High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment

Abstract. High resolution observations of sporadic-E layers using a new experiment with the EISCAT (European Incoherent SCATter) Svalbard radar (ESR) are presented. The observations were made by means of a new type of hardware, which was connected in parallel with the standard receiver. The radar beam was aligned with the geomagnetic field. The experiment applies a new modulation principle. Two phase codes, one with 22 bits and the other with 5 bits, were transmitted at separate frequencies. Each bit was further modulated by a 5-bit Barker code. The basic bit length of both transmissions was 6 µs. Instead of storing the lagged products of the ionospheric echoes in the traditional way, samples of both the transmitted pulses and the ionospheric echoes were taken at intervals of 1 µs and stored on hard disk. The lagged products were calculated later in an off-line analysis. In the analysis a sidelobe-free Barker decoding technique was used. The experiment produces range ambiguities, which were removed by mathematical inversion. Sporadic-E layers were observed at 105–115 km altitudes, and they are displayed with a 150-m range resolution and a 10-s time resolution. The layers show sometimes complex shapes, including triple peaked structures. The thickness of these sublayers is of the order of 1–2 km and they may be separated by 5 km in range. While drifting downwards, the sublayers merge together to form a single layer. The plasma inside a layer is found to have a longer correlation length than that of the surrounding plasma. This may be an indication of heavy ions inside the layer. The field-aligned ion velocity is also calculated. It reveals shears in the meridional wind, which suggests that shears probably also exist in the zonal wind. Hence the wind shear mechanism is a possible generation mechanism of the layer. However, observations from the coherent SuperDARN radar indicate the presence of an ionospheric electric field pointing in the sector between west and north. Thus, the layer could also be produced by the electric field mechanism. This means that both mechanisms may be active simultaneously. Their relative importance could not be determined in this study. Key words. Ionosphere; polar ionosphere, instruments and techniques

[1]  J. D. WHITEHEAD,et al.  Formation of the Sporadic E Layer in the Temperate Zones , 1960, Nature.

[2]  J. Mathews.  Sporadic E: current views and recent progress , 1998 .

[3]  C. Gardner,et al.  Lidar, radar and airglow observations of a prominent sporadic Na/sporadic E layer event at Arecibo during AIDA-89 , 1993 .

[4]  M. A. Macleod.  Sporadic E Theory. I. Collision-Geomagnetic Equilibrium , 1966 .

[5]  W. Axford.  The formation and vertical movement of dense ionized layers in the ionosphere due to neutral wind shears , 1963 .

[6]  Peter. Dyson,et al.  On the role of electric field direction in the formation of sporadic E-layers in the southern polar cap ionosphere , 1998 .

[7]  Philip J. Erickson,et al.  MIDAS-W: a workstation-based incoherent scatter radar data acquisition system , 2000 .

[8]  T. Hagfors,et al.  Methods for the measurement of vertical ionospheric motions near the magnetic equator by incoherent scattering , 1969 .

[9]  W. Axford,et al.  Vertical movement of temperate‐zone sporadic E layers , 1968 .

[10]  T. Nygrén,et al.  Ion composition in sporadic E layers measured by the EISCAT UHF radar , 1988 .

[11]  L. Smith,et al.  Reflection of radio waves by sporadic-E layers , 1977 .

[12]  Asko Huuskonen,et al.  Fractional lags in alternating codes: Improving incoherent scatter measurements by using lag estimates at noninteger multiples of baud length , 1996 .

[13]  T. Nygrén,et al.  The role of electric field and neutral wind direction in the formation of sporadic E-layers , 1984 .

[14]  William A. Bristow,et al.  Effect of the large-scale convection electric field structure on the formation of thin ionization layers at high latitudes , 1994 .

[15]  Asko Huuskonen,et al.  General incoherent scatter analysis and GUISDAP , 1996 .

[16]  L. Smith,et al.  Incoherent scatter radar observations of irregular structure in mid‐latitude sporadic E layers , 1978 .

[17]  J. Whitehead.  Recent work on mid-latitude and equatorial sporadic-E , 1989 .

[18]  E. A. Mechtly,et al.  Rocket observations of sporadic-E layers. , 1972 .

[19]  William A. Bristow,et al.  Incoherent scatter observations of thin ionization layers at Sondrestrom , 1993 .

[20]  Gudmund Wannberg,et al.  The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design , 1997 .

[21]  Michael P. Sulzer,et al.  Recent incoherent scatter techniques , 1989 .

[22]  P. Perillat,et al.  Aspects of layer electrodynamics inferred from high‐resolution ISR observations of the 80–270 km ionosphere , 1997 .

[23]  J. Whitehead.  Production and prediction of sporadic E , 1970 .

[24]  J. Young,et al.  POSITIVE ION COMPOSITION OF A TEMPERATE-LATITUDE SPORADIC E LAYER AS OBSERVED DURING A ROCKET FLIGHT. , 1967 .

[25]  U. Zahn,et al.  On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers , 1991 .

[26]  L. Smith,et al.  Sporadic-E layers and unstable wind shears , 1980 .