Journey 'Round the Sun: STEREO Science and Spacecraft Performance Results

The Solar TErrestrial RElations Observatory (STEREO) was originally designed as a two to five year heliocentric orbit mission to study coronal mass ejections (CMEs), solar energetic particles (SEPs), and the solar wind. After over ten years of continuous science data collection, the twin NASA STEREO observatories have significantly advanced the understanding of Heliophysics. This mission was the first to image CMEs all the way from the Sun to Earth and to observe the entire sphere of the Sun at one time. STEREO has demonstrated the importance of a point of view beyond the Sun-Earth line to significantly improve CME arrival time estimates and in understanding CME structure and trajectories and the longitudinal distribution of SEPs. STEREO was also the first to use one launch vehicle to insert two spacecraft into opposing heliocentric orbits, undergo a 3.5 month long superior solar conjunction, implement unattended daily science operations on two deep space observatories, maintain 7 arcsec continuous pointing without gyros, and detect and attempt to recover a spacecraft after a 22-month long communications anomaly at a range of 2 AU. This paper discusses the significant performance results after the first ten years of operations of the STEREO mission from its journey around the Sun

Journey 'Round the Sun: STEREO Science and Spacecraft Performance Results

The Solar TErrestrial RElations Observatory (STEREO) was originally designed as a two- to five-year heliocentric orbit mission to study coronal mass ejections (CMEs), solar energetic particles (SEPs), and the solar wind. After over ten years of continuous science data collection, the twin NASA STEREO observatories have significantly advanced the understanding of Heliophysics. This mission was the first to image CMEs all the way from the Sun to Earth and to observe the entire sphere of the Sun at one time. STEREO has demonstrated the importance of a point of view beyond the Sun-Earth line to significantly improve CME arrival time estimates and in understanding CME structure and trajectories and the longitudinal distribution of SEPs. STEREO was also the first to use one launch vehicle to insert two spacecraft into opposing heliocentric orbits, undergo a 3.5-month-long superior solar conjunction, implement unattended daily science operations on two deep space observatories, maintain 7 arcsec continuous pointing without gyros, and detect and attempt to recover a spacecraft after a 22-month long communications anomaly at a range of 2 AU (Astronomical Units). This paper discusses the significant performance results after the first ten years of operations of the STEREO mission from its journey around the Sun

Radiation dose during relativistic electron precipitation events at the International Space Station

AbstractWe provide a quantitative estimate of the radiation dose during relativistic electron precipitation (REP) events at the International Space Station (ISS). To this goal, we take advantage of the data collected by the CALorimetric Electron Telescope, the Monitor of All‐sky X‐ray Image, and the Space Environment Data Acquisition equipment‐Attached Payload. The three ISS detectors offer complementary REP observations, including energy spectra and flux directional information, during a period of approximately 2.5 years, from November 2015 to March 2018. We have identified 762 REP events during this period from which we obtain the distribution of radiation dose, relevant to extravehicular activities outside the ISS

BurstCube: A CubeSat for Gravitational Wave Counterparts

BurstCube will detect long GRBs, attributed to the collapse of massive stars, short GRBs (sGRBs), resulting from binary neutron star mergers, as well as other gamma-ray transients in the energy range 10-1000 keV. sGRBs are of particular interest because they are predicted to be the counterparts of gravitational wave (GW) sources soon to be detectable by LIGO/Virgo. BurstCube contains 4 CsI scintillators coupled with arrays of compact low-power Silicon photomultipliers (SiPMs) on a 6U Dellingr bus, a flagship modular platform that is easily modifiable for a variety of 6U CubeSat architectures. BurstCube will complement existing facilities such as Swift and Fermi in the short term, and provide a means for GRB detection, localization, and characterization in the interim time before the next generation future gamma-ray mission flies, as well as space-qualify SiPMs and test technologies for future use on larger gamma-ray missions. The ultimate configuration of BurstCube is to have a set of $\sim10$ BurstCubes to provide all-sky coverage to GRBs for substantially lower cost than a full-scale mission.Comment: In the 35th International Cosmic Ray Conference, Busan, Kore

Can the number of relativistic solar proton 1 AU crossings be determined from neutron monitor data?

Energetic protons released during solar eruptive events experience scattering during their interplanetary propagation and may cross the spherical surface of radius 1 AU multiple times. Knowledge of Ncross, the average number of 1 AU crossings per particle, is therefore important to deduce the total number of protons in interplanetary space during solar energetic particle events, for example for comparison with the number of interacting protons at the Sun during gamma-ray flares. It has been proposed that for relativistic protons Ncross can be obtained by comparing them relative fluences measured in the sunward and anti-sunward directions by the worldwide network of neutron monitors during ground level enhancements (GLEs). For five recent GLE events, we use neutron monitor data to derive Ncross using the latter approach and we compare the results with those of full-orbit test particle simulations of relativistic protons in a Parker spiral magnetic field, including the effects of scattering and drifts. We show that the approach based on neutron monitor data significantly underestimates Ncross during highly-anisotropic SEP events. This is due to the data sampling only a very small portion of the 1 AU sphere

Energetic proton back-precipitation onto the solar atmosphere in relation to long-duration gamma-ray flares

Context. Gamma-ray emission during long-duration gamma-ray flare (LDGRF) events is thought to be caused mainly by >300 MeV protons interacting with the ambient plasma at or near the photosphere. Prolonged periods of the gamma-ray emission have prompted the suggestion that the source of the energetic protons is acceleration at a coronal mass ejection (CME)-driven shock, followed by particle back-precipitation onto the solar atmosphere over extended times. Aims. We study the latter hypothesis using test particle simulations, which allow us to investigate whether scattering associated with turbulence aids particles in overcoming the effect of magnetic mirroring, which impedes back-precipitation by reflecting particles as they travel sunwards. Methods. The instantaneous precipitation fraction, P, the proportion of protons that successfully precipitate for injection at a fixed height, r_i, is studied as a function of scattering mean free path, lambda, and r_i. Upper limits to the total precipitation fraction, P_bar, were calculated for eight LDGRF events for moderate scattering conditions lambda=0.1 au). Results. We find that the presence of scattering helps back-precipitation compared to the scatter-free case, although at very low lambda values outward convection with the solar wind ultimately dominates. For eight LDGRF events, due to strong mirroring, P_bar is very small, between 0.56 and 0.93% even in the presence of scattering. Conclusions. Time-extended acceleration and large total precipitation fractions, as seen in the observations, cannot be reconciled for a moving shock source according to our simulations. Therefore, it is not possible to obtain both long duration gamma ray emission and efficient precipitation within this scenario. These results challenge the CME shock source scenario as the main mechanism for gamma ray production in LDGRFs

Cosmic-Ray Positrons: Are There Primary Sources?

Cosmic rays at the Earth include a secondary component originating in collisions of primary particles with the diffuse interstellar gas. The secondary cosmic rays are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.Comment: 15 pages, Latex, epsfig and aasms4 macros required, to appear in Astroparticle Physics (1999