Abstract. In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22-23, 1999, with the hourly Dst index being -167 nT at -2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of -231 nT at -0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS) -

Abstract. Satellite scintillation, all-sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9øN, 11.8øE; 75.7øN corrected geomagnetic latitude, over the period January 4-15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillation of 1.6-GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual-frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250-MHz scintillation observations explored the daytime auroral ionosphere 2 ø poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600-1500 magnetic local time. The period of observations included the magnetic storm of January 10-11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3 x 1016 m -2 and large-scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2 x 1016 m -2 min -1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large-scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.

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Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD‐enhanced all‐sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field‐aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earth's magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.

Abstract. Sounding rocket experiments were conducted during the NASA EQUIS II campaign on Kwajalein Atoll designed to elucidate the electrodynamics and layer structure of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each comprised of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets deployed trails of trimethyl aluminum (TMA) which yielded wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. Electron density profiles were also measured with rocket beacons and an ionosonde. Strong plasma shear flow was evident in both experiments. Bottom-type scattering layers were observed mainly in the valley region, below the shear nodes, in westward-drifting plasma strata. The layers were likely produced by wind-driven interchange instabilities as proposed by Kudeki and Bhattacharyya (1999). In both experiments, the layers were patchy and distributed periodically in space. Their horizontal structure was similar to that of the largescale plasma depletions that formed later at higher altitude during ESF conditions. We argue that the bottom-type layersCorrespondence to: D. L. Hysell (dlh37@cornell.edu) were modulated by the same large-scale waves that seeded the ESF. A scenario where the large-scale waves were themselves produced by collisional shear instabilities is described.