Interplanetary Research Articles

Analysis of the Interplanetary Sources for the Main Phase of the Geomagnetic Storm During October 18---19, 1998

采用高时间分辨率的地磁指数SYM-<EM>H</EM>, 同时考虑日地连线引力平衡点(L1点)太阳风地磁效应的滞后性, 精确分析了1998年10月18---19日大磁暴主相的行星际源. 分析结果表明, 磁暴主相的行星际源仅为行星际激波和行星际日冕物质抛射之间的太阳风(Sheath), 磁云对磁暴主相没有贡献. 这个磁暴事例的研究表明, 行星际磁场南向分量与太阳风动压的乘积是影响磁暴主相发展的关键参数.

STEREO and Wind observations of a fast ICME flank triggering a prolonged geomagnetic storm on 5–7 April 2010

On 5 April 2010 an interplanetary (IP) shock was detected by the Wind spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP coronal mass ejection (ICME). During the subsequent moderate geomagnetic storm (minimum Dst = −72 nT, maximum Kp = 8−), communication with the Galaxy 15 satellite was lost. We link images from STEREO/ SECCHI to the near–Earth in situ observations and show that the ICME did not decelerate much between Sun and Earth. The ICME flank was responsible for a long storm growth phase. This type of glancing collision was for the first time directly observed with the STEREO Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled with approaches assuming an invariant direction. These observations confirm the hypotheses that parts of ICMEs classified as (1) long–duration MCs or (2) magnetic–cloud–like (MCL) structures can be a consequence of a spacecraft trajectory through the ICME flank.

Polar cap flow channel events: spontaneous and driven responses

Abstract. We present two case studies of specific flow channel events appearing at the dusk and/or dawn polar cap boundary during passage at Earth of interplanetary (IP) coronal mass ejections (ICMEs) on 10 January and 25 July 2004. The channels of enhanced (&gt;1 km/s) antisunward convection are documented by SuperDARN radars and dawn-dusk crossings of the polar cap by the DMSP F13 satellite. The relationship with Birkeland currents (C1–C2) located poleward of the traditional R1–R2 currents is demonstrated. The convection events are manifest in ground magnetic deflections obtained from the IMAGE (International Monitor for Auroral Geomagnetic Effects) Svalbard chain of ground magnetometer stations located within 71–76° MLAT. By combining the ionospheric convection data and the ground magnetograms we are able to study the temporal behaviour of the convection events. In the two ICME case studies the convection events belong to two different categories, i.e., directly driven and spontaneous events. In the 10 January case two sharp southward turnings of the ICME magnetic field excited corresponding convection events as detected by IMAGE and SuperDARN. We use this case to determine the ground magnetic signature of enhanced flow channel events (the NH-dusk/By&lt;0 variant). In the 25 July case a several-hour-long interval of steady southwest ICME field (Bz&lt;0; By&lt;0) gave rise to a long series of spontaneous convection events as detected by IMAGE when the ground stations swept through the 12:00–18:00 MLT sector. From the ground-satellite conjunction on 25 July we infer the pulsed nature of the polar cap ionospheric flow channel events in this case. The typical duration of these convection enhancements in the polar cap is 10 min.

Modeling proton intensity gradients and radiation dose equivalents in the inner heliosphere using EMMREM: May 2003 solar events

[1] Solar energetic particles (SEPs) provide a significant radiation hazard for manned and unmanned interplanetary (IP) space missions. In order to estimate these hazards, it is essential to quantify the gradients of SEP intensities in the IP medium. The Earth-Moon-Mars Radiation Exposure Module (EMMREM) is a new project aimed at characterizing the time-dependent radiation exposure in IP space. In this paper, we utilize EMMREM to study the radial dependence of proton peak intensities, event fluences, and radiation dose equivalents of 27–31 May 2003 SEP events at eight different locations between 1 and 4.91 AU at energies between ∼1.5 MeV and ∼130 MeV. We have modeled onset times and intensity profiles of the SEP events at Mars and Ulysses and found very good agreement at different energies. We report observations of energetic particles at locations with magnetic field line footprints that are separated by ∼90° in heliolongitude, possibly indicating very large coronal mass ejection sizes and/or high cross-field diffusion at large radial distances. Our results show that radial dependencies of proton peak intensities exhibit a broken power law between 1 to 2.5 AU and 2.5 to 4.91 AU, ranging between R−2.52 ± 0.42 and R−5.97 ± 0.32 for 25 MeV and between R−2.13 ± 0.36 and R−5.21 ± 0.29 for 52 MeV, where R is the radial distance from the Sun in units of AU. Event fluences exhibit a similar behavior but with a harder spectra. Radiation dose calculations show that these events did not pose a short-term radiation hazard to humans in the IP space.

