Probing geomagnetic storm-driven
magnetosphere-ionosphere dynamics in D-region via
propagation characteristics of very low frequency radio
signals
Victor U. J. Nwankwo
a
, Sandip K. Chakrabarti
a,b
, Olugbenga
Ogunmodimu
c
a
S. N. Bose National Centre for Basic Sciences, Kolkata 700098, India
b
Indian Centre for Space Physics, Kolkata 700084, India
c
Department of Physics, Lancaster University, LA1 4YW, UK
Abstract
The amplitude and phase of VLF/LF radio signals are sensitive to changes
in electrical conductivity of the lower ionosphere which imprints its signature
on the Earth-ionosphere waveguide. This characteristics makes it useful in
studying sudden ionospheric disturbances, especially those related to prompt
X-ray flux output from solar flares and gamma ray bursts (GRBs). However,
strong geomagnetic disturbance and storm conditions are known to produce
large and global ionospheric disturbances, which can significantly affect VLF
radio propagation in the D region of the ionosphere. In this paper, using
the data of three propagation paths at mid-latitudes (40
◦
- 54
◦
), we analyze
the trend of aspects of VLF diurnal signal under varying solar and geomag-
netic space environmental conditions in order to identify possible geomag-
netic footprints on the D region characteristics. We found that the trend of
variations generally reflect the prevailing space weather conditions in various
time scales. In particular, the ‘dipping’ of mid-day signal amplitude (MDP)
of VLF always occurs after geomagnetic perturbed or storm conditions in the
time scale of 1-2 days. The mean signal before sunrise (MBSR) and mean
signal after sunset (MASS) also exhibit storm-induced dipping, but they ap-
pear to be influenced by event’s exact occurrence time and highly variable
conditions of dusk-to-dawn ionosphere. We observed fewer cases of the sig-
nals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic
event, though this effect may be related to storms associated phenomena or
Preprint submitted to Atmospheric and Solar-Terrestrial Physics March 10, 2016
effects arising from sources other than solar origin. The magnitude of in-
duced dipping (or rise) significantly depends on the intensity and duration of
event(s), as well as the propagation path of the signal. The post-storm day
signal (following a main event, with lesser or significantly reduced geomag-
netic activity), exhibited a tendency of recovery to pre-storm day level. In
the present analysis, We do not see a well defined trend of the variations of
the post-storm sunrise terminator (SRT) and sunset terminator (SST). The
SRT and SST signals show more post-storm dipping in GQD-A118 propa-
gation path but generally an increase along DHO-A118 propagation path.
Thus the result could be propagation path dependent and detailed modeling
is required to understand these phenomena.
Keywords: D-region ionosphere, Geomagnetic storm, Ionospheric response,
magnetosphere-ionosphere dynamics, VLF radio signals
1. Introduction1
Although separated by thousands of kilometers, the magnetosphere and2
ionosphere are known to be physically connected through the Earth’s mag-3
netic field into one global system. The ionosphere responds to (a) prompt4
changes in solar energetic events, mainly the solar flare associated bursts5
in EUV, X-ray and relativistic particles (Mitra, 1974; Bounsanto, 1999; Al-6
fonsi et al., 2008), (b) delayed changes mainly due to geomagnetic storm7
conditions with time scale from several hours to 1-3 days (Lastovika, 1996;8
Bounsanto, 1999; Kutiev, 2013), and (c) periodic changes with time scales of9
several days to months, and those of several solar cycles (Alfonsi, 2008; Ku-10
tiev, 2013). The ionosphere also exhibits diurnal (day/night) and seasonal11
(summer/winter) variations (Miller and Brace, 1969; Zhang et al., 1999).12
Solar and geomagnetic induced phenomena drive changes in magnetosphere13
conditions, whose coupling effects modify ionospheric signatures including14
atmospheric density distribution, total electron content (TEC), ionospheric15
current system, ionisation rates, and crucial D-region parameters such as con-16
ductivity gradient and reference height (Wait, 1959; Wait and Spies, 1964;17
Mitra, 1974; Buonsanto, 1999; Burke, 2000; Simoes et al., 2012; Nwankwo18
and Chakrabarti, 2014b). The dynamics of ionospheric response to changes in19
