Gudmund Wannberg

Meteor observations with the European Incoherent Scatter UHF Radar

The European Incoherent Scatter (EISCAT) UHF radar, which operates at a nominal frequency of 930 MHz, is introduced as a powerful meteor radar. Its high sensitivity is utilized to detect transient enhanced ionization trails caused by meteors of all orientations, in contrast to conventional HF and VHF backscatter radars, which observe only the meteor trails oriented approximately normal to the radar beam. A comparison shows that EISCAT observes almost as high hourly rates of meteors as meteor radars do, in spite of its beamwidth being only 0.5° compared to their 100°. Two different kinds of echoes are seen in the data. About three fourths of them are strongly Doppler-shifted transient echoes, which we interpret as head echoes, since they move with meteor velocities. The remaining echoes are long-lived. They come from the ionized trails left behind the meteors, which drift slowly or not at all. There are fundamental differences between the scattering process producing both types of echoes and the scattering from trails observed with the meteor radars. For example, we have never seen the trail echoes appear after a head echo. All our shower echoes are also strongly nonspecular. We postulate that some time is required for the trails to expand and for the ionization within them to approach the thermal equilibrium state in order to give detectable incoherent echoes. Due to the very small beamwidth our radar cannot follow the motion of an individual meteor, as the conventional radars can, but we can obtain a statistical profile of what happens at different heights as the meteoroids penetrate the atmosphere. The altitude distribution of ionized trails shows that they are observable over the whole measurement range from about 65 to 165 km, while the head echoes can be detected only within a narrow altitude range. The latter effect is probably related to the radio meteor ceiling effect.

Three‐dimensional radar observation of a submillimeter meteoroid fragmentation

A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration. This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust. The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle. A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg.

On the meteoric head echo radar cross section angular dependence

We present radar cross section (RCS) measurements of meteor head echoes observed with the tristatic 930 MHz EISCAT UHF radar system. The three receivers offer a unique possibility to accurately com ...

Enhanced ion-acoustic echoes from meteor trails

Abstract Some spectral echoes with short-living asymmetric ion-acoustic peaks, observed with the EISCAT UHF radar, are interpreted to be of meteoric origin. Under certain conditions meteor trails can scatter radio waves incoherently. Already a meteor of about 3 mm size causes enough ionisation that a diffusing trail after tens of seconds still can have enhanced ionisation. By this time the expansion rate of the trail radius per experiment integration time is small compared to the total radius. The enhanced plasma inside the trail approaches thermal equilibrium and can sustain ion-acoustic waves. A qualitative model for incoherent scatter from meteor trails is proposed, and a few examples of trail echoes are presented. They show strongly asymmetric incoherent ion-line spectra, which we assume to result from the Doppler shift caused by heavy meteoric ions in the drifting trail. The relation to other observations of enhanced ion-acoustic waves in connection to auroral displays is discussed. The applicability of the proposed model to the data is evaluated.

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.

The EISCAT_3D arbitrary waveform exciter and polyphase constant amplitude codes for EISCAT VHF and ESR D/E layer experiments

The performance specifications for the EISCAT_3D third-generation phased-array incoherent-scatter (IS) radar mandate a transmitter subsystem with full arbitrary-waveform capabilities and a power bandwidth of more than 5 MHz. An exciter meeting these requirements, based on the AD9957 digital up-converter chip, is currently under development at the Swedish Institute of Space Physics. An overview of the 3D exciter system and the AD9957 and its capabilities, with emphasis on its performance in an IS system, is given. To verify the capabilities of the AD9957 in a real radar environment, two polyphase codes with good D- and E-layer performance have been developed, viz. a QPSK-coded eight-element set of complementary [A, B] sequences and a complementary-coded set of 71-baud Chu sequences. Features of the two codes are described; results from on-the-air tests will be shown.

A Critical Ionization Velocity Experiment on the ARGOS Satellite

Abstract : We report on a xenon gas release experiment conducted on the Advanced Research and Global Observations (ARGOS) Satellite in the F-region ionosphere above the European Incoherent Scatter (EISCAT) radar at Tromso, Norway, Oct 20, 2000. In this experiment, xenon gas was released in the ram direction of the satellite. This was intended to induce ionization through the critical ionization velocity (CIV) process proposed by Alfven in his theory of the formation of the planets in the solar system. If the CIV process had been operational and efficient, ionization of the xenon cloud might have been observed. Radar observations by EISCAT showed no detectable enhancement of the ambient plasma in the velocity of the satellite. We present a simple model calculation which predicts that the overall yield of xenon ions in the release would be low, owing merely to the initially high density of the rapidly expanding xenon cloud.