Intense Magnetic Storms Research Articles

An investication into geomagnetic and atmospheric response associated with the storm of April 12-14, 1981

An investigation is made of the intense storms of April 12- 14, 1981(Dst =-311nT) in an attempt to contribute to the current understanding of solar wind structures and magnetospheric processes that generate intense magnetic storms, as well as explain the F2 region response associated with these interplanetary structures and magnetospheric processes. The interplanetary and geomagnetic data used in this study consists of hourly values of proton density, solar wind flow speed, interplanetary magnetic field B component, and the low-latitude magnetic index, Dst. The ionospheric data are hourly values of foF2 obtained from a network of ionosonde stations located in the East Asian sector: Yakutsk, Magadan, Khabarovsk, Wakkanai, Akita, Kokubunji, Okinawa and Manila. The study shows that the present storm is double step, and the leading single magnetospheric process that was responsible for both the first and second Dst decrease is the enhancement of the plasma sheet. An enhanced solar wind density drove, under southward B conditions, the plasma sheet density leading to the injection of the ring current. In regards to the F2-region response, it appears the ionosphere in the East Asian zone is characterized by the occurrence of strong negative phase at the low latitude station of Manila before the beginning of the geomagnetic storm, absence of positive ionospheric storm effects at high and mid latitudes on the dayside during the initial phase of the magnetic storm, and simultaneous intense depletion of foF2 at all latitudes at ~ 20:00 UT, April 12. The simultaneous depletion of foF2 at all latitudes does not appear to support the previously held notion that the depletion of F2-region plasma density is due to changes in neutral composition resulting from neutral wind produced predominantly by Joule heating in the aurora zone, but rather suggests that particle precipitation does contribute to depletion of foF2 at all latitudes during intense magnetic storms. Keywords: Geomagneticstorm, ionospheric Fz region, negative storm, plasma sheet density and ring current Nigerian Journal of Physics Vol. 18 (2) 2006: pp. 171-180

Energy balance during intense and super-intense magnetic storms using an Akasofu ε parameter corrected by the solar wind dynamic pressure

Geomagnetic storms are large disturbances in the Earth's magnetosphere caused by enhanced solar wind–magnetosphere energy transfer. One of the main manifestations of a geomagnetic storm is the ring current enhancement. It is responsible for the decrease in the geomagnetic field observed at ground stations. In this work, we study the ring current dynamics during two different levels of magnetic storms. Thirty-three events are selected during the period 1981–2004. Eighteen out of 33 events are very intense (or super-intense) magnetic storms ( Dst ⩽−250 nT) and the remaining are intense magnetic storms (−250< Dst ⩽−100 nT). Interplanetary data from spacecraft in the solar wind near Earth's orbit (ACE, IMP-8, ISEE-3) and geomagnetic indices ( Dst and Sym-H) are analyzed. Our aim is to evaluate the interplanetary characteristics (interplanetary dawn–dusk electric field, interplanetary magnetic field component B S), the ε parameter, and the total energy input into the magnetosphere ( Wε) for these two classes of magnetic storms. Two corrections on the ε energy coupling function are made: the first one is an already known correction in the magnetopause radius to take into account the variation in the solar wind pressure. The second correction on the Akasofu parameter, first proposed in this work, accounts for the reconnection efficiency as a function of the solar wind ram pressure. Geomagnetic data/indices are also employed to study the ring current dynamics and to search for the differences in the storm evolution during these events. Our corrected ε parameter is shown to be more adequate to explain storm energy balance because the energy input and the energy dissipated in the ring current are in better agreement with modern estimates as compared with previous works. For super-intense storms, the correction of the Akasofu ε is on average a scaling factor of 3.7, whilst for intense events, this scaling factor is on average 3.4. The injected energy during the main phase using corrected ε can be considered a criterion to separate intense from very intense storms. Other possibilities of cutoff values based on the energy input are also investigated. A threshold value for the input energy is much more clear when a new classification on Dst=−165 nT is considered. It was found that the energy input during storms with Dst<−165 nT is double of the energy for storms with Dst>−165 nT.

