Ion Drift Meter Research Articles

Lower midlatitude ionospheric disturbances and the perkins instability

This paper presents a preliminary study of ionospheric disturbances at dip latitudes less than 40° as seen by the ion drift meter and the retarding potential analyzer on board Atmosphere Explorer E during a period of low solar activity. The altitude of observation was relatively low, mostly below 300 km. The emphasis is on the midlatitude region, where some features resemble equatorial bubbles; no clear demarcation in latitude could be recognized between the bubbles and other midlatitude disturbances. Excellent evidence was found that the Perkins instability is responsible for very structured disturbances which were frequently observed. In most cases where regions of inward and outward E × B drift were encountered, diffusive motion up or down the field line partially cancelled the vertical component of the cross-field drift. In some cases cancellation was almost perfect, but in others it appeared not to occur at all (probably cases involving rapid changes). Within the larger structures caused by the Perkins instability there were places where the secondary gradient drift instability was also active.

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude ( E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn/ n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

The ionospheric signatures of rapid subauroral ion drifts

Subauroral ion drifts (SAID) are latitudinally narrow regions of rapid westward ion drift located in the evening sector and centered on the equatorward edge of the diffuse aurora. Observations of SAID as identified by the ion drift meters on the Atmosphere Explorer C and Dynamics Explorer B spacecraft are utilized to determine their effect on the F region ion composition, their relationship to the mid‐latitude trough, and their temporal evolution. At altitudes near the F peak a deep ionization trough is formed in regions of large ion drift where the O+ concentration is considerably depleted and the NO+ concentration is enhanced, while at higher altitudes the trough signature is considerably mitigated or even absent. SAID have been observed to last longer than 30 min but less than 3 hours, and their latitudinal width often becomes narrower as time progresses. The plasma flows westward equatorward of the SAID and becomes more westward as invariant latitude increases. Poleward of the SAID, the flow is, on average, westward throughout the auroral zone in the evening, while near midnight it becomes eastward.

The geomagnetic mass spectrometer— mass and energy dispersions of ionospheric ion flows into the magnetosphere

NASA's Dynamics Explorer (DE) mission was designed to study the coupling between the Earth's magnetosphere, ionosphere and neutral thermosphere1. One area of major interest is the outflow of ionospheric plasma into the magnetosphere, the scale and significance of which is only now becoming apparent with the advent of mass-resolving, low-energy ion detectors. Here we compare observations of ion flows in the polar magnetosphere, made by the retarding ion mass spectrometer (RIMS)2 on DE1, with those made simultaneously in the topside ionosphere by the ion drift meter (IDM)3 on the lower-altitude DE2 spacecraft. The results show the dayside auroral ionosphere to be a significant and highly persistent source of plasma for the magnetosphere. The upwelling ionospheric ions are spatially dispersed, according to both their energy and mass, by the combined actions of the geomagnetic field and the dawn-to-dusk convection electric field, in an effect analogous to the operation of an ion mass spectrometer.

Bottomside sinusoidal irregularities in the equatorial F region: 2. Cross‐correlation and spectral analysis

Equatorial bottomside sinusoidal (BSS) irregularities have been studied by applying techniques of cross‐correlation and spectral analysis to the Atmosphere Explorer data set. In order to determine the spatial displacements between the density and transverse velocity components of the irregularities, we have cross‐correlated the ion density data from the retarding potential analyzer with the pitch and yaw ion velocity components as measured by the ion drift meter. The analysis indicates that the pitch velocity component is displaced by somewhat more than 90° (referred to the wavelength of the spectral peak) to the west with respect to the ion density fluctuations. However, the yaw drift component is almost in phase with the density fluctuations. Spectra of the pitch velocity fluctuations show a sharp peak centered at about 1 km E‐W wavelength, and a power law spectral index near +2 in the wavelength range from 10 km to 1 km. Yaw velocity and ion density spectra are also peaked at 1 km, but have more power at long wavelengths than do the pitch velocity spectra. The occurrence frequency of these irregularities maximizes at the solstices, and has a seasonally‐dependent longitude distribution. BSS events are detected most frequently in the local summer hemisphere. Many of these new results can be explained by fairly simple theoretical arguments that emphasize the importance of the ion Larmor flux, interhemispheric plasma flows, and variations in the lower F region Pedersen conductivity. The latter are thought to provide an important boundary condition on the Rayleigh‐Taylor instability dynamics.

