Lightning

Spacecraft missions in the 1970s and 1980s revealed that Jupiter experiences lightning in its clouds much as Earth does. W. Borucki and M. Williams (1986) calculated that the lightning must occur at the 5-bar level, far below the clouds visible to us, in giant thunderstorms with the same dynamics as those on Earth. Indeed, spacecraft detected lightning photographically on Jupiter's night side, and heard the lightning as 'whistlers' with a plasma wave experiment [262]. More recently, the Galileo atmospheric probe detected lightning when it descended into the cloud tops of Jupiter. This probe 'heard the radio bursts of 50,000 lightning flashes during the 57.6 minutes of its descent' [263].

I think it is fascinating that lightning occurs on a planet other than Earth. The fact that the physical conditions that cause lightning can exist on another planet so much unlike Earth is truly a wonder. But, there is an important reason for studying the lightning on Jupiter. Lightning is diagnostic of dynamics, chemical composition, and heat exchange within the Jovian atmosphere [264]. Although lightning on Earth is evenly distributed geographically, lightning on Jupiter appears to be confined to high latitudes [265]. On Earth we experience ground strike lightning but also discharges between clouds, which are far more common. Since Jupiter has no 'ground', lightning there is presumed to be from cloud to cloud. The average lightning on Jupiter is ten times stronger than the average lightning on Earth [266].

The Galileo spacecraft searched for lightning on Jupiter's dark side and detected a chain of flashing thunderheads just south of the westward moving jetstream at 46° north latitude (Fig. 5.8). Almost all the lightning detected by Voyager had also been detected near the latitude of a westward moving jetstream. The lightning was

2 min

Fig. 5.8. Changing Lightning Storms on Jupiter. Galileo spacecraft images showing lightning storms in three different locations on Jupiter's night side. Each panel shows multiple lightning strikes, coming from different parts of the storm. The lightning originates in Jupiter's water cloud, which is 50-75 km (30-45 miles) below the ammonia cloud. The latter acts as a translucent screen, diffusing the light over an area proportionate to the depth of the lightning. The lightning strikes are hundreds of times brighter than lightning on Earth. The bottom row shows the same three storms as the top row, but the bottom-row images were taken two minutes later. The panels are 8,000 km on a side. North is at the top of the picture. (Credit: NASA/JPL-Caltech).

flashing far below the visible ammonia cloud deck that was acting like a translucent screen, diffusing the light upward. The apparent width of the flash could be used as an indicator to infer its depth of 75 km, a depth that would be consistent with water clouds [267]. Previously, the Voyager observations determined that Jupiter's lightning occurred at a depth of ~5 bar in water clouds [268].

Voyager observed that the Great Red Spot (GRS) spawned a series of rapidly expanding white clouds resembling large thunderheads, which were carried away by the prevailing jetstream. Indeed, a telescope of just 6-in. aperture can reveal the large bright wake on the following side of the GRS. Later, the Galileo spacecraft detected lightning in these GRS-following 'thunderheads' and confirmed they are intensely convective features. Galileo made multi-filter observations of a cloud in one of these amorphous cyclonic regions, and the observation of lightning was consistent with this observation (Fig. 5.9). Multiple lightning strikes confirmed that this was a site of moist convection in the saturated environment at a depth of 75 km. The bright cloud Galileo observed resembled a site of convective upswelling in Earth's atmosphere [269]. As Cassini passed Jupiter on its way out to Saturn, it also made observations of Jovian lightning. Its observations of lightning in the Ha line of the lightning spectrum also suggested that the lightning Cassini observed was at depths of more than 5 bars in the atmosphere [270]. We know there is great turbulence following the GRS. The most interesting lightning seen by the Galileo

18 16 14 12 10

West Longitude

Fig. 5.9. Jovian Lightning and the Daytime Storm. A Galileo spacecraft image of a convective storm (left panel) and the associated lightning (right panels) in Jupiter's atmosphere. The left image shows the daytime view. The right images show the area highlighted (box) in the dayside view as it appeared 110 min later during the night. Multiple lightning strikes are visible in the nightside images, which were taken 3 min and 38 s apart. The bright, cloudy area in the dayside view is similar in appearance to a region of upwelling in Earth's atmosphere. The dark, clear region to the west (left) appears similar to a region of downwelling in Earth's atmosphere. (Credit: NASA/JPL-Caltech).

