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
The ionospheric variability is to a large extent driven by the solar and
geomagnetic activity and is influenced by plasma transport, electric fields
and currents, and neutral winds and atmospheric waves
In September 2016, a COSPAR Capacity Building workshop on space weather and its effects on the Earth's system was held in Paratunka, Kamchatka, Russia. During the workshop students were grouped into several teams in order to analyze complex solar–terrestrial events. One team led by Ilya Edemskiy and supervised by Jan Lastovicka was asked to analyze the solar–terrestrial events of August 2015. One of them, the event of 15 August 2015, revealed strong and unexpected localized enhancement of TEC observed near local noon in a region between Africa and Antarctica – i.e., at southern higher midlatitudes. This was a by-product of the requested analysis but a scientifically interesting puzzle. This paper deals with analysis of that puzzle.
Section 2 describes the data and methods used. Section 3 presents results and discussion. Section 4 contains conclusions.
Variation of geomagnetic indices Kp and Dst, and solar wind parameters (proton density and Bz) during August 2015. Colored columns mark the three geomagnetic storms. I, M, R – onset of initial, main and recovery phases of geomagnetic storms, respectively.
Three-day “satellite environment” around 15 August 2015 (NOAA/SWPC Boulder, CO, USA). Panels from top to bottom: proton fluxes, electron fluxes at geosynchronous altitude, GOES Hp, Kp. I, M, R – onset of initial, main and recovery phases of geomagnetic storms, respectively.
Geomagnetic indices (Kp and Dst) data used to characterize the general
geophysical situation were taken from the NASA Coordinated Data Analysis
Web. CDAWeb: SOHO:
The ionosphere is primarily characterized by TEC, taken from the Bern (CODE)
hourly global ionospheric maps (GIMs) GIM:
TRIGNET: COSMIC: SuperDARN plots: RBSP:
SANSA:
wdc.kugi.kyoto-u.ac.jp/igrf/gggm/index.html
To find local TEC enhancement (LTE) in GIM maps we developed a method based
on the analysis of deviations of individual TEC values
To describe the overall solar–terrestrial situation in August 2015 we are using geomagnetic indices Kp and Dst. Figure 1 shows that during August 2015 the disturbed geomagnetic conditions occurred around 15 August, 22–23 August and 26–29 August. The event of 22–23 August was weak. The event of 15 August was relatively rapid, which indicates CME origin, whereas the third event was rather slow, which suggests a different origin. Here we shall focus on the event of 15 August, because it was accompanied by an unexpected phenomenon in the Southern Hemisphere (SH) ionosphere.
Global total electron density (TEC) distribution at 12:00 UT for
the
quiet day of 5 August 2015
TEC distribution at the Southern Hemisphere based on TRIGNET data.
Electron density profiles for the quiet day of 5 August 2015
Solar data confirm the existence of a CME probably responsible for the
15 August geomagnetic storm. The 15 August geomagnetic storm was induced by
the 12 August partial-halo CME, which was visible on
SOHO/LASCO C2 imagery. SOHO:
Such a geomagnetic storm should give rise to an ionospheric storm. To
investigate the storm-induced ionospheric disturbances, TEC data have been
used, because this is the only continuously monitored ionospheric parameter
with global and reasonably dense coverage. GIMs of TEC taken from Bern Centre
for Orbit Determination in Europe (CODE GIM:
Dynamics of
Global hourly TEC maps for the first appearance of the southern
higher latitude TEC enhancement plumes (05:00 UT,
All the above ionospheric data confirm an anomalous enhancement of electron
density in the two plumes between Africa and Antarctica around 12:00 UT.
However, how do these two regions evolve with time? We present global CODE
TEC maps at 05:00, 13:00 and 15:00 UT in Fig. 7 to demonstrate the
development of this enhancement. Figure 7 shows that a weak but detectable
enhancement of TEC appears in two plumes at 05:00 UT. The southern plume
appears near the southwest coast of Australia (top panel) at longitudes
corresponding to early afternoon (
Estimation of polar plasma convection intensity and Heppner–Maynard
boundary position (
Energy flux of precipitating electrons and ions over the south polar region at 12:00 UT on 15 August 2015 by OVATION Prime data.
