The Helios Faraday Rotation Data Archive
M.K. Bird, H. Volland , G.S. Levy, C.T. Stelzried, B.L. Seidel† and A.I. Efimov,
V.E. Andreev, L.N. Samoznaev
Radioastronomisches Institut, Universität Bonn, 53121 Bonn, Germany
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Inst. for Radio Engineering & Electronics, Russian Academy of Science, Moscow, 101999, Russia
†
Abstract. The Helios Faraday Rotation (FR) Experiment, a passive radio science investigation requiring no on-board
hardware other than the existing spacecraft radio subsystem, was designed to study the dynamic and quiescent structure
of the magnetic fields and electron density in the solar corona. Measurements of coronal Faraday rotation were derived from
the linearly polarized S-band downlink carrier signal, which probed otherwise inaccessible regions of the corona in the radial
range from 2 to 15 solar radii during the regularly recurring solar conjunctions. More than 1250 hours of Helios FR data were
recorded over the duration of the Helios 1 (1974-84) and Helios 2 (1976-80) missions. The time scales of FR variations provide
information on various physical phenomena: (a) slowly-varying rise and fall associated with the changing ray path offset,
combined with the rotation of the quasi-static corona; (b) ubiquitous random oscillations with higher fluctuation amplitude
at smaller solar offset distances, probably caused by coronal Alfvén waves; (c) occasional nearly discontinous jumps in the
polarization angle, most likely caused by transient events such as coronal mass ejections (CMEs). The Helios FR data, aspects
of which have been reported in more than forty publications to date, have now been systematically collected in a data archive
for public dissemination. A brief review of the main results of the Helios FR Experiment are presented, together with some
suggestions for possible use of the archive for continued solar wind research.
INTRODUCTION
The primary goal of the Helios Faraday Rotation experiment was to investigate the magnetic field B in the solar
corona (Volland et al., 1977, 1984; Levy et al., 1980; Bird
and Edenhofer, 1990). The passive radio science experiment required no on-board hardware other than the existing spacecraft radio communications subsystem. Helios
polarization angle (FR) data, recorded using the linearly
polarized S-band downlink carrier signal during times of
superior conjunction, have been made available in an online archive. Selected results gleaned from the unique
Helios FR data set with relevance for the coronal magnetic field and MHD-wave environment, as well as some
possible future uses of the archive, are presented.
EXPERIMENT DESCRIPTION
A significant component of linear polarization is necessary for recording FR measurements with spacecraft radio signals. Only Pioneers 6 and 9 had been used for this
purpose prior to Helios (Levy et al., 1969; Stelzried et al.,
1970). In the more recent era, the Galileo spacecraft was
optimally equipped with dual-frequency linear polarization (Howard et al., 1990), but prospects for conducting
FR experiments were dashed prior to solar conjunction
when the high gain antenna could not be deployed. The
Helios FR data volume thus remains the largest collection of radio sounding data sensitive to the coronal magnetic field in the region of the solar wind transition to
supersonic velocities.
Linear polarized electromagnetic waves propagating
through a magnetized plasma undergo ‘Faraday rotation’
of their plane of polarization, which can be used to deduce the magnetic structure of the propagation medium.
For the case of the cylindrical despun Helios antenna,
the electric vector of the transmitted linearly polarized
carrier signal was oriented perpendicular to the ecliptic. Measurements of the coronal contribution to signal
Faraday rotation (Ω) were obtained by tracking the received signal polarization angle q, referenced to the local
horizon, and transforming this value back to the ecliptic frame from the known parallactic angle p, as shown
schematically in Fig. 1. Coronal FR is related to the longitudinal component of the magnetic field along the ray
path Bs B ŝ and electron density Ne by
Ω
K
f2
H
Ne s Bs s ds radians
where K 236 10 4 in both MKS and cgs units, f is the
radio frequency (2.296 GHz), and the integral in Eq. (1)
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
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(1)
rate to decrease from 14 Æ per year (Fig. 1, right) to 6 Æ
per year for Helios 2. From its launch on 15 Jan 1976,
Helios 2 was tracked regularly until it ceased operating
on 3 Mar 1980 (formal EOM: 8 Jan 1981).
