The Space Weather Prediction Center has a pdf file
Basic Facts about the Ionosphere from Windows to the Universe
The National Geophysical Data Center's Ionosphere Page
The High Frequency Active Auroral Research Project (HAARP) Ionosphere Page
A perspective of military exploitation from the Global Policy Forum about Argus, Starfish and HAARP
The upper atmosphere (especially section 3) in the Physics of the Aurora webcast
University of Oulu's (Finland) Virtual Library on Aeronomy. A links page to related material.

Discovery of the Ionosphere

1820	Hans Christian Oersted discovers that electricity and magnetism are related
1864 - 73	James Clerk Maxwell describes theory of electromagnetic radiation and predicts existence of radiowaves
1882	Balfour Stewart suggests winds drive electric currents in the upper atmosphere
1887	Heinrich Hertz proves existence of radiowaves
1895	Guglielmo Marconi demonstrates wireless (radio) communication in Bologna, Italy
1899	Marconi transmits radio signal across English Channel
Dec. 12, 1901	Marconi transmits radio signal across Atlantic Ocean from Cornwall, England to St. John's, Newfoundland
1902	Oliver Heaviside; Arthur Kennelly propose existence of conducting layer in upper atmosphere
1909	Marconi awarded Nobel Prize
1924	Edward Appleton and others develop the ionosonde and begin ground-based soundings; prove existence of ionosphere
1925	Appleton discovers second layer (the F region)
1926	Robert Watson-Watt (later developer of radar) coins word "ionosphere"
1927	Sydney Chapman describes theory for formation of ionosphere
1947	Appleton awarded Nobel Prize
1948	Rocket probes begin
1950's	Riometer (Relative Ionospheric Opacity Meter using Extra Terrestrial Electromagnetic Radiation) developed
1958	Incoherent Scatter Radar developed
1962	Alouette 1 satellite makes first topside soundings

Radio Communication

Radio Reception

• Amplitude Modulated (AM) radio (540 - 1600 kHz)
  
  • ground wave - received (local reception)
    
  • sky wave -
    
    • DAY: absorbed in D-region of ionosphere
      
    • NIGHT: reflected by E-region
      
      • interference from distant transmitters
        
      • fading in/out of long distance reception
• Short Wave (SW) radio (2 - 26 MHz)
  
  • ground wave - received (local reception)
    
  • sky wave - reflected by F-region (enables long distance reception)
• Frequency Modulated (FM) radio (88 - 108 MHz)
  
  • ground wave - received (local reception)
    
  • sky wave - transmitted through ionosphere into space

Some Milestones in the History of Radio

A few internet resources on the history of radio:
Early developments
Timeline of Communication History
Appliances That Changed The Home Forever
Old Time Radio Show Catalog: A website with info and samples of old programs.
Another site for enthusiasts of early radio

A few webites (with some interesting photos of early radio equipment) for radio museums around the world:
U.S. Marconi Museum in Bedford, NH
AWA Electronic Communication Museum in Bloomfield, NY
Marconi Canadian Historic Site in Glace Bay, Nova Scotia, CANADA
Hammond Museum of Radio in Guelph, Ontario, CANADA
The Popov Central Museum of Communication in St. Petersburg, RUSSIA
Popov Museum of Radio in Yekaterinburg, RUSSIA
Marconi Foundation's Historical Museum in Bologna, ITALY

The Big Question: Who invented radio?
I used to think Star Trek's Ensign Chekov's view of history was just comic relief. Little did I know.

Ionospheric Structure

The number of electrons (or electron density) varies spatially in altitude and latitude. The structure also varies temporally in a largely regular manner due to:

• time of day (diurnal variation)
• time of year (seasonal variation)
• solar cycle (11-yr variation)

Some of the regular structure and behavior of the ionosphere was not expected by theory in the 1930's. These phenomena are referred to as Classical anomalies and include:

• the equatorial electrojet, an eastward daytime electric current over the geomagnetic dip equator around 60 to 70 miles (100 to 115 km) altitude
• the mid-latitude trough (a.k.a. the Canadian border effect), a band of decreased ionization around 60 degrees magnetic latitude occurring during night hours
• the equatorial anomaly (a.k.a. the fountain effect), the Maximum Usable Frequency (MUF) is lower during the day at the dip equator than at latitudes just to the north and south.
• the winter anomaly, the MUF is lower in summer than in winter at mid latitudes, i.e. the F₂ electron density is smaller in summer

The ionosphere is also susceptible to largely unpredictable variations due to "weather" from below the ionosphere, intrusions by meteors and solar disturbances. These will be presented under the topic of Ionospheric Storms and "Space Weather".

Formation of the Ionosphere

• D-region (about 60 to 90 km or 35 to 55 miles altitude)
  • production: daytime ionization of nitric oxide (NO) by solar Lyman alpha (121 nanometer wavelength) and of air (N₂, O₂) by solar X-rays (less than 20 nm). Molecular ions react with water vapor to produce water cluster ions
  • destruction: electrons recombine rapidly with water cluster ions & also attach to O₂ to make negative ions (but rapidly detach again in daylight)
  • balance: layer disappears at night (within several minutes) as production essentially ceases and electrons undergo rapid recombination and attachment
• E-region (about 90 to 140 km or 55 to 90 miles altitude)
  • production: daytime ionization of molecular oxygen (O₂) by extreme ultraviolet solar radiation (90-103 nm), ionization of meteoric vapors
  • destruction: electrons recombine with molecular ions (O₂⁺ and NO⁺)
  • balance: layer persists, although diminishes, during night due to slower recombination (than in D-region) and presence of atomic metallic ions such as Na⁺ (sodium) and Fe⁺ (iron). Electrons recombine with atomic ions (such as Na⁺ or O⁺) very inefficiently.
• F-region (above 140 km or 90 miles altitude)
  
  • production: daytime ionization of atomic O by extreme ultraviolet (EUV) solar radiation (20 - 90 nm). O⁺ converted to NO⁺ by molecular nitrogen (N₂)
    * F₁-region (about 140 to 180 km or 90 to 115 miles altitude)
    
  • destruction: controlled by recombination of NO⁺ ions with electrons.
  • balance: layer diminishes at night as electrons recombine with NO⁺
    * F₂-region (peak about 300 to 400 km or 180 to 250 miles altitude)
    
  • destruction: controlled by O⁺ reaction with molecular nitrogen (N₂), electrons recombine quickly with ion product (NO⁺) as it is created
    
  • balance: layer persists through night (becoming simply the F-region) since the small supply of N₂ leads to slow conversion of O⁺ to NO⁺ and hence only a small reduction in the number of electrons