Pop pop quiz: what’s hot, trendy and inescapable for about three months of the year? Hint: this year her name was Nelly, last year she went by Mariah, and two summers ago, a little crooner named Usher took the crown.

Hits may come and go, but the summer pop anthem is a yearly staple, the one song that has defined the humid, heady months of freedom for young people of all stripes ever since Jefferson Airplane ushered in the Summer of Love with their psychedelic smash “Somebody to Love.”

This year, however, a seismic shift is underfoot, largely thanks to the influence of legal and convenient music downloading services. The summer anthem — indeed, the enduring pop hit — may soon go the way of bell-bottoms and love beads.

Never before has the footprint of digital downloading been felt so prominently on the hallowed slots of the Billboard Hot 100 Chart, the definitive measure of the most popular songs in the country. Up until a few years ago, the chart was tallied by a pair of factors: radio spins and sales of the physical single itself. In the 1950s and ’60s, music was largely a singles business, but when the Beatles and other genre-defining acts crystallized the notion of a full-length rock album, the physical single became a blip on our collective radar. No single has ever sold more than 10 million copies, compared to the dozens of albums that have passed that benchmark.

For most of the past 40 years, the Hot 100 has been a radio haven, a place where those nauseatingly catchy, blasting-on-every-station denizens of the American cultural mindset came to proclaim their dominance. The formula used to be simple: radio would catch a hit and debut it near the bottom of the chart; it would grow and ascend as the weeks passed, finally peaking at No. 1 if it was lucky, after which it would slowly descend to last-summer’s-leftovers status.

But without a doubt, 2006 has been one of the most tumultuous years for the venerable chart, as single after single displaced one another from the pole position, often vaulting up dozens of positions in a single week. Rihanna’s “SOS” zoomed from No. 34 to No. 1 in a single week; the inexplicably successful “Breaking Free” from “High School Musical” made a hard one-week charge to the top five from — get this — No. 84. And just a few weeks ago, soul singer Akon careened from No. 95 to lucky No. 7, breaking every chart record in the process. Since the Hot 100 just started including digital sales into its tallies last year, popular artists releasing new singles don’t need to wait for radio promotion to reach chart heights.

Singles don’t smash these milestones with radio play alone — the radio world is too slow to move, too entrenched in focus groups and corporate hand-holding to recognize the true potential hits. In 2005, digital music was still coming of age. Though last year’s “Hollaback Girl” was the first digital single to crest the one million sales mark, radio still determined the course of pop music. Starting with Mario’s “Let Me Love You” and closing with Mariah Carey’s “Don’t Forget About Us,” there were a paltry seven No. 1 singles in 2005, compared with an unbelievable 15 top songs in 2006 — and we still have nearly three months to go.

Which brings us back to the summer anthem. In a word, it’s disappearing: While a glut of radio love propelled Mariah Carey’s “We Belong Together” to number one for a whopping 14 non-consecutive weeks last year, Nelly Furtado’s “Promiscuous” stole the summit for six measly summer weeks this year. Instead of resting comfortably on its airplay peak, it was boxed in on all sides by songs whose sales moxie eclipsed their radio savvy by leaps and bounds — Furtado’s successor to the throne, Fergie’s “London Bridge,” made one of those massive sales leaps that seems to be the defining characteristic of the 2006 pop scene: No. 84 to a two-week reign at No. 1 in three short, hot summer weeks.

It’s a messy and hectic world out there; it’s also the best thing that could happen to the stagnant music industry. Though album sales are still plummeting, overall music sales — digital and standard — are up a fraction of a percent. Beyond that, though, the ascendance of digital music wrenches the power of promotion from the radio gurus and places it squarely back in the hands of the music-buying public. Like no other time in pop history, people have a direct link to their favorite new songs, point-and-click access via the Internet at a fraction of traditional record-store prices. Let the chaos continue.

Michael Gold doesn’t want you to know what he’s downloading right now; we don’t want to know either. Shhhh. It may or may not be ‘legal.’