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Radar detection of interplanetary shocks: scattering by anisotropic Langmuir turbulence
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
2010 (English)In: Advances in Space Research, ISSN 0273-1177, Vol. 45, no 6, 804-811 p.Article in journal (Refereed) Published
Abstract [en]

Earth-directed interplanetary shocks associated with coronal mass ejections (CMEs) are known to have a severe impact on the magnetosphere, causing strong geomagnetic storms and substorms. Hence, early detection of such shocks is important. Here we study the feasibility of radar detection of interplanetary shocks. We consider a scattering mechanism, which is based on the induced scattering t + l ⇄ t of a radar wave by anisotropic Langmuir turbulence, being generated by the shock-accelerated electrons. The problem is studied for an arbitrary angle between the electron beam and the incident radar wave, vb kt, and special emphasis is put on the study of a dependence of the scattering process on this angle. We obtain and analyze analytical expressions for the frequency shift, scattering cross-section of the turbulence, coefficient of absorption (due to scattering), and the optical depth. We show that the detection of such shocks is possible if the turbulence energy density exceeds W = 10- 5 nT (nT is the thermal energy density of a plasma), which is quite realistic according to our estimations. The altitudes in the solar corona where reflections may occur depend on the angle vb kt. If expressed in local plasma frequencies, ωp, the altitudes span is ωt / 8 ≲ ωp ≤ ωt for vb kt = π and ωt / 120 ≲ ωp ≤ ωt for vb kt = π / 2, where ωt is a frequency of the transmitted radar wave. Thus the scattering occurs much closer to the radar in the second case than in the first. Detection of the scattered signal, in the general case, requires a remote receiver, since the radar wave backscatters only for vb kt = π.

Place, publisher, year, edition, pages
2010. Vol. 45, no 6, 804-811 p.
Keyword [en]
Coronal mass ejection, Plasma turbulence, Radar, Scattering of electromagnetic radiation in plasmas, Solar radar, Space weather
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-94977DOI: 10.1016/j.asr.2009.12.005ISI: 000275974300011OAI: oai:DiVA.org:uu-94977DiVA: diva2:169018
Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2010-12-14Bibliographically approved
In thesis
1. Radar Probing of the Sun
Open this publication in new window or tab >>Radar Probing of the Sun
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is dedicated to the theory of solar radar experiments. The Sun exhibits a variety of interesting and complicated physical phenomena, examined mainly through analysis of its radiation. Active solar probing by radar provides an alternative possibility to study the Sun. This concept was tested originally in the 1960's by solar radar experiments at El Campo, Texas, but due to an insufficient level of technology at that time the experimental results were of a poor quality and thus difficult to interpret. Recently, the space weather program has stimulated interest in this topic. New experimental proposals require further development of the theory of solar radar experiments to meet the current knowledge about the Sun and the modern level of technology.

Three important elements of solar radar experiments are addressed in this thesis: i) generation of wave turbulence and radiation in the solar corona, ii) propagation of the radar signal to the reflection point, and iii) reflection (scattering) of the incident radar signal from the Sun.

It is believed that the radio emission of solar type II and III bursts occurs due to conversion of Langmuir waves, generated by electron beams, into electromagnetic radiation (plasma emission mechanism). The radar signal propagating through the emission source region can get scattered by the Langmuir turbulence and finally deliver the observer insights of the physics of this turbulence. Such process of scattering is considered in this thesis in the weak turbulence limit by means of the wave-kinetic theory. Scattering frequency shifts, scattering cross-sections, efficiency of scattering (the coefficient of absorption due to scattering), optical depths, and the spectra of the scattered signal are estimated.

Type II solar radio bursts are known to be associated with the electron beams accelerated by interplanetary shocks. From their dynamic spectra the properties of the shocks and regions in the vicinity of the shock are usually inferred by assuming a plasma emission mechanism. In situ observations of the source region of type II burst, presented in this thesis, suggest that an additional emission mechanism may be present. This mechanism is related to energetic particles crossing the shock front, known in electrodynamics as transition radiation.

Plasma density fluctuations are known to scatter radio waves and thus broadening their angular dispersion. In the thesis this process is studied in the solar wind and terrestrial electron and ion foreshocks on the basis of in situ observations of density fluctuations. It is shown that the angular broadening of the radar signal is negligible in this regions.

The results of this thesis can be applied for the preparation of future solar radar experiments and interpretation of experimental data.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 83 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 231
Space and plasma physics, solar radar, active experiments, space physics, solar corona, plasma, Rymd- och plasmafysik
urn:nbn:se:uu:diva-7192 (URN)91-554-6681-8 (ISBN)
Public defence
2006-11-04, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:00
Available from: 2006-10-06 Created: 2006-10-06Bibliographically approved

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Department of Astronomy and Space PhysicsDepartment of Physics and AstronomySwedish Institute of Space Physics, Uppsala Division
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