Spread Echoes Research Articles

MARSIS Observations of Field‐Aligned Irregularities and Ducted Radio Propagation in the Martian Ionosphere

AbstractKnowledge of Mars's ionosphere has been significantly advanced in recent years by observations from Mars Express and lately Mars Atmosphere and Volatile EvolutioN. A topic of particular interest are the interactions between the planet's ionospheric plasma and its highly structured crustal magnetic fields and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument on Mars Express. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp‐like) magnetic fields and spread echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination, and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to interhemispheric ducting in the Earth's ionosphere.

Daytime low‐altitude quasi‐periodic echoes at Gadanki: Understanding of their generation mechanism in the light of their Doppler characteristics

In this paper, we investigate the genesis of low‐altitude quasi‐periodic (LQP) E region echoes observed during daytime by the Gadanki radar. We show that such echoes reflect much wider Doppler spectra compared to that of typical daytime low‐altitude E‐region echoes. Further, the Doppler spread resembles that of meteor induced spread echoes. Considering that the meteor spread echoes are generated by the gradient drift instability on the sharp density gradient associated with the plasma trail, we invoke plasma density gradient and a similar physical process to account for the LQP spread echoes. Taking into account all the pertinent issues related to the formation of plasma density structures in the collision dominated lower E region, we surmise that the required plasma density structures are generated by a weakly driven Kelvin‐Helmholtz instability. These structures get polarized by the background electric field or neutral wind and become gradient drift unstable manifesting LQP echoes.

A global study of nighttime midlatitude topside spread echoes

Range spreading is frequently evident in midlatitude topside ionograms from the ISS‐B satellite. These ionograms provide a geographically unbiased perspective for the first global investigation of nighttime spread echo events that focuses solely on the midlatitude domain. The topside echo occurrence probability data show significant longitudinal variations, along with very large winter/equinox asymmetries in several geographic regions. The spread echoes are divided into two types and found to have different geophysical dependences. Small‐scale events that appear as spreading in one or two consecutive ionograms have a marked seasonal dependence on magnetic declination and no obvious correlation with auroral indices. Larger‐scale events in which the spreading is observed over spatial domains of 1000 km or more have less dependence on magnetic declination but are anticorrelated with the AE index in periods 6–8 hours prior to their appearance. The declination dependence for the small‐scale events suggests that the irregularities are most prevalent when the E region conductivity is low at both ends of the flux tube. This in turn suggests that electric fields mapping from the E region up to F region altitudes are likely involved in their generation. The correlation of the larger‐scale events with depressed AE signatures suggests that traveling ionospheric disturbances caused by auroral heating are not the most likely cause of these large features at midlatitudes.

Interferometry observations of VHF backscatter from plasma irregularities induced by meteor in sporadic E region

showed that the meter-scale plasma irregularities can be generated within the meteor trail through gradient drift/ Farley Buneman instability. Dyrud et al. [2001] further showed that these small scale irregularities will produce an anomalous cross-field diffusion of the meteor trail, leading to substantially reduction in the lifetime of the meteor trail echoes. [3] With spatial domain interferometry techniques, Chapin and Kudeki [1994b] and Reddi et al. [2002] found that the range spread echo regions are nearly parallel to the radar beam and the directions of the angle of arrival were not necessarily perpendicular to the geomagnetic field lines. Their results imply that the range spread meteor echoes are unlikely to come from the field-aligned irregularities due to appreciably large aspect angle. However, with beam swing technique, Zhou et al. [2001] found that numerous longlived range spread echoes were detected when the radar beam was pointed normal to magnetic field line, and the echoes were largely absent when the beam was pointed vertically and parallel to magnetic field line directions. They inferred that the plasma irregularities responsible for the range spread echoes are field-aligned, irrespective of no aspect sensitivity of the backscatter was measured. We will provide concrete evidence that, using interferometry methods to accurately position the targets in the echoing region, the meteor-induced range spread echoes may be highly aspect sensitive and the corresponding irregularities are field-aligned.

Analysis and simulation of land and rain clutter for PRC CW radar at X-band

It is pointed out that rain, which can be treated as object in the atmosphere, will cause time spread of radar echo signals, as does the land when the radar grazing angle is low. For the pseudo-random code continuous wave (PRC CW) radar, the time-spread echo signals at a specified moment will be the superposition of some spread echo signals in a period of time before this specified moment. A superposition algorithm is proposed for calculating the PRC CW radar signal. Based on previous results on the characteristics of rain and land clutter, the power spectra of the echo signal of rain and land clutter for the PRC CW radar are investigated with theoretical analysis and a simulation method.