Response of the equatorial ionosphere at dusk to penetration electric fields during intense magnetic storms

We investigate the response of the equatorial ionosphere at dusk to the intense magnetic storms of 30 October 2003 and 20 November 2003, different aspects of which have been widely studied by the community. We present here a very complete set of space and ground‐based diagnostics that provide the vertical and latitudinal structures of the ionosphere within the South Atlantic magnetic anomaly (SAMA) region and the contiguous parts of South America and Africa. We show that for both storms, the dusk sector corresponding to the universal time (UT) interval between the fast decrease of the SYM‐H index and minimum SYM‐H value determines uniquely the longitude interval populated by equatorial plasma bubbles and depletions. Further, we find that the UT of these storms is such that the ionospheric density perturbations occur in the SAMA region, which are most extended in latitude and altitude compared with other regions of the globe. In the dusk sector, the eastward penetration electric field, associated with rapid SYM‐H decrease, adds to the postsunset eastward E‐field because of the F region dynamo, which may be specially enhanced in this longitude interval because of the increased zonal conductivity gradient caused by energetic particle precipitation. This enhanced E‐field at dusk causes a rapid uplift of the ionosphere and sets off plasma instabilities to form bubbles or bite‐outs. The decreased ion density seen in the Defense Meteorological Satellite Program (DMSP) in situ data at 840 km indicates that the ionospheric plasma has been lifted above the DMSP altitude and transported away from the region by diffusion along magnetic field lines. Plasma bubbles and bite‐outs impact satellite communication and navigation systems by introducing scintillations and steep density gradients. This paper corroborates that intense magnetic storms follow the framework, developed by Su. Basu et al. (2001) for moderate storms, that specifies the longitude interval in which such disturbances are most likely to occur.

Extreme polar cap density enhancements along magnetic field lines during an intense geomagnetic storm

Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite are used to derive electron number density distributions along magnetic field lines in the polar cap magnetosphere during an intense magnetic storm. It is shown that electron densities along magnetic field lines in the polar cap magnetosphere were greatly enhanced on both the dayside and nightside during the storm, compared to the electron density profiles measured during periods of lower geomagnetic activities. The electron density enhancements were observed extending to 7 Earth radii (RE) in altitude on the dayside, with the electron density value reaching about 10 cm−3 at 7 RE altitude. The observed density enhancements were likely due to the enhanced cleft ion fountain during the storm although some of nightside density enhancements might be caused by the increased ion outflows locally in the polar cap. The strongest electron density enhancements observed on the dayside are possibly further associated with storm‐time transport of plasma from the midlatitude ionosphere and plasmasphere to high latitudes, which manifests as a plasma plume intruding to dayside high latitudes as seen from total electron content (TEC) maps. With an enhanced source population supplied by the plasma plume, acceleration and heating processes in the dayside cusp/auroral region may produce a large flux of outflowing plasma along magnetic field lines while the outflowing plasma is convected anti‐sunward toward the polar cap. These processes lead to strongly enhanced cleft ion fountain and thus greatly raised electron densities at magnetospheric altitudes in the polar cap. The present study captures an event of a massive redistribution of the magnetospheric and ionospheric plasma during a geomagnetic storm caused by extreme solar wind/interplanetary magnetic field (IMF) conditions.

