Monitoring the Ionosphere with GPS Space Weather “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 atmos- phere and beyond. One of these tools is GPS. 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 elec- trons 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 radia- tion, 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 significant- ly 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. S pace weather and associated distur- bances in Earth’s magnetic field can produce large gradients in the total elec- tron content (TEC) in the mid-latitudes. For single-frequency GPS users, these large gradients in the TEC are of con- cern because they can make the ionos- phere difficult to model and remove, thereby affecting GPS-derived position accuracy. The presence of these gradi- ents can also affect carrier-phase differ- ential GPS (DGPS) and real-time kine- matic (RTK) applications because the ionospheric term in the observation equa- tions may not cancel, thus making unknown ambiguities difficult to resolve. In addition, large gradients in the TEC are frequently associated with ionos- pheric scintillation events that can cause amplitude and phase fluctuations of the received signal. In severe conditions, these fluctuations can cause the receiv- er 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 com- bining 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 geo- magnetic 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 pre- sents a brief background of space weath- er and its effects on GPS. This section also reviews how GPS observables are used to measure TEC. The second sec- tion 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 phe- nomenon known as storm enhanced den- sity (SED), which is driven by process- es 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 obser- vations and connect the observed SED phenomenon with other space-weather processes. The final section discusses some specific effects on GPS observa- tions that arose from the March 31 storm. 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 blow- ing outward from the solar atmosphere. Earth’s magnetosphere, which is the region of space influenced by Earth’s magnet- ic field, shields Earth from most of this erupted material. However, some of the energy from solar disturbances does enter the magnetosphere, disrupting its con- figuration, 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 elec- trons (plasma) located approximately 100–2,000 kilometers above Earth’s sur- face 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 inner- most regions of Earth’s magnetosphere. The plasmasphere is the high-altitude extension of the ionosphere. The inter- action of the solar wind, magnetosphere, plasmasphere, and ionosphere during storm-time conditions is complex and coupled, and our understanding of the Monitoring the Ionosphere with GPS Space Weather Anthea Coster, John Foster, and Philip Erickson 42 GPS World May 2003 www.gpsworld.com Innovation