Meteor trail induced backscatter in MST radar echoes

The amplitude variations and Doppler spectra of the 50 MHz radar echoes from meteor trails are investigated. The radar echoes from underdense meteor trails particulary during nighttime do not always fit the conventional specular reflection model. The echo was found to spread, sometimes, over as large as 8–10 km in range. In each range bin, the spread echo from the trail is produced with an altitude dependent time delay after the passage of the meteoroid producing the ionized trail. The Doppler spectra of the spread echo in each range bin is asymmetric with a high frequency wing comprised of narrow spectral peaks and successive spectral peaks amplitude decrease with frequency. These observations are interpreted as due to multiple reflections of the radar pulse between the striations generated parallel to the geomagnetic field along the length of the meteor trail. There is evidence to show that within the dynamo region the EW and NS components of the currents at lower and higher altitudes could be in opposite directions during the early morning hours of Sq electric field reversal.

Amplitude statistics of signals reflected from the ionosphere

Statistical analysis methods used to define the amplitude distributions of signals returned from the ionosphere are discussed in this paper. Emphasis is placed on determining accurately the parameter B, which is the ratio of steady to random components present in a signal. Thus B > 1 if the signal is dominated by the steady component, and B < 1 when the random components dominate. This study investigates the characteristics of B for F-region and E-region ionospheric echoes, as well as some types of spread- F, observed at the southern mid-latitude station Beveridge (37.3 S and 144.6 E). The results indicate that amplitude measurements obtained in approximately 100 s are adequate for determining B. The results also illustrate some effects that the E-region can have on F-region echoes. It is found that frequency spreading, the most common type of spreading observed at Beveridge, displays strong specular reflections and some signal variation due to interference at the leading edge of the F-region echo (i.e. B > 2). Within the spread echo B fluctuates between 0 and about 1.5 but is typically less than 1. The autocorrelation function of signal amplitude has a relatively large coherence interval, suggesting that this type of spread- F is due to interference of specular reflections from coherent irregularity structures with horizontal scale sizes of tens of kilometres rather than scattering from small scale irregularities. A second form of spread- F which would generally be classified as frequency spreading on standard ionoerams is actually due to off-vertical reflections from patches ol irregularities which originate south (poleward) of Beveridge. Echoes within this oblique spread- F (OS- F) do not exhibit coherence indicating that the irregularities responsible are of a smaller scale than those producing normal frequency spread. Finally, the phenomenon of spreading occurring on the second hop, but not the first hop trace is studied. It is shown that the form of the second hop echoes can be reproduced using a simple geometric model of ground scatter. The interpretation is supported by the fact that B for spread second hop echoes is less than 1 whereas it is much greater than 1 for the corresponding first hop echoes.

Frequency and duration of disturbances in the mid‐latitude F region of the ionosphere

Disturbances of the ionospheric F region at two well‐separated mid‐latitude stations were identified on rapid run ionograms by spread echoes, blackouts, and anomalies in structure or critical frequency. The diurnal variation in the probability of disturbance consistently shows minima around sunrise and sunset. Details such as a brief postsunset enhancement in winter are revealed by the fine time resolution of the data. Disturbance probabilities were lowest in autumn and spring (8% and 9%), while that for winter (33%) exceeded the summer level (20%). Spread F constituted 32% of the events, and disturbances producing sharply defined ionogram signatures 77%, with some overlapping of types. The majority of disturbances were not correlated with geomagnetic phenomena. Disturbances at the two stations were essentially independent (spatial correlation of 0.15). Spread F events were of longer duration (25 min) than sharp disturbances (20 min). TIDs were the exception, with a most probable duration of 80 min, but accounted for only 8.3% of the sharp disturbances. The absence of disturbances in the F region at dusk and especially at dawn coincides with periods of instability in the E region.

ISS-b experimental results on global distributions of ionospheric parameters and thunderstorm activity

The experimental results obtained from the Ionosphere Sounding Satellite-b (ISS-b) launched in February 1978 are given with particular emphasis on the global distributions of the ionospheric parameters such as the ionospheric critical frequency (foF2), occurrence probability of the spread echoes, nighttime electron temperature and ionic constituents of the ambient plasma, and also of the thunderstorms detected by means of radio technique. The global maps of foF2 are obtained based on the topside sounding experiment, and comparison between the ISS-b results and CCIR model is discussed. The global maps of the occurrence probability of the spread echoes on the topside ionograms are obtained, and longitudinal variability in the occurrence of the equatorial spread echoes is discussed. The global maps of the nighttime electron temperature of the ambient plasma are obtained, and appearance of the temperature troughs near the magnetic equator is discussed. The geomagnetic vs local time distribution characteristics of the ionic constituents, H +, He + and O +, of the ambient plasma are investigated, and winter bulge in the daytime He + density is found. The global maps of the occurrence rate of the thunderstorms are obtained based on the radio noise measurement, and they indicate existence of peculiar regions of high thunderstorm activity.

