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Solar wind oscillations with a 1.3 year period

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J. D. Richardson, K. I. Paularena, J. W. Belcher, and A. J. Lazarus
Center for Space Research, M.I.T., Cambridge, Massachusetts, USA

Table of Contents:

Abstract.

The IMP-8 and Voyager 2 spacecraft have recently detected a very strong modulation in the solar wind speed with an approximately 1.3 year period. Combined with evidence from long-term auroral and magnetometer studies, this suggests that fundamental changes in the Sun occur on a roughly 1.3 year time scale.

Introduction

The Sun emits a continuous stream of ionized particles called the solar wind. This wind is not constant, but varies due to changes on the Sun. Strong periodicities in the solar wind linked with the solar rotation period (roughly 25 days) [Neugebauer and Snyder, 1966] and the solar cycle [Neugebauer, 1975; Bridge, 1977; Lazarus and McNutt, 1990] have been observed. Periodicities ranging from 51 to 256 days have been reported both in solar wind and in other solar observations (see Villanueva [1994] and references therein). Solar cycle variations of sunspot numbers and auroral activity are well established [see review by Silverman, 1992].

The IMP-8 and Voyager 2 spacecraft have obtained solar wind data since 1973 and 1977, respectively. These long sets of solar wind observations permit the study of long-term trends in the solar wind. These trends may give an indication of underlying variations in the Sun. Comparison of IMP-8 data to Voyager 2 data gives information on the radial evolution of the solar wind (IMP-8 orbits Earth, whereas Voyager 2 is now at 44 AU). This paper reports IMP-8 and Voyager 2 observations of a large amplitude oscillation of the solar wind radial velocity with a period of about 1.3 years.

Observations

Both the IMP-8 and Voyager 2 instruments use data from Faraday cups to determine the velocity, density, and temperature of the solar wind. Voyager 2 is moving out from the Sun at a slowly changing solar ecliptic longitude while IMP-8 orbits the Sun with Earth. The solar wind is highly variable on time scales of minutes to solar cycles. In addition, the solar wind is often characterized by time-varying, high-velocity streams of ions originating from specific solar longitudes. The regions of open magnetic field lines from which the high speed streams originate are called coronal holes. To facilitate the search for solar variations on time scales longer than the 25-day solar rotation period and to make possible IMP-8 to Voyager 2 comparisons, we average data over 50 days, approximately two solar rotations. Figure 1 shows 50-day boxcar averages of the solar wind velocity from IMP-8 and Voyager 2 from 1987 through 1993. Voyager 2 data are time-shifted to compensate for the propagation time of the solar wind from Earth (and IMP-8) to Voyager 2. The time shift is calculated by dividing the distance of Voyager 2 beyond Earth's orbit by a running 100-day average of the solar wind velocity. This shift produces a reasonably good alignment of features observed by the two spacecraft.

Figure 1: (GIF format (8K)) or (PS format (44K)): Fifty day averages of the radial speed of the solar wind versus time from IMP-8 (dotted line) and Voyager 2 (solid line). Voyager 2 observations are time-shifted to compensate for the transit time of the solar wind plasma from IMP-8 to Voyager.

The first point we make is the excellent correlation between IMP-8 and Voyager 2 velocity features as Voyager 2 moved from 20 to 44 AU. The Voyager 2 data are generally smoother, indicative of the processing which occurs as the solar wind moves outward: faster streams of solar wind catch up to slower streams, resulting in the formation of shocks and equalization of stream speeds.

The most striking features in the velocity data are the 1.3-year period oscillations observed in the velocity profiles after the 1987 solar sunspot minimum. The amplitude of these oscillations is approximately 100 km/s for both IMP-8 and Voyager 2. Five of these oscillations have now been observed; prior to 1987, structure with this period was not apparent.