Case study of nightside magnetospheric magnetic field response to interplanetary shocks

Observations show that the geosynchronous magnetic field in midnight sector sometimes decreases when an interplanetary (IP) fast forward shock (FFS) passes Earth, even though the magnetosphere is always compressed. We perform case studies of the response observed by the GOES spacecraft at geosynchronous orbit near midnight to two IP shocks passing Earth. One shock produces a decrease in BZ (a negative response) and the other an increase in BZ (a positive response). A global 3D MHD code is run to reproduce the responses at geosynchronous orbit, and to further provide information on the initiation and development of BZ variations in the entire magnetosphere. The model reveals that when a FFS sweeps over the magnetosphere, there exist mainly two regions, a positive response region caused by the compressive effect of the shock and a negative response region which is probably associated with the temporary enhancement of earthward convection in the nightside magnetosphere. The spacecraft may observe an increase or decrease of the magnetic field depending on which region it is in. The numerical results reproduce the main characters of the geosynchronous magnetic field response to IP shocks for these two typical cases.

Geomagnetic storms caused by shocks and ICMEs

We performed an event‐by‐event study of 47 geomagnetic storms (GSs) that occurred during the ascending phase of solar cycle 23. All the GSs are associated with the passage of a shock and an interplanetary coronal mass ejection (ICME). For each event, we identified the section in the interplanetary (IP) medium causing the GS (the sheath behind the shock, the main body of the ICME or the combination of both). On average, the most intense GSs are caused by sheaths, followed by sheath‐ICME combinations and by ICMEs. We obtained the correlation coefficients between the intensity of each GS (minimum Dst) and several solar wind parameters. We found that the well‐known correlation between the GS intensity and the solar wind convected electric field, Ey, stands for the GSs caused by ICMEs (CC = −0.88) and sheath‐ICME combinations (CC = −0.95), but it is very low for the GSs caused by sheaths (CC = −0.44). In contrast, we found a very good correlation between the GSs caused by sheaths and the total convected electric field (ΣEy) (CC = −0.89). On the other hand, we estimated the total perpendicular pressure (Pt) for each IP event associated with the GSs and identified the three different types of Pt profiles. The most intense GSs are related with IP events with Pt = 1, but moderate and less intense storms are associated with the three Pt profiles. The correlations between the Dst and the solar wind parameters results that the CCs decrease significantly for IP events having a Pt profile of 3.

Interplanetary shock characteristics and associated geosynchronous magnetic field variations estimated from sudden impulses observed on the ground

Interplanetary (IP) shocks disturb the magnetosphere‐ionosphere system resulting in geosynchronous magnetic field changes and sudden impulses observed by ground‐based magnetometers. We extend the implications of a previous statistical study and show that sudden impulses (SIs) can be used to estimate some parameters at the L1 point and geosynchronous orbit, including the change of the square root of solar wind dynamic pressure across the shock and the associated geosynchronous magnetic field changes near the subsolar region. It should be pointed out that the relationship between magnetospheric field change and SIs amplitude and the solar wind dynamic pressure is not a single valued one, but a statistical relationship is useful in cases when interplanetary data are not available. Empirical formulae deduced from observations can be used to estimate certain IP shock characteristics and geosynchronous magnetic field changes from sudden impulse data observed on the ground, with the prediction efficiency as high as 90% and 86%, respectively. These estimates are useful for studying historic, pre‐space era data or if the L1 and geosynchronous data are not available at some future time.