Enhancement of OI 630.0 nm emission at mid-latitudes during an intense magnetic storm

All-sky camera (ASC), Global Positioning System (GPS), and ionosonde measurements were used to investigate the upper atmospheric variations at mid-latitude during the strong geomagnetic storm on October 29–31, 2003. An arc-shaped 630.0 nm emission was observed in the northern sky on all-sky images taken at Mt. Bohyun (36.2°N, 128.9°E, GMLAT=29°N) in Korea during 17:48–8:58 UT in the main phase of the geomagnetic storm on October 29. The NmF 2 and hmF 2 from the ionosonde show strong disturbances at that time. The vertical profiles of electron densities, calculated by the ionospheric tomographic method using ground-based GPS slant total electron contents measurements, show the largest value at ∼440 km height at 18:30 UT on October 29 when the enhancements of OI 630.0 nm emission were observed. The arc-shaped red emission observed during the main phase of the magnetic storm is likely a low-latitude red aurora due to its short duration of ∼1 h. The result implies that the plasmapause was at L=1.4–1.6 during the geomagnetic storm. The fact that the arc did not follow a constant L-value appears to suggest that neutral precipitation and a traveling ionospheric disturbance could also be the cause of the arc.

Study of ionospheric irregularities during intense magnetic storms

The effects of two intense magnetic storms over ionospheric irregularities were analyzed using GPS signal scintillation data from the stations of São Luís (2.57ºS, 44.21ºW, dip latitude 1.73ºS) in the equatorial region, São José dos Campos (23.07ºS, 45.86ºW, dip latitude 18.01ºS) and Cachoeira Paulista (22.57ºS, 45.07ºW, dip latitude 18.12ºS) both under the Equatorial Ionization Anomaly (EIA), and São Martinho da Serra (29.28ºS, 53.82ºW, dip latitude 18.57ºS), located in the South of Brazil. Total Electron Content (TEC) data for São Luís and São José dos Campos, were also analyzed. The analyzed storms occurred on October 28-31, 2003 and on November 7-11, 2004. Both storm periods presented two main phases. In the nights of 29/30 and 30/31 of October, during the two storm main phase, it was observed that TEC over São José dos Campos reached higher values than the TEC for the magnetically quiet day of October 10, due to the effect of eastward electric field prompt penetration to magnetic equator that intensified the EIA. Compared to a quiet day (Oct 10), scintillation in the GPS signal amplitude due to ionospheric irregularity, quantified by the scintillation index S4, was stronger for Cachoeira Paulista (under EIA) during the night of 30/31but not for the night of 29/30 and for São Martinho da Serra was stronger during the nights of 29/30 and 30/31. Scintillation for the nights of 29/30 and 30/31 at these two stations lasted longer than on October 10, reaching the post midnight time sector. During the November 7-11 storm, TEC kept the behavior of a quiet day except during days 10 and 11 (up to 9 UT), when a large TEC decrease was observed. The GPS scintillation, compared to the quiet day November 19, was larger at the equatorial station of São Luís during the nights of 7/8 and 8/9 and it was completely inhibited for the São Luís and São José dos Campos stations during the nights of 9/10 and 10/11, probably due to action of westward disturbance dynamo electric field penetration to equator.

From pre-storm activity to magnetic storms: a transition described in terms of fractal dynamics

Abstract. We show that distinct changes in scaling parameters of the Dst index time series occur as an intense magnetic storm approaches, revealing a gradual reduction in complexity. The remarkable acceleration of energy release – manifested in the increase in susceptibility – couples to the transition from anti-persistent (negative feedback) to persistent (positive feedback) behavior and indicates that the occurence of an intense magnetic storm is imminent. The main driver of the Dst index, the VBSouth electric field component, does not reveal a similar transition to persistency prior to the storm. This indicates that while the magnetosphere is mostly driven by the solar wind the critical feature of persistency in the magnetosphere is the result of a combination of solar wind and internal magnetospheric activity rather than solar wind variations alone. Our results suggest that the development of an intense magnetic storm can be studied in terms of "intermittent criticality" that is of a more general character than the classical self-organized criticality phenomena, implying the predictability of the magnetosphere.

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.

