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Ionospheric Data

Introduction to the Ionosphere

The ionosphere is that part of the upper atmosphere where free electrons occur in sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves. This ionization depends primarily on the Sun and its activity. Ionospheric structures and peak densities in the ionosphere vary greatly with time (sunspot cycle, seasonally, and diurnally), with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions), and with certain solar-related ionospheric disturbances.

The major part of the ionization is produced by solar X-ray andultraviolet radiation and by corpuscular radiation from the Sun. The most noticeable effect is seen as the Earth rotates with respect to the Sun; ionization increases in the sunlit atmosphere and decreases on the shadowed side. Although the Sun is the largest contributor toward the ionization, cosmic rays make a small contribution. Any atmospheric disturbance effects the distribution of the ionization.

The ionosphere is a dynamic system controlled by many parameters including acoustic motions of the atmosphere, electromagnetic emissions, and variations in the geomagnetic field. Because of its extreme sensitivity to atmospheric changes, the ionosphere is a very sensitive monitor of atmospheric events.

In some circles it is thought that there is persuasive evidence of an ionospheric precursor to large earthquakes that can be used a predictor. Besides the obvious acoustic waves generated before and during an earthquake, a part of the preparation process of large earthquakes is the generation of electromagnetic emissions (EMEs). These EMEs have been detected in the ionosphere up to six days prior to a large earthquake, such as with the May 1960, Chilean 8.3 earthquake.

The most accurate way of measuring the ionosphere is with a ground-based ionosonde, which records data as ionograms

Vertical Incidence Soundings (Ionograms)

Ionograms are recorded tracings of reflected high frequency radio pulses generated by an ionosonde. Unique relationships exist between the sounding frequency and the ionization densities which can reflect it. As the sounder sweeps from lower to higher frequencies, the signal rises above the noise of commercial radio sources and records the return signal reflected from the different layers of the ionosphere. These echoes form characteristic patterns of "traces" that comprise the ionogram. Radio pulses travel more slowly within the ionosphere than in free space, therefore, the apparent or "virtual" height is recorded instead of a true height. For frequencies approaching the level of maximum plasma frequency in a layer, the virtual height tends to infinity, because the pulse must travel a finite distance at effectively zero speed. The frequencies at which this occurs are called the critical frequencies. Characteristic values of virtual heights (designated as h'E, h'F, and h'F2, etc.) and critical frequencies (designated as foE, foF1, and foF2, etc.) of each layer are scaled, manually or by computer, from these ionograms. Typically, an ionosonde station obtains one ionogram recording every 15 minutes. When the scaling is done manually only the hourly recordings are routinely reduced to numerical data. Modern ionosondes with computer-driven automatic scaling procedures routinely scale all the ionograms recorded. The resulting numerical values, along with the original ionograms and station reports, are archived at five World Data Centers (WDCs) for Ionosphere.

The ionosphere is divided into four broad regions called D,E, F, and topside. These regions may be further divided into several regularly occurring layers, such as F1 or F2.D Region: The region between about 75 and 95km above the Earth in which the relatively weak) ionization is mainly responsible for absorption of high-frequency radio waves. E Region: The region between about 95 and 150km above the Earth that marks the height of the regular daytime E layer. Other subdivisions isolating separate layers of irregular occurrence within this region are also labeled with an E prefix, such as the thick layer, E2, and a highly variable thin layer, Sporadic E. Ions in this region are mainly O2+. F Region: The region above about 150km in which the important reflec-ting layer, F2, is found. Other layers within this region are also described using the prefix F, such as a temperate-latitude regular stratification, F1, and a low-latitude, semi-regular stratification, F1.5. Ions in the lower part of the F layer are mainly NO+ and are predominantly O+ in the upper part. The F layer is the region of primary interest for radio communications.

Topside: This part of the ionosphere starts at the height of the maximum density of the F2 layer of the ionosphere and extends upward with decreasing density to a transition height where O+ ions become less numerous than H+ and He+ ions. The transition height varies but seldom drops below 500km at night or 800km in the daytime, although it may lie above 1000km. Above the transition height, the weak ionization has little influence on transionospheric radio signals.

