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Overall diurnal universal time (UT) variation of hourly number of DstMin of the storms in Kyoto Dst (blue) and USGS Dst (red); horizontal lines represent diurnal means.
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A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT...
Citations
... To improve the time and spatial resolutions, the SymH index of 1-min resolution was developed (Iyemori et al. 1992) using the H-component data from up to 6 stations of MLS ~ 70°. The Dst and SymH indices have been used for studying not only the geomagnetic storms (e.g., Russell et al. 1973;Burton et al. 1975a;Ebihara et al. 2003Ebihara et al. , 2005Gonzalez et al. 2011;Gopalswamy et al. 2015;Yermolaev et al. 2021;Balan et al. 2021;Manu et al. 2022Manu et al. , 2023 but also the disturbed upper atmosphere, ionosphere and magnetosphere (e.g., Fuller-Rowell et al. 1994;Manuucci et al. 2005;Tulasiram et al. 2010;Balan et al. 2013). For reviews, see Akasofu (1981Akasofu ( , 2021, Proless (1995), Daglis (1997), Luhr et al. (2017) and Zong et al. (2021). ...
We notice that the important early decreasing part of the main phase (MP) from the positive main phase onset (MPO) to 0-level of Dst and SymH indices is missed in the treatment of the main phase (MP) of geomagnetic storms. We correct this inconsistency in 848 storms having positive MPO (out of 1164 storms) in SymH during 1981–2019 by raising the 0-level of SymH to the MPO-level. The correction considers the full range of the main phase, increases the corrected (revised) storm intensity (SymHMin*) and impulsive strength (IpsSymH*) by up to − 149 nT and − 134 nT, respectively, and seems important for all aspects of global space weather. For example, the corrected SymHMin* changes the conventional storm identification and classification and corrected IpsSymH* clearly identifies all 3 severe space weather (SvSW) events from over 1100 normal space weather (NSW) events with a separation of 52 nT; it also identifies all 8 minor-system-damage space weather (MSW) events from the NSW events.
... The storms are more frequent and more intense at equinoxes than in solstices (e.g., Svalgaard, 2011;Newell et al., 2001;Balan, Tulasiram, et al., 2017;Franco et al., 2021). The occurrence time of the storm peak intensity (maximum negative value of Dst during main phase (MP) of the storm) exhibits a quasi-semidiurnal type variation at low latitudes (e.g., Ahn et al., 2002;Balan et al., 2021). The seasonal and diurnal UT variations are understood in terms of equinoctial hypothesis (Bartels, 1932) and Russell-McPherron effect (Russell & McPherron, 1973). ...
The weakest solar cycle 24 (SC24, 2010–2019) in 100 years was 1/3rd less active compared to the previous solar cycle 23 (SC23, 1996–2009). We identify 135 and 61 ICME (interplanetary coronal mass ejection) driven clear geomagnetic storms (DstMin ≤ −50 nT) in SC23 and SC24, respectively, giving a reduction of 55% storms in SC24, and present the double superposed epoch analysis (DSEA) of the storms/activities in SC23 and SC24 using the Dst, symmetric H (SymH), Kp and AE indices. The DSEA method for the corresponding solar wind velocity V, north‐south component of the interplanetary magnetic field (IMF Bz) and the product VBz are also presented. Compared to SC23, the maximum storm/activity intensity in SC24 reduces by 52%, 12%, and 45% at low, mid and high latitudes and the corresponding maxima in ‐VBz, V, and ‐Bz reduce by 39%, 17%, and 38%, respectively. The epoch average storm/activity intensity reduces by 27%, 11%, and 4% at low, mid and high latitudes and average maxima in ‐VBz, V, and ‐Bz reduce by 24%, 14%, and 13%, respectively. The results seem to reveal that the average reduction in the main driver ‐VBz (∼24%) might have caused nearly the same and equal average storm/activity intensity reductions in all latitudes (∼25%), though the irregular nature of the AE index makes the reduction very small (4%) at high latitudes, and small (∼11%) at mid latitudes mainly due to the small (0–9) quasi logarithmic scale of the Kp index.
