K. I. Paularena

Solar wind oscillations with a 1.3 year period

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.

Radial evolution of the solar wind from IMP 8 to Voyager 2

Voyager 2 and IMP 8 data from 1977 through 1994 are presented and compared. Radial velocity and temperature structures remain intact over the distance from 1 to 43 AU, but density structures do not. Temperature and velocity changes are correlated and nearly in phase at 1 AU, but in the outer heliosphere temperature changes lead velocity changes by tens of days. Solar cycle variations are detected by both spacecraft, with minima in flux density and dynamic pressure near solar maxima. Differences between Voyager 2 and IMP 8 observations near the solar minimum in 1986–1987 are attributed to latitudinal gradients in solar wind properties. Solar rotation variations are often present even at 40 AU. The Voyager 2 temperature profile is best fit with a R−0.49±0.01 decrease, much less steep than an adiabatic profile.

Plasma and magnetic field correlations in the solar wind

Data from multiple spacecraft are used to determine solar wind plasma and interplanetary magnetic field (IMF) correlation coefficients. These correlation coefficients provide information on solar wind scale lengths and the predictive capability of upstream monitors for space weather purposes. Previous work has looked at plasma and IMF correlation coefficients independently and used much smaller databases than those used in this study. We use data sets from 1977–1984 and 1994–1998 and calculate plasma and IMF correlation coefficients. The IMF correlation coefficients are, on average, slightly higher than those for plasma. Dependence of the correlation coefficients on the spatial separation of the spacecraft is important for placement of upstream monitors; we find a small dependence on the radial separation of the spacecraft but a very strong dependence on spacecraft separation in the YZ (GSE) plane. Scale lengths perpendicular to the flow are about 45 Earth radii (RE) for the IMF components, 70 RE for the speed and IMF magnitude, and over 100 RE for the density. Radial scale lengths are of the order of 400 RE. The plasma and IMF correlation coefficients are larger when values of the density standard deviation are high and when the IMF direction is perpendicular to X (GSE). Front orientations are similar for both plasma and IMF features and are more perpendicular than the average field direction.

Low-latitude dusk flank magnetosheath, magnetopause, and boundary layer for low magnetic shear: Wind observations

We have studied in detail a Wind spacecraft crossing of the low-latitude dusk flank magnetosheath, magnetopause (MP), and the low-latitude boundary layer (LLBL) when the local magnetic shear across the MP was low (<30°) and the interplanetary magnetic field (IMF) was northward. We find that the magnetosheath flow tangential to the MP slows down initially as one moves from the bow shock toward the MP. However, close to the MP this flow speeds up as the MP is approached. The source of flow acceleration is likely to be the magnetic force associated with draping of the field lines around the MP. Magnetic flux pile-up and a plasma depletion layer are also observed next to the flank MP indicating that the level of magnetic flux transfer across the entire dayside low-latitude MP via reconnection is low. The MP is characterized by changes in the plasma properties. The electron parallel temperature is enhanced across the MP and continues to increase across the LLBL, while the perpendicular temperature is constant across the MP. This constancy of the perpendicular temperature suggests that the transfer of plasma takes place across the local MP. In the LLBL, the ion and electron temperatures are well correlated with the density. In addition, the flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath, and the flow speed tangential to the MP decreases gradually with decreasing LLBL density. The behavior of the particle distributions suggests that the entire LLBL was on closed field lines. In essence, our findings on the topology and on the LLBL plasma characteristics suggest that even in the absence of reconnection at the local low-shear MP, the LLBL is locally coupled to the adjacent magnetosheath. The smooth variations of the plasma parameters with the density are consistent with the LLBL spatial profiles being gradual. This may suggest that diffusion processes play a role in the formation and dynamics of the LLBL. Finally, the magnetic field and the state of the plasma in the plasma sheet adjacent to the flank MP/LLBL appear to be functions of the IMF direction. Thus the IMF may control both the external (magnetosheath) and the internal (plasma sheet) boundary conditions for the flank MP processes.

A dawn‐dusk density asymmetry in Earth's magnetosheath

Magnetosheath data from IMP 8 and solar wind data from ISEE 1, ISEE 3, and WIND are used to investigate Earths magnetosheath structure. Magnetosheath data are transformed into a coordinate system which allows data from regions with similar physical histories to be binned together. The results show a significant dawn-dusk asymmetry in Earths magnetosheath near solar maximum but not near solar minimum, with larger densities on the dawnside than on the duskside. We use a magnetohydrodynamic (MHD) simulation to model the Parker spiral interplanetary magnetic field (IMF) case; an asymmetry does develop in the same sense as observed in the data but with a somewhat smaller magnitude. Near solar minimum the observed density asymmetry is dependent on the IMF orientation, but this is not true near solar maximum, apparently ruling out both foreshock effects and different compressions/deflections by parallel and perpendicular shocks as causes. Earths magnetosphere thus could be a contributor to this effect.