The International Union of Radio Science (URSI) Ionospheric Informatic Working Group (IIWG) has produced a data exchange format to address problems such as scaled time resolutions greater than the traditional hour-based usage found in previously accepted formats. For a full description of the data format and the philosophy behind its creation see "Ionogram Characteristics at Uneven Data Rates," by Gamache and Reinisch [1989].

The IDD software can access data only in the IIWG format. This format allows data from all sampling rates to be included in the database. Even though most of the data on these CDs contain only hourly values, data taken on autoscaling equipment produce fully scaled data for every ionogram. Most future data will come from equipment which automatically scales the ionograms. The IIWG format allows the archiving of all data regardless of the resolution.

Five positions are reserved for scaled values: 3 numeric for the value, 1 alpha for qualifier, and 1 alpha for descriptor. Valid combinations are: a full weight value (123__), an unqualified, but described value (123_A), a qualified and described value (123UA), or a replaced value(____A).

QUALIFICATION Letters

A less than (used only fbEs in case of total blanketing).

D greater than.

E less than.

I interpolated.

J deduced from x component.

M mode uncertain.

T smoothed from sequence.

U uncertain.

Z deduced from z component.

DESCRIPTIVE Letters

A blanketing.

B absorption.

C non-ionospheric (equipment).

D above upper frequency range of equipment.

E below lower frequency range of equipment.

F frequency spread.

G ionization density too small.

H stratification.

K particle E layer.

L no sufficiently defined cusp between F layers.

M mode uncertain.

N superimposed layers.

O measurement refers to o component.

P man-made perturbation of parameters.

Q range spread.

R attenuation near critical frequency.

S interference.

T interpolated.

V forked trace.

W above height range.

X measurement refers to x component.

Y lacuna (tilt).

Z measurement refers to z component.

When qualifier and descriptive letters are not used with the data, it must be clear that their absence does not indicate full weight values. The IIWG has developed a code to designate this condition properly, as indicated in the table below:

Note QD = Qualification and Descriptive Letters

For more details on qualification and description letters, see W.R. Piggott and K. Rawer, editors, URSI Handbook of Ionogram Interpretation and Reduction, 2nd edition (Report UAG-23, World Data Center A for Solar-Terrestrial Physics, Boulder, CO, 1972).

Ionospheric Vertical Incidence Parameters

F Layer Parameters

Parameter	URSI code #	Dimension	Description
foF2	00	.1 MHz	F2 layer o-mode (ordinary) critical frequency
fxF2	01	.1 MHz	F2 layer x-mode (extraordinary) critical frequency
fzF2	02	.1 MHz	F2 layer z-mode critical frequency
M3000F2	03	.01 MHz	F2 layer M factor (the ratio of the maximum usable frequency divided by the critical frequency)
h'F2	04	km	F2 layer o-mode minimum virtual height
hpF2	05	km	An estimate of the true height of the F2 layer (measurement of the ordinary mode virtual height at a frequency of 83.4% of foF2)
h'Ox	06	km	F layer minimum virtual height of the x-mode trace at a frequency equal to the foF2
MUF3000F2	07	.1 MHz	F2 layer maximum usable frequency for a 3000km path
hc	08	km	The height of the maximum obtained by fitting a theoretical h'F curve for the parabola of best fit to the observed ordinary mode trace near foF2 and correcting for under-lying ionization
qc	09	km	F layer scale height
foF1	10	.01 MHz	F1 layer o-mode critical frequency
fxF1	11	.01 MHz	F1 layer x-mode critical frequency
M3000F1	13	.01 MHz	F1 layer M factor (see URSI code 03)
h'F1	14	km	F1 layer o-mode minimum virtual height
h'F	16	km	F layer o-mode minimum virtual height
MUF3000F1	17	.1 MHz	F1 layer maximum usable frequency (see URSI code 07)

E Layer Parameters

Es Layer Parameters

Other Parameters

Spread E/Oblique Parameters

N(h) Parameters

REFERENCE: Definition of Characteristic extracted from UAG23 (URSI Handbook of Ionogram Interpretation and Reduction, November, 1972)