Occurrence of LTEs over the South African midlatitude sector during
the period 2002–2015
Occurrence of LTEs over the South African midlatitude sector during the
period 2012–2015
What might be the reason for the appearance of these two plumes? The Southern Hemisphere has two specific regions, the South Atlantic magnetic anomaly and the Weddell Sea anomaly. However, when approaching the Weddell Sea anomaly, the southern TEC cell rapidly weakens and it disappears within about 2 h. The core of the South Atlantic magnetic anomaly is located more northward but it can affect energetic particle fluxes even at magnetic latitudes corresponding to the southern TEC plume. However, the ionospheric anomaly was “born” too eastward to be excited/created by the South Atlantic magnetic anomaly related particle precipitation and other processes.
Conditions in the magnetosphere during the storm period can be analyzed with
SuperDARN measurements allowing us to see dynamics of plasma convection over
the southern high latitudes. The SuperDARN provides pattern qualitatively
expected for such a magnetic storm. Measured velocities show that the most
intense convection during the magnetic storm is observed at the same time as
the TEC disturbance. Estimated values of a potential across the polar cap are
higher than those for the previous days. Figure 8 shows the SuperDARN
measurements of plasma velocities and estimated values of electric field
potential at 12:00 UT of 15 August. The estimated location of the
Heppner–Maynard boundary
Figure 9 presents the particle flux at 12:00 UT of 15 August according to the
Ovation Prime data. The precipitation zone is located mostly over the Pacific
Ocean and it reveals no pronounced flux increase near the center of the
southern plume location (
The southern plume is located at relatively high latitudes. So what is its
position with respect to the auroral oval? OVATION map (Fig. 9) shows that
the center of the southern plume at 12 UT (
We showed above that LTEs observed in GIMs have also been observed by ionosondes and by COSMIC electron density profiles, at least in the South African region. So we can rely on GIMs to make preliminary conclusions on the LTE occurrence rate and when looking on other possible LTE events. Above, we analyzed one event associated with magnetic storm of 15 August 2015. Now we shall search for the possible occurrence of other LTE events.
Using the method described in Sect. 2 we analyzed GIMs for the period
2002–2016 to find LTEs over the South African midlatitude sector. Figure 10
presents variations of the Dst index (top panel) and of F10.7 (middle panel), and
days of detected LTEs (bottom panel). It is evident from Fig. 10 that LTEs
occur only during higher solar activity periods. We have not detected any LTE
during the low solar activity period 2006–2009. It is important to notice
that not all the strong negative Dst events (Dst
Ionospheric effects were investigated during a moderate geomagnetic storm of 15 August 2015, which was probably caused by a CME of 12 August. An unexpected localized TEC enhancement (LTE) in the form of two plumes centered southward of Africa, observed slightly after noontime, was confirmed by all available ionospheric measurements. The event began at 05:00 UT near the southwestern coast of Australia. It was associated with local time slightly after noontime. The southern plume moved with the Sun (the northern plume slightly slower), it peaked near 13:00 UT southward of South Africa, and it disappeared after about 10 h (the northern plume 2 h later) over the South Atlantic Ocean. A similar LTE does not occur in the Northern Hemisphere, it is a Southern Hemisphere phenomenon. Its origin is not understood yet.
Analysis of GIMs over the period 2002–2015 shows no occurrence of LTE events during the solar activity minimum period 2006–2009. In 2012–2015 we found altogether 26 LTEs and all of them were associated with a southward excursion of Bz. However, the negative Bz excursion is a necessary but not sufficient condition as during some geomagnetic storms associated with the negative Bz excursions the LTE events did not appear.
Future work will include a deeper analysis of collected data set of LTE events for 2002–2015, particularly towards finding physical processes behind the occurrence of LTEs. Figure 3 shows a very weak two-plume structure at somewhat lower latitudes and slightly different longitudes for the quiet day of 5 August; therefore a possible quiet-time effect will also be investigated.
Geomagnetic indices:
IE did most of the work; JL came up with the leading idea and substantially contributed to writing the paper; DB analyzed and interpreted ionosonde data, prepared Fig. 5 and contributed to writing the final version; JBH analyzed and interpreted TRIGNET TEC data, prepared Fig. 3 and contributed to writing the final version; and IN helped with data mining and evaluation.
The authors declare that they have no conflict of interest.
Authors thank David John McComas at SWRI, Joseph H. King and Natalia Papitashvilli at ADNET,
NASA GSFC and CDAWeb for providing space weather data
(