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Behavior of the Total Electron Content over the Arctic and Antarctic sectors during several intense geomagnetic storms
Geodesy and Geodynamics 10 (2019) 26e36
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Behavior of the Total Electron Content over the Arctic and Antarctic sectors during several intense geomagnetic storms Gustavo A. Mansilla a, b, * n, Av. Independencia 1800, 4000, San Miguel de Departamento de Física, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucuma n, Argentina Tucuma b Consejo Nacional de Investigaciones científicas y T ecnicas, Godoy Cruz 2290, CABA, Argentina a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 June 2018 Accepted 18 January 2019 Available online 31 January 2019
In this paper, the behavior of TEC at three stations located in the Arctic and Antarctic sectors during some intense geomagnetic storms in the period 2012e2016 is analyzed. The results show the opposite storm effects in the Arctic and Antarctic regions. Both the positive and negative TEC disturbances presented more fluctuations over the Arctic stations than over the Antarctic stations. Moreover, the positive TEC disturbances were more significant in winter. The negative disturbances were generally long-lasting, sometimes interrupted by short-duration positive disturbances. Overall, the increases and decreases in TEC can be mainly attributed to changes (i.e., increase and decreases in the O/N2 ratio respectively) in the thermospheric composition, but prompt penetration electric field could be responsible for the initial TEC disturbances. The thermospheric circulation and the disturbance dynamo, which are maintained due to prolonged high-energy input at high latitudes, can also play important roles at the end of main phase and during recovery phase. © 2019 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Geomagnetic storms TEC Arctic and Antarctic sectors
1. Introduction The ionospheric storms (disturbances in the terrestrial ionosphere produced by geomagnetic storms) cause significant disturbances for technological systems, such as the static or dynamical positioning with GNSS satellites, and others [1], which depend on the transionospheric communications. Basically, in comparison with under quiet conditions, the electron density can increase or decrease during geomagnetic storm periods. These changes, which have been called positive ionospheric storm or positive storm effect and negative ionospheric
* Corresponding author. Departamento de Física, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucum an, Av. Independencia 1800, 4000, San n, Argentina. Miguel de Tucuma E-mail address: [email protected]. Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
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storm or negative storm effect respectively, occur because there is significant energy input (from the solar wind) into the polar ionosphere, usually over a period of several hours to a day [2,3]. Several driver forces have been used to explain the ionospheric effects during storms at different latitudes. For example, it is believed that horizontal convection dominates in the polar caps, and composition changes, particle precipitation and electric fields dominate in the auroral zones, while electric fields, meridional winds and composition changes dominate at equatorial and low latitudes (e.g., [1,2,4e8]). In the auroral and polar ionosphere, those irregularities at a different scale have a common feature during geomagnetic storms, which causes fluctuations in the Total Electron Content (TEC) [9]. In turn, the small scale ionospheric plasma density irregularities produce rapid fluctuation in the amplitude and phase of transionospheric radio signals, which is known as scintillation [10]. The large-scale irregularities and associated TEC fluctuations can complicate phase ambiguity resolution, increase the number of uncorrected cycle slips and losses of signal lock in GNSS (e.g., [11,12]). Because there are relatively sparse results for TEC at sub-auroral, auroral and polar latitudes [13], in this paper we analyze the
https://doi.org/10.1016/j.geog.2019.01.004 1674-9847/© 2019 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
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behavior of TEC at three stations located in the Arctic and Antarctic sectors during five intense geomagnetic storms in the period 2012e2016. We also analyze possible physical mechanisms related with the observed ionospheric disturbances.
positive (negative) TEC disturbances correspond to the positive (negative) values of DTEC.
2. Data and methodology
3.1. Geomagnetic storm on 14e16 July 2012
Hourly TEC data from three stations located at high and subauroral latitudes (Kangerlussuaq, Scorebysund/Ittoqqortoomiit and O'Higgins, respectively) during five storms between 2012 and 2016 are considered. TEC data were obtained from the IONOLAB web site (www.ionolab.org). The information of three stations is given in Table 1. Fig. 1 shows the geographical locations of the three stations. The different phases of the storms, namely initial phase, main phase and recovery phase, are determined by the variation of the Dst index. Hourly values of Dst and AE indices were obtained from the World Data Center (WDC) Kyoto, Japan website (http://swdc. kugi.kyoto-u.ac.jp/dstdir). Bz component of the interplanetary magnetic field (IMF) was obtained from the NASA Space Science Data Coordinated Archive website (http://omniweb.gsfc.nasa.gov). Similarly to Blagoveshchensky et al. [14]and Adimula et al. [15], the relative change in TEC (DTEC) was calculated as follows:
Fig. 2 shows the variations of the Dst and AE indices, the Bz component of the interplanetary magnetic field (IMF) and the DTEC for the geomagnetic storm of 14e16 July 2012. The storm onset was at 0600 UT on 15 July (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3. html) and reached the maximum disturbance (approximately e 118 nT) at about 1000 UT on 15 July. The AE index abruptly increased at about 1700 UT on 14 July and reached a peak value (~1300 nT) at 2100 UT. Later, it have decreased for a few hours during the main phase (between about 00 UT and 0600 UT on 15 July) and increased again at the end of the main phase and during the recovery phase, with fluctuations between 1200 UT on 15 July and 1800 UT on 16 July. The interplanetary magnetic field (IMF) Bz component turned southward at about 0230 UT on 15 July and remained largely negative during the main and recovery phases until about 1500 UT on 16 July (peak value ~ -19 nT). The storm onset occurred after local midnight. The increases in TEC can be seen before the storm onset (between about 1800 UT and 2300 UT on 14 July), in association with the sharp increase in AE. The Northern Hemisphere (NH) stations presented similar behavior in response to the storm: irregular long-duration negative disturbances since a few hours before the storm commencement, which remained during 15 and 16 July. The Southern Hemisphere (SH) station initially presented the decrease in TEC. A significant positive disturbance was observed (DTEC higher than 150%) between local
DTEC ¼ TECobs TECavg
TECavg 100%
where TECobs is the observed value of the TEC and TECavg is the average value of those TECobs in the five quietest days of the month of the storm. In general, the positive and negative storm effects are referred to the changes in the electron density at the height of peak concentration (i.e. based on the foF2 or NmF2 parameter). Here, the
3. Results of observations
Table 1 The information of three stations.
O'Higgins (OHI) Kangerlussuaq (KAN) Scorebysund/Ittoqqortoomiit (SCO)
Geog. Latitude
Geog. Longitude
Geomag. Latitude
Geomag. Longitude
63.32oS 67.02oN 70.48oN
57.89oW 50.72oW 21.95oW
53.7oS 75.4oN 74.6oN
11.2oE 35.6oE 78.9oE
Fig. 1. The geographical locations of the three stations.
