166 research outputs found
A zone of preferential ion heating extends tens of solar radii from Sun
The extreme temperatures and non-thermal nature of the solar corona and solar
wind arise from an unidentified physical mechanism that preferentially heats
certain ion species relative to others. Spectroscopic indicators of unequal
temperatures commence within a fraction of a solar radius above the surface of
the Sun, but the outer reach of this mechanism has yet to be determined. Here
we present an empirical procedure for combining interplanetary solar wind
measurements and a modeled energy equation including Coulomb relaxation to
solve for the typical outer boundary of this zone of preferential heating.
Applied to two decades of observations by the Wind spacecraft, our results are
consistent with preferential heating being active in a zone extending from the
transition region in the lower corona to an outer boundary 20-40 solar radii
from the Sun, producing a steady state super-mass-proportional
-to-proton temperature ratio of . Preferential ion heating
continues far beyond the transition region and is important for the evolution
of both the outer corona and the solar wind. The outer boundary of this zone is
well below the orbits of spacecraft at 1 AU and even closer missions such as
Helios and MESSENGER, meaning it is likely that no existing mission has
directly observed intense preferential heating, just residual signatures. We
predict that {Parker Solar Probe} will be the first spacecraft with a perihelia
sufficiently close to the Sun to pass through the outer boundary, enter the
zone of preferential heating, and directly observe the physical mechanism in
action.Comment: 11 pages, 7 figures, accepted for publication in the Astrophysical
Journal on 1 August 201
Radiation modeling in the Earth and Mars atmospheres using LRO/CRaTER with the EMMREM Module
Abstract We expand upon the efforts of Joyce et al. (2013), who computed the modulation potential at the Moon using measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft along with data products from the Earth-Moon-Mars Radiation Environment Module (EMMREM). Using the computed modulation potential, we calculate galactic cosmic ray (GCR) dose and dose equivalent rates in the Earth and Mars atmospheres for various altitudes over the course of the LRO mission. While we cannot validate these predictions by directly comparable measurement, we find that our results conform to expectations and are in good agreement with the nearest available measurements and therefore may be used as reasonable estimates for use in efforts in risk assessment in the planning of future space missions as well as in the study of GCRs. PREDICCS (Predictions of radiation from REleASE, EMMREM, and Data Incorporating the CRaTER, COSTEP, and other solar energetic particles measurements) is an online system designed to provide the scientific community with a comprehensive resource on the radiation environments of the inner heliosphere. The data products shown here will be incorporated into PREDICCS in order to further this effort and daily updates will be made available on the PREDICCS website (http://prediccs.sr.unh.edu). Key Points We model GCR dose and dose equivalent rates in Earth and Mars atmospheres Dose rates are in reasonable agreement with nearby measurements Data products will soon be made available on PREDICCS website
Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration?
Abstract
The Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show stopper for long-duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% risk of exposure-induced death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23â24 minimum. The time to 3% REID is estimated to be âŒ20% lower in the coming cycle 24â25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations
Radiation environment at the Moon: Comparisons of transport code modeling and measurements from the CRaTER instrument
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER), an instrument carried on the Lunar Reconnaissance Orbiter spacecraft, directly measures the energy depositions by solar and galactic cosmic radiations in its silicon wafer detectors. These energy depositions are converted to linear energy transfer (LET) spectra. High LET particles, which are mainly highâenergy heavy ions found in the incident cosmic ray spectrum, or target fragments and recoils produced by protons and heavier ions, are of particular importance because of their potential to cause significant damage to human tissue and electronic components. Aside from providing LET data useful for space radiation risk analyses for lunar missions, the observed LET spectra can also be used to help validate space radiation transport codes, used for shielding design and risk assessment applications, which is a major thrust of this work. In this work the Monte Carlo transport code HETCâHEDS (HighâEnergy Transport CodeâHuman Exploration and Development in Space) is used to estimate LET contributions from the incident primary ions and their charged secondaries produced by nuclear collisions as they pass through the three pairs of silicon detectors. Also in this work, the contributions to the LET of the primary ions and their charged secondaries are analyzed and compared with estimates obtained using the deterministic space radiation code HZETRN 2010, developed at NASA Langley Research Center. LET estimates obtained from the two transport codes are compared with measurements of LET from the CRaTER instrument during the mission. Overall, a comparison of the LET predictions of the HETCâHEDS code to the predictions of the HZETRN code displays good agreement. The code predictions are also in good agreement with the CRaTER LET measurements above 15âkeV/”m but differ from the measurements for smaller values of LET. A possible reason for this disagreement between measured and calculated spectra below 15âkeV/”m is an inadequate representation of the light ion spectra in HETCâHEDS and HZETRN code calculations. It is also clear from the results of this work that Vavilov distributions need to be incorporated into the HETCâHJEDS code before it will be able to recreate the observed LET spectra measured by the CRaTER instrument. Key Points Vavilov corrections should be incorporated into simulated results The predictions of the transport codes reasonably agree with the CRaTER LET The observed LET can be used to help validate space radiation transport codesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108081/1/swe20145.pd
