Implementation of the ground level enhancement alert software at NMDB database

The European Commission is supporting the real-time database for high-resolution neutron monitor measurements (NMDB) as an e-Infrastructures project in the Seventh Framework Programme in the Capacities section. The realization of the NMDB will provide the opportunity for several applications most of which will be implemented in real-time. An important application will be the establishment of an Alert signal when dangerous solar particle events are heading to the Earth, resulting into a ground level enhancement (GLE) registered by neutron monitors (NMs). The cosmic ray community has been occupied with the question of establishing such an Alert for many years and recently several groups succeeded in creating a proper algorithm capable of detecting space weather threats in an off-line mode. A lot of original work has been done to this direction and every group working in this field performed routine runs for all GLE cases, resulting into statistical analyses of GLE events. The next step was to make this algorithm as accurate as possible and most importantly, working in real-time. This was achieved when, during the last GLE observed so far, a real-time GLE Alert signal was produced. In this work, the steps of this procedure as well as the functionality of this algorithm for both the scientific community and users are being discussed. Nevertheless, the transition of the Alert algorithm to the NMDB is also being discussed.     

First Analysis of Ground-Level Enhancement (GLE) 72 on 10 September 2017: Spectral and Anisotropy Characteristics

Using data obtained with neutron monitors and space-borne instruments, we analyzed the second ground-level enhancement (GLE) of Solar Cycle 24, namely the event of 10 September 2017 (GLE 72), and derived the spectral and angular characteristics of associated GLE particles. We employed a new neutron-monitor yield function and a recently proposed model based on an optimization procedure. The method consists of simulating particle propagation in a model magnetosphere in order to derive the cutoff rigidity and neutron-monitor asymptotic directions. Subsequently, the rigidity spectrum and anisotropy of GLE particles are obtained in their dynamical evolution during the event on the basis of an inverse-problem solution. The derived angular distribution and spectra are discussed briefly.     

The 26 December 2001 Solar Eruptive Event Responsible for GLE63: III. CME,Shock Waves,and Energetic Particles

The SOL2001-12-26 moderate solar eruptive event (GOES importance M7.1, microwaves up to 4000 sfu at 9.4 GHz, coronal mass ejection (CME) speed 1446 km?s^?1) produced strong fluxes of solar energetic particles and ground-level enhancement (GLE) of cosmic-ray intensity (GLE63). To find a possible reason for the atypically high proton outcome of this event, we study multi-wavelength images and dynamic radio spectra and quantitatively reconcile the findings with each other. An additional eruption probably occurred in the same active region about half an hour before the main eruption. The latter produced two blast-wave-like shocks during the impulsive phase. The two shock waves eventually merged around the radial direction into a single shock traced up to \(25~\mathrm{R}_{\odot}\) as a halo ahead of the expanding CME body, in agreement with an interplanetary Type II event recorded by the Radio and Plasma Wave Investigation (WAVES) experiment on the Wind spacecraft. The shape and kinematics of the halo indicate an intermediate regime of the shock between the blast wave and bow shock at these distances. The results show that i) the shock wave appeared during the flare rise and could accelerate particles earlier than usually assumed; ii) the particle event could be amplified by the preceding eruption, which stretched closed structures above the developing CME, facilitated its lift-off and escape of flare-accelerated particles, enabled a higher CME speed and stronger shock ahead; iii) escape of flare-accelerated particles could be additionally facilitated by reconnection of the flux rope, where they were trapped, with a large coronal hole; and iv) the first eruption supplied a rich seed population accelerated by a trailing shock wave.     

Effective Rigidity of a Polar Neutron Monitor for Recording Ground-Level Enhancements

The “effective” rigidity of a neutron monitor for a ground-level enhancement (GLE) event is defined so that the event-integrated fluence of solar energetic protons with rigidity above it is directly proportional to the integral intensity of the GLE as recorded by a polar neutron monitor, within a wide range of solar energetic-proton spectra. This provides a direct way to assess the integral fluence of a GLE event based solely on neutron-monitor data. The effective rigidity/energy was found to be 1.13?–?1.42 GV (550?–?800 MeV). A small model-dependent, systematic uncertainty in the value of the effective rigidity is caused by uncertainties in the low-energy range of the neutron-monitor yield function, which requires more detailed computations of the latter.     