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G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36 50
1600
16 July 2012
17 July 2012
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18 July 2012
AE
0
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KAN
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0
2
4
6
8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22
UT
Fig. 2. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field (IMF) and DTEC for the period of 14e16 July 2012 storm.
noon and 2200 LT (during the recovery phase), and changed to negative during the recovery phase. The disturbance amplitude in TEC was greater over the station of the SH than at high latitudes of the NH. 3.2. Geomagnetic storm on 26e28 August 2014 Fig. 3 shows the variations of the Dst and AE indices, the Bz component of the interplanetary magnetic field and the DTEC for the period of 26e28 August 2014. The main phase of the storm started at about 0200 UT on 27 August and lasted until 19 UT on 27 August, then Dst reached a minimum value of - 79 nT. Afterwards, Dst showed a slow recovery. The AE index started to increase at about 0300e0400 UT on 27 August and reached a maximum value of 1075 nT near to the end of the main phase. After that, AE initiated an oscillating behavior with lower peaks than before. Bz turned southward nearly simultaneously with the onset of the main phase and remained negative during the main and recovery phases. The maximum value (about e 13 nT) in negative direction appeared at 1630 UT on 27 August (2e3 h before the end of the main phase). The main phase occurred at all the stations between local midnight and afternoon hours. The common feature of the NH stations was irregular increases in DTEC during the main phase (in the daytime hours) and irregular decreases during the recovery phase. The SH station showed an oscillating increase in DTEC since about 1000 UT (0600 LT) on 27 August, which remained during the rest of the main phase and the recovery phase. 3.3. Geomagnetic storm on 21e23 June 2015 Fig. 4 shows the variations of the Dst and AE indices, the Bz component and DTEC for the storm period of 21e23 June 2015. The main phase of the storm started at about 2000 UT on 22 June. The
Dst index reached a minimum value of 204 nT at 0500 UT on 23 June (end of the main phase), then initiated a slow recovery. AE index initially showed a secondary peak at about 0800 UT on 22 June. AE started to increase at about 1400 UT, exhibiting strong fluctuations between 1600 UT on 22 June and 1200 UT on 23 June, then abruptly decreased. The Bz component presented an oscillating behavior during the main and recovery phases. It sharply turned southward at about 1730 UT on 22 June and reached a minimum value of e 26 nT at 1930 UT, but became northward from 2030 UT on 22 June to0130 UT on 23 June. Then, Bz turned again southward during 5 h simultaneously with the end of the main phase. During the recovery phase, Bz became positive for about 2 h and then negative for about 4 h. The main phase of the storm can be seen at all the stations between morning hours on 22 June and local midnight. The NH stations presented similar ionospheric effects: negative disturbances with increasing amplitude, which started at about 2200 UT on 22 June (during the main phase). The maximum deviations (~60%) occurred at the end of the main phase and during the first stage of the recovery. The SH station also presented an oscillating behavior in DTEC. A negative disturbance was observed prior to the storm onset, and changed to positive during the main phase and first part of the recovery, with two irregular peaks between about 1300 UT on 22 June and 0700 UT on 23 June. Then, this positive disturbance changed to negative again during the rest of the recovery phase (amplitude of ~60%). 3.4. Geomagnetic storm on 19e21 December 2015 Fig. 5 shows the variations of the Dst and AE indices, the Bz component of the IMF and the DTEC for the storm period of 19e21 December 2015. The sudden commencement (SC) was at 1616 UT on 19 December. The Dst started to decrease at about 2300 UT and reached a minimum of e 170 nT at about 2200 UT on 20 December
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
29 1200
40
27 August 2014
26 August 2014
Dst
28 August 2014
AE
20
-20
600
-40
AE (nT)
Dst (nT)
1000 800
0
400 -60
200
-80 -100
0
20
Bz (nT)
10
0
-10
-20 150
OHI SCO KAN
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DTEC (%)
50
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2
4
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8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22
UT
Fig. 3. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of26e28 August 2014 storm.
50
22 June 2015
21 June 2015
23 June 2015
Dst AE
0
1800 1600 1400
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1000
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8 10 12 14 16 18 20 22 0
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8 10 12 14 16 18 20 22
UT
Fig. 4. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 21e23 June 2015 storm.
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G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36 1600
20 December 2015
19 December 2015
21 December 2015
20
Dst
1400
AE
1200
1000 800
-80
AE (nT)
Dst (nT)
-30
600
400
-130
200
0
-180
20
Bz (nT)
10
0
-10
-20
250
OHI SCO KAN
200
DTEC (%)
150 100 50 0 -50 -100
0
2
4
6
8 10 12 14 16 18 20 22 0
2
4
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8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22
UT
Fig. 5. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 19e21 December 2015 storm.
(end of the main phase), then started a rapid recovery. AE gradually increased at about 1600 UT on 19 December, displaying three peaks (1200e1400 nT) at 0600, 1300 and 1800 UT on 20 December (during the main phase). Afterwards, AE irregularly decreased. Nearly simultaneously with the SC, the Bz component showed an abrupt northward peak for about 2 h, remaining positive till 0330 UT on 20 December, then abruptly turned southward, remaining negative till about 1630 UT on 21 December. The minimum Bz of about e 19 nT occurred around the end of the main phase. The storm commencement occurred near local midday, which can be seen at the three stations. Irregular increases in TEC were observed at the NH stations since the storm onset. These irregular positive disturbances (amplitudes higher than 150%) remained during the main phase (between afternoon hours on 19 December and premidnight on 20 December) and changed to negatives during the first part of the recovery. During the main phase, the SH station presented a negative disturbance followed by a positive one: the negative disturbance occurred between 0400 UT and 1600 UT on December 20 (from local midnight to noon) and the positive disturbance appeared in the afternoon hours (amplitudes of about 100% at around 20 UT). About 2 h before the end of the main phase a long-duration negative disturbance was initiated, and remained on 21 December. 3.5. Geomagnetic storm on 19e21 January 2016 Fig. 6 shows the variations of the Dst and AE indices, the Bz component of the IMF and the DTEC for the storm period of 19e21 January 2016. The storm commencement was at 1000 UT on January 22. The Dst index started to decrease at about 0100 UT on 20 January and reached a minimum of e 104 nT at 1600 UT (end of the main phase). Afterwards, the Dst initiated a slow recovery. AE index showed an increase between about 0000 UT and 1300 UT on 19 January followed by a quiet period. At about 06 UT on 20 January,
AE started to increase again, showing a secondary peak at about 08 UT (~600 nT) and a main peak at 1600 UT (~1250 nT). On 21 January, AE showed an oscillating behavior with values between 200 nT and 400 nT. The Bz component, northward prior to the main phase onset, turned southward (negative) from about 0530 UT on 20 January to 0400 UT on 21 January (during the main phase and first part of the recovery). The minimum value of about e 12 nT occurred nearly simultaneously with the end of the main phase. The main phase of the storm was from local midnight to about noon hours. The NH stations began to appear irregular positive disturbances before the storm onset. These positive effects remained at the two stations during the main phase and recovery phase, but with smaller amplitude. At the SH station, a negative disturbance was observed before the storm commencement, and remained during the main phase, then changed to the positive disturbance during the first stage of the recovery (between local noon and predawn hours). This positive disturbance changed to negative again after about 22 UT on 20 June, remaining so during the rest of the recovery (~60% maximum change). 4. Discussion and conclusions In response to five intense geomagnetic storms in the period of 2012e2016, this paper analyzes TEC perturbations observed at two stations located in the Arctic sector and one station located in the Antarctic sector. The main results can be summarized as follows: - In general, opposite storm effects were observed in the Arctic and Antarctic sectors during the main phase and the early stage of the recovery. The maximum disturbances in TEC occurred within 24 h after the storm commencement. - Both the positive and negative disturbances presented more intense fluctuations over the NH stations than over the SH station.
