VLF/ELF Remote Sensing of Ionospheres and Magnetospheres

Editorial Board (July 2024)
Editor-in-Chief
Takeshi Sagiya, Nagoya University, Japan

Vice Editors-in-Chief
Masahito Nosé, Nagoya City University, Japan

Aitaro Kato, University of Tokyo, Japan

Editors
Koki Aizawa
Hideo Aochi
Elvira Astafyeva
Pascal Audet
Kiyoshi Baba
Toshitaka Baba
Joseph B.H. Baker
Maxwell C. Brown
Stephan Christoph Buchert
Sonia Calvari
Xiaowei ChenPeter Chi
Kuo-En Ching
Phil Cummins
Hao Dong
Guangyu Fu
Hidenori Genda
Alexander Grayver
Aditya Gusman

Takeshi Hasegawa
Stuart Henrys
Yuji Himematsu
Hauke Hussmann
Mie Ichihara
Hidekatsu Jin
Art Jolly
Nobuki Kame
Katsuya Kaneko
Ikuo Katayama
Yoko Kebukawa
Kunihiro Keika
Kostas Konstantinou
Juanjo Ledo
Koji Matsuo
Masaki Matsushima
Yasunobu Miyoshi
Hiroshi Nagaoka
Norihiro Nakamura

Atsuko Namiki
Yasuhito Narita
Kazunari Nawa
Kiwamu Nishida
Hiroyo Ohya
Duggirala Pallamraju
Severine Rosat
Susumu Saito
David Shelly
Wataru Suzuki
Youichiro Takada
Sunny Tam
Naoki Uchida
Yanbin Wang
Yih-Min Wu
Yusuke Yokota
Yohei Yukutake
Seiji Zenitani

Hiroo Kanamori
Jun'ichiro Kawaguchi
Hitoshi Mizutani
Yasuo Ogawa

Toshitaka Tsuda
Zhongliang Wu
Kiyoshi Yomogida

Advisory Board
Benjamin Fong Chao
Bernard Chouet
Yoshimori Honkura
Toshihiko Iyemori

Journal Scope
Earth, Planets and Space (EPS) is the official journal of Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
covering scientific articles in the entire field of earth and planetary sciences, particularly geomagnetism,aeronomy, space science, seismology, volcanology, geodesy, and planetary science. EPS also welcomesarticles in new and interdisciplinary subjects, and technical reports on instrumentation and software.
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Earth, Planets and Space(2024) 76:53
Omura et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-024-02000-z

Open Access

PREFACESpecial issue “VLF/ELF remote sensingof ionospheres and magnetospheres”
Yoshiharu Omura1*, Jacob Bortnik2, Mark Clilverd3, Andrei Demekhov4 and Yohei Miyake5

Graphical Abstract

Extremely Low Frequency (ELF: 300–3 kHz) and Very
Low Frequency (VLF: 3–30 kHz) radio waves, both naturally and artificially generated. We had 59 invited oral
papers and 83 poster papers presented during 10 sessions consisting of morning sessions (9–12 am JST) andevening sessions (9–12 pm JST) with 174 registered participants from 20 countries. Two years after the 9th VERSIM meeting the 10th VERSIM Workshop was held as a
hybrid meeting, online and in-person at Sodankylä, Finland in November 2022.
We solicited papers for the special issue of the 9th
VERSIM Workshop, and currently have 13 papers in it.
Among them, we asked Daniel Baker to write a review
paper (Baker 2021) summarizing the achievements
by the NASA Radiation Belt Storm Probes program(renamed the “Van Allen Probes” mission in November 2012). We received 5 other papers (Foster et al.

The present collection of papers is the result of collaborations and discussions fostered in the 9th VLF/ELF
Remote Sensing of Ionospheres and Magnetospheres(VERSIM) Workshop, which was held successfully as
a virtual meeting during the week of 16–20 November
2020. The VERSIM working group is an internationalgroup of scientists interested in studying the behavior of the magnetosphere and ionosphere by means of*Correspondence:
Yoshiharu Omura
omura@rish.kyoto-u.ac.jp
1
Kyoto University, Kyoto, Japan
2
UCLA, Los Angeles, USA
3
British Antarctic Survey, Cambridge, UK
4
Institute of Applied Physics of the Russian Academy of Sciences, Nizhny
Novgorod, Russia
5
Kobe University, Kobe, Japan© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

1

VLF/ELF Remote Sensing of Ionospheres and Magnetospheres
Omura et al. Earth, Planets and Space(2024) 76:53

Page 2 of 22021; Martinez-Calderon et al. 2021; Parrot et al. 2022;Bezděková et al. 2022; Briand et al. 2022; Redoplado et al.
2022) related to spacecraft and grand-based observationsof VLF/ELF waves. We also asked David Nunn to write
a paper (Nunn 2021) describing the detailed technicalissues surrounding the Vlasov Hybrid Simulation code
which he has been developing for more than 30 years
to reproduce whistler-mode chorus emissions. We also
received 4 other papers (Grach et al. 2021; Liu et al. 2021;
Fujiwara et al. 2022; Katoh et al. 2023) on particle simulations describing wave-particle interactions and relatedparticle dynamics taking place in the background dipole
magnetic field. We also have a paper by Kikukawa et al.
(2022) on developing compact plasma particle detectors
for use on board future satellite missions.
We would like to thank the reviewers of the articles inthe special issue. Their names are listed below in alphabetical order: Anton Artemyev, Stas Barabash, Mark A.
Clilverd, Morris Cohen, Andrei Demekhov, Philip Erickson, Hitoshi Fujiwara, Essam Ghamry, Miroslav Hanzelka, Liming He, Yasuhide Hobara, Yuto Katoh, Janos
Lichtenberger, Lann-Yeng Liu, Jyrki Manninen, Hiroyo
Ohya, Michael Rycroft, David Shklyar, S. Sripathi, Danny
Summers, Jicheng Sun, Bruce Tsurutani, Shoichiro
Yokota.

Katoh Y, Rosendahl PS, Ogawa Y et al (2023) Effect of the mirror force on
the collision rate due to energetic electron precipitation: Monte Carlosimulations. Earth Planets Space 75:117. https://doi.org/10.1186/
s40623-023-01871-y
Kikukawa M, Asamura K, Zushi T et al (2022) Development of miniaturized pick-up amplification circuit for plasma particle detectors on
board satellites. Earth Planets Space 74:188. https://doi.org/10.1186/
s40623-022-01746-8
Liu Y, Omura Y, Hikishima M (2021) Simulation study on parametric dependence of whistler-mode hiss generation in the plasmasphere. Earth Planets
Space 73:230. https://doi.org/10.1186/s40623-021-01554-6
Martinez-Calderon C, Manninen JK, Manninen JT et al (2021) A review of
unusual VLF bursty-patches observed in Northern Finland for Earth.
Planets Space Earth Planets Space 73:191. https://doi.org/10.1186/
s40623-021-01516-yNunn D (2021) The numerical simulation of the generation of lower-band VLF
chorus using a quasi-broadband vlasov hybrid simulation code. Earth
Planets Space 73:222. https://doi.org/10.1186/s40623-021-01549-3
Parrot M, Němec F, Cohen MB et al (2022) On the use of ELF/VLF emissions triggered by HAARP to simulate PLHR and to study associated MLR events. Earth Planets Space 74:4. https://doi.org/10.1186/
s40623-021-01551-9
Redoblado PAAL, Kumar S, Kumar A et al (2022) Lightning evolution and
VLF perturbations associated with category 5 TC Yasa in the South
Pacific Region. Earth Planets Space 74:65. https://doi.org/10.1186/
s40623-022-01632-3

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Author contribution
YO, JB, MC, AD, and YM served as guest editors for this special issue.

Declarations
Competing interests
The authors declare that they have no competing interests.
Received: 15 March 2024 Accepted: 29 March 2024

References
Baker DN (2021) Wave–particle interaction effects in the Van Allen belts. Earth
Planets Space 73:189. https://doi.org/10.1186/s40623-021-01508-yBezděková B, Němec F, Manninen J (2022) Ground-based VLF wave intensity
variations investigated by the principal component analysis. Earth Planets
Space 74:30. https://doi.org/10.1186/s40623-022-01588-4
Briand C, Clilverd M, Inturi S et al (2022) Role of hard X-ray emission in
ionospheric D-layer disturbances during solar flares. Earth Planets Space
74:41. https://doi.org/10.1186/s40623-022-01598-2
Foster JC, Erickson PJ, Omura Y (2021) Subpacket structure in strong VLF
chorus rising tones: characteristics and consequences for relativistic
electron acceleration. Earth Planets Space 73:140. https://doi.org/10.
1186/s40623-021-01467-4
Fujiwara Y, Nogi T, Omura Y (2022) Nonlinear triggering process of whistlermode emissions in a homogeneous magnetic field. Earth Planets Space
74:95. https://doi.org/10.1186/s40623-022-01646-x
Grach VS, Demekhov AG, Larchenko AV (2021) Resonant interaction of relativistic electrons with realistic electromagnetic ion–cyclotron wave packets.
Earth Planets Space 73:129. https://doi.org/10.1186/s40623-021-01453-w

2

Earth, Planets and SpaceFRONTIER LETTER

Open AccessWave–particle interaction effects in the Van Allen belts
Daniel N. Baker
Earth, Planets and Space 2021, 73:189 DOI: 10.1186/s40623-021-01508-y
Received: 4 June 2021, Accepted: 24 August 2021, Published: 19 October 2021

Abstract
Discovering such structures as the third radiation belt (or “storage ring”) has been a major observational achievement of theNASA Radiation Belt Storm Probes program (renamed the “Van Allen Probes” mission in November 2012). A goal of that
program was to understand more thoroughly how high-energy electrons are accelerated deep inside the radiation belts—andultimately lost—due to various wave–particle interactions. Van Allen Probes studies have demonstrated that electrons
ranging up to 10 megaelectron volts (MeV) or more can be produced over broad regions of the outer Van Allen zone ontimescales as short as a few minutes. The key to such rapid acceleration is the interaction of “seed” populations of ~ 10–200
keV electrons (and subsequently higher energies) with electromagnetic waves in the lower band (whistler-mode) chorus
frequency range. Van Allen Probes data show that “source” electrons (in atypical energy range of one to a few tens of keV energy) produced bymagnetospheric substorms play a crucial role in feeding free energy into
the chorus waves in the outer zone. These chorus waves then, in turn,rapidly heat and accelerate the tens to hundreds of keV seed electrons
injected by substorms to much higher energies. Hence, we often see thatgeomagnetic activity driven by strong solar storms (coronal mass ejections,or CMEs) commonly leads to ultra-relativistic electron production throughthe intermediary step of waves produced during intense magnetosphericsubstorms. More generally, wave–particle interactions are of fundamental
importance over a broad range of energies and in virtually all regions of
the magnetosphere. We provide a summary of many of the wave modes
and particle interactions that have been studied in recent times.
Courtesy of NASA

Keywords: Plasma waves, Energetic particles, Radiation belts, Acceleration

Graphical abstract

Corresponding author: Daniel N. Baker, daniel.baker@lasp.colorado.edu

FULL PAPER

Open AccessResonant interaction of relativistic electrons with realistic
electromagnetic ion–cyclotron wave packets
Veronika S. Grach*, Andrei G. Demekhov and Alexey V. Larchenko
Earth, Planets and Space 2021, 73:129 DOI: 10.1186/s40623-021-01453-w
Received: 25 March 2021, Accepted: 5 June 2021, Published: 21 June 2021

Abstract
We study the influence of real structure of electromagnetic ion-cyclotron wave packets in the Earth’s radiation belts on
precipitation of relativistic electrons. Automatic algorithm is used to distinguish isolated elements (wave packets) and obtain
their amplitude and frequency profiles from satellite observations by Van Allen Probe B. We focus on rising-tone EMIC wave
packets in the proton band, with a maximum amplitude of 1.2–1.6 nT. The resonant interaction of the considered wave
packets with relativistic electrons 1.5–9 MeV is studied by numerical simulations. The precipitating fluxes are formed as aresult of both linear and nonlinear interaction; for energies 2–5 MeV precipitating fluxes are close to the strong diffusion limit.
The evolution of precipitating fluxes is influenced by generation of higher-frequency waves at the packet trailing edge near
the equator and dissipation of lower-frequency waves in the H＋ cyclotron resonance region at the leading edge. The wave
packet amplitude modulation leads to a significant(a)change of precipitated particles energy spectrum
during short intervals of less than 1 minute. For
short time intervals about 10–15 s, theapproximation of each local amplitude maximumof the wave packet by a Gaussian amplitude profile
(b)
24
21
1
22
6 8 9 11 13 14 16
30 31
27
7 28
and a linear frequency drift gives a satisfactory
3
19 20
1
4 5
25
29
12
7
23
2
1
10
26
description of the resonant interaction.
15
0.1
3. 0
2. 5

f [Hz]

1. 5

1 0- 2

1. 0
0. 5

1 0- 3|Bw | [nT]0. 0Keywords: Resonant interaction, Radiation belts,
Precipitation, Relativistic electrons,
EMIC waves

0.01
17

18

0.001

Fig. 1*Corresponding author: Veronika S. Grach, vsgrach@ipfran.ru
3

PSD [nT2 /Hz]

1 0- 1

2. 0

VLF/ELF Remote Sensing of Ionospheres and Magnetospheres

FULL PAPER

Open AccessSubpacket structure in strong VLF chorus rising tones:
characteristics and consequences for relativistic electron
acceleration
John C. Foster*, Philip J. Erickson and Yoshiharu Omura
Earth, Planets and Space 2021, 73:140 DOI: 10.1186/s40623-021-01467-4
Received: 16 February 2021, Accepted: 21 June 2021, Published: 7 July 2021

Abstract
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 innermagnetosphere. Analysis of the frequency gap between lower and upper chorus-band waves identifies fceEQ, the electron
gyrofrequency in the equatorial wave generation region. Initialsubpackets in these strong chorus rising-tone elements begin at afrequency near 1/4 fceEQ and exhibit smooth gradual frequency
increase across their > 10 ms temporal duration. A second muchstronger 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 theinitial 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 theirnonlinear interaction with a single chorus element.