18 16 14 12 10

West Longitude

Fig. 5.9. Jovian Lightning and the Daytime Storm. A Galileo spacecraft image of a convective storm (left panel) and the associated lightning (right panels) in Jupiter's atmosphere. The left image shows the daytime view. The right images show the area highlighted (box) in the dayside view as it appeared 110 min later during the night. Multiple lightning strikes are visible in the nightside images, which were taken 3 min and 38 s apart. The bright, cloudy area in the dayside view is similar in appearance to a region of upwelling in Earth's atmosphere. The dark, clear region to the west (left) appears similar to a region of downwelling in Earth's atmosphere. (Credit: NASA/JPL-Caltech).

spacecraft occurred in the GRS wake. Apparently, this region of unusually strong turbulent eddies in the atmospheric flow is favorable for unusually strong lightning. One of the four storms observed by Cassini resided in the turbulent wake of the GRS [271].

Jupiter's lightning appears to occur in clusters, also called storms (Fig. 5.10). The Galileo spacecraft observed multiple flashes in these storms and that the storms are separated by large distances. Most of the storms observed by Galileo occur in cyclonic shear zones. The only exceptions are the storms between 40° and 50° north, which seem to be clustered near the center of westward jets [272]. Both the Voyager and Galileo spacecraft observed considerably fewer storms in the southern hemisphere of Jupiter.

Since lightning on Jupiter is expected to occur in water clouds, lighting deeper than 5 bars suggests that the water clouds themselves exist deeper than 5 bars on Jupiter. Thus, we see demonstrated the benefit of observing Jupiter's lightning, as no other investigative technique can observe much below the 5 bar level on Jupiter. To support water clouds at depths more than 5 bars and the corresponding temperature associated with that depth, water abundance in the deep Jovian atmosphere should be more than 1x solar abundances [273, 274].

The Cassini spacecraft observed four lightning spots on Jupiter as it passed by, and these were correlated with four unusually bright, small (~1,000 km in size)

Fig. 5.10. Water Cloud Thunderstorm on Jupiter. A false color image of a convective thunderstorm northwest of the GRS taken by the Galileo spacecraft. The white cloud in the center is a tall, thick cloud 1,000 km (620 miles) across, standing 25 km (15 miles) higher than the surrounding clouds. (Credit: NASA/California Institute of Technology/Cornell University).

C = 3 ui (A n clouds that Cassini had previously observed a few hours earlier on the day side. Apparently the bright, small clouds are quite rare, with only a few on the planet at any given time. Visible and near infrared (IR) spectra suggested that these clouds were dense, vertically extended, and contained unusually large particles. This is also typical of terrestrial (Earth bound) thunderstorms [275]. Lightning in the Voyager 2 observations were not always correlated with small bright clouds. Because Cassini at the distance it passed could observe very few and only the most powerful lightning storms, the correlation of the clouds with Cassini lightning may suggest that only the most powerful thunderstorms with bright lightning will penetrate the troposphere up to the levels where they are easily observable in reflected light [276]. The Cassini observations also confirmed the Voyager conclusions that Jovian thunderstorms may generate lightning even when the clouds do not extend to the top of the troposphere and expose themselves as bright clouds. Such deep storms may develop the high tops several hours or even days after or before the deep thunderstorm stage, or they may remain at deep levels through their whole life span [277].

We can now be fairly certain that Jovian lightning occurs from cloud to cloud discharge, as most lightning occurs on Earth, and that Jovian lightning occurs in water clouds. The Jovian lightning gives us an indication of the depth of water clouds on Jupiter and, by estimating the temperature at these depths allows us to make an estimate of the Jovian water abundance ratio compared to the solar ratio. We have learned much from the spacecraft missions mentioned above; yet, we have so much to learn about Jovian lightning and the mechanisms that drive it.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment