Solar activity is associated with the appearance, temporal evolution and spatial modifications of the magnetic field at the solar surface and its interactions with plasma flows.
Global properties of solar activity have been extensively investigated and much information has been acquired especially for large flares, where ALL manifestations are visible and the collective phenomena may alter the final observables.
If the various manifestations of solar activity can be attributed to the same basic physical mechanism (viz., the currently fashionable magnetic reconnection), then the study of the small spatial scale and short time-constant phenomena is particularly valuable for a better understanding of solar activity, since the basic mechanisms will be primarily at work.
In this context, minor transient activity (such as Ellermann bombs, persistent bright points, microflares, etc.) are interesting targets of investigation due to their importance in clarifying basic mechanisms of energy storage, release and transfer occurring in an apparently simple structure. In other words, they may be considered as the low energy tail of a wide phenomena distribution in which major flares represent the high energy asymptote.
An international coordinated observing campaign was organized during September and October 1992 by some of the proponents, mainly using the observing capabilities of the NSO-Sacramento Peak Observatory Vacuum Tower Telescope (see MAXFACTS, no.8, July 1992); 17 research groups actively cooperated together with the YOHKOH SXT experiment in targeting minor emission phenomena in ARs. The same observing campaign has been repeated at the beginning of February 1995, although the cooperation was limited to the NSO observing sites (SP and KP), the Hawaii Mees Observatory and YOHKOH S/C.
One of the first results of these campaigns was the detection of sudden upward mass motions occurring at the onset of Ellerman bombs ( - 5 Km/s), changing to downward motion of + 6 Km/s few minutes after the maximum emission. Similar results were also detected in flares, where upward motions were measured in small localized areas with values of about - 20 Km/s at the flare onset, changing to downward motions of about + 40 Km/s within few tens of seconds.
Coronal counterparts of the Ellerman bombs have not been clearly identified but there are indications that the weak coronal loops overlying the Ellerman bomb were ``activated" some tens of minutes before the Ellerman bomb development.
The primary limitation of all the existing studies of minor solar phenomena is that they have been observed at isolated height regimes using ground-based techniques; at photospheric and low chromospheric layers, where the plasma parameter is 1 and, at the coronal layers where the is << 1. It is also of crucial importance to follow the height variations of these phenomena in the intermediary layers but maintaining the highest possible spatial resolution.