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
31 1400
40
19 January 2016
21 January 2016
20 January 2016
1200
20
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AE
Dst (nT)
1000 800
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600
AE (nT)
0 -20
-60
400
-80 -100
200
-120
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20
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OHI SCO KAN
DTEC (%)
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50
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2
4
6
8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22 0
2
4
6
8 10 12 14 16 18 20 22
UT
Fig. 6. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 19e21 January 2016 storm.
- Positive TEC fluctuations were more pronounced in the winter hemisphere. - The negative disturbances generally last for a long time, sometimes interrupted by short-duration positive disturbances. The greater amplitude of TEC fluctuations at the NH stations than at the SH station was possibly due to the higher geomagnetic latitude. In general, the higherTEC fluctuations took place when the Bz component was southward [12]. However, TEC fluctuations were observed when Bz turned northward or varied around zero, which occurred prior to the storm onset (e.g., two storms in August 2014 and January 2016). It is well known that geomagnetic storms are triggered by impulsive changes in parameters of the solar wind. The increased solar wind compresses the Earth's magnetosphere, and when the component Bz is negative (southward) a reconnection or coupling with the geomagnetic field is produced, which results in a large amount of energy deposition into the high latitude regions of the Earth [16]. Such high latitude heating, in turn, create strong prompt penetration electric fields (PPEFs), which appear almost immediately in the Earth's ionosphere and magnetosphere after they have been convected to the magnetosphere by the disturbed solar wind [17]. The PPEFs are directed eastward (dawn to dusk) on the dayside and westward (dusk to dawn) on the nightside [18]. A feature of these high-latitude electric fields is that they may penetrate to lower latitudes before the ring current has developed (e.g., [19e23]). At high latitudes, these intense electric fields produce a horizontal plasma convection because the magnetic field lines are near perpendicular to the Earth's surface. Therefore, the storm-time induced electric fields do not produce vertical drifts, but there is a vertical component where the magnetic field is tilted a little from the vertical.
To analyze this mechanism, we considered the eastward prompt penetration electric field model developed by the Cooperative Institute for Research in Environmental Sciences (CIRES, http:// geomag.org/models/PPEFM/RealtimeEF.html). This model is based on the data from the Advanced Composition Explorer (ACE) satellite, the JULIA radar, and the magnetometer onboard the CHAMP satellite. As example, Fig. 7 shows the PPEF variation for the storm period of 21e23 June 2015. It can be seen that PPEF was significantly disturbed after about 18 UT on 22 June and affected TEC over the Antarctic station. Because the O'Higgins station is a lower latitude station than the Arctic stations, it may occur an uplifting of the ionospheric regions to lower recombination heights. Possibly the significant increase in TEC could be related with a prompt penetration electric field directed eastward, which occurred with sudden southward turning of Bz. The PPEF effects in the ionosphere seem to be produced after 1e2 h of their existence. However, the oscillating behavior of the PPEF observed in the period of 22 June to 23 June could make the ionospheric electron density lower. In some storms (not shown here), PPEFs are not significant. Possibly in these cases the electric fields are not intense enough to produce significant ionospheric effects and the changes of composition (discussed below) substitute or promote the electric field effect. The enhancement of AE (indicative of auroral region activity) before the storm commencement could be a possible cause for the pre-storm enhancements. The particle precipitation possibly affects the electron density close to the ionospheric F layer and it could be an important source of ionization in the winter hemisphere, although this region remains in the darkness. However, this mechanism seems to be not always valid because positive effects were observed during the storm of August 2014 and no significant activity of the auroral electrojet (measured by the AE index)
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1,2
21 June 2015
El ectri c fi el d (mV/m)
0,9
22 June 2015
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0,6 0,3 0 -0,3 -0,6 -0,9 -1,2
0
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12
16
20
0
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12
16
20
0
4
8
12
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UT quiet
quiet + penetraƟon
Fig. 7. The PPEF variation for the period of 21e23 June 2015 storm.
occurred. The pre-storm enhancements have been already studied and suggested as storm precursor (e.g., [24e26]). The positive effects observed before the geomagnetic storm commencement could be part of “meteorological origin” [27]. However, it is unlikely that strong positive effects a few tens of hours before the onset of a geomagnetic disturbance could be associated to meteorological effects or a natural quiet-time day-to-day variability. Buresova and Lastovicka [26] concluded that the dominant mechanism of the pre-storm enhancements remains to be uncovered. So, there could be more than one mechanism and the cause may be different for each individual event. The above mentioned energy deposition in the high latitude region produces considerable heating of the ionized and neutral gases, which leads to an expansion of the thermosphere. That produces pressure gradients and drives a global circulation: the winds flow from high to low latitudes [1,28,29]. The disturbed thermospheric circulation leads to changes in the neutral gas composition and moves the ionospheric plasma up and down along magnetic field lines, changing the rates of production and recombination of the ionized gas. At the same time, polarization electric fields are generated by the dynamo action of disturbed neutral winds due to the collisions with the plasma [14].
The widely accepted mechanism is that the change in the thermospheric composition, i.e. increases in the N2/O ratio, causes the negative storm effects [2,3,5,7,30e34]. The global storm-time circulation, in which neutral winds blow from high to low latitudes, transports a bulge of increased mean molecular mass toward midlatitudes, especially in the summer hemisphere, where the meridional circulation is already equatorward (e.g., [2,35]). Since the molecular species N2 and O2 determine the loss rate of ions, their increases at middle latitudes contribute to the decreases of the electron density. We used the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP, https://ccmc.gsfc.nasa.gov/models/ctip.php#description) model to verify the relation between TEC disturbances and changes in the O/N2 ratio at high latitudes. We did not use the data from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite because there are no measurements at the Antarctic latitudes during the considered storms. The CTIP model simulates the time-dependent global structure of the wind vector, temperature, and density of the neutral thermosphere by numerically solving the non-linear primitive equations of momentum, energy, and continuity on a 3D spherical polar grid rotating with the Earth. Fig. 8 shows the global map of the O/N2 ratio for the August 2014 storm. Four representative moments of the storm are
Fig. 8. The global map of the O/N2 ratio for the August 2014 storm.
