Main Phase Development Research Articles

GEOINDUCED CURRENTS DURING GEOMAGNETIC STORM ON 27-28 SEPTEMBER 2017

The complex space weather events of September 2017 included interplanetary coronal mass ejections (ICMEs), magnetic clouds (MCs), Sheaths, Corotating Interaction Regions (CIRs), solar wind high-speed streams (HSSs), fast forward shocks. This month can be divided into four events with different space weather conditions: first period on 7-8 September, second period on 12-13 September, third period on 14-17 September and fourth period on 27-28 September. In this report we considered the increasing of geoinduced currents (GICs) during moderate magnetic storm (SYM/H ~ −74 nT) occurring on September 27-28 caused by CIR connected with the high-speed streams (HSS). This storm was characterized by a gradual, multi-step main phase development with maximum at ~06 UT on 28 September. At the background of the storm another geomagnetic activities were also registered – substorms and geomagnetic pulsations – which caused intense GICs on the Vykhodnoy (VKH), Revda (RVD) and Kondopoga (KND) stations in the North-West of Russia and on the Mantsala (MAN) in the South of Finland. The increasing of GICs were controlled by data from registration system on Karelian-Kola powerline in the auroral zone (eurisgic.ru) and in Finland obtained from gas pipeline near Mantsala in the subauroral zone. The fine spatiotemporal structure of electrojet development during substorms were analyzed using the maps of the equivalent currents of the MIRACLE system and IMAGE magnetometers data. It was shown that intense GICs observed in the subauroral and auroral zones were associated with two sources - the movement and intensification of the substorm westward electrojet during the expansion phase of the substorm (in the premidnight sector) and Pc5 pulsations during the recovery phase of the substorm (in the morning sector).

Evidence for potential and inductive convection during intense geomagnetic events using normalized superposed epoch analysis

AbstractThe relative contribution of storm‐time ring current development by convection driven by either potential or inductive electric fields has remained an unresolved question in geospace research. Studies have been published supporting each side of this debate, including views that ring current buildup is entirely one or the other. This study presents new insights into the relative roles of these storm main phase processes. We perform a superposed epoch study of 97 intense (DstMin < –100 nT) and 91 moderate (–50 nT > DstMin > –100 nT) storms using OMNI solar wind and ground‐based data. Instead of using a single reference time for the superpositioning of the events, we choose four reference times and expand or contract each phase of every event to the average length of this phase, creating a normalized timeline for the superposed epoch analysis. Using the bootstrap method, we statistically demonstrate that timeline normalization results in better reproduction of average storm dynamics than conventional methods. Examination of the Dst reveals an inflection point in the intense storm group consistent with two‐step main phase development, which is supported by results for the southward interplanetary magnetic field and various ground‐based magnetic indices. This two‐step main‐phase process is not seen in the moderate storm timeline and data sets. It is determined that the first step of Dst development is due to potential convective drift, during which an initial ring current is formed. The negative feedback of this hot ion population begins to limit further ring current growth. The second step of the main phase, however, is found to be a more even mix of potential and inductive convection. It is hypothesized that this is necessary to achieve intense storm Dst levels because the substorm dipolarizations are effective at breaking through the negative feedback barrier of the existing inner magnetospheric hot ion pressure peak.

Are there optical differences between storm-time substorms and isolated substorms?

Abstract. We have performed an extensive analysis of auroral optical events (substorms) that occurred during the development of the main phase of magnetic storms. Using images from the Earth Camera on the Polar spacecraft (Frank et al., 1995), we compared the optical emission features of substorms occurring during 16 expansion phases of magnetic storms with the features of isolated substorms occurring during non-storm times. The comparison used two techniques, visual inspection and statistical comparisons. The comparisons were based on the common characteristics seen in isolated substorms that were initially identified by Akasofu (1964) and quantified by Gjerloev et al. (2008). We find that when auroral activity does occur during main phase development the characteristics of the aurora are very dissimilar to those of the classical isolated substorm. The primary differences include the lack of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion phases, and greater intensities. Since a surge/bulge and bifurcation of the aurora are characteristics of the existence of a substorm current wedge, a key component of the magnetosphere-ionosphere current system during substorms, the lack of this component would indicate that the classical substorm model does not apply to the storm time magnetosphere-ionosphere current system. Rather several of the analyses suggest that the storm-time substorms are associated more closely with the auroral oval, at least spatially, and, therefore, probably with the plasma sheet dynamics during the main phase development. These results then must call into question the widely held assumption that there is no intrinsic difference between storm-time substorms and classical isolated substorms.

