Thursday Jan. 13, 2011
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First day of class.  We started with some information about the course, a project that you will have the option of doing, and mentioned some books that might be used during the class.  A list of topics that might be covered during the semester wasn't covered at the beginning of the class but we came back to it briefly during the period.  Note the that examples of midterm exams and final exams from previous editions of the course are accessible via links on the main class webpage.


A quick overview of the Global Electric Circuit is a good way to introduce much of what we will be covering this semester.  The following notes are a more neatly drawn version of what we did in class.




Point 1  Together, the earth's surface and the ionosophere resemble a spherical capacitor   The two electrodes are the ground and the ionosphere.  The ground is negatively charged during fair weather.  Positive space charge is found in the air between the ground and the ionosphere (most of the charge is near the ground) instead of on the second conductor.  The positive charge is attached to particulates in the air (aerosols) and is relatively immobile (due to the large size and large inertia of the particles).  These are called "large ions."

Point 2  You would find a downward pointing electric field of 100 to 300 Volts/meter at the ground during normal fair weather conditions.  We will assume that the ground is a perfect conductor in many of the problems we do, so the field will be perpendicular to the ground.  Also we will often just consider the ground to be flat because we will just be looking a small portion of the globe and because the atmosphere is much thinner than the radius of the earth.  That's why we used Ez in the picture instead of Er.

Points 3 & 4  Air is a very poor conductor but does have a finite conductivity.  A very weak current flows from the ionosphere to the ground.  Jz in the figure is current density ( amperes/meter2 ).


In a wire it is the motions of electrons that carry current from one point to another.  Charge carries of both polarites carry current in the atmosphere.  Small ions are charged clusters of molecules that are much smaller and more mobile than large ions.  We will have a look at some point in the semester at what creates small ions, what factors determine small ion concentrations, and that sort of thing (small ion concentration will affect conductivity).



Point 5  Estimate of Jz.

We can multiply this by the area of the earth's surface to determine to total current flowing between the ionosphere and the earth's surface.


Point 6  a bit of a detour.  Why don't you feel the 400 volt potential difference between your head and toes when standing outside on a fair day?

Because air has a high resistance, a very weak current flowing through air can produce a large potential difference.  The resistance of a human body is much lower.  The person is effectively a short circuit and there is little or no head-to-toe potential difference.

Point 7  The potential of the ionosphere ranges from 150 kV to 600 kV relative to the earth's surface (see Table 15.1 in The Earth's Electrical Environment i.e. the "yellow book")  We'll use an average value of 280,000 volts (instead of the 200,000 volts used in class).

We can divide the surface-ionosphere potential difference by the current flowing between the ionosphere and the surface to determine an effective resistance of the atmosphere.


Point 8  The following equation shows the relationship between surface charge density and electric field at the surface of a conductor (we'll derive this expression soon in class, if I remember right it is a simple application of Gauss' Law)


We'll multiply by the area of the surface of the earth to determine the total charge on the earth's surface

The following calculation shows that it wouldn't take very long for the current flowing between the ionosphere and the ground, I, to neutralize the charge on the earth's surface, Q.


This doesn't happen.  The obvious question is what maintains the surface-ionsphere potential difference?  What keeps the spherical capacitor charged up?


Point 9  The original answer was lightning.  Most cloud-to-ground lightning carries negative charge to the ground. 

At some point it became clear that lightning alone wasn't enough.  The thinking then became thunderstorms in general.  Point (b) shows an upward current flowing from the top of the thunderstorm and also from point discharge currents on the ground.  These currents aren't quite sufficient.

The current thinking is that thunderstorms and non-thundery but electrified clouds are needed to produce sufficient charging current.


Points 1-8 in the figure at the tops of today's notes (yellow) form what might be called "fair weather atmospheric electricity."  We'll spend a significant portion  of the class (40% perhaps, maybe somewhat less depending on the interests of the students in the class) discussing this topic. 

Point 10  The majority of the class will be devoted to stormy weather electricity, i.e. thunderstorms, lightning, and related topics.  We'll look at how thunderstorms become electrified (doesn't it seem surprising that electrical charge is created and separated in the cold wet interior of thunderstorms?).  We'll spend quite a bit of time looking at the sequence of events that make up negative cloud-to-ground lightning.  We'll also look at other times of lightning (intracloud lightning, positive cloud-to-ground lightning, upward and triggered lightning).  Lightning currents can be measured directly and indirectly (from the radiated fields).  Some knowledge of lightning currents characteristics is needed to be able to design effective lightning protection equipment.  And we'll cover lightning protection and lightning safety.


I'm planning to doing some basic demonstrations and also bringing in some examples of working instrumentation used in thunderstorm and lightning research.  The plasma globe that appeared at the end of today's class seemed appropriate for the first day of class.  You'll find a clear and basic explanation how plasma globes work here.