Spread F and ionospheric F‐region irregularities

The results of ionospheric observations by means of a variety of techniques are summarized to show that magnetic field‐aligned irregularities exist in the ionosphere. The irregularities appear in patches having horizontal dimensions up to 1000 km in the east‐west direction and several hundred kilometers in the north‐south direction. Size of the irregularities is typically about 1 km transverse to the magnetic field and elongated along the field line. Irregularities associated with spread F occur in the altitude range from 250 km up to the height of Fmax. Correlations between ionogram spread F and other ionospheric observations indicate that spread echoes are closely related to the field‐aligned irregularities. A critique of theories explaining why the irregularities cause spread echoes is given. Statistical studies of the geographic and temporal variations in spread‐F occurrence are summarized. The fundamental question of how irregularities causing spread F are produced in the ionosphere is discussed.

Polar ionospheric spread echoes and radio frequency properties of ice shelves

Records from three stations in the Antarctic and one Arctic drifting station show that downward radiation from an ionosonde aerial is reflected from sea water under the ice on which the station is built. As the ice is several wavelengths thick, an interference pattern is established which makes it possible to derive (a) the thickness and dielectric constant of the ice and (b) the direction from which ionospheric spread echoes arise. Spread F frequently occurs at vertical incidence in the polar regions.

Flutter Fading of Short-Wave Radio Signals in Equatorial Regions and its Connection with Spread Echoes, Magnetic Storms and the Radiation Belt

AbstractThe phenomena of ‘spread echoes’ on vertical incidence ionospheric soundings and ‘flutter fading’ on short-wave C.W. transmissions are of frequent occurrence in the low equatorial latitudes. Both the phenomena are predominant after local sunset and are inhibited during periods of strong magnetic activity associated with magnetic storms. A physical explanation is offered in this paper to the origin of these two phenomena in the following manner: Marked increase in the height of the F region of the ionosphere is almost a daily occurrence immediately after local sunset in low equatorial latitudes where the F region often ascends beyond 500 to 600 km. in the evening hours. The Allen radiation belt, recently discovered from satellite explorations, reaches down to a level of 500 miles or so from the surface of the earth over the equatorial regions. The F layer, during its. upward ascent, is likely to touch the lower fringe of this radiation belt and since this belt consists of high energy charged partic...

Theory of spreadFbased on aspect-sensitive backscattered echoes

To explain spread echoes from the F region the concept of aspect sensitivity was incorporated in our derivations of h′f curves using the assumption that the magnetoionic components can be treated separately. The shape of the h′f curves, as well as the extent of range and frequency spreading, was deduced for ionospheric stations located at various magnetic latitudes. The requirements of aspect sensitivity restrict the solid angle containing the echo rays to a wedge with its apex line running east-west through the transmitter location and with its angle determined by the dip angle. At the magnetic north the wedge opens flat, and at the magnetic equator the wedge degenerates into an east-west flat fan. The model does not lead to an explanation of a large class of observed spread F in the northern and middle latitudes. At the magnetic equator, however, the model is potentially capable of clarifying the observed spread F of a type shown in this paper.

Recombination and Diffusion and Spread Echoes from the Ionosphere

This paper is concerned with the explanation of the spread F echoes often obtained on frequency-sweep records of equivalent height in the ionosphere, especially at night and in the auroral regions. It is shown in the Appendix that the production of a mountain range of echoes from a single incident pulse can be accounted for if there is a periodic structure of ionization in the F layer. There are two mechanisms of ionization in the ionosphere, namely the incidence of ultra-violet radiation from the sun, and the influx of a stream of charged particles that are deflected by the earth's magnetic field towards the geomagnetic poles. The ultra-violet light produces well defined layers, the E, F1 and F2 layers, with a single maximum of ionic density within them, and they may be satisfactorily explained by Chapman's theory, taking into account the recombination process but neglecting diffusion effects. In the case of particle ionization, however, the effect of diffusion becomes predominant. It is shown in the paper that the inclusion of the diffusion term in the differential equation of layer formation leads to the possibility of a periodic structure within the F layer of the kind needed to explain the spread echoes. This is because the differential equation has as its solution a doubly periodic elliptic function that provides the required periodicity of ionic density as a function of height. The form of the mathematics gives the essentially periodic nature of the solution without any detailed appeal to the initial conditions, but reference is made to the auroral curtains indicating the existence of sources of charged particles that maintain the periodic structure of ionization in between the curtains. Whereas the ultra-violet light causes layers to be formed that are many wavelengths thick, the particle ionization causes very thin layers only two or three wavelengths thick embedded in the background of ionization.