The Sun’s Solar Wind Stopped Fading and Is Growing Stronger

For more than two decades up to 2008, multiple studies reported a gradual weakening of the solar wind and its embedded magnetic field, prompting speculation that the Sun might be heading toward a prolonged quiet spell similar to the Dalton or Maunder minima. The new analysis by Jamie M. Jasinski and Marco Velli shows that the trend of decline stopped in 2008, and that a steady recovery has continued through 2025, with a range of solar wind and interplanetary magnetic field parameters increasing by substantial percentages. The study uses the long, intercalibrated OMNI-2 dataset and focuses on whole-rotation averages measured near 1 astronomical unit (au).

Quick summary of the measured changes

Using linear trend fits from the start of 2008 through early 2025, the scientists report increases in key proton and magnetic parameters observed at 1 au. The principal changes are approximately:

• Solar wind proton speed, up about 6 percent.
• Proton density, up about 26 percent.
• Proton temperature, up about 29 percent.
• Thermal pressure, up about 45 percent.
• Mass flux, up about 27 percent.
• Dynamic pressure (momentum flux), up about 34 percent.
• Energy flux, up about 40 percent.
• Interplanetary magnetic field magnitude, up about 31 percent, and its radial component up about 33 percent.

These percentages and the underlying values are derived from trend fits to OMNI-2 measurements spanning 2008.92 to 2025.13. The fitted dynamic pressure, for example, rose from about 1.39 nPa in 2008 to about 1.86 nPa in 2025.

Why this question matters

Scientists keep close watch on long-term changes in the solar wind because those changes shape the heliosphere, the vast bubble carved by the Sun that shields the solar system from incoming galactic cosmic rays. Variations in the solar wind also change where a planet’s magnetopause sits, and they alter the conditions for magnetic reconnection, a process that governs how energy and particles are exchanged between the solar wind and a planet’s magnetic environment. Put simply, the Sun’s outflow affects space weather for satellites, planetary atmospheres, and even the interstellar boundary of our solar system. The finding that the Sun reversed a long-term decline is therefore consequential for understanding the Sun’s future behavior and the response of the heliosphere and planetary magnetospheres.

The problem the researchers addressed

Since roughly the 1990s, sunspot maxima and several measures of solar activity declined, culminating in an unusually weak solar minimum around 2008 and a weak solar cycle 24. Earlier work, notably by McComas and colleagues, documented declines in mass flux, dynamic pressure, and energy flux and raised the possibility that the Sun might be entering a multi-decade low-activity episode similar to historical minima. That raised two linked questions: was the Sun undergoing a long-term descent into a new grand minimum, and were the observed declines a persistent trend or a temporary perturbation? Jasinski and Velli set out to test whether the weakening continued after 2008 or whether the solar wind regained strength. Their goal was to quantify more recent behavior using the best available, intercalibrated in-ecliptic data.

How the researchers approached the problem

The scientists used the OMNI-2 dataset, a long, intercalibrated compilation of solar wind and magnetic field data measured near 1 au by multiple spacecraft and merged into a consistent record. To reduce sampling biases that arise because spacecraft see different mixes of fast and slow wind at different times, the scientists averaged values over full solar rotations (about 27 days). They then fit linear trends to the rotation-averaged time series beginning at the deep minimum in 2008 (2008.92 in their notation) and continuing through early 2025 (2025.13), using chi-square minimization on the mean rotation-averaged values. This approach emphasizes whole-Sun, rotation-averaged behavior rather than day-to-day fluctuations that are dominated by transients and local sampling.

Why that approach is sensible

Averaging over a full solar rotation reduces the influence of short-lived structures and the sampling bias introduced when a single spacecraft favors either fast or slow solar wind streams. The OMNI-2 intercalibration helps ensure that trends are not artifacts of instrument drift or differences between spacecraft. The linear fits provide a clear, quantitative measurement of long-term trending behavior from the 2008 minimum onward. The scientists also compare their fitted trends to historic averages from 1974–1994 to place the recent changes in a longer-term context.

The breakthrough discovery explained

The main discovery is that the long decline in solar wind strength ended in 2008, after which many plasma and magnetic parameters have increased steadily. The size of the increases is not uniform: thermal pressure, energy flux and dynamic pressure show the largest relative gains, while the asymptotic speed increased only modestly. The modest change in speed, relative to larger changes in density and temperature, has implications for energy partitioning in the solar wind, and it suggests that the net energy per particle has not changed as dramatically as the particle number and thermal energy have. The radial magnetic field component and the total interplanetary magnetic field have both increased substantially, implying a stronger magnetic connection between the Sun and interplanetary space than during the deep minimum.