Keywords: VLF chorus, Subpackets, Radiation belt, Nonlinear
interaction, Electron acceleration

Graphical abstract*Corresponding author: John C. Foster, jcfoster@mit.edu

FULL PAPER

Open AccessA review of unusual VLF bursty-patches observed in
Northern Finland for Earth, Planets and SpaceClaudia Martinez-Calderon*, Jyrki K. Manninen, Jemina T. Manninen and Tauno Turunen
Earth, Planets and Space 2021, 73:191 DOI: 10.1186/s40623-021-01516-y
Received: 1 June 2021, Accepted: 7 September 2021, Published: 19 October 2021

Abstract
Using numerical filtering techniques allowing us to reduce noise from sferics, we are able to clearly study a new type ofdifferently 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 theelectron 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 underinvestigation. This paper unifies the nomenclature by regrouping allthese waves detected at frequencies higher than the local equatorial
0.5 fce at the L-shell of observation under the name of VLF burstypatches. 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 withvarying periodicity and shape. They are sometimes characterized bysingle bursts covering very large frequency ranges of several kHz.
We also give a review of the different characteristics of VLF burstypatches observed at Kannuslehto, which at the moment, is thestation with the highest observation rate. We present recent
observations between 2019 and 2021.

Keywords: VLF waves, Magnetospheric waves, ELF/VLF, VLF
bursty-patches

Graphical abstract*Corresponding author: Claudia Martinez-Calderon, claudia@isee.nagoya-u.ac.jp
4Earth, Planets and Space

FULL PAPER

Open Access

The numerical simulation of the generation of lower-band
VLF chorus using a quasi-broadband Vlasov Hybrid
Simulation code
David Nunn
Earth, Planets and Space 2021, 73:222 DOI: 10.1186/s40623-021-01549-3
Received: 7 August 2021, Accepted: 19 November 2021, Published: 11 December 2021

Abstract
In this paper, we perform the numerical modelling of lower-band VLF chorus in the earth’s magnetosphere. Assuming parallel
propagation the 1d3v code has one spatial dimension z along the ambient magnetic field, which has a parabolic z
dependence about the equator. The method used is Vlasov Hybrid Simulation (VHS) also known in the literature as themethod of Kinetic Phase Point Trajectories (Nunn in Computer Physics Comms 60:1–25, 1990, J Computational Phys
108(1):180–196, 1993; Kazeminezhad et al. in Phys Rev E67:026704, 2003). The method is straightforward and easy to program,and robust against distribution function filamentation. Importantly, VHS does not invoke unphysical smoothing of the
distribution function. Previous versions of the VLF/VHS code had a narrow bandwidth ~ 100 Hz, which enabled simulation of awide variety of discrete triggered emissions. The present
quasi-broadband VHS code has a bandwidth of ~ 3000
Hz, which is far more realistic for the simulation of chorus
in its entirety. Further, the quasi-broadband code does
not require artificial saturation, and does not need to
employ matched filtering to accommodate large spatial
frequency gradients. The aim of this paper which has
been achieved is to produce VLF chorus Vlasov
simulations employing a systematic variety of triggering
input signals, namely key down, single pulse, PLHR, andGraphical abstract
broadband hiss.
Corresponding author: David Nunn, dave@djn14.plus.com

FULL PAPER

Open Access

Simulation study on parametric dependence ofwhistler-mode hiss generation in the plasmasphere
Yin Liu*, Yoshiharu Omura and Mitsuru Hikishima
Earth, Planets and Space 2021, 73:230 DOI: 10.1186/s40623-021-01554-6
Received: 13 July 2021, Accepted: 3 December 2021, Published: 16 December 2021