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
presented: 0900 and 1600 UT on 27 August (during the main phase), 0100 and 0600 UT on 28 August (during the recovery phase). The O/N2 ratio showed a decrease over the NH stations, and increased with the storm development, in association with the storm-time negative disturbances. Instead, over theSH station the increases of the O/N2 ratio were observed during the course of the storm. The greatest values of the O/N2 ratio (on 28 August) were produced simultaneously with the important positive storm effect (~150% change). So, it verified the well-known relation between negative disturbances in TEC and decreases in the O/N2 ratio. Moreover, the
33
results show a good correlation between positive storm effects and increases of the O/N2 ratio in the SH. Since the production rate is directly proportional to the oxygen atomic density, an increase of O density increases the ionization production rate and leads to the positive disturbances in TEC. Fig. 9 presents the evolution of the O/N2 ratio for some selected moments during the storm period of 22e23 June 2015. The moments considered are: 2000 UT on 22 June, 0000 and 0300 UT on 23 June (main phase of the storm). At the SH station, the O/N2 ratio at 2000 UT increased, while at the NH stations the O/N2 ratio decreased in association with the decreases in TEC. At
Fig. 9. The evolution of the O/N2 ratio during the period of 22e23 June 2015 storm.
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0000 UT on 23 June, the decreases of the O/N2 ratio occurred over O'Higgins simultaneously with the decreases of TEC, but at 2000 UT on 22 June the O/N2 ratio increased again in association with the positive effect in TEC. During the main phase theNH stations showed the decreases of O/N2 values (in association with the negative effect in TEC). During the recovery phase (not shown here), the depressed O/N2 values appeared in the Northern and Southern Hemispheres. Fig. 10 presents the global behavior of the O/N2 ratio for the December 2015 storm. Three moments are shown: 0600 UT, 2000 UT on 20 December (during the main phase) and 0600 UT on 21 December (during the first part of the recovery). At the considered latitudes, during the main phase (0600 UT on 20 December) the O/ N2 ratio was greater in the NH than in the SH. These results suggested that the increase of the O/N2ratio can be sometimes a possible cause to produce (or contribute to) the positive storm effects. The relation between TEC positive disturbances and increases in the O/N2 ratio seems to be not always valid, as can be seen at 20 UT on 20 December and 06 UT on 21 December. It is likely that other mechanism was also operative to produce the long-duration positive storm effects. Storm-time
thermospheric winds, disturbance dynamo electric fields (DDEFs), and plasmaspheric downward fluxes have been also proposed as possible causes for the increases in the ionospheric electron density (e.g., [4e6,22,32,35e40]). Maruyama et al. [41] observed that an eastward directed disturbance dynamo electric field can persist for many hours due to increased energy deposition into the high-latitude ionosphere (indicated by the AE enhancement). That could maintain the upward drift and thereby the F2-layer lifting at the O'Higgins station or produce large scale motions (convection) at higher latitudes, which are able to influence the F-region behavior via the recombination coefficient [2,42]. For the maintenance of the positive storm effects, the large-scale change in the thermospheric circulation caused by heating in the high-latitude region can be also important. During prolonged periods of southward orientation of the Bz component of the interplanetary magnetic field (which is favorable for the reconnection with the geomagnetic field), a significant energy transfer from solar wind to the magnetosphere-ionosphere system is produced, which makes the thermosphere and ionosphere to remain disturbed for longer time periods, hence resulting in the long-duration storm effects.
Fig. 10. The global behavior of the O/N2 ratio for the December 2015 storm.
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
In summary, we observed in general opposite storm effects in the Arctic and Antarctic sectors during the considered storms. Both the positive and negative TEC disturbances presented more fluctuations over the NH stations than over the SH station. Moreover, the positive TEC disturbances were more significant in winter. The negative disturbances were generally long-lasting, sometimes interrupted by short-duration positive disturbances. Both the positive and the negative storm effects in TEC during the main and recovery phases of storms could be ascribed to neutral composition changes (i.e., increase and decrease in the O/N2 ratio respectively). However, prompt penetration electric fields could also play an important role initially. The effects of the thermospheric circulation and the disturbance dynamo electric fields also play important roles several hours after the storm commencements because they can persist for many hours due to the sustained input of energy into the high-latitude ionosphere. However, more studies are necessary to explain the ionospheric disturbances because they are very complex phenomena not yet fully understood. Among the ionospheric effects are the positive disturbances prior to the storm commencement, the long-duration positive disturbances and the change (interface) between positive and negative disturbances (or vice versa). In addition, more studies are required near the South Atlantic Magnetic Anomaly region where the processes could be changed due to the energetic particle precipitation. Conflicts of interest The authors declare that there is no conflicts of interest. Acknowledgments The author thanks the IONOLAB web site for providing TEC data, World Data Center (WDC) Kyoto for the Dst and AE indexes data, NASA Space Science Data Coordinated Archive for the Bz data, the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model for the global maps of the O/N2 ratio and CIRES for the eastward prompt penetration electric field model. The author also thanks the referees for their valuable suggestions to improve the manuscript. References [1] M.J. Bounsanto, Ionospheric storms - a review, Space Sci. Rev. 88 (1999) 563e601. [2] A.D. Danilov, F2-region response to geomagnetic disturbances, J. Atmos. Sol. Terr. Phys. 63 (2001) 441e449. [3] T.J. Fuller-Rowell, M.V. Codrescu, R.J. Moffett, S. Quegan, Response of the thermosphere and ionosphere to geomagnetic storms, J. Geophys. Res. 99 (1994) 3893e3914. € lss, Ionospheric F-region storms, in: Handbook of Atmospheric [4] G.W. Pro Electrodynamics, vol. 2, CRC Press/Boca Raton, Volland, 1995, pp. 195e248. [5] A.D. Danilov, Ionospheric F-region response to geomagnetic disturbances, Adv. Space Res. 52 (2013) 343e366. [6] M. Blanc, A.D. Richmond, The ionospheric disturbance dynamo, J. Geophys. Res. 85 (1980) 1669e1686, https://doi.org/10.1029/JA085iA04p01669. [7] H.G. Mayr, I. Harris, N.W. Spencer, Some properties of upper atmosphere dynamics, Rev. Geophys. Space Phys. 16 (1978) 539e565. [8] C.M. Huang, Disturbance dynamo electric fields in response to geomagnetic storms occurring at different universal times, J. Geophys. Res. Space Phys. 118 (2013) 496e501, https://doi.org/10.1029/2012JA018118. [9] A. Krankowsky, I. Shagimuratov, L.W. Baran, I.I. Ephishov, Study of TEC fluctuations in Antarctic ionosphere during storm using GPS observations, Acta Geophys. Pol. 53 (2005) 205e218. [10] V. Sreeja, M. Aquino, Statistics of ionospheric scintillation occurrence over European high latitudes, J. Atmos. Sol. Terr. Phys. 120 (2014) 96e101. [11] N. Jakowski, S.M. Stankov, D. Klaehn, Operational space weather service for GNSS precise positioning, Ann. Geophys. 23 (2005) 3071e3079. [12] I.I. Shagimuratov, A. Krankowski, I. Ephishov, Yu. Cherniak, P. Wielgosz, I. Zakharenkova, High latitude TEC fluctuations and irregularity oval during geomagnetic storms, Earth Planets Space 64 (2012) 521e529.
