RESUMO
The previous three solar cycles have ended in progressively more quiescent conditions, suggesting a continual slide into an ever deeper minimum state. Although the Sun's magnetic field is undoubtedly responsible for this quiescence, it is not clear how changes in its structure and strength modulate the properties of the solar wind. In this study, we compare the statistical properties of the solar wind during the three most recent minima (08/1996, 12/2008, and 12/2019) and develop global MHD model solutions to help interpret these observations. We find that, counter-intuitively, the statistical properties of the solar wind for the most recent minimum lie midway between the 08/1996 and 12/2008 minima. For example, while the minimum speed dropped by 40 km s-1 between 08/1996 and 12/2008, they rose by 20 km s-1 around the 12/2019 minimum. From the model results, we infer that the 12/2008 and 12/2019 minima were structurally similar to one another, with the presence of corotating interaction regions driven by equatorial coronal holes, while the 08/1996 minimum represented a more "standard" tilted dipole configuration associated with those of earlier space age minima. Comparison of the statistical properties derived from the model results with data suggest several opportunities to improve model parameters, as well as to apply more sophisticated modeling approaches. Overall, however, the model results capture the essential features of the observations and, thus, allow us to infer the global structure of the inner heliosphere, of which the in-situ measurements provide only a glimpse.
RESUMO
Solar eruptions are the main driver of space-weather disturbances at the Earth. Extreme events are of particular interest, not only because of the scientific challenges they pose, but also because of their possible societal consequences. Here we present a magnetohydrodynamic (MHD) simulation of the 14 July 2000 "Bastille Day" eruption, which produced a very strong geomagnetic storm. After constructing a "thermodynamic" MHD model of the corona and solar wind, we insert a magnetically stable flux rope along the polarity inversion line of the eruption's source region and initiate the eruption by boundary flows. More than 1033 ergs of magnetic energy are released in the eruption within a few minutes, driving a flare, an EUV wave, and a coronal mass ejection (CME) that travels in the outer corona at ≈1500 km s-1, close to the observed speed. We then propagate the CME to Earth, using a heliospheric MHD code. Our simulation thus provides the opportunity to test how well in situ observations of extreme events are matched if the eruption is initiated from a stable magnetic-equilibrium state. We find that the flux-rope center is very similar in character to the observed magnetic cloud, but arrives ≈8.5 hours later and ≈15° too far to the North, with field strengths that are too weak by a factor of ≈1.6. The front of the flux rope is highly distorted, exhibiting localized magnetic-field concentrations as it passes 1 AU. We discuss these properties with regard to the development of space-weather predictions based on MHD simulations of solar eruptions.