Tauno Turunen

Unusually high frequency natural VLF radio emissions observed during daytime in Northern Finland

Geomagnetic field variations and electromagnetic waves of different frequencies are ever present in the Earths environment in which the Earths fauna and flora have evolved and live. These waves are a very useful tool for studying and exploring the physics of plasma processes occurring in the magnetosphere and ionosphere. Here we present ground-based observations of natural electromagnetic emissions of magnetospheric origin at very low frequency (VLF, 3–30 kHz), which are neither heard nor seen in their spectrograms because they are hidden by strong impulsive signals (sferics) originating in lightning discharges. After filtering out the sferics, peculiar emissions are revealed in these digital recordings, made in Northern Finland, at unusually high frequencies in the VLF band. These recently revealed emissions, which are observed for several hours almost every day in winter, contain short (~1–3 min) burst-like structures at frequencies above 4–6 kHz, even up to 15 kHz; fine structure on the 1 s time scale is also prevalent. It seems that these whistler mode emissions are generated deep inside the magnetosphere, but the detailed nature, generation region and propagation behaviour of these newly discovered high latitude VLF emissions remain unknown; however, further research on them may shed new light on wave-particle interactions occurring in the Earths radiation belts.

A new type of daytime high-frequency VLF emissions at auroral latitudes (“bird emissions”)

This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto (L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird’s chirping to a human ear; for that reason, they were called “bird emissions.” The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth’s surface.

Lower Ionosphere Sensitivity to Solar X‐ray Flares Over a Complete Solar Cycle Evaluated From VLF Signal Measurements

The daytime lower ionosphere behaves as a solar X-ray flare detector, which can be monitored using Very Low Frequency (VLF) radio waves that propagate inside the Earth-ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X-ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X-ray flare up to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval December-January from 2007 to 2016 (Solar Cycle 24). The computation of FXmin uses the X-ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB) and USA (NAA); which were recorded in Brazil, Finland and Peru. The main result of this study is that the long-term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7J/m2. Our result suggests that FXmin is anti-correlated with the lower ionosphere sensitivity, confirming that the long-term variation of the ionospheric sensitivity is anti-correlated with the level of solar activity. This result is important to identify the minimum X-ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.

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

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 (fHe) is about 20 kHz. The generated VLF waves could be ducted in the plasmapause at frequencies lower than a half of fHe, 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.