Margaret D. Wilson

Present status of the recovery phase of cosmic ray 11‐year modulation

The recovery from cosmic ray 11-year modulation follows two distinct repetitive patterns [Ahluwalia, 1994]. When the magnetic polarity of the Sun in the northern hemisphere is negative (qA 0) the recovery period is reduced to less than half as much. These repetitive patterns have been observed for the last five solar activity cycles (17 to 21). The recovery for cycle 22 is still in progress. By the end of 1994, the intensity monitored with the nuetron detectors at Deep River, Climax, and Huancayo continues to be significantly below the cosmic ray maxium observed in March 1987. The physical significance of our results is discussed.

Trapped energetic ions in Jupiter's inner magnetosphere

Ion measurements made with the high-flux telescope at ∼1 MeV/nucleon during the passage of the Ulysses spacecraft through Jupiters inner magnetosphere are presented. Stable pancake-shaped pitch angle distributions were observed for protons inside ∼17 RJ. They are fitted by a time-independent model in which an assumed directional flux at the magnetic equator is transformed to the spacecrafts position using Liouvilles equation and the O6CS magnetic field model. Nongyrotropic features are fitted with a flow in the corotation direction. The derived equatorial pitch angle distribution is approximately independent of radial distance and has broad shoulders. However, it is not as isotropic as that predicted for strong diffusion. From a flux-composition fitting analysis, we find (1) the equatorial proton omnidirectional flux decreases approximately exponentially with magnetic equatorial distance RM and is nearly longitudinally symmetric, (2) the equal energy per nucleon abundance ratios for O/H, S/H, and S/O decrease with RM, (3) the ion spectral index softens linearly with RM, (4) the phase space densities of protons and oxygen at a constant magnetic moment increase rapidly with RM, and (5) the sulfur phase space density turns up inside 10 RJ. The data are best fitted with similar spectral indices for oxygen and sulfur. Thus there is likely a source of energetic sulfur in the Io torus. Particle losses throughout the region are required to fit the radial variation of the phase space densities with a physically reasonable radial diffusion coefficient. Further work is needed to determine whether these losses are consistent with weak equatorial pitch angle diffusion.

Pitch Angles and Spectra of Particles in the Outer Zone Near Noon

Precipitation patterns of electrons on the dayside have been studied for many years using ground based evidence and satellite experiments. This article will summarize some of the more recent work, and attempt to extend and refine our understanding of these regions.

Microscale structure of the interplanetary medium and solar wind control of the time lags in the auroral zone geomagnetic response

While the level of auroral zone geomagnetic activity, as measured by the AE index, is determined by the value of the southward component, Bs, of the interplanetary magnetic field (IMF), the delay in the geomagnetic response is modified by the microscale fluctuations of the IMF vector B. The fluctuations are characterized by the square root σz of the variance of the solar magnetospheric (SM) z component of B. For extreme values of Bs(Bs > 5 nT and Bs = 0) the geomagnetic disturbances associated with small values of σz last longer than those associated with large σz. Conversely, for other values of Bs, the magnetosphere responds to the quasi-stationary interplanetary conditions with a shorter delay than to the time dependent solar wind conditions. Autocorrelations of the solar wind (SW), parameters evaluated for various time lags are always stronger when the initial value of σz is small than when it is large. Analysis of the autocorrelations and of the geomagnetic responses corresponding to them suggests that, for positive but not extremely large values of Bs, the temporal fluctuations of B trigger the internal magnetospheric (or the SW-magnetospheric coupling) processes which increase the observed time lags in the auroral zone geomagnetic activity. The value of σz is the SW parameter indicating whether the magnetosphere responds to SW conditions almost immediately (within less than ∼1 hour) or whether the magnetospheric response is more gradual and is delayed for another ∼1 to ∼4 hours. The data analyzed in this study indicate the existence of magnetospheric processes delaying the geomagnetic response to the SW conditions for Bs = 0 but they do not allow us to draw conclusions on the effects of σz when Bs is extremely high or equal to zero. Future short-term forecasts of geomagnetic activity are likely to be based on the SW data obtained from an upwind located satellite and appropriate adjustments of the forecasts are in order to account for the effects of the fluctuations.