Fate of a Radio Wave

SOLAR AND IONOSPHERIC EFFECTS ON RADIO-WAVE SYSTEMS (Communications, Tracking, Navigation, etc.)

A. Wave penetrates the ionospheric layer

B. Wave is absorbed within the ionosphere

C. Wave is scattered in random directions by plasma irregularities

D. Wave is reflected normally by the ionosphere

ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere 2

Ionospheric Plasma

A plasma is a gaseous mixture of electrons, ions, and neutral particles. The ionosphere is a weakly ionized plasma.

If, by some mechanism, electrons are displaced from ions in a plasma the resulting separation of charge sets up an electric field which attempts to restore equilibrium. Due to their momentum, the electrons will overshoot the equilibrium point, and accelerate back. This sets up an oscillation.

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--EThe frequency of this oscillation is called the

plasma frequency: p = 2f = (4Nee2/me)1/2

which depends upon the properties of the

particular plasma under study

Radio Waves in an Ionospheric Plasma

A radio wave consists of oscillating electric and magnetic fields. When a low-frequency radio wave (i.e., frequency less than the plasma frequency) impinges upon a plasma, the local charged particles have sufficient time to rearrange themselves so as to “cancel out” the oscillating electric field and thereby “screen” the rest of the plasma from the oscillating E-field.

This low frequency radio wave cannot penetrate the plasma, and is reflected (D).

For a high frequency wave (i.e., frequency greater than the plasma frequency), the particles do not have time adjust themselves to produce this screening effect, and the wave passes through (A).

The critical frequency of the ionosphere (foF2) represents the minimum radio frequency capable of passing completely through the ionosphere.

Mathematically, the behavior of a radio wave in a plasma can be viewed in terms of an index of refraction (see Tascione, Section 9.2).

Index of Refraction

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n = 1−8.1×10−5Ne

ν 2

The index of refraction for a radio wave in a plasma is given by

Here Ne is the electron density in cm-3 and is the wave frequency in MHz.

n2 < n1

n1

Snell’s Law

As a radio wave propagates up into an ionosphere with increasing Ne, n decreases and the ray bends as shown:

For a given Ne, a high-frequency wave will suffer less refraction than a lower frequency wave, since n is closer to 1 for a high-frequency wave.

The above expression for refractive index becomes imaginary for certain values of Ne / , implying reflection of the radio wave. For a vertically-propagating wave, this occurs at the critical value (where n = 0):

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c = 9 ×10−3 Nec

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foF2 = 9 ×10−3 NmF2

Using our previous notations, then, the lowest frequency radio wave capable of penetrating the ionosphere is

ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere 5

Ionospheric Disturbances

This variation can result from geomagnetic activity (and the influences of the neutral atmosphere), or it can be the direct result of X-rays and EUV produced by a solar flare.

Ionospheric disturbances occur when the Earth’s ionosphere (50 – 500 km) experiences a temporary fluctuation in degree of ionization.

A Sudden Ionospheric Disturbance (SID) is a disturbance that occurs almost simultaneously with a flare’s X-ray emission (generally constrained to dayside).

MUF depends on foF2 and the angle of incidence of the radio wave

LUF determined by amount of absorption in the D and E-Regions

Short Wave FadeoutWhen collisions between oscillating electrons and ions and neutral particles becomes sufficiently frequent (as in the D-region, 60 – 90 km), these collisions “absorb” energy from the radio wave leading to what is called radio wave absorption.

Short Wave Fade (SWF) is a particular type of SID that can severely hamper HF radio users (up to 20 – 30 MHz) by causing increased ionization and signal absorption which may last for up to 1-2 hours.

SHORT-WAVE FADOUT

X-rays fromflare

EnhancedD-region

ionization

F-region

D-region

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Solar Particle Events and Polar Cap Absorption

Part of the energy released in solar flares are in the form of accelerating particles (mostly proton and electrons) to high energies and released into space.

PCA events occur when high energy protons spiral along the Earth’s magnetic field lines towards the polar ionosphere’s D-region (50 – 90 km altitude).