Concrete numbers to anchor the result

To make the change tangible, the scientists report that the fitted proton speed at 1 au rose from about 400.7 km/s at the end of 2008 to 423.3 km/s in 2025, an increase of about 6 percent. Proton density increased from approximately 5.42 cm^-3 to 6.83 cm^-3, about 26 percent higher. Thermal pressure rose by roughly 45 percent and the interplanetary magnetic field magnitude increased by about 31 percent. Dynamic pressure, the quantity most directly tied to magnetospheric compression, rose from about 1.39 nPa to about 1.86 nPa over the interval. These fitted values and percentage changes are taken directly from the trend analysis in the study.

What the shift means for the heliosphere and planets

A sustained rise in dynamic pressure implies that the heliosphere, the Sun’s protective bubble, is likely to expand. Observations have previously linked dynamic pressure pulses to measurable shifts in the heliosphere’s size. When dynamic pressure is higher, the heliopause, where solar and interstellar pressures balance, moves outward. For planetary magnetospheres, higher dynamic pressure compresses magnetopauses closer to planetary surfaces, reducing the size of a planet’s magnetosphere. The consequences differ by planet because magnetospheric compressibility varies: giant planets such as Jupiter and Saturn are more compressible than Earth, meaning their magnetospheric boundaries will change more for the same increase in solar wind pressure. Increased interplanetary magnetic field strength also affects magnetic reconnection rates and how effectively the solar wind can couple to planetary magnetic fields, potentially changing the frequency and intensity of space weather events around planets.

Context and caveats the scientists emphasize

The study is careful in its language, noting that although the recovery since 2008 is steady, the solar wind parameters have not yet returned to the higher values recorded at the end of the 20th century. For example, the dynamic pressure averaged about 2.36 nPa in the 1974–1994 historic baseline, compared to the fitted average of about 1.86 nPa in 2025. Thus, while the recent trend is upward, the Sun has not fully recovered to those earlier levels. The scientists further note that proton plasma-beta, the ratio of thermal to magnetic pressure, has continued to decline because the magnetic field increased more rapidly than the temperature recovered. This selective recovery in different parameters highlights that the Sun’s behavior is nuanced and that not every metric moves in lockstep.

Limitations and what remains uncertain

No single dataset can fully resolve every subtlety of long-term solar behavior. Sampling biases remain even after rotation averaging, because the mix of coronal structures that produce fast and slow wind can vary across cycles. The OMNI-2 compilation minimizes inter-spacecraft inconsistencies but relies on careful intercalibration. The linear fit model captures a simple, first-order trend, but the underlying physical drivers may be nonlinear and modulated by the full 22-year Hale cycle and other multi-cycle variability. The scientists point out that continuous monitoring will be necessary to determine whether the upward trend continues, stabilizes, or reverses again in future cycles. In other words, the 2008 reversal appears real and persistent through 2025, but whether it marks a permanent regime shift or a multi-decade oscillation remains an open question.

How this fits with solar dynamo ideas and cycle predictions

The observed recovery is consistent with predictions from expanded views of the Hale cycle that emphasize longer-term phasing of magnetic activity bands and the role of so-called terminators, the times when opposite-hemisphere bands meet at the equator. Some recent work suggested that cycle 25 would be stronger than cycle 24, and the measured upward trend in solar wind strength is broadly compatible with that perspective. The study therefore supports the interpretation that the weak solar cycle 24 was an anomalous low point within a longer oscillation, rather than the start of a sustained grand minimum. Nonetheless, linking measured interplanetary changes directly to detailed dynamo physics requires further theoretical and observational work.

What to watch next

Scientists will continue to monitor OMNI and other long-term datasets, including in-ecliptic and out-of-ecliptic measurements as they become available. Key things to watch are whether dynamic pressure and magnetic field strength continue to climb through the remainder of cycle 25 and into cycle 26, and whether complementary observations, such as changes in the heliopause location or in the incidence of galactic cosmic rays at Earth, reflect the inferred expansion of the heliosphere. Improved solar dynamo models and continued observations of surface magnetic patterns, torsional oscillations, and coronal structure will help connect interplanetary trends to the Sun’s internal magnetic processes.

Takeaway

The Sun’s behavior is not a single number but the sum of many interlinked parameters. The evidence presented by Jasinski and Velli shows that the multi-decade weakening of the solar wind ended in 2008, and that proton density, thermal pressure, dynamic pressure, energy flux and magnetic field strength have increased steadily through early 2025. While the Sun has not returned to the higher values seen late in the 20th century, the recovery implies that the exceptionally weak cycle 24 was most likely a recent outlier rather than the start of a new grand minimum. Continued observations and careful analysis will determine whether this upward trend stabilizes or continues, and what that will mean for the heliosphere, planetary environments, and space weather.

The research was published in The Astrophysical Journal Letters on September 8, 2025.