The lag between eruption and expansion is the time it takes the burst of solar particles to traverse 93 million miles of space. At times, superstorms on the Sun can drive the northern lights almost to the Equator and cause widespread havoc in radars, radios and power grids. As a rule, experts say, auroras that surge toward the Equator tend to be more colorful and intense. Predicting the peak of the action can be difficult. The length of the sunspot cycle can vary anywhere from 8 to 15 years, averaging 11 years, and stormy times can crest sooner rather than later, or vice versa.
Cycles also vary in strength.
Cycle 19, which peaked in 1958, was a blockbuster. The current one, cycle 23, looks more routine so far. Dr. Evans, the Hudson Bay researcher, an auroral physicist at the National Oceanic and Atmospheric Administration's Space Environment Center, in Boulder, Colo., said chances of seeing an aurora in the New York region were now likely to occur about once a month. The odds rise in higher latitudes, he added, and fall in lower ones. The Moon is now waning, so nights this week and next will become increasingly favorable for auroral viewing.