a comprehensive analysis of secular variation of the South Atlantic Anomaly. The center location of the South Atlantic Anomaly (SAA) is derived from the proton flux measurements of different energy channels by SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer)/PET (Proton/Electron Telescope). The results are compared with National Oceanic and Atmospheric Administration/Medium Energy Proton and Electron Detector observations and the International Geomagnetic Reference Field 12 (IGRF12) geomagnetic field model. It is found that different energy protons have different behaviors while the center location of the SAA changes with altitude. The IGRF12 magnetic field can explain the observations to a limited extent. Possible reasons for the differences are discussed. It is confirmed that center location of the SAA is also affected by atmospheric density at SAMPEX altitudes. The results could be helpful for constructing inner radiation belt models.

SAMPEX observations of the South Atlantic anomaly secular drift during solar cycles 22–24

AbstractIt is observed that charged particle intensities are very high near the South Atlantic anomaly (SAA) and are a potential hazard to spacecraft passing through the region. In this study, we examine the secular drift of the SAA location at ∼400–600 km altitude over nearly two solar cycles, using particle count rates to trace the geomagnetic field lines in the region near the SAA. We use data from the Low‐Energy Ion Composition Analyzer sensor on board the SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) spacecraft to measure both the longitudinal and latitudinal drifts of the SAA. We find that the longitudinal drift rate is 0.20 ± 0.04° west per year and that the latitudinal drift rate is 0.11 ± 0.01° south per year. These measurements are compared with the IGRF12 (International Geomagnetic Reference Field) model calculations based on an analysis of magnetic field minima in the region of the SAA. Our results, which are in good agreement with model results and prior measurements when declining spacecraft altitude is taken into account, have important space weather implications.

Fast injection of the relativistic electrons into the inner zone and the formation of the split‐zone structure during the Bastille Day storm in July 2000

AbstractDuring the July 2000 geomagnetic storm, known as the Bastille Day storm, Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX)/Heavy Ion Large Telescope (HILT) observed a strong injection of ~1 MeV electrons into the slot region (L ~ 2.5) during the storm main phase. Then, during the following month, electrons were clearly seen diffusing inward down to L = 2 and forming a pronounced split structure encompassing a narrow, newly formed slot region around L = 3. SAMPEX observations are first compared with electron and proton observations on HEO‐3 and NOAA‐15 to validate that the observed unusual dynamics was not caused by proton contamination of the SAMPEX instrument. The time‐dependent 3‐D Versatile Electron Radiation Belt (VERB) simulation of 1 MeV electron flux evolution is compared with the SAMPEX/HILT observations. The results show that the VERB code predicts overall time evolution of the observed split structure. The simulated split structure is produced by pitch angle scattering into the Earth atmosphere of ~1 MeV electrons by plasmaspheric hiss.

The causes of the hardest electron precipitation events seen with SAMPEX

AbstractWe studied the geomagnetic, plasmaspheric, and solar wind context of relativistic electron precipitation (REP) events seen with the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), Proton Electron Telescope (PET) to derive an exponential folding energy E0 for each event. Events with E0&lt; 400 keV peak near midnight, and with increasing E0, the peak magnetic local time (MLT) moves earlier but never peaks as early as the MLT distribution of electromagnetic ion cyclotron (EMIC) waves in the outer belt, and a distinct component near midnight remains. Events with E0&gt;750 keV near dusk (1400 &lt; MLT &lt; 2000) show correlations with solar wind dynamic pressure and proton density, AE index, negative Dst index, and an extended plasmasphere, all supporting an EMIC wave interpretation. Events with 500 keV &lt;E0&lt; 600 keV near midnight (MLT 2200–0200) do not show these correlations. Comparing these two samples to all events with E0&gt;500 keV (“hard REP”), we estimate that roughly 45% of the whole population has the distributions of geomagnetic and solar wind parameters associated with EMIC waves, while 55% does not. We hypothesize that the latter events may be caused by current sheet scattering (CSS), which can be mistaken for EMIC wave scattering in that both simultaneously precipitate MeV electrons and keV protons. Since a large number of MeV electrons are lost in the near‐midnight hard REP events, and in the large number of E0&lt; 400 keV events that show no dusk‐like peak at all, we conclude that CSS should be studied further as a possibly important loss channel for MeV electrons.

