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.