Storms is Earth-space seem to occur in response to the passage of a disturbance in the outflowing 'solar wind.', especially in response to fast coronal mass ejections going by. The details of the Earth-space response are quite complex, and the explanations for it only partially complete. When a fast CME passes, its leading shock causes the sudden onset of a variety of magnetospheric activity. A large disturbance of the magnetic field near the equator signals the injection of a newly accelerated particle population (a 'ring current') into the magnetosphere. New, temporary 'radiation belts' may also appear. There is an intensification of auroral activity, which is caused by electrons and ions raining or 'precipitating' from the magnetosphere into the upper atmosphere. The magnetosphere as a whole is energized by a much stronger interaction than usual with the disturbed solar wind, resulting in enhanced ionospheric currents and increased coupling between the magnetosphere and ionosphere. The latter is particularly true if the disturbed magnetic field in the coronal ejecta or the piled up interplanetary field has a long period (hours to days) of southward-oriented magnetic field. Manifestation of this closer coupling includes a substantial increase in the numbers of ionospheric ions appearing in the magnetosphere, as well as a significantly disturbed high latitude ionosphere. (WHAT FIGS HERE???)
Space weather storms could in principle be predicted from observations of fast coronal mass ejections coming toward the Earth. At the typical speeds of a fast CME, this would allow about 2 days warning if the initiation was seen at the Sun, or about 1 hours' warning if the iinterplanetary disturbance was detected upstream of Earth at the location of most solar wind spacecraft. (FIG?)
'Sub storms' are in some ways like small versions of the above storms but they can occur without the passage of a major interplanetary disturbance. They are considered to build up when the undisturbed interplanetary field has a southward component. Current thinking is that there is a gradual transfer of solar wind energy into the magnetosphere under these circumstances, and then any small perturbation, like a solar wind pressure increase, or a change in the interplanetary field orientation, can release that energy in storm-like ways. In this case the ultimate driver is the magnetosphere's limit on storing energy derived from the solar wind. Substorms can also be embedded within larger storms, modulating the longer term and more intense activity associated with them. (FIG???)
Although the most remarkable ionospheric disturbances occur in relation to the auroral activity and strong solar wind-magnetosphere connection related to storms and substorms, others of note can happen apart from geomagnetic activity. Since the energetic electromagnetic radiation bursts (ultraviolet and x-rays) accompanying flares on the Sun travel at the speed of light, they arrive at Earth well ahead of any coronal ejecta. Moreover, the passage of photons is not affected by the presence of the magnetosphere. The direct response of the upper atmosphere to the energetic solar flare photon emissions is transient increased ionization called a 'Sudden Ionospheric Disturbance'. (FIG??)
As mentioned above, particles of 'cosmic-ray-like' energies are accelerated by the interplanetary shock preceding a fast coronal mass ejection and possibly in the vicinity of a solar flare site. These appear as a transient population in interplanetary space outside the magnetosphere, and also in the Earth's polar regions where they can gain access to low altitudes. Sometimes solar particles also find their way into the deeper magnetosphere by means of other transport processes. In the polar caps, they cause atmospheric ionization at levels that disturb the transmission of signals to and from space. These are called 'Polar Cap Absorption' events.
Other Items of interest:
[There is a UT page on Total Electron Content of the Ionosphere]
[Also perhaps include a plot of electron fluxes from Rice University]
