Space Weather
May 1, 2003 By: Anthea Coster, John Foster, Philip Erickson GPS WorldMonitoring the Ionosphere with GPS
"Stormy today, clearing up tomorrow." That may sound like a typical forecast given by your local TV meteorologist, but it could just as well be a forecast of space weather. Here on Earth, high winds, heavy rains, deep snow, and other forms of severe weather can disrupt our daily lives. Conditions on the Sun and in the solar wind, magnetosphere, and the ionosphere can also affect our lives through the effects they have on satellites, communications, navigation, and power systems. Scientists are now studying space weather with a wide range of tools to try to learn more about the physical and chemical processes taking place in the upper atmosphere and beyond. One of these tools is GPS.
 Figure 1 A map of total electron content determined by a network of GPS receivers on March 31, 2001, at 19:30 UTC shows the plume of storm-enhanced density stretching across the Great Lakes and into Canada. |
The signals from the GPS satellites travel through the ionosphere on their way to receivers on or near Earth's surface. The free electrons populating this region of the atmosphere affect the propagation of the signals, changing their speed and direction of travel. By processing the data from a dual-frequency GPS receiver, it's actually possible to estimate just how many electrons were encountered by the signal along its travel path - the total electron content (TEC). TEC is the number of electrons in a column with a cross-sectional area of one square meter centered on the signal path. If a regional network of ground-based GPS receivers is used, then a map of TEC above the region can be constructed. The TEC normally varies smoothly from day to night as Earth's dayside atmosphere is ionized by the Sun's extreme ultraviolet radiation, while the nightside ionosphere electron content is reduced by chemical recombination. But the ionosphere can experience stormy weather just as the lower atmosphere does. Smooth variations in TEC are replaced by rapid fluctuations, and some regions experience significantly higher or lower TEC values than normal. In this month's column, we look at how GPS is being used to study such storms and how it is furthering our understanding of the Earth-Sun environment. - R.B.L.
Space weather and associated disturbances in Earth's magnetic field can produce large gradients in the total electron content (TEC) in the mid-latitudes. For single-frequency GPS users, these large gradients in the TEC are of concern because they can make the ionosphere difficult to model and remove, thereby affecting GPS-derived position accuracy. The presence of these gradients can also affect carrier-phase differential GPS (DGPS) and real-time kinematic (RTK) applications because the ionospheric term in the observation equations may not cancel, thus making unknown ambiguities difficult to resolve. In addition, large gradients in the TEC are frequently associated with ionospheric scintillation events that can cause amplitude and phase fluctuations of the received signal. In severe conditions, these fluctuations can cause the receiver to lose lock.
Until now, the physical mechanisms that cause these large TEC gradients to form in the mid-latitudes have been poorly understood. However, by combining data from the global network of continuously operating dual-frequency GPS receivers, the development of these TEC perturbations can be monitored, and our understanding of the physical processes involved has been greatly enhanced.
This article discusses the effects of geomagnetic storms on GPS observations and measurements and focuses on one in particular, the March 31, 2001, storm. The first of the article's four sections presents a brief background of space weather and its effects on GPS. This section also reviews how GPS observables are used to measure TEC. The second section presents a map of the TEC over North and South America based on GPS data collected during this geomagnetic storm. The TEC map clearly illustrates a phenomenon known as storm enhanced density (SED), which is driven by processes in Earth's magnetosphere and is associated with large gradients in the ionospheric and plasmaspheric TEC. The next section presents data from additional sensors that support the GPS TEC observations and connect the observed SED phenomenon with other space-weather processes. The final section discusses some specific effects on GPS observations that arose from the March 31 storm.
 Figure 2 Intense sunward ion flux was measured by the Millstone Hill incoherent scatter radar at 19:30 UTC on March 31, 2001. The color coding indicates a range of flux values from 1013 to 1015 electrons/meter2/second. The red contours indicate the area in the GPS-measured total electron content (TEC) map where TEC values exceed 50 TEC units. |
Background
Space weather - the variability of Earth's space environment - can adversely affect the integrity and performance of man-made systems such as GPS. Solar drivers of space weather include solar flares, coronal holes, and coronal mass ejections (CMEs). These disturbances are the key ingredients of strong geomagnetic storms on Earth. Outbursts of charged particles and electromagnetic energy from CMEs and solar flares propagate through the solar wind, the tenuous material blowing outward from the solar atmosphere. Earth's magnetosphere, which is the region of space influenced by Earth's magnetic field, shields Earth from most of this erupted material. However, some of the energy from solar disturbances does enter the magnetosphere, disrupting its configuration, particle populations, and the important coupling between the outer magnetosphere and the inner layers of Earth's atmosphere.Regions of particular importance are Earth's ionosphere and plasmasphere. The ionosphere is the region of free electrons (plasma) located approximately 100-2,000 kilometers above Earth's surface and created by the action of extreme ultraviolet (EUV) sunlight on the gases of the upper neutral atmosphere. The plasmasphere is a doughnut-shaped region of low-temperature plasma that co-rotates with Earth and is located in the innermost regions of Earth's magnetosphere. The plasmasphere is the high-altitude extension of the ionosphere. The interaction of the solar wind, magnetosphere, plasmasphere, and ionosphere during storm-time conditions is complex and coupled, and our understanding of the many processes involved is continually evolving.
For GPS users, the impact of space weather can usually be attributed to disturbances in the ionosphere as well as the plasmasphere, which in turn can cause degradation in range measurements and in severe circumstances, loss of lock by the receiver of the GPS signal. As GPS signals propagate through the ionosphere, the propagation speed and direction of the GPS signal are changed in proportion to the varying electron density along the line of sight between the receiver and the satellite. The accumulated effect, by the time the signal arrives at the receiver, is proportional to the integrated TEC, the number of electrons in a column stretching from the receiver to the satellite with a cross-sectional area of one square meter. This in turn affects the GPS range observable: a delay is added to the code measurements and an advance to the phase measurements. To achieve very precise positions from GPS, this ionospheric delay/advance must be taken into account.
 Figure 3a, b, c Data collected at approximately 21:20 UTC on March 31, 2001, by the IMAGE satellite and by the ground-based GPS network show the correlation of observed storm-time features: (a) IMAGE satellite extreme ultraviolet image of plasmasphere and plasmaspheric plume; (b) equatorial projection of storm-enhanced density (SED) feature and plasmapheric plume; (c) GPS total electron content map showing SED feature. |
One can take advantage of the very same ionospheric delay/advance that causes problems for the majority of GPS users. By measuring this delay, properties of the ionosphere can be inferred, and these properties can be used to monitor space-weather events. The ionospheric range delay/advance is (to first order) a function of both the TEC along the line of sight from the receiver to the satellite and the inverse carrier frequency squared. By computing the difference in the range measurements made at two frequencies, the TEC is measured.
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