These particles cause significant increased ionization levels, resulting in severe absorption of HF radio waves used for communication and some radar systems.

This phenomenon, sometimes referred to as “polar cap blackout ”, is often accompanied by widespread geomagnetic and ionospheric disturbances.

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In addition, LF and VLF

systems may experience

phase advances

when operating in or

through the polar cap

during a PCA event due to

changes in the Earth-

ionosphere waveguide.

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Solar Effects on Radio Wave Reception

Radio Noise Storms. Sometimes an active region on the Sun can produce increased noise levels, primarily at frequencies below 400 MHz. This noise may persist for days, occasionally interfering with communication systems using an affected frequency.

Solar Radio Bursts. Radio wavelength energy is constantly emitted from the Sun; however, the amount of radio energy may increase significantly during a solar flare.

Solar radio noise and bursts may interfere with radar, HF (3 – 30 MHz) and VHF (30 – 300 MHz) radio, or satellite communication systems.

IONOSPHERIC EFFECTS ON NAVIGATION SYSTEMS

In the past, terrestrial navigation systems such as Loran-C and Omega were primarily employed that depended on the ionosphere for their operation.

More recently, space-based systems such as GPS have become the pre-eminent methods of navigation. These systems use radiofrequencies that pass through the ionosphere but nonetheless are hindered by it.

Ground stations monitor the information from the satellites and upload changes as needed to ensure that the navigation information is accurate.

In addition to reflection and absorption, there are three ways in which the ionosphere can alter the propagation of a radio wave:

2. The phase velocity increases as

vφ =c

1− p

2

2

producing “phase advance”.

3. Plasma density irregularities scatter radio signals, which then add constructively and destructively at the receiver, causing “scintillations.

Paul Kintner: Beginner’s Guide to Space Weather and GPS, Cornell U. GPS Laboratory http://gps.ece.cornell.edu/

QuickTime™ and aTIFF (Uncompressed) decompressor

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1. The group velocity decreases as vg =c 1− p

2

2

where c is the speed of light, p is the plasma frequency and is the wave frequency (typically 1-10 MHz). This yields“group delay” or “code delay”.

GPS frequencies are 1.2, 1.5 GHz

TEC is the total electron content, i.e., the number of electrons in a tube of 1m2 unit cross-section extending from the receiver to the satellite:

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TEC = neS

R

∫ (l)dl

Where ne(l) is the variable electron density along the signal path, and the integration is along the signal path from the satellite to the receiver.

Changes in the ionosphere through which GPS signals propagate can cause the phase of the carrier signal to advance and the phase of the modulating signal to delay (group delay) by the amount:

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Δτ =40.3⋅TECcf 2

c = speed of light in vacuum

f = wave frequency

TEC is measured in TEC Units (TECU), defined as 1016 electrons m-2. If the integral is with respect to height, the above integral is called TECV. One TECU produces about 16 cm of range error.

Typical values of TECV range between 1 and 150.

Typical variations in TECV about monthly averages for a given site are about ~25%.

By using a dual-frequency system, it is possible to remove the effects of the ionosphere. In this case, the values of TEC and TECV so obtained can be used for research purposes to study the ionosphere and develop improved models of TEC.

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The carrier-to-noise ratios (signal strength) of two GPS satellite signals, one of which is scintillating while the other is not.

Paul Kintner: Beginner’s Guide to Space Weather and GPS, Cornell U. GPS

Laboratory http://gps.ece.cornell.edu/

QuickTime™ and aTIFF (Uncompressed) decompressor

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Example of an equatorial plasma bubble and amplitude scintillations.Left: Elevation-azimuth plot of satellite PRN 14.Right: Signal power and TEC.

A A

B

B

At low elevations (A & B), the oblique slant path through the ionosphere produces larger TEC values.

The satellite signal encounters a bubble between 2200 LT and 2400 LT where TEC values are structured and depressed.

The lower signal power at A and B is a result of the antenna gain pattern, which is generally lower at lower elevations.

Ionospheric irregularities (and hence scintillations) generally occur in association with steep plasma density gradients (i.e.,

within plasma “bubbles” over the equator where plasma instabilities

occur.