Ring Current Dynamics

This chapter reviews the current understanding of ring current dynamics. The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ∼10,000 to 60,000 km. Enhancements in this current are responsible for global decreases in the Earth’s surface magnetic field, which have been used to define geomagnetic storms. Intense geospace magnetic storms have severe effects on technological systems, such as disturbances or even permanent damage of telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the ring current are positive ions, with energies from ∼1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere into the inner magnetosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while active processes in geospace tend to increase the abundance (both absolute and relative) of O+ ions, which are of ionospheric origin. During intense geospace magnetic storms, the O+ abundance increases dramatically. This increase has been observed to occur concurrently with the rapid intensification of the ring current in the storm main phase and to result in O+ dominance around storm maximum. This compositional change can affect several dynamic processes, such as species-and energy-dependent charge-exchange and wave-particle scattering loss.

In search of a new ULF wave index: Comparison of Pc5 power with dynamics of geostationary relativistic electrons

A new ULF wave index, characterizing the turbulent level of the geomagnetic field, has been calculated and applied to the analysis of relativistic electron enhancements during space weather events in March–May 1994 and September 1999. This global wave index has been produced from the INTERMAGNET, MACCS, CPMN, and Greenland dense magnetometer arrays in the northern hemisphere. A similar ULF wave index has been calculated using magnetometer data from geostationary (GOES) and interplanetary (Wind, ACE) satellites. During the periods analyzed several magnetic storms occurred, and several significant increases of relativistic electron flux up to 2–3 orders of magnitude were detected by geostationary monitors. However, these electron enhancements were not directly related to the intensity of magnetic storms. Instead, they correlated well with intervals of elevated ULF wave index, caused by the occurrence of intense Pc5 pulsations in the magnetosphere. This comparison confirmed earlier results showing the importance of magnetospheric ULF turbulence in energizing relativistic electrons. In addition to relativistic electron energization, a wide range of space physics and geophysics studies will benefit from the introduction of the ULF wave index. The ULF index database is freely available via anonymous FTP for all interested researchers for further validation and statistical studies.

Convection electric field in the near-Earth tail during the super magnetic storm of November 20–21, 2003

[1] We examine the electric field observations made by the Geotail spacecraft in the magnetotail during the intense magnetic storm of November 20–21, 2003. During this storm, Geotail traversed the magnetotail at R ∼ 9–12 Re from dusk to dawn through the midnight sector at ∼17 UT on November 20 and observed the convection electric field in the near-Earth plasma sheet for the main phase and early recovery phase. While the electric field in the plasma sheet during the storm interval shows highly turbulent characteristics in a wide range of local time, the average intensity of the duskward electric field is not so different in magnitude from those observed during other small to moderate level storms. A comparison with those calculated using the empirical models reveals that the independent behavior of the tail convection intensity with regard to the storm driver intensity still holds for an intense storm case. These results give a further support to the hypothesis suggested by our previous study that the tail convection supplies the ring current particles rather constantly regardless of the solar wind condition.

The Effect of the Heliospheric Current Sheet on Interplanetary Shocks

Using 180 interplanetary (IP) shock events associated with coronal mass ejections (CMEs) during 1997 – 2005, we investigate the influence of the heliospheric current sheet (HCS) upon the propagation and geoeffectiveness of IP shocks. Our preliminary results are: (1) The majority of CME-driving IP shocks occurred near the HCS. (2) The numbers of shock events and related geomagnetic storms observed when the Earth and the solar source are located on the same side of the HCS, represented by f SS and f SG, respectively, are obviously higher than those when the Earth and the solar source are located on the opposite sides of the HCS, denoted by f OS and f OG, with f SS/f OS=126/54, f SG/f OG = 91/36. (3) Parameter jumps across the shock fronts for the same-side events are also higher than those for the opposite-side events, and the stronger shocks (Δ V ≥ 200 km s−1) are mainly attributed to be same-side events, with f SSh/f OSh = 28/15, where f SSh and f OSh are numbers of stronger shocks which belong to same-side events and opposite-side events, respectively. (4) The level of the geomagnetic disturbances is higher for the same-side events than for the opposite-side events. The ratio of the number of intense magnetic storms (Dst < −100) triggered by same-side events to those triggered by opposite-side events is 25/10. (5) We propose an empirical model to predict the arrival time of the shock at the Earth, whose accuracy is comparable to that of other prevailing models. These results show that the HCS is an important physical structure, which probably plays an important role in the propagation of interplanetary shocks and their geoeffectiveness.

