24 research outputs found
A recurrent neural network approach to quantitatively studying solar wind effects on TEC derived from GPS; preliminary results
Application of neural networks to South African GPS TEC modelling
The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa
Statistical analysis of the correlation between the equatorial electrojet and the occurrence of the equatorial ionisation anomaly over the East African sector
This study presents statistical quantification of the correlation between
the equatorial electrojet (EEJ) and the occurrence of the equatorial ionisation
anomaly (EIA) over the East African sector. The data used were for quiet
geomagnetic conditions (Kp ≤ 3) during the period 2011–2013. The
horizontal components, H, of geomagnetic fields measured by magnetometers
located at Addis Ababa, Ethiopia (dip lat. ∼ 1° N), and Adigrat,
Ethiopia (dip lat. ∼ 6° N), were used to determine the EEJ
using differential techniques. The total electron content (TEC) derived from Global
Navigation Satellite System (GNSS) signals using 19 receivers located along
the 30–40° longitude sector was used to determine the EIA
strengths over the region. This was done by determining the ratio of TEC over
the crest to that over the trough, denoted as the CT : TEC ratio. This technique
necessitated characterisation of the morphology of the EIA over the region. We
found that the trough lies slightly south of the magnetic equator
(0–4° S). This slight southward shift of the EIA trough
might be due to the fact that over the East African region, the general
centre of the EEJ is also shifted slightly south of the magnetic equator. For the
first time over the East African sector, we determined a threshold daytime
EEJ strength of ∼ 40 nT that is mostly associated with prominent EIA
occurrence during a high solar activity period. The study also revealed that
there is a positive correlation between daytime EEJ and EIA strengths, with a
strong positive correlation occurring during the period 13:00–15:00 LT
Investigating the performance of neural network backpropagation algorithms for TEC estimations using South African GPS data
In this work, results obtained by investigating the application of
different neural network backpropagation training algorithms are presented.
This was done to assess the performance accuracy of each training algorithm
in total electron content (TEC) estimations using identical datasets in
models development and verification processes. Investigated
training algorithms are standard backpropagation (SBP), backpropagation with
weight delay (BPWD), backpropagation with momentum (BPM) term,
backpropagation with chunkwise weight update (BPC) and backpropagation for
batch (BPB) training. These five algorithms are inbuilt functions within the
Stuttgart Neural Network Simulator (SNNS) and the main objective was to find
out the training algorithm that generates the minimum error between the TEC derived from Global Positioning System (GPS) observations and the modelled TEC
data. Another investigated algorithm is the MatLab based
Levenberg-Marquardt backpropagation (L-MBP), which achieves convergence after
the least number of iterations during training. In this paper, neural network
(NN) models were developed using hourly TEC data (for 8 years: 2000–2007)
derived from GPS observations over a receiver
station located at Sutherland (SUTH) (32.38° S, 20.81° E), South
Africa. Verification of the NN models for all algorithms considered was
performed on both "seen" and "unseen" data. Hourly TEC values over SUTH
for 2003 formed the "seen" dataset. The "unseen" dataset consisted of
hourly TEC data for 2002 and 2008 over Cape Town (CPTN) (33.95° S,
18.47° E) and SUTH, respectively. The models' verification showed that all
algorithms investigated provide comparable results statistically, but differ
significantly in terms of time required to achieve convergence during
input-output data training/learning. This paper therefore provides a guide to
neural network users for choosing appropriate algorithms based on the
availability of computation capabilities used for research
A recurrent neural network approach to quantitatively studying solar wind effects on TEC derived from GPS; preliminary results
This paper attempts to describe the search for the parameter(s) to represent
solar wind effects in Global Positioning System total electron content (GPS
TEC) modelling using the technique of neural networks (NNs). A study is
carried out by including solar wind velocity (Vsw), proton number
density (Np) and the Bz component of the interplanetary magnetic field
(IMF Bz) obtained from the Advanced Composition Explorer (ACE) satellite
as separate inputs to the NN each along with day number of the year (DN),
hour (HR), a 4-month running mean of the daily sunspot number (R4) and the
running mean of the previous eight 3-hourly magnetic A index values (A8).
