Nighttime sensitivity of ionospheric VLF measurements to X‐ray bursts from a remote cosmic source

Abstract

AbstractOn 22 January 2009, a series of X‐ray bursts were emitted by the soft gamma ray repeater SGR J1550‐5418. Some of these bursts produced enhanced ionization in the nighttime lower ionosphere. These ionospheric disturbances were studied using X‐ray measurements from the Anti‐Coincidence Shield of the Spectrometer for Integral onboard the International Gamma‐Ray Astrophysics Laboratory and simultaneous phase and amplitude records from two VLF propagation paths between the transmitter Naval Radio Station, Pearl Harbor (Hawaii) and the receivers Radio Observatorio do Itapetinga (Brazil) and Estação Antarctica Commandante Ferraz (Antarctic Peninsula). The VLF measurements have been obtained with an unprecedented high time resolution of 20 ms. We find that the illumination factor I (illuminated path length times the cosine of the zenith angle), which characterizes the propagation paths underlying the flaring object, is a key parameter which determines the sensitivity threshold of the VLF detection of X‐ray bursts from nonsolar transients. For the present VLF measurements of bursts from SGR J1550‐5418, it is found that for I ≥ 1.8 Mm, all X‐ray bursts with fluence in the 25 keV to 2 MeV range larger than F25_min ~ 1.0 × 10−6 erg/cm2 produce a measurable ionospheric disturbance. Such a lower limit of the X‐ray fluence value indicates that moderate X‐ray bursts, as opposed to giant X‐ray bursts, do produce ionospheric disturbances larger than the sensitivity limit of the VLF technique. Therefore, the frequency of detection of such events could be improved, for example by increasing the coverage of existing VLF receiving networks. The VLF detection of high‐energy astrophysical bursts then appears as an important observational diagnostic to complement their detection in space. This would be especially important when space observations suffer from adverse conditions, like saturation, occultation from the Earth, or the passage of the spacecraft through the South Atlantic anomaly.