Electric field and plasma density instrumentation on board a sounding rocket launched from Punta Lobos, Peru, detected intense electrostatic waves indicative of plasma instabilities in the daytime equatorial electrojet. Simultaneous measurements taken by the Jicamarca radar showed strong 3‐m type 1 electrojet echoes as well as evidence of kilometer scale horizontally propagating waves. The in situ electric field wave spectra displayed three markedly different height regions within the unstable layer: (1) a two‐stream region on the topside between 103 and 111 km where the electron current was considered to be strongest, (2) a gradient drift region between 90 and 106.5 km where the upward directed, zero‐order electron density gradient was unstable, and (3) an “interaction” region between 103 and 106.5 km where both of these instabilities were linearly unstable. The unstable altitudes and differentiation showed good agreement with the simultaneous 3‐m Jicamarca backscatter radar observations. In the region where the density gradient was unstable, large‐amplitude waves with large scale sizes (wavelengths of roughly 1–2 km) were observed. These kilometer scale waves dominated the observed in situ spectrum despite the fact that the peak in the linear gradient drift growth rate occurred at wavelengths of only a few hundred meters. Comparisons of the measured δE and δn/n components of the large‐scale waves verify the basic process inherent to the gradient drift instability: density enhancements were observed coincident with westward electric fields, and density depletions were associated with eastward fields. The amplitudes (10–15 mV/m) of these horizontal waves were strong enough to drive vertical two‐stream secondary waves. In the region where these waves existed and the electrojet current was strongest, evidence of wave steepening was seen, and the resulting waveforms of the large structures displayed a “flat‐topped” nature. In the lower region of the electrojet the irregularity power occurred over a broad range of wavelengths, estimated to be in the range of tens of meters to kilometers, and fell off rapidly for the shorter wavelengths. Throughout the gradient drift region, shorter‐scale waves often occurred in bursts which appeared controlled by the larger electric field structures.

During the Structure and Atmospheric Turbulence Environment (STATE) campaign in June 1983, three small rockets (Super Arcas) containing dc probes to measure electron density irregularities with high spatial resolution were launched at Poker Flat, Alaska. The rockets were launched at three different times when the nearby MST (mesospheric, stratospheric, and tropospheric) radar showed intense regions of backscatter in the mesosphere. The first and third flights (STATE 1 and STATE 3) were perfectly successful, providing high‐quality electron density measurements; STATE 2 did not produce any useful results. When the electron density measurements are compared with the radar echo power as a function of altitude for STATE 1 and 3, large fluctuations and strong gradients in the electron density profiles are observed in the region of most intense backscatter. The electron density profiles show different characteristics in the peak scattering region with respect to altitude, electron density gradients, and irregularities. Power spectra of the electron density spatial fluctuations were derived from the measured electron densities for the region from approximately 65 to 90 km for several height intervals, with the smallest being approximately 100 m. In the region of most intense backscatter, the spectral power over the entire frequency range increases by almost 3 orders of magnitude for both rocket data sets. For STATE 1 a linear fit to the log‐log power spectral plots between 1.0–80 Hz (i.e., spatial scales from about 500 to 5 m) can be approximated by a power law with an index of about –(5/3), as would be expected in an inertial subrange of homogeneous, isotropic turbulence. The spectra, moreover, show a continuous steepening of the spectral slope in the viscous subrange at frequencies above 100 Hz (approximately 4.5–0.5 m), giving a much higher spectral index. The STATE 3 spectra, on the other hand, show a steeper spectral index near −2.0 in the inertial subrange but steepening at the higher frequencies, as do the STATE 1 data. A detailed intercomparison of the probe data is presented, followed by an absolute comparison between the radar and rocket measurements. Reasonable agreement is seen between the observed echo power profile and the profile calculated using the 3‐m electron density fluctuations obtained from the rocket data.

