  In this work the possibility of detecting solar electron antineutrinos
produced by a solar core magnetic field from the KamLAND recent observations is
investigated. We find a scaling of the antineutrino probability with respect to
the magnetic field profile in the sense that the same probability function can
be reproduced by any profile with a suitable peak field value. In this way the
solar electron antineutrino spectrum can be unambiguosly predicted. We use this
scaling and the negative results indicated by the KamLAND experiment to obtain
upper bounds on the solar electron antineutrino flux. We get
$\phi_{\bar\nu}<3.8\times 10^{-3}\phi(^8B)$ at 95% CL. For 90% CL this becomes
$\phi_{\bar\nu}<3.4\times 10^{-3}\phi(^8B)$, an improvement by a factor of 3-5
with respect to existing bounds. These limits are independent of the detailed
structure of the magnetic field in the solar interior. We also derive upper
bounds on the peak field value which are uniquely determined for a fixed solar
field profile. In the most efficient antineutrino producing case, we get (95%
CL) an upper limit on the product of the neutrino magnetic moment by the solar
field $\mu B< 2.8\times 10^{-19}$ MeV or $B_0 \leq 4.9 \times 10^7 G$ for
$\mu_\nu=10^{-12}\mu_B$.
