Global measurement of electron content using GPS receivers
Electron number increase (e.g. during storms) generates interference for GPS and communication systems (change of signal velocity and
Electron sensing (diagnostics) /characterization
Use of scattering (incoherent scatter RADAR), interferometry (heterodyne interferometry for determination of phase shift), spectroscopy/spectral broadening
of atomic lines (hydrogen Stark broadening, Doppler broadening and resonance broadening)
Total Electron Content (TEC) prediction using GPS signals
"The signals from the GPS satellites travel through the ionosphere on their way to receivers on the Earth’s surface. The free electrons populating this region of the
atmosphere affect the propagation of the signals, changing their velocity and direction of travel as shown at figure 1."
"The parameter of ionosphere that produces most of the effects on radio signals is total electron content (TEC)."
"Ionospheric delay correction is carried out through modelling the TEC along each satellite signal path." GPS satellites are often used.
"GPS satellites transmit electromagnetic waves for positioning on two frequencies which is L1 (1575.42 MHz) and L2 (1227.60 MHz) (...)". This enables us to extract
the ionosphere TEC (total electron content) along the line of sight, from satellite to receiver."
The "TEC map has gained much attention in the recent years because of the ionospheric effects to the GPS-based navigation application. A range delay caused by the
ionosphere during quiet and disturbed geomagnetic days can be approximated using the measurements of TEC map."
One of the used techniques mentioned can be performed at cm-level.
Ionosphere Total Electron Content
"Red is high electron counts, blue is low, gray where there is no data. From the pre-storm state, we see relatively low electron counts. As the storm intensity increases, so do the number of
electrons. The increase will generate more interference for communications systems, GPS, etc."
"Using the Global GPS Network and Other Satellite Data to Monitor Ionospheric Total Electron Content"
"A globally distributed network of dual-frequency global positioning system (GPS) receivers is the primary source of data used to measure atmospheric total electron content (TEC) on global
"The GPS network currently contains over 50 stations, covering a latitude range from 77S to 78N degrees, each tracking, between 4 and 8 GPS satellites simultaneously."
"In addition to this GPS resource, the TEC data set can be augmented using other dual-frequency tracking systems, such as the DORIS system developed by the French space agency CNES."
"The DORIS network of 50 beacons sequentially transmit a signal to a small number of compatible satellites, nottably the TOPEX/POSEIDON ocean altimetry mission."
Electron characterization - Spectral line broadening mechanisms: Doppler broadenings vs Stark, van der Waals, and resonance broadenings
Determining electron temperature and density using the Stark effect (Stark broadening of emission lines)
"A Stark broadening method to determine simultaneously the electron temperature and density in high-pressure microwave plasmas" https://iopscience.iop.org/article/10.1088/0022-3727/40/19/023:
"Usually, when the electron temperature is previously known, the Stark broadening of certain spectral lines spontaneously emitted by the
plasma is used to determine the electron density in a rapid and inexpensive way. However, comparing two or more broadening of lines can allow us to diagnose the electron density and temperature
simultaneously. To carry out this cross-point method, we must know the Stark broadening dependence on the electron temperature and density for different lines. In this work we have used the first
three Balmer series hydrogen lines, whose Stark broadenings were calculated by means of a recent micro-field model existing in the bibliography."
"2.2.1. Free-electron diagnostics based on active processes"
Laser scattering. "In Thomson scattering, the incident light on the plasma is scattered by free charged particles, which are predominantly electrons, and it is one
of the most well-established and most straightforward diagnostic methods for monitoring electron information."
Laser interferometry: "Interferometry for electron characterization is generally called heterodyne interferometry because the heterodyne technique is used to
determine the phase shift of the probing laser."
"2.2.2. Free-electron diagnostics based on passive optical emission"
Line broadening. "The spectral broadening of the specific atomic line emissions contains a convenient quantity for n(e) measurements." Variety of broadening
mechanisms. Doppler broadenings - Gaussian profile; Stark, van der Waals, and resonance broadenings - Lorentzian profile; "thus, the spectral line broadening approach should be carefully considered
to make accurate measurements."
"Stark broadening results from Coulomb interactions between the radiator and free electrons in the plasma (the role of ions in the broadening is negligible because
of their relatively low speeds). Fortunately, broadening caused by the Stark effect depends explicitly on the electron number density, whereas other broadenings do not."
"In general, hydrogen or hydrogen-like ions are the most useful radiators in plasma for diagnostics. The Hβ line especially is widely selected because it is a common
radiative species in the visible range. From the Hβ line shape, the Stark broadening width is given as a function of n(e), and it shows weak temperature dependence, which was calculated using
classical electrodynamics in 1960s ."
"Multiple diagnostics in a high-pressure hydrogen microwave plasma torch"
Incoherent scatter RADAR (ISR) determines electron density, ion composition, electron and ion temperature and plasma velocity
Scattering from electrons: complete theory describes the spectrum of the scattered signal and inexpensive signal processing is possible
W. Gordon "proposed that individual ionospheric electrons might react to an incoming radar signal by oscillating at the same frequency, thus producing an
(incoherent) electromagnetic signal"
The idea was confirmed experimentally. Also, it was determined "that each ion in the upper ionosphere is surrounded by a cloud of electrons and scattering was
essentially from the clouds".
"Soon it was stated that the incoherent scatter method allows measurement of ionospheric electron density, ion temperature and electron temperatures, ion composition
and plasma velocity. A new powerful tool for studying the Earth’s ionosphere was invented."
"ISR has remained a useful technique for ionospheric studies, because a complete, accurate, and elegant theory describes the spectrum of the scattered signal, and
because inexpensive and easily implemented digital signal processing makes the use of new radar techniques practical and allows new and better data analysis methods."
Passive Characterization of the Ionosphere
IARPA Challenge "Passive Ionospheric Non-Characterized Sounding" (PINS)
"Solvers are challenged to characterize the ionosphere with selected digitized radio-frequency (RF) spectrum recordings from sounder receiver data, but not any transmitter data."