【天体物理学】木星の雷によって生じる電磁波の高周波成分が測定された

New insights into the distribution and rate of lightning emissions on Jupiter, revealed by the Juno spacecraft, are reported in two papers published in Nature and Nature Astronomy this week.

In 1979, the Voyager 1 spacecraft detected very low-frequency radio emissions around Jupiter that are dubbed whistlers because they resemble descending, whistled tones. Whistlers are thought to originate from lightning emissions in the atmosphere of the gas giant. Lightning has been detected on Jupiter by most visiting spacecraft but only through night-side optical imaging or radio waves at low frequencies (in the kilohertz range), despite a search for signals in the megahertz range.

Writing in Nature, Shannon Brown and colleagues present observations of high-frequency Jovian lightning atmospherics (or sferics, as these broadband electromagnetic impulses are known) at 600 megahertz. The detections suggest that Jovian lightning discharges are not as dissimilar from their terrestrial counterparts - which can reach frequencies in the gigahertz range - as previously thought. The authors show that lightning on Jupiter is more common near the poles and in the northern hemisphere and absent at the equator, suggesting that there is increased moist convection polewards. These findings could help to improve our understanding of the composition, circulation and energy flows on Jupiter.

In the second paper, published in Nature Astronomy, Ivana Kolmasova and colleagues present the largest database of lightning-generated whistlers from Jupiter to date, including more than 1,600 signals - almost ten times the number recorded by Voyager 1. They detected peak rates of four lightning strokes per second - similar to the rates observed in thunderstorms on Earth, and six times higher than the peak values detected by Voyager 1.