The Dependence of the Geoeffectiveness of Interplanetary Flux Rope on Its Orientation,with Possible Application to Geomagnetic Storm Prediction

Interplanetary magnetic clouds (MCs) are one of the main sources of large non-recurrent geomagnetic storms. With the aid of a force-free flux rope model, the dependence of the intensity of geomagnetic activity (indicated by Dst index) on the axial orientation (denoted by θ and φ in GSE coordinates) of the magnetic cloud is analyzed theoretically. The distribution of the Dst values in the (θ, φ) plane is calculated by changing the axial orientation for various cases. It is concluded that (i) geomagnetic storms tend to occur in the region of θ<0°, especially in the region of θ≲−45°, where larger geomagnetic activity could be created; (ii) the intensity of geomagnetic activity varies more strongly with θ than with φ; (iii) when the parameters B ₀ (the magnetic field strength at the flux rope axis), R ₀ (the radius of the flux rope), or V (the bulk speed) increase, or |D| (the shortest distance between the flux rope axis and the x-axis in GSE coordinates) decreases, a flux rope not only can increase the intensity of geomagnetic activity, but also is more likely to create a storm, however the variation of n (the density) only has a little effect on the intensity; (iv) the most efficient orientation (MEO) in which a flux rope can cause the largest geomagnetic activity appears at φ∼0° or ∼ 180°, and some value of θ which depends mainly on D; (v) the minimum Dst value that could be caused by a flux rope is the most sensitive to changes in B ₀ and V of the flux rope, and for a stronger and/or faster MC, a wider range of orientations will be geoeffective. Further, through analyzing 20 MC-caused moderate to large geomagnetic storms during 1998 – 2003, a long-term prediction of MC-caused geomagnetic storms on the basis of the flux rope model is proposed and assessed. The comparison between the theoretical results and the observations shows that there is a close linear correlation between the estimated and observed minimum Dst values. This suggests that using the ideal flux rope to predict practical MC-caused geomagnetic storms is applicable. The possibility of the long-term prediction of MC-caused geomagnetic storms is discussed briefly.     

Development of four magnetic storms in February 1972

Four magnetic storms were observed in February 1972, with instruments on the Explorer 45 satellite in the evening quadrant of the inner magnetosphere. The magnitude of the storms ranged from small, D_st ? ?40 γ, to moderate, D_st ? ?80 γ. During the development of the storms several substorms occurred. At the beginning of the substorms there was evidence of a partial ring current above L = 5. After the expansion phase of several substorms there was evidence of enhancement of a partial ring below L = 5. Distortions of the field in the east-west direction were observed, in conjunction with substorm expansions, that can be interpreted as due to field aligned currents flowing from the ionosphere. A substantial symmetric ring current, at L～4, developed during the largest storm. Very little additional ring current was contributed by the smallest storm. Relations between the magnetosphere inflation and ring current protons, plasmaspheric hiss, and ULF waves also measured on Explorer 45 were noted.     

The Automatic Predictability of Super Geomagnetic Storms from halo CMEs associated with Large Solar Flares

We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D _st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90^∘ in the case of the southward orientation and ≥ 90^∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms (D _st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms.     

Comparison of the Characteristics of Magnetic Clouds and Magnetic Cloud-Like Structures for the Events of 1995 – 2003

