TY - JOUR

T1 - Centennial evolution of monthly solar wind speeds

T2 - Fastest monthly solar wind speeds from long-duration coronal holes

AU - Lukianova, Renata

AU - Holappa, Lauri

AU - Mursula, Kalevi

PY - 2017/3/1

Y1 - 2017/3/1

N2 - High-speed solar wind streams (HSSs) are very efficient drivers of geomagnetic activity at high latitudes. In this paper we use a recently developed ΔH parameter of geomagnetic activity, calculated from the nightside hourly magnetic field measurements of the Sodankylä observatory, as a proxy for solar wind (SW) speed at monthly time resolution in 1914–2014 (solar cycles 15–24). The seasonal variation in the relation between monthly ΔH and solar wind speed is taken into account by calculating separate regressions between ΔH and SW speed for each month. Thereby, we obtain a homogeneous series of proxy values for monthly solar wind speed for the last 100 years. We find that the strongest HSS-active months of each solar cycle occur in the declining phase, in years 1919, 1930, 1941, 1952, 1959, 1973, 1982, 1994, and 2003. Practically all these years are the same or adjacent to the years of annual maximum solar wind speeds. This implies that the most persistent coronal holes, lasting for several solar rotations and leading to the highest annual SW speeds, are also the sources of the highest monthly SW speeds. Accordingly, during the last 100 years, there were no coronal holes of short duration (of about one solar rotation) that would produce faster monthly (or solar rotation) averaged solar wind than the most long-living coronal holes in each solar cycle produce.

AB - High-speed solar wind streams (HSSs) are very efficient drivers of geomagnetic activity at high latitudes. In this paper we use a recently developed ΔH parameter of geomagnetic activity, calculated from the nightside hourly magnetic field measurements of the Sodankylä observatory, as a proxy for solar wind (SW) speed at monthly time resolution in 1914–2014 (solar cycles 15–24). The seasonal variation in the relation between monthly ΔH and solar wind speed is taken into account by calculating separate regressions between ΔH and SW speed for each month. Thereby, we obtain a homogeneous series of proxy values for monthly solar wind speed for the last 100 years. We find that the strongest HSS-active months of each solar cycle occur in the declining phase, in years 1919, 1930, 1941, 1952, 1959, 1973, 1982, 1994, and 2003. Practically all these years are the same or adjacent to the years of annual maximum solar wind speeds. This implies that the most persistent coronal holes, lasting for several solar rotations and leading to the highest annual SW speeds, are also the sources of the highest monthly SW speeds. Accordingly, during the last 100 years, there were no coronal holes of short duration (of about one solar rotation) that would produce faster monthly (or solar rotation) averaged solar wind than the most long-living coronal holes in each solar cycle produce.

KW - coronal holes

KW - geomagnetic activity

KW - high-speed streams

KW - solar wind

KW - space climate

UR - http://www.scopus.com/inward/record.url?scp=85014144136&partnerID=8YFLogxK

U2 - 10.1002/2016JA023683

DO - 10.1002/2016JA023683

M3 - Article

AN - SCOPUS:85014144136

VL - 122

SP - 2740

EP - 2747

JO - Journal of Geophysical Research: Biogeosciences

JF - Journal of Geophysical Research: Biogeosciences

SN - 0148-0227

IS - 3

ER -