J. Space Weather Space Clim.
Volume 11, 2021
Topical Issue - Space climate: The past and future of solar activity
|Number of page(s)||22|
|Published online||22 January 2021|
Long-term studies of photospheric magnetic fields on the Sun
National Solar Observatory, 3665 Discovery Drive, 3rd Floor, Boulder, CO 80303, USA
2 Central Astronomical observatory of Russian Academy of Sciences at Pulkovo, 196140 Saint Petersburg, Russia
3 St Petersburg State University of Aerospace Instrumentation, Bol’shaya Morskaya ul. 67, 190000 Saint Petersburg, Russia
4 Kislovodsk Mountain Astronomical Station of Pulkovo Observatory, 357700 Kislovodsk, Russia
5 Institute for Solar-Terrestrial Physics, PO Box 291, Lermontov st., 126a, 664033 Irkutsk, Russia
* Corresponding author: email@example.com
Accepted: 18 November 2020
We briefly review the history of observations of magnetic fields on the Sun, and describe early magnetograps for full disk measurements. Changes in instruments and detectors, the cohort of observers, the knowledge base etc may result in non-uniformity of the long-term synoptic datasets. Still, such data are critical for detecting and understanding the long-term trends in solar activity. We demonstrate the value of historical data using studies of active region tilt (Joy’s law) and the evolution of polar field and its reversal. Using the longest dataset of sunspot field strength measurements from Mount Wilson Observatory (1917-present) supplemented by shorter datasets from Pulkovo (1956–1997) and Crimean (1956-present) observatories we demonstrate that the magnetic properties of sunspots did not change over the last hundred years. We also show that the relationship between the sunspot area and its magnetic flux can be used to extend the studies of magnetic field in sunspots to periods with no direct magnetic field measurements. Finally, we show how more recent full disk observations of the vector magnetic field can be used to study the long-term (solar cycle) variations in magnetic helicity on the Sun.
Key words: Sun: magnetic fields / sunspots / polar fields / solar cycle / helicity
© A.A. Pevtsov et al., Published by EDP Sciences 2021
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