A review of unusual VLF bursty-patches observed in Northern Finland for Earth, Planets and Space

Using 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). These emissions are intriguing, since they are detected at frequencies above half the electron gyrofrequency in the equatorial plane (fce) for the L-shell of Kannuslehto (fce ~ 5–6 kHz). They are commonly observed at Kannuslehto, but have also been infrequently reported at other stations, sometimes under different names. Their possible common origin and manner of propagation is still under investigation. This paper unifies the nomenclature by regrouping all these waves detected at frequencies higher than the local equatorial 0.5 fce at the L-shell of observation under the name of VLF bursty-patches. While these waves have different spectral features, they appeared mostly composed of hiss bursts with durations of a few seconds to several minutes. They also show periodic features with varying periodicity and shape. They are sometimes characterized by single bursts covering very large frequency ranges of several kHz. 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. We present recent observations between 2019 and 2021.

Geometric electrostatic particle-in-cell algorithm on unstructured meshes

We present a geometric particle-in-cell (PIC) algorithm on unstructured meshes for studying electrostatic perturbations with frequency lower than electron gyrofrequency in magnetized plasmas. In this method, ions are treated as fully kinetic particles and electrons are described by the adiabatic response. The PIC method is derived from a discrete variational principle on unstructured meshes. To preserve the geometric structure of the system, the discrete variational principle requires that the electric field is interpolated using Whitney 1-forms, the charge is deposited using Whitney 0-forms and the electric field is computed by discrete exterior calculus. The algorithm has been applied to study the ion Bernstein wave (IBW) in two-dimensional magnetized plasmas. The simulated dispersion relations of the IBW in a rectangular region agree well with theoretical results. In a two-dimensional circular region with fixed boundary condition, the spectrum and eigenmode structures of the IBW are obtained from simulations. We compare the energy conservation property of the geometric PIC algorithm derived from the discrete variational principle with that of previous PIC methods on unstructured meshes. The comparison shows that the new PIC algorithm significantly improves the energy conservation property.

Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron acceleration

Van Allen Probes in situ observations are used to examine detailed subpacket structure observed in strong VLF (very low frequency) rising-tone chorus elements observed at the time of a rapid MeV electron energization in the inner magnetosphere. Analysis of the frequency gap between lower and upper chorus-band waves identifies fceEQ, the electron gyrofrequency in the equatorial wave generation region. Initial subpackets in these strong chorus rising-tone elements begin at a frequency near 1/4 fceEQ and exhibit smooth gradual frequency increase across their > 10 ms temporal duration. A second much stronger subpacket is seen at frequencies around the local value of 1/4 fce with small wave normal angle (< 10°) and steeply rising df/dt. Smooth frequency and phase variation across and between the initial subpackets support continuous phase trapping of resonant electrons and increased potential for MeV electron acceleration. The total energy gain for individual seed electrons with energies between 100 keV and 3 MeV ranges between 2 and 15%, in their nonlinear interaction with a single chorus element.

A Review of Unusual VLF Bursty-Patches Observed in Northern Finland

Using numerical filtering techniques allowing to reduce noise from sferics, we are able to clearly study a new type of differently structured very low frequency (VLF) waves above f = 4 kHz at the ground station of Kannuslehto in northern Finland (KAN, MLAT=64.4°N, L=5.5). These emissions are intriguing since they are detected above the local equatorial electron gyrofrequency for the L-shell of Kannuslehto (fce ~ 5- 6 kHz). They are commonly observed at Kannuslehto, but have also been infrequently reported at other stations, sometimes under different names. Their possible common origin and manner of propagation is still under investigation. This paper unifies the nomenclature by regrouping all these waves detected at frequencies higher than the local equatorial 0.5 fce at the L-shell of observation under the name of VLF bursty-patches. We also give a review of the different characteristics of VLF bursty-patches observed by Kannuslehto, which at the moment, is the station with the highest observation rate. We will present recent observations between 2019 and 2021. While these waves have different spectral features, they appeared mostly composed of hiss bursts with durations of a few seconds to several minutes. They also show periodic features with varying periodicity and shape. They are sometimes characterized by single bursts covering very large frequency ranges of several kHz.

