  The million degree plasma of the solar corona must be supplied by the
underlying layers of the atmosphere. The mechanism and location of energy
release, and the precise source of coronal plasma, remain unresolved. In
earlier work we pursued the idea that warm plasma is supplied to the corona via
direct heating of the chromosphere by nanoflares, contrary to the prevailing
belief that the corona is heated in-situ and the chromosphere is subsequently
energized and ablated by thermal conduction. We found that single
(low-frequency) chromospheric nanoflares could not explain the observed
intensities, Doppler-shifts, and red/blue asymmetries in Fe XII and XIV
emission lines. In the present work we follow up on another suggestion that the
corona could be powered by chromospheric nanoflares that repeat on a timescale
substantially shorter than the cooling/draining timescale. That is, a single
magnetic strand is re-supplied with coronal plasma before the existing plasma
has time to cool and drain. We perform a series of hydrodynamic experiments and
predict the Fe XII and XIV line intensities, Doppler-shifts, and red/blue
asymmetries. We find that our predicted quantities disagree dramatically with
observations and fully developed loop structures cannot be created by
intermediate- or high-frequency chromospheric nanoflares. We conclude that the
mechanism ultimately responsible for producing coronal plasma operates above
the chromosphere, but this does not preclude the possibility of a similar
mechanism powering the chromosphere; extreme examples of which may be
responsible for heating chromospheric plasma to transition region temperatures
(e.g. type II spicules).
