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Title: COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES

Abstract

Here we use the PFSS model and photospheric data from Wilcox Solar Observatory, SOHO /MDI, SDO/HMI, and SOLIS to compare the coronal field with heliospheric magnetic field measured at 1 au, compiled in the NASA/NSSDC OMNI 2 data set. We calculate their mutual polarity match and the power of the radial decay, p , of the radial field using different source surface distances and different number of harmonic multipoles. We find the average polarity match of 82% for the declining phase, 78%–79% for maxima, 76%–78% for the ascending phase, and 74%–76% for minima. On an average, the source surface of 3.25 R{sub S} gives the best polarity match. We also find strong evidence for solar cycle variation of the optimal source surface distance, with highest values (3.3 R{sub S}) during solar minima and lowest values (2.6 R{sub S}–2.7 R{sub S}) during the other three solar cycle phases. Raising the number of harmonic terms beyond 2 rarely improves the polarity match, showing that the structure of the HMF at 1 au is most of the time rather simple. All four data sets yield fairly similar polarity matches. Thus, polarity comparison is not affected by photospheric field scaling, unlike comparisons of themore » field intensity.« less

Authors:
; ;  [1]
  1. University of Oulu, P.O. Box 3000, FI-90014 Oulu (Finland)
Publication Date:
OSTI Identifier:
22664033
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; DECAY; DISTANCE; HARMONICS; HELIOSPHERE; MAGNETIC FIELDS; MULTIPOLES; NASA; PHOTOSPHERE; SOLAR CORONA; SOLAR CYCLE; SUN; SURFACES

Citation Formats

Koskela, J. S., Virtanen, I. I., and Mursula, K., E-mail: jennimari.koskela@oulu.fi. COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/1/63.
Koskela, J. S., Virtanen, I. I., & Mursula, K., E-mail: jennimari.koskela@oulu.fi. COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES. United States. doi:10.3847/1538-4357/835/1/63.
Koskela, J. S., Virtanen, I. I., and Mursula, K., E-mail: jennimari.koskela@oulu.fi. Fri . "COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES". United States. doi:10.3847/1538-4357/835/1/63.
@article{osti_22664033,
title = {COMPARING CORONAL AND HELIOSPHERIC MAGNETIC FIELDS OVER SEVERAL SOLAR CYCLES},
author = {Koskela, J. S. and Virtanen, I. I. and Mursula, K., E-mail: jennimari.koskela@oulu.fi},
abstractNote = {Here we use the PFSS model and photospheric data from Wilcox Solar Observatory, SOHO /MDI, SDO/HMI, and SOLIS to compare the coronal field with heliospheric magnetic field measured at 1 au, compiled in the NASA/NSSDC OMNI 2 data set. We calculate their mutual polarity match and the power of the radial decay, p , of the radial field using different source surface distances and different number of harmonic multipoles. We find the average polarity match of 82% for the declining phase, 78%–79% for maxima, 76%–78% for the ascending phase, and 74%–76% for minima. On an average, the source surface of 3.25 R{sub S} gives the best polarity match. We also find strong evidence for solar cycle variation of the optimal source surface distance, with highest values (3.3 R{sub S}) during solar minima and lowest values (2.6 R{sub S}–2.7 R{sub S}) during the other three solar cycle phases. Raising the number of harmonic terms beyond 2 rarely improves the polarity match, showing that the structure of the HMF at 1 au is most of the time rather simple. All four data sets yield fairly similar polarity matches. Thus, polarity comparison is not affected by photospheric field scaling, unlike comparisons of the field intensity.},
doi = {10.3847/1538-4357/835/1/63},
journal = {Astrophysical Journal},
number = 1,
volume = 835,
place = {United States},
year = {Fri Jan 20 00:00:00 EST 2017},
month = {Fri Jan 20 00:00:00 EST 2017}
}
  • Magnetic field observations by Pioneer 11 and ISEE 3/ICE are used to investigate the large scale temporal behavior and spatial gradients in the heliospheric magnetic field. The authors extent and refine previous analyses by expanding the data set to include observations out to a radial distance of 24 AU and over a longer time interval which includes the recent solar minimum. The spatial gradients are investigated by removing the solar cycle variations with data obtained at 1 AU at the corresponding time. To first order, the gradients in the magnetic field magnitude and in the azimuthal component are found tomore » agree with the Parker model. Furthermore, this analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of approximately 1%/AU, so that at 20 AU the relative deficit is of the order of 20%. The solar wind speed measured at 1 AU as well as at Pioneer 11 has been analyzed, but it is found that the small variations observed between the two locations affect this result only minimally. The average field strength at Pioneer 11 at a distance of 16 AU has been compared with a Voyager 2 average obtained at nearly the same radial distance and time interval but at a significantly different latitude. They agree very closely and both exhibit a deficit of {approx}20% relative to the 1-AU baseline. This agreement demonstrates that B did not vary significantly with latitude at this time (1984-1985 near solar minimum).« less
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