MHD analysis of propagation of an interplanetary shock across magnetospheric boundaries

An important problem of the Space Weather Program is the interaction of interplanetary (IP) shocks with Earth's magnetosphere because their interaction often (but not always) leads to major geomagnetic storms. Since the huge interaction region can be covered by simultaneous spacecraft observations only sporadically, global MHD modeling can help in our understanding of the interaction process. We have developed a procedure that clearly distinguished magnetospheric boundaries in an output of the global MHD model and compare its results to spacecraft observations. Using one IP shock observed by Geotail, we compare its passage through the magnetosphere with predictions of two modifications of the global BATS-R-US MHD code. We demonstrate the complexity of the shock interaction with the bow shock, magnetopause, and oscillations of the whole system toward a new equilibrium state with a duration of 10–12 min. Moreover, based on the changes of the magnetopause and bow shock locations in the nightside region, we suggest that the information about the IP shock hitting the subsolar magnetopause reaches the nightside magnetopause earlier than the IP shock can arrive. This information is, in our view, mediated by fast magnetosonic waves in the inner magnetosphere. Although we have found generally a good agreement, we discuss possible sources of deviations of modeled and observed locations of the boundaries.

Data storage and real time observations at the Mexican Array Radio Telescope

The tracking of large-scale interplanetary (IP) disturbances traveling from the Sun to the Earth is a key issue in space weather studies. The Mexican Array Radio Telescope (MEXART) applies the Interplanetary Scintillation (IPS) technique to detect these solar wind disturbances and it will participate in a global warning network of space weather forecasting. We describe the data storage and computational processes carried out to manage the instrument’s real time data. These procedures are important for the MEXART calibration, operation and the scientific data reduction.

PRECURSORS OF THE FORBUSH DECREASE ON 2006 DECEMBER 14 OBSERVED WITH THE GLOBAL MUON DETECTOR NETWORK (GMDN)

We analyze the precursor of a Forbush Decrease (FD) observed with the Global Muon Detector Network on December 14, 2006. An intense geomagnetic storm is also recorded during this FD with the peak Kp index of 8+. By using the "two-dimensional map" of the cosmic ray intensity produced after removing the contribution from the diurnal anisotropy, we succeed in extracting clear signatures of the precursor. A striking feature of this event is that a weak loss-cone signature is first recorded more than a day prior to the Storm Sudden Commencement (SSC) onset. This suggests that the loss-cone precursor appeared only 7 hours after the Coronal Mass Ejection (CME) eruption from the Sun, when the Interplanetary (IP) shock driven by the Interplanetary Coronal Mass Ejection (ICME) located at 0.4 AU from the Sun. We find the precursor being successively observed with multiple detectors in the network according to the Earth's spin and confirmed that the precursor continuously exists in space. The long lead time (15.6 hours) of this precursor which is almost twice the typical value indicates that the IMF was more quiet in this event than a typical power spectrum assumed for the IMF turbulence. The amplitude (-6.45 %) of the loss-cone anisotropy at the SSC onset is more than twice the FD size, indicating that the maximum intensity depression behind the IP shock is much larger than the FD size recorded at the Earth in this event. We also find the excess intensity from the sunward IMF direction clearly observed during ~10 hours preceding the SSC onset. It is shown that this excess intensity is consistent with the measurement of the particles accelerated by the head-on collisions with the approaching shock. This is the first detailed observation of the precursor due to the shock reflected particles with muon detectors.