... In the century since Bartels' original deduction, a very large number of papers have discussed UT variations in the magnetosphere which, given that many magnetospheric processes take place in limited magnetic local time regions (in particular, substorm phenomena take place in the sector around local midnight), gives potential longitudinal variations in space weather and also means that the effects of a given disturbance in interplanetary space depend upon its time of arrival at Earth. UT variations have been reported in geomagnetic indices in a great many studies (Waldo-Lewis and McIntosh, 1953;McIntosh, 1959;Nicholson and Wulf, 1961;Davis and Sugiura, 1966;Berthelier, 1976;Aoki, 1977;Mayaud, 1978;Russell, 1989;Berthelier, 1990;Saroso et al., 1993;Takalo et al., 1995;de La Sayette and Berthelier, 1996;Siscoe and Crooker, 1996;Hajkowicz, 1998;Ahn et al., 2000;Cliver et al., 2000;Lyatsky et al., 2001;O'Brien and McPherron, 2002;Ahn and Moon, 2003;Karinen and Mursula, 2005;Wang and Lühr, 2007;Yakovchouk et al., 2012;Chu et al., 2015;Lockwood et al., 2020a;Lockwood et al., 2020b;Lockwood et al., 2020c;Balan et al., 2021;Lockwood et al., 2021;Wang et al., 2021). A problem for all these studies is that if the longitudinal distribution of magnetometer stations employed is not even, then a spurious UT variation is introduced into the geomagnetic data (Mayaud, 1978;Mayaud, 1980;Takalo and Mursula, 2001;Lockwood et al., 2019b). ...
... This implies there is some net inter-hemispheric current flow as well as between dawn and dusk. It also implicates the UT variations in the ring current (that connects to the R2 FACs) that have reported in large ring current storms by Balan et al. (2021). An anticorrelation is seen for the converse combination (Northern dusk R1 and Southern dawn R1; Figure 17F) but is not as strong. ...
We study the dependencies of Earth’s magnetosphere on Universal Time, UT. These are introduced because Earth’s magnetic axis is not aligned with the rotational axis and complicated because it is eccentric, which makes the offset of the magnetic and rotation poles considerably greater in the Southern hemisphere and the longitudinal separation of the magnetic poles less than 180°: hence consequent UT variations in the two hemispheres are not in equal in amplitude nor in exact antiphase and do not cancel, as they would for a geocentric dipole. We use long series of a variety of geomagnetic data to demonstrate the inductive effect of motions of the polar caps in a “geocentric-solar” frame, which is phase-locked to the Russell-McPherron (R-M) effect on solar-wind magnetosphere coupling. This makes the response of the magnetosphere-ionosphere system different for the two polarities of the Y-component of the Interplanetary Magnetic Field in the GSEQ reference frame, explaining the difference in response to the March and September equinox peaks in solar wind forcing. The sunward/antisunward pole-motion effect is detected directly in satellite transpolar voltage data and is shown to have a greater effect on the geomagnetic data than the full dipole tilt effect which generates the equinoctial pattern, the potential origins of which are discussed in terms of the dipole tilt effect on ionospheric conductivities and the stability of the near-Earth tail. Persistent UT variations in Region-1 and Region-2 field-aligned currents and in partial ring current indices are presented: their explanation is an important challenge for numerical modelling of the magnetosphere-ionosphere-thermosphere system which we need to quantify the relative contributions of the various mechanisms and to give understanding of the effect of arrival time on the response of the system to large, geoeffective disturbances in interplanetary space.
Plain language summary: The effect on terrestrial space weather of Earth’s magnetic axis not being aligned with the rotational axis is investigated. It is complex because not only do these two axes not align in direction (the “dipole tilt”), the magnetic axis does not pass through the centre of the Earth, which sets a requirement for an “eccentric” model of the field and not the commonly-used “geocentric” one. For many years, it has been known that the dipole tilt gives a peak in geomagnetic activity at the equinoxes (the semi-annual variation) through the “Russell-McPherron” (R-M) effect. However, although the variation with Universal Time is consistent with the R-M effect for the September equinox, it is not for the March equinox. We here solve this long-standing puzzle by investigating the effects of the motions of the two poles in a frame fixed with respect to both the Earth and the Sun for an eccentric dipole model. But solving one puzzle generates many others. We present observations of the Universal Time variations that these mechanisms combine to generate, which set an important challenge to the numerical modelling of the near-Earth space environment.
... The storms are also more frequent and more intense at equinoxes than in solstices (e.g., Balan, Tulasiram, et al., 2017;Newell et al., 2001;Svalgaard, 2011). Similar to substorms in high latitudes, the occurrence time of the storm intensity at low latitudes also exhibits a quasi-semidiurnal type variation (e.g., Ahn et al., 2002;Balan et al., 2021). The seasonal and diurnal UT variations are understood in terms of the equinoctial hypothesis (Bartels, 1932) and Russell-McPherron effect (Russell & McPherron, 1973). ...
