Assar Westman

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 ...

Tristatic EISCAT‐UHF measurements of the HF modified ionosphere for low background electron temperatures

The effects of a powerful high-frequency electromagnetic wave transmitted in the ordinary mode from the European Incoherent Scatter (EISCAT) ionospheric modification facility up to the daytime ionospheric F region have been studied with the EISCAT-UHF radar system. This is one of the first tristatic EISCAT-UHF incoherent scatter experiments, in which the excited Langmuir and ion acoustic fluctuations are measured simultaneously at scattering angles near parallel (Tromso site), ∼22° (Kiruna site), and ∼31° (Sodankyla site) to the geomagnetic field. We present measurements of the spectral structure and temporal evolution with a maximum resolution of 13 ms, at the different scattering angles. For the first time, up to five cascade lines in addition to the decay line in the Tromso HF-enhanced plasma line and well-developed cascade type plasma lines in Sodankyla were observed. In spite of low background electron temperature, and hence negligible electron Landau damping parallel to the geomagnetic field, the excited plasma turbulence varied considerably.

Strong E region ionization caused by the 1767 trail during the 2002 Leonids

Intensive E region ionization extending up to 140 km altitude and lasting for several hours was observed with the European Incoherent Scatter (EISCAT) UHF radar during the 2002 Leonids meteor shower maximum. The level of global geomagnetic disturbance as well as the local geomagnetic and auroral activity in northern Scandinavia were low during the event. Thus, the ionization cannot be explained by intensive precipitation. The layer was 30–40 km thick, so it cannot be classified as a sporadic E layer which are typically just a few kilometers wide. Incoherent scatter radars have not to date reported any notable meteor shower-related increases in the average background ionization. The 2002 Leonids storm flux, however, was so high that it might have been able to induce such an event. The Chemical Ablation Model is used to estimate deposition rates of individual meteors. The resulting electron production, arising from hyperthermal collisions of ablated atoms with atmospheric molecules, is related to the predicted Leonid flux values and observed ionization on 19 November 2002. The EISCAT Svalbard Radar (ESR) located at some 1000 km north of the UHF site did not observe any excess ionization during the same period. The high-latitude electrodynamic conditions recorded by the SuperDARN radar network show that the ESR was within a strongly drifting convection cell continuously fed by fresh plasma while the UHF radar was outside the polar convection region maintaining the ionization.

E region ionization enhancement over northern Scandinavia during the 2002 Leonids

Intensive E-region ionization was observed with the EISCAT UHF radar during the 2002 Leonids meteor shower. The levels of the geomagnetic disturbance were low during the event. Thus the ionization cannot be explained by intensive precipitation. The layer was 30-40 km thick, so it cannot be classified as a sporadic E-layer (often associated to ions of meteoric origin). These are typically only about km-wide. Incoherent scatter radars have never so far reported any notable meteor shower-related increases in the average background ionization. The 2002 Leonid storm flux, however, was so high that it, if any, might be able to induce such an event. Whether meteors in general can cause such an excess E-region ionization during an intensive shower is discussed. The University of Leeds CABMOD model is used to estimate deposition rates of individual meteors and to relate the results to the predicted Leonid flux values in free space and observed ionization on November 19, 2002.

IPY 2007–2008 EISCAT Svalbard radar event studies for the MISW FP7 project

Satellite navigation at high latitudes suffers from disturbances (scintillation) caused by the structured and rapid ionospheric electron density variations caused by magnetospheric particle precipitation and solar energetic particle events. The MISW FP7 project (MItigation of Space Weather threats to GNSS services) aims at a better understanding of such effects and their mitigation.

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.

The EISCAT High Power HF Radar Capability

The EISCAT HF facility or ionospheric heater near Tromsø, Norway, has recently been upgraded with the capability to function as a radar. One of the antenna arrays has been converted for receiving use, covering the frequency range 5.5-8 MHz. The transmitters now have direct digital synthesizers and are controlled by the same hardware and software systems used by the other EISCAT radars. With the new two-channel HF receiver the facility is capable of operating simultaneously as a heater and a HF radar.

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.