Conditions in solar wind and magnetosphere during the nontypical VLF hiss burst on December 8, 2013

Abstract

The features of the dynamics of solar wind and IMF parameters were studied during the initial phase of the weak magnetic storm of December 8, 2013, where a nontypical two hour lasting wide-band (∼4–10 kHz) hiss burst was recorded during VLF observations in auroral latitudes near Sodankyla observatory (L ∼ 5.5), which differed from classical auroral hiss. A similar VLF hiss burst was recorded at Russian Lovozero observatory, which is located ∼400 km to the east. In contrast to a typical auroral hiss, the VLF emissions at both points were left-polarized and arrived at the observation point from the southeast. Although the VLF hiss burst coincided in time with the development of a substorm and the appearance of zenithal bright auroras near the stations traveling north-south, the excitation of the VLF hiss apparently has no relation to the auroras. It is suggested that the VLF emissions were generated due to cyclotron instability far to the east of Scandinavia in the region of plasmapause at L ∼ 3.5, where the equatorial gyrofrequency (f He) is about 20 kHz. The generated VLF waves could be ducted in the plasmapause at frequencies lower than a half of f He, i.e., below ∼10 kHz, they arrive at the Earth’s surface near the projection of their source and propagate in the Earth-ionosphere waveguide to large distances, as left-polarized waves. A sharp increase in the solar wind dynamic pressure was noted during the hiss burst under study, which resulted in a significant contraction of the daytime magnetosphere, a shift of the plasmapause and radiation belt to lower L shells, and the development of a substorm and southward travel of auroral arcs. The VLF hiss may have been generated in the region where energetic particles of the radiation belt crossed the plasmapause. The fact that the hiss under study was generated not in the early morning sector (Scandinavian meridian), but much further to the east, could be indirectly confirmed by quasiperiodic modulation of the VLF noise intensity in the range of Pc5 geomagnetic pulsations, which were observed in this period at eastern stations but not at the Scandinavian meridian.