Abstract
We conduct electromagnetic particle simulations to examine the applicability of nonlinear wave growth theory to the generation
process of plasmaspheric hiss. We firstly vary the gradient of the background magnetic field from a realistic model to a rather steep
gradient model. Under such variation, the threshold amplitude in the nonlinear theory increases quickly and the overlap between
threshold and optimum amplitude disappears correspondingly, the nonlinear process is suppressed. In the simulations, as we enlarge
the gradient coefficient of the background magnetic field, waves generated near the equator do not grow through propagation. By
examining the range of suitable values of inhomogeneity factor S (i.e., |S| < 2),(c) Case 3(a) Case 1(b) Case 2
we find the generation of wave packets is limited to the equatorial region when
the background field is steep, showing a good agreement with what is indicatedby critical distance in the theory. We then check the dependence of generationof hiss emissions on different hot electron densities. Since the overlap between
threshold and optimum amplitude vanishes, the nonlinear process is weakened
when hot electron density becomes smaller. In the simulation results, we findsimilar wave structures in all density cases, yet with different magnitudes. The
existence of suitable S values implies that the nonlinear process occurs even at alow level of hot electron density. However, by examining JE that closely relates
to the wave growth, we find energy conveyed from particles to waves is muchlimited in small density cases. Therefore, the nonlinear process is suppressed
when hot electron density is small, which agrees with the theoretical analysis.

Keywords: Plasmaspheric hiss, Nonlinear, Particle simulation, Gradientcoefficient, Electron density

Graphical abstract*Corresponding author: Yin Liu, liu.yin.52z@st.kyoto-u.ac.jp
5

VLF/ELF Remote Sensing of Ionospheres and Magnetospheres

FULL PAPER

Open AccessOn the use of ELF/VLF emissions triggered by HAARP to
simulate PLHR and to study associated MLR events
Michel Parrot*, Frantisěk Němec*, Morris B. Cohen* and Mark Gołkowski*
Earth, Planets and Space 2022, 74:4 DOI: 10.1186/s40623-021-01551-9
Received: 15 September 2021, Accepted: 25 November 2021, Published: 4 January 2022

Abstract
A spectrogram of Power Line Harmonic Radiation (PLHR) consists of a set of lines with frequency spacing corresponding exactly to 50 or 60
Hz. It is distinct from a spectrogram of Magnetospheric Line Radiation (MLR) where the lines are not equidistant and drift in frequency. PLHR
and MLR propagate in the ionosphere and the magnetosphere and are recorded by ground experiments and satellites. If the source of PLHR isevident, the origin of the MLR is still under debate and the purpose of this paper is to understand how MLR lines are formed. The ELF waves
triggered by High-frequency Active Auroral Research Program (HAARP) in the ionosphere areused to simulate lines (pulses of different lengths and different frequencies). Several receivers
are utilized to survey the propagation of these pulses. The resulting waves are simultaneously
recorded by ground-based experiments close to HAARP in Alaska, and by the low-altitudesatellite DEMETER either above HAARP or its magnetically conjugate point. Six cases arepresented which show that 2-hop echoes (pulses going back and forth in themagnetosphere) are very often observed. The pulses emitted by HAARP return in the
Northern hemisphere with a time delay. A detailed spectral analysis shows that sidebands
can be triggered and create elements with superposed frequency lines which drift infrequency during the propagation. These elements acting like quasi-periodic emissions are
subjected to equatorial amplification and can trigger hooks and falling tones. At the end allthese known physical processes lead to the formation of the observed MLR by HAARP pulses.
It is shown that there is a tendency for the MLR frequencies of occurrence to be around 2 kHz
although the exciting waves have been emitted at lower and higher frequencies.

Keywords: Ionospheric heating, Power Line Harmonic Radiation, Magnetospheric Line
Radiation, HAARP, DEMETER

Graphical abstract*Corresponding author: Michel Parrot, mparrot@cnrs-orleans.fr; Frantisěk Němec, frantisek.nemec@gmail.com; Morris B. Cohen,
mcohen@gatech.edu; Mark Gołkowski, mark.golkowski@ucdenver.edu

FULL PAPER

Open Access

Ground-based VLF wave intensity variations investigatedby the principal component analysis
Barbora Bezděková*, František Němec and Jyrki Manninen
Earth, Planets and Space 2022, 74:30 DOI: 10.1186/s40623-022-01588-4
Received: 28 November 2021, Accepted: 28 January 2022, Published: 14 February 2022

Abstract
Very low frequency wave intensity variations measured by the Kannuslehto station, Finland in the frequency range 0–12 kHz between
2016 and 2020 are analyzed by the principal component analysis (PCA). As the analyzed ground-based measurements are basicallycontinuous, the length of individual basis vectors entering into PCA is fundamentally arbitrary. To better characterize both long- andshort-period variations, two PCAs with different lengths of the basis vectors are eventually performed. Specifically, either dailyfrequency–time spectrograms or individual frequency spectra are chosen as the PCA basis vectors. Analysis of the first three principal
components shows substantial variations of the wave
intensity due to seasonal and local time effects.
(a)
(b)
(c)
Intensity variations related to the geomagnetic activity
characterized by Kp and AE indices and standard
deviation of the magnetic field magnitude are lesssignificant. Moreover, PCA allows one to distinguishbetween nighttime and daytime Kannuslehto
variations and study them independently. Solar andgeomagnetic activity effects on the daytime and
(d)
(e)
(f)
nighttime measurements are discussed. Wave intensity
variations related to substorm occurrence are also
analyzed.
100

