The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth's magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be ...accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA's Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.
Over the last decade, the solar wind has exhibited low densities and magnetic field strengths, representing anomalous states that have never been observed during the space age. As discussed by ...Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084), the cycle 23–24 solar activity led to the longest solar minimum in more than 80 years and continued into the “mini” solar maximum of cycle 24. During this weak activity, we observed galactic cosmic ray fluxes that exceeded theERobserved small solar energetic particle events. Here we provide an update to the Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter. The Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) study examined the evolution of the interplanetary magnetic field and utilized a previously published study by Goelzer et al. (2013, https://doi.org/10.1002/2013JA019404) projecting out the interplanetary magnetic field strength based on the evolution of sunspots as a proxy for the rate that the Sun releases coronal mass ejections. This led to a projection of dose rates from galactic cosmic rays on the lunar surface, which suggested a ∼20% increase of dose rates from one solar minimum to the next and indicated that the radiation environment in space may be a worsening factor important for consideration in future planning of human space exploration. We compare the predictions of Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) with the actual dose rates observed by CRaTER in the last 4 years. The observed dose rates exceed the predictions by ∼10%, showing that the radiation environment is worsening more rapidly than previously estimated. Much of this increase is attributable to relatively low‐energy ions, which can be effectively shielded. Despite the continued paucity of solar activity, one of the hardest solar events in almost a decade occurred in September 2017 after more than a year of all‐clear periods. These particle radiation conditions present important issues that must be carefully studied and accounted for in the planning and design of future missions (to the Moon, Mars, asteroids, and beyond).
Plain Language Summary
We examine the evolution of fluxes from galactic cosmic rays and recent solar energetic particle events to evaluate the recent evolution of radiation hazards in space and their implications for human and robotic exploration.
Key Points
GCR radiation doses are rising faster than predicted previously
SEP radiation events are large despite low solar activity
Radiation environment is a significant factor for mission planning
Seven years of measurements from the Polar spacecraft are surveyed to monitor the variations of plasma density within the magnetospheric cusps. The spacecraft's orbital precession from 1998 through ...2005 allows for coverage of both the northern and southern cusps from low altitude out to the magnetopause. In the mid- and high- altitude cusps, plasma density scales well with the solar wind density (n(sub cusp)∕n(sub sw) approximately 0.8). This trend is fairly steady for radial distances greater then 4 R(sub E). At low altitudes (r less than 4R(sub E)) the density increases with decreasing altitude and even exceeds the solar wind density due to contributions from the ionosphere. The density of high charge state oxygen (O(greater +2) also displays a positive trend with solar wind density within the cusp. A multifluid simulation with the Block-Adaptive-Tree Solar Wind Roe-Type Upwind Scheme MHD model was run to monitor the relative contributions of the ionosphere and solar wind plasma within the cusp. The simulation provides similar results to the statistical measurements from Polar and confirms the presence of ionospheric plasma at low altitudes.
Early observations from the first orbit of Parker Solar Probe (PSP) show recurrent stream interaction regions that form close to the Sun. Energetic particle enhancements were observed on the ...320th–326th day of the year 2018, which corresponds to ~1–7 days after the passage of the stream interface between faster and slower solar wind. Energetic particles stream into the inner heliosphere to the PSP spacecraft near 0.33 au (71 solar radii) where they are measured by the Integrated Science Investigation of the Sun (IS⊙IS). The large 6-day time interval over which energetic particles are observed after the stream passage provides a unique perspective on the development of stream interactions within the heliosphere. The long duration of energetic particle enhancements suggests that particles stream in through the inner heliosphere more directly along magnetic field lines that form a sub-Parker spiral structure due to magnetic footpoint motion at the Sun and shearing of the magnetic field in the rarefaction region behind the stream interface. The strong build-up of energetic particle fluxes in the first 3 days after the passage of the stream interface indicates that suprathermal populations are enhanced near the interaction region through compression or other acceleration processes in addition to being diffusively accelerated. The early increases in energetic particle fluxes (in the first 3 days) in the formation of these events allows for the characterization of the acceleration associated with these suprathermal seed populations. Thus, we show that the time history of energetic particle fluxes observed by IS⊙IS provides a new view of particle acceleration at stream interaction regions throughout the inner heliosphere.
Dynamic ion spectral features in the inner magnetosphere are the observational signatures of ion acceleration, transport, and loss in the global magnetosphere. We report “trunk‐like” ion structures ...observed by the Van Allen Probes on 2 November 2012. This new type of ion structure looks like an elephant's trunk on an energy‐time spectrogram, with the energy of the peak flux decreasing Earthward. The trunks are present in He+ and O+ ions but not in H+. During the event, ion energies in the He+ trunk, located at L = 3.6–2.6, magnetic local time (MLT) = 9.1–10.5, and magnetic latitude (MLAT) = −2.4–0.09°, vary monotonically from 3.5 to 0.04 keV. The values at the two end points of the O+ trunk are energy = 4.5–0.7 keV, L = 3.6–2.5, MLT = 9.1–10.7, and MLAT = −2.4–0.4°. Results from backward ion drift path tracings indicate that the trunks are likely due to (1) a gap in the nightside ion source or (2) greatly enhanced impulsive electric fields associated with elevated geomagnetic activity. Different ion loss lifetimes cause the trunks to differ among ion species.
