Ingemar Häggström

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.

Electron energization in the topside auroral ionosphere: on the importance of ion-acoustic turbulence

Abstract EISCAT observations show enhancements of electron temperatures up to 8000 K in the topside ionosphere during active auroral conditions. These temperature enhancements correlate with electron density enhancements in the altitude region 170–230 km, which indicate that auroral electron precipitation in the 100–500 eV range is associated with the electron heating. Electron densities in other altitude intervals show no such correlation and thereby indicate that auroral particles outside the energy range 100–500 eV are not the major regular cause for this bulk electron heating. We also present observations of ion acoustic turbulence in connection with the bulk electron heating events. Such turbulence will give a greatly enhanced Joule heating in the presence of field-aligned currents because of the resulting anomalous resistivity by the turbulently fluctuating electric fields. This heating will occur in addition to the classical collisonal heating by precipitating particles, and is probably a major heat source for the topside ionospheric electrons. In fact, the precipitating electrons in the 100–500 eV range may themselves be runaway suprathermal electrons produced by the ion-acoustic turbulence. An ion-ion two-stream instability is suggested to be the cause of the observed enhanced ion-acoustic fluctuations.

Synoptic observations of auroras along the postnoon oval: a survey with all-sky TV observations at Zhongshan, Antarctica

Abstract All-sky TV data obtained at Zhongshan, Antarctica, have been used to survey auroral displays along the postnoon auroral oval. The auroral occurrence peak around 15 MLT, which was previously shown by satellite observations, is confirmed to exist in ground observations as well. The so called ‘midday gap’ of discrete aurora, however, is not confirmed by ground observations. This survey reveals that the noon region appears to involve another auroral occurrence peak. The noon auroral peak observed from the ground is dominated by an aurora termed as dayside corona in the present study. A dayside corona is usually weak and changing rapidly in its appearance, luminosity and locale. The electron precipitation causing the dayside corona might be too soft, have reduced flux and/or have too rapid a motion of its rayed structures which would result in less luminosity, all of which could account for the ‘midday gap’ in satellite observations. We thus argue for a new synoptic picture of auroral displays along the postnoon oval, in which beside the 15 MLT peak, the noon region is filled with the dayside corona rather than a ‘gap’ in discrete aurora.

Upper atmosphere cooling over the past 33 years

Theoretical models and observations have suggested that the increasing greenhouse gas concentration in the troposphere causes the upper atmosphere to cool and contract. However, our understanding of the long-term trends in the upper atmosphere is still quite incomplete, due to a limited amount of available and well-calibrated data. The European Incoherent Scatter radar has gathered data in the polar ionosphere above Tromso for over 33 years. Using this long-term data set, we have estimated the first significant trends of ion temperature at altitudes between 200 and 450 km. The estimated trends indicate a cooling of 10–15 K/decade near the F region peak (220–380 km altitude), whereas above 400 km the trend is nearly zero or even warming. The height profiles of the observed trends are close to those predicted by recent atmospheric general circulation models. Our results are the first quantitative confirmation of the simulations and of the qualitative expectations.

Near‐Earth substorm onset: A coordinated study

We present simultaneous satellite and ground-based measurements of a substorm. Throughout the initial substorm expansion, southward drifting arcs are observed poleward of the expanding substorm aurora, indicating two independent systems of particle precipitation. Freja passes the brightening onset arc in the topside ionosphere near the moment of the substorm onset, observing an Alfven wave, field aligned current and oxygen ion outflow. The substorm onset occurs at low magnetospheric L-shells, near the poleward edge of the region of trapped particles. The location and time for the substorm injection are confirmed by geostationary spacecraft together with magnetometers, all-sky cameras and radar on the ground. We believe that the substorm onset may be triggered by modification of the oxygen content of the inner magnetosphere during the growth-phase caused by ionospheric ion outflow.

Optical and ionospheric phenomena at EISCAT under continuous X‐mode HF pumping

We present experimental results from multiinstrument observations in the high-latitude ionospheric F2 layer at the EISCAT (European Incoherent Scatter Scientific Association) heating facility. The results come from a set of experiments, when an X-polarized HF pump wave at high heater frequencies (fH > 6.0 MHz) was injected into the F region of the ionosphere toward the magnetic zenith. Experiments were carried out under quiet magnetic conditions with an effective radiated power of 458–548 MW. HF pumping was produced at different heater frequencies, away from electron gyroharmonic frequencies, and different durations of heater pulses. We show the first experimental evidence of the excitation of artificial optical emissions at red (630 nm) and green (557.7 nm) lines in the high-latitude ionospheric F2 layer induced by an X-polarized HF pump wave. Intensities at red and green lines varied in the range 110–950 R and 50–350 R, respectively, with a ratio of green to red line of 0.35–0.5. The results of optical observations are compared with behaviors of the HF-enhanced ion and plasma lines from EISCAT UHF incoherent scatter radar data and small-scale field-aligned artificial irregularities from Cooperative UK Twin Located Auroral Sounding System observations. It was found that the X-mode radio-induced optical emissions coexisted with HF-enhanced ion and plasma lines and strong artificial field-aligned irregularities throughout the whole heater pulse. It is indicative that parametric decay or oscillating two-stream instabilities were not quenched by fully established small-scale field-aligned artificial irregularities excited by an X-mode HF pump wave.