Ground level enhancements of cosmic rays in solar cycle 24

Using data from ground-based observations of cosmic rays (CRs) on the worldwide network of stations and spacecraft, we have investigated the proton spectra and the CR anisotropy during the ground level enhancements of CRs on May 17, 2012 (GLE71) and January 6, 2014 (GLE72) occurred in solar cycle 24 by the spectrographic global survey method. We provide the CR rigidity spectra and the relative changes in the intensity of CRs with a rigidity of 2 GV in the solar–ecliptic geocentric coordinate system in specific periods of these events. We show that the proton acceleration during GLE71 and GLE72 occurred up to rigidities R ~ 2.3?2.5 GV, while the differential rigidity spectra of solar CRs are described neither by a power nor by an exponential function of particle rigidity. At the times of the events considered the Earth was in a loop-like structure of the interplanetary magnetic field.     

On the possible mechanism of the first ground level enhancement in cosmic ray intensity of solar cycle 24

We have carried out this work to comprehend the possible mechanisms of the first ground level enhancement (GLE71 17 May 2012 01:50 UT) in cosmic ray intensity of the solar cycle 24. For this, the cosmic ray intensities registered by neutron monitors at several sites have been analyzed and studied with concurrent solar flares of different energy channels. To assess empirically whether the GLE might have been caused by the energy released from solar flare or CME-driven shock, we identify the possible time line in terms of the lowest spectral index determined from proton fluxes. If the GLE is caused by the energy released from particle acceleration in solar flare, the intensive phase of the flare representing the extreme emission should exist within/around the possible time line. In this respect, it is observed that the possible time line lies within the prominent phase of CME-driven shock. For better understanding, we have checked the possible relativistic energy with respect to solar flare as well as CME-driven shock. As witnessed, if the extreme emission phase of the flare is considered as the reason for the causation of GLE peak, the flare components procured insufficient amount of energy (≤～0.085 GeV) to produce a GLE. If the extreme emission phase of the flare is also considered as the dominator along GLE onset, the possible energy procurement (≤～0.414 GeV) is still not adequate to produce a GLE. In contrast, the CME-driven shock is capable of procuring enough possible relativistic energy (≥～1.21 GeV) that is sufficient amount of the energy for a GLE production. Any amount of the energy (<0.414 GeV) released from preceding flare components is supposed to have been contributed to the shock process. Thus, it is assumed that the GLE71 was possibly caused by the energy released from the shock acceleration, which might have been boosted by the energy emanated from preceding flare.     

Solar Energetic Particle Events with Protons Above 500 MeV Between 1995 and 2015 Measured with SOHO/EPHIN

The Sun is an effective particle accelerator that produces solar energetic particle (SEP) events, during which particles of up to several GeVs can be observed. These events, when they are observed at Earth with the neutron monitor network, are called ground-level enhancements (GLEs). Although these events with their high-energy component have been investigated for several decades, a clear relation between the spectral shape of the SEPs outside the Earth’s magnetosphere and the increase in neutron monitor count rate has yet to be established. Hence, an analysis of these events is of interest for the space weather and for the solar event community.In this article, SEP events with protons accelerated to above 500 MeV were identified using data obtained with the Electron Proton Helium Instrument (EPHIN) onboard the Solar and Heliospheric Observatory (SOHO) between 1995 and 2015. For a statistical analysis, onset times were determined for the events and the proton energy spectra were derived and fitted with a power law.As a result, we present a list of 42 SEP events with protons accelerated to above 500 MeV measured with the EPHIN instrument onboard SOHO. The statistical analysis based on the fitted spectral slopes and absolute intensities is discussed, with special emphasis on whether an event has been observed as a GLE. Furthermore, we are able to determine that the derived intensity at 500 MeV and the observed increase in neutron monitor count rate are correlated for a subset of events.     

A study on the dynamic spectral indices for SEP events on 2000 July 14 and 2005 January 20

We have studied the dynamic proton spectra for the two solar energetic particle(SEP) events on2000 July 14(hereafter GLE59) and 2005 January 20(hereafter GLE69). The source locations of GLE59 and GLE69 are N22 W07 and N12 W58 respectively. Proton fluxes 30 Me V have been used to compute the dynamic spectral indices of the two SEP events. Our results show that spectral indices of the two SEP events increased more swiftly at early times, suggesting that the proton fluxes 30 Me V might be accelerated particularly by the concurrent flares at early times for the two SEP events. For the GLE69 with source location at N12 W58, both flare site and shock nose are well connected with the Earth at the earliest time. However, only the particles accelerated by the shock driven by eastern flank of the CME can propagate along the interplanetary magnetic field line to the Earth after the flare. For the GLE59 with source location at N22 W07, only the particles accelerated by the shock driven by western flank of the associated CME can reach the Earth after the flare. Our results also show that there was slightly more than one hour during which the proton spectra for GLE69 are softer than that for GLE59 after the flares, suggesting that the shock driven by eastern flank of the CME associated with GLE69 is weaker than the shock driven by the western flank of the CME associated with GLE59. The results support that quasi-perpendicular shock has stronger potential in accelerating particles than the quasi-parallel shock. These results also suggest that only a small part of the shock driven by western flank of the CME associated with the GLE59 is quasi-perpendicular.     

