High‐resolution backscatter power observations of 440‐MHz E region coherent echoes at Millstone Hill

Abstract

A 40-/•s pulse length has been used to provide 10-s temporal and 6-kin range resolution observations of E region coherent backscatter from the premidnight eastward electrojet region to the north of Millstone Hill. The sensitivity of the Millstone UHF system is such that coherent returns can be observed over a 80-dB dynamic range and at levels down to the incoherent scatter background. Our observations can be divided into two categories: strong events in which the backscattered amplitude nears saturation and weak events in which spatial structure and large-amplitude variations are common. Calibrated observations find a typical volume scattering coefficient of ---10-1Ira-1 at 440 MHz during strong events with a maximum level of 9x10-10m -1 observed for brief intervals. During less intense events the radar backscatter is modulated by --30 dB in amplitude at Pc 5 frequencies (150 -500 s) by waves with spatial wavelength 50 -100 kin. Our observations support the premise that the weak irregularities grow linearly with electric field strength and reach a saturation amplitude beyond which the oscillating electric field of the Pc pulsation has little effect. The observed variation of backscattered power with range is interpreted using a geometrical model which accounts for the detailed antenna beam pattern, a magnetic aspect angle sensitivity of -10 dB per degree, and a thin layer of irregularities centered at 110 km altitude. For strongly driven conditions a comparison of the range variation of backscattered power with our thin layer model suggests that the signal power becomes increasingly dominated by strong scatterers confined to a narrower altitude range. The apparent altitude extent of the strongest irregularities decreases by a factor of 2 as the amplitude of the backscattered signal increases by a factor of 10. INTRODUCTIONauroral E region, submitted to Journal of Geophysical Research, 1990) (hereinafter referred to as del Pozo et al., 1990), empha-The Millstone Hill 440-MHz UHF radar receives intense sized data taken during synoptic experiments [Foster et aL, 1985] backscatter from 30 cm irregularities when viewing E region which used long pulse lengths and antenna scanning at 1ow-eleheights to the north at aspect angles near perpendicular to the magnetic field [St. Maurice et aL, 1989, del Pozo, 1988]. Early observations from Millstone Hill using an L band (1295 MHz) radar have been reported by Abel and Newell [1969] and Hagfors [1972]. Although the Millstone Hill site is situated at subauroral latitude (288.5 ø E longitude, 42.6 ø N. latitude, 55øA), the appropriate aspect angle for coherent backscatter is attained some 5 ø -8 ø poleward of this using low-elevation angles (4 ø -10ø). When the electric field in the E region exceeds a threshold value of the order of 20 mV/m [Moorcroft, 1979, 1980], electron drifts higher than the local ion acoustic speed lead to the onset of the two-stream instability [Farley, 1963, Bunernan, 1963] and the generation of a spectrum of irregularities which produce vation angle to d...

Degradation of GPS performance in geomagnetically disturbed conditions

“…E region coherent scatter is aspect angle sensitive. Therefore, GPS phase slips might be due to a mechanism rooted in two-stream FarleyBuneman instabilities (Foster and Tetenbaum 1991). Using simultaneous measurements of backscatter signal characteristics from the Irkutsk incoherent scatter radar (Kurkin et al 1999) and existing models for such irregularities, Afraimovich et al (2001a) estimated the order of magnitude of the expected phase fluctuations of the GPS signal.…”

Inferred electric field variability in the polarization jet from Millstone Hill E region coherent scatter observations

“…In order to interpret the observed range returns correctly, we use a model which simulates the radar range‐intensity profile that would be observed from an assumed irregularity latitude and altitude layer structure, on the basis of radar pointing azimuth and elevation, the 46 m antenna beam pattern, and magnetic aspect angle sensitivity. Such a model has been described by Foster and Tetenbaum [1991] and has been used for several studies of coherent echoes [ Foster et al , 1992; Foster and Erickson , 2000]. In the current version of the model we integrate over the antenna beam pattern out to 10° from the main beam pointing direction and determine the contribution to the total power at a given range from every point at which the constant‐range surface intersects the irregularity layer.…”

“…Millstone's high power (2–5 MW peak), narrow beam (1.2° full width at half maximum), and receiver sensitivity down to incoherent scatter levels (−170 dBW) give it a unique capability for wide dynamic range, high‐resolution E region backscatter measurements. Several previous studies using Millstone data have probed the relationship between E region backscatter cross section, phase velocity, and, more recently, electric field strength [ St.‐Maurice et al , 1989; Foster and Tetenbaum , 1991, 1992; Foster and Erickson , 2000].…”

Application of an imaging HF riometer for the observation of scintillations of discrete cosmic sources

“…The effect is well known in the literature [e.g., Gershman et al, 1984] and has been detected more than once by the writers in HF radar sounding of the field-aligned irregularities stimulated by powerful HF signals . Also, typical examples of the effect are coherent echoes observed when sounding natural ionospheric irregularities in the E region with the use of fully steerable incoherent scatter radars for lines of sight perpendicular to geomagnetic field lines [Foster and Tetenbaum, 1991]. The aspect-sensitive scattering is by tens of decibels more efficient than the isotropic scattering.…”