Evidence from galactic cosmic rays that the sun has likely entered a secular minimum in solar activity

Since the beginning of the space age, the Sun has been in a multi-cycle period of elevated activity (secular maximum). This secular maximum is the longest in the last 9300 years. Since the end of solar cycle 21 (SC21), however, the Sun has shown a decline in overall activity, which has remarkably increased the fluxes of galactic cosmic rays (GCRs). Here, we investigate the correlation between the modulation of GCRs, the heliospheric magnetic field, and solar wind speed for the last 24 solar cycles to find trends that can potentially be used to predict future solar activity. Specifically, we develop a tool for predicting future magnetic field intensity, based on the hysteresis in the GCR variation, during the last phases of the current cycle. This method estimates that SC25 will be as weak as or weaker than SC24. This would mean that the Sun has likely entered a secular minimum, which, according to historical records, should last for another two cycles (SC25 and SC26)

Update on the Worsening Particle Radiation Environment Observed by CRaTER and Implications for Future Human Deep‐Space Exploration

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 SummaryWe 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 PointsGCR radiation doses are rising faster than predicted previouslySEP radiation events are large despite low solar activityRadiation environment is a significant factor for mission planningPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143683/1/swe20567_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143683/2/swe20567.pd

Galactic cosmic radiation in the interplanetary space through a modern secular minimum

Recent solar conditions indicate a persistent decline in solar activity—possibly similar to thepast solar grand minima. During such periods of low solar activity, the fluxes of galactic cosmic rays(GCRs) increase remarkably, presenting a hazard for long-term crewed space missions. We used data fromthe Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter(LRO) to examine the correlation between the heliospheric magnetic field, solar wind speed, and solarmodulation potential of the GCRs through Cycle 24. We used this correlation to project observations frompast secular solar minima, including the Dalton minimum (1790–1830) and the Gleissberg minimum(1890–1920), into the next cycle. For the case of conditions similar to the Dalton (or Gleissberg) minimum,the heliospheric magnetic field could drop to 3.61 (or 4.06) nT, leading to a dose rate increase of75% (or34%). We showed that accounting for a floor in the modulation potential, invoked by the Badhwar-O'Neill2014 model, moderates the projected radiation levels in Cycle 25. We used these results to determine themost conservative permissible mission duration (PMD,290.4+37.7−35.9and 204.3+26.6−25.2days for 45-year-old maleand female astronauts, respectively) based on a 3% risk of exposure-induced death (REID) at the upper95% confidence interval in interplanetary space

Radiation Pressure from Interstellar Hydrogen Observed by IBEX through Solar Cycle 24

As the Sun moves through the local interstellar medium (LISM), neutral atoms travel through the heliosphere and can be detected by IBEX. We consider interstellar neutral (ISN) hydrogen atoms with a drifting Maxwellian distribution function in the LISM that travel on almost hyperbolic trajectories to the inner heliosphere. They are subject to solar gravity and radiation pressure, as well as ionization processes. For ISN H, the radiation pressure, which exerts an effective force comparable to gravitation, decelerates individual atoms and shifts the longitude of their observed peak relative to that of ISN He. We used the peak longitude of the observed flux in the lowest energy channel of IBEX-Lo to investigate how radiation pressure shifts the ISN H signal over almost an entire solar cycle (2009–2018). Thus, we have created a new methodology to determine the Lyα effective radiation pressure from IBEX ISN H data. The resulting effective ratio of the solar radiation pressure and gravitation (μeff=1.074±0.038), averaged over cycle 24, appears to agree within the uncertainties with simulations based on total irradiance observations7 while being higher by ∼21%. Our analysis indicates an increase of μeff with solar activity, albeit with substantial uncertainties. Further study of IBEX H response functions and future Interstellar Mapping and Acceleration Probe data should provide significant reduction of the uncertainties and improvements in our understanding of the effects of radiation pressure on ISN atoms

Interstellar Neutral Hydrogen in the Heliosphere: New Horizons Observations in the Context of Models

Interstellar neutral (ISN) hydrogen is the most abundant species in the outer heliosheath and the very local interstellar medium (VLISM). Charge-exchange collisions in the outer heliosheath result in filtration, reducing the ISN hydrogen density inside the heliosphere. Additionally, these atoms are intensively ionized close to the Sun, resulting in a substantial reduction of their density within a few astronomical units from the Sun. The products of this ionization—pickup ions (PUIs)—are detected by charged particle detectors. The Solar Wind Around Pluto instrument on New Horizons provides, for the first time, PUI observations from the distant heliosphere. We analyze the observations collected between 22 and 52 au from the Sun to find the ISN hydrogen density profile and compare the results with predictions from global heliosphere models. We conclude that the density profile derived from the observations is inconsistent with steady-state model predictions. This discrepancy is not explained by time variations close to the Sun and thus may be related to the temporal evolution of the outer boundaries or VLISM conditions. Furthermore, we show that the cold and hot models of ISN hydrogen distribution are not a good approximation closer to the termination shock. Therefore, we recommend a new fiduciary point based on the available New Horizons observations at 40 au from the Sun, at ecliptic direction (285.°62, 1.°94), where the ISN hydrogen density is 0.11 cm ^−3 . The continued operation of New Horizons should give better insight into the source of the discussed discrepancy

Model-free Maps of Interstellar Neutral Hydrogen Measured with IBEX between 2009 and 2018

The Interstellar Boundary Explorer (IBEX) is a NASA satellite in Earth orbit, dedicated to observing interstellar neutral (ISN) atoms entering the heliosphere and energetic neutral atoms from the heliosheath from 11 eV to 6 keV. This work presents comprehensive maps of ISN hydrogen observed with IBEX at energies between 11 and 41 eV, covering almost an entire solar cycle from 2009 to 2018. ISN hydrogen measurements can provide nformation on the interstellar medium and on the heliosphere that modifies the incoming ISN flow. Whereas hydrogen is the dominant species in the unperturbed interstellar medium, most ISN hydrogen atoms crossing into the heliosphere do not reach the inner solar system: some are filtered out around the heliopause, while others are held off by solar radiation pressure or may be ionized as they approach the Sun. This paper presents and evaluates several approaches for generating model-free maps of ISN hydrogen from IBEX measurements. We discuss the basic implications of our results for ISN hydrogen inflow and outline the remaining discrepancies between observations and model predictions. Our maps show, during weak solar activity from 2009 to 2011, a clear signal of ISN hydrogen for ecliptic longitudes between 240° and 310°, roughly one month after the signal of ISN helium has peaked. When the solar activity approached its maximum around 2014, the ISN hydrogen signal weakened and dropped below the detection threshold because of increasing solar radiation pressure and ionization. The ISN hydrogen signal then reappeared in 2017