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Dr. Gustavo Adolfo Mansilla, Professor of Physics, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de n, Argentina. Doctor (Ph. D) en Física, Department Tucuma of Physics, Universidad Nacional de Tucuman, Argentina.Researcher of the National Council of Scientific Researches and Techniques (CONICET), Argentina.
Contents lists available at ScienceDirect
Geodesy and Geodynamics journal homepage: http://www.keaipublishing.com/geog
Behavior of the Total Electron Content over the Arctic and Antarctic sectors during several intense geomagnetic storms Gustavo A. Mansilla a, b, * n, Av. Independencia 1800, 4000, San Miguel de Departamento de Física, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucuma n, Argentina Tucuma b Consejo Nacional de Investigaciones científicas y T ecnicas, Godoy Cruz 2290, CABA, Argentina a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 June 2018 Accepted 18 January 2019 Available online 31 January 2019
In this paper, the behavior of TEC at three stations located in the Arctic and Antarctic sectors during some intense geomagnetic storms in the period 2012e2016 is analyzed. The results show the opposite storm effects in the Arctic and Antarctic regions. Both the positive and negative TEC disturbances presented more fluctuations over the Arctic stations than over the Antarctic stations. Moreover, the positive TEC disturbances were more significant in winter. The negative disturbances were generally long-lasting, sometimes interrupted by short-duration positive disturbances. Overall, the increases and decreases in TEC can be mainly attributed to changes (i.e., increase and decreases in the O/N2 ratio respectively) in the thermospheric composition, but prompt penetration electric field could be responsible for the initial TEC disturbances. The thermospheric circulation and the disturbance dynamo, which are maintained due to prolonged high-energy input at high latitudes, can also play important roles at the end of main phase and during recovery phase. © 2019 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Geomagnetic storms TEC Arctic and Antarctic sectors
1. Introduction The ionospheric storms (disturbances in the terrestrial ionosphere produced by geomagnetic storms) cause significant disturbances for technological systems, such as the static or dynamical positioning with GNSS satellites, and others [1], which depend on the transionospheric communications. Basically, in comparison with under quiet conditions, the electron density can increase or decrease during geomagnetic storm periods. These changes, which have been called positive ionospheric storm or positive storm effect and negative ionospheric
* Corresponding author. Departamento de Física, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucum an, Av. Independencia 1800, 4000, San n, Argentina. Miguel de Tucuma E-mail address: [email protected]. Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
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storm or negative storm effect respectively, occur because there is significant energy input (from the solar wind) into the polar ionosphere, usually over a period of several hours to a day [2,3]. Several driver forces have been used to explain the ionospheric effects during storms at different latitudes. For example, it is believed that horizontal convection dominates in the polar caps, and composition changes, particle precipitation and electric fields dominate in the auroral zones, while electric fields, meridional winds and composition changes dominate at equatorial and low latitudes (e.g., [1,2,4e8]). In the auroral and polar ionosphere, those irregularities at a different scale have a common feature during geomagnetic storms, which causes fluctuations in the Total Electron Content (TEC) [9]. In turn, the small scale ionospheric plasma density irregularities produce rapid fluctuation in the amplitude and phase of transionospheric radio signals, which is known as scintillation [10]. The large-scale irregularities and associated TEC fluctuations can complicate phase ambiguity resolution, increase the number of uncorrected cycle slips and losses of signal lock in GNSS (e.g., [11,12]). Because there are relatively sparse results for TEC at sub-auroral, auroral and polar latitudes [13], in this paper we analyze the
https://doi.org/10.1016/j.geog.2019.01.004 1674-9847/© 2019 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
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behavior of TEC at three stations located in the Arctic and Antarctic sectors during five intense geomagnetic storms in the period 2012e2016. We also analyze possible physical mechanisms related with the observed ionospheric disturbances.
positive (negative) TEC disturbances correspond to the positive (negative) values of DTEC.
2. Data and methodology
3.1. Geomagnetic storm on 14e16 July 2012
Hourly TEC data from three stations located at high and subauroral latitudes (Kangerlussuaq, Scorebysund/Ittoqqortoomiit and O'Higgins, respectively) during five storms between 2012 and 2016 are considered. TEC data were obtained from the IONOLAB web site (www.ionolab.org). The information of three stations is given in Table 1. Fig. 1 shows the geographical locations of the three stations. The different phases of the storms, namely initial phase, main phase and recovery phase, are determined by the variation of the Dst index. Hourly values of Dst and AE indices were obtained from the World Data Center (WDC) Kyoto, Japan website (http://swdc. kugi.kyoto-u.ac.jp/dstdir). Bz component of the interplanetary magnetic field (IMF) was obtained from the NASA Space Science Data Coordinated Archive website (http://omniweb.gsfc.nasa.gov). Similarly to Blagoveshchensky et al. [14]and Adimula et al. [15], the relative change in TEC (DTEC) was calculated as follows:
Fig. 2 shows the variations of the Dst and AE indices, the Bz component of the interplanetary magnetic field (IMF) and the DTEC for the geomagnetic storm of 14e16 July 2012. The storm onset was at 0600 UT on 15 July (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3. html) and reached the maximum disturbance (approximately e 118 nT) at about 1000 UT on 15 July. The AE index abruptly increased at about 1700 UT on 14 July and reached a peak value (~1300 nT) at 2100 UT. Later, it have decreased for a few hours during the main phase (between about 00 UT and 0600 UT on 15 July) and increased again at the end of the main phase and during the recovery phase, with fluctuations between 1200 UT on 15 July and 1800 UT on 16 July. The interplanetary magnetic field (IMF) Bz component turned southward at about 0230 UT on 15 July and remained largely negative during the main and recovery phases until about 1500 UT on 16 July (peak value ~ -19 nT). The storm onset occurred after local midnight. The increases in TEC can be seen before the storm onset (between about 1800 UT and 2300 UT on 14 July), in association with the sharp increase in AE. The Northern Hemisphere (NH) stations presented similar behavior in response to the storm: irregular long-duration negative disturbances since a few hours before the storm commencement, which remained during 15 and 16 July. The Southern Hemisphere (SH) station initially presented the decrease in TEC. A significant positive disturbance was observed (DTEC higher than 150%) between local
DTEC ¼ TECobs TECavg
TECavg 100%
where TECobs is the observed value of the TEC and TECavg is the average value of those TECobs in the five quietest days of the month of the storm. In general, the positive and negative storm effects are referred to the changes in the electron density at the height of peak concentration (i.e. based on the foF2 or NmF2 parameter). Here, the
3. Results of observations
Table 1 The information of three stations.