Unusual locations of Earth's bow shock on September 24–25, 1987: Mach number effects

ISEE 1 and IMP 8 data are used to identify 19 crossings of Earths bow shock during a 30-hour period following 0000 UT on September 24, 1987. Apparent standoff distances for the shock are calculated for each crossing using two methods and the spacecraft location; one method assumes the average shock shape, while the other assumes a ram pressure-dependent shock shape. The shocks apparent standoff distance, normally ∼ 14 RE, is shown to increase from near 10 RE initially to near 19 RE during an 8-hour period, followed by an excursion to near 35 RE (where two IMP 8 shock crossings occur) and an eventual return to values smaller than 19 RE. The Alfven MA and fast magnetosonic Mms Mach numbers remain above 2 and the number density above 4 cm−3 for almost the entire period. Ram pressure effects produce the initial near-Earth shock location, whereas expansions and contractions of the bow shock due to low Mach number effects account, qualitatively and semiquantitatively, for the timing and existence of almost all the remaining ISEE crossings and both IMP 8 crossings. Significant quantitative differences exist between the apparent standoff distances for the shock crossings and those predicted using the observed plasma parameters and the standard model based on Spreiter et al.s (1966) gasdynamic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at low MA and Mms, or variations in shock shape with MA and Mms (becoming increasingly “puffed up” with decreasing MA and Mms, as expected theoretically), or by a combination of both effects. Global MHD simulations, to be presented elsewhere, confirm that both effects occur and are significant. Ram pressure-induced changes in the shocks shape are discussed but found to be quantitatively unimportant for the shock crossings analyzed. Approximate estimates of both the deviation of the shocks standoff distance from the standard model and of the shocks shape are determined independently (but not consistently) for Mms ∼ 2.4. The estimates imply substantial changes in standoff distance and/or shock shape at low MA and Mms. Mach number effects can therefore be quantitativwely important in determining and predicting the shape and location of the bow shock, even when MA and Mms remain above 2. This study confirms and generalizes previous studies of Mach number effects on Earths bow shock. Statistical studies and simulations of the bow shocks shape and location should be performed as a function of Mach number, magnetic field orientation, and ram pressure.

Evidence for a solar wind slowdown in the outer heliosphere

Voyager 2 and IMP 8 plasma data are used to look for the predicted slowdown of the solar wind with heliospheric distance. Decreases of roughly 7% in the radial velocity and of the same order in the flux are found if the Voyager 2 and IMP 8 velocities are normalized to agree in the inner heliosphere. This decrease is consistent with a pickup ion density equal to 8% of the total ion density, similar to predictions and other determinations of this density. Comparison with published model results allows us to infer an interstellar neutral density of 0.05 cm−3.

Coincident 1.3‐year periodicities in the ap geomagnetic index and the solar wind

Recent observations show an approximately 1.3-year period in the speed of the solar wind detected by the IMP 8 and Voyager 2 spacecraft. A similar period is also seen in the north-south (GSE) component of the magnetic field observed by IMP 8. Since both parameters are commonly used as input to models of geomagnetic activity, the ap index (a measure of geomagnetic disturbance) is examined to look for this periodicity. The Lomb-Scargle periodogram method is used on the ap, plasma, and magnetic field data during the 1973–1994 time range. A dynamic FFT periodogram method is also used to analyze the ap data during this time, as well as to look for periods present between 1932 and 1972. A clear 1.3-year periodicity is present in the post-1986 data when the same period is observed in the plasma and field data. The V2Bzsm and V2Bs proxies for geomagnetic activity also show this periodicity. However, the southward (GSM) component of the magnetic field does not have a 1.3-year period, and neither do solar wind or ap data from 1973–1985. This demonstrates that the ap geomagnetic index can act as a proxy for solar wind periodicities at this time scale. Historic ap data are examined, and show that a similar periodicity in ap exists around 1942. Since auroral data show a 1.4-year periodicity, all these similar periods may result from a common underlying solar mechanism.

Solar wind plasma correlations between L1 and Earth

Solar wind plasma data from ISEE 3 at the L1 point and IMP 8 in Earth orbit are compared to determine how well an Ll monitor predicts plasma conditions at Earth. These data cover the time period August 1978 to February 1980, approaching solar maximum. Data are divided into 6-hour blocks, time shifted to compensate for the radial separation of the spacecraft, and interpolated to provide identically sampled time series; then linear correlation coefficients are calculated as a function of lag. The average correlations of solar wind speed, density, and flux are all about 0.6. The most important factors determining the degree of correlation are the radial separation of the spacecraft and the standard deviation of the density. For the largest standard deviations of density, the correlation coefficients are 0.85. The lag between observation of solar wind features at the two spacecraft varies with both the radial and azimuthal separation of the spacecraft, indicating that both radial propagation of the solar wind and the rotation of the Sun are important effects for determining solar wind arrival time. Plasma structures seem to be less well correlated than magnetic field structures.

Solar wind plasma correlations between IMP 8, INTERBALL‐1, and WIND

Solar wind plasma flux correlations between data from three spacecraft (IMP 8, WIND, and INTERBALL-1) were analyzed for approximately 4 months during late 1995 and mid 1996 (near solar minimum) in order to investigate the local homogeneity of the solar wind. The data were split into 6-hour segments, resulting in a total of 397 segments where data from at least one pair of spacecraft could be correlated. The results show that the average flux correlation was 0.7 over distances ranging from 0 to 220 RE in the radial direction and up to 80 RE perpendicular to the Earth-Sun line. 43% of the segments studied had correlation coefficients of at least 0.8, while only 19% of the segments had correlation coefficients less than 0.5. The additional lags, after performing radial advection shifts at the plasma bulk speed, cluster near zero (71% of the best correlations occur with lags under 10 min), implying that the advection shift is a good approximation of the propagation time for the structures being correlated. There appeared to be no dependence of the correlation on spacecraft separation in either XGSE or YGSE. The best organizers of the flux correlation appear to be the value of the flux and the standard deviations of the flux and the density.