60

2016/20172017/20182018/20192019/202050

2016/20172017/20182018/20192019/202040

30

2016/20172017/20182018/20192019/2020

20

0

PC3

PC2

PC1

20
0

10
0

-20

-50

-10

-40

-100
9

10

11

12

Month

1

2

3

9

4

150

20

100

10

11

12

Month

1

2

3

-20
9

4

10

11

12

Month

1

2

3

15
10
5

50

PC3

0

PC2

PC1

10

0

0

-10

Keywords: ELF/VLF wave intensity, Principal

component analysis, Geomagnetic
activity, Solar activity, Seasonal variations

-5

-50

-20

-100
-150
0

5

10

UT

15

20

-30
0

-10

5

10

UT

15

20

Graphical abstract*Corresponding author: Barbora Bezděková, barbora.bezdekova@mff.cuni.cz
6

-15
0

5

10

UT

15

20

4

Earth, Planets and Space

FULL PAPER

Open AccessRole of hard X-ray emission in ionospheric D-layer
disturbances during solar flares
Carine Briand*, Mark Clilverd, Srivani Inturi and Baptiste Cecconi
Earth, Planets and Space 2022, 74:41 DOI: 10.1186/s40623-022-01598-2
Received: 26 November 2021, Accepted: 23 February 2022, Published: 15 March 2022

Abstract∆t (s)

Any disturbance of the ionosphere may affect operational activities based on HF communication. The electron density is a
critical parameter that controls levels of HF-signal absorption. A significant part of the HF absorption takes place in the
D-layer. The increase of X radiations during solar flares generates noticeable perturbations of the electron density of the
D-layer. However, the ionosphere reacts with some delay to the solar forcing. Several studies have addressed this question ofionospheric sluggishness from the time delay between VLF narrow-band transmissions and soft X-ray emissions during solar
flares. Our study initially considers the interpretation of the VLF amplitude time profile. In particular, we show that the
maximum of X-ray emission can be associated with a reversal in the VLF amplitude variation with time, i.e. exhibiting a peak or
a trough. Then, building on this insight, we perform estimates of the time delay between VLF and soft X-rays during 67 events
between 2017 and 2021, thus including the major flares of 2017. We show that the time delay can become negative for flares
above X2, proving that soft X-rays are not the initial source of ionization in the case of major flares. From a careful analysis of
RHESSI data for some events of September
600
2017, we demonstrate that radiation above 40
400200
keV (i.e. hard X-rays) is an important forcing
0
GQD
source of the ionosphere. This is of crucial
-200
NRK
NSY
-400
interest in the frame of space weather
Mean
-600
400
Basak
forecasting since the hard X-rays are produced
200
Tan
0
several minutes before the peak of soft X-rays.
10-5

10-4

10-3

10-4

10-3

∆t (s)

10-6-200

Keywords: VLF, Ionosphere, Solar flares,

-400
10-6

X-ray

10-5

Flare Flux (W/m2)

Graphical abstract*Corresponding author: Carine Briand, carine.briand@obspm.fr

FULL PAPER

Open Access

Lightning evolution and VLF perturbations associatedwith category 5 TC Yasa in the SouthRedoblado
Pacific
Regionet al. Earth,Planets and Space(2022) 74:65

Paolo A. A. L. Redoblado, Sarwan Kumar, Abhikesh Kumar* and Sushil Kumar
Earth, Planets and Space 2022, 74:65 DOI: 10.1186/s40623-022-01632-3
Received: 12 November 2021, Accepted: 14 April 2022, Published: 28 April 2022

Abstract
In this paper, we present the D-region ionospheric response during the lifespan (10–19 December 2020) of a severe category 5
tropical cyclone (TC) Yasa in the South Pacific by using the very low frequency (VLF, 3–30 kHz) signals from NPM, NLK, and JJI
transmitters recorded at Suva, Fiji. Results indicate enhanced lightning and convective activity in all three regions (eyewall,inner rainbands, and outer rainbands) during the TC Yasa that are also linked to the wave-sensitive zones of these transmitter–receiver great circle paths. Of the three regions, the outer rainbands showed the maximum lightning occurrence; hence
convective activity. Prominent eyewall lightning was observed just before the TC started to weaken following its peakintensity. Analysis of VLF signals amplitude showed both negative and positive perturbations (amplitudes exceeding ± 3σmark) lasting for more than 2 h with maximum change in the daytime and nighttime signal amplitudes of − 4.9 dB (NPM) and− 19.8 dB (NLK), respectively. The signal perturbations were wave-like, exhibiting periods of oscillations between ~ 2.2 and 5.5
h as revealed by the Morlet wavelet analysis. Additionally, the LWPC modeling of the signal perturbations indicated a 10 km
increase in the daytime D-region reference height, H' , and a
12 km decrease in the nighttime D-region H' during TC Yasa.
The D-region density gradients (sharpness), β, showed small
perturbations of 0.01–0.14 km−1 from its normal values. We
suggest that the observed changes to the D-region
parameters are due to the enhanced convection during TC
Yasa which excites atmospheric gravity waves producing
traveling ionospheric disturbances to the D-region.

