Comparing coronal and heliospheric magnetic fields over several solar cycles
|Author:||Koskela, J. S.1,2; Virtanen, I. I.1,2; Mursula, K.1,2|
1ReSoLVE Centre of Excellence, Space Climate Research Unit, University of Oulu
2University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland
|Online Access:||PDF Full Text (PDF, 1 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe201704035938
|Publish Date:|| 2017-04-03
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 RS 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 RS) during solar minima and lowest values (2.6 RS–2.7 RS) 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.
|Type of Publication:||
A1 Journal article – refereed
|Field of Science:||
115 Astronomy and space science
We acknowledge the financial support by the Academy of
Finland to the ReSoLVE Centre of Excellence (project No.
|Academy of Finland Grant Number:||
272157 (Academy of Finland Funding decision)
Wilcox Solar Observatory data used in this study
were obtained via the website http://wso.stanford.edu, courtesy
of J.T. Hoeksema. Data were acquired by SOLIS
instruments operated by NISP/NSO/AURA/NSF. SOHO/
MDI is a project of international cooperation between ESA and
NASA. HMI data are courtesy of the Joint Science Operations
Center (JSOC) Science Data Processing Team at Stanford
University. The OMNI data were obtained from the GSFC/
SPDF OMNIWeb interface at http://omniweb.gsfc.nasa.gov.
© 2017. The American Astronomical Society. All rights reserved. Published in this repository with the kind permission of the publisher.