Catalogue of ${>}\,55$ MeV Wide-longitude Solar Proton Events Observed by SOHO,ACE, and the STEREOs at ≈1${\approx}\,1$ AU During 2009 – 2016

Based on energetic particle observations made at \({\approx}\,1\) AU, we present a catalogue of 46 wide-longitude (\({>}\,45^{\circ}\)) solar energetic particle (SEP) events detected at multiple locations during 2009?–?2016. The particle kinetic energies of interest were chosen as \({>}\,55\) MeV for protons and 0.18?–?0.31 MeV for electrons. We make use of proton data from the Solar and Heliospheric Observatory/Energetic and Relativistic Nuclei and Electron Experiment (SOHO/ERNE) and the Solar Terrestrial Relations Observatory/High Energy Telescopes (STEREO/HET), together with electron data from the Advanced Composition Explorer/Electron, Proton, and Alpha Monitor (ACE/EPAM) and the STEREO/Solar Electron and Proton Telescopes (SEPT). We consider soft X-ray data from the Geostationary Operational Environmental Satellites (GOES) and coronal mass ejection (CME) observations made with the SOHO/Large Angle and Spectrometric Coronagraph (LASCO) and STEREO/Coronagraphs 1 and 2 (COR1, COR2) to establish the probable associations between SEP events and the related solar phenomena. Event onset times and peak intensities are determined; velocity dispersion analysis (VDA) and time-shifting analysis (TSA) are performed for protons; TSA is performed for electrons. In our event sample, there is a tendency for the highest peak intensities to occur when the observer is magnetically connected to solar regions west of the flare. Our estimates for the mean event width, derived as the standard deviation of a Gaussian curve modelling the SEP intensities (protons \({\approx}\,44^{\circ}\), electrons \({\approx}\,50^{\circ}\)), largely agree with previous results for lower-energy SEPs. SEP release times with respect to event flares, as well as the event rise times, show no simple dependence on the observer’s connection angle, suggesting that the source region extent and dominant particle acceleration and transport mechanisms are important in defining these characteristics of an event. There is no marked difference between the speed distributions of the CMEs related to wide events and the CMEs related to all near-Earth SEP events of similar energy range from the same time period.     

A New Version of the Neutron Monitor Based Anisotropic GLE Model: Application to GLE60

In this work we present a cosmic ray model that couples primary solar cosmic rays at the top of the Earth’s atmosphere with the secondary ones detected at ground level by neutron monitors during Ground-Level Enhancements (GLEs). The Neutron Monitor Based Anisotropic GLE Pure Power Law (NMBANGLE PPOLA) model constitutes a new version of the already existing NMBANGLE model, differing in the solar cosmic ray spectrum assumed. The total output of the model is a multi-dimensional GLE picture that reveals part of the characteristics of the big solar proton events recorded at ground level. We apply both versions of the model to the GLE of 15 April 2001 (GLE60) and compare the results.     

An Extreme Solar Event of 20 January 2005: Properties of the Flare and the Origin of Energetic Particles

The famous extreme solar and particle event of 20 January 2005 is analyzed from two perspectives. Firstly, using multi-spectral data, we study temporal, spectral, and spatial features of the main phase of the flare, when the strongest emissions from microwaves up to 200 MeV gamma-rays were observed. Secondly, we relate our results to a long-standing controversy on the origin of solar energetic particles (SEP) arriving at Earth, i.e., acceleration in flares, or shocks ahead of coronal mass ejections (CMEs). Our analysis shows that all electromagnetic emissions from microwaves up to 2.22 MeV line gamma-rays during the main flare phase originated within a compact structure located just above sunspot umbrae. In particular, a huge (≈ 10⁵ sfu) radio burst with a high frequency maximum at 30 GHz was observed, indicating the presence of a large number of energetic electrons in very strong magnetic fields. Thus, protons and electrons responsible for various flare emissions during its main phase were accelerated within the magnetic field of the active region. The leading, impulsive parts of the ground-level enhancement (GLE), and highest-energy gamma-rays identified with π ⁰-decay emission, are similar and closely correspond in time. The origin of the π ⁰-decay gamma-rays is argued to be the same as that of lower-energy emissions, although this is not proven. On the other hand, we estimate the sky-plane speed of the CME to be 2 000 – 2 600 km s⁻¹, i.e., high, but of the same order as preceding non-GLE-related CMEs from the same active region. Hence, the flare itself rather than the CME appears to determine the extreme nature of this event. We therefore conclude that the acceleration, at least, to sub-relativistic energies, of electrons and protons, responsible for both the major flare emissions and the leading spike of SEP/GLE by 07 UT, are likely to have occurred nearly simultaneously within the flare region. However, our analysis does not rule out a probable contribution from particles accelerated in the CME-driven shock for the leading GLE spike, which seemed to dominate at later stages of the SEP event. S.N. Kuznetsov deceased 17 May 2007.     

