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
remainder of this class.
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.
Together, the earth's surface and the ionosophere resemble a charged
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 (the charge would normally be found on the
electrode in a typical capacitor). The positive charge is
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." Most of the positive charge is found 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) electric field (E
ground during normal fair weather conditions. Soil and ocean
water are much better conductors than air so we'll often assume
the ground is a perfect conductor in many of the problems that we look
In that case the E field will be perpendicular to the ground. Also
atmosphere is much
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
system) in the
picture instead of Er
Note that the ground is normally positively charged underneath a
thunderstorm (Point 9). The electric field 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 carries of both
current. These charge carriers are called "small ions" and
consist of charged clusters of
molecules that 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
term which depends on the strength of the electric
field (E) and the conductivity
of the air (lambda 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.
5 We can assume reasonable values for the strength of the
"fair weather" electric field and the conductivity of the air to
We can multiply this current density by the area of the earth's
determine to total current flowing between the ionosphere and the
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
Why don't we feel this?
Air has a very low conducitivity (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 wet or
dry the body is). Compared to air the person is
effectively a short circuit and there really is very little or no
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.
We'll multiply by the area of the surface of the earth to
determine the total charge on the earth's surface
8 The following equation shows the relationship between
surface charge density (Coulombs per unit area) and electric field at
the surface of the earth
(we'll derive this expression soon in this class, it's
a simple application of Gauss' Law)
The earth's surface is charged, but a weak current flows through the
atmosphere to the earth trying to neutralize the charge on the earth.
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
It would only take about 10 minutes
to discharge the earth's surface. This doesn't happen
obvious question is what maintains the surface-ionsphere potential
difference? What keeps the earth-ionosphere spherical capacitor
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. But these
aren't quite sufficient either.
The current thinking is that thunderstorms and non-thundery (Point (c)
electrified clouds are needed to produce sufficient charging current.
Points 1-8 in the figure at the
beginning of today's notes constitute
what might be called "fair weather atmospheric electricity."
We'll spend a significant portion of this class discussing this
Point 10 Most of the
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 windy
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 types of lightning
(intracloud lightning, positive cloud-to-ground lightning, upward and
triggered lightning). We'll look at
how lightning current
characteristics can be measured. This is important because some
lightning currents characteristics is needed to to design
effective lightning protection equipment. Lightning protection of
structures and electrical systems is something else we'll cover.
We'll also look at new ground- and satellite-based sensors being used
to detect lightning as it occurs around the globe.
We try to include as many basic demonstrations and examples
instrumentation used in thunderstorm and
lightning research in the classroom version of this course because they
are entertaining and educational. As much as possible
we'll try to do the same in this online course.
Along those lines, the
flow of electricity between the ionosphere and the surface of the earth
in some 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.