AREAS OF INVESTIGATION
FIGURE 1. Polarization angle reference in local and ecliptic
systems, as viewed from an observer looking toward Helios.
is taken along the ray path from Helios (H) to the ground
station on Earth (), assumed to be along a straight line.
The quantity Ω can be plus or minus depending on the
integrated product of electron density and component of
magnetic field along the ray path (quasi-longitudinal approximation). The direction of rotation is called positive
(counterclockwise) when the magnetic field points toward the observer, as sketched in Fig. 1, and negative
(clockwise) when directed away from the observer.
The short-period, zero-inclination, heliocentric orbits
of the two identical Helios spacecraft were ideally designed for conducting radio science investigations of the
solar corona during the recurring solar occultations. Helios 1, with an original design lifetime of 18 months, was
launched 10 Dec 1974 and underwent a total of 10 solar
conjunctions before its demise in 1985. The official end
of mission (EOM) was declared as 15 Mar 1986.
Fig. 2 shows an ecliptic plane view of the Helios 1 orbit in a Sun/Earth fixed system during the first year after
launch (left panel) and for all orbits through the end of
September 1981 (right panel). The shaded wedge-shaped
area denotes the region of space where interplanetary
spacecraft attain a solar elongation within 3 Æ , roughly the
region where coronal Faraday rotation of an S-band radio signal becomes measureable. Dots are placed at 10
day intervals along the trajectory. Although most solar
occultations proceeded diametrically along a solar radial
from West to East limb, an occultation near spacecraft
aphelion, such as the very first solar conjunction shown
in Fig. 2 (left panel), could extend over several weeks.
The counterpart of Fig. 1 for Helios 2 is very similar,
except that the orbital perihelion was lowered to 0.29 AU
from the 0.31 AU of Helios 1. The associated change in
the orbital period caused the clockwise orbital precession
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The time scales of FR variations, observed during virtually all occultations, provide information on various
physical phenomena in the solar corona:
Slowly-varying background. The slow rise and fall in
FR is associated with the changing ray path offset, combined with the rotation of the quasi-static corona. Fig. 3
shows such slow FR variations observed at solar offsets R 3 R near and during the overlap interval of
two tracking stations (Volland et al., 1977). Pätzold et
al. (1987) utilized the absolute FR measurements from
1975-76 for a determination of the mean coronal magnetic field BR during solar minimum. A more general
study using all FR data to study BR at other solar cycle
phases would be a valuable extension of this work.
Random oscillations. This ubiquitous phenomenon,
with higher FR fluctuation amplitude at smaller solar
offset distances, is probably caused by coronal Alfvén
waves (Hollweg et al., 1982; Efimov et al., 1993, 2000;
Andreev et al., 1996, 1997a,b). Fig. 4 shows simultaneous recordings at two tracking stations during an interval
of quite obvious wavelike FR fluctuations. A preliminary
study determined that ca. 30% of the velocities determined from two-station cross correlations were directed
radially inward toward the Sun (Bird et al., 1992). A
more exhaustive study using the entire FR data set should
be undertaken to confirm these results.
FR discontinuities. These occasional abrupt jumps in
the polarization angle are assumed to be caused by transient events such as coronal mass ejections (CMEs). A
search for white-light CMEs during the 1979 Helios solar conjunctions yielded 5 distinct events which produced
obvious abrupt FB signatures associated with their passage through the signal ray path (Bird et al., 1985). These
‘FR transients’ were first seen at solar maximum (Levy
et al., 1969), even before their optical counterparts were
widely publicized from the Skylab observations. Not restricted to epochs of solar activity maximum, an example from 1976 presented by Bird et al. (1977) is shown
in Fig. 5. Some recent work has associated the FR transients with interplanetary sector boundaries (Woo, 1997),
rather than CMEs (Pätzold and Bird, 1998). The Helios
FR archive offers by far the largest data source for pursuit
of this controversy.
