# Centennial evolution of monthly solar wind speeds: Fastest monthly solar wind speeds from long-duration coronal holes

## TL;DR

This study reconstructs a century of monthly solar wind speeds using geomagnetic data, revealing that the highest speeds are linked to long-lived coronal holes during the declining phases of solar cycles.

## Contribution

It introduces a new proxy method for estimating historical solar wind speeds from geomagnetic measurements, highlighting the link between long-lasting coronal holes and peak solar wind speeds.

## Key findings

- Peak monthly solar wind speeds occur during the declining phase of solar cycles.
- Longest-lasting coronal holes produce the highest average solar wind speeds.
- No short-duration coronal holes have produced faster monthly solar wind speeds in the last century.

## Abstract

High speed solar wind streams (HSSs) are very efficient drivers of geomagnetic activity at high latitudes. In this paper we use a recently developed $\Delta{H}$ parameter of geomagnetic activity, calculated from the night-side hourly magnetic field measurements of the Sodankyl\"a 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 $\Delta{H}$ and solar wind speed is taken into account by calculating separate regressions between $\Delta{H}$ 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.