Keywords: VLF perturbations, Lightning, D-region, Tropical
cyclones

Graphical abstract*Corresponding author: Abhikesh Kumar, abhikesh.kumar@usp.ac.fj
7

VLF/ELF Remote Sensing of Ionospheres and Magnetospheres

FULL PAPER

Open Access

Nonlinear triggering process of whistler-mode emissions
in a homogeneous magnetic fieldYuya Fujiwara*, Takeshi Nogi and Yoshiharu Omura
Earth, Planets and Space 2022, 74:95 DOI: 10.1186/s40623-022-01646-x
Received: 16 July 2021, Accepted: 15 May 2022, Published: 18 June 2022

Abstract
We perform an electromagnetic particle simulation of triggered emissions in a uniform magnetic field for understanding of nonlinear wave–
particle interaction in the vicinity of the magnetic equator. A finite length of a whistler-mode triggering wave packet with a constant frequency
is injected by oscillating an external current at the equator. We find that the first subpacket of triggered emissions is generated in thehomogeneous magnetic field. By analyzing resonant currents and resonant electron dynamics in the simulation, we find that the formation ofan electron hole in a velocity phase space results in resonant currents, and the currents cause wave amplification and frequency increase. We
obtain the interaction time of counter-streaming resonant electrons in a triggering wave packet with a finite width. By changing the duration
time of the triggering pulse, we evaluate the interaction
time necessary for formation of an electron hole. We
conduct 4 runs with different duration times of thetriggering pulse, 980, 230, 105, 40 Ω−1e , which correspondto cases with interaction times, 370%, 86%, 39%, and 15%of the nonlinear trapping period, respectively. We findgeneration of triggered emissions in the three cases of
370%, 86%, and 39%, which agrees with the conventional
nonlinear model that the nonlinear transition time, which
is necessary for formation of resonant currents, is about aquarter of the nonlinear trapping period.

Keywords: Wave–particle interaction, Whistler-mode
waves, Triggered emissions, Cyclotron
resonance, Phase-bunching, Particle
simulation

Graphical abstract*Corresponding author: Yuya Fujiwara, fujiwara.yuya.53v@kyoto-u.jp

EXPRESS LETTER

Open Access

Effect of the mirror force on the collision rate due toenergetic electron precipitation: Monte Carlo simulations
Yuto Katoh*, Paul Simon Rosendahl, Yasunobu Ogawa, Yasutaka Hiraki and Hiroyasu Tadokoro
Earth, Planets and Space 2023, 75:117 DOI: 10.1186/s40623-023-01871-y
Received: 16 June 2022, Accepted: 10 July 2023, Published: 2 August 2023

Abstract
We study the effect of the mirror force on the collision rate due to the energetic electron precipitation
into the ionosphere. We solve the motion of individual precipitating electrons with the mirror force,
where collisions with neutral gas are computed by the Monte Carlo method. By comparing the resultswith those without the mirror force, we examine the effect of the mirror force on the altitude profile ofthe ionization rate. First, we carry out simulations of mono-energetic precipitation of 3 keV electrons
whose initial pitch angle is 70 degrees at 400 km at L = 6.45. We find that the collision rate peaks at around
120 km altitude and that the duration of the collision is scattered in time with a delay of about 5 mscompared with the result without mirror force. Next, we perform mono-energetic precipitation of thedifferent energy and pitch angle ranges. Simulation results demonstrate that larger kinetic energy lowers
the altitude profiles of the collision rate, consistent with previous studies. We also find that the upward
motion of electrons bounced back from their mirror points results in the upward broadening of thealtitude profile of the collision rate. Simulation results for electrons with kinetic energies above 100 keVshow that a secondary peak of the collision rate is formed near the mirror point. The formation of thesecondary peak can be explained by the stagnation of electrons around the mirror point at 130 km
altitude, because the relatively long duration of staying in neutral gas increases the number of collisions.
Simulation results show that under the precipitation of electrons in the kinetic energy range larger thantens of keV with the pitch angle close to the loss cone, the maximum collision rate in the altitude range
lower than 100 km becomes one order of the magnitude smaller. The results of the present study suggest
the importance of the mirror force for the precise modeling of ionospheric response due to the energetic
electron precipitation caused by the pitch angle scattering through wave–particle interactions.

