Ionospheric Density Gradients Research Articles

Negative shuttle charging during TSS 1R

We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased from −17 to −245 V as tether length, L, increased to 2.6 km. Current in the circuit depended strongly on ionospheric density. With the 25 kΩ resistor in place, Φs ≈ −300 V in the low density, nightside ionosphere with L = 5.1 km. Near local noon Φs ≈ −80 V with L = 17.2 km. The shuttle charged to ∼−600 V during two dawn terminator crossings, one with and one without thruster firings. As on TSS 1, firings of two aft vernier thrusters significantly increased |Φs|. In the case without thruster firings, simultaneous variations of Φs, tether current, and the inferred satellite potential are consistent with strong azimuthal and vertical ionospheric density gradients. These are the first known direct measurements of strong negative shuttle charging.

Linear theory and modeling of the Rayleigh‐Taylor instability leading to the occurrence of equatorial spread F

In a test of the generally accepted Rayleigh‐Taylor (R‐T) instability mechanism for equatorial spread F (ESF), a linear instability growth rate γRT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γRT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R‐T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night‐to‐night and even an hour‐to‐hour basis.

Reply [to “Comments on ‘Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets’ by W. L. Oliver et al.”

Reply [to “Comments on ‘Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets’ by W. L. Oliver et al.”

Comments on “Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets” by W. L. Oliver et al.

Jacobson [this issue] has written a comment on our article Oliver et al. [1994], subsequently referred to as OEA, describing various geometric effects neglected in our analysis of middle and upper atmosphere (MU) radar multiplebeam observations of ionospheric gravity waves. Jacobson [this issue] shows that these effects can be important in the assessment of wave structure and, in the worst case, possibly void the findings altogether. We wish here to acknowledge the validity of the effects discussed by Jacobson and to comment on their applicability to our results. The MU radar in Japan can observe in multiple beams simultaneously. OEA used this capability to look at electron density differences between beams and considered these differences and their time sequences to be evidence of horizontal gradients caused by gravity waves. The first F region gravity wave seen with the MU radar occurred during the large magnetic storm of February 6-8, 1986. It had a horizontal wavelength of 2500 km, large compared with our 200-km beam spacing, and we have tacitly assumed since that time that the MU radar beams enclose only a small portion of a wave. Jacobson points out that this was not true for all waves reported in OEA. The waves reported there had horizontal wavelengths between 260 and 1400 km. The longer of these waves are appropriately seen with beams 200 km apart, but the shorter are not. Jacobson also points to the problem arising from beams that intersect the wave fronts at different aspect angles (see his Figure 1); one beam may cut across wave trough and crest while another may lie entirely within a trough or crest. Again, this problem is exacerbated when the wavelength is small.

Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.

A preliminary experimental test of ionospheric tomography

A preliminary experiment to test the application of computerized tomography (CT) to ionospheric imaging is described. Co-ordinated observations are presented in which radio transmissions from NNSS satellites were used to measure the total electron content required for the tomographic image of electron density. Simultaneous measurements of electron density by the EISCAT ionospheric radar facility were made for comparison. The results show that a large-scale ionospheric electron density gradient reconstructed by the tomographic technique is also seen in the EISCAT observations.

Observation of two preferred propagation paths for whistler mode VLF signals received at a non-conjugate location

Whistler mode signals from NLK (24.8 kHz) were received at Dunedin, New Zealand during 1989–1990. The arrival bearing of the signals shows a bimodal distribution which is consistent with the results of Allcock and McNeill (1966), who suggested the existence of two paths with less total transmission loss than other paths. PATH 1 signals (which have a duct entry region just south of Dunedin's conjugate) are observed at all times of the year, while PATH 2 signals (which have a duct entry region just south of the NLK transmitter) show an annual variation in occurrence with a maximum in May and a minimum in November which may be a consequence of horizontal ionospheric electron density gradients near the duct entry region. Similarly preferred paths have been observed at Faraday, Antarctica from the NSS transmitter.

The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.

Generation of ELF/ULF waves in the ionosphere by dynamo processes

It is shown that amplitude modulated HF heating in the ionospheric E-region, can produce substantial power in the ELF/UI range even in the absence of ambient ionospheric currents. The signals are associated with spontaneous generation of magnetic fields caused by the coupling of the hot spot temperature gradient to the ambient ionospheric density gradient (Vn x VT). The mechanism produces values for PC5 micropulsations consistent with those observed by Stubbe and Kopka (1981).

Correcting Low-Frequency Solar Radio Source Positions for Ionospheric Refraction

We describe a method for reducing displacements in radio source positions caused by ionospheric refraction. Our method is an improvement on that used by Wild et al. (1959), who assumed all frequencies in the 40-70 MHz band came from the same position above the Sun and applied an f-2 correction for ionospheric refraction. Our method retains the dispersion of source position with frequency, which is inherent in the radio source, but allows for the f-2 ionospheric effect. We also discuss ways of estimating the absolute source positions from a knowledge of ionospheric density gradients. A typical example of ionospheric variations on solar records is shown in Figure 1(a). Here we have plotted the observed source positions on an f-2 scale. The measurements refer to a solar storm continuum burst which occurred on 1981 May 9 following an importance 2B flare at 08°N., 38°E., heliographic coordinates. From previous observations of such events we think it highly likely that the true source positions are stationary and displaced with decreasing frequency outwards along a line close to the radial direction above the flare. Figure 1(a) shows that: (a) the 160, 80 and 43 MHz source displacements vary as f-2; (b) there is a systematic slow drift of the source positions towards the south and east which increases with increasing hour angle; (c) superimposed on this steady drift is a quasi-periodic variation in source position with a period ~20 min.

Amplification of electrostatic noise in cyclotron resonance with an adiabatic auroral beam

The adiabatic evolution of the auroral beam in the convergent geomagnetic field gives rise to a region of ∂F/∂ν⊥ >0 in the electron distribution function. The altitude dependence of this source of free energy is examined, and its ability to drive electrostatic waves toward instability is considered. General linear properties of the possible instabilities are discussed, and a detailed numerical calculation based on a collisionless height dependent model of an auroral arc (Maggs and Lotko, this issue) is described. Of the three wave modes considered, the upper hybrid mode is optimally amplified at altitudes below about 1000 km. Oblique electron Bernstein modes may be weakly unstable at the lowest altitudes, and the whistler mode appears to be stable at all altitudes. Wave amplification is limited by a finite resonance time which is determined by refraction of parallel wavenumbers by the topside ionospheric density gradient. Slow beams originating at relatively high altitudes and nearly perpendicularly propagating waves with frequencies away from cyclotron harmonics are favored by this instability mechanism.

Experimental study of ionospheric electron density gradients and their effect on VLF propagation

Ionospheric electron density gradients in the mid-latitude ionosphere have been studied experimentally through their effect on the wave-normal direction of upgoing artificial VLF waves. From the diurnal and seasonal variations of the east-west and north-south components of the unit wave normal, the horizontal gradients of electron density are deduced. It is shown that the direction of propagation of the waves is closely related to certain broad features of the ionospheric structure, namely its variations with longitude or local time which act upon the E-W component of the wave normal and its latitudinal variations which influence the N-S component. These data shed some light on the choice of the initial wave-normal directions for use in magnetospheric ray-tracing studies.