Earthward directed high‐density Birkeland currents observed by HILAT

An intense (94 µA/m²) earthward directed Birkeland current was detected by the HILAT satellite on July 23, 1983, less than one month after launch. It was located at the equatorward edge of a large‐scale earthward flowing current in the late evening sector. This current is the most intense ever to be reported that is consistent with upward flowing thermal electrons. Simultaneous vacuum ultraviolet images from the auroral imager mapper show that the large‐scale current is embedded within the diffuse aurora. Energetic electron measurements in the range 20 eV to 20 keV indicate an increase in downward flux from ∼5×107 to ∼7×109 (cm² sec sr)−1 in ∼0.75 s. This increase in flux results in a gradient of ∼2 mhos/km in the height‐integrated Pedersen conductivity (Σp). In the presence of a typical dc electric field (∼40 mV/m), this sharp gradient in the ionospheric conductivity is consistent with the inferred high‐density current. Measurement of horizontal ion drift from the ion drift meter (IDM) reveals strong turbulence present for ∼10 km on both sides of the region of intense Birkeland current. The IDM also measured an ion density (ni) of ∼2×104 cm−3; thus, if ne ≃ ni, the parallel electron drift velocity derived from J = ne Vd q is ∼30 km/s, resulting in a net electron flux of ∼6×1010 cm−2 s−1. A flux of electrons of this magnitude is sufficient to destabilize O+ ion cyclotron waves. The velocity of 30 km/s coupled with an F region ion density gradient scale length of ∼3 km is also comparable to criteria required to drive the current convective instability.

Ion‐neutral coupling in the high‐latitude F region: Evaluation of ion heating terms from Dynamics Explorer 2

We study a set of south (summer) polar passes from the Dynamics Explorer 2 data base that provide measurements of ion and neutral velocities along the track of the spacecraft for various geomagnetic conditions. The ion velocities are obtained from the ion drift meter and the retarding potential analyzer instruments; the neutral velocities are obtained from the Fabry‐Perot interferometer and the wind and temperature spectrometer instruments. These data enable a direct determination of the in situ ion‐neutral difference velocity. Simultaneous measurements of atmospheric composition using the neutral atmosphere composition spectrometer and of electron temperatures and densities using the Langmuir probe enable the ion heating and cooling rates to be determined. The rates are discussed in relationship to values for the electron, ion, and neutral kinetic temperatures. The comprehensive nature of the data set allows for a quantitative study of ion‐neutral coupling and, in particular, affords a more rigorous test of the common assumptions used in the ion energy equation than has been previously possible. From our study we find the following: (1) The ion‐neutral difference velocities, which drive the Joule heating, have a complex morphology that depends on the stability of the high‐latitude ion convection and the inertia of the neutral gas. (2) Ionospheric “hot spots” are observed to be related to large perpendicular electric fields that give rise to frictional (Joule) heating. (3) These events are often seen to occur in conjunction with troughs in the electron density and with decreases in the ratio of atomic oxygen to molecular nitrogen. In such cases, momentum transfer to the neutral gas is reduced, and the ion‐neutral velocity difference and associated Joule heating can be maintained over longer time periods. (4) An approximate balance between local frictional heating of the ions and local cooling of the ions to the neutrals is seen in regions of strong ion convection. The commonly used approximate form of the ion energy equation involving only local processes appears at times, however, to be inadequate for large regions of the summer polar cap. The ion temperatures observed in these regions are higher than expected from simple local heating/local cooling arguments. Additional source(s) of ion heating would be required to provide energy balance.

F region electron density irregularity spectra near auroral acceleration and shear regions

Spectral characteristics of auroral F region irregularities were studied by the use of high‐resolution (∼35 m) density measurements made by the retarding potential analyzer (RPA) on board the Atmosphere Explorer D (AE‐D) satellite during two orbits when the satellite was traversing the high‐latitude ionosphere in the evening sector. Coordinated DMSP passes provided synoptic coverage of auroral activity. The auroral energy input was estimated by intergrating the low‐energy electron (LEE) data on AE‐D. It was found that the one‐dimensional in situ spectral index (p1) of the irregularities at scale lengths of <1 km showed considerable steepening in regions of large auroral acceleration events with p1 values of ∼ −3. This is interpreted as resulting from the effects of E region conductivity on the F region irregularity structure. The regions in between the precipitation structures, where presumably the E region conductivity was small, were generally associated with large shears in the horizontal E‐W drifts and large velocities, as measured by the ion drift meter on board AE‐D. The maximum drifts measured were ∼2 km s−1, corresponding to an electric field of 100 mV m−1. The large‐velocity regions were also associated with substantial ion heating and electron density depletions. The largest shear magnitudes observed were ∼80 m s−1 km−1, and the shear gradient scale lengths were ∼10 km, which was approximately the resolution of the ion drift meter data set used. The spectral characteristics of irregularities in the large, variable flow regions were very different, with p1 being ∼−1. Since these regions were also associated with the largest irregularity amplitudes, it seemed probable that either the velocity shears or, alternatively, the large velocities provided a source of irregularities in the auroral F region ionosphere. Current work on plasma instabilities related to velocity shears and two‐dimensional fluid turbulence is briefly summarized and critically compared to the findings of the present study.

