The tweek resonances were received on the loop antenna in the
garden but not on the E-field antenna of the (somewhat whitened out)
city grabber
http://df6nm.bplaced.net/VLF/spherics/dk7fc_wideband_150718_0330.jpg.
This corroberates the notion of near vertical incidence and horizontal H-field
polarisation. According to the literature, tweek tails are usually circular
polarized as only one sense of rotation exhibits a high reflection coefficient.
They are predominately excited by horizontal current components in intra-cloud
lightnings.
Of course the resonances will also there in quiet nights without
spherics, so they could probably be employed to enhance fieldstrength (up
to a factor of Q) for medium-range VLF communication experiments using magnetic
transmit and receive antennas. When Stefan still had his earth dipole
we already discussed a 2 kHz tweek-mode experiment, which for various
reasons hasn't taken place yet. Now it looks like one could even employ the
fifth mode near 8.5 kHz...