Using nine years of solar wind plasma and magnetic field data from the Wind mission, we investigated the characteristics of both magnetic clouds (MCs) and magnetic cloud-like structures (MCLs) during 1995 – 2003. A MCL structure is an event that is identified by an automatic scheme (Lepping, Wu, and Berdichevsky, Ann. Geophys. 23, 2687, 2005) with the same criteria as for a MC, but it is not usually identifiable as a flux rope by using the MC (Burlaga et al., J. Geophys. Res. 86, 6673, 1981) fitting model developed by Lepping, Jones, and Burlaga (Geophys. Res. Lett. 95(11), 957, 1990). The average occurrence rate is 9.5 for MCs and 13.6 for MCLs per year for the overall period of interest, and there were 82 MCs and 122 MCLs identified during this period. The characteristics of MCs and MCL structures are as follows: (1) The average duration, Δt, of MCs is 21.1 h, which is 40% longer than that for MCLs (Δt=15 h); (2) the average (minimum B _z found in MC/MCL measured in geocentric solar ecliptic coordinates) is −10.2 nT for MCs and −6 nT for MCLs; (3) the average Dst_min (minimum Dst caused by MCs/MCLs) is −82 nT for MCs and −37 nT for MCLs; (4) the average solar wind velocity is 453 km s⁻¹ for MCs and 413 km s⁻¹ for MCLs; (5) the average thermal speed is 24.6 km s⁻¹ for MCs and 27.7 km s⁻¹ for MCLs; (6) the average magnetic field intensity is 12.7 nT for MCs and 9.8 nT for MCLs; (7) the average solar wind density is 9.4 cm⁻³ for MCs and 6.3 cm⁻³ for MCLs; and (8) a MC is one of the most important interplanetary structures capable of causing severe geomagnetic storms. The longer duration, more intense magnetic field and higher solar wind speed of MCs, compared to those properties of the MCLs, are very likely the major reasons for MCs generally causing more severe geomagnetic storms than MCLs. But the fact that a MC is an important interplanetary structure with respect to geomagnetic storms is not new (e.g., Zhang and Burlaga, J. Geophys. Res. 93, 2511, 1988; Bothmer, ESA SP-535, 419, 2003).     

Latitudinal characteristics of the geomagnetic field during the storm of 15–16 July 2000

In this paper geomagnetic disturbances at middle and low latitudes are discussed by using geomagnetic data of the magnetic storm of 15–16 July 2000. This storm is a response to the solar Bastille Day flare on 14 July. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric–ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), and the region I field-aligned currents (FA). Our results show that: (1) The northward turning of IMF-Bz started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned anti-parallel to the geomagnetic axis from a 15° deviation, and the local time (Λ) pointed by the RC-field stayed at 16:00. After that, Λ rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but 5°–10° deviated. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.     

Dynamics of the earth radiation belts during strong magnetic storms based on <Emphasis Type="Italic">CORONAS-F</Emphasis> data

The results of an experimental study of the variations in the intensity of the fluxes of the Earth radiation belt (ERB) particles in 0.3–6 and 1–50 MeV energy intervals for electrons and protons, respectively, are reported. ERBs were studied during strong magnetic storms from August 2001 through November 2003. The results of the CORONAS-F mission obtained during the magnetic storms of November 6 (D _st = ?257 nT) and November 24, 2001 (D _st = ?221 nT), October 29–30 (D _st = ?400 nT) and November 20, 2003 (D _st = ?465 nT) are analyzed. The electron flux is found to decrease abruptly in the outer radiation belt during the main phase of the magnetic storms under consideration. During the recovery phase, the outer radiation belt is found to recover much closer to Earth, near the boundary of the penetration of solar electrons during the main phase of the magnetic storm. We associate the decrease in the electron flux with the abrupt decrease of the size of the magnetosphere during the main phase of the storm. Note that, in all cases studied, the Earth radiation belts exhibited rather long (several days) variations. In those cases where solar cosmic-ray fluxes were observed during the storm, protons with energies 1–5 MeV could be trapped to form an additional maximum of protons with such energies at L >2.     

Relationships Among Magnetic Clouds, CMES, and Geomagnetic Storms

During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team using Wind (1995 – 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs (17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections (CMEs), and geomagnetic storms. During the period 1996 – 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number. When we included “magnetic cloud-like structures” (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs, because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most severe storms caused by MCs/MCLs with Dst _min≤ −100 nT occurred in the active solar period.     

Solar Wind Streams And Intense Geomagnetic Storms Observed During 1986–1991

We have analyze the set of 70 intense geomagnetic storms associatedwith D_st decrease of more than 100 nT, observed duringthe period (1986–1991). We have compile these selected intensegeomagnetic storm events and find out their association with twotypes of solar wind streams and different interplanetary parameters.We concluded that the maximum numbers of intense geomagneticstorms are associated with transient disturbances in solar wind streams,which causes strong interplanetary shocks in interplanetary medium.The association of supersonic shocks and magnetic clouds with intensegeomagnetic storms have also been discussed.     

