Interplanetary Field Enhancements Research Articles

Interplanetary Magnetic Field Enhancements and Their Association with the Asteroid 2201 Oljato

Comparison of the times of occurrence of a newly discovered type of disturbance in the interplanetary magnetic field at 0.72 astronomical unit with the passage of the small Venus-crossing asteroid, 2201 Oljato, reveals a possible association, but the source of these disturbances appears to be associated with outgassing material in the Oljato orbit some distance behind the asteroid and not with the asteroid itself. This suggested association can account for one quarter of the total number of events seen in eight Venus years.

Interplanetary field enhancements in the solar wind: Evidence for cometesimals at 0.72 and 1.0 AU?

A new class of interplanetary magnetic disturbance has been identified which consists of a nearly symmetric rise and fall of the magnetic field surrounding a cusp-shaped maximum. These disturbances have been hypothesized to be caused by the mass-loading of the solar wind by small outgassing bodies. The clustering of these events in space suggests that not all the events are independent. Clustering is greatest at 0.72 AU because of one very strong family of events associated with the orbit of the asteroid 2201 Oljato. The events are larger at 0.72 AU than at 1 AU. The timing of the disturbances at both 1 AU and 0.72 AU relative to Oljato suggests the presence of outgassing debris both in front of and behind the asteroid.

Cosmic ray modulations related to the interplanetary magnetic field intensity

Data covering a complete solar cycle (1965–1976) have been analyzed in a comprehensive investigation of the nature of transient modulations of relativistic galactic cosmic rays associated with interplanetary magnetic field intensity excursions in the vicinity of the earth. It had been recognized earlier that the correlation coefficient between the cosmic ray intensity and the IMF magnitude is small. In fact, it remains less than 0.5 throughout the solar cycle. In order to understand the nature of field‐related modulations and to minimize the complications arising from the effects of other features of solar plasma, the data are here analyzed by the method of superposed epochs. Following newly developed procedures for evaluating the statistical significance of the results obtained with this type of analysis, it is demonstrated that, while the days characterized by high IMF magnitude B are associated with intensity decreases, the IMF rarefactions (low B) cause an increase in the flux. In contrast to the case of geomagnetic disturbances, which respond more rapidly, maximum cosmic ray intensity variations associated with both high and low B occur 1 day after the key days characterized by field departures from average values. This indicates that the modulation mechanism becomes efficient only when the plasma engulfment of the earth extends at least a gyroradius beyond 1 AU. Finally, while the geomagnetic index Ap follows the interplanetary magnetic field changes rather closely, the polar nucleonic intensity shows a prolonged recovery time (7–10 days) following magnetic enhancements as well as rarefactions. These results suggest that, on average, interplanetary magnetic field intensity variations which are presumably related to disturbances traveling from the sun are effective transient modulators of the relativistic particles and that the corotation of the modulating region plays a significant role in the recovery of the cosmic ray intensity. An investigation of the spectra of the cosmic ray intensity variations produced by interplanetary magnetic field enhancements and rarefactions, respectively, over the rigidity range covered by ground‐based nucleonic intensity detectors has revealed that a power law of the form Kp−1 fits the data in both cases. This result is relevant to further theoretical studies of cosmic ray modulation processes.