Ionospheric scintillations are one of the earliest known effects of space weather. Caused by ionization density irregularities, scintillating signals change phase unexpectedly and vary rapidly in amplitude. GPS signals are vulnerable to ionospheric irregularities and scintillate with amplitude variations exceeding 20 dB. GPS is a weak signal system and scintillations can interrupt or degrade GPS receiver operation. For individual signals, interruption is caused by fading of the in‐phase and quadrature signals, making the determination of phase by a tracking loop impossible. Degradation occurs when phase scintillations introduce ranging errors or when loss of tracking and failure to acquire signals increases the dilution of precision. GPS scintillations occur most often near the magnetic equator during solar maximum, but they can occur anywhere on Earth during any phase of the solar cycle. In this article we review the subject of GPS and ionospheric scintillations for scientists interested in space weather and engineers interested in the impact of scintillations on GPS receiver design and use.

[1] GPS L1 C/A signal scintillation data were collected at the equatorial anomaly over a period of three months using five receivers spaced on magnetic east-west and north-south axes to examine the speed, orientation, shape, width, and duration of GPS scintillation fade patterns. The nighttime speeds were primarily eastward in the range of 100-200 m/s with a significant spread to both larger values and negative (westward) values as expected, given the known behavior of ionospheric drifts and GPS signal path movement. The characteristic velocity was found to be small so that the true velocity was equal to the apparent velocity to a very good approximation. The orientation of the scintillation fade patterns was organized by a simple projection model of the magnetic field along the GPS signal path onto the horizontal plane when the signal paths were aligned no closer than 60°from the magnetic field. The shape of the scintillation fade pattern was greatly elongated in the magnetic north-south direction, and no change could be detected over a distance of 1 km. The east-west widths of the scintillation fade patterns were variable, but after normalizing to the elevation angle, accounting for the fade orientation, and eliminating signal paths within 60°of the magnetic field, an organized scale length of about 450 m was determined. The duration of the scintillation fade patterns was examined using the optimal cross-correlation amplitude as a measure of change. For a 5 s duration, 49% of the optimal cross-correlation amplitudes exceed a value of 0.8.

Since the 1920s it has been known that the equatorial ionosphere can become highly disturbed in the post‐sunset period. We have placed a new array of instruments on top of the Haleakala Volcano in Hawaii to study these disturbances from midlatitudes. Two airglow imagers provide two‐dimensional snapshots of the development of these disturbances during the night, while two GPS receivers can quantify their severity. Here we report on the spectacular February 16–17, 2002 disturbance, which reached an altitude of 1500 km over the magnetic equator and mapped magnetically to latitudes well north of Hawaii. The signals from every Global Positioning System satellite in the field of view from Maui were severely disturbed whenever the corresponding look direction passed through one of the turbulent features. We also present an example from March 19, 2002 in which the spread‐F activity is not as severe to demonstrate that the instrumentation still provides valuable information.

Abstract. Over 300 nights of airglow and GPS scintillation data collected between January 2002 and August 2003 (a period near solar maximum) from the Haleakala Volcano, Hawaii are analyzed to obtain the seasonal trends for the occurrence of equatorial plasma bubbles in the Pacific sector (203 • E). A maximum probability for bubble development is seen in the data in April (45%) and September (83%). A broad maximum of occurrence is seen in the data from June to October (62%). Many of the bubbles observed from June through August occur later in the evening, and, as seen in the optical data, tend to be "fossilized." This suggests that the active growth region during these months is to the west of the observing location. These seasonal trends are consistent with previous data sets obtained both from other groundbased and satellite studies of the occurrence of equatorial bubbles in the Pacific sector. However, our data suggests a much greater probability of bubble occurrence than is seen in other data sets, with bubbles observed on over 40% of the nights studied.

[1] First observations of intense GPS L1 amplitude scintillation activity in the midlatitude ionosphere at latitudes corresponding to the northeastern United States have been made. Moderate to severe, these scintillations result from space weather effects due to a disturbed ionosphere. Moderate to severe scintillations can degrade or even disrupt communication and navigation systems relying upon transionospheric radio wave propagation. A modified GPS receiver was used to record GPS satellite signal strength at Cornell University (53.2°m agnetic latitude) during a magnetospheric disturbance on September 25 -26, 2001 from 0000 -0400 UTC. This disturbance (K p = 6, minimum D st = À110 nT) prompted the ionospheric trough to move equatorward over the northeastern U.S. and produced large plasma densities and gradients attributed to storm-time effects. This disturbance resulted in intense L-band amplitude scintillations (!20 dB, S 4 % 0.8) which are highly uncharacteristic at this magnetic latitude. Concurrent measurements of TEC showed steep density gradients ($30 TEC/deg) and evidence of irregularity structuring.