Introduction to Atmospheric Science at Arecibo Observatory Mike Sulzer AO Summer Student Talk June 8, 2011

What can one talk about in one hour? When we study many human activities, we say “Follow the money.” For studying a physical system, we might say “Follow the energy.”

So, let’s look at energy in the atmosphere, and then concentrate on the radar measurements we can make in the ionosphere.

1. Introductory Atmospheric Concepts

If the sun irradiates the earth, what is the temperature of the earth, assuming it is a black body?

How do pressure and density vary as a function of altitude? (Simple case, just gravity) Effect of gravity Equation of State Scale Height

Integrating the last equation gives the pressure as a function of height. Remember, this is a simplification. For example, a vertical velocity (usually small) could alter these equations. H variable in height H fixed in height

So how does the temperature actually vary? Suppose we have an atmosphere in Hydrostatic equilibrium. How does the temperature vary as a function of height? Let’s assume that the ground is heated by the sun, but that the atmosphere does not absorb any solar radiation. We assume that the atmosphere just above the ground is heated to ground temperature, and vertical motion occurs to distribute the heat, resulting in some equilibrium. We further assume that the motion is adiabatic; that is, a moving parcel of air holds onto it heat, neither losing nor gaining. The resulting temperature gradient is (skipping the derivation): This defines the Adiabatic lapse rate. So how does the temperature actually vary?

Radiative Equilibrium with absorption in the atmosphere The concepts here are not very difficult, but this can get very complicated. Everyone is aware of some of the consequences in the lower atmosphere due to the absorption and re-emission of infrared (so called green house effect, etc.) But what about the upper atmosphere? What are the effects of the higher energy solar photons? First, let’s look briefly at another kind of emission from the sun: the solar wind.

1I. Studying the Ionosphere with Incoherent Scatter Radar This plot show the number density of electrons versus altitude. What else does one want to know?

What is incoherent Scatter? Consider a population of electrons; the locations and velocities are random processes. If this population is illuminated by an electromagnetic (EM) wave well above the plasma frequency, a very small amount of EM radiation is emitted (scattered) in a broad directional pattern. Why is it useful? It is a method for sensing the state of the scattering medium: 1. The power of the scattered signal is a measure of the number density of electrons. 2. The time history of the scattered signal, when measured in a statistical sense, determines a number of useful parameters, in our case, including one to three different temperatures and velocities of the constituents of the plasma, as well as the composition of the plasma.

What is radar? For our purposes, radar is a remote sensing technique in which the reflected or scattered signals from radio frequency transmissions are received and analyzed in order to study the medium. The term “incoherent scatter” is used when the scattering properties of the medium are primarily associated with thermal fluctuations in the medium, that is, the more or less random location and possibly organized motion of the atomic and molecular constituents of a plasma. An incoherent scatter radar is one that uses large antennas and high transmitted power in order to study the weak return from the ionospheric plasma.

Significant scatter comes from electrons, not ions because the electrons are thousands of times lighter, and thus much easier to accelerate. So why is it that the scattered signal contains information about the ions, such as temperature, mass, and velocity? This fundamental question is very interesting. It can be answered in an hour in only an intuitive way using analogies.

The Hardware IS radars are expensive, and there are not very many. As a result they have been designed especially for a given application, and each one is different. Standardization of design is now becoming possible in some cases.

Jicamarca Radio Observatory Field of coco antennas, 64 individual sections

The Poker Flat Incoherent Scatter Radar: A phased Array

The AMISR is a Phased Array A Phased Array is composed of modules. In the language for the AMISR, a module is called an AEU (Antenna Element Unit) Antenna Low level rf in Control info. in/out Receiver out Box containing a transmitter, a receiver (front end), and some control circuits

Range Time Diagram

The radar signal illuminates many electrons, each with its own velocity and Doppler shift along the radar line of sight.

The Signal is the vector sum from many electrons.

The Radar Experiment

(Informal analysis)

The previous viewgraphs describe scattering from many electrons, but the ionosphere is a plasma and so there is much more going on: 1. It has other particles, ions. 2. The particles are charged (electrostatic force. 3. Collective effects happen (wave propagation, etc.) 4. Another important collective effect is shielding: charge fluctuations tend to be reduced or eliminated.

* * the fluctuations N, not the power (<|N|^2>)

The Radar Experiment