  The very low frequency (VLF) regime below 30 MHz in the electromagnetic
spectrum has presently drawing global attentions in radio astronomical research
due to its potentially significant science outcomes exploring many unknown
extragalactic sources, transients, and so on. However, the non-transparency of
the Earth's ionosphere, ionospheric distortion and artificial radio frequency
interference (RFI) have made it difficult to detect the VLF celestial radio
emission with ground-based instruments. A straightforward solution to overcome
these problems is a space based VLF radio telescope, just like the VLF radio
instruments onboard the Chang'E 4 spacecraft. But building such a space
telescope would be inevitably costly and technically challenging. The
alternative approach would be then a ground based VLF radio telescope.
Particularly, in the period of post 2020 when the solar and terrestrial
ionospheric activities are expected to be in a 'calm' state, it will provide us
a good chance to perform VLF ground-based radio observations. Anticipating such
an opportunity, we built an agile VLF radio spectrum explorer co-located with
the currently operational Mingantu Spectra Radio Heliograph (MUSER). The
instrument includes four antennas operating in the VLF frequency range 1-70
MHz. Along with them, we employ an eight-channel analog and digital receivers
to amplify, digitize and process the radio signals received by the antennas. We
present in the paper this VLF radio spectrum explorer and the instrument will
be useful for celestial studies of VLF radio emissions.
