Skip to main content

Currently Skimming:

5 Active Earth Remote Sensing for Space Physics
Pages 103-121

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.


From page 103...
... Ionospheric studies using radio transmitters date to the early part of the last century and are intimately tied to the early development of radio communications. Marconi's early attempts at long-distance radio transmissions suggested the presence of a reflecting layer in the upper atmosphere,1 which was published nearly simultaneously by Arthur Kennelly and Oliver Heaviside in 1902.
From page 104...
... In order to provide the reader with background to understand the application of active remote sensing to space physics, this chapter begins with a discussion of the scientific disciplines. The discussion begins with the ionosphere/thermosphere, followed by the magnetosphere, radio science, and, finally, space weather.
From page 105...
... Table 5.1 summarizes the spectrum usage based upon currently operating instruments. The chapter concludes with a brief discussion of radio spectrum issues experienced by space physics active remote sensing.
From page 106...
... 106 Active Remote Sensing Amid Increased Demand for Radio Spectrum TABLE 5.1  Characteristics of Space Physics Remote Sensing Transmitters Frequency Radar Location (Bandwidth) Power License Arecibo HF Arecibo, Puerto Rico 5.1 MHz and 8.175 MHz 600 kW NTIA Arecibo ISR Arecibo, Puerto Rico 430 MHz, 2.5 MW NTIA 500 kHz BW Digisonde Global 2 MHz–30 MHz 300 W GPS/GNSS Global/space-based 1100 MHz to 1600 MHz HAARP Gakona, Alaska 2.6 MHz-9.995 MHz 3.6 MW CW NTIA instantaneous BW 200 kHz Homer VHF Homer, Alaska 29.795 MHz 15 kW FCC experimental 100 kHz BW noninterference Jicamarca ISR Jicamarca, Peru 49.92 MHz 6 MW Peruvian 1 MHz BW Millstone Hill Westford, 440.0, 440.2, 440.4 MHz, 1.7 2.5 MW NTIA noninterference ISR Massachusetts MHz BW PFISR Poker Flat, 449.5 MHz 2 MW NTIA primary Alaska 1 MHz BW RISR Resolute Bay, 442.9 MHz 2 MW Industry Canada Nunavut, Canada 4 MHz BW Sondrestron Kangerlussuaq, 1287-1293 MHz 3.5 MW Greenland 1 MHz BW SuperDARN Global 8 MHz-20 MHz instantaneous 10 kW Within United States, BW 60 kHz FCC experimental noninterference NOTE: Acronyms are defined in Appendix D
From page 107...
... In addition to creating ionization, these particles are responsible for creating the aurora. While solar EUV radiation and energetic particle precipitation produce ioniza tion, the resulting distribution of plasma density is determined by various chemi cal reactions taking place in the ionosphere, along with plasma motions, which can transport the plasma across latitudes, longitudes, and altitudes.
From page 108...
... Remote sensing of the ionospheric plasma density and temperature profiles coupled with observations of the solar irradiance at altitudes above the ionosphere 4  C.T. Fallen, J.A.
From page 109...
... ISRs are single-frequency radars that observe backscatter from thermal fluctuations of the plasma at all altitudes with significant density up to the point where returns become too weak to detect. These observa tions, when coupled with the well-developed theory of incoherent-scattering from plasmas, provide altitude profiles of a number of parameters, including plasma density, plasma temperature, plasma velocity, ionic composition, and ion-neutral collision frequency.
From page 110...
... Magnetospheric Physics The magnetosphere is the volume of space formed by the interaction of Earth's magnetic field with the solar wind and the interplanetary magnetic field (IMF)
From page 111...
... It is a region of strong interaction between the solar wind/IMF and the magnetosphere and drives much of the dynamics of the entire magnetosphere. The primary goal of magnetospheric physics is to understand the complex chain of interactions that couple solar wind energy and momentum into the upper atmosphere.
From page 112...
... Ground-based active remote sensing uses observations of the ionosphere to study the magnetosphere, which is possible because the patterns of particle pre cipitation and plasma velocities observed in the ionosphere can be mapped to the magnetosphere. As discussed in the section covering ionospheric science, energetic particle precipitation from the magnetosphere produces plasma in the ionosphere that can be sensed using ISRs.
From page 113...
... Space Weather Space weather is a relatively new discipline that encompasses the other disci plines of space physics but focuses on societal impacts and on prediction of events with potentially adverse effects. Examples of such effects include increased level of ionospheric scintillation causing outages of GPS signals, induced currents in power
From page 114...
... As discussed above, the high con ductivity of the magnetospheric magnetic field lines makes these observations possible. The same convection and particle precipitation measurements used in basic magnetospheric and ionospheric research apply to space weather as well.
From page 115...
... Magnetospheric research using the VLF band has been continued by using HF band heating facilities to generate the signals by modulating electrical currents flowing in the ionosphere. Ionospheric research using VLF signals currently relies on radio navigation and time signals transmitted by various government agencies in the 15-20 kHz band.
From page 116...
... As described in the preceding section, "Scientific and Other Applications," ionosondes measure the altitude profile of electron density in the lower ionosphere by observing reflec tions of swept frequency signals. When a wave propagates into a region where the plasma frequency is comparable to but below the wave frequency, the propagating wave experiences a decrease in velocity and a change in direction.
From page 117...
... For example, under suitable ionospheric conditions, operating the transmitter in a mode that deposits energy into the D-region and lower E-region can alter the ionospheric conductivity and modulate the naturally occurring electrojet currents, which as a result radiate electromagnetic waves at the modulation frequency. ELF/VLF waves generated in this manner have been measured with significant amplitudes both on the ground and in space. Other effects include the generation of field-aligned irregularities in the plasma, artificial auroras, large-scale modification of the plasma density, and stimulated electro magnetic emissions.
From page 118...
... The total scattering cross section is approximately the product of the classical electron scattering cross section and the number of electrons in the scattering volume, which results in very weak scattering, requiring transmit powers of megawatts and antenna gains of 30-50 dB or more. A well-developed theory of ISR describes the shape of the frequency spectrum of the radar returns.6 The spectrum is a function of several ionospheric parameters, which can be estimated by fitting theoretical curves to the observed spectra.
From page 119...
... These effects relate to the total time a signal takes to propagate from the satellite to the ground, and how the amplitude and phase of the signals vary in time. Both effects are related to propaga tion through ionospheric plasma and can be used to infer ionospheric properties.
From page 120...
... Finding 5.2: The choice of frequencies and associated bands used in space physics active remote sensing are determined primarily by the physical properties of the medium being probed. Finding 5.3: The information provided by ionospheric active remote sensing can not be obtained by other means.
From page 121...
... Recommendation 5.3: NASA and the National Science Foundation should con duct a formal survey of the space physics research community to determine future spectrum needs.


This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More information on Chapter Skim is available.