The occurrence and intensity of the geomagnetic storms/activity in the weak solar cycle 24 (SC24) were studied mainly for low latitudes, with intensity being the maximum value of the activity. The impulsive strength of the activity giving its mean value during the main phase, which can better indicate the effect of the activity on utility systems, has not received much attention at any latitude. In this paper, we investigate the intensity and impulsive strength of the 179 and 85 clear geomagnetic activities (DstMin ≤ −50 nT) identified in the low, mid and high latitude indices (SymH, Kp, and AE) and corresponding solar wind velocity V, IMF Bz, and the product V × Bz in solar cycles 23–24 (1996–2019) for the first time. Compared to SC23, the total intensity and total impulsive strength in SC24 in all latitudes reduce by nearly equal amounts (∼60%) as the reduction in the number of activities (∼53%). The average intensity and average impulsive strength, however, reduce by nearly equal and largest amounts in low latitudes (∼23%), which is close to the reduction in the combination <−(V × Bz)MP > by ∼30%. At mid and high latitudes, the average intensity and average impulsive strength reduce by only small amounts (∼7.5%). Only the impulsive strength of the geomagnetic activity at low latitudes (IpsSymH) and the combination <−(V × Bz)MP> identify the two power outages happened in SC23. The correlation between IpsSymH and <−(V × Bz)MP> is also high (0.83) in SC23.
... Here we discuss the largest GIC events in 4 seasons. Figure 3 shows the GIC (3a) data at September equinox (29-30 October 2003) when the largest GICmax (57.05A) and largest number (11) of GICmax > 25A occurred in the 21 years of this study (Table 1). This case is associated with the rst of a super double geomagnetic storm (SymHMin − 391 nT, Kpmax 9 and AEmax 4056 nT; 3b-3c), fastest solar wind velocity (2242 km/s; 3d) (e.g., Skoug et al. 2004) and largest negative V×Bz (-97. . ...
The association of GIC (geomagnetically induced current) with various solar and geophysical conditions has been known. However, what determines the time of occurrence and amplitude of the largest GIC during geomagnetic storms, which during extreme storms can cause sudden damage of vulnerable utility systems, is not yet known. We address this important question by analyzing the GIC data measured in Finland for 21 years (1999–2019) during 106 geomagnetic activities (DstMin ≤-50 nT) at low, mid and high latitudes and the corresponding solar wind velocity V, dynamic pressure P, north-south component of interplanetary magnetic field (IMF Bz), and the products V×Bz and P×Bz. The results show for the first time that the largest GIC (≥ 10 A) occurs at the time of the largest -(V×Bz) in all seasons and solar activity levels with its time determined by the time of the largest -Bz and magnitude determined by both V and -Bz, except in one case. The two power outages happened in the 21-year period (06 November 2001 and 30 October 2003) also occurred at the UT time of the largest GICmax. The correlation of largest GICmax is also highest (0.92) with the largest -(V×Bz) at September equinox. The results highlight the importance of the single station GIC measurements and possibility of improving the forecasting of the rate of change of the local horizontal geomagnetic field (dH/dt) directly related to GIC.
... In the century since Bartels' original deduction, a very large number of papers have discussed UT variations in the magnetosphere which, given that many magnetospheric processes take place in limited magnetic local time regions (in particular, substorm phenomena take place in the sector around local midnight), gives potential longitudinal variations in space weather and also means that the effects of a given disturbance in interplanetary space depend upon its time of arrival at Earth. UT variations have been reported in geomagnetic indices in a great many studies (Waldo-Lewis and McIntosh, 1953;McIntosh, 1959;Nicholson and Wulf, 1961;Davis and Sugiura, 1966;Berthelier, 1976;Aoki, 1977;Mayaud, 1978;Russell, 1989;Berthelier, 1990;Saroso et al., 1993;Takalo et al., 1995;de La Sayette and Berthelier, 1996;Siscoe and Crooker, 1996;Hajkowicz, 1998;Ahn et al., 2000;Cliver et al., 2000;Lyatsky et al., 2001;O'Brien and McPherron, 2002;Ahn and Moon, 2003;Karinen and Mursula, 2005;Wang and Lühr, 2007;Yakovchouk et al., 2012;Chu et al., 2015;Lockwood et al., 2020a;Lockwood et al., 2020b;Lockwood et al., 2020c;Balan et al., 2021;Lockwood et al., 2021;Wang et al., 2021). A problem for all these studies is that if the longitudinal distribution of magnetometer stations employed is not even, then a spurious UT variation is introduced into the geomagnetic data (Mayaud, 1978;Mayaud, 1980;Takalo and Mursula, 2001;Lockwood et al., 2019b). ...