Measurement of inner radiation belt electrons with kinetic energy above 1 MeV

AbstractData from the Proton‐Electron Telescope on the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, taken during 1992–2009, are analyzed for evidence of inner radiation belt electrons with kinetic energy E &gt; 1 MeV. It is found that most of the data from a detector combination with a nominal energy threshold of 1 MeV were, in fact, caused by a chance coincidence response to lower energy electrons or high‐energy protons. In particular, there was no detection of inner belt or slot region electrons above 1 MeV following the 2003 Halloween storm injection, though they may have been present. However, by restricting data to a less‐stable, low‐altitude trapping region, a persistent presence of inner belt electrons in the energy range 1 to 1.6 MeV is demonstrated. Their soft, exponential energy spectra are consistent with extrapolation of lower energy measurements.

Rapid MeV electron precipitation as observed by SAMPEX/HILT during high‐speed stream‐driven storms

AbstractA number of relativistic electron loss processes exist in the inner magnetosphere, and the extent to which MeV electron precipitation into Earth's atmosphere plays a role in radiation belt dynamics is a topic of much debate. In this work, we investigate the contribution of electron precipitation to radiation belt losses, looking at what times and locations precipitation is important. Through high‐cadence low‐altitude measurements from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, we examine the distributions of millisecond (microburst) as well as longer‐duration (band‐type) precipitation and the relative contributions of these two precipitation types to radiation belt dynamics during high‐speed stream (HSS)‐driven storms. Different local time and radial distributions between microbursts and precipitation bands suggest different scattering mechanisms as the causes of the two precipitation types. In a superposed epoch study of 42 HSS‐driven storms, enhanced main and recovery phase losses to the atmosphere are observed. Microburst occurrence rates peak in the recovery phase of the storms, while their magnitudes remain fairly constant over storm phase. Precipitation bands show an increase in both occurrence and magnitude at storm onset, particularly at the inner edge of the outer radiation belt. The observations, enabled by the high time resolution and large geometric factor and field of view of the SAMPEX/Heavy Ion Large Telescope (HILT) instrument, reveal when and where microburst and band‐type precipitation are contributing to radiation belt dynamics during HSS‐driven storms.

On long decays of electrons in the vicinity of the slot region observed by HEO3

AbstractLong decay periods of electron counts, which follow abrupt rises and last from weeks to months, have been observed by the HEO3 spacecraft in the vicinity of the slot region between the years 1998 and 2007. During the most stable decay periods as selected, e‐folding timescales are extracted and statistically analyzed from observations as a function of L‐shell and electron energy. A challenge is to reproduce the observed timescales from simulations of pitch angle diffusion by three acting waves—the plasmaspheric hiss, lightning‐generated whistlers, and VLF transmitter waves. We perform full numerical simulations to accurately compute electron lifetimes. We choose to use the method and wave parameters proposed by Abel and Thorne in 1998 with the goal to assess whether they can reproduce lifetimes extracted from HEO observations. We show how hiss dominantly affect high energy electrons (E &gt; 2 MeV) for L in [2, 3.5] and VLF transmitter waves control residency times of low energy electrons (&lt;0.4 MeV) around L = 2. These interactions induce characteristic shapes of the lifetime profiles that will be discussed. We show how the wave amplitudes can be adjusted for the particular energy particles that are dominantly affected by one wave type only. Using these amplitudes, mean HEO lifetimes are reproduced within a factor 2 to 5. VLF occurrence rates and hiss amplitude turn out significantly higher than those proposed by Abel and Thorne. The wide energy response of the sensors complicates the analysis because it blurs the electron lifetime dependence on energy, increases the overall lifetimes, and reduces the differences between the different channel lifetimes. In particular, our simulations suggest the flux measured by an integrated energy sensor aboard HEO has a variable slope, i.e., a variable lifetime, during 10–20 days in our data, due to the faster decay of the low residency time particles while slower decaying particles control the steady decay. It can explain some of the multi‐slopes decays observed by HEO. HEO electron long decay timescales are also compared to the timescales previously observed from Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) and Combined Release and Radiation Effects Satellite (CRRES) with differences attributed to factors such as instrument characteristic and different satellite orbits.