Hourly GPS TEC values derived from a dual frequency receiver located at
Sutherland (32.38° S, 20.81° E), South Africa for 8 years
(2000–2007) have been used to train the Elman neural network (ENN) and the
result has been used to predict TEC variations for a GPS station located at
Cape Town (33.95° S, 18.47° E). Quantitative results indicate
that each of the parameters considered may have some degree of influence on
GPS TEC at certain periods although a decrease in prediction accuracy is also
observed for some parameters for different days and seasons. It is also
evident that there is still a difficulty in predicting TEC values during
disturbed conditions. The improvements and degradation in prediction
accuracies are both close to the benchmark values which lends weight to the
belief that diurnal, seasonal, solar and magnetic variabilities may be the
major determinants of TEC variability
Daytime twin-peak structures observed at southern African and European middle latitudes on 8–13 April 2012
Daytime twin-peak structures, also known as bite-out or diurnal double-maxima
structures, are ionospheric phenomena in which the diurnal ionospheric trend
shows two peaks (instead of the normal one) during the daytime. This study
reports on first simultaneous observations of these structures in the Global
Positioning System and ionosonde measurements from the southern African and
European middle-latitude stations during a mostly quiet geomagnetic condition
period of 8–13 April 2012, which indicates that their occurrence and
therefore driving mechanism(s) may not be localised. It is found that the
daytime twin-peak structures generally appear later in the Northern
Hemisphere with a 1–3 h latency although they propagate mostly equatorward
in both hemispheres. Proxies of meridional neutral winds were calculated from
available manually scaled ionosonde measurements and used to explore their
potential as drivers of the structures. Bite-out events were linked to
downward drifts of the vertical component of equivalent neutral winds causing
plasma depletions. In addition, evidence of sporadic E layers at the same time
as enhancements of daytime twin-peak structures suggests that the tides had
influence via the meridional wind shear in generating these structures
through the dynamo electric field which resulted in upward
<b>E</b> × <b>B</b> drifts
Determinations of ionosphere and plasmasphere electron content for an African chain of GPS stations
The confluence of recent instrumentation
deployments in Africa with developments for the determination of plasmasphere
electron content using Global Positioning System (GPS) receivers has provided
new opportunities for investigations in that region. This investigation,
using a selected chain of GPS stations, extends the method (SCORPION)
previously applied to a chain of GPS stations in North America in order to
separate the ionosphere and plasmasphere contributions to the total electron
content (TEC) during a day (24 July) in 2011. The results span latitudes from
the southern tip of Africa, across the Equator, to the southern Arabian
Peninsula, providing a continuous latitudinal profile for both the ionosphere
and plasmasphere during this day.The peak diurnal vertical ionosphere electron content (IEC) increases from
about 14 TEC units (1 TEC unit = 1016 electrons m−2) at the
southernmost station to about 32 TEC units near the geographic equator, then
decreases to about 28 TEC units at the Arabian Peninsula. The peak diurnal
slant plasmasphere electron content (PEC) varies between about 4 and
7 TEC units among the stations, with a local latitudinal profile that is
significantly influenced by the viewing geometry at the station location,
relative to the magnetic field configuration. In contrast, the peak vertical
PEC varies between about 1 and 6 TEC units among the stations, with a
more uniform latitudinal variation.Comparisons to other GPS data analyses are also presented for TEC,
indicating the influence of the PEC on the determination of latitudinal TEC
variations and also on the absolute TEC levels, by inducing an overestimate
of the receiver bias. The derived TEC latitudinal profiles, in comparison to
global map profiles, tend to differ from the map results only about as much
as the map results differ among themselves. A combination of ionosonde IEC
and alternative GPS TEC measurements, which in principle permits a PEC
determination through their difference, was compared to the composite and
separate ionosphere and plasmasphere contributions derived solely by the
SCORPION method for one station. Although there is considerably more scatter
in the PEC values derived from the difference of the GPS TEC and ionosonde
IEC measurements compared to the PEC values derived by the SCORPION method,
the average overhead values for this day are comparable for the two methods,
near 2 TEC units, at the South African site examined.This initial investigation provides a basis for day-to-day TEC monitoring
for Africa, with separate ionosphere and plasmasphere electron content
determinations
Unexpected Southern Hemisphere ionospheric response to geomagnetic storm of 15 August 2015
Geomagnetic storms are the most pronounced phenomenon of space weather. When
studying ionospheric response to a storm of 15 August 2015, an unexpected
phenomenon was observed at higher middle latitudes of the Southern
Hemisphere. This phenomenon was a localized total electron content (TEC)
enhancement (LTE) in the form of two separated plumes, which peaked southward
of South Africa. The plumes were first observed at 05:00 UT near the
southwestern coast of Australia. The southern plume was associated with local
time slightly after noontime (1–2 h after local noon). The plumes moved
with the Sun. They peaked near 13:00 UT southward of South Africa. The
southern plume kept constant geomagnetic latitude (63–64° S); it
persisted for about 10 h, whereas the northern plume persisted for about
2 h
more. Both plumes disappeared over the South Atlantic Ocean. No similar
LTE event was observed during the prolonged solar activity minimum period of
2006–2009. In 2012–2016 we detected altogether 26 LTEs and all of them were
associated with the southward excursion of Bz. The negative Bz excursion is a
necessary but not sufficient condition for the LTE occurrence as during some
geomagnetic storms associated with negative Bz excursions the LTE events did
not appear
Estimating the geoeffectiveness of halo CMEs from associated solar and IP parameters using neural networks
Estimating the geoeffectiveness of solar events is of significant
importance for space weather modelling and prediction. This paper describes
the development of a neural network-based model for estimating the
probability occurrence of geomagnetic storms following halo coronal mass
ejection (CME) and related interplanetary (IP) events. This model
incorporates both solar and IP variable inputs that characterize geoeffective
halo CMEs. Solar inputs include numeric values of the halo CME angular width
(AW), the CME speed (<I>V</I><sub>cme</sub>), and the comprehensive flare index (cfi), which
represents the flaring activity associated with halo CMEs. IP parameters used
as inputs are the numeric peak values of the solar wind speed (<I>V</I><sub>sw</sub>) and the
southward Z-component of the interplanetary magnetic field (IMF) or <I>B</I><sub>s</sub>. IP
inputs were considered within a 5-day time window after a halo CME eruption.
The neural network (NN) model training and testing data sets were constructed
based on 1202 halo CMEs (both full and partial halo and their properties)
observed between 1997 and 2006. The performance of the developed NN model was
tested using a validation data set (not part of the training data set) covering
the years 2000 and 2005. Under the condition of halo CME occurrence, this
model could capture 100% of the subsequent intense geomagnetic storms
(Dst ≤ −100 nT). For moderate storms (−100 < Dst ≤ −50), the model is
successful up to 75%. This model's estimate of the storm occurrence rate
from halo CMEs is estimated at a probability of 86%