... This implies there is some net inter-hemispheric current flow as well as between dawn and dusk. It also implicates the UT variations in the ring current (that connects to the R2 FACs) that have reported in large ring current storms by Balan et al. (2021). An anticorrelation is seen for the converse combination (Northern dusk R1 and Southern dawn R1; Figure 17F) but is not as strong. ...
Universal Time variations have long been known in the AE(12) auroral
electrojet indices and have generally been attributed to longitudinal
structure in the ring of 12 AE geomagnetic observatories. Such problems
are even more extreme in equivalent indices from more extensive networks
of magnetometers because of the lack of stations in the great oceans.
However, this explanation may have masked a real UT variation in the
magnetosphere. We present evidence for a UT variation in the transpolar
voltage, as derived for both polar caps using the SuperDARN radars with
the matched potential technique and from a statistical survey of polar
orbiting satellite data. The results are consistent with a UT
dependence in the b2i tail stretching index. We suggest that
reconnection in the magnetotail is supressed at certain UT by the
longitudinal warp of the near-Earth edge of the cross-tail current
giving reduced transpolar voltage and enhanced tail stretching at such
times. However, not everything fits with this interpretation: a survey
of the open flux from global FUV images did not find a corresponding
rise at the relevant UT.
In this study, we compare two significant geomagnetic storms of the 21st century: the well-known Halloween geomagnetic storm of 2003 (Kp index 9) and a somewhat milder storm of September 2017 (Kp 8). Both events caused exceptionally high values of geomagnetically induced currents (GIC) and earned a place among the top ten with respect to the measured GIC in the Finnish natural gas pipeline.. We analyze solar wind and geomagnetic data as well as modeled geoelectric fields during these two events to better understand the drivers behind these strong GIC. We discover certain geographic locations that experienced stronger magnetic field time derivatives during the 2017 storm. This is interesting because in terms of magnetic indices, the 2017 storm was a weaker event. We use equivalent currents to get a view of the ionospheric and induced currents in the Fennoscandian region. We find that the interplay between different structures of ionospheric currents and the three-dimensional ground conductivity leads to a complex behaviour of the geoelectric field. This study improves knowledge in space weather preparedness by identifying location-specific risks for geoelectric hazards, which can create severe problems in the high-voltage power grid.
We notice an inconsistency in the geomagnetic storms having positive main phase onset (MPO >0 nT). The inconsistency makes the values of the storm indices (Dst and SymH) during the main phase and recovery phase significantly less than their actual values. We correct the inconsistency in 848 such storms (out of 1164 storms) during 1981-2019 by raising the 0-level of SymH to the MPO-level. The physically meaningful correction considers the full range of the main phase, increases the storm intensity (SymHMin) and impulsive strength (IpsSymH) by up to -149 nT and -139 nT, respectively, and seems important for all aspects of global space weather. For example, the corrected SymHMin changes the conventional storm identification and classification and corrected IpsSymH clearly identifies all 3 severe space weather events and 8 minor-system-damage space weather events from over 1100 normal space weather events.
In this work, the relation between Dst* (solar wind pressure corrected Dst) and solar wind parameters (north–south component of the interplanetary magnetic field Bz and east–west component of the electric field Ey) in the main phase of the single-step geomagnetic storms was analyzed on their peak (Dst*p and Bzp/Eyp) and difference values (ΔDst*p and ΔBz/ΔEy). From 1995 to 2019, 133 storms with peak Dst* ≤ −50 nT were selected. It was found that the correlations between Dst*p and Bzp/Eyp (r = 0.60 and −0.69) were higher than those between Dst*p and ΔBz/ΔEy (r = −0.49 and −0.45, respectively). The correlation between the variations of ΔDst* and ΔBz/ΔEy were intermediate between those ranges (r = 0.52 and 0.45, respectively). Multiple linear correlation of Dst*p with Bzp/ΔBz and Eyp/ΔEy explains 38 and 48% of the variance of Dst*p, respectively, with a larger effect of peak values of solar wind parameters. There were twice more single-step storms in cycle 23 than in cycle 24, but the average values of peaks and variations did not show significant differences. In terms of Eyp criteria for moderate (−100 nT < Dst*p ≤ −50 nT) and intense (Dst*p ≤ 100 nT) geomagnetic storms, >93% of the moderate storms had Eyp > 3 mV·m⁻¹, while 96% of intense storms had Eyp > 5 mV·m⁻¹. On ΔEy , 87% of the moderate storms had ΔEy ≤ 10 mV·m⁻¹, while 72% of the intense storms had ΔEy > 10 mV·m⁻¹. Thus it may be concluded that Bzp and Eyp were important for Dst*p of the single-step storms and that Eyp and ΔEy values could be interplanetary criteria for the moderate and intense storms.