Discussion

The obvious question is whether this period is related to a fundamental oscillation related to structural changes on the Sun. A search of the literature reveals two studies of solar-wind-related phenomena over long times periods which show similar periods. One [Shapiro, 1967] looks at values of Ci, a measure of the disturbance of Earth's magnetic field as determined from a ground-based magnetometer network, obtained between 1884 and 1964. This work finds a 1.4-year variation in Ci, but at a marginal confidence level. The second study [Silverman and Shapiro, 1983] performs a power spectral analysis on a data set of visual observations of aurorae in Sweden between 1721 and 1943. This analysis finds a peak at 1.4 years at between the 90 and 95% confidence levels. The importance of this 1.4-year peak varies with a roughly 65-year period, the phase of which would imply this peak was of lesser importance in the current epoch, with a projected minimum in 1980. This variation in importance may be consistent with the observations of solar wind described above in which the 1.3-year oscillation is obvious only after 1987.

A 1.3- to 1.4-year oscillation corresponds to no obvious variation in the orientation of Earth with respect to the solar wind, so by implication this period must correspond to a time scale for solar processes. Solar wind velocity structure is closely related to the magnetic topology of coronal holes and, in particular, to the divergence rate of magnetic flux tubes in the solar corona [Wang and Sheeley, 1990a]. Thus one possibility is that this period is related to the formation rate and lifetime of such open magnetic structures, which in turn are determined by the flux generation processes inside the Sun and the subsequent transport of the erupted active-region flux over the solar surface [Wang and Sheeley, 1990b]. Correlation of these data with solar observations may provide more information on the origin of such long-period solar wind oscillations. This study illustrates the value of continuous long-term monitoring of the solar wind plasma for detecting long-period solar variations.

Acknowledgements

We thank G. L. Siscoe and Y.-M. Wang for their suggestions which assisted this work. This work was supported by NASA under contract 959203 from JPL to MIT (Voyager) and by NASA contracts NAGW-1550 (SR&T) and NAG5-584 (IMP) to MIT.

References

Bridge, H. S., Solar cycle manifestations in the interplanetary medium, in Physics of Solar Planetary Environments, edited by D. J. Williams, pp. 47-62, AGU, Washington, D. C., 1976.

Lazarus, A. J. and R. L. McNutt, Jr., Plasma observations in the distant heliosphere: A view from Voyager, in Physics of the Outer Heliosphere, edited by S. Grzedzielski and D. E. Page, Pergamon Press, New York, 1990.

Neugebauer, M., Large scale solar cycle variations of the solar wind, Space. Sci. Rev., 17, 221, 1975.

Neugebauer, M. and C. W. Snyder, Mariner 2 observations of the solar wind, 1, average properties, J. Geophys. Res. 71, 4469- 4484, 1966.

Shapiro, R., Interpretation of the subsidiary peaks at periods near 27 days in power spectra of geomagnetic disturbance indices, J. Geophys. Res. 72, 4945-4949, 1967.

Silverman, S. M., Secular variation of the aurora for the past 500 years, Rev. Geophys., 30, 333-351, 1992.

Silverman, S. M. and R. Shapiro, Power spectral analysis of auroral occurrence frequency, J. Geophys. Res. 88, 6310-6316, 1983.

Villanueva, L. A., A study of the solar wind from the Voyager spacecraft, 1977-1992, Ph.D. thesis, Physics Dept., Mass. Inst. of Technol., Cambridge, Mass. USA, 1994.

Wang, Y.-M. and N. R. Sheeley, Jr., Solar wind speed and coronal flux tube expansion, Ap. J. 355, 726-732, 1990a.

Wang, Y.-M. and N. R. Sheeley, Jr., Magnetic flux transport and the sunspot-cycle evolution of coronal holes and their wind streams, Ap. J. 3655, 372-386, 1990b.

Copyright

Copyright 1994 by the American Geophysical Union.
Paper number 94GL06113
0148-0227/94GL-06113$05.00
(Received March 11, 1994; accepted April 11, 1994 )

J. W. Belcher, A. J. Lazarus, K. I. Paularena, and J. D. Richardson, Center for Space Research, Cambridge, MA 02139 USA (e-mail: jdr@space.mit.edu)

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