O'Higgins (OHI) Kangerlussuaq (KAN) Scorebysund/Ittoqqortoomiit (SCO)
Geog. Latitude
Geog. Longitude
Geomag. Latitude
Geomag. Longitude
63.32oS 67.02oN 70.48oN
57.89oW 50.72oW 21.95oW
53.7oS 75.4oN 74.6oN
11.2oE 35.6oE 78.9oE
Fig. 1. The geographical locations of the three stations.
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G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36 50
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16 July 2012
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Fig. 2. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field (IMF) and DTEC for the period of 14e16 July 2012 storm.
noon and 2200 LT (during the recovery phase), and changed to negative during the recovery phase. The disturbance amplitude in TEC was greater over the station of the SH than at high latitudes of the NH. 3.2. Geomagnetic storm on 26e28 August 2014 Fig. 3 shows the variations of the Dst and AE indices, the Bz component of the interplanetary magnetic field and the DTEC for the period of 26e28 August 2014. The main phase of the storm started at about 0200 UT on 27 August and lasted until 19 UT on 27 August, then Dst reached a minimum value of - 79 nT. Afterwards, Dst showed a slow recovery. The AE index started to increase at about 0300e0400 UT on 27 August and reached a maximum value of 1075 nT near to the end of the main phase. After that, AE initiated an oscillating behavior with lower peaks than before. Bz turned southward nearly simultaneously with the onset of the main phase and remained negative during the main and recovery phases. The maximum value (about e 13 nT) in negative direction appeared at 1630 UT on 27 August (2e3 h before the end of the main phase). The main phase occurred at all the stations between local midnight and afternoon hours. The common feature of the NH stations was irregular increases in DTEC during the main phase (in the daytime hours) and irregular decreases during the recovery phase. The SH station showed an oscillating increase in DTEC since about 1000 UT (0600 LT) on 27 August, which remained during the rest of the main phase and the recovery phase. 3.3. Geomagnetic storm on 21e23 June 2015 Fig. 4 shows the variations of the Dst and AE indices, the Bz component and DTEC for the storm period of 21e23 June 2015. The main phase of the storm started at about 2000 UT on 22 June. The
Dst index reached a minimum value of 204 nT at 0500 UT on 23 June (end of the main phase), then initiated a slow recovery. AE index initially showed a secondary peak at about 0800 UT on 22 June. AE started to increase at about 1400 UT, exhibiting strong fluctuations between 1600 UT on 22 June and 1200 UT on 23 June, then abruptly decreased. The Bz component presented an oscillating behavior during the main and recovery phases. It sharply turned southward at about 1730 UT on 22 June and reached a minimum value of e 26 nT at 1930 UT, but became northward from 2030 UT on 22 June to0130 UT on 23 June. Then, Bz turned again southward during 5 h simultaneously with the end of the main phase. During the recovery phase, Bz became positive for about 2 h and then negative for about 4 h. The main phase of the storm can be seen at all the stations between morning hours on 22 June and local midnight. The NH stations presented similar ionospheric effects: negative disturbances with increasing amplitude, which started at about 2200 UT on 22 June (during the main phase). The maximum deviations (~60%) occurred at the end of the main phase and during the first stage of the recovery. The SH station also presented an oscillating behavior in DTEC. A negative disturbance was observed prior to the storm onset, and changed to positive during the main phase and first part of the recovery, with two irregular peaks between about 1300 UT on 22 June and 0700 UT on 23 June. Then, this positive disturbance changed to negative again during the rest of the recovery phase (amplitude of ~60%). 3.4. Geomagnetic storm on 19e21 December 2015 Fig. 5 shows the variations of the Dst and AE indices, the Bz component of the IMF and the DTEC for the storm period of 19e21 December 2015. The sudden commencement (SC) was at 1616 UT on 19 December. The Dst started to decrease at about 2300 UT and reached a minimum of e 170 nT at about 2200 UT on 20 December
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29 1200
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Fig. 3. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of26e28 August 2014 storm.
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Fig. 4. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 21e23 June 2015 storm.
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20 December 2015
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21 December 2015
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Fig. 5. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 19e21 December 2015 storm.
(end of the main phase), then started a rapid recovery. AE gradually increased at about 1600 UT on 19 December, displaying three peaks (1200e1400 nT) at 0600, 1300 and 1800 UT on 20 December (during the main phase). Afterwards, AE irregularly decreased. Nearly simultaneously with the SC, the Bz component showed an abrupt northward peak for about 2 h, remaining positive till 0330 UT on 20 December, then abruptly turned southward, remaining negative till about 1630 UT on 21 December. The minimum Bz of about e 19 nT occurred around the end of the main phase. The storm commencement occurred near local midday, which can be seen at the three stations. Irregular increases in TEC were observed at the NH stations since the storm onset. These irregular positive disturbances (amplitudes higher than 150%) remained during the main phase (between afternoon hours on 19 December and premidnight on 20 December) and changed to negatives during the first part of the recovery. During the main phase, the SH station presented a negative disturbance followed by a positive one: the negative disturbance occurred between 0400 UT and 1600 UT on December 20 (from local midnight to noon) and the positive disturbance appeared in the afternoon hours (amplitudes of about 100% at around 20 UT). About 2 h before the end of the main phase a long-duration negative disturbance was initiated, and remained on 21 December. 3.5. Geomagnetic storm on 19e21 January 2016 Fig. 6 shows the variations of the Dst and AE indices, the Bz component of the IMF and the DTEC for the storm period of 19e21 January 2016. The storm commencement was at 1000 UT on January 22. The Dst index started to decrease at about 0100 UT on 20 January and reached a minimum of e 104 nT at 1600 UT (end of the main phase). Afterwards, the Dst initiated a slow recovery. AE index showed an increase between about 0000 UT and 1300 UT on 19 January followed by a quiet period. At about 06 UT on 20 January,
AE started to increase again, showing a secondary peak at about 08 UT (~600 nT) and a main peak at 1600 UT (~1250 nT). On 21 January, AE showed an oscillating behavior with values between 200 nT and 400 nT. The Bz component, northward prior to the main phase onset, turned southward (negative) from about 0530 UT on 20 January to 0400 UT on 21 January (during the main phase and first part of the recovery). The minimum value of about e 12 nT occurred nearly simultaneously with the end of the main phase. The main phase of the storm was from local midnight to about noon hours. The NH stations began to appear irregular positive disturbances before the storm onset. These positive effects remained at the two stations during the main phase and recovery phase, but with smaller amplitude. At the SH station, a negative disturbance was observed before the storm commencement, and remained during the main phase, then changed to the positive disturbance during the first stage of the recovery (between local noon and predawn hours). This positive disturbance changed to negative again after about 22 UT on 20 June, remaining so during the rest of the recovery (~60% maximum change). 4. Discussion and conclusions In response to five intense geomagnetic storms in the period of 2012e2016, this paper analyzes TEC perturbations observed at two stations located in the Arctic sector and one station located in the Antarctic sector. The main results can be summarized as follows: - In general, opposite storm effects were observed in the Arctic and Antarctic sectors during the main phase and the early stage of the recovery. The maximum disturbances in TEC occurred within 24 h after the storm commencement. - Both the positive and negative disturbances presented more intense fluctuations over the NH stations than over the SH station.
