G.O.L. Jones

EISCAT observations of topside ionospheric ion outflows during auroral activity: Revisited

New EISCAT observations of large field-aligned bulk ion outflows from the topside ionosphere during auroral activity are presented. The ions (mainly O+) start their outflows from a variable altitude and may reach field-aligned outward velocities of up to 1500 m s−1 in the altitude region 900–1500 km. The observed ion fluxes are about a factor of 10 larger than previously observed reaching 2×1014 m−2 s−1, and in some cases is nonconstant with altitude. Two different types of ion outflows have been identified. The first type is related to periods of strong perpendicular electric fields, enhanced and anisotropic ion temperatures, and low electron densities below 300 km, indicating small amounts of auroral precipitation. The second type is related to auroral arcs and enhanced electron temperatures. The exact mechanism causing the ion outflows is still not yet understood, but additional mechanisms other than thermal expansion are required to explain the observations presented here.

Regions of strongly enhanced perpendicular electric fields adjacent to auroral arcs

Abstract In a joint campaign involving EISCAT, the Cornell University Portable Radar Interferometer (CUPRI), and sounding rockets, we have observed short-lived elevations of E -region electron temperatures, indicating the presence of strong electric fields. The use of a new pulse-code technique has considerably improved our EISCAT data in regions of low ionospheric electron densities. It has been found that strong and apparently short-lived enhancements of electric fields and associated E - region electron temperatures occur more commonly than long-lived ones. However, earlier EISCAT data with simultaneous optical recordings (and also some CUPRI radar data from the ERRRIS campaign) indicate that many of these events are, in fact, not short-lived, but occur in localized regions and are associated with drifting auroral forms. We show that the observed elevations of electron temperatures are created by very intense electric fields which can be found within narrow regions adjacent to auroral arcs. We discuss our observations against the background of models for electric field suppression or enhancement in the vicinity of auroral precipitation.

The measured relationship between electric field strength and electron temperature in the auroral E-region

When strong electric fields are applied to the auroral E-region they cause a modified two-stream instability which heats the electron population. The theory of electron-plasmon collisions [Robinson (1986) J. atmos. terr. Phys. 48, 417] predicts the relationship between the strength of the applied field and the corresponding increase in electron temperature at different heights. This relationship is non-linear and to make a valid comparison of the predicted increases in Te with the values observed accurate measurements must be made with a time resolution which matches the rapid variations in field strength. The measurements must also be corrected for the effects of neutral velocity, and care must be taken to minimise statistical bias when the results are averaged, including the effect of small errors in the assumed direction of the magnetic field. Results from EISCAT using alternating codes show a very good agreement between theory and observation. However, the significance of this comparison is limited by uncertainties in the assumed values of the electron-neutral collision frequency and the electron cooling rate.

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Abstract Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of 1. (i) a southward neutral wind 2. (ii) a vertical wind driven by Joule heating 3. (iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

Large field-aligned velocities observed by EISCAT

Abstract Large upward ion fluxes are observed near the polar cap/auroral zone boundary in the midnightdawn sector. Results from a recent model show that large upward velocities will occur in response to strong frictional heating and these results are consistent with the observations presented in this paper. There is particularly good agreement between the spatial and temporal morphology of the heating and the upward flux of ions which favours this mechanism to explain the observations. However, the time and location are also those associated with other acceleration mechanisms which cause upward motion of energised ions and these may also contribute to the large velocities observed.

High-resolution measurements of magnetospheric electric fields

Abstract Observations made at EISCAT suggest that the plasma velocity measured in the F-region above Tromso can vary substantially on a timescale of a minute or so. The high-resolution measurements made using alternating codes during the ERRRIS experiment have confirmed this result by showing that the rapid variations of plasma velocity measured directly correspond exactly to the variations of ion temperature in the rmupper-E and lower-F region caused by frictional heating, and the variations of electron temperature in the E-region, caused by wave turbulence heating.

Measuring ion temperatures and studying the ion energy balance in the high-latitude ionosphere

Abstract Data are presented for a nighttime ion heating event observed by the EISCAT radar on 16 December 1988. In the experiment, the aspect angle between the radar beam and the geomagnetic field was fixed at 54.7°, which avoids any ambiguity in derived ion temperature caused by anisotropy in the ion velocity distribution function. The data were analyzed with an algorithm which takes account of the non-Maxwellian line-of-sight ion velocity distribution. During the heating event, the derived spectral distortion parameter ( D ∗ ) indicated that the distribution function was highly distorted from a Maxwellian form when the ion drift increased to 4 km s −1 . The true three-dimensional ion temperature was used in the simplified ion balance equation to compute the ion mass during the heating event. The ion composition was found to change from predominantly O 4 to mainly molecular ions. A theoretical analysis of the ion composition, using the MSIS86 model and published values of the chemical rate coefficients, accounts for the order-of-magnitude increase in the atomic/molecular ion ratio during the event, but does not successfully explain the very high proportion of molecular ions that was observed.

Observations of large field-aligned flows of thermal plasma in the auroral ionosphere

Large upward field-aligned ion flows have previously been observed in the high latitude ionosphere in response to frictional heating of the local ion population. Results from a recent experiment using the EISCAT radar show similar features but allow, for the first time, determination of the field-aligned profiles of plasma parameters during these events. The upflows occur during frictional heating. The flows are shown to be transient plasma upwellings, from regions where the ion temperature has been elevated by the motion of a convection shear over the observed field line.

Methods of measuring plasma velocity with EISCAT

Abstract Using the EISCAT system it is possible to determine the total vector of plasma velocity—and hence the electric field—both in the “monostatic” and “tristatic” mode. Results are presented for the evening of 18 May 1982 during the reversal of plasma velocity from westward to eastward. A comparison demonstrates that the random errors in the measured velocity are smallest in the monostatic mode using data taken at Tromso, where the signal-to-noise ratio is highest. There is, however, a systematic error in the monostatic measurements due to horizontal gradients in plasma velocity.

A statistical study of polar mesosphere summer echoes observed by EISCAT

A comprehensive survey of data on ‘Polar Mesosphere Summer Echoes’, observed by the EISCAT VHF radar during 1988–1993, confirms that (1) these echoes are a summer phenomenon, with a season lasting from June to August; (2) PMSE occur most frequently around noon and midnight, and thus follow a semidiurnal pattern; (3) PMSE occur at a mean height of 85±2 km; (4) there is often a good correlation between the vertical Doppler velocity and the rate of change of echo height, which suggests that the echoing structures move bodily, perhaps in response to gravity waves. Previous results on the lack of correlation between the occurrence of PMSE and noctilucent clouds are reinforced.