Variations of the magnetopause position versus the level of geomagnetic activity (according to data of the INTERBALL-1 Satellite for 1995–1997)

The variations in the deviation of the observed position of the magnetosphere boundary from its mean position predicted by the Shue at al., 1997 (Sh97) model [7] are studied as a function of the substorm activity level (the AE-index value) and magnetic storm intensity (the value of the corrected Dst* index). The results obtained make it possible to state that the amplitude of motion of the magnetospheric boundary on the dayside and in the low-latitude tail is small. It is likely that the position of the boundary is either independent of the AE and Dst* indices or this dependence is weak. At the same time, the boundary of the high-latitude tail shifts inward on the average by 1.5RE with an increase of the AE-index in the case of absence of magnetic storms (contraction of the magnetospheric tail). On the contrary, in the presence of magnetic storms, this boundary shifts outward by up to 3RE with an increase of the AE-index (inflation of the magnetospheric tail). It is also shown that the boundary of the high-latitude tail moves outward with an increase of the Dst* index, both at low substorm activity and in periods of high substorm activity. The amplitude of the outward motion of the high-latitude tail of the magnetosphere is by a factor of two higher for moderate magnetic storms with strong substorms than for moderate magnetic storms with weak substorms.

Auroral electrojets and boundaries of plasma domains in the magnetosphere during magnetically disturbed intervals

Abstract. We investigate variations in the location and intensity of the auroral electrojets during magnetic storms and substorms using a numerical method for estimating the equivalent ionospheric currents based on data from meridian chains of magnetic observatories. Special attention was paid to the complex structure of the electrojets and their interrelationship with diffuse and discrete particle precipitation and field-aligned currents in the dusk sector. During magnetospheric substorms the eastward electrojet (EE) location in the evening sector changes with local time from cusp latitudes (Φ~77°) during early afternoon to latitudes of diffuse auroral precipitation (Φ~65°) equatorward of the auroral oval before midnight. During the main phase of an intense magnetic storm the eastward currents in the noon-early evening sector adjoin to the cusp at Φ~65° and in the pre-midnight sector are located at subauroral latitude Φ~57°. The westward electrojet (WE) is located along the auroral oval from evening through night to the morning sector and adjoins to the polar electrojet (PE) located at cusp latitudes in the dayside sector. The integrated values of the eastward (westward) equivalent ionospheric current during the intense substorm are ~0.5 MA (~1.5 MA), whereas they are 0.7 MA (3.0 MA) during the storm main phase maximum. The latitudes of auroral particle precipitation in the dusk sector are identical with those of both electrojets. The EE in the evening sector is accompanied by particle precipitation mainly from the Alfvén layer but also from the near-Earth part of the central plasma sheet. In the lower-latitude part of the EE the field-aligned currents (FACs) flow into the ionosphere (Region 2 FAC), and at its higher-latitude part the FACs flow out of the ionosphere (Region 1 FAC). During intense disturbances, in addition to the Region 2 FAC and the Region 1 FAC, a Region 3 FAC with the downward current was identified. This FAC is accompanied by diffuse electron precipitation from the plasma sheet boundary layer. Actually, the triple system of FAC is observed in the evening sector and, as a consequence, the WE and the EE overlap. The WE in the evening sector comprises only the high-latitude periphery of the plasma precipitation region and corresponds to the Hall current between the Region 1 FAC and Region 3 FAC. During the September 1998 magnetic storm, two velocity bursts (~2–4 km/s) in the magnetospheric convection were observed at the latitudes of particle precipitation from the central plasma sheet and at subauroral latitudes near the ionospheric trough. These kind of bursts are known as subauroral polarization streams (SAPS). In the evening sector the Alfvén layer equatorial boundary for precipitating ions is located more equatorward than that for electrons. This may favour northward electric field generation between these boundaries and may cause high speed westward ions drift visualized as SAPS. Meanwhile, high speed ion drifts cover a wider range of latitudes than the distance between the equatorward boundaries of ions and electrons precipitation. To summarize the results obtained a new scheme of 3-D currents in the magnetosphere-ionosphere system and a clarified view of interrelated 3-D currents and magnetospheric plasma domains are proposed.

