Michael C. Kelley

On the role of charged aerosols in polar mesosphere summer echoes

Submicron aerosols, as evidenced by the occurrence of polar mesospheric and noctilucent clouds, exist at heights from which polar mesosphere summer echoes (PMSE) are observed. We investigate the role of positively and negatively charged aerosols in the scattering processes proposed in the literature. These aerosols, if charged substantially, can account for the remarkably high radar reflectivity at both VHF and UHF by raising the electron Schmidt number through the ambipolar effect. A positively charged component may be responsible for enhanced UHF radar scatter by increasing the incoherent scatter power through a dressed dust effect, although such a process is not realistic as an explanation for VHF scatter during PMSE. Such an enhanced UHF scatter will be associated with extremely narrow backscatter spectra. We propose a model in which both negatively and positively charged aerosols are present to explain both the radar properties and the rocket probe observations of charged particle depletions. Finally, we point out that the Poker Flat 50-MHz long-term data, which contrary to accepted dynamical theory show average downward velocities in the summertime upper mesosphere, can be attributed to the fall speed of the aerosols responsible for PMSE.

Turbulent upwelling of the mid‐latitude ionosphere: 1. Observational results by the MU radar

In this paper we present the detailed results of a series of experiments designed to study the coherent backscatter of 50-MHz radar waves from the mid-latitude F region. Data were obtained with the active phased-array MU radar in Japan and include some auxiliary E region coherent echoes as well. As in other turbulent ionospheric phenomena the intense nonthermal scatter comes from irregularities oriented parallel to B. The strongest echoes correspond to irregularities at least 20 dB stronger than thermal backscatter at the same frequency from typical F region densities at the same range. Simultaneous observations with ionosondes show that these echoes occur during strong mid-latitude spread F. As defined by ionosondes, the latter phenomenon is certainly much more widespread than the turbulent upwelling events described here, but we believe that in some sense these correspond to the most violent mid-latitude spread F. The strongest echoes occur in large patches which display away Doppler shifts corresponding to irregularity motion upward and northward from the radar. At the edges of these patches there is often a brief period of toward Doppler before the echoing region ceases. On rare occasions comparable patches of strong away and toward Doppler are detected, although in such cases the Doppler width of the toward echoes is much narrower than that of the away echoes. The away patches often are characterized by mean velocities well over 250 m/s and Doppler widths (full width at half maximum) of 50 m/s. The multiple beam capability at MU allowed us to track the patches in the zonal direction on two days. The patches moved east to west in both cases at velocities of 125 m/s and 185 m/s, respectively. There is a distinct tendency for the bottom contour of the scattering region to be modulated at the same period as the patch occurrence frequency as well as at higher frequencies. This higher-frequency component may correspond to substructures in the large patches and to the E region coherent scatter patches which were detected simultaneously in several multiple beam experiments. In the companion paper (Kelley and Fukao, this issue), we explore a number of possible explanations for this phenomenon in more detail.

Polar mesosphere summer radar echoes: Observations and current theories

The remarkably strong radar echoes from the summer polar mesosphere have been an enigma to atmospheric and radar scientists since their discovery more than a decade ago. Since then, more sophisticated radar experiments and in situ rocket measurements have shed some light on the underlying physics and chemistry, and theories have been formulated to explain the generation of the intense radar backscatter and the remarkable physical conditions associated with it. First, we review the key observations and examine the proposed theories. We then evaluate the progress that has been made in understanding this phenomenon and explore its connection to global change, to the newly recognized material referred to as a dusty plasma, and to the highest clouds in the Earths atmosphere. Finally, we end with suggestions for future research.

Airglow observations of mesoscale low‐velocity traveling ionospheric disturbances at midlatitudes

This paper presents a summary of 630.0 nm emission observations made by the Cornell All-Sky Imager that have revealed an abundance of structure in the midlatitude thermosphere. Some events were so bright that the weaker 557.7 nm thermospheric line was readily visible and produced sharper images because of the shorter excitation lifetime. Global Positioning System observations show that the airglow features are traveling ionospheric disturbances (TIDs). The remarkable feature of the data is the overwhelming tendency for these low-velocity TIDs to develop with a northwest to southeast orientation and to propagate in the southwest direction. Speeds ranged from 50 to 170 m/s, and wavelengths ranged from 50 to 500 km. The Perkins instability is investigated as a possible explanation for the structures. The linear theory, including both winds and electric fields, predicts a positive but small growth rate. However, the real part of the dispersion relation gives the wrong sign for the wave propagation. Furthermore, the growth rate seems too small to amplify a seed gravity wave significantly during one period of neutral gas oscillation. We conclude that this class of low-velocity TID is not yet explained theoretically.

Two dimensional spectral analysis of mesospheric airglow image data

A technique to analyze short-period (<1 hour) gravity wave structure in all-sky images of the airglow emissions is described. The technique involves spatial calibration, star removal, geographic projection, regridding, and flat fielding of the data prior to the determination of the horizontal wave parameters (wavelength, velocity, and period), by use of standard two-dimensional Fourier analysis techniques. The method was developed to exploit the information that is now available with wide-field solid state imaging systems. This technique permits interactive and quantitative investigations of large, complex data sets. Such studies are important for investigating gravity wave characteristics, their interaction with the airglow emissions, and their geographic and seasonal variability. We study one event of this type here and present possible evidence of a nonlinear wave-wave interaction in the upper atmosphere.

