Cosmic ray modulation with a Fisk-type heliospheric magnetic field and a latitude-dependent solar wind speed

Hitge, Mariette; Burger, R. A.

doi:10.1016/j.asr.2009.07.024

Cited by 33 publications

(33 citation statements)

References 35 publications

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“…However, as a long time average, e.g., over a solar rotation or solar cycle 23, the Parker prediction still is a useful approximation. Indeed, although the extracted B ϕ ( R s , ϕ s ) are small in comparison to the B ϕ ( R s , ϕ s ), they have a strong influence on the direction of the magnetic field at the solar surface and for all ( r , ϕ ) in the heliosphere, increasing with r , as shown in section 4 and by other authors [e.g., Fisk , 1996; Jokipii and Kóta , 1989; Hitge and Burger , 2010].…”

Section: Predictions For B(r ϕ) At the Photosphere And 25 Solar Radiimentioning

confidence: 52%

Nonzero azimuthal magnetic fields at the solar source surface: Extraction, model, and implications

2012

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[1] A recent two-dimensional (radial distance r and solar longitude f) model for the solar wind is driven using 1 h average data from the Wind spacecraft. We extend the treatment of the Sun's magnetic field in comparison with previous models to allow a nonzero intrinsic azimuthal component B f at the solar source surface, in addition to the radial component B r . We find nonzero azimuthal magnetic fields at the source surface which dominate the variability in B(r, f) at 1 AU and beyond. The averages 〈|B f (R s )|〉 and 〈|B r (R s )|〉 inferred over solar cycle 23 were 0.44 AE 0.48 mT (4.4 AE 4.8 mG) and 120 AE 30 mT (1.2 AE 0.3 G) at the photosphere, respectively, where R s equals one solar radius. While the surface magnetic field is closely radial on average as expected, it is sometimes over 20 from radial. Both B f and B r vary smoothly on periods of order of a day, with evidence for relatively narrow current sheets, and vary differently with the solar cycle: B r (R s ) is correlated with the sunspot number while B f (R s ) has a two-level behavior. The typical fields |B r (R s )| are consistent with previous estimates. This consistency, the finding of a linear relation between the data-smoothing time period and the average transition time for each component and other arguments support the contention that the B f (R s ) fields found are intrinsic. Our results and model can account naturally for non-Parker-like magnetic field directions at 1 AU and all r above the source surface.

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Section: Predictions For B(r ϕ) At the Photosphere And 25 Solar Radiimentioning

confidence: 52%

Nonzero azimuthal magnetic fields at the solar source surface: Extraction, model, and implications

2012

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“…The extracted azimuthal surface fields are large enough to perturb the distant polar field ( r ≫ 1 AU), since B ϕ ∝ r −1 while B r ∝ r −2 , and lead to several important consequences for solar modulation of cosmic rays [e.g., Fisk , 1996; Jokipii and Kóta , 1989; Hitge and Burger , 2009]. Since the polar entry of cosmic rays into the inner solar system depends on the magnitude and direction of the B , this entry may be strongly affected by the nonzero B ϕ ( R s ) fields found here and the different component B ϕ ( r , ϕ ) predicted for B ϕ ( R s ) ≠ 0.…”

Section: Discussionmentioning

confidence: 99%

Modeling 1 AU solar wind observations to estimate azimuthal magnetic fields at the solar source surface

Bäumen¹,

Cairns²,

Robinson³

2011

Geophys. Res. Lett.

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.[1] A recent two-dimensional (radial distance r and solar longitude f) model for the solar wind is driven using 1-hour average data from the Wind spacecraft. We extend the treatment of the Sun's magnetic field to allow a nonzero azimuthal component B f at the source surface, assumed to be at the photosphere where r = R s , in addition to the radial component B r . We find nonzero azimuthal magnetic fields at the source surface with important consequences for the more distant heliosphere. The averages jB f (R s )j and jB r (R s )j inferred over solar cycle 23 are 0.44 AE 0.48 mT (4.4 AE 4.8 mG) and 120 AE 30 mT (1.2 AE 0.3 G) at the photosphere, respectively. Both components vary with time by more than an order of magnitude, with jB f (R s )j ≤ jB r (R s )j. While the surface magnetic field is closely radial on average it is sometimes 20°from radial. Both B f and B r vary smoothly on periods of 10 hours, with evidence for relatively narrow current sheets, and vary with the solar cycle: B r (R s ) is correlated with the sunspot number, but with a time lag of 20 months, while B f (R s ) has a two level behavior, decreasing near solar maximum and increasing near solar minimum. Our results and model can account naturally for non-Parker-like magnetic field directions at 1 AU since the B f fields inferred at the source surface lead to the B f and B r fields at 1 AU having similar average magnitudes and large variability.Citation: Schulte in den Bäumen, H., I. H. Cairns, and P. A. Robinson (2011), Modeling 1 AU solar wind observations to estimate azimuthal magnetic fields at the solar source surface, Geophys.

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“…The Heaviside step function H is used to switch the field's direction across the HCS at = ′ . Note that a Fisk field with latitude-dependent solar wind speed should be used in 3-D modeling, but Hitge and Burger [2010] found that the solar wind speed does not significantly influence cosmic ray transport in most conditions. Therefore, for the simplicity purpose, here we use Parker field with constant solar wind speed.…”

Section: Modulation Modelmentioning

confidence: 99%

Modulation of galactic cosmic rays during the unusual solar minimum between cycles 23 and 24

et al. 2014

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During the recent solar minimum between cycles 23 and 24 (solar minimum P23/24), the intensity of galactic cosmic rays (GCR) measured at the Earth was the highest ever recorded since space age. It is the purpose of this paper to resolve the most plausible mechanism for this unusually high intensity. A GCR transport model in three‐dimensional heliosphere based on a simulation of Markov stochastic process is used to find the relation of cosmic ray modulation to various transport parameters, including solar wind (SW) speed, distance of heliospheric boundary, magnitude of interplanetary magnetic field (IMF) at the Earth, tilt angle of heliospheric current sheet, and values of parallel and perpendicular diffusion coefficients. We calculate GCR proton energy spectra at the Earth for the last three solar minima P21/22, P22/23, and P23/24, with the transport parameters obtained from observations. Besides weak IMF magnitude and slow SW speed, we find that a possible low magnetic turbulence, which increases the parallel diffusion and reduces the perpendicular diffusion in the polar direction, might be an additional possible mechanism for the high GCR intensity in the solar minimum P23/24.

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Cosmic ray modulation with a Fisk-type heliospheric magnetic field and a latitude-dependent solar wind speed

Cited by 33 publications

References 35 publications

Nonzero azimuthal magnetic fields at the solar source surface: Extraction, model, and implications

Nonzero azimuthal magnetic fields at the solar source surface: Extraction, model, and implications

Modeling 1 AU solar wind observations to estimate azimuthal magnetic fields at the solar source surface

Modulation of galactic cosmic rays during the unusual solar minimum between cycles 23 and 24

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