Low-latitude geomagnetic signatures during major solar energetic particle events of solar cycle-23

Abstract. The frequency of occurrence of disruptive transient processes in the Sun is enhanced during the high solar activity periods. Solar cycle-23 evidenced major geomagnetic storm events and intense solar energetic particle (SEP) events. The SEP events are the energetic outbursts as a result of acceleration of heliospheric particles by solar flares and coronal mass ejections (CMEs). The present work focuses on the geomagnetic variations at equatorial and low-latitude stations during the four major SEP events of 14 July 2000, 8 November 2000, 24 September 2001 and 4 November 2001. These events have been reported to be of discernible magnitude following intense X-ray flares and halo coronal mass ejections. Low-latitude geomagnetic records evidenced an intense main phase development subsequent to the shock impact on the Earth's magnetosphere. Satellite observations show proton-flux enhancements associated with solar flares for all events. Correlation analysis is also carried out to bring out the correspondence between the polar cap magnetic field perturbations, AE index and the variations of low-latitude magnetic field. The results presented in the current study elucidate the varying storm development processes, and the geomagnetic field response to the plasma and interplanetary magnetic field conditions for the energetic events. An important inference drawn from the current study is the close correspondence between the persistence of a high level of proton flux after the shock in some events and the ensuing intense magnetic storm. Another interesting result is the role of the pre-shock southward IMF Bz duration in generating a strong main phase.

Radial dependence of relativistic electron fluxes for storm main phase development

Satellite observations have revealed that the flux variations of outer belt relativistic electrons during the main phase of a magnetic storm exhibit a strong radial dependence. This L dependence is characterized as a small increase or decrease near the inner edge of the belt (L ∼ 3) and a large decrease inits outer region (L ∼ 5). Extending the study by Kim and Chan [1997] of relativistic electron flux decreases at geostationary orbit, we investigate the characteristic radial dependence in terms of the fully adiabatic response of relativistic electrons to magnetic field perturbations. Using Liouville's theorem and the conservation of the first and third adiabatic invariants, we calculate storm main phase fluxes of equatorially mirroring electrons by adiabatically evolving the prestorm values. A quiet time electron distribution model is constructed from the CRRES satellite data. The radial structure of magnetic field perturbations and the spatial and energy dependence of the quiet time electron distribution are found to affect the main phase fluxes through adiabatic processes. In response to the field perturbations, adiabatic flux changes become larger at higher L shells where electrons can experience strong deceleration and considerable radial displacement. The nonmonotonic energy spectrum at the inner edge of the outer belt, which is featured in our quiet time electron model, can yield a slight flux increase in that region even during adiabatic deceleration. The results of this study suggest that a fully adiabatic treatment can provide an important component of the explanation for the general pattern of large scale changes in the radial profile of relativistic electron fluxes during the storm main phase.

A study of magnetic storms development in two or more steps and its association with the polarity of magnetic clouds

The study of the response of the magnetosphere, measured by the Dst index, to different interplanetary magnetic field configurations observed in magnetic clouds has led us to conclude that about 20% of intense magnetic storms have the growth of their main phases in three or more steps. We also have found that the magnetic clouds with south-to-north magnetic field rotation tend to lead to moderate or intense magnetic storms with a two-step main phase development due to the closer southward magnetic fields in the sheath and in the cloud. On the other hand, magnetic clouds with a north-to-south magnetic field rotation mostly seem to lead to magnetic storms with one-step main phase development, due to the larger separation between the southward magnetic fields in the sheath and in the cloud. Furthermore, the magnetic clouds with a substantial tilt to the ecliptic plane appear to lead to intense magnetic storms when they have a prolonged southward axial field.