Statistical Comparison of Magnetic Clouds with Interplanetary Coronal Mass Ejections for Solar Cycle 23

We compare the number and characteristics of interplanetary coronal mass ejections (ICMEs) to those of magnetic clouds (MCs) by using in-situ solar wind plasma and magnetic field observations made at 1 AU during solar cycle 23. We found that ≈ 28% of ICMEs appear to contain MCs, since 103 magnetic clouds (MCs) occurred during 1995  – 2006, and 307 ICMEs occurred during 1996 – 2006. For the period between 1996 and 2006, 85 MCs are identified as part of ICMEs, and six MCs are not associated with ICMEs, which conflicts with the idea that MCs are usually a subset of ICMEs. It was also found that solar wind conditions inside MCs and ICMEs are usually similar, but the linear correlation between geomagnetic storm intensity (Dst _min) and relevant solar wind parameters is better for MCs than for ICMEs. The differences between average event duration (Δt) and average proton plasma β (〈β〉) are two of the major differences between MCs and ICMEs: i) the average duration of ICMEs (29.6 h) is 44% longer than for MCs (20.6 hours), and ii) the average of 〈β〉 is 0.01 for MCs and 0.24 for ICMEs. The difference between the definition of a MC and that for an ICME is one of the major reasons for these average characteristics being different (i.e., listed above as items i) and ii)), and it is the reason for the frequency of their occurrences being different.     

Analysis on the interplanetary causes of the great magnetic storms in solar maximum (2000-2001)

In this paper, we analyze the interplanetary causes of eight great geomagnetic storms during the solar maximum (2000-2001). The result shows that the interplanetary causes were the intense southward magnetic field and the notable characteristic among the causal mechanism is compression. Six of eight great geomagnetic storms were associated with the compression of southward magnetic field, which can be classified into (1) the compression between ICMEs (2) the compression between ICMEs and interplanetary medium. It suggests that the compressed magnetic field would be more geoeffective. At the same time, we also find that half of all great storms were related to successive halo CMEs, most of which originated from the same active region. The interactions between successive halo CMEs usually can lead to greater geoeffectiveness by enhancing their southward field B_s interval either in the sheath region of the ejecta or within magnetic clouds (MCs). The types of them included: the compression between the fast speed transient flow and the slow speed background flow, the multiple MCs, besides shock compression. Further, the linear fit of the D_st versus gives the weights of and Δt as α=2.51 and β=0.75, respectively. This may suggest that the compression mechanism, with associated intense B_s, rather than duration, is the main factor in causing a great geomagnetic storm.     

RING CURRENT DYNAMICS DURING THE 13–18 JULY 2000 STORM PERIOD

We study the development of the terrestrial ring current during the time interval of 13–18 July, 2000, which consisted of two small to moderate geomagnetic storms followed by a great storm with indices Dst=−300 nT and Kp=9. This period of intense geomagnetic activity was caused by three interplanetary coronal mass ejecta (ICME) each driving interplanetary shocks, the last shock being very strong and reaching Earth at ∼ 14 UT on 15 July. We note that (a) the sheath region behind the third shock was characterized by B _z fluctuations of ∼35 nT peak-to-peak amplitude, and (b) the ICME contained a negative to positive B _z variation extending for about 1 day, with a ∼ 6-hour long negative phase and a minimum B _z of about −55 nT. Both of these interplanetary sources caused considerable geomagnetic activity (Kp=8 to 9) despite their disparity as interplanetary triggers. We used our global ring current-atmosphere interaction model with initial and boundary conditions inferred from measurements from the hot plasma instruments on the Polar spacecraft and the geosynchronous Los Alamos satellites, and simulated the time evolution of H⁺, O⁺, and He⁺ ring current ion distributions. We found that the O⁺ content of the ring current increased after each shock and reached maximum values of ∼ 60% near minimum Dst of the great storm. We calculated the growth rate of electromagnetic ion cyclotron waves considering for the first time wave excitation at frequencies below O⁺ gyrofrequency. We found that the wave gain of O⁺ band waves is greater and is located at larger L shells than that of the He⁺ band waves during this storm interval. Isotropic pitch angle distributions indicating strong plasma wave scattering were observed by the imaging proton sensor (IPS) on Polar at the locations of maximum predicted wave gain, in good agreement with model simulations.     

