Scientific rationale
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 $\beta$
plasma parameter is $\approx$ 1 and, at the coronal layers where the $\beta$
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.