Stow (1969) gives the total transfer of positive charge from the ionosphere to earth due to currents in fine weather as about \(90 \mathrm{C} \mathrm{km}^{-2}\) per year. What is the approximate fine weather current falling on \(1 \mathrm{~mm}^{2}\) of the earth? How many clectronic charges per second does this represent?

To understand electric current in this context, imagine a stream of water flowing over a cliff to create a small waterfall. Likewise, electric current represents the flow of electric charge from one point to another, in this case, from the ionosphere to the Earth. This flow occurs when there is a difference in electric potential, like the difference in height for water. Our goal with electric current calculation is similar to measuring how much water falls over the edge in a set amount of time.

In the given exercise, we calculated the fine weather electric current by determining the amount of electric charge transferred to a tiny area of 1 square millimeter over a year. We used the given total charge transfer rate, converted the area from square kilometers to square millimeters, and then divided by the seconds in a year. This gave an extremely small current of approximately 2.85 x 10^-18 amperes, which symbolizes the very gentle and almost imperceptible trickle of charge during fine weather conditions.

It's also essential to recognize that the current is not constant over the year, and fine-tuning the calculation to account for temporal variations could provide a more accurate picture, like how a waterfall's flow can vary from season to season.

The concept of 'fine weather atmospheric electricity' refers to the electric field and associated electric current present in the atmosphere under clear, calm weather conditions. Unlike thunderstorms, which can create large and rapid transfers of charge, fine weather electrical currents are much more subdued and steady.

These electric currents are significant for multiple reasons, and one important aspect is their role in the Earth's global electric circuit. Constant, albeit small, currents like the one described in the exercise help to maintain the potential difference between the Earth's surface and the ionosphere.

Understanding these fine weather currents helps us comprehend a larger picture of electrical processes happening on a planetary scale. Scientists study such phenomena to learn more about Earth's weather systems, climate, and even interactions with solar and cosmic radiation.

At the core of calculating electric currents is the concept of the 'elementary charge', which is a fundamental constant in physics. It represents the smallest unit of electric charge that cannot be divided into smaller parts. This charge is the electric charge carried by a single proton or, equivalently, the negative of the charge carried by a single electron.

With an approximate value of 1.602 x 10^-19 coulombs, the elementary charge serves as the building block for understanding electric phenomena on both macroscopic and microscopic scales. When we talked about the number of electronic charges per second in the exercise, we were essentially counting how many 'buckets' (electrons) of charge pass our point of interest (1 square millimeter area of Earth) every second. In our exercise, this translated to approximately 18 elementary charges, or electrons, per second – a tiny but measurable procession of nature's fundamental particles contributing to Earth's atmospheric electric currents.