As one of Japan’s most active volcanoes, Sakurajima often dazzles with spectacular displays of volcanic lightning set against an ash-filled sky. But the volcano can also produce much smaller, invisible bursts of electrical activity that mystify and intrigue scientists.Tiny ash particles rub together, gaining and losing electrons, which creates positive and negative charges that tend to clump together in pockets of like charge. To neutralize this unstable electrical field, lightning zigzags between the charged clusters, says Cassandra Smith, a volcanologist at the Alaska Volcano Observatory in Anchorage.
Experiments have shown that you can’t get lightning without some amount of ash in the system, Smith says. “So if you’re seeing volcanic lightning, you can be pretty confident in saying that the eruption has ash.”
Vent discharges, on the other hand, are relatively newly detected bursts of electrical activity, which produce a continuous, high-frequency signal for seconds — an eternity compared with lightning. These discharges can be measured using specialized equipment.
By focusing on small explosions from Sakurajima, defined as those with plume heights of 3 kilometers or less and with a duration of less than five minutes, Smith and colleagues examined silicate charging, plume dynamics and the relationship between volcanic lightning and vent discharges. As expected, the team found that lightning at Sakurajima occurred in plumes replete with ash. Vent discharges, however, occurred only when ash-rich plumes with volcanic lightning rocketed skyward at velocities greater than about 55 meters per second.
Experiments have shown that you can’t get lightning without some amount of ash in the system, Smith says. “So if you’re seeing volcanic lightning, you can be pretty confident in saying that the eruption has ash.”
Vent discharges, on the other hand, are relatively newly detected bursts of electrical activity, which produce a continuous, high-frequency signal for seconds — an eternity compared with lightning. These discharges can be measured using specialized equipment.
By focusing on small explosions from Sakurajima, defined as those with plume heights of 3 kilometers or less and with a duration of less than five minutes, Smith and colleagues examined silicate charging, plume dynamics and the relationship between volcanic lightning and vent discharges. As expected, the team found that lightning at Sakurajima occurred in plumes replete with ash. Vent discharges, however, occurred only when ash-rich plumes with volcanic lightning rocketed skyward at velocities greater than about 55 meters per second.