Consequences of the neural network investigation for Dst-AL relationship

Several recent studies have suggested that most of the Dst main phase variations and of the AL variations similarly respond to a certain type of solar wind condition although the processes are independent of each other. This similarity suggests that some consistency between the Dst main phase development and AL variations exists, regardless of the existence of causality. In what situations this consistent relationship really exists or collapses has been examined with the technique of an Elman recurrent neural network. The network was trained with the Dst and hourly averaged AL indices for 70 storm events from 1967 to 1981, and tested for nine storms that occurred in 1982. The result shows that the Dst-AL relationship can be categorized into two types: high correlative mapping for which 80% and more of the Dst peak in the main phase is reproduced by AL, and partially correlative mapping where only about a half of the Dst peak is reproduced. It is found that whether the correlation is high or partial is determined by whether the Dst main phase develops smoothly or with a discontinuity, i.e., for storms having a discontinuity in the main phase, the coherency collapses. The discontinuity in the Dst main phase is associated with the rapid southward IMF change after the northward excursion. We suggest that it is this IMF variation to which storms and/or substorms respond in a highly complex manner and that such a complex response can be associated with about a half of the maximum ring current intensity.

RING CURRENTS AND INTERNAL PLASMA SOURCES

The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres.

Recent findings on angular distributions of dayside ring current energetic ions

Angular distributions of dayside ring current energetic ions are studied with the Medium Energy Particle Analyzer (MEPA) on the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft. The anisotropy index n for pancake distribution during 10 geomagnetically quiet days is examined for the first time in the intermediate energies (50–1000 keV) for which compositional information is now available. It is shown that n for protons and helium ions increases with L whereas n for CNO ions appears to be independent of L. The empirical formula n(E) = no+k log10 (E(MeV)), for the energy dependence given by Fritz and Spjeldvik (1982), is extended to these energies and ion species. The parameter no is found to increase generally with ion mass and L, except for CNO ions which show no noticeable L dependence. The parameter k appears to be independent of L and ion mass. We have also surveyed the varieties of pitch angle distributions. Two unusual distributions were detected during the September 4–7, 1984, geomagnetic storm. The first is a net field‐aligned transport of ring current energetic ions at 6 ≲ L ≲ 9 and 1030–1300 MLT. Such a signature is shown most prominently by the heavy ions (mostly oxygen; E > 137 keV), to a lesser extent by helium ions (E > 72 keV), and only slightly by protons (E > 56 keV) which are the dominant ionic species. The second new feature is a sharp intensity enhancement over a pitch angle range centered at 90° which occurs in all ionic species and energy passbands monitored by MEPA at 3 ≲ L ≲ 4 and 1700–1800 MLT. Its characteristics suggest that it represents the addition of a freshly injected ring current population in the main phase development of the storm, and not due to L shell splitting or wave‐particle interaction.

Solar wind and magnetosheath observations during the January 13-14, 1967, geomagnetic storm

The interplanetary, magnetosheath, and magnetotail plasmas were observed with electrostatic analyzers on the Vela 3A and 3B satellites at ∼18 RE during the January 13–14, 1967, geomagnetic storm. Various parts of the storm phenomenology were observed. The sudden commencement at 1202 UT on January 13 was caused by an interplanetary shock that passed the earth with a speed of ∼463 km sec−1, considerably lower than the probable average propagation speed from the sun of ∼720 km sec−1. During the initial phase and main phase development the magnetotail was either compressed or tilted up, or both. Just before the inward movement of the magnetopause observed with ATS 1 the solar wind velocity decreased and suddenly increased again by ∼55 km sec−1. During the main phase development the solar wind plasma population as observed by the Vela satellites was distinctly different from the initial phase population, as shown by the arrival of plasma with substantially increased density and alpha particle abundance. Near the peak of the main phase and later, magnetopause crossings showed that the magnetosphere was inflated.

Measurements of the interplanetary solar wind during the large geomagnetic storm of April 17-18, 1965

During the large geomagnetic storm of April 17–18, 1965, measurements of the solar wind were made with electrostatic analyzers on the Vela 2 satellites. A number of the observed solar wind changes can be related to worldwide geomagnetic changes; the most impressive event occurred after the main phase development when a fivefold increase in the solar wind density resulted in a sudden geomagnetic impulse. During the initial phase of the storm the plasma measurements were not complete, but the evidence indicates that the wind pressure increased at the time of the sudden commencement, and other features of the initial phase seem to have been related to solar wind changes. However, the main phase of the storm developed during a period when the solar wind remained relatively steady; further, there was nothing abnormal about the magnitude of the velocity, temperature, or proton density at that time. The α-particle flux during this period was higher than usual. The storm recovery was characterized by a period of rising solar wind velocity and temperature.