Field-aligned Observations Research Articles

Ionospheric altitude profiles in the main ionospheric trough as observed by field-aligned EISCAT incoherent scatter radar observations

EISCAT field-aligned observations for winter and equinoctial seasons under quiet geomagnetic conditions were used to study electron concentration Ne(h) distribution in the evening sector of the main ionospheric trough. Both morphological results and physical interpretation are considered. A typical variation of the F2-layer maximum height is hmF2 increase in the trough minimum followed by hmF2 sharp decrease in the polar wall of the trough and in the auroral oval. The effective scale height Heff of plasma distribution in the topside F2-region increases in the trough minimum. Specific Ne(h) profiles with Heff decreasing with height observed in the trough minimum are due to upward flow of plasma from the F2-region. Such Ne(h) distributions are associated with narrow deep troughs, but only if the trough is formed in non-sunlit conditions.

Plasma convection at high latitudes using the EISCAT VHF and ESR incoherent scatter radars

Abstract. The recent availability of substantial data sets taken by the EISCAT Svalbard Radar allows several important tests to be made on the determination of convection patterns from incoherent scatter radar results. During one 30-h period, the Svalbard Radar made 15 min scans combining local field aligned observations with two, low elevation positions selected to intersect the two beams of the Common Programme Four experiment being simultaneously conducted by the EISCAT VHF radar at Tromsø. The common volume results from the two radars are compared. The plasma convection velocities determined independently by the two radars are shown to agree very closely and the combined three-dimensional velocity data used to test the common assumption of negligible field-aligned flow in this regime.Key words: Ionosphere (auroral ionosphere; polar ionosphere) - Magnetospheric physics (plasma convection)

Plasma convection at high latitudes using the EISCAT VHF and ESR incoherent scatter radars

The recent availability of substantial data sets taken by the EISCAT Svalbard Radar allows several important tests to be made on the determination of convection patterns from incoherent scatter radar results. During one 30-h period, the Svalbard Radar made 15 min scans combining local field aligned observations with two, low elevation positions selected to intersect the two beams of the Common Programme Four experiment being simultaneously conducted by the EISCAT VHF radar at Tromsø. The common volume results from the two radars are compared. The plasma convection velocities determined independently by the two radars are shown to agree very closely and the combined three-dimensional velocity data used to test the common assumption of negligible field-aligned flow in this regime.Key words: Ionosphere (auroral ionosphere; polar ionosphere) - Magnetospheric physics (plasma convection)

A statistical study of diurnal, seasonal and solar cycle variations of F-region and topside auroral upflows observed by EISCAT between 1984 and 1996

Abstract. A statistical analysis of F-region and topside auroral ion upflow events is presented. The study is based on observations from EISCAT Common Programmes (CP) 1 and 2 made between 1984 and 1996, and Common Programme 7 observations taken between 1990 and 1995. The occurrence frequency of ion upflow events (IUEs) is examined over the altitude range 200 to 500 km, using field-aligned observations from CP-1 and CP-2. The study is extended in altitude with vertical measurements from CP-7. Ion upflow events were identified by consideration of both velocity and flux, with threshold values of 100 m s–1 and 1013 m–2 s–1, respectively. The frequency of occurrence of IUEs is seen to increase with increasing altitude. Further analysis of the field-aligned observations reveals that the number and nature of ion upflow events vary diurnally and with season and solar activity. In particular, the diurnal distribution of upflows is strongly dependent on solar cycle. Furthermore, events identified by the velocity selection criterion dominate at solar minimum, whilst events identified by the upward field-aligned flux criterion dominated at solar maximum. The study also provides a quantitative estimate of the proportion of upflows that are associated with enhanced plasma temperature. Between 50 and 60% of upflows are simultaneous with enhanced ion temperature, and approximately 80% of events are associated with either increased F-region ion or electron temperatures.Key words. Ionosphere (auroral ionosphere; particle acceleration)