Key Points
A new type of ion spectral structure—“trunk‐like”—is reported
The trunk structures are present in He+ and O+ ions but not in H+
Simulations are performed to gain insight into the trunk formation mechanism
Energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly, irrespective of conditions that engender reconnection and boundary-normal magnetic fields. ...A signature observed by the Magnetospheric Multiscale (MMS) mission, simultaneous monohemispheric streaming of multiple species (electrons, H+, Hen+), is reported here as unexpectedly common in the dayside, dusk quadrant of the magnetosheath even though that region is thought to be drift-shadowed from energetic electrons. This signature is sometimes part of a pitch angle distribution evolving from symmetric in the magnetosphere, to asymmetric approaching the magnetopause, to monohemispheric streaming in the magnetosheath. While monohemispheric streaming in the magnetosheath may be possible without a boundary-normal magnetic field, the additional pitch angle depletion, particularly of electrons, on the magnetospheric side requires one. Observations of this signature in the dayside dusk sector imply that the static picture of magnetospheric drift-shadowing is inappropriate for energetic particle dynamics in the outer magnetosphere.
Abstract
At the end of 2020 November, two coronal mass ejections (CMEs) erupted from the Sun and propagated through the interplanetary medium in the direction of Parker Solar Probe while the ...spacecraft was located at ∼0.81 au. The passage of these interplanetary CMEs (ICMEs) starting on November 29 (DOY 334) produced the largest enhancement of energetic ions and electrons observed by the Integrated Science Investigation of the Sun (IS⊙IS) energetic particle instrument suite on board Parker Solar Probe during the mission’s first eight orbits. This was also the first spatially widespread solar energetic particle event observed in solar cycle 25. We investigate several key characteristics of the energetic electron event including the time profile and anisotropy distribution of near-relativistic electrons as measured by IS⊙IS’s low-energy Energetic Particle Instrument (EPI-Lo) and compare these observations with contextual data from the Parker Solar Probe Fields Experiment magnetometer. These are the first electron anisotropy measurements from IS⊙IS/EPI-Lo, demonstrating that the instrument can successfully produce these measurements. We find that the electron count rate peaks at the time of the shock driven by the faster of the two ICMEs, implying that the shock parameters of this ICME are conducive to the acceleration of electrons. Additionally, the angular distribution of the electrons during the passage of the magnetic clouds associated with the ICMEs shows significant anisotropy, with electrons moving primarily parallel and antiparallel to the local magnetic field as well as bidirectionally, providing an indication of the ICME’s magnetic topology and connectivity to the Sun or magnetic structures in the inner heliosphere.
Abstract
We present observations of ≳10–100 keV nucleon
−1
suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of <0.1 au ...from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second crossing; (2) ion arrival times exhibit no velocity dispersion; (3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ∼10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic farther from the HCS; (4) the He spectrum steepens either side of the HCS, and the He, O, and Fe spectra exhibit power laws of the form ∼
E
−4
–
E
6
; and (5) maximum energies
E
X
increase with the ion’s charge-to-mass (
Q
/
M
) ratio as
E
X
/
E
H
∝
(
Q
X
/
M
X
)
δ
, where
δ
∼ 0.65–0.76, assuming that the average
Q
states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to
E
/
Q
but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.
Simulations from our newly expanded ring current‐atmosphere interactions model with self‐consistent magnetic field (RAM‐SCB), now valid out to 9 RE, are compared for the first time with Van Allen ...Probes observations. The expanded model reproduces the storm time ring current buildup due to the increased convection and inflow of plasma from the magnetotail. It matches Magnetic Electron Ion Spectrometer (MagEIS) observations of the trapped high‐energy (>50 keV) ion flux; however, it underestimates the low‐energy (<10 keV) Helium, Oxygen, Proton, and Electron (HOPE) observations. The dispersed injections of ring current ions observed with the Energetic particle, Composition, and Thermal plasma (ECT) suite at high (>20 keV) energy are better reproduced using a high‐resolution convection model. In agreement with Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) observations, RAM‐SCB indicates that the large‐scale magnetic field is depressed as close as ∼4.5 RE during even a moderate storm. Regions of electromagnetic ion cyclotron instability are predicted on the duskside from ∼6 to ∼9 RE, indicating that previous studies confined to geosynchronous orbit may have underestimated their scattering effect on the energetic particles.
Key PointsExpanded RAM‐SCB model reproduces well high‐energy (>50 keV) MagEIS observationsThe magnetic field is depressed as close as ∼4.5 RE during even a moderate stormEMIC wave growth extends on duskside from ∼6 to ∼9 RE during storm main phase
Energetic ions observed in the cusp have been explained as a result of processes within the magnetosphere, but also proposed as a driver of some of those same processes. This study assesses potential ...connections between energetic ions observed in the cusp and geomagnetic storm and substorm activity. These connections may suggest sources of cusp energetic particles (CEPs), or imply effects of these particles on magnetospheric dynamics. We identify CEPs from six years of cusp crossings by the Polar satellite, relating them to storm and substorm onsets. CEPs showed no significant dependence on storms but did show a weak, statistically significant, increase after substorm onsets. CEPs had no significant association with subsequent storm or substorm onsets. We conclude that substorm acceleration may contribute to CEPs but CEPs are unlikely to contribute to global magnetospheric dynamics.
PDF
