solar wind
© unknown

I initially wrote this article using data only from David Archibald, but within a couple of minutes I was given some broader data from Leif Svalgaard, so I have rewritten this to include both resources in the interest of seeing the broader perspective. - Anthony

Last September WUWT covered NASA's press conference on the state of the sun. One of the announcements was this:
Sept. 23, 2008: In a briefing today at NASA headquarters, solar physicists announced that the solar wind is losing power.

"The average pressure of the solar wind has dropped more than 20% since the mid-1990s," says Dave McComas of the Southwest Research Institute in San Antonio, Texas. "This is the weakest it's been since we began monitoring solar wind almost 50 years ago."
From Wiki:
The solar wind is a stream of charged particles - a plasma - ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particles varies in temperature and speed with the passage of time. These particles are able to escape the sun's gravity, in part because of the high temperature of the corona, but also because of high kinetic energy that particles gain through a process that is not well-understood.

The solar wind creates the Heliosphere, a vast bubble in the interstellar medium surrounding the solar system. Other phenomena include geomagnetic storms that can knock out power grids on Earth, the aurorae such as the Northern Lights, and the plasma tails of comets that always point away from the sun.
Solar Wind Flow Pressure is something that is tracked daily by the Space Weather Prediction Center (SWPC) For example they display a nifty solar wind dashboard gauge on Space Weather Now that shows "dynamic pressure":
space weather solar wind

Explanation of readings.

Dynamic Pressure Dial:

Ranges from 0.1 to 100 nPa. The scale is log10 over the full range. If the density or speed data are missing, the arrow will not appear. The arrow will move to the location on the scale corresponding to the actual value of the latest 15 minute average of the Dynamic Pressure P of the solar wind. Dynamic Pressure is a function of speed and density.

David Archibald writes:
Robert Bateman's graphic of the solar wind sent me in search of a longer time series. I found a longer one, and one that is a more accurate indication of the force that is pushing the galactic cosmic rays out from the inner planets of the solar system. It is the three month smoothed, 27 day average of the solar wind flow pressure. The data is from the Omniweb site.
solar wind pressure
© unknown

The narrow downtrend channel that started in 2005 is quite evident. Before that it was trendless, and didn't change with solar cycle amplitude. The volatility within the downtrend is much less than it was prior to 2005. Also evident is a big oscillation in 2004, which may be an artefact of a switch that changed the mode.

From this chart, solar activity is still falling until the downtrend channel is broken. As the solar wind takes a year to reach the heliopause, the Oulu neutron count will continue to rise for the next year. But just as the Earth's atmosphere has shrunk, the heliopause will also be shrinking.
However this Archibald graph only shows a narrow slice of the entire data picture, Leif Svalgaard has an OMNI2 dataset that tracks back to 1963:
OMNI2 dataset graph
© unknown

While we can indeed see the current downtrend since 1997, we have had periods before where the solar wind has been almost as low . Though NASA said last year "This is the weakest it's been since we began monitoring solar wind almost 50 years ago.".

There is an overall down trend since 1992, with a short plateau at the last solar max around year 2000-2004, followed by another downtrend starting about 2005.

In terms on the sun's history (if it were compared to a day) we have about a microsecond worth of data out of that day on display above. So what conclusion, if any, can we draw from it? The only one I can see is it showing reduced solar activity, but nothing profound (in terms of the solar wind data we have) except that. We see a low period of similar amplitude around 1970, but it is noisier. The trend we've seen since 2005 is less noisy, which is inline with the quiet sun we have observed recently.

Let's hope sol gets the magneto revved up again.

UPDATE: I had written to David Archibald, saying that "the broader data set to 1963 didn't agree with your conclusions", and he wrote back within about 15 minutes and provided a new graph:
Anthony, Agreed, and thankyou.

I went back to find the larger data set, as follows:
solar wind pressure 1970-2009
© unknown

It is evident that the longer picture is more complicated. The correlation with solar minima and maxima is quite poor. Activity did not recover into Solar Cycle 23.

Yours sincerely,

David Archibald
So now we have all the makings of a good debate.