A statistical study of diurnal, seasonal and solar cycle variations of F-region and topside auroral upflows observed by EISCAT between 1984 and 1996

<strong class="journal-contentHeaderColor">Abstract.</strong> A statistical analysis of F-region and topside auroral ion upflow events is presented. The study is based on observations from EISCAT Common Programmes (CP) 1 and 2 made between 1984 and 1996, and Common Programme 7 observations taken between 1990 and 1995. The occurrence frequency of ion upflow events (IUEs) is examined over the altitude range 200 to 500 km, using field-aligned observations from CP-1 and CP-2. The study is extended in altitude with vertical measurements from CP-7. Ion upflow events were identified by consideration of both velocity and flux, with threshold values of 100 m s^–1 and 10¹³ m^–2 s^–1, respectively. The frequency of occurrence of IUEs is seen to increase with increasing altitude. Further analysis of the field-aligned observations reveals that the number and nature of ion upflow events vary diurnally and with season and solar activity. In particular, the diurnal distribution of upflows is strongly dependent on solar cycle. Furthermore, events identified by the velocity selection criterion dominate at solar minimum, whilst events identified by the upward field-aligned flux criterion dominated at solar maximum. The study also provides a quantitative estimate of the proportion of upflows that are associated with enhanced plasma temperature. Between 50 and 60% of upflows are simultaneous with enhanced ion temperature, and approximately 80% of events are associated with either increased F-region ion or electron temperatures.<br><br><b>Key words.</b> Ionosphere (auroral ionosphere; particle acceleration)

The BEAN experiment - An EISCAT study of ion temperature anisotropies

Abstract. Results are presented from a novel EISCAT special programme, SP-UK-BEAN, intended for the direct measurement of the ion temperature anisotropy during ion frictional heating events in the high-latitude F-region. The experiment employs a geometry which provides three simultaneous estimates of the ion temperature in a single F-region observing volume at a range of aspect angles from 0° to 36°. In contrast to most previous EISCAT experiments to study ion temperature anisotropies, field-aligned observations are made using the Sodankylä radar, while the Kiruna radar measures at an aspect angle of the order of 30°. Anisotropic effects can thus be studied within a small common volume whose size and altitude range is limited by the radar beamwidth, rather than in volumes which overlap but cover different altitudes. The derivation of line-of-sight ion temperature is made more complex by the presence of an unknown percentage of atomic and molecular ions at the observing altitude and the possibility of non-Maxwellian distortion of the ion thermal velocity distribution. The first problem has been partly accounted for by insisting that a constant value of electron temperature be maintained. This enables an estimate of the ion composition to be made, and facilitates the derivation of more realistic line-of-sight ion temperatures and temperature anisotropies. The latter problem has been addressed by assuming that the thermal velocity distribution remains bi-Maxwellian. The limitations of these approaches are discussed. The ion temperature anisotropies and temperature partition coefficients during two ion heating events give values intermediate between those expected for atomic and for molecular species. This result is consistent with an analysis which indicates that significant proportions of molecular ions (up to 50%) were present at the times of greatest heating.

Field aligned observations of transequatorial bubbles from Rarotonga in 1969‐70

A raster scanning photometer with 0.5° field of view was used in 1969‐70 to make images in 6300 Å and 5577 Å emission from Rarotonga of tropical F region trans‐equatorial ‘bubbles’ or ‘depletions.’ From this site the F region plasma structures in the emission layer can be viewed in a magnetic field aligned configuration. A spatial resolution of approximately 15 km was achieved, and revealed a variety of shapes in the bubble cross sections, including ‘bifurcations’ and ‘pinched off’ bubbles. Eastward drift velocities averaged 125 ms−1. Tilts were usually westward at about 45° in the equatorial plane, but some were near zero and some were eastward. The tops reached apex heights of over 800 km. The structures appeared a little less diffuse in 5577 Å emission than in 6300 Å owing to less diffusion of O¹S with its 0.75 sec. lifetime, than O¹D with its 150 sec. lifetime.