The excitation of magnetospheric whistler-mode chorus in response to interplanetary (IP) shocks is investigated using wave data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. As an example, we show a typical chorus wave excitation following an IP shock event that was observed by THEMIS in the postnoon sector near the magnetopause on 3 August 2010. We then analyze characteristic changes during this event and perform a survey of similar events during the period 2008-2014 using the THEMIS and OMNI data set. Our statistical analysis demonstrates that the chorus wave excitation/intensification in response to IP shocks occurs only at high L shells (L > 8) on the dayside. We analyzed the variations of magnetic curvature following the arrival of the IP shock and found that IP shocks lead to more homogeneous background magnetic field configurations in the near-equatorial dayside magnetosphere; and therefore, the threshold of nonlinear chorus wave growth is likely to be reduced, favoring chorus wave generation. Our results provide the observational evidence to support the concept that the geomagnetic field line configuration plays a key role in the excitation of dayside chorus. Key Points Excitation of dayside chorus waves is observed These waves are in response to IP shocks IP shocks change the magnetic field line configuration.

Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10-30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8-1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.