They are commonly observed at Kannuslehto, but have also been infrequently reported at other stations, sometimes under different names. These emissions are intriguing, since they are detected at frequencies above half the electron gyrofrequency in the equatorial plane ( f ce) for the L-shell of Kannuslehto ( f ce ~ 5–6 kHz). Due to the configuration of the Earths magnetic field lines, the most intense electromagnetic waves that are typically observed in near-Earth space occur near the geomagnetic poles, so thatUsing numerical filtering techniques allowing us to reduce noise from sferics, we are able to clearly study a new type of differently structured very low frequency (VLF) radio waves above f = 4 kHz at the ground station of Kannuslehto in northern Finland (KAN, MLAT = 64.4°N, L = 5.5). And Very Low Frequency (VLF) waves are used to remotely sense the otherwise-inaccessible regions of the Earths upper atmosphere.
We present recent observations between 20.B-field VLF Receiver. We also give a review of the different characteristics of VLF bursty-patches observed at Kannuslehto, which at the moment, is the station with the highest observation rate. They are sometimes characterized by single bursts covering very large frequency ranges of several kHz. They also show periodic features with varying periodicity and shape. While these waves have different spectral features, they appeared mostly composed of hiss bursts with durations of a few seconds to several minutes. This paper unifies the nomenclature by regrouping all these waves detected at frequencies higher than the local equatorial 0.5 f ce at the L-shell of observation under the name of VLF bursty-patches.
Loop antennas couple to the magnetic field component of the.Earth’s magnetic field interacts with the solar wind to create a cavity called the magnetosphere, protecting us from harmful energetic particles of solar origin. The sources are lightning flashes and high-power VLF transmitters.Low Frequency (VLF) and Extremely Low Frequency (ELF) signals. The VW signals of interest in this research originate in the air space below the ionosphere and cover the frequency range - 0.3 to - 30 kHz. It uses a single turn loop for the antenna, however.In this report we present and interpret very-low-frequency (Vm) signals observed with the polar orbiting 060-4 satellite.
For example, emissions showing discrete elements with rising or falling frequency tones in the timescale of tenths of seconds are known as chorus (Sazhin and Hayakawa 1992 Santolik 2008 and references therein). 2000).Historically, ELF/VLF waves have been categorized by their spectral features (Helliwell 1965). Their names come from the frequency band in which they are detected: between 0.3 and 3 kHz for ELF and 3 to 30 kHz for VLF (Barr et al. These emissions are plasma waves in the whistler-mode that propagate below the local gyrofrequency of electrons (Kennel and Petschek 1966 Helliwell 1965 Gurnett and Bhattacharjee 2017). Through resonant cyclotron interactions, electrons up to several tens of keV are behind the generation of extremely low (ELF) and very low frequency (VLF) emissions. These particles are then trapped around the Earth in what is commonly known as the radiation belts or Van Allen belts.
If the waves present a periodic time modulation of their intensity in the orders of several seconds up to minutes they are classified as quasi-periodic (QP) (e.g., Sazhin and Hayakawa 1994 Smith et al. 1993 Hayakawa and Sazhin 1992). On the other hand, noise-like broadband emissions with no discernible discrete features are called hiss (see reviews by Sazhin et al.
These were short duration bursty emissions observed above 4 kHz, sometimes at frequencies higher than the usual half gyrofrequency of electrons ( f ce) in the equatorial plane of the magnetosphere at the L-shell of ATH (0.5 f ce eq. = 5.5 kHz). ( 2014) reported the existence of ‘Bursty-Patch’ emissions at the ground station of Athabasca, Canada (ATH, MLAT = 64.5, L ~ 4.3) during geomagnetically disturbed periods. While this process has ameliorated the observations of usual VLF emissions, such as chorus and hiss on the ground, it also allowed for the clear study of frequencies above 5 kHz, where a new type of VLF emissions was observed.Using data from the VLF-CHAIN campaign in February 2012, Shiokawa et al. To improve the study of VLF waves by ground-based receivers, the Sodankylä Geophysical Observatory (SGO) in Finland developed a sferics filtering system (briefly explained in Manninen et al. During the periods of high lightning storm activity these sferics can cover all the frequencies down to 1 kHz obscuring the observations of VLF emissions. In particular, sferics originating from lightning discharges present themselves as a multitude of vertical lines almost completely covering frequencies above 4 or 5 kHz on spectrograms.
Using polarization analysis, Martinez-Calderon et al. Since bursty-patches consisted of short duration rising-tone elements and were consistent with the expected upper frequency band of chorus, they believed these two emissions to be linked. It is believed that in the latter case, detection on the ground becomes more difficult as the waves might have more difficulties crossing the ionosphere, because their wave normal angles with respect to the vertical can become very large. 2017 Martinez-Calderon et al. At f > 0.5 f ce, the waves would be traveling with oblique wave angles with respect to the local magnetic field lines, also known as unducted propagation (e.g., Němec et al.
( 2014), but also detected multiple other types at unusually higher frequencies, above the local equatorial gyrofrequency ( f ce eq. = 5.4 kHz). KAN not only observed emissions similar to the bursty-patches as described by Shiokawa et al. Since ATH data is usually obscured by sferics, further study of these type of emissions could not be accurately completed.Thankfully, the data from the VLF receiver at Kannuslehto in Northern Finland, has been analyzed with a sferics filter allowing for clearer observations at frequencies above the local 0.5 f ce eq at the L-shells of KAN (0.5 f ce eq = 2.7 kHz). This analysis could not confirm with certainty if bursty-patches and chorus were linked or generated by a separate source. In addition, bursty-patches were usually detected at ATH as coming from very similar directions.
( 2021) described these type of emissions as high-frequency VLF patches or VLF ‘birds’ and focused on their relationship to temporal and spatial details of wave–particle interactions. More recently, Manninen et al. In some cases, they are shaped like bullets with sudden stops or show wing-like features, while in other cases, we note gradual frequency increase or showing multiple periodic features (Manninen et al.
The station is located at a latitude of 67.74 N, a longitude of 26.27 E, and at the L-shell of 5.5. These unusual type of high frequency emissions observed at f > f ce will be known as VLF bursty-patches.In this study we present data from a VLF receiver located in the auroral zone of Northern Finland at KAN. This paper also aims to unify the nomenclature for these new group of VLF emissions which have often been called by different names in several papers and presentations: RREs, ‘birds’, ‘bursty-patches’, VLF bursts, VLF patches and KHF-VLF. We will discuss some features that have not been previously reported, as well as a closer look at a few particular types of periodic emissions and consider possible mechanisms behind the observed characteristics. While we understand a bit more on these waves, the reasons behind this variety of shapes or periodicity mechanisms are still unknown.In this paper, we will focus on remarkable observations of high frequency VLF emissions at the ground station of KAN from 2019 to 2021, and particularly those detected above 6 kHz, i.e., f > f ce. They concluded that VLF-patches could indirectly indicate a local enhancement of electron fluxes and could be generated even if they were not being detected on the ground.
Meaning that while we are able to reduce the noise from sferics, we do not remove them all as otherwise the VLF emissions at these frequencies would also be removed. When we apply the sferics filter during the data analysis, we allow between 15 and 20% data loss. The configuration and location of the receiver has been the same since 2006.Data from KAN benefits from a unique sferics filtering system.
0 kommentar(er)