Multi-Scale Analysis of the Geomagnetic Symmetric Index (sym)

In this work we present a methodology to estimate the geomagnetic symmetric index (Sym) based on the wavelet analysis of the time series of the H component of the geomagnetic field measured at mid-latitude stations localized at Kakioka (KAK), Honolulu (HON), Hermanus (HER) and San Juan (SJG). A case study of the intense geomagnetic storm of 17–22 February 1999, caused by intense southward magnetic fields just behind an interplanetary shock driven by a magnetic cloud, is shown as an example of the procedure of derivation of the symmetric index and the capabilities of this analysis to improve the study of the coupling of the solar wind and the Earth's magnetosphere. Other examples are shown in order to demonstrate the applicability of the methodology to different magnetospheric conditions. It is shown that the long period variations of the symmetric index are linearly correlated to variations at the same periods of the H component of the geomagnetic field and that the contribution of short period variations to the symmetric index are biased by localized current systems such as the partial ring current and the field aligned currents.     

Evaluation of the STORM-Time Ionospheric Empirical Model for the Bastille Day event

Recent theoretical model simulations of the ionospheric response to geomagnetic storms have provided the understanding for the development of an empirical storm-time ionospheric model (STORM). The empirical model is driven by the previous time-history of a _p, and is designed to scale the quiet-time F-layer critical frequency (f _o F ₂) to account for storm-time changes in the ionosphere. The model provides a useful, yet simple tool for modeling of the perturbed ionosphere. The quality of the model prediction has been evaluated by comparing with the observed ionospheric response during the Bastille Day (July 2000) storm. With a maximum negative D _st of −290 nT and an a _p of 400, this magnetic perturbation was the strongest of year 2000. For these conditions, the model output was compared with the actual ionospheric response from all available stations, providing a reasonable latitudinal and longitudinal coverage. The comparisons show that the model captures the decreases in electron density particularly well in the northern summer hemisphere. In winter, the observed ionospheric response was more variable, showing a less consistent response, imposing a more severe challenge to the empirical model. The value of the model has been quantified by comparing the root mean square error (RMSE) of the STORM predictions with the monthly mean. The results of this study illustrate that the STORM model reduces the RMSE at the peak of the disturbance from 0.36 to 0.22, a significant improvement over climatology.     

The challenge of predicting the occurrence of intense storms

Geomagnetic super-storms of October and November 2003 are compared in order to identify solar and interplanetary variables that influence the magnitude of geomagnetic storms. Although these superstorms (D_ST < -300 nT) are associated with high speed CMEs, their DST indices show large variation. The most intense storm of November 20, 2003 (D_St∼ - 472 nT) had its source in a comparatively small active region and was associated with a relatively weaker, M-class flare, while the others had their origins in large active regions and were associated with strong X-class flares. An attempt has been made to implement a logistic regression model for the prediction of the occurrence of intense/superintense geomagnetic storms. The model parameters (regression coefficients) were estimated from a training data-set extracted from a data-set of 64 geo-effective CMEs observed during 1996–2002. The results indicate that logistic regression models can be effectively used for predicting the occurrence of major geomagnetic storms from a set of solar and interplanetary factors. The model validation shows that 100% of the intense storms (-200 nT < D_St < -100 nT) and only 50% of the super-intense (DST < -200 nT) storms could be correctly predicted.     

Cyclic Reversal of Magnetic Cloud Poloidal Field

Coronal mass ejections (CMEs) carry magnetic structure from the low corona into the heliosphere. The interplanetary CMEs (ICMEs) that exhibit the topology of helical magnetic fluxropes are traditionally called magnetic clouds (MCs). MC fluxropes with axis of low (high) inclination with respect to the ecliptic plane have been referred to as bipolar (unipolar) MCs. The poloidal field of bipolar MCs has a solar cycle dependence. We report a cyclic reversal of the poloidal field of low inclination MC fluxropes during 1976 to 2009. The MC poloidal field cyclic reversal on the same time scale of the solar magnetic cycle is evident over three sunspot cycles. Approximately 48% of ICMEs are MCs, and 40% of IMCs are bipolar MCs during solar cycle 23. The speed of the bipolar MCs has essentially the same distribution as all ICMEs, which implies that they are not from any special type of CMEs in terms of the solar origin. Although CME fluxropes may undergo a number of complications during the eruption and propagation, a significant group of MCs retains sufficient similarity to the source region magnetic field to posses the same cyclic periodicity in polarity reversal. The poloidal field of bipolar MCs gives the out-of-ecliptic-plane field or B _z component in the IMF time series. MCs with southward B _z field are particularly effective in causing geomagnetic disturbances. During the solar minima, the B _z field IMF sequence within MCs at the leading portion of a bipolar MC is the same with the solar global dipole field. Our finding shows that MCs preferentially remove the like polarity of the solar dipole field, and it supports the participation of CMEs in the solar magnetic cycle.     

