Gamma-ray Burst Effects on the Ionosphere
Cosmic gamma-ray bursts (GRBs) are bursts of energetic gamma radiation produced by the violent explosions of distant stars. There is a huge amount of interesting astrophysics involved that is far beyond the scope of this document. From the perspective of the VLF group at Stanford, GRBs are interesting because of their effects on the Earth's atmosphere and the near-Earth space environment.
The burst of gamma-rays from a GRB interacts with the upper atmosphere and lower ionosphere, depositing its energy and ionizing atoms and molecules. This ionization is in addition to natural ionization due to ultraviolet light from the sun and cosmic rays and acts to change the conductivity of the upper atmosphere. The change in conductivity is visible as a change in the way VLF radio waves reflect from the ionosphere and thus can be detected by monitoring transmitted VLF signals from long distances.
A good example of this phenomenon was observed on December 27, 2004, when a GRB from an object known as soft gamma-ray repeater (SGR) 1806-20 hit the Earth's atmosphere. The disturbance of the Earth's atmosphere caused VLF radio signals transmitted from Hawaii to Palmer Station, Antarctica, to decrease in amplitude by over 20 dB, as shown in Figure 1 (Inan et al., 2007).
Physically, GRB effects on the atmosphere break down nicely into three phases: gamma-ray ionization of the upper atmosphere, the chemistry of the atmosphere, and radio wave propagation. The VLF group at Stanford has models of all of these processes. First, gamma-ray interaction with the atmosphere is simulated with a model of the atmosphere implemented in CERN's GEANT4 particle physics Monte Carlo code. This simulation determines the energy deposition of various GRB spectra. The ionization produced by this energy deposition is then used as an input to a model of atmospheric chemistry. The atmospheric chemistry model solves the differential equations governing the recovery of the atmosphere after ionization by the GRB. This determines the electron and ion densities as the atmosphere recovers. The electron and ion densities are then used to determine the radio wave propagation characteristics of the atmosphere. The Long Wavelength Propagation Capability (LWPC) code is useful for this. Together, these simulations give the time evolution of VLF radio signal amplitude and phase before, during, and after a gamma-ray burst. A sample plot of simulation results is shown in Figure 2. Comparison of the results of these models to data from our receivers allows us to test our models and determine properties of the GRB, atmospheric chemistry, and the ionosphere.
This material is based upon work supported by the National Science Foundation under Grant No. ATM-0836326.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.