Solar Cycle Variations of the Magnetosphere: Causes and Consequences II
Presiding: V K Jordanova, University of New Hampshire; I Richardson, NASA Goddard Space Flight Center
SM43D-01 INVITED 13:30h
The Solar Cycle Dependence of the Solar Wind Sources of Geomagnetic Activity
Variations of average geomagnetic activity levels occur during the solar cycle, as measured by long term (much greater than a solar rotation) averages of geomagnetic indices such as aa. These variations are generally not dominated by energetic solar events (which may be intense but brief) but by changes in the background solar wind, including the effects of changes in the solar open magnetic flux and solar wind speed. They also do not closely follow the solar activity cycle. In particular, activity tends to be enhanced during the declining phase of the cycle due to the presence of corotating solar wind streams. In addition, the period right at solar (sunspot) maximum is often marked by a decrease in activity levels, apparently associated with the weaker interplanetary magnetic fields around the time of solar field reversal, a temporary lull in the occurrence of energetic solar events, and the general absence of fast solar wind. Considering the sources of geomagnetic storms, the most intense storms as almost invariably associated with the passage of interplanetary coronal mass ejections, the associated shocks and compressed post-shock plasma. The presence of a strong southward magnetic field is an important parameter determining the storm size; speed shows a much weaker dependence. Such events predominantly occur at higher activity levels, and typically show two occurrence rate peaks, before and after a temporary decline right at solar maximum. Weaker storms are produced by both ICMEs and corotating streams at high activity levels, and predominantly by corotating streams at lower activity levels. We illustrate these points using observations from 1972 to present from an analysis of solar wind structures inferred from in-situ data.
SM43D-02 13:50h
A Systematic Search of Solar Sources of Major Geomagnetic Storm
A systematic process of identifying solar sources of major geomagnetic storms is presented. From 1996 to 2004, there are 77 major geomagnetic storms (defined as Dst <= -100). For each of the 77 events, we make use combined remote-sensing solar observations and in-situ solar-wind data to identify their possible sources. The data include (1) solar wind plasma and magnetic observations in near-Earth space from ACE and WIND experiments, (2) solar CME observations from the LASCO (Large Angle and Spectrometric Coronagraph) on SOHO, (3) coronal observations from the EIT (Extreme Ultraviolet Imaging Telescope) on SOHO, and (4) other synoptic solar observations including flares and filament eruptions. Solar wind data are necessary in the identification process because it provides constraints on the duration of backward search window. A major geomagnetic storm may be caused by (1) a single halo CME, (2) multiple halo CMEs, or (3) CIR (corotating interaction region). The properties of those responsible solar events will be discussed.
SM43D-03 INVITED 14:05h
Solar Cycle Effects on Solar Wind-Magnetosphere Coupling
It is well established that solar wind coupling to the magnetosphere is a maximum at the equinoxes. This is a geometric effect partially caused by the projection of the IMF onto the GSM Z-axis. In the Russell-McPherron effect activity is a maximum at equinox according to the rule "spring to fall away". Recently it has been shown that the equinoctial maximum is also a result of dipole tilt towards and away from the Sun. Coupling is less efficient when this angle departs from 90 degrees. The actual disturbance measured on the ground also depends on ionospheric conductivity so that it is unclear which factors are dominant is determining the strength of ground disturbances. Since conductivity depends on solar UV it is likely that coupling to the magnetosphere is modulated by the solar cycle. In addition we have recently found evidence that coupling depends on the Mach number of the solar wind. This property is modulated by the solar cycle through its dependence on the occurrence of CMEs. These considerations lead us to speculate that there should be an obvious solar cycle dependence of the coupling of the solar wind to the magnetosphere. We will investigate this hypothesis by calculating the prediction filters that relate the solar wind electric field to several different magnetic indices including AL, sym-H, and PC. Filters will be calculated at equinox and solstice for each year of the solar cycle. The sum of the filter coefficients will be used as a measure of coupling strength.
SM43D-04 INVITED 14:25h
Solar Cycle Dependence of the Outer Radiation Belt
Earth's outer electron radiation belt is an enormous reservoir of energetic particles that is dynamically driven by the solar wind. The radiation belt particles originate predominately in the solar wind and are accelerated in various stages within the magnetosphere. The solar wind controls the efficiency of its own entry into the magnetosphere and the energization and loss of the particles within the magnetosphere. Under certain conditions, the radiation belts are found to be remarkably sensitive to changes in the solar wind, while at other times quite insensitive to the solar wind. This presentation will describe the observed variability of the outer electron radiation belt to solar wind dynamics over the most recent solar cycle. This will include the long-term variability, on solar cycle to seasonal time scales, as well as the short-term variability driven by transient disturbances in the solar wind.
SM43D-05 14:45h
Solar cycle changes of energetic particle properties in the Earth's radiation belts
The near-Earth responds powerfully to changes of the Sun and subsequently the solar wind. The Earth's radiation belts and inner magnetosphere show pronounced differences in their characteristics as the Sun's magnetic and solar wind plasma properties change. Solar coronal holes produce regular, recurrent solar wind streams in geospace, often enhancing highly relativistic electrons and causing recurrent magnetic storms. These phenomena are characteristics of the approach to solar minimum. This is the phase of the 11-year solar cycle in which we presently are situated. On the other hand, major geomagnetic disturbances associated with aperiodic coronal mass ejections occur most frequently around solar maximum. We describe the observational and modeling results that describe differences throughout the inner part of geospace during the course of the 11-year solar cycle. We place particular emphasis on long-term, homogeneous data sets from the SAMPEX and POLAR missions.