Analysis of Observations near the Fourth Electron Gyrofrequency Heating Experiment in EISCAT

We present the observations of the artificial ionospheric heating experiment of EISCAT (European Incoherent Scatter Scientific Association) on 22 February 2012 in Tromsø, Norway. When the pump is operating near the fourth electron gyrofrequency, the UHF radar observation shows some strong enhancements in electron temperature, electron density, ion line, and the outshifted plasma lines. Based on some existing theories, we find the following: first, Langmuir waves scattering off lower hybrid density fluctuations and strong Langmuir turbulence (SLT) in the Zakharov model cannot completely explain the outshifted plasma lines, but the data suggest that this phenomenon is related to the cascade of the pump wave and should be researched further; second, the spatiotemporal consistency between the enhancement in electron density/electron temperature reaches up to three to four times that of the undisturbed state and HF-enhanced ion lines (HFILs) suggest that SLT excited by parametric instability plays a significant role in superthermal electron formation and electron acceleration; third, some enhancements in HFILs and HF-induced plasma lines (HFPLs) are generated by parametric decay instability (PDI) during underdense heating in the third cycle, we suggest that this is due to the existence of a second cut-off in the upper hybrid dispersion relation as derived from a kinetic description.

Subpacket Structure in Strong VLF Chorus Rising Tones: Characteristics and Consequences for Radiation Belt Acceleration

Van Allen Probes in situ observations are used to examine detailed subpacket structure observed in strong VLF (very low frequency) rising tone chorus elements observed at the time of a rapid MeV electron energization in the inner magnetosphere. Analysis of the frequency gap between lower and upper chorus-band waves identifies fceEQ, the electron gyrofrequency in the equatorial wave generation region. Initial subpackets in these strong chorus rising-tone elements begin at a frequency near 1/4 fceEQ, exhibit smooth gradual frequency increase across their > 10 ms temporal duration. A second much stronger subpacket is seen at frequencies around the local value of 1/4 fce with small wave normal angle (< 10 deg) and steeply rising df/dt. Smooth frequency and phase variation across and between the initial subpackets supports continuous phase trapping of resonant electrons and increased potential for MeV electron acceleration. The total energy gain for seed electrons with energies between 100 keV and 3 MeV ranges between 2 % and 15 %, in their nonlinear interaction with a single chorus element.

Unexpected high-frequency “birds”-type VLF emissions.

The new typeof daytime natural VLF whistler mode emissions of the magnetospheric origin was recently found in the VLF observations at Kannuslehto station (L ~ 5.5) in Northern Finland.These VLF events occurred at the frequencies above 4-5 kHzeven up to 15 kHz. Here we present the different spectra of this peculiar daytime high-frequency VLF emissions observed under quiet geomagnetic conditions at auroral latitudes at Kannuslehto (Finland) and Lovozero (Russia) stations. These high-frequency waves cannot be attributed to typical well known VLF chorus and hiss. They became visible on the spectrograms only after the filtering out sferics originating by the lightning discharges and hiding all natural high-frequency signals. After this filtering, it was found a large collection of different natural VLF signals observed as a sequence of right-polarized short (less than 1-2 minutes) patches at frequencies above 4-5 kHz, i.e. at higher frequencythan a half the equatorial electron gyrofrequency at the L-shell of Kannuslehto and Lovozero. These emissions were called “birds” due to their chirped sounds. It was established that the “birds” are typically occur during the daytime only under quiet space weather conditions. But in this time, small magnetic substorms were could be observed in the night sector of the Earth. Here we also show the recently observed series of the “bird-mode” emissions with various bizarre quasi-periodic dynamic spectra, sometimes consisting of two (and even more) frequency bands. The “birds” occur simultaneously at Kannuslehto and Lovozero with similar spectral structure demonstrating their common source. It seems that the “birds” emissions are generated deep inside the magnetosphere at the low L-shells. But the real nature, the generation region and propagation behavior of these VLF emissions remain still unknown. Moreover, nobody can explain how the waves could reach the ground at the auroral latitudes like Kannuslehto and Lovozero as well as which magnetospheric driver could generate this very complicated spectral feature of the emissions.