Soliton-induced spectrally uniform ion line power enhancements at the ionospheric F region peak

We present European Incoherent Scatter (EISCAT) observations of spectrally uniform ion line power enhancements (SUIPE), where the up- and downshifted shoulder and the spectral valley between them are enhanced simultaneously and equally. We have identified 48 cases of this type of ion line enhancement in data from the EISCAT Svalbard radar taken during the International Polar Year (extending from March 2007 to the end of February 2008). The SUIPEs are observed at altitudes between 210 km and 280 km with a standard deviation of 9% of the average occurrence height 230 km. The power enhancements are one order of magnitude above the thermal level. The SUIPEs occur at the ionospheric F region density peak with 85% of the cases located within 10 km of the peak. These characteristics are in good agreement with the predictions of a recently published model for soliton-induced ion-line enhancements at the F region peak. The SUIPE occurrence shows a clear preference for magnetically disturbed conditions, with the likelihood of occurrence increasing with increasing K index. A majority of the events occur in the magnetic evening to pre-midnight sector.

Thermospheric winds around the cusp region

An equatorward wind has been observed first by the balloon-borne Fabry-Perot interferometer called High-Altitude Interferometer Wind Observation on the equatorward side of the cusp near the local noon, which is opposite to the typical direction of neutral wind driven by the day-night pressure gradient. However, this dayside equatorward wind was not reproduced by the standard Thermosphere Ionosphere Electrodynamics General Circulation Model under the resolution of 5° longitude by 5° latitude (5°×5°). In this study, the Global Ionosphere Thermosphere Model has been run in different cases and under different resolutions to investigate the neutral dynamics around the cusp region. First, we compare the simulations with and without additional cusp energy inputs to identify the influence of cusp heating. Both runs have a resolution of 5°×1° (longitude × latitude) in order to better resolve the cusp region. After adding in the cusp energy, the meridional wind in simulation turns to be equatorward on the dayside, which is consistent with the observation. It indicates that strong heating in the cusp region causes changes in the pressure gradient around the cusp and subsequent variations in the neutral winds. The simulations with the same cusp heating specifications are repeated, but with different horizontal resolutions to examine the influence of resolution on the simulation results. The comparisons show that the resolution of 5°×1° can resolve the cusp region much more stably and consistently than the 5°×5° resolution.

Seasonal variation and solar activity dependence of the quiet‐time ionospheric trough

We have conducted a statistical analysis of the ionospheric F region trough, focusing on its seasonal variation and solar activity dependence under geomagnetically quiet and moderate conditions, using plasma parameter data obtained via Common Program 3 observations performed by the European Incoherent Scatter (EISCAT) radar between 1982 and 2011. We have confirmed that there is a major difference in frictional heating between the high- and low-latitude sides of the EISCAT field of view (FOV) at ~73°0′N–60°5′N (geomagnetic latitude) at an altitude of 325 km, which is associated with trough formation. Our statistical results show that the high-latitude and midlatitude troughs occur on the high- and low-latitude sides of the FOV, respectively. Seasonal variations indicate that dissociative recombination accompanied by frictional heating is a main cause of trough formation in sunlit regions. During summer, therefore, the occurrence rate is maintained at 80–90% in the postmidnight high-latitude region owing to frictional heating by eastward return flow. Solar activity dependence on trough formation indicates that field-aligned currents modulate the occurrence rate of the trough during the winter and equinox seasons. In addition, the trough becomes deeper via dissociative recombination caused by an increased ion temperature with F10.7, at least in the equinox and summer seasons but not in winter.

Application of alternating codes for EISCAT observations during the ERRRIS campaign for E-region plasma irregularities

Abstract The first extensive use of alternating codes in incoherent scatter experiments took place during the ERRRIS campaign in March and April 1988. Alternating codes provide a very powerful technique which in theory offers time resolution a factor of 5 shorter than is possible with the commonly used multipulse codes for low signal-to-noise ratios. For quiet conditions during the ERRRIS campaign this was confirmed by the remarkable consistency of measured parameters which were adjacent in time or height, indicating a low level of random error even for integration times as short as 45 s. This gave confidence in the value of the parameters measured during disturbed periods when the electron temperature at 110km reached values of several thousand kelvin for short periods, with simultaneous increases in ion temperature at greater heights.