Geomagnetic activity triggered by interplanetary shocks

Interplanetary (IP) shocks can greatly disturb the Earth's magnetosphere, causing the global dynamic changes in the electromagnetic fields and the plasma. In order to investigate this, we have systematically analyzed 106 IP shock events based on OMNI data, GOES, and Los Alamos National Laboratory satellite observations during 1997−2007. It is revealed that the median value of IMF Bz keeps negative/positive prior to shock arrival and becomes more negative/positive following the shock arrival. The statistical analysis shows that IP shocks with southward interplanetary magnetic field (IMF) (46%) are likely to increase AE (AL, AU) and PC indices significantly. The amplitude of AE index increases from 200 to 600 nT, AU from 100 to 200 nT, AL from 50 to 400 nT, and PC from 1.5 to 3 approximately in 10 min, which could be a signature of geomagnetic activity/substorms onset (or substorm further intensification). Meanwhile, there is a strong injection of energetic electrons in the dawn region following the shock arrival and a strong depletion in the dusk region 30 min later, showing a clear dawn‐dusk asymmetry. On the other hand, there is only the typical shock compression effect for IP shocks with northward IMF (54%). The median value of AE index increased from 80 to 150 nT, AU from 50 to 90 nT, AL index decreased from −30 to −40 nT, and PC index increased from 0.6 to 1.2 in ∼10 min following the shock arrival. Both individual cases and statistical studies indicate that the magnetosphere‐ionosphere system must be preconditioned for a substorm‐like geomagnetic activity to be triggered by an IP shock with southward IMF impact, whereas IP shock with northward IMF precondition shows only compression effect.

Polar cap convection/precipitation states during Earth passage of two ICMEs at solar minimum

Abstract. We report important new aspects of polar cap convection and precipitation (dawn-dusk and inter-hemisphere asymmetries) associated with the different levels of forcing of the magnetosphere by two interplanetary (IP) magnetic clouds on 20 November 2007 and 17 December 2008 during solar minimum. Focus is placed on two intervals of southward magnetic cloud field with large negative By components (Bx=−5 versus 0 nT) and with high and low plasma densities, respectively, as detected by spacecraft Wind. The convection/precipitation states are documented by DMSP spacecraft (Southern Hemisphere) and SuperDARN radars (Northern Hemisphere). The (negative) By component of the cloud field is accompanied by a newly-discovered flow channel (called here FC 2) threaded by old open field lines (in polar rain precipitation) at the dusk and dawn sides of the polar cap in the Northern and Southern Hemispheres, respectively, and a corresponding Svalgaard-Mansurov (S-M) effect in ground magnetic deflections. On 20 November 2007 the latter S-M effect in the Northern winter Hemisphere appears in the form of a sequence of six 5–10 min long magnetic deflection events in the 71–74° MLAT/14:30–16:00 MLT sector. The X-deflections are consistent with the flow direction in FC 2 (i.e. caused by Hall currents) in both IP cloud cases. The presence of a lobe cell and associated polar arcs in the Southern (summer) Hemisphere in the low density (1–2 cm−3) and Bx=0 ICME case is accompanied by the dropout of polar rain precipitation in the dusk-side regime of sunward polar cap convection and inward-directed Birkeland current. The low-altitude observations are discussed in terms of momentum transfer via dynamo processes in the high- and low-latitude boundary layers and Birkeland currents located poleward of the traditional R1-R2 system.

INTERPLANETARY SHOCKS LACKING TYPE II RADIO BURSTS

We report on the radio-emission characteristics of 222 interplanetary (IP) shocks. A surprisingly large fraction of the IP shocks (~34%) is radio quiet (i.e., the shocks lacked type II radio bursts). The CMEs associated with the RQ shocks are generally slow (average speed ~535 km/s) and only ~40% of the CMEs were halos. The corresponding numbers for CMEs associated with radio loud (RL) shocks are 1237 km/s and 72%, respectively. The RQ shocks are also accompanied by lower peak soft X-ray flux. CMEs associated with RQ (RL) shocks are generally accelerating (decelerating). The kinematics of CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock The RQ shocks seem to be mostly subcritical and quasi-perpendicular. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.