An intense geomagnetic storm associated with slow solar wind

Results of earlier studies on intense geomagnetic storms appear to suggest the interplanetary manifestation of fast (v = 500km/s) coronal mass ejections (CMEs) as the dominant interplanetary phenomenon causing intense magnetic storms. However, the intense storm of April 1-2, 1973 appears to be due to a slow solar wind. Presently a study has been made of the April 1-2, 1973 geomagnetic storm in order to identify the probable roles played by the various solar and interplanetary factors in the generation of major geomagnetic storms. The storm is summarized using the low-latitude magnetic index, Dst and is interpreted using available interplanetary data: proton number density, solar wind flow speed, plasma temperature, interplanetary magnetic field southward component Bz, plasma beta and dawn-dusk electric field. Our results show the magnetic storm is a double step event: a moderate-to- weak storm which occurred during the period 20:00 UT March 31- 13:00 UT April 1 and is due to the weakly compressed magnetic fields in the sheath, and the April 1-2 intense storm which is due a magnetic cloud. Furthermore, the present results show that the peak solar wind velocity occurred on April 2, a day Dst after attained its peak value of -211 nT and confirms the results of Sawyer and Haurwitz (1976) that geomagnetic activity is highest on the day preceding peak velocity in the high-speed stream. Keywords: solar wind, Interplanetary Magnetic Field (IMF), coronal mass ejections (CME), low-latitude magnetic index (Dst), geomagnetic storm, Plasma beta Nigerian Journal of Physics Vol. 17, 2005: 27-34

Introduction to special section on corotating solar wind streams and recurrent geomagnetic activity