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
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Fig. 6. The temporal variations of the Dst, AE, Bz component of the interplanetary magnetic field and DTEC for the period of 19e21 January 2016 storm.
- Positive TEC fluctuations were more pronounced in the winter hemisphere. - The negative disturbances generally last for a long time, sometimes interrupted by short-duration positive disturbances. The greater amplitude of TEC fluctuations at the NH stations than at the SH station was possibly due to the higher geomagnetic latitude. In general, the higherTEC fluctuations took place when the Bz component was southward [12]. However, TEC fluctuations were observed when Bz turned northward or varied around zero, which occurred prior to the storm onset (e.g., two storms in August 2014 and January 2016). It is well known that geomagnetic storms are triggered by impulsive changes in parameters of the solar wind. The increased solar wind compresses the Earth's magnetosphere, and when the component Bz is negative (southward) a reconnection or coupling with the geomagnetic field is produced, which results in a large amount of energy deposition into the high latitude regions of the Earth [16]. Such high latitude heating, in turn, create strong prompt penetration electric fields (PPEFs), which appear almost immediately in the Earth's ionosphere and magnetosphere after they have been convected to the magnetosphere by the disturbed solar wind [17]. The PPEFs are directed eastward (dawn to dusk) on the dayside and westward (dusk to dawn) on the nightside [18]. A feature of these high-latitude electric fields is that they may penetrate to lower latitudes before the ring current has developed (e.g., [19e23]). At high latitudes, these intense electric fields produce a horizontal plasma convection because the magnetic field lines are near perpendicular to the Earth's surface. Therefore, the storm-time induced electric fields do not produce vertical drifts, but there is a vertical component where the magnetic field is tilted a little from the vertical.
To analyze this mechanism, we considered the eastward prompt penetration electric field model developed by the Cooperative Institute for Research in Environmental Sciences (CIRES, http:// geomag.org/models/PPEFM/RealtimeEF.html). This model is based on the data from the Advanced Composition Explorer (ACE) satellite, the JULIA radar, and the magnetometer onboard the CHAMP satellite. As example, Fig. 7 shows the PPEF variation for the storm period of 21e23 June 2015. It can be seen that PPEF was significantly disturbed after about 18 UT on 22 June and affected TEC over the Antarctic station. Because the O'Higgins station is a lower latitude station than the Arctic stations, it may occur an uplifting of the ionospheric regions to lower recombination heights. Possibly the significant increase in TEC could be related with a prompt penetration electric field directed eastward, which occurred with sudden southward turning of Bz. The PPEF effects in the ionosphere seem to be produced after 1e2 h of their existence. However, the oscillating behavior of the PPEF observed in the period of 22 June to 23 June could make the ionospheric electron density lower. In some storms (not shown here), PPEFs are not significant. Possibly in these cases the electric fields are not intense enough to produce significant ionospheric effects and the changes of composition (discussed below) substitute or promote the electric field effect. The enhancement of AE (indicative of auroral region activity) before the storm commencement could be a possible cause for the pre-storm enhancements. The particle precipitation possibly affects the electron density close to the ionospheric F layer and it could be an important source of ionization in the winter hemisphere, although this region remains in the darkness. However, this mechanism seems to be not always valid because positive effects were observed during the storm of August 2014 and no significant activity of the auroral electrojet (measured by the AE index)
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1,2
21 June 2015
El ectri c fi el d (mV/m)
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0
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Fig. 7. The PPEF variation for the period of 21e23 June 2015 storm.
occurred. The pre-storm enhancements have been already studied and suggested as storm precursor (e.g., [24e26]). The positive effects observed before the geomagnetic storm commencement could be part of “meteorological origin” [27]. However, it is unlikely that strong positive effects a few tens of hours before the onset of a geomagnetic disturbance could be associated to meteorological effects or a natural quiet-time day-to-day variability. Buresova and Lastovicka [26] concluded that the dominant mechanism of the pre-storm enhancements remains to be uncovered. So, there could be more than one mechanism and the cause may be different for each individual event. The above mentioned energy deposition in the high latitude region produces considerable heating of the ionized and neutral gases, which leads to an expansion of the thermosphere. That produces pressure gradients and drives a global circulation: the winds flow from high to low latitudes [1,28,29]. The disturbed thermospheric circulation leads to changes in the neutral gas composition and moves the ionospheric plasma up and down along magnetic field lines, changing the rates of production and recombination of the ionized gas. At the same time, polarization electric fields are generated by the dynamo action of disturbed neutral winds due to the collisions with the plasma [14].
The widely accepted mechanism is that the change in the thermospheric composition, i.e. increases in the N2/O ratio, causes the negative storm effects [2,3,5,7,30e34]. The global storm-time circulation, in which neutral winds blow from high to low latitudes, transports a bulge of increased mean molecular mass toward midlatitudes, especially in the summer hemisphere, where the meridional circulation is already equatorward (e.g., [2,35]). Since the molecular species N2 and O2 determine the loss rate of ions, their increases at middle latitudes contribute to the decreases of the electron density. We used the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP, https://ccmc.gsfc.nasa.gov/models/ctip.php#description) model to verify the relation between TEC disturbances and changes in the O/N2 ratio at high latitudes. We did not use the data from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite because there are no measurements at the Antarctic latitudes during the considered storms. The CTIP model simulates the time-dependent global structure of the wind vector, temperature, and density of the neutral thermosphere by numerically solving the non-linear primitive equations of momentum, energy, and continuity on a 3D spherical polar grid rotating with the Earth. Fig. 8 shows the global map of the O/N2 ratio for the August 2014 storm. Four representative moments of the storm are
Fig. 8. The global map of the O/N2 ratio for the August 2014 storm.
G.A. Mansilla / Geodesy and Geodynamics 10 (2019) 26e36
presented: 0900 and 1600 UT on 27 August (during the main phase), 0100 and 0600 UT on 28 August (during the recovery phase). The O/N2 ratio showed a decrease over the NH stations, and increased with the storm development, in association with the storm-time negative disturbances. Instead, over theSH station the increases of the O/N2 ratio were observed during the course of the storm. The greatest values of the O/N2 ratio (on 28 August) were produced simultaneously with the important positive storm effect (~150% change). So, it verified the well-known relation between negative disturbances in TEC and decreases in the O/N2 ratio. Moreover, the
33
results show a good correlation between positive storm effects and increases of the O/N2 ratio in the SH. Since the production rate is directly proportional to the oxygen atomic density, an increase of O density increases the ionization production rate and leads to the positive disturbances in TEC. Fig. 9 presents the evolution of the O/N2 ratio for some selected moments during the storm period of 22e23 June 2015. The moments considered are: 2000 UT on 22 June, 0000 and 0300 UT on 23 June (main phase of the storm). At the SH station, the O/N2 ratio at 2000 UT increased, while at the NH stations the O/N2 ratio decreased in association with the decreases in TEC. At
Fig. 9. The evolution of the O/N2 ratio during the period of 22e23 June 2015 storm.