Precise Detections of Solar Particle Events and a New View of the Moon
We have invented a new method for detecting solar particle events using data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Using a simple function of the total particle detection rates from four of CRaTERâs six detectors, we can precisely identify solar energetic particle event periods in the CRaTER data archive. During solar quiet periods we map the distribution of a mareâassociated mixture of elements in the lunar regolith using this new method. The new map of the moon probably reflects an asâyet unknown combination of lunar albedo protons, neutrons, and gamma rays, and most closely resembles Lunar Prospector maps of gamma rays characteristic of thorium and iron. This result will lead to multiple followâup studies of lunar albedo particles and may also contribute to the study of diurnally varying hydrogenation of the lunar regolith.Key PointsThe CRaTER instrument on LRO can detect and quantify small solar particle events with a simple new analysis techniqueOur new lunar map of albedo radiation resembles gamma ray maps from Lunar ProspectorFollowâup studies will investigate contributions from neutrons, protons, and gamma rays, and signatures of hydrogen in lunar regolithPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152796/1/grl60033_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152796/2/grl60033.pd
Testing the Solar Probe Cup, an Instrument Designed to Touch the Sun
Solar Probe Plus will be the first, fastest, and closest mission to the sun, providing the first direct sampling of the sub-Alfvenic corona. The Solar Probe Cup (SPC) is a unique re-imagining of the traditional Faraday Cup design and materials for immersion in this high temperature environment. Sending an instrument of this type into a never-seen particle environment requires extensive characterization prior to launch to establish sufficient measurement accuracy and instrument response. To reach this end, a slew of tests for allowing SPC to see ranges of appropriate ions and electrons, as well as a facility that reproduces solar photon spectra and fluxes for this mission. Having already tested the SPC at flight like temperatures with no significant modification of the noise floor, we recently completed a round of particle testing to see if the deviations in Faraday Cup design fundamentally change the operation of the instrument. Results and implications from these tests will be presented, as well as performance comparisons to cousin instruments such as those on the WIND spacecraft
Turbulence Transport Modeling and First Orbit Parker Solar Probe (PSP) Observations
Parker Solar Probe (PSP) achieved its first orbit perihelion on November 6,
2018, reaching a heliocentric distance of about 0.165 au (35.55 R).
Here, we study the evolution of fully developed turbulence associated with the
slow solar wind along the PSP trajectory between 35.55 R and 131.64
R in the outbound direction, comparing observations to a theoretical
turbulence transport model. Several turbulent quantities, such as the
fluctuating kinetic energy and the corresponding correlation length, the
variance of density fluctuations, and the solar wind proton temperature are
determined from the PSP SWEAP plasma data along its trajectory between 35.55
R and 131.64 R. The evolution of the PSP derived turbulent
quantities are compared to the numerical solutions of the nearly incompressible
magnetohydrodynamic (NI MHD) turbulence transport model recently developed by
Zank et al. (2017). We find reasonable agreement between the theoretical and
observed results. On the basis of these comparisons, we derive other
theoretical turbulent quantities, such as the energy in forward and backward
propagating modes, the total turbulent energy, the normalized residual energy
and cross-helicity, the fluctuating magnetic energy, and the correlation
lengths corresponding to forward and backward propagating modes, the residual
energy, and the fluctuating magnetic energy
Small-scale Magnetic Flux Ropes in the First two Parker Solar Probe Encounters
Small-scale magnetic flux ropes (SFRs) are a type of structures in the solar
wind that possess helical magnetic field lines. In a recent report (Chen & Hu
2020), we presented the radial variations of the properties of SFR from 0.29 to
8 au using in situ measurements from the Helios, ACE/Wind, Ulysses, and Voyager
spacecraft. With the launch of the Parker Solar Probe (PSP), we extend our
previous investigation further into the inner heliosphere. We apply a
Grad-Shafranov-based algorithm to identify SFRs during the first two PSP
encounters. We find that the number of SFRs detected near the Sun is much less
than that at larger radial distances, where magnetohydrodynamic (MHD)
turbulence may act as the local source to produce these structures. The
prevalence of Alfvenic structures significantly suppresses the detection of
SFRs at closer distances. We compare the SFR event list with other event
identification methods, yielding a dozen well-matched events. The cross-section
maps of two selected events confirm the cylindrical magnetic flux rope
configuration. The power-law relation between the SFR magnetic field and
heliocentric distances seems to hold down to 0.16 au.Comment: Accepted by ApJ on 2020 Sep 1
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