section_path: Climate/Storms
section_name: Storms
paragraph_text: Storms
  
  In 1989, the Great Dark Spot, an anticyclonic storm system spanning 13,000 km × 6,600 km (8,100 mi × 4,100 mi), was discovered by NASA 's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. Some five years later, on 2 November 1994, the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in Neptune's northern hemisphere.
  
  The Scooter is another storm, a white cloud group farther south than the Great Dark Spot. This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when they were observed moving at speeds faster than the Great Dark Spot (and images acquired later would subsequently reveal the presence of clouds moving even faster than those that had initially been detected by Voyager 2). The Small Dark Spot is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It was initially completely dark, but as Voyager 2 approached the planet, a bright core developed, which can be seen in most of the highest-resolution images. In 2018, a newer main dark spot and smaller dark spot were identified and studied. In 2023, the first ground-based observation of a dark spot on Neptune was announced.
  
  Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter cloud features, so they appear as holes in the upper cloud decks. As they are stable features that can persist for several months, they are thought to be vortex structures. Often associated with dark spots are brighter, persistent methane clouds that form around the tropopause layer. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator or possibly through some other, unknown mechanism.
  
  In 1989, Voyager 2 's Planetary Radio Astronomy (PRA) experiment observed around 60 lightning flashes, or Neptunian electrostatic discharges emitting energies over 7 × 10 J. A plasma wave system (PWS) detected 16 electromagnetic wave events with a frequency range of 50–12 kHz at magnetic latitudes 7–33˚. These plasma wave detections were possibly triggered by lightning over 20 minutes in the ammonia clouds of the magnetosphere.
  
  During Voyager 2 ’s closest approach to Neptune, the PWS instrument provided Neptune’s first plasma wave detections at a sample rate of 28,800 samples per second. The measured plasma densities range from 10 – 10 cm. Neptunian lightning may occur in three cloud layers, with microphysical modelling suggesting that most of these occurrences happen in the water clouds of the troposphere or the shallow ammonia clouds of the magnetosphere. Neptune is predicted to have 1/19 the lightning flash rate of Jupiter and to display most of its lightning activity at high latitudes. However, lightning on Neptune seems to resemble lightning on Earth rather than Jovian lightning.