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0000 UT on 23 June, the decreases of the O/N2 ratio occurred over O'Higgins simultaneously with the decreases of TEC, but at 2000 UT on 22 June the O/N2 ratio increased again in association with the positive effect in TEC. During the main phase theNH stations showed the decreases of O/N2 values (in association with the negative effect in TEC). During the recovery phase (not shown here), the depressed O/N2 values appeared in the Northern and Southern Hemispheres. Fig. 10 presents the global behavior of the O/N2 ratio for the December 2015 storm. Three moments are shown: 0600 UT, 2000 UT on 20 December (during the main phase) and 0600 UT on 21 December (during the first part of the recovery). At the considered latitudes, during the main phase (0600 UT on 20 December) the O/ N2 ratio was greater in the NH than in the SH. These results suggested that the increase of the O/N2ratio can be sometimes a possible cause to produce (or contribute to) the positive storm effects. The relation between TEC positive disturbances and increases in the O/N2 ratio seems to be not always valid, as can be seen at 20 UT on 20 December and 06 UT on 21 December. It is likely that other mechanism was also operative to produce the long-duration positive storm effects. Storm-time
thermospheric winds, disturbance dynamo electric fields (DDEFs), and plasmaspheric downward fluxes have been also proposed as possible causes for the increases in the ionospheric electron density (e.g., [4e6,22,32,35e40]). Maruyama et al. [41] observed that an eastward directed disturbance dynamo electric field can persist for many hours due to increased energy deposition into the high-latitude ionosphere (indicated by the AE enhancement). That could maintain the upward drift and thereby the F2-layer lifting at the O'Higgins station or produce large scale motions (convection) at higher latitudes, which are able to influence the F-region behavior via the recombination coefficient [2,42]. For the maintenance of the positive storm effects, the large-scale change in the thermospheric circulation caused by heating in the high-latitude region can be also important. During prolonged periods of southward orientation of the Bz component of the interplanetary magnetic field (which is favorable for the reconnection with the geomagnetic field), a significant energy transfer from solar wind to the magnetosphere-ionosphere system is produced, which makes the thermosphere and ionosphere to remain disturbed for longer time periods, hence resulting in the long-duration storm effects.
Fig. 10. The global behavior of the O/N2 ratio for the December 2015 storm.
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In summary, we observed in general opposite storm effects in the Arctic and Antarctic sectors during the considered storms. Both the positive and negative TEC disturbances presented more fluctuations over the NH stations than over the SH station. Moreover, the positive TEC disturbances were more significant in winter. The negative disturbances were generally long-lasting, sometimes interrupted by short-duration positive disturbances. Both the positive and the negative storm effects in TEC during the main and recovery phases of storms could be ascribed to neutral composition changes (i.e., increase and decrease in the O/N2 ratio respectively). However, prompt penetration electric fields could also play an important role initially. The effects of the thermospheric circulation and the disturbance dynamo electric fields also play important roles several hours after the storm commencements because they can persist for many hours due to the sustained input of energy into the high-latitude ionosphere. However, more studies are necessary to explain the ionospheric disturbances because they are very complex phenomena not yet fully understood. Among the ionospheric effects are the positive disturbances prior to the storm commencement, the long-duration positive disturbances and the change (interface) between positive and negative disturbances (or vice versa). In addition, more studies are required near the South Atlantic Magnetic Anomaly region where the processes could be changed due to the energetic particle precipitation. Conflicts of interest The authors declare that there is no conflicts of interest. Acknowledgments The author thanks the IONOLAB web site for providing TEC data, World Data Center (WDC) Kyoto for the Dst and AE indexes data, NASA Space Science Data Coordinated Archive for the Bz data, the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model for the global maps of the O/N2 ratio and CIRES for the eastward prompt penetration electric field model. The author also thanks the referees for their valuable suggestions to improve the manuscript. References [1] M.J. Bounsanto, Ionospheric storms - a review, Space Sci. Rev. 88 (1999) 563e601. [2] A.D. Danilov, F2-region response to geomagnetic disturbances, J. Atmos. Sol. Terr. Phys. 63 (2001) 441e449. [3] T.J. Fuller-Rowell, M.V. Codrescu, R.J. Moffett, S. Quegan, Response of the thermosphere and ionosphere to geomagnetic storms, J. Geophys. Res. 99 (1994) 3893e3914. € lss, Ionospheric F-region storms, in: Handbook of Atmospheric [4] G.W. Pro Electrodynamics, vol. 2, CRC Press/Boca Raton, Volland, 1995, pp. 195e248. [5] A.D. Danilov, Ionospheric F-region response to geomagnetic disturbances, Adv. Space Res. 52 (2013) 343e366. [6] M. Blanc, A.D. Richmond, The ionospheric disturbance dynamo, J. Geophys. Res. 85 (1980) 1669e1686, https://doi.org/10.1029/JA085iA04p01669. [7] H.G. Mayr, I. Harris, N.W. Spencer, Some properties of upper atmosphere dynamics, Rev. Geophys. Space Phys. 16 (1978) 539e565. [8] C.M. Huang, Disturbance dynamo electric fields in response to geomagnetic storms occurring at different universal times, J. Geophys. Res. Space Phys. 118 (2013) 496e501, https://doi.org/10.1029/2012JA018118. [9] A. Krankowsky, I. Shagimuratov, L.W. Baran, I.I. Ephishov, Study of TEC fluctuations in Antarctic ionosphere during storm using GPS observations, Acta Geophys. Pol. 53 (2005) 205e218. [10] V. Sreeja, M. Aquino, Statistics of ionospheric scintillation occurrence over European high latitudes, J. Atmos. Sol. Terr. Phys. 120 (2014) 96e101. [11] N. Jakowski, S.M. Stankov, D. Klaehn, Operational space weather service for GNSS precise positioning, Ann. Geophys. 23 (2005) 3071e3079. [12] I.I. Shagimuratov, A. Krankowski, I. Ephishov, Yu. Cherniak, P. Wielgosz, I. Zakharenkova, High latitude TEC fluctuations and irregularity oval during geomagnetic storms, Earth Planets Space 64 (2012) 521e529.
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Dr. Gustavo Adolfo Mansilla, Professor of Physics, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de n, Argentina. Doctor (Ph. D) en Física, Department Tucuma of Physics, Universidad Nacional de Tucuman, Argentina.Researcher of the National Council of Scientific Researches and Techniques (CONICET), Argentina.