The fair weather electric field at the Earth's surface is approximately
Answer: D
The ionosphere sits at a positive potential of several hundred kilovolts relative to the ground. The resulting field is present everywhere in fair weather and is part of the global circuit. It is easily measured with a field mill.
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Assertion (A): Thunderstorms are regarded as the generators of the global electric circuit.
Reason (R): They transport negative charge to the ground and maintain the ionospheric potential.
Answer: A
Both are true and the reason gives the role, and fair weather conduction currents discharge the circuit continuously. Without ongoing thunderstorm activity the potential would decay within minutes. Roughly ten minutes is the usual figure quoted.
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Consider the following statements about the global electric circuit.
1. Thunderstorms act as the generators of the circuit.
2. Fair weather regions carry the return conduction current.
3. The circuit would persist unchanged without thunderstorms.
Answer: B
The first two are correct and describe the circuit. The third is wrong, since conduction would discharge the ionosphere within about ten minutes were the generators to stop. Continuous thunderstorm activity is therefore essential.
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The Carnegie curve describes
Answer: C
The field varies through the day in step with global thunderstorm activity regardless of local time. It is among the strongest pieces of evidence for the global circuit concept. The curve was established from ocean measurements.
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Assertion (A): The fair weather field over the ocean varies with universal rather than local time.
Reason (R): It responds to global thunderstorm activity rather than to conditions locally.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the cause. The fair weather field is maintained by the world’s thunderstorms acting as a global generator, so it peaks when the total global thunderstorm activity peaks - at about 1900 universal time, when the storms over the Americas are most active. Over the ocean, away from local sources of variation, that global signal is what is measured. It is called the Carnegie curve.
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Global lightning activity is estimated at approximately how many flashes per second?
Answer: A
Correct answer: 45 flashes per second
Satellite observation gives a global average near 45 flashes a second, or some 1.4 billion a year, with the great majority over land in the tropics. The figure varies by a factor of two through the day and the year as the global thunderstorm activity varies, and it is that variation which the fair weather field over the ocean follows.
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Assertion (A): Lightning is far more frequent over land than over the ocean.
Reason (R): Land heats rapidly and supports the vigorous updraughts that charging requires.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Land has a low heat capacity and warms quickly in the sun, driving strong surface heating and the vigorous updraughts that carry supercooled water above the freezing level. The sea warms slowly and its surface temperature varies little through the day, so oceanic convection is generally weaker and the lightning rate lower by a factor of ten.
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Three of the following are regions of high lightning activity. Which is the odd one out?
Answer: C
Correct answer: the central Pacific ocean
The central Pacific has among the lowest flash rates on earth, for exactly the reason given by the previous item - weak convection over water whose surface temperature barely varies. The other three are among the world’s lightning maxima: the Congo basin, which holds the global record, northern Argentina and Paraguay, and central Florida, the most active region of the United States.
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Sprites are
Answer: D
They occur in the mesosphere at fifty to ninety kilometres and last only milliseconds. They are usually triggered by large positive cloud to ground flashes. Their existence was only confirmed in the late twentieth century.
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Match each transient luminous event with where it occurs.
List I
A. Sprite
B. Blue jet
C. Elve
D. Gigantic jet
List II
1. a rapidly expanding ring in the lower ionosphere
2. mesosphere above a thunderstorm
3. from cloud top all the way to the ionosphere
4. upward from the cloud top into the stratosphere
Answer: A
Correct answer: A-2, B-4, C-1, D-3
The four transient luminous events occupy different altitudes. A sprite is a red discharge in the mesosphere, 50 to 90 km up, triggered by a large charge transfer below. A blue jet shoots upward from the cloud top into the stratosphere. An elve is a rapidly expanding ring in the lower ionosphere, produced by the electromagnetic pulse of the flash. A gigantic jet bridges the whole gap from cloud top to ionosphere.
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