The shielding of galactic cosmic rays by the solar magentic field in the inner heliosphere
The shielding of galactic cosmic rays by the solar magentic field in the inner heliosphere
The origin of the eleven-year variation in Galactic Cosmic Ray (GCR) flux at Earth is examined in light of recent re-evaluations of the role of scattering processes in the heliosphere. A statistical survey is carried out of the relationship between GCR time variations and solar parameters which describe various forms of solar activity. persistently high anti-correlation coefficients at lags of order 25 to 50 days (1-2 Carrington rotations) between the interplanetary magnetic field (IMF) variations measured at Earth and GCRs. The IMF field strength enhancements, B, occur systematically one rotation before the great GCR decreases, related to the passage of propagating diffusive barriers. The very low lags show that the large GCR depletions occur in the very inner heliosphere (within 5-10 Aus of the Sun where AU is an Astronomical Unit). This is at odds with the concept, in widespread use at present, that the background shielding is fully explained by structures forming at 40-50 AU called global merged interaction regions (GMIRs).
The flux of open field lines connected to the low-latitude regions of the photosphere, (FS/Low) has a significantly higher anti-correlation with GCRs than any other solar parameter, both on quasi-decadal and quasi-annual time-scales.
The response of GCRs to an isolated FS/Low enhancement is investigated during simple solar minimum conditions using full magnetohydrodynamic (MHD) models of the corona and inner heliosphere. The effect of the latitudinal extent of interacting solar wind streams of differing speed in the region of FS/Low enhancement is found to be a key controlling factor of GCR intensities. FS/Low is known to regulate the open flux content of the heliosphere during solar maximum years. Its role in controlling the other determining component of the IMF strength, namely the solar wind speed fluctuations, is investigated using a potential field source surface model of the solar coronal field substantiated by semi-analytical and full MHD modelling of the interplanetary stream magnetism. It is found that the latitude, θ, of occurrence of IMF enhancements increases at solar maximum providing large structures extending from the equator to θ > 60° in agreement with out-of-ecliptic measurements of the Ulysses spacecraft during solar maximum years. FS/Low is highly correlated with the extent of medium and large structures. The better anti-correlation between GCRs with FS/Low than with the local IMF strength, B appears, therefore, to be related to the latitudinal extent of FS/Low which is not accounted for by variations in B.
University of Southampton
2007
Rouillard, Alexis Paul
(2007)
The shielding of galactic cosmic rays by the solar magentic field in the inner heliosphere.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The origin of the eleven-year variation in Galactic Cosmic Ray (GCR) flux at Earth is examined in light of recent re-evaluations of the role of scattering processes in the heliosphere. A statistical survey is carried out of the relationship between GCR time variations and solar parameters which describe various forms of solar activity. persistently high anti-correlation coefficients at lags of order 25 to 50 days (1-2 Carrington rotations) between the interplanetary magnetic field (IMF) variations measured at Earth and GCRs. The IMF field strength enhancements, B, occur systematically one rotation before the great GCR decreases, related to the passage of propagating diffusive barriers. The very low lags show that the large GCR depletions occur in the very inner heliosphere (within 5-10 Aus of the Sun where AU is an Astronomical Unit). This is at odds with the concept, in widespread use at present, that the background shielding is fully explained by structures forming at 40-50 AU called global merged interaction regions (GMIRs).
The flux of open field lines connected to the low-latitude regions of the photosphere, (FS/Low) has a significantly higher anti-correlation with GCRs than any other solar parameter, both on quasi-decadal and quasi-annual time-scales.
The response of GCRs to an isolated FS/Low enhancement is investigated during simple solar minimum conditions using full magnetohydrodynamic (MHD) models of the corona and inner heliosphere. The effect of the latitudinal extent of interacting solar wind streams of differing speed in the region of FS/Low enhancement is found to be a key controlling factor of GCR intensities. FS/Low is known to regulate the open flux content of the heliosphere during solar maximum years. Its role in controlling the other determining component of the IMF strength, namely the solar wind speed fluctuations, is investigated using a potential field source surface model of the solar coronal field substantiated by semi-analytical and full MHD modelling of the interplanetary stream magnetism. It is found that the latitude, θ, of occurrence of IMF enhancements increases at solar maximum providing large structures extending from the equator to θ > 60° in agreement with out-of-ecliptic measurements of the Ulysses spacecraft during solar maximum years. FS/Low is highly correlated with the extent of medium and large structures. The better anti-correlation between GCRs with FS/Low than with the local IMF strength, B appears, therefore, to be related to the latitudinal extent of FS/Low which is not accounted for by variations in B.
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Published date: 2007
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Local EPrints ID: 466244
URI: http://eprints.soton.ac.uk/id/eprint/466244
PURE UUID: 24947bb0-97d7-4031-94f7-b6851508d656
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Date deposited: 05 Jul 2022 04:54
Last modified: 05 Jul 2022 04:54
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Author:
Alexis Paul Rouillard
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