Magnetic Clouds Observed at 1 Au During the Period 2000–2003

In this work we have performed a systematic study of all the magnetic clouds identified in the time interval 2000–2003. The study shows that the non force-free model of Hidalgo is a good approximation to the magnetic topology of the MCs in the interplanetary medium. This conclusion is reached based on the good fits obtained with the model for most of the clouds, in spite of the variety of profiles found in the magnetic field strength and in every of its components. The model incorporates the distortion and expansion of the cross-section of the MCs. We have compared, when available, the results obtained with those in literature. The unique published global study of the MCs at the same time interval has been provided by Lepping using the circular cross-section model of Burlaga, and the results are available in his web page. From all the parameters he obtained, only the longitude, φ, the latitude, θ, and the distance of maximum approach of the spacecraft to the cloud axis, y₀, may be compared with those obtained by Hidalgo's model. As we show, the main discrepancy between both models refers to the longitude values. Concerning the comparison with other models of literature, only the Bastille day and October 2003 magnetic clouds have been studied by other authors.     

Analysis of plasma and field conditions during some intensely geo-effective transient solar/interplanetary disturbances of solar cycle 23

The problem of solar wind-magnetosphere coupling is investigated for intense geomagnetic storms (Dst < -100nT) that occurred during solar cycle 23. For this purpose interplanetary plasma and field data during some intensely geo-effective transient solar/interplanetary disturbances have been analysed. A geomagnetic index that represents the intensity of planetary magnetic activity at subauroral latitude and the other that measures the ring current magnetic field, together with solar plasma and field parameters (V, B, Bz, σB, N, and T) and their various derivatives (BV,-BVz, BV², -BzV², B²V, Bz²V, NV²) have been analysed in an attempt to study mechanism and the cause of geo-effectiveness of interplanetary manifestations of transient solar events. Several functions of solar wind plasma and field parameters are tested for their ability to predict the magnitude of geomagnetic storm.     

Effects of Geometries and Substructures of ICMEs on Geomagnetic Storms

To better understand geomagnetic storm generations by ICMEs, we consider the effect of substructures (magnetic cloud, MC, and sheath) and geometries (impact location of flux-rope at the Earth) of the ICMEs. We apply the toroidal magnetic flux-rope model to 59 CDAW CME–ICME pairs to identify their substructures and geometries, and select 20 MC-associated and five sheath-associated storm events. We investigate the relationship between the storm strength indicated by minimum Dst index \((\mathrm{Dst}_{\mathrm{min}})\) and solar wind conditions related to a southward magnetic field. We find that all slopes of linear regression lines for sheath-storm events are steeper (\({\geq}\,1.4\)) than those of the MC-storm events in the relationship between \(\mathrm{Dst}_{\mathrm{min}}\) and solar wind conditions, implying that the efficiency of sheath for the process of geomagnetic storm generations is higher than that of MC. These results suggest that different general solar wind conditions (sheaths have a higher density, dynamic and thermal pressures with a higher fluctuation of the parameters and higher magnetic fields than MCs) have different impact on storm generation. Regarding the geometric encounter of ICMEs, 100% (2/2) of major storms (\(\mathrm{Dst}_{\mathrm{min}} \leq -100~\mbox{nT}\)) occur in the regions at negative \(P_{Y}\) (relative position of the Earth trajectory from the ICME axis in the \(Y\) component of the GSE coordinate) when the eastern flanks of ICMEs encounter the Earth. We find similar statistical trends in solar wind conditions, suggesting that the dependence of geomagnetic storms on 3D ICME–Earth impact geometries is caused by asymmetric distributions of the geoeffective solar wind conditions. For western flank events, 80% (4/5) of the major storms occur in positive \(P_{Y}\) regions, while intense geoeffective solar wind conditions are not located in the positive \(P_{Y}\). These results suggest that the strength of geomagnetic storms depends on ICME–Earth impact geometries as they determine the solar wind conditions at Earth.     