INTERPLANETARY SHOCKS LACKING TYPE II RADIO BURSTS

We report on the radio-emission characteristics of 222 interplanetary (IP) shocks detected by spacecraft at Sun–Earth L1 during solar cycle 23 (1996 to 2006, inclusive). A surprisingly large fraction of the IP shocks (∼34%) was radio quiet (RQ; i.e., the shocks lacked type II radio bursts). We examined the properties of coronal mass ejections (CMEs) and soft X-ray flares associated with such RQ shocks and compared them with those of the radio-loud (RL) shocks. The CMEs associated with the RQ shocks were generally slow (average speed ∼535 km s−1) and only ∼40% of the CMEs were halos. The corresponding numbers for CMEs associated with RL shocks were 1237 km s−1 and 72%, respectively. Thus, the CME kinetic energy seems to be the deciding factor in the radio-emission properties of shocks. The lower kinetic energy of CMEs associated with RQ shocks is also suggested by the lower peak soft X-ray flux of the associated flares (C3.4 versus M4.7 for RL shocks). CMEs associated with RQ CMEs were generally accelerating within the coronagraph field of view (average acceleration ∼+6.8 m s−2), while those associated with RL shocks were decelerating (average acceleration ∼–3.5 m s−2). This suggests that many of the RQ shocks formed at large distances from the Sun, typically beyond 10 Rs, consistent with the absence of metric and decameter—hectometric (DH) type II radio bursts. A small fraction of RL shocks had type II radio emission solely in the kilometric (km) wavelength domain. Interestingly, the kinematics of the CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock Mach numbers at 1 AU shows that the RQ shocks are mostly subcritical, suggesting that they were not efficient in accelerating electrons. The Mach number values also indicate that most of these are quasi-perpendicular shocks. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. About 18% of the IP shocks do not have discernible ejecta behind them. These shocks are due to CMEs moving at large angles from the Sun–Earth line and hence are not blast waves. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.

On Cosmic Rays, IP Structures and Geospace Consequences During WHI

AbstractThis work presents some observations during the period of the Whole Heliosphere Interval (WHI) of the effects of interplanetary (IP) structures on the near-Earth space using three sets of observations: magnetic field and plasma from the Advanced Composition Explorer (ACE) satellite, ground-based cosmic ray data from the Global Muon Detection Network (GMDN) and geomagnetic indices (Disturbance storm-time, Dst, and auroral electrojet index, AE). Since WHI was near minimum solar activity, high speed streams and corotating interaction regions (CIRs) were the dominant structures observed in the interplanetary space surrounding Earth. Very pronounced geomagnetic effects are shown to be correlated to CIRs, especially because they can cause the so-called High-Intensity Long-Duration Continuous AE Activity (HILDCAAs) - Tsurutani and Gonzalez (1987). At least a few high speed streams can be identified during the period of WHI. The focus here is to characterize these IP structures and their geospace consequences.

Low‐frequency whistler waves and shocklets observed at quasi‐perpendicular interplanetary shocks

We present observations of low‐frequency waves (0.25 Hz &lt; f &lt; 10 Hz) at five quasi‐perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20°–50° and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low‐frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave‐particle interactions in the solar wind.

Simultaneous ground-satellite optical observations of postnoon shock aurora in the Southern Hemisphere

[1] On 14 June 2005, a transient postnoon shock aurora, induced by an interplanetary (IP) shock, was observed simultaneously with the FUV imager onboard the IMAGE satellite and the all-sky imager (ASI) at the South Pole Station (−74.3° magnetic latitude (MLAT), ∼15 magnetic local time). The global evolution of the shock aurora was identified by the FUV, whereas the detailed spatial-temporal structure was identified by the ASI. Both optical emissions for the shock aurora showed a reasonable agreement in a common field of view. During the transient auroral brightenings in the dayside oval detected by the FUV, a two-step development of the shock aurora was identified by the ASI in the afternoon sector of the oval. Just after the IP shock, the ASI first observed a diffuse 557.7 nm aurora expanding duskward at the equatorward edge of the oval (−70° to −73° MLAT). About 5 min later, new discrete auroral forms (arcs) were detected in the middle of the oval (−73° to −76° MLAT) by both 557.7 nm and 630.0 nm ASI images. The discrete arcs developed with relatively brighter emissions, and had a lifetime of the order of 10 min, during the main impulse (MI) of geomagnetic sudden commencement. The spectrographic auroral imagers onboard the IMAGE satellite indicated that proton precipitation played an effective role in the first shock auroral emission observed with the ASI, while intense electron precipitation played an effective role in the second one. Mechanisms of the first and second postnoon shock auroras presented here are speculated to be associated with the wave-particle interaction process, and the field-aligned acceleration process in the region of the MI-related upward field-aligned currents, respectively.