[1] This special section of the Journal of Geophysical Research (Space Physics) is devoted to research papers on corotating solar wind streams, corotating interaction regions (CIRs), and many facets of the associated recurrent geomagnetic storms and/or activity. The topics cover the physics related to these events at the Sun all the way to their effects in the Earth’s ionosphere. This portion of space weather is complementary to the interplanetary and geomagnetic activity produced by solar flares and/or coronal mass ejections (CMEs), which occur most predominantly during the maximum sunspot phase of the solar cycle. Collisionless shocks ahead of fast interplanetary CMEs (ICMEs) accelerate energetic protons and higher Z ions to flux levels that can be hazardous to satellites and to humans in space. ICMEs and their upstream plasma sheaths cause intense and dramatic magnetic storms and auroras noted at Earth. On the other hand, high-speed streams associated with large coronal holes occur predominantly in the declining and minimum phases of the solar cycle. The interaction of the high-speed streams with upstream (further from the Sun) slow-speed streams leads to the formation of the magnetic compression regions called CIRs. Collisionless shocks typically form at CIR boundaries at distances greater than 1.5 to 2.0 AU from the Sun, so energetic particle radiation formed at these (corotating) shocks is not a hazard at Earth. CIR interactions with the Earth’s magnetosphere are gentler and produces only weak to moderate intensity magnetic storms. [2] The history of recurrent magnetic storms and their solar origin is a long one and goes back for more than a century. Maunder [1904] noted a strong 27 day recurrence in geomagnetic activity and speculated that this could be caused by particle streams emanating from the Sun. Chree [1912, 1913] was the first to quantitatively establish a 27 day quasi-periodicity in geomagnetic activity using a superposed epoch method of analyses (this is the origin of what we now call ‘‘Chree analyses’’). Bartels [1934], not being able to observe any obvious solar features that could be responsible for the recurrent storms, suggested that these ‘‘time patterns’’ were caused by unseen ‘‘M-regions’’ on the Sun (M stood for ‘‘magnetically active’’). Later, beginning in the space age, knowledge of the cause of these storms and their effects accelerated rapidly. Direct, in situ, measurements of the time variability of the solar wind were first made by Mariner 2, which revealed a 27-day modulation of the solar wind speed [Snyder et al., 1963; Neugebauer and Snyder, 1966]. Krieger et al. [1973] demonstrated that the source of these high-speed streams were ‘‘coronal holes’’ [Altschuler and Perry, 1972; Munro and Withbroe, 1972]. Coronal holes are detectable in soft X rays and not in visible light, thus solving what Bartels’ mysterious M-regions were. These coronal hole streams, or some facet of them, were clearly the cause of the recurrent geomagnetic activity at Earth [Sheeley et al., 1976]. For the reader interested in more details concerning the early history of this subject, we recommend the comprehensive Chapman and Bartels [1940] books and the references therein. An abridged and more recent historical account is given by Crooker and Cliver [1994]. [3] As previously mentioned, the high-speed solar wind streams do not simply propagate unimpeded to the Earth, but interact with upstream slower speed streams to form CIRs [Smith and Wolfe, 1976]. The antisunward portions of CIRs are compressed and accelerated slow stream plasma and the sunward portions are compressed and decelerated high-speed stream plasma [Pizzo, 1985]. It is the CIRs that cause recurrent magnetic storm main phases [Tsurutani et al., 1995]. These recurrent magnetic storms are unique in that their ‘‘recovery’’ phases are exceptionally long, lasting from many days to weeks. These storm phases occur within the high speed streams characterized by large amplitude Alfven waves. It has been shown that the southward component of the Alfven waves leads to magnetic reconnection with magnetopause magnetic fields. The latter results in bursts of plasma injections into the magnetosphere sustaining the storm ‘‘recoveries’’ [Soraas et al., 2004]. The resultant geomagnetic activity is called high-intensity, longduration continuous AE activity or HILDCAAs [Tsurutani and Gonzalez, 1987, 1997]. It has been argued that at times, more solar wind energy is introduced into the magnetoJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A07S00, doi:10.1029/2006JA011745, 2006 Click Here for Full Article

Geoeffectiveness of corotating interaction regions as measured by Dst index

Corotating interaction regions (CIRs) are structures formed when high‐speed solar wind streams overtake slow solar wind streams as they propagate outward. These structures produce regions of enhanced density and magnetic field strength in the solar wind near the ecliptic plane. In this paper, the geoeffectiveness of CIRs, as measured by the geomagnetic Dst index, is assessed during the solar wind observational period 1964–2003. A catalogue of CIRs is constructed by consulting high‐speed plasma streams (HSPS) lists present in the literature and by analyzing solar wind parameters for each HSPS event. The geoeffectiveness of CIRs is analyzed by determining the number of intense (Dst ≤ −100 nT), moderate (−100 &lt; Dst ≤ −50 nT), or weak (−50 &lt; Dst ≤ −30 nT) magnetic storms that followed each CIR event. Statistical distributions of CIR parameters (maximum solar wind speed, maximum convection electric field, southward magnetic field peak) and geoeffectiveness (Dst peak) are obtained. Correlation analyses of Dst index with various solar wind parameters are presented. A comparison with the geoeffectiveness of other interplanetary structures such as shocks, magnetic clouds, and sector boundaries is performed. Our results show that 33% of CIRs are followed by moderate/intense magnetic activity (Dst &lt; −50 nT), i.e., approximately one third of the CIR events observed near Earth are geoeffective.

