GEOPHYSICAL RESEARCH LETTERS, VOL. 11, NO. 3, PAGES 279-282, MARCH 1984 LARGE SCALE TEMPORAL AND RADIAL GRADIENTS IN THE IMF: HELIOS 1,2, ISEE-3, AND PIONEER 10, 11 J.A. Slavin, E.J. Smith, and B.T. Thomas Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Abstract. Recent investigations using AU. Accordingly, the field quantities examined measurements at 1 AU have discovered three types are rZBr, rB• , and •2/(l+r 2) r2B. of long term variation in the interplanetary Burlaga fnd King (1979) have shown that the magnetic field: solar minimum decreases, solar distribution of IMF intensities obeys log normal maximum enhancements, and small decreases around statistics. Accordingly, by working with the solar reversal. In this study we have examined logarithm of field magnitude we may utilize the 1972-1982 Helios 1,2, ISEE-3, and Pioneer standard deviations and standard errors in the 10,11 observations between 0.3 and 12 AU to mean in the usual ways. Figure 1 presents further investigate these changes. It was found logarithms of hourly averages of IMF magnitude that all three IMF solar cycle effects are also at Helios 1 and Pioneer 11 after they have been present in the Helios and Pioneer measurements, scaled to 1 AU. The distributions demonstrate confirming that these variations occur that the aletrended field strength continues to throughout the low latitude heliosphere. In obey log normal statistics inward to the orbit addition, the comparison of measurements by of Helios, perihelion 0.3 AU, and outward to identical magnetometers on ISEE-3, Pioneer 10, Pioneer11's 1982 distance, 12 AU. For •he and Pioneer 11 has revealed a more rapid purpose of calculating standard errors (Burlaga decrease in IMF intensity than predicted by and King, 1979), log averages will be used when classical Parker theory. Causes and examining total field magnitude. ramifications of both the long term variations The widths of the distributions in Figure 1 and steeper than expected radial gradients in provide a measure of the relative power in the the interplanetary magnetic field are discussed. fluctuations and how it grows with distance from the sun. The standard deviations vary from Introduction about 20% of the mean inside 1 AU to 30-50% in the outer solar system. The most probable cause Only recently has the data base of spacecraft of the increase is the presence of corotating measurements begun in the 1960's become interaction regions beyond the orbit of Mars sufficient to detect solar cycle changes in the (Smith and Wolfe, 1979). The increase in the solar wind and interplanetary magnetic field. width of the IMF distributions may be closely During solar cycle 20, 1964-1976, the IMF related to the radial variations in plasma ion strength was relatively constant except for a temperature which decreases much more slowly possible decrease during the 1964-5 sunspot than expected due to the effects of dissipation minimum (King, 1979). In this study we further in shocks and CIR's (Smith and Wolfe, 1979). investigate the solar cycle 21 variations in the These structures transfer energy from smooth interplanetary magnetic field that have been flows and fields into heat and shorter found in the 1 AU observations. They consist of wavelength fluctuations. IMF magnitude decreases around solar minimum Figure 2 displays annual averages of the 1 AU (King, 1979), solar maximumenhancements (King, equivalent IMF magnitudes measured by Helios 1980; Slavin and Smith, 1983), and small 1,2, IMP - ISEE-3, and Pioneer 10,11 over the decreases near solar reversal (Slavin and Smith, years 1966-1982. A representative standard 1983). If these effects originate with large error in the annual means is shown in the lower scale alterations of the sun's magnetic field, right hand corner. The long term trends in the then they should be observed throughout the IMF magnitude at 1 AU appear to be well hellosphere. With this in mind, Helios 1,2, reproduced both closer to the sun in the Helios ISEE-3, and Pioneer 10,11 observations are used observations and in the outer heliosphere by to examine the long term variations in the Pioneer 11. Similary, the increase in field interplanetary magnetic field. In addition, the strength between 1976 and 1982 with a dip in radial gradients in the IMF are investigated 1980 is also very clear. The discrepancy with the long term temporal variations removed between Pioneer 10,11 and the 1 AU measurements using the 1 AU baseline observations. is discussed in a later section on radial gradients. Overall, the basic solar cycle Temporal Variations variations reported by King (1979;1981) and Slavin and Smith (1983), solar minimum decrease, The changing distances between Helios 1,2, solar maximum increase, and solar reversal Pioneer 10,11, and the sun make it difficult to decrease in 1980, appear to to have been present examine long term temporal variations in the IMF throughout the entire low latitude heliosphere. unless the radial changes are suppressed. In These variations therefore appear to have been this study we have used the Parker (1963) radial temporal in nature and not associated with dependences to scale the Helios and Pioneer effects limited to 1 AU. hourly averaged magnetic field values back to 1 This paper is not subject to U.S. copyright. Pub- lished in 1984 by the American Geophysical Union. Paper number 4L0137. Solar Wind Variations The long term changes in solar wind velocity are also of interest both in themselves and because of their influence on IMF spiral angle. 2?9