solar and geomagnetic conditions, involve the exchange of particles and elec-20
tromagnetic energy (absorbed, reprocessed and deposited in the ionosphere21
by the magnetosphere) b etween magnetically connected regions (Burke, 2000;22
2
Streltsov and Lotko, 2004; Goldstein et al., 2006; Russell et al., 2010; Russell23
and Wright, 2012 Leonard et al., 2012; Kutiev et al., 2013).24
1.1. The ionosphere at a glance25
The ionosphere is composed of three distinct space regions [D (50 km to26
90 km), E (90 km to 120 km), and the F (from 120 km up to 500 km), which27
often split into two layers, namely, F1 and F2]. Its existence is primarily28
due to ionisation by solar ultraviolet (UV) radiation and X-ray wavelength29
(Kelley, 1989; Prolss, 2004; McRae and Thomson, 2004; Raulin et al., 2006;30
Heikkila, 2011) and isotropic cosmic rays. Recombination also occurs when31
free electrons are captured by positive ions. Ionisation and recombination32
efficiency controls the overall electron density at every instant of time. The33
D region ionosphere highly active during the day (roughly between the local34
sunrise and sunset) due to high rate of ionisation, but its density fall signif-35
icantly at night largely due to rapid recombination at the altitude. The E36
region also maintains the same dynamics (night/day fluctuations) as the D37
region but ionisation state persists longer due to slower rate of recombination38
at lower density. Thus, the reflection of signals mainly occurs at the bottom39
of the nighttime E region (Han and Cummer, 2010a and references therein).40
The F region is present both day and night; air density and recombination41
rate is very low in the region. Therefore, ionisation persists in the nighttime42
(also see Mimno, 1937; Poole, 1999; Prolss, 2004). In general, these layers43
are severely disturbed by phenomena of solar and geomagnetic origin, as well44
as planetary and tidal waves, thermospheric tides and stratospheric warming45
(Pancheva et al., 2008; Leonard et al., 2012; Chen et al., 2013; Goncharenko46
et al., 2012; Polyakova et al., 2014). However, effects at different heights, lo-47
cations or latitudes vary in development, depending on time and intensity (of48
driving force). Ionospheric signature variations reflect different mechanisms49
and aspects of solar and other induced phenomena.50
1.2. VLF propagation in the Earth-ionosphere waveguide51
The velocity, direction and amplitude of most electromagnetic waves are52
distinctly affected when propagating through the ionosphere. This character-53
istics makes Radio waves one of the ideal tools for ionospheric study (Prolss,54
2004). Very low frequency (VLF) radio waves in the 3-30 kHz are effective55
in the investigation of solar induced variable conditions in the ionosphere56
(especially the D region) because their amplitude and phase are sensitive to57
changes in electrical conductivity of the lower ionosphere (Wait and Spies,58
3
1964; Mitra, 1974; Alfonsi et al., 2008). VLF radio signals are reflected59
alternately by the D region and the Earth’s surface due to high conductiv-60
ity (Mimno, 1937; Poole, 1999). The transmitted wave is thus guided be-61
tween the Earth and the ionosphere enabling the signal to propagate globally62
through the Earth-Ionosphere waveguide. The signal is then received at var-63
ious receivers across the world. Variations in daytime VLF signal amplitude64
and phase appear to be well correlated with solar X-ray output, with almost65
prompt responses. Hence, it has been used by many researchers to study66
sudden ionospheric disturbances and changes in the atmosphere (e.g., Araki,67
1974; Hayakawa et al., 1996; Molchanov et al., 1998; Kleimenova et al., 2004;68
McRae and Thomson, 2004; Thomas et al., 2004; Chakrabarti et al., 2005;69
Grubor et al., 2005; Peter et al., 2006; Sasmal et al., 2009; Chakrabarti et70
al., 2010; Clilverd et al., 2010; Basak et al., 2011; Pal et al., 2012; Palit et71
al., 2013; Ray et al, 2013; Raulin et al., 2013; Nwankwo and Chakrabarti,72
2014b). Other methods used for ionospheric studies include observational and73
experimental techniques and tools such as Global Navigation Satellite system74
(GNSS) receivers, vertical and oblique sounding, Riometers, incoherent scat-75
ter radars (e.g., EISCAT), coherent scatter radars (e.g., Goose Bay radar,76