A high-performance rocket carrying a four-frequency, phase-coherent beacon and full complement of in situ diagnostic instrumentation was launched into active equatorial spread F on July 17, 1979. In this paper we report the results of spectrally analyzing the beacon phase-scintillation and Langmuir probe data. By using simultaneous backscatter data from the Altair radar we were able to establish that the scintillation develops in high-density regions adjacent to the prominent plume structures and associated depletions. In these high-density regions the in situ spectra show a pronounced change in the power law slope near a spatial wavelength of 500 m. Larger scale structures admit a systematically varying power law index that is generally less than 2, in good agreement with a large body of Wideband satellite data and recently analyzed Atmospheric Explorer E data. Smaller-scale structures admit a spectral index much larger than 2. A single, overall power law near k -2 was found only in low-density regions that did not contribute significantly to the scintillation. The results presented here and in a companion paper suggest that refinements in the current theories of equatorial spread F near and above the F region peak are needed.

During the PLUMEX I rocket flight from Kwajalein Island, plasma density and electric field fluctuations were measured in situ, simultaneous with ground‐based radar backscatter measurements at 0.96‐m and 0.36‐m wavelengths. The rocket penetrated an extremely turbulent topside region which had associated intense backscatter. As measured by the radar the backscatter power was decaying with time during and after the flight. The intermediate wavelength (0.1–10 km) in situ electron density measurements are described in a companion paper, while here we report the transitional and short wavelength results (λ < 100 m). These data include the first in situ equatorial spread F measurements of the electric field component of electrostatic fluctuations with wavelengths less than 1 m. At all altitudes above about 280 km, a repeatable form for the wave‐number spectrum was found for the electron density and electric field fluctuations at wavelengths less than about 100 m. The density spectrum varies approximately as k−5 and the electric field spectrum as k−3. The steepness of the density spectrum corresponds to an absence of steep edges in the density waveform on the scale of 100 m and less. These two spectral forms are shown to be consistent with an explanation involving low‐frequency waves with finite wave numbers parallel to the magnetic field (k∥). Both theory and laboratory experiments show a power law density fluctuation spectrum for gradient‐driven drift waves with negative index in the range 4.5–6.0. Since such waves do have finite k∥, and since sharp gradients exist in the spread F environment, we conclude that at sufficiently high altitudes, drift waves act on the steep gradients caused by a primary longer‐wavelength instability to create the observed spectral form. These waves may then create an anomalous diffusion as discussed by Huba and Ossakow (1981b). At lower altitudes a shallower spectral index was observed in the tens of meters range, which may be related to a collisional damping regime. This suggests an altitude threshold for the drift waves that is probably related to ion neutral collisions. The power law spectra show no marked change near k⊥ri ≈ 1 where ri is the ion gyroradius. Since low‐frequency drift waves are linearly stable for k⊥ri ≳ 1, it seems that a wave‐wave interaction (cascade) operates to deposit energy in a range where waves are linearly damped. There is a slight suggestion of a spectral change (smaller negative index) for k⊥re ≳ 0.2 which may be due to excitation of a lower‐hybrid drift wave and which may be related to the observed enhanced backscatter at wavelengths on the order of 1 m. A stability analysis shows that the plasma is near but on the stable side of the marginal stability boundary for the lower‐hybrid drift wave in the most intense region of backscatter. In regions devoid of drift waves, evidence is found for an exponential inner‐scale cutoff at a wavelength of 200 m.

Estimates of galaxy distances based on indicators that are independent of cosmological redshift are fundamental to astrophysics. Researchers use them to establish the extragalactic distance scale, to underpin estimates of the Hubble constant, and to study peculiar velocities induced by gravitational attractions that perturb the motions of galaxies with respect to the "Hubble flow" of universal expansion. In 2006 the NASA/IPAC Extragalactic Database (NED) began making available a comprehensive compilation of redshift-independent extragalactic distance estimates. A decade later, this compendium of distances (NED-D) now contains more than 100,000 individual estimates based on primary and secondary indicators, available for more than 28,000 galaxies, and compiled from over 2000 references in the refereed astronomical literature. This paper describes the methodology, content, and use of NED-D, and addresses challenges to be overcome in compiling such distances. Currently, 75 different distance indicators are in use. We include a figure that facilitates comparison of the indicators with significant numbers of estimates in terms of the minimum, 25th percentile, median, 75th percentile, and maximum distances spanned. Brief descriptions of the indicators, including examples of their use in the database, are given in an appendix.