Keywords: Energetic electron precipitation, Collision rate, Elastic scattering, Numerical simulation*Corresponding author: Yuto Katoh, yuto.katoh@tohoku.ac.jp
8Graphical abstract

Earth, Planets and SpaceTECHNICAL REPORT

Open Access

Development of miniaturized pick-up amplification circuit
for plasma particle detectors on board satellites
Motoyuki Kikukawa*, Kazushi Asamura, Takahiro Zushi, Satoshi Kurita and Hirotsugu Kojima
Earth, Planets and Space 2022, 74:188 DOI: 10.1186/s40623-022-01746-8
Received: 27 May 2022, Accepted: 2 December 2022, Published: 23 December 2022

Abstract
Plasma particles and waves are important observation targets in space plasmas for understanding the mechanisms of energy
and momentum transfer between waves and particles because space plasmas are essentially collisionless. Multi-point
observations are crucial for understanding the spatial–temporal variations of space plasmas. To realize such observations by alarge number of satellites, onboard instruments should be miniaturized to reduce their required resources. This paper
proposes a small amplifier for plasma particle detectors onboard satellites. This charge-sensitive amplifier converts an electron
cloud emitted from the detector, for example a microchannel plate, to a current pulse that can be handled by a time-of-flightmeasurement circuit to determine the particle velocity and thus mass. The amplifier is realized using application-specificintegrated circuit technology to minimize size. Its dimensions are estimated to be 2120 µm × 1680 µm, which are muchsmaller than those of a conventional amplifier.
The response time of the proposed amplifier
has a variation of less than 1.2 ns over the rangeof expected input levels. The amplifier canhandle up to 2 × 107 signals per second andhas a sensitivity of 1.5 V/pC at 20°C.

Keywords: ASIC, Multi-point observation,
Amplifier for time-of-flighttechnique

Graphical abstract*Corresponding author: Motoyuki Kikukawa, kikukawa.motoyuki.75u@st.kyoto-u.ac.jp

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Contents
Special issue “VLF/ELF remote sensing of ionospheres and magnetospheres”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yoshiharu Omura, Jacob Bortnik, Mark Clilverd, Andrei Demekhov and Yohei Miyake
Wave–particle interaction effects in the Van Allen belts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Daniel N. Baker
Resonant interaction of relativistic electrons with realistic electromagnetic ion–cyclotron wave packets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Veronika S. Grach, Andrei G. Demekhov and Alexey V. Larchenko
Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron
acceleration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . John C. Foster, Philip J. Erickson and Yoshiharu Omura
A review of unusual VLF bursty-patches observed in Northern Finland for Earth, Planets and Space. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Claudia Martinez-Calderon, Jyrki K. Manninen, Jemina T. Manninen and Tauno Turunen
The numerical simulation of the generation of lower-band VLF chorus using a quasi-broadband Vlasov Hybrid
Simulation code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . David Nunn
Simulation study on parametric dependence of whistler-mode hiss generation in the plasmasphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yin Liu, Yoshiharu Omura and Mitsuru Hikishima
On the use of ELF/VLF emissions triggered by HAARP to simulate PLHR and to study associated MLR events. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Michel Parrot, Frantisěk Němec, Morris B. Cohen and Mark Gołkowski
Ground-based VLF wave intensity variations investigated by the principal component analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barbora Bezděková, František Němec and Jyrki Manninen
Role of hard X-ray emission in ionospheric D-layer disturbances during solar flares. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carine Briand, Mark Clilverd, Srivani Inturi and Baptiste Cecconi
Lightning evolution and VLF perturbations associated with category 5 TC Yasa in the South Pacific Region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paolo A. A. L. Redoblado, Sarwan Kumar, Abhikesh Kumar and Sushil Kumar
Nonlinear triggering process of whistler-mode emissions in a homogeneous magnetic field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuya Fujiwara, Takeshi Nogi and Yoshiharu Omura
Effect of the mirror force on the collision rate due to energetic electron precipitation: Monte Carlo simulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuto Katoh, Paul Simon Rosendahl, Yasunobu Ogawa, Yasutaka Hiraki and Hiroyasu Tadokoro
Development of miniaturized pick-up amplification circuit for plasma particle detectors on board satellites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Motoyuki Kikukawa, Kazushi Asamura, Takahiro Zushi, Satoshi Kurita and Hirotsugu Kojima

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Cover image: Omura, Y. et al., Earth, Planets and Space 2024, 76:53 DOI: 10.1186/s40623-024-02000-z