Wednesday Jan. 14, 2015

Introductory information
Welcome to ATMO/ECE 489/589 - Atmospheric Electricity.  This is a one-semester course that is taught every two years.  It seems to attract upper division-undergraduate and graduate students from a variety of disciplines.  Because of the variety of student interests and backgrounds the class is taught at what I would judge to be the level of an upper division undergraduate physics class. 

We began this first class with a quick look at information about the course, and a project that you will have the option of doing.  This class does not have a textbook, these online lecture notes will be reading matter for the course.  Information covered in the course will be drawn from several books and articles in scientific journals (some of the books are available for short term checkout from the instructor or the Atmospheric Sciences Department library). 

An online version of the class has been developed and is available to you in its entirety.    This semester's class will probably be very similar to the course that was taught in Spring 2013 (that was a T Th class so the material may be split a little different that this semester).  As you read through that material please let me know if you find errors or sections that aren't clear and need improvement.   Please have a look at the list of topics that we plan to cover during the class and let me know if there are one or more areas that you are especially interested, especially if they aren't on the topics list.

Copies of midterm exams and final exams from several past courses are available online.  ATMO students would have access to this material from fellow students and it only seemed fair to make the material available to students from other departments.

We'll begin this course with a quick look at the global atmospheric electrical circuit.  This is a way to introduce some of the topics that we will be covering in more depth during the remainder of this class.  I would also encourage you to read "Electricity in the Atmosphere," chapter 9 in Volume II of the Feynman Lectures on Physics (the entire 3 volume series is available here). 

Global electric circuit pt. 1
The figure below shows the earth's surface and the bottom 100 km or so of the earth's atmosphere - the part extending from the ground up to the bottom of the ionosphere.  In the ionosphere there is enough ionization of air (mainly by ultraviolet radiation) to make the atmosphere an electrically conducting layer.  The figure is not drawn to scale, the 100 km thick layer of atmosphere has been greatly magnified (and thunderstorms aren't nearly as tall as the figure below would imply).



Point 1  Together, the earth's surface and the ionosphere resemble a charged spherical capacitor, i.e. two oppositely charged conducting electrodes with an insulator in between.   The ground is normally negatively charged during fair weather.  Positive charge is found in the air between the ground and the ionosphere (this positive charge would normally be found on the second electrode in a typical capacitor).  The positive charge is attached to small particles in the air (aerosols) and is relatively immobile (compared to air molecules due to the large size and large inertia of the particles).  These are called "large ions."  The positive space charge density is highest near the ground.

Point 2  Negative charge on the ground and positive charge in the air above means that there is a downward pointing, 100 to 300 volts/meter (V/m) amplitude, electric field (E field) at the ground during normal fair weather conditions.  Soil and ocean water are much better conductors than air so we'll often assume that the ground is a perfect conductor in many of the problems that we look at.  In that case the E field will be perpendicular to the ground.  Also because the atmosphere is much thinner than the radius of the earth, we will usually be able to just consider the ground to be flat and ignore the fact that it is curved.  That's why we used Ez (cartesian coordinate system) in the picture above instead of Er (spherical polar coordinates).

Note that the ground is normally positively charged underneath a thunderstorm (Point 9).  The electric fields at the ground under a thunderstorm are normally more intense, 1000s of V/m.

Points 3 & 4  Air is not a perfect insulator, it does have a very small but finite conductivity.  A very weak current flows from the ionosphere to the ground. 

Conduction of electricity in the atmosphere is a little different from what happens in a wire.  In a wire it is the motions of free electrons alone that carry current from one point to another.  In the atmosphere charge carriers of both polarities carry current.  These charge carriers are called "small ions" and consist of charged oxygen and nitrogen molecules (N2 and O2) that have water vapor molecules clustered around them.  Small ions are much smaller and more mobile than large ions.  During the class we will have a look at how small ions are created (and destroyed).


Current flowing to the ground is denoted Jz in the figure.  J stands for current density which has units of amperes/meter2.  A conduction current term which depends on the strength of the electric field (E) and the conductivity of the air (λ in the figure below) is often the main component of Jz .

 

We'll find that Ez decreases, conductivity increases, and current density remains about constant with increasing altitude (under steady state conditions).

Point 5  We can assume reasonable values for the strength of the "fair weather" electric field and the conductivity of the air to estimate Jz.

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



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 )  We'll use an average value of 280,000 volts.

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 6  Let's step backward briefly.  An electric field of 200 V/m would mean there would be a 400 volt difference between the ground and a point 2 meters above the ground.  That's about a 400 volt difference between our head and our toes when we step outside.  Why don't we feel this?


Air has a very low conductivity (high resistance), a very weak current flowing through air can produce a large potential difference.  The resistance of a human body is much lower (I don't really know what the resistance of a human body is, perhaps 1000 ohms up to as much as 100,000 ohms depending on how hydrated the body is).  Compared to air the person is effectively a short circuit and there really is very little or no head-to-toe potential difference.

This is about as far as we were able to go on this first day of class.  The remainder of the material on the global electric circuit has been moved to the Jan. 16 lecture notes.

The following wasn't mentioned in class today
We try to include as many basic demonstrations and examples of working instrumentation used in thunderstorm and lightning research in this course as we can.  They are entertaining and educational and they also take up time that otherwise would be spent on lecture. 

Along those lines, the flow of electricity between the ionosphere and the surface of the earth in some respects (probably very few respects) resembles the visible discharges in a plasma globe.  Some photos are shown below (source).







You'll find a clear and basic explanation of how plasma globes work here.

References and optional reading
R.P. Feynman, R.B. Leighton, M. Sands, "Electricity in the Atmosphere," chap. 9 in the Feynman Lectures on Physics, Vol. II, Addison-Wesley Publ. Co., 1964.

E.A. Bering III, A.A. Few, J.R. Benbrook, "The Global Electric Circuit", Physics Today, 51, 24-30, 1998.

R.G. Roble and I. Tzur, "The Global Atmospheric-Electrical Circuit," chap 15. in The Earth's Electrical Environment, National Academy Press, Washington, 1986.

E.R. Williams, "The global electrical circuit: A review," Atmospheric Research, 91, 140-152, 2009.