\subsection{Solar neutrinos}

  Water \v{C}herenkov detectors have measured the high energy tail 
of the solar $^{8}$B neutrino flux using electron-neutrino 
elastic scattering \cite{Smy:2002rz}. 
Since such detectors could record the time of an interaction and reconstruct 
the energy and direction of the recoiling electron, unique information 
of the spectrum and time variation of the solar neutrino flux was extracted. 
This provided further insights into the ``solar neutrino problem'', 
the deficit of the neutrino flux (measured by several experiments) 
with respect to the flux expected by the standard solar models. 
It also constrained the neutrino flavor oscillation solutions in a fairly 
model-independent way.

The recoiling electrons from solar neutrino interactions are low in energy 
and produce few \v{C}herenkov photons. However, if at least 20 \% of the 
detection surface is photo-sensitive then solar neutrinos above 10 MeV 
could be detected even with a modest photo-sensor efficiency. 
A detector with larger size than any existing  Water \v{C}erenkov  has 
the potential to measure spectrum and time-variation of the high-energy 
solar neutrino flux more precisely, if systematic uncertainties can be 
kept small.
 For example, Super-Kamiokande's measurements obtained from 1258 days 
of data could be repeated in about half a year (the seasonal flux variation 
measurement requires of course a full year). In particular, a first 
measurement of the flux of the rare hep neutrinos may be possible.
Elastic neutrino-electron scattering is strongly forward peaked. 
To separate the solar neutrino signal from background events, this 
directional correlation is exploited. Angular resolution is limited 
by multiple scattering.  The reconstruction algorithm first reconstructs 
the vertex from the PMT times and then the direction assuming a single 
Cherenkov cone originating from the reconstructed vertex. 
Reconstructing 7 MeV events in a 400 kton 
fiducial volume water \v{C}erenkov (UNO,MEMPHYS,...) seems not to be a problem.

This means we are able to make improvements in solar neutrino detection 
with a megaton-scale \v{C}erenkov
detector: even if it is not the main goal of such a detector 
it could be an excellent by-product.

