Heliospheric current sheet
© Public DomainHeliospheric current sheet

Many readers are familiar with a number of solar proxies used to gauge the activity of the sun, the most familiar being sunspot counts and type. However they aren't the only metric you can use to determine when one cycle ends and another begins. The Heliospheric Current Sheet sounds a bit like a "newsletter" and in a sense it is, because it can announce the true end of solar cycle 23.

From Wikipedia:
The heliospheric current sheet (HCS) is the surface within the Solar System where the polarity of the Sun's magnetic field changes from north to south. This field extends throughout the Sun's equatorial plane in the heliosphere.The shape of the current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind). A small electrical current flows within the sheet, about 10−10 A/m². The thickness of the current sheet is about 10,000 km.

The underlying magnetic field is called the interplanetary magnetic field, and the resulting electric current forms part of the heliospheric current circuit.[4] The heliospheric current sheet is also sometimes called the interplanetary current sheet.
What the Heliospheric Current Sheet is telling us.

David Archibald writes:
One of the things that the now disbanded NASA Solar Cycle 24 Prediction Panel told us was that is that solar minimum is marked by a flat heliospheric current sheet. The heliospheric current sheet can be found here, (Link).

The site provides two data series - the classic and the radial, and notes that the radial may be possibly more accurate. Plotting up the radial data, the following chart is generated:
Heliospheric current sheet graph
© unknown

The heliospheric current sheet, for the last three minima, has got down to 3°. The last reading was 8.7°. It has been declining at an average of 8.6° per annum. If it holds that rate, solar minimum will be in August 2009. If it holds to the orange bounding line, solar minimum could be as late as April 2010. The last reading on the classic series is 22.8° and this series got down to 10° on average in previous solar minima. At its decline rate, solar minimum will be in another 1.9 years, which is late 2010.

To paraphrase a popular aphorism, Solar Cycle 23 isn't over until the heliospheric current sheet has flattened, and it has a way to go yet.