Inward shift of outer radiation belt electrons as a function of Dst index and the influence of the solar wind on electron injections into the slot region

The radial positioning of radiation belt electrons as a function of the Dst index and the controlling solar wind parameters for deep penetration of energetic electrons into the inner magnetosphere are investigated. Using 2–6 MeV electron data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) from January 1995 to June 2004, we examine the L location of energetic electron deepest penetration, L location of the maximum flux, and L location of the maximum flux enhancement and variation over the duration of 119 moderate and strong isolated geomagnetic storms. We find that the L location of deepest penetration, L location of the maximum flux at the end of electron injections, and L location of the maximum flux variation have strong correlations with the minimum Dst index. Although the variation and radial positioning of radiation belt electrons is clearly associated with the Dst index, their deep penetration into the slot region (2 &lt; L &lt; 3) is not directly correlated with the Dst index in an obvious way. It is also known that the Dst index can be accurately predicted based on solar wind parameters. Thus, the necessary and sufficient combination of solar wind conditions for the occurrence of injection events into the slot region, between L = 2–3, are also investigated. We find a total of 23 injection events at L = 2.5 from 1995 to 2004 and that when certain solar wind conditions are met, an injection event at L = 2.5 is ensured. We also find that the electron flux preconditioning is an important factor influencing electron injections at L = 2.5.

A small spacecraft mission with large accomplishments

A remarkable era of space research will end soon when, after 20 years of space‐based observations, the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft will reenter Earth's atmosphere. This will result from the resurgence of the Sun's activity and the related increase of atmospheric drag produced by increasing solar ultraviolet radiation. The best estimate is that SAMPEX will succumb to drag forces (and burn up on reentry) in late September 2012, but this could occur as early as August or as late as December 2012 [see Baker et al., 2012]. SAMPEX has been a pacesetting mission since its inception. It was selected in 1989 for flight as NASA's first spacecraft in the “Small Explorer” (SMEX) program [Baker et al., 1993]. The SMEX program was intended both to accomplish forefront science (at a very affordable cost) as well as to provide a training ground in the best space development practices for a new generation of scientists, engineers, and managers. As its full name suggests, SAMPEX was always intended to perform multiple duties and was geared toward making measurements in space of moderate to very high energy particles [see Baker et al., 1993]. A few of the key contributions made by the SAMPEX program are summarized below.

SAMPEX to Reenter Atmosphere: Twenty‐Year Mission Will End

SAMPEX to Reenter Atmosphere: Twenty‐Year Mission Will End

Dynamic variations of the outer radiation belt during magnetic storms for 1.5–6.0 MeV electrons

The CME’s structure of solar wind (interplanetary magnetic field) is different from CIR’s. The two processes in which plasma and solar wind energy are injected into the Earth’s inner magnetosphere are not the same. So, the variations of energetic electrons flux in the radiation belts are different between the storms associated with CMEs and CIRs. By using data from SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) satellite, we have investigated the dynamic variations of the outer radiation belt for 1.5–6.0 MeV electrons during 54 CME-driven storms and 26 CIR-driven recurrent storms. According to the superposed epoch analysis, for CME- and CIR-driven storms, when the Dst index reaches the minimum, the locations of the outer boundary move to L=4 and L=5.5, respectively. In the recovery phases, the locations of the outer boundary of the outer radiation belt are generally lower than and slightly higher than those before CME- and CIR-driven storms, respectively. We have found that the logarithmically decaying 1/e cut-off L-shell is a satisfying indicator of the outer boundary of the outer radiation belt. Furthermore, our study shows that the logarithmically decaying 1/e cut-off latitude is dependent on the Kp index in the main phases of CME- and CIR- driven storms, while in the recovery phases, there is no obvious correlation. In addition, it has been shown that the locations of the peak electron flux are controlled by the minimum Dst index in the main phases of CME-driven storms. The influences of multiple storms on the electron flux of outer radiation belt have also been investigated.

