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.

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The wave experiment of the Viking satellite frequently detected dynamic small-scale (=100 m), large-amplitude, rarefactive (~An/n ( &50%) solitary waves of negative potential (~|S~& 2 V) moving upwards along the magnetic field lines (v =5 to & 50 km/s). The structures, which resemble ion holes, often have an upward-directed net potential drop (& l V) and are then interpreted as weak double layers. PACS numbers: 52.35.Mw, 52.35.gz, 52.35.Sb, 52.70.Ds There is currently an interest in the appearance in space plasmas of small (Debye) scale solitary structures, such as ion acoustic solitions, ion holes, phase-space vortices, and weak double layers. While several laboratory studies, ' computer simulations, and theoretical investigations 5 7 have been performed, partly with space applications in mind, in situ space experiments revealing such structures are rare. The low-frequency wave experiment of the Swedish Viking satellite (launched 22 February 1986, apogee 13527 km, perigee 817 km, inclination 98. 8') has verified the occurrence in the magnetospheric plasma of structures described as solitary waves (SW, with no net potential drop) and weak double layers (WDL, with a small net potential drop pi &kT,/e) first reported from the S3-3 satellite.Our experiment has provided new information on their characteristics, i.e. , spatial and temporal scales, direction of motion, typical velocities, density depletions, and the relation between density and potential variations.The structures are of interest per se, but also for particle energizing processes which may be of universal occurrence. Specifically, it has been proposed that the simultaneous presence of a large number of WDL's along the geomagnetic field lines might, through their cumulative effect, contribute to the auroral particle acceleration, and stochastic variations might provide a mechanism to explain observed energy spectra.The new, two-point measurements of relative plasma density variations (hn/n) provided by two Langmuir probes on Viking proved particularly useful. Two spherical probes of diameter 10 cm were located 80 m apart at the end of wire booms in the spin plane (spin rate 3 rpm) of the spacecraft (inset in Fig. 1). The probes were biased positively ( & 16 V), so that photoelectron currents were negligible, in order to collect electron currents proportional to the plasma density (n). The influence of electron temperature variations is discussed below, and other possible interference sources, such as

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A comparative study of upstreaming energetic ions in the kilovolt energy range and electrostatic hydrogen cyclotron (EHC) waves has been made using the ion mass spectrometer and plasma wave receiver data sets for the first 1200 orbits of the S3‐3 spacecraft. The upstreaming energetic ions and EHC waves are found to coincide in over 90% of the events studied. In addition, both EHC waves and upstreaming ions with energies greater than 500 eV exhibit a lower border in their altitude distribution near 5000 km. The nearly exact correlation suggests either that the upstreaming ions are producing the EHC waves or that the EHC waves are heating the ions. One example of EHC waves and upstreaming energetic ions is analyzed to test the two hypotheses. Evidence that EHC waves are heating ions is presented in the form of conic ion distributions which some theories predict are the consequence of this process. Perpendicular ion heating to at least 6 keV is found to coincide with EHC waves. Evidence that the upstreaming ions are the source of free energy for the EHC waves is presented in the form of an ion distribution function with ∂f/∂ν > 0. However, the stability of that ion distribution function is considered and found to be stable unless other conditions such as filamentation or electron drift are invoked. There also exists the possibility that the source of free energy for EHC waves is drifting thermal electrons. For the one example studied the drifting electron process is consistent with data from the S3‐3 magnetometer. However, it is inconsistent with the S3‐3 electron spectrometer which indicates that the current is carried by keV electrons, not thermal electrons. Consequently, the source of free energy for EHC waves is not yet unambiguously determined.