University of Oulu

Ilpo I. Virtanen et al 2020 ApJL 889 L28.

Abrupt shrinking of solar corona in the late 1990s

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Author: Virtanen, Ilpo I.1; Koskela, Jennimari S.1; Mursula, Kalevi1
Organizations: 1ReSoLVE Centre of Excellence Space Climate research unit, University of Oulu, P.O. Box 3000, FIN-90014, University of Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.2 MB)
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Language: English
Published: IOP Publishing, 2020
Publish Date: 2020-05-06


We derive the longest uniform record of rotational intensities solar coronal magnetic field since 1968 and compare it with the heliospheric magnetic field (HMF) observed at the Earth. We scale the Mount Wilson Observatory and Wilcox Solar Observatory observations of the photospheric magnetic field to the level of the Synoptic Optical Long-term Investigations of the Sun/Vector Spectro Magnetograph and apply the potential field source surface model to calculate the coronal magnetic field. We find that the evolution of the coronal magnetic field during the last 50 yr agrees with the HMF observed at the Earth only if the effective coronal size, the distance of the coronal source surface of the HMF, is allowed to change in time. We calculate the optimum source surface distance for each rotation and find that it experienced an abrupt decrease in the late 1990s. The effective volume of the solar corona shrunk to less than one half during a short period of only a few years. We note that this abrupt shrinking coincides with other changes in solar magnetic fields that are likely related to the decrease of the overall solar activity, i.e., the demise of the Grand Modern Maximum.

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Series: Astrophysical journal letters
ISSN: 2041-8205
ISSN-E: 2041-8213
ISSN-L: 2041-8205
Volume: 889
Issue: 2
Article number: L28
DOI: 10.3847/2041-8213/ab644b
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: We acknowledge the financial support by the Academy of Finland to the ReSoLVE Centre of Excellence (project No. 307411).
Academy of Finland Grant Number: 307411
Detailed Information: 307411 (Academy of Finland Funding decision)
Dataset Reference: Wilcox Solar Observatory data used in this study were obtained via the website courtesy of J.T. Hoeksema. This study includes data from the synoptic program at the 150 Foot Solar Tower of the Mt. Wilson Observatory, which is acknowledged. Data were acquired by SOLIS instruments operated by NISP/NSO/AURA/NSF. Data used in this study was obtained from the following web sites: WSO: MWO: SOLIS/VSM:
Copyright information: © 2020. The American Astronomical Society.