Influence of the foreshock of the Earth’s bow shock on the interplanetary shock propagation during their mutual interaction

Abstract Interplanetary shocks have been recognized as a very efficient source of geomagnetic disturbances. We present a short study of the propagation of one interplanetary (IP) shock observed by five spacecraft located in the solar wind far upstream of the Earth’s bow shock as well as in its close vicinity. The IP shock normal was highly inclined from the Sun-Earth line and thus the IP shock-bow shock interaction started at the flank. We have found a significant evolution of IP shock parameters during its motion along the bow shock. This modification is discussed and attributed to the presence of strong fluxes of energetic particles in the foreshock.

COMPOSITION AND SPECTRAL PROPERTIES OF THE 1 AU QUIET-TIME SUPRATHERMAL ION POPULATION DURING SOLAR CYCLE 23

We have surveyed the spectral and compositional properties of suprathermal heavy ions during quiet times from 1995 January 1 to 2007 December 31 using Wind/Energetic Particles: Anisotropy, Composition, and Transport/SupraThermal-through-Energetic Particle Telescope and Advanced Composition Explorer/Ultra-low Energy Isotope Spectrometer at energies between 0.04 and 2.56 MeV nucleon–1. We find the following. (1) Quiet-time Fe/O and C/O abundances are correlated with solar cycle activity, reflecting corresponding values measured in solar energetic particle and interplanetary (IP) shock events during solar maximum, and those measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions. (2) The 3He/4He ratio lies in the 3%-8% range during the quiet times of 1998-2004 with finite 3He detected on ~27.4% of the days. This ratio drops to 0.3%-1.2% during 2005-2007 and finite 3He is detected on ~5% of the days. (3) All heavy-ion species exhibit suprathermal tails between 0.04 and 0.32 MeV nucleon–1 with spectral indices ranging from ~1.27 to 2.29. These tails sometimes extend above ~2 MeV nucleon–1 with Fe spectra rolling over at lower energies than those of CNO. (4) The suprathermal tail spectral indices of heavier species (i.e., Fe) are harder than those of the lighter ones (i.e., CNO). These indices do not exhibit a clear solar cycle dependence and for ~50% of the time, they deviate significantly from the 1.5 value. These compositional observations provide evidence that even during the quietest times in IP space, the suprathermal population (3He and C-through-Fe) consists of ions from different sources whose relative contributions vary with solar activity. The heavy-ion energy spectra exhibit suprathermal tails with variable spectral indices that do not exhibit the spectral index of 1.5 predicted by some recent models.

Interplanetary fast forward shocks and energetic storm particle events above 1.5 MeV

Abstract. Interplanetary (IP) shock passages are usually identified by abrupt changes in the plasma parameters, but sometimes they are also associated with energetic storm particles (ESPs). The maximum observed energies of ESPs usually reach a few MeVs per nucleon and occasionally even a few hundred MeVs per nucleon. We have carried out a statistical study of ESP events observed by SOHO/ERNE above 1.5 MeV during the seven-year period between May 1996 and April 2003. In the first stage, we gathered a comprehensive database of IP shock candidates using several ready-made shock lists. We defined a qualitative classification for the ESP signals and studied their association with fast forward shocks. We present a survey of the overall statistics of ESP associations with fast forward shocks and the yearly amount of the shocks and associated ESP events during the 7-year study period. Our most important findings are that only 40% of the observed interplanetary fast forward shocks accelerate ESPs to energies greater than 1.5 MeV and that the high-energy ESP-effectiveness of the fast forward shocks has a solar cycle dependence. The yearly ESP-effectiveness varied from 11%, in May 1996–April 1997 (~activity minimum), to 53% in May 2000–April 2001 (~activity maximum). We also performed a quantitative analysis of the proton power law spectra at the time of the shock passage. We found that the average spectral index of ESPs was −3.6 with the standard deviation of the distribution of 1.3. The ESP events had significantly larger power law factors than the reference spectra, calculated every day at a certain time for comparison.