Storm time occurrence of relativistic electron microbursts in relation to the plasmapause

Microbursts, or short‐duration bursts of precipitating relativistic electrons observed at low altitudes, are associated with whistler mode chorus and have been proposed as one of the primary mechanisms for storm time depletion of the outer radiation belt. Whistler chorus occurs outside the plasmapause and is generated by wave‐particle interactions involving soft electrons (1–100 keV). We use low‐altitude identifications of the plasmapause from the DMSP satellites and microburst observations from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite to identify the relationship between the plasmapause and microburst locations. Microbursts move radially inward during geomagnetic storms in concert with plasmapause movement, with the microbursts consistently located outside the plasmapause. Near storm peak, as Dst reaches minimum values, microbursts occur at a given L value within hours of erosion of the plasmasphere. This time delay between erosion of the plasmasphere and the appearance of microbursts is positively correlated with L value. We find the occurrence of relativistic microbursts to be associated with a combination of daily averaged 1.5–6 MeV electron fluxes exceeding about 1 particle s−1 sr−1 cm−2 (observed at SAMPEX altitudes) and the presence of soft electrons from the plasma sheet. These observations are consistent with the suggestion that microbursts are associated with chorus and contribute significantly to storm time radiation belt depletion.

Coupling Processes in the Inner Magnetosphere: Inner Magnetosphere Coupling (IMC) Workshop; Espoo, Finland, 28 July to 1 August 2008

In recent years, satellite measurements (e.g., Combined Release and Radiation Effects Satellite (CRRES); Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE); Polar; Highly Elliptical Orbit (HEO); Cluster; Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX); and Time History of Events and Macroscale Interactions During Substorms (THEMIS)) and theoretical and modeling studies have significantly advanced the understanding of the dynamics of various plasma populations in the inner magnetosphere. Observations and modeling have revealed that the dynamics of the inner magnetospheric plasma populations not only depend on solar wind conditions but also are coupled to each other bymeans of the electromagnetic field, currents, and wave‐particle interactions.The understanding of coupling processes between the plasmasphere, ring current, radiation belts, magnetic field, and waves was the subject of discussion at the recent Inner Magnetospheric Coupling (IMC) workshop, which brought together more than 50 researchers studying various plasma populations and processes that may influence the inner magnetosphere, as well as the effects of the inner magnetosphere on the ionosphere, atmosphere, and global magnetospheric interactions.

Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

Slot region electron loss timescales due to plasmaspheric hiss and lightning‐generated whistlers

Energetic electrons (E &gt; 100 keV) in the Earth's radiation belts undergo Doppler‐shifted cyclotron resonant interactions with a variety of whistler mode waves leading to pitch angle scattering and subsequent loss to the atmosphere. In this study we assess the relative importance of plasmaspheric hiss and lightning‐generated whistlers in the slot region and beyond. Electron loss timescales are determined using the Pitch Angle and energy Diffusion of Ions and Electrons (PADIE) code with global models of the spectral distributions of the wave power based on CRRES observations. Our results show that plasmaspheric hiss propagating at small and intermediate wave normal angles is a significant scattering agent in the slot region and beyond. In contrast, plasmaspheric hiss propagating at large wave normal angles and lightning‐generated whistlers do not contribute significantly to radiation belt loss. The loss timescale of 2 MeV electrons due to plasmaspheric hiss propagating at small and intermediate wave normal angles in the center of the slot region (L = 2.5) lies in the range 1–10 days, consistent with recent Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) observations. Wave turbulence in space, which is responsible for the generation plasmaspheric hiss, thus leads to the formation of the slot region. During active periods, losses due to plasmaspheric hiss may occur on a timescale of 1 day or less for a wide range of energies, 200 keV &lt; E &lt; 1 MeV, in the region 3.5 &lt; L &lt; 4.0. Plasmaspheric hiss may thus also be a significant loss process in the inner region of the outer radiation belt during magnetically disturbed periods.

Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location

A remarkable correlation between the inner edge of the outer radiation belt electrons and the innermost plasmapause location is demonstrated by using 12 years of measurements of MeV electrons from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Combined Radiation and Release Experiment Satellite (CRRES) during its entire mission (July 1990–October 1991). An empirical model of the plasmapause location, Lpp, by O'Brien and Moldwin (2003) as well as the measured Lpp during the CRRES mission is used for the comparison. This correlation suggests that the innermost Lpp is the innermost limit of the outer radiation belt penetration. We discuss the possible scenarios that might explain this correlation.