Evolution of large CMEs and associated shocks.
The WIND and SOHO spacecraft demonstrated capability to provide advanced warning of oncoming shocks associated with CMEs will be continued and enhanced during GGS/SOLARMAX. Shock associated Type II emissions can be tracked from the time the CME becomes invisible to SOHO at about 15 RS, through the interplanetary medium, to Earth and beyond. Magnetic clouds are also known to be related to CMEs which, in turn, may be related to disappearing filaments or solar flares. The high quality SOHO measurements in collaboration with detailed GGS solar wind measurements will provide an important opportunity to fill in this important link.
Connection between CMEs and Solar Energetic Particle events
In the 1999-2000 time frame, just as the solar cycle approaches maximum, the ULYSSES spacecraft will be in the inner heliosphere and well positioned relative to WIND to make simultaneous observations of CME shock fronts and together provide true 3D localization measurements as the CMEs propagate through the interplanetary medium. This capability will also serve as an invaluable operational asset to the future Solar Stereo Mission. The eventual launch of CLUSTER by ESA in FY2000 will add high time resolution, 3D measurement capabilities in the high latitude, near-Earth environment, providing additional information about solar wind phenomena and its coupling to the global processes.
During the past few years it has become clear that high-energy particles in the largest solar energetic particle (SEP) events are accelerated at shock waves driven out from the Sun by CMEs. Only the fastest ~1% of CMEs produce large SEP events. Multi-spacecraft observations have shown that SEP events can form vast structures with particles streaming out ahead of the shock and trapped behind it, to fill a large fraction of the heliosphere. The particle intensities are highest at the 'nose" of the shock, ahead of the CME driver gas, and they decrease around on the "flanks". Quantitatively the angular extent of the SEP event depends upon the angular extent of the CME and profile of the fast shock. However, it is not yet possible to make quantitative predictions of SEP intensities from images of a CME. Since SEP events present a significant risk to astronauts on lunar, interplanetary and high inclination Earth orbiting missions, accurate predictions of the particle intensities will be of considerable practical value. Although the WIND instrumentation is two orders of magnitude more sensitive than that flown on ISEE 3 during the previous solar cycle, in 2.5 years of operation near solar minimum WIND has seen only a few events, most near the lower limit of WIND detectability. The combination of coronagraph observations from SOHO with the energetic particle measurements on WIND provide the proper tools for studying large events.
Great Magnetic Storms at Solar Maximum
While typical magnetic storms during solar minimum exhibit auroral emissions that are found at latitudes poleward of the U.S.- Canadian border (Figure 3.3a), the aurora during great magnetic storms have been seen in the southern states, such as Florida and Texas (Figure 3.3b). From these observations we infer that the magnetosphere undergoes substantial distortions during the storms but such auroral images of past events only provide tantalizing suggestions as to the state of our near-Earth environment at solar maximum.
While the coupling of large amounts of energy from the solar wind to the magnetosphere is certainly necessary for the development of a major storm, it is probably not sufficient in itself for the creation of an intense, low L-value ring current. Nor do past results allow us simply to assume that convection, compression and radial diffusion of the preexisting plasma sheet and quiet-time ring current give the full story of the formation of the storm ring current. Strong and continuous coupling to the ionosphere clearly must be occurring, as it appears that the percentage of oxygen in the ring current increases with increasing storm intensity. From the ion plasma and wave in-situ measurements from POLAR, supplemented by detailed CLUSTER and lower altitude measurements from FAST, SAMPEX and AKEBONO, we will obtain measurements of the latitudinal and local time distribution of energy input to the top of the ionosphere and the intensity of outflowing ions on a time scale appropriate for storm development. Horizontal and vertical cuts through the ring current by Equator-S, CLUSTER and POLAR respectively at various local times and Dst values as the solar cycle waxes will directly yield the composition changes to the ring current during the solar cycle.
Role of heavy ions
How thermal oxygen ions from the ionosphere are energized, move up lines of force into the equatorial magnetosphere and become trapped is not currently known. Wave heating via ULF waves and pitch angle scattering are almost certainly important processes, but by which modes and what instabilities? The L-value of the source of oxygen is not at all understood. The suggestion has been made that substorms occur at lower and lower magnetic latitudes as the storm main phase develops, providing a wide variety of L-shells which supply the oxygen to the ring current. Simultaneous auroral and ENA imaging from POLAR (the latter superseded by higher quality ENA images from IMAGE) during the large storms of solar maximum will provide a powerful tool for studying this phenomenon.
Role of magnetic reconnection
Magnetospheric physics has played a leading role in the recognition and understanding of the fundamental plasma physics process known as magnetic reconnection, which can alter drastically the magnetic field topology of the system. This process, which is undoubtedly important in all cosmic plasmas, converts the magnetic energy of a plasma into highly non-Maxwellian particle distributions with energetic tails and fast flowing particles. On the Sun reconnection is also likely to be important in the production of CMEs and solar flares. While excellent progress has been made in our studies of this fundamental process through the detailed measurements by GGS spacecraft and earlier by ISEE, more opportunities to engage in reconnection events in the magnetosphere will result in a much better understanding of this phenomenon by the end of GGS/SOLARMAX. The causes of the abrupt onset of reconnection will be sought. In the magnetotail, reconnection sometimes occurs but does not evolve into a complete substorm (a pseudo onset). On the Sun CMEs may or may not develop out of seemingly similar situations. Thus an understanding of the cause of onsets and pseudo-onsets is important for predictive capabilities of solar events.
Since the ionosphere provides a closure path for magnetospheric current systems, its character is fundamentally important to magnetosphere-ionosphere coupling processes. Within this context, the F-region ionosphere, at altitudes of 200 to 1500 km, is perhaps the most solar-cycle dependent region in the entire magnetosphere-ionosphere system. During solar maximum, enhanced levels of EUV and X-ray radiation from the sun increase the scale height of the ionosphere as well as the ionization level. The densities of O+ at F-region altitudes may be enhanced by an order of magnitude, providing a much larger reservoir of ionospheric ions to populate the magnetosphere. It is fundamentally important to understand the initial energization processes of these ions, which is only possible through ionospheric measurements of vertical ion density profiles and temperatures. These measurements can best be provided by ground-based coherent scatter and incoherent radar systems such as SuperDARN and at Sondrestromfjord. The low latitude extent of ionospheric current systems especially during great storms can only be measured by the various magnetometer chains like CANOPUS.
Enhanced wave activity - Wave-particle interactions
WIND, POLAR and GEOTAIL can make remotely-sensed observations at low radio frequencies of regions within the magnetosphere where wave-particle interactions are taking place. Auroral kilometric radiation, low frequency bursts, and Earth’s continuum radiation all respond to and are dramatic indicators of enhancements in solar wind input and, especially, southward turnings of the IMF. During solar maximum, the frequency and intensity of such enhancements will be increased and because WIND will be spending additional time within the magnetosphere, true 3D tracking (with GEOTAIL) of greatly amplified wave-particle interaction regions will be possible.
Local sensing of plasma wave activity by POLAR, GEOTAIL and eventually CLUSTER with its high time resolution, microscale measurement capabilities will provide further insight into underlying wave-particle interactions that mediate diffusion, acceleration and scattering of plasma populations that play a fundamental role in the global reconfiguration of the flow of mass, energy and momentum within geospace. It is expected that during solar maximum dramatic increases in the levels of electrostatic and electromagnetic turbulence will occur.
Figure 3-3b. Dynamics Explorer-1 SAI imaging of the July 30, 1989 storm.
Figure 3-3a. Polar VIS imaging of the January 10-11, 1997 storm.