In summary, the Helios Faraday rotation experiment
provided valuable clues about the quiet and disturbed
magnetic structure of the solar corona in the critical
FIGURE 2.
Helios 1 orbit in a Sun/Earth fixed system during the first year (left) and through the end of 1981 (right).
FIGURE 3. Overlapping FR measurements at Effelsberg and
Goldstone during the second solar conjunction of Helios 1
(from Volland et al., 1977).
FIGURE 4. Simultaneous two station Faraday rotation measurements. The constant offset between the values of FR at DSS
14 and 43 is due to different local ionospheric contributions.
radial range 2 R R 15 R . The unexpectedly long
lifetime of the Helios probes enabled studies to extend
from deepest solar activity minimum into the following
maximum and beyond.
of the ray path’s point of solar closest approach (the "solar offset"). Significant coronal FR at levels easily distinguishable from the diurnal variation of the Earth’s ionosphere is typically obtained for R 15R . It is important
to continually monitor the FR during an occultation (see
Fig. 3), avoiding gaps between ground station transitions.
This is because any isolated measurement of polarization angle is known only to modulo 180 Æ. It follows that
the actual value of coronal FR can become ambiguous
by n180 Æ if the tracking is interrupted too long. This
condition was actually realized on a number of occasions
during the many solar conjunctions (e.g., Fig. 5).
Helios FR data exist for the years 1975-1984 during
most of the periods when the ray path offset was below
15 R . Only the large Goldstone station (DSS 14) of the
FR DATA ARCHIVE STRUCTURE
The FR data consist of ground-received, time-tagged
measurements of the locally referenced polarization angle using automatic tracking polarimeters. This angle is
transformed to the ecliptic plane (see Fig. 1) to determine
the coronal Faraday rotation given by Eq. (1) as a function of (R Θ Φ), the heliocentric spherical coordinates
162
REFERENCES
FIGURE 5. FR measurements during the 1976 solar conjunction of Helios 2 (occultation egress, west solar limb). The
inset shows the data of DOY 200 at an enlarged time scale
featuring a Faraday rotation transient event (possible CME).
NASA Deep Space Network (DSN) and the 100 m Effelsberg Radio Telescope were used during the first Helios mission years. The experiment could be conducted
on a global basis starting in 1977 when the DSN added
automatic tracking polarimeters at the 64-m stations in
Canberra (DSS 43) and Madrid (DSS 63). These polarimeters were removed from the DSN ground stations
in 1984, effectively ending the Helios FR experiment a
few months before Helios 1 operations were terminated.
Data from the Helios FR archive, comprised of 227
tracking passes with 1277 hours of FR data as well as
short statistical summaries of each tracking pass, are
now available for downloading at the following URL:
http://www.astro.uni-bonn.de/˜mbird/helios_html/
ACKNOWLEDGMENTS
The Helios Faraday Rotation Experiment could not have
been performed without valuable contributions from the
Helios Project Team, the German Space Operations Center (GSOC, Oberpfaffenhofen), the Max-Planck-Institute
für Radioastronomie (MPIfR, Bonn), the NASA Deep
Space Network (DSN), and the Radio Science Support
Team at Jet Propulsion Laboratory (JPL, Pasadena). We
wish to extend special thanks to Dr. H. Porsche for his
continued supportive efforts and to Dr. M. Pätzold for
many helpful suggestions. The work in Bonn was supported by the Deutsche Forschungsgemeinschaft (DFG)
and the Deutsches Zentrum für Luft- und Raumfahrt
(DLR), known during the Helios days as the Deutsche
Forschungs- und Versuchsanstalt für Luft- und Raumfahrt (DFVLR).
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