The peak frequency source of Saturn’s Kilometric Radiation

&lt;p&gt;Before to ultimately plunge into Saturn&amp;#8217;s atmosphere, the Cassini spacecraft explored between 2016 and 2017 the auroral regions of Saturn&amp;#8217;s magnetosphere, where rises the Saturn&amp;#8217;s Kilometric Radiation (SKR). This powerful, nonthermal, radio emission analog to Earth&amp;#8217;s Auroral Kilometric Radiation, is radiated through the Cyclotron Maser Instability (CMI) by mildly relativistic electrons at frequencies close to the local electron gyrofrequency. The typical SKR spectrum, which ranges from a few kHz to ~1MHz, thus corresponds to auroral magnetic flux tubes populated by radiosources at altitudes ranging from ~4 kronian radii (Rs) down to the planetary ionosphere.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;During the F-ring orbital sequence, Cassini probed the outer part of both northern and southern auroral regions, ranging from ~2.5 to ~4 Rs altitudes, and crossed several SKR low frequency sources (~10-30 kHz). Their analysis showed that the radiosources strongly vary with time and local time, with the lowest frequencies reached on the dawn sector. They were additionally colocated with the UV auroral oval and controlled by local time-variable magnetospheric electron densities, with importants consequences for the use SKR low frequency extensions as a proxy of magnetospheric dynamics. Along the proximal orbits, Cassini then explored auroral altitudes below ~2.5 Rs and crossed numerous, deeper, SKR sources at frequencies close to, or within the emission peak frequency (~80-200 kHz). Here, we present preliminary results of their survey analysis. Understanding how the CMI operates in the widely different environments of solar system magnetized planets has direct implications for the ongoing search of radio emissions from exoplanets, ultracool dwarves or stars.&lt;/p&gt;

A Neural Network Model of Three-dimensional Magnetospheric Chorus Waves

&lt;p&gt;The evolution of chorus waves is important in the inner magnetosphere since it is closely related to the loss and acceleration of radiation belt electrons. In this study, we develop neural-network-based models for upper-band chorus (UBC; 0.5 f&lt;sub&gt;ce &lt;/sub&gt;&lt; f &lt;&amp;#160; f&lt;sub&gt;ce &lt;/sub&gt;) waves and lower-band chorus (LBC; 0.05 f&lt;sub&gt;ce &lt;/sub&gt;&lt; f &lt; 0.5 f&lt;sub&gt;ce&lt;/sub&gt;) waves, where f&lt;sub&gt;ce&lt;/sub&gt; is the equatorial electron gyrofrequency. We establish a root-mean-square amplitude database for both UBC and LBC using Van Allen Probe levels 2 and 3 data products from the EMFISIS payload between October 1, 2012 and January 14, 2018. Based on the database, we construct an artificial neural network with corresponding L, magnetic local time, magnetic latitude, solar wind parameters and geomagnetic indices on different time windows as model inputs. Additionally, we adopt several different feature selection techniques to determine the most important features of magnetospheric chorus waves, reduce training or running time and improve the model accuracy. Our study suggests that the model results using the machine learning technique have the great potential to highly improve current understanding of the radiation belt dynamics.&lt;/p&gt;

Conditions for topside ionline enhancements

&lt;p&gt;High frequency (HF) enhanced ion line spectra as a response to magnetic field aligned HF pumping of the polar ionosphere in an O-mode polarization can be observed at the top and bottomside F-region ionosphere under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in magnetic zenith on 18th and 19th October 2017 while stepping the pump frequency of the EISCAT Heating facility across the double resonance frequency of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 2-min-on, 2-min-off pump cycle, stepping both upward and downward in frequency. We present observations of two separate cases of topside HF enhanced ion lines (THFIL). THFIL simultaneous to bottomside HFIL (BHFIL) and conditioned by the relative proximity to the double resonance frequency, consistent with previous observations \citep{Rexer2018} were observed for heating pulses on 19th October. Recurring THFIL with a second set of characteristics were observed on 18th October, appearing independently from BHFIL and possibly conditioned by the proximity of the topside double resonance frequency. Propagation of the pump wave to the topside ionosphere is consistent with L-mode wave propagation facilitated by density striations in the plasma. We consider the conditions for the occurrence of THFIL for two cases/types of observations.&amp;#160;&lt;/p&gt;