Bottomside sinusoidal irregularities in the equatorial F region

A new category of equatorial F region plasma irregularities having properties easily distinguished from those of plasma bubbles, depletions, or broadband turbulence has been identified in the data from the retarding potential analyzer (RPA) and the ion drift meter (IDM) on the Atmosphere Explorer satellites AE‐C and AE‐E. These irregularities are observed in the bottomside at night in a narrow belt extending approximately ±12° from the dip equator and up to 7500 km or more in the east–west direction. High‐resolution data from the RPA reveal nearly sinusoidal waves in the ion number density Ni with peak to peak ΔNi/Ni amplitudes that span more than three orders of magnitude, from values less than 0.1% to more than 100%. These distinctive “bottomside sinusoidal” (BSS) irregularities typically have a central wavelength lying in the range from 300 m to 3 km, and relatively narrow spectra. The power spectral index is often large (5 or 6) at the shorter wavelengths, but the spectrum usually decays much less rapidly on the longer wavelength side. The vertical drift velocity also has a wavelike structure that appears to be anticorrelated to within a phase uncertainty of approximately ±90° with the Ni variations, such that regions of lower density plasma move upward and higher density regions move downward with respect to the mean ionospheric drift. Anticorrelated wavelike drift and density fluctuations are consistent with plasma that is Rayleigh‐Taylor unstable. Consideration of altitude localization and cross‐field ambipolar diffusion effects can introduce at least a slight maximum in the R‐T growth rate near 1 km and also sharply limit irregularity growth at smaller scale sizes, thus providing a tentative explanation of the peaked Ni spectrum observed. Ionograms from Huancayo display a typical “mid‐latitude” frequency spread signature when BSS is present.

Dynamics Explorer observations of equatorial spread F: Evidence for drift waves

Recent DE‐2 data from the Langmuir probe, vector electric field, and ion drift meter instruments are employed to study equatorial spread F in the frequency regime of the low frequency drift and the lower hybrid drift instabilities. Strong electron density gradients observed along the orbital path through the topside equatorial F region correspond to regions of high electric field waves and large ion drift velocities. The electric field waves are seen in two distinct wavelength ranges which correspond to the parameter regimes of the low frequency drift and the lower hybrid drift instabilities. In the smaller of the two wavelength ranges the lower hybrid drift instability is found to be unstable, based on the ion drift velocity and the other plasma parameters measured on DE‐2, and using published theory. Thus there is experimental evidence that the lower hybrid drift instability may produce the observed short wavelength waves without invoking a cascading mechanism.

An upper limit on the aperture separation of ion drift meters

An upper limit has been calculated on the effective aperture separation or detector thickness of ion drift meters of two fundamental types. The limit applies to meters which compare currents collected by detectors with different view directions at the same retarding potential and to meters which measure the entire thermal ion distribution function. For both types, a single scanned detector may be employed instead of multiple detectors, in which case the limit applies to twice the radius of curvature of the path followed by the detector aperture during a scan, (i.e. the diameter of a spinning payload). The limit was found to be important in two cases. First, in the F region on spacecraft with stringent electrostatic cleanliness requirements, the 10% error limit was found to be 40 cm. Second, in the E region, the limit was found to be ≲1 cm.

An instrument for total ion drift velocity measurements aboard the ‘Intercosmos-Bulgaria-1300’ satellite

The ‘INTERCOSMOS-BULGARIA-1300’ satellite was launched on Aug. 7/81, to investigate ionospheric plasma dynamics. It had a perigee of 825 km, an apogee of 906 km and orbit inclination of 81.2°. The satellite was three axis stabilized within ±1° on each axis. The ion driftmeter, ID-1, aboard this satellite was intended to measure ion density irregularities, the ion drift velocity and photoelectron fluxes. The purpose of this paper is to present a brief description of the ID-1 instrumentation and to show the first results obtained from the flight instrument.