SuperDARN), magnetometers, etc. (Greenwald et al., 1995, 1996; Honary77
et al., 1995; Lastovicka, 1996; Wild et al., 2003; Burke, 2000; Danilov and78
Lastovicka, 2001; Goldstein et al., 2005; Ruohoniemi and Greenwald, 2005;79
Alfonsi et al., 2008).80
1.3. VLF signal detection mechanism of sudden ionospheric disturbances81
The D region ionosphere is maintained by Lyman-α radiation at a wave-82
length of about 121.5nm, which ionises neutral nitric oxide (NO). With high83
solar activity, hard X-ray (λ < 1nm) may ionise N
2
and O
2
. Galactic cosmic84
rays are also responsible for the ionisation of the lowest part of the lower85
ionosphere and the low-lying atmosphere down to the troposphere (also, see86
Mitra, 1974; Lastovika, 1996). A huge amount of energy is released during87
solar flare in the form of highly energetic ultraviolet radiation, mainly X-ray88
flux enhancement. The radiation penetrates the D region where it increases89
ionisation rate (of dominant neutral NO molecules), and enhances electron90
density. These processes enhance the ’thickness’ of the D region, thereby91
decreasing the reflection height (h) in the waveguide. This is normally de-92
tected as a sudden change (usually an increase) in the amplitude and phase93
enhancement of a VLF signal. VLF dusk-to-dawn signal exhibit high vari-94
ability (or, fluctuation) during the night due to a significant fall in density95
4
of the D region. The signal is also sensitive to phenomena other than those96
originating from the Sun. Day time VLF signal is primarily controlled by97
the Sun.98
1.4. Geomagnetic induced variations of the ionosphere and effects99
Geomagnetic disturbances and storms are also known to produce signifi-100
cant global disturbances in the ionosphere, including the middle atmosphere101
and troposphere (Lastovika, 1996; Danilov and Lastovika 2001). Geomag-102
netic storms are the products of highly variable solar wind speeds and density103
and associated shock waves (Lastovika, 1986; Baker, 1996, 2000; Borovsky104
and Denton, 2006; Tsurutani et al., 2006; Kozyra et al., 2006). The ef-105
fects of geomagnetic storms on the ionosphere manifest mainly through en-106
ergetic particles precipitation, which lose their energy by impact and X-ray107
bremsstrahlung production (Lastovika, 1996). There is also a consequent and108
significant enhancement of electron density (Chenette et al., 1993; Stoker109
1993; Lastovika, 1996), causing significant increase in radio wave absorp-110
tion and subsequent disappearance of radio signals in MF/HF values (Las-111
tovika, 1996). Galactic cosmic ray flux (which are modulated by geomagnetic112
storms) and global electric circuit and atmosphere electricity (affected by lo-113
cal changes of conductivity and ionosphere/magnetosphere electric fields and114
currents), are assumed to be the processes for ionospheric effects of geomag-115
netic storms (Danilov and Lastovika, 2001). VLF signals can be significantly116
affected by geomagnetic disturbances and storms induced ionosphere per-117
turbations (Kikuchi and Evans, 1983). Nevertheless, a few researchers have118
used it to study these perturbations with insightful findings (e.g., Araki,119
1974; Kleimenova et al., 2004; Peter et al., 2006; Clilverd et al., 2010; Ku-120
mar and Kumar, 2014; Tatsuta et al., 2015).121
122
Apart from X-ray flux induced enhancement of amplitude and phase,123
anomalies in diurnal VLF signature may convey other important informa-124
tion, especially those related to geomagnetic disturbance or storm-induced125
ionospheric variations. If substantiated, such information could be instruc-126
tive and resourceful to the study and understanding of the complex dynamics127
of Earth’s ionosphere. Thus, in addition to well correlated VLF signal am-128
plitude variation and phase enhancement with X-ray flux induced sudden129
ionospheric disturbances (SID), this work seeks to understand possible ge-130
omagnetic activity footprints in the D region of the ionosphere and their131
dependence on the propagation path of VLF radio waves. First, the analysis132
5
![Figure 1: VLF signal propagation paths (PP) used in the study: A118 receiver (thick red circle), DHO transmitter (red star), GQD (brown star), ICV (blue star) [adopted from A118 SID station Web page]](/figures/figure-1-vlf-signal-propagation-paths-pp-used-in-the-study-14ibydw1.webp)
