The Sun Reversed Its Decades-long Weakening Trend in 2008
Jamie M. Jasinski1
aa and Marco Velli1,2
aa 1 NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; jasinski@jpl.nasa.gov 2 Department of Earth, Planetary and Space Science, University of California, Los Angeles, CA, USA
Received 2025 May 30; revised 2025 July 15; accepted 2025 July 16; published 2025 September 8


Abstract

Over the course of two decades until 2008, the solar wind became significantly weaker with a constant declining
trend in many important solar wind parameters, and solar cycle 24 being the weakest on record since the start of
the space age. Here we show that since 2008, the Sun has reversed this long-term weakening trend with a steady
increase in various solar wind proton parameters observed at 1 au. Furthermore, comparison of values from a
fitted trend to data between 2008 and 2025 show the following increases in solar wind proton parameters: speed
(~6%), density (~26%), temperature (~29%), thermal pressure (~45%), mass flux (~27%), momentum flux or
dynamic pressure (~34%), energy flux (~40%), interplanetary magnetic field magnitude (~31%), and the radial
component of the magnetic field (~33%). This has important implications on long-term solar trends, implying that
the exceptional weakness of solar cycle 24 was most likely a recent outlier and that the Sun is not entering a
modern era Maunder/Dalton-like minimum phase in its solar variation, but is instead recovering from a ~20 yr
decline. Presently, the trending average solar wind dynamic pressure of ~1.9 nPa in the current solar cycle,
however, is still lower than the recorded ~2.4 nPa at the end of the 20th century. Continuous future measurements
will reveal whether this increase will continue in upcoming solar cycles or whether these parameters will remain
stable.
Unified Astronomy Thesaurus concepts: Solar cycle (1487); The Sun (1693); Solar activity (1475)