Density depletions at the 10‐m scale induced by the Arecibo heater

In June 1992 a NASA sponsored sounding rocket was flown through the Arecibo heater beam to study the structure of the heated volume. The rocket carried an instrument payload and traversed the 5.1-MHz reflection height at 268.5 km. Data from the plasma density probe are presented in this paper. The rocket passed through several regions of disturbed plasma both above and below the reflection level. In these regions, over 180 deep filamentary density depletions were detected. Measured perpendicular to the magnetic field, these depletions or filaments have a mean width at half maximum of 7 m which is roughly equal to twice the ion gyroradius (O + ) and a mean depletion depth of 6%. The ratio of parallel to perpendicular scale for these structures exceeds 20,000, and the spacing between the filaments is around 15 m. A power spectrum of the rocket data clearly shows the spectral content of the filaments and also reveals peaks at longer wavelengths which we interpret as the spacing between the bunches and between sets of filaments within a given bunch. We believe that previous scintillation and satellite measurements emphasized these longer wavelengths. The power spectrum measured by the rocket instrumentation falls off as k −4 for wavenumber k larger than 0.4/m and remains above the system noise for structure down to 1 m. It is clear that VHF backscatter from these structures can be explained by our data, as can many features of heater-related, field-aligned irregularities found in the literature.

Investigations of thermospheric-ionospheric dynamics with 6300-Å images from the Arecibo Observatory

Pilot observations were conducted at Arecibo, Puerto Rico, using an all-sky, image-intensified CCD camera system in conjunction with radar, ionosonde, and Global Positioning System (GPS) diagnostic systems during the periods January 19–28, 1993, and February 21 to August 22, 1995. These represent the first use of campaign mode operations of an imager at Arecibo for extended periods of F region observations. The January 1993 period (the so-called “10-day run”) yielded a rich data set of gravity wave signatures, perhaps the first case of direct imaging of thermospheric wave train properties in the F region. The 6-month 1995 campaign revealed two additional optical signatures of F region dynamics. A brightness wave in 6300 A passing rapidly through the field of view (FOV) has been linked to meridional winds driven by the midnight temperature maximum (MTM) pressure bulge. On May 3, 1995, during a period of geomagnetic activity, a 6300-A airglow depletion pattern entered the Arecibo FOV. Such effects represent the optical signatures of equatorial spread F instabilities that rise above the equator to heights near 2500 km, thereby affecting Arecibos L = 1.4 flux tube.

Turbulent upwelling of the mid‐latitude ionosphere: 2. Theoretical framework

In a companion paper, data from the MU radar have been presented which show that during sunspot minimum conditions in Japan the mid-latitude ionosphere is sometimes characterized by regions of rapid and turbulent upwelling. In this paper we explore possible mechanisms for these events. The most likely process seems to be the instability of the equilibrium which occurs when the mid-latitude plasma is supported against gravity either by an eastward electric field component or by a southward neutral wind, as was proposed by Perkins (1973). We show, for example, that the growth rate determined by Perkins is considerably higher in sunspot minimum conditions than at sunspot maximum for comparable altitudes of the ionospheric F layer. The growth rate is not very large, however, and we argue here that the observed structures must evolve from preexisting undulations of the bottomside of the F region which are generated by gravity waves. That is, the gravity waves create finite amplitude structures which are amplified by the plasma instability. An intriguing feature of the gravity wave role in this process is that the echoing patches detected by the MU radar and the height bands detected by the Arecibo radar, which we believe to be related phenomena, all seem to propagate to the west. This is the same direction reported for the angle-of-arrival measurements of classic mid-latitude spread F by a number of researchers using ionosonde techniques. Since the MU radar detects the upwelling regions from nonthermal 3-m irregularities there must be mechanism to create such tracers. We propose that secondary structures are created at intermediate scales via the E×B instability operating on the dome of the upwelling structure and by the neutral wind-driven process on either the west or the east wall of the structure, depending on the direction of the zonal wind. The 3-m waves are then created in a cascade process which brings energy into a range of k space in which the structures are linearly damped. Finally, we discuss the fine structure of the echoing patches and suggest several plausible mechanisms, two of which involve E region coupling and one which deals with the vertical structure of the gravity wave seeding process.

Nonlinear evolution of equatorial spread F: 1. On the role of plasma instabilities and spatial resonance associated with gravity wave seeding

We have used a computer simulation to study properties of large-scale equatorial F region irregularities produced by gravity waves by separating such different processes as the spatial resonance effect and the Rayleigh-Taylor instability. Our purpose is to show their relative importance in the production of strong ionization perturbations. When a gravity wave propagates perpendicular to the magnetic field, it generates a polarization electric field. If there is no amplification by the Rayleigh-Taylor instability, the amplitude of the electric field remains almost constant for a long time. If spatial resonance occurs, after one wave period strong ionization perturbations with a relative density amplitude of 58% are produced. However, the associated electric field is limited and the gravity wave-induced perturbation does not grow into topside plasma bubbles when the Rayleigh-Taylor instability is absent. A zonally propagating gravity wave can, however, initiate the Rayleigh-Taylor instability in the bottomside F region. The initiation does not depend on the spatial resonance mechanism. After initiation, the Rayleigh-Taylor instability amplifies nonlinearly the perturbations induced by the seed gravity wave, and results in topside plasma bubbles. Spatial resonance can speed up the formation of bubbles. It is thus concluded that the Rayleigh-Taylor instability mechanism is the most important for production and rise of plasma bubbles and that seeding by gravity waves can occur even without the spatial resonance effect.

Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an elect.rolyiia.iuically driven traveling ionospheric disturbance which passed over Arecibo observatory between 22 and 24 AST on.January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a therniospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determinatiou: tie use of G300 A airglow images front the coupling, energetics, and dynamics of atmospheric regions (CEDAR) all-sky imager to track the two-dimensional, mesoscale dynamics of the disturbance and the rise of a portable ionosonde to simultaneonsly measure the field line integrated ionospheric conductivity in the conjugate hemisphere. we have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].