Universität zu Köln

Efimov, A. I., Lukanina, L. A., Rogashkova, A. I., Samoznaev, L. N., Chashei, I. V., Bird, M. K. and Paetzold, M. (2015). Coronal Radio Occultation Experiments with the Helios Solar Probes: Correlation/Spectral Analysis of Faraday Rotation Fluctuations. Sol. Phys., 290 (9). S. 2397 - 2409. DORDRECHT: SPRINGER. ISSN 1573-093X

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Abstract

The coronal Faraday rotation (FR) experiments using the linearly polarized signals of the Helios-1 and Helios-2 interplanetary probes remain a unique investigation of the magnetic field of the solar corona and its aperiodic and quasi-periodic variations. The unexpectedly long lifetime of these spacecraft (1974 -aEuro parts per thousand 1986) enabled studies from very deep solar-activity minimum (1975 -aEuro parts per thousand 1976) into the strong activity maximum (1979). Important experimental data were also obtained for the rising (1977 -aEuro parts per thousand 1978) and declining (1980 -aEuro parts per thousand 1984) branches of the solar-activity cycle. Previous publications have presented results of the initial experimental data only for coronal-sounding experiments performed during individual solar-conjunction opportunities. This report is a more detailed analysis of the Helios FR measurements for the entire period 1975 -aEuro parts per thousand 1984. Radial profiles of the FR fluctuation (FRF) intensity recorded during the deepest solar-activity minimum in 1975 -aEuro parts per thousand 1976 are shown to differ distinctly from those during the strong solar-activity maximum in 1979. In particular, the decrease of the FRF intensity with solar-offset distance is substantially steeper in 1979 than in 1975/1976. In all cases, however, the FR data reveal quasi-periodic wave-like fluctuations in addition to the random background with a power-law spectrum. The dominant period of these fluctuations, recorded during 35 % of the total measurement time, is found to be close to five minutes. Large-scale FR variations at considerably longer periods (1.1 -aEuro parts per thousand 2.7 hours) were observed during 20 % of the measurement time. Knowing the intrinsic motion of the radio ray path from spacecraft to Earth and making a reasonable assumption about the solar-wind velocity, FRF observations at widely spaced ground stations have been used to estimate the velocity of coronal Alfv,n waves. The velocity values range between 290 and 550 km s(-1) at heliocentric distances between 3.5 and 4.5 R-aS (TM) and are marginally lower (150 -aEuro parts per thousand 450 km s(-1)) at distances between 5.5 and 6.5 R-aS (TM). Occasional FR variations with a period near 160 minutes and harmonics with periods 60, 30, and 20 minutes were also observed.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Efimov, A. I. UNSPECIFIED UNSPECIFIED UNSPECIFIED Lukanina, L. A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rogashkova, A. I. UNSPECIFIED UNSPECIFIED UNSPECIFIED Samoznaev, L. N. UNSPECIFIED UNSPECIFIED UNSPECIFIED Chashei, I. V. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bird, M. K. UNSPECIFIED UNSPECIFIED UNSPECIFIED Paetzold, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-393738
DOI:	10.1007/s11207-015-0687-y
Journal or Publication Title:	Sol. Phys.
Volume:	290
Number:	9
Page Range:	S. 2397 - 2409
Date:	2015
Publisher:	SPRINGER
Place of Publication:	DORDRECHT
ISSN:	1573-093X
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/39373