Statistical relationship between solar wind conditions and geomagnetic storms in 1998-2008

Applying ACE data and pressure-corrected Dst index (Dst*), annual distributions of solar wind structures detected at L1 point (the first Lagrangian point between solar-terrestrial interval) and correlations between solar wind structures and geomagnetic storms in 1998-2008 have been studied. It was found that, within the Earth's upstream solar wind, the dominant feature was interplanetary coronal mass ejections (ICMEs), primarily magnetic clouds, during solar maximum period but corotating interaction regions (CIRs) at solar minimum. During rising and declining phases, solar wind features became unstable for the complicated solar corona transition processes between the maximum and minimum phases, and there was a high CIR occurrence rate in 2003, the early period of the declining phase, for the Earth's upstream solar wind was dominated by high-speed southern coronal-hole outflows at that time. The occurrence rate of sector boundary crossing (SBC) events was evidently higher at the late half of declining phase and minimum period. ICMEs mainly centered on the maximum period but CIRs on all the declining phase. The occurrence rate of ICMEs was 1.3 times of that of CIRs, and more than half of ICMEs were magnetic clouds (MCs). Half of magnetic clouds could drive interplanetary shock and played a crucial role for geomagnetic storms generation, especially intense storms (Dst*≤100 nT), in which 45% were jointly induced by sheath region and driving MC structure. Sixty percent of intense storms were totally induced by shock-driving MCs; moreover, 74% of intense storms were driven by magnetic clouds, 81% of them driven by ICMEs. Shock-driving MC was the most geoeffective interplanetary source for four fifths of it able to lead to storms and more than one-third to intense storms. The rest of intense storms (19%) were induced just by 3% of all detected CIRs, and most of CIRs (53%) were corresponding to nearly 40% moderate and small storms (−100 nT<Dst*≤−30 nT). The true sector boundary crossing (SBC) events actually had no obvious geoeffectiveness, just 6% of them corresponding to small storms.     

Average Thickness of Magnetosheath Upstream of Magnetic Clouds at 1 AU versus Solar Longitude of Source

Starting with a large number (N=100) of Wind magnetic clouds (MCs) and applying necessary restrictions, we find a proper set of N=29 to investigate the average ecliptic plane projection of the upstream magnetosheath thickness as a function of the longitude of the solar source of the MCs, for those cases of MCs having upstream shock waves. A few of the obvious restrictions on the full set of MCs are the need for there to exist a driven upstream shock wave, knowledge of the MC’s solar source, and restriction to only MCs of low axial latitudes. The analysis required splitting this set into two subsets according to average magnetosheath speed: slow/average (300 – 500 km s⁻¹) and fast (500 – 1100 km s⁻¹) speeds. Only the fast set gives plausible results, where the estimated magnetosheath thickness (ΔS) goes from 0.042 to 0.079 AU (at 1 AU) over the longitude sector of 0° (adjusted source-center longitude of the average magnetic cloud) to 40° off center (East or West), based on N=11 appropriate cases. These estimates are well determined with a sigma (σ) for the fit of 0.0055 AU, where σ is effectively the same as (chi-squared) for the appropriate quadratic fit. The associated linear correlation coefficient for ΔS versus |Longitude| was very good (c.c.=0.93) for the fast range, and ΔS at 60° longitude is extrapolated to be 2.7 times the value at 0°. For the slower speeds we obtain the surprising result that ΔS is typically more-or-less constant at 0.040±0.013 AU at all longitudes, indicating that the MC as a driver, when moving close to the normal solar wind speed, has little influence on magnetosheath thickness. In some cases, the correct choice between two candidate solar-source longitudes for a fast MC might be made by noting the value of the observed ΔS just upstream of the MC. Also, we point out that, for the 29 events, the average sheath speed was well correlated with the quantity ΔV[=(〈V _MC〉−〈V _UPSTREAM〉)], and also with both 〈V _MC〉 and 〈V _MC,T〉, where 〈V _MC〉 is the first one-hour average of the MC speed, 〈V _MC,T〉 is the average MC speed across the full MC, and 〈V _UPSTREAM〉 is a five-hour average of the solar wind speed just upstream of the shock.