In situ observation atL= 2.3–5 by the Akebono satellite of the plasmaspheric depletion during the September 1998 magnetic storm

[1] From ground-based observations at L = 2.07 of the field line resonance (FLR) during an intense magnetic storm on 25 September 1998, Chi et al. (2000) estimated that the equatorial plasma density at L = 2.07 dropped to 25% of the prestorm value. Such depletion very close to the Earth is unusual. Two possible interpretations of the result are: (1) the plasmapause moved inward past L = 2.07, and (2) the plasmapause remained outside L = 2.07, but the density within the plasmasphere decreased. To distinguish between these possibilities, we examine in situ observations of the electron density made by the Akebono satellite at L = 2.3–5 on four passes during the same storm. The electron density measured by Akebono at a reference L shell of L ∼2.5 changed with time in a manner consistent with the FLR-based estimates. On three of these passes, the plasmapause was located at L > 3, and if the plasmaspheric L profile of the Akebono density data is extrapolated inward, for each pass, its value at L = 2.07 matches the FLR-based estimates. However, on the pass at 2323–2351 UT on 25 September that corresponded to the severe density decrease detected at L = 2.07, Akebono did not detect a plasmapause in the L range (>2.3) for which the electron density data were available. For the same pass, inward extrapolation of the density measured at L > 2.3 was a factor of ∼5 smaller than the FLR-based estimate. This implies that there was a sharp density inward gradient (the plasmapause) at 2.07 < L < 2.3, supporting the second interpretation.

Interplanetary origin of magnetic storms and their F2-region responses at the equatorial anomaly

The interplanetary origin of 102 storms ( D st ⩽ −50 nT) of varying strength that occurred during the period of 1997–2001, have been studied. The shock compression, magnetic clouds, and shock compression followed by magnetic clouds are most probable sources of intense southward component B S of IMF in the interplanetary medium causing the magnetic storms. Thirty five percent of magnetic storms were associated with the magnetic clouds. The main phase of 60% intense and very intense magnetic storms associated with the magnetic clouds, occurred through the multi-step growth in the ring current during the main phase evolution of the storms. The storms were associated with enhanced IMF intensity, proton bulk velocity, ion dynamic pressure and ion density, in the solar wind, in general, with increase in strength of the storms, but not linearly. The storm-time effects in the ionospheric F2-region during 51 magnetic storms during 1997–2001, at Ahmedabad (23.2°N, 72.4°E, dip angle 30.7° at sub-ionospheric points), a station at the ionization anomaly crest in the Indian region, are also studied. The negative ionospheric effects are more pronounced than the positive ionospheric effects. While the prompt penetration of high-latitude electric fields and the electric fields generated by the disturbance dynamo play important role in F2-region response by changing E × B drifts at low latitude, the storm-induced transport of gas with depleted ratio of [O]/[N2] may also be responsible for the post-midnight large decrease in f oF 2, during strong magnetic storms ( D st < 100 nT).

A statistical study of magnetic cloud parameters and geoeffectiveness

A statistical study of magnetic cloud parameters and geoeffectiveness is presented in this work, based on the analysis of 149 magnetic clouds during the period 1966–2001. The distributions of maximum magnetic field strength, solar wind speed and southward magnetic field inside the clouds were determined for the whole data set and for subsets classified according to the magnetic cloud polarity (rotation in Z or Y directions). The geoeffectiveness was determined by classifying the number of magnetic clouds followed by intense, moderate and weak magnetic storms, and by calm periods. It was found that around 77% of the magnetic clouds are geoeffective, i.e., they were followed by intense or moderate geomagnetic storms ( Dst ⩽ - 50 nT ) . Considering also weak storms ( Dst ⩽ - 30 nT ) , 97% of MCs were followed by geomagnetic activity. When considering polarity, each magnetic cloud subset has a slightly different geoeffectiveness, which is in agreement with the differences observed in their parameter distributions. The NSY - was observed to be the less geoeffective of the MC subsets, with 66.6% of this class events being followed by intense or moderate storms, against 73.0% of SNY + , 80.0% of the Y, 83.3% of the NSY - and 85.7% of the SNY - . The B peak – V peak relation was confirmed for the magnetic clouds with rotation in Z direction.