] You can hear sferics, tweeks, whistlers and other VLF
] radio sounds at any time of the day, but the hours
] around dawn and dusk are generally best. Nighttime is
] also better than daytime. In Huntsville, AL, where
] our online receiver is located, dawn happens at about
] 1200 UT and dusk is ten hours later at 2200 UT.

NASA online VLF receiver