Relationship of ground level enhancements with solar,interplanetary and geophysical parameters

Cosmic rays registered by Neutron Monitor on the surface of the Earth are believed to originate from outer space, and sometimes also from the exotic objects of the Sun. Whilst the intensities of the cosmic rays are observed to be enhanced with sudden, sharp and short-lived increases, they are termed as ground level enhancements (GLEs). They are the occurrences in solar cosmic ray intensity variations on short-term basis, so different solar factors erupted from the Sun can be responsible for causing them. In this context, an attempt has been made to determine quantitative relationships of the GLEs having peak increase >5% with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006, thereby searching the responsible factors which seem to cause the enhancements. Results suggest that GLE peaks might be caused by solar energetic particle fluxes and solar flares. The proton fluxes which seemed to cause GLE peaks were also supported by their corresponding fluences. For most of the flares, the time integrated rising portion of the flare emission refers to the strong portion of X-ray fluxes which might be the concern to GLE peak. On an average, GLE peak associated X-ray flux (0.71×10⁻⁴ w/m²) is much stronger than GLE background associated X-ray flux (0.11×10⁻⁶ w/m²). It gives a general consent that the GLE peak is presumably caused by the solar flare. Coronal mass ejection alone does not seem to cause GLE. Coronal mass ejection presumably causes geomagnetic disturbances characterized by geomagnetic indices and polarities of interplanetary magnetic fields.     

The 26 December 2001 Solar Event Responsible for GLE63. I. Observations of a Major Long-Duration Flare with the Siberian Solar Radio Telescope

Ground level enhancements (GLEs) of cosmic-ray intensity occur, on average, once a year. Because they are rare, studying the solar sources of GLEs is especially important to approach understanding their origin. The SOL2001-12-26 eruptive-flare event responsible for GLE63 seems to be challenging in some aspects. Deficient observations limited our understanding of it. Analysis of additional observations found for this event provided new results that shed light on the flare configuration and evolution. This article addresses the observations of this flare with the Siberian Solar Radio Telescope (SSRT). Taking advantage of its instrumental characteristics, we analyze the detailed SSRT observations of a major long-duration flare at 5.7 GHz without cleaning the images. The analysis confirms that the source of GLE63 was associated with an event in active region 9742 that comprised two flares. The first flare (04:30?–?05:03 UT) reached a GOES importance of about M1.6. Two microwave sources were observed, whose brightness temperatures at 5.7 GHz exceeded 10 MK. The main flare, up to an importance of M7.1, started at 05:04 UT and occurred in strong magnetic fields. The observed microwave sources reached a brightness temperature of about 250 MK. They were not static. After appearing on the weaker-field periphery of the active region, the microwave sources moved toward each other nearly along the magnetic neutral line, approaching the stronger-field core of the active region, and then moved away from the neutral line like expanding ribbons. These motions rule out an association of the non-thermal microwave sources with a single flaring loop.     

A Simultaneous Forbush Decrease Associated with an Earthward Coronal Mass Ejection Observed by STEREO

The intensity?Ctime profile of Forbush decrease (FD) events observed by neutron monitors (NMs) looks like that of a geomagnetic storm as defined by the Dst index. Oh, Yi, and Kim (J.?Geophys. Res. 113, A01103, 2008) and Oh and Yi (J.?Geophys. Res. 114, A11102, 2009) classified FD events based on the amount of overlap and simultaneity of their main phase in Universal Time (UT). Oh and Yi define an FD event as simultaneous if the main phases observed by NMs distributed evenly around the Earth overlap in UT, and nonsimultaneous if they overlap only in the local time of some stations. They suggested that the occurrence mechanisms of two types of FD events may be related to interplanetary (IP) magnetic structures such as IP shocks and magnetic clouds. In their model, the simultaneity of FD events depends on the strength and propagation direction of magnetic structures overtaking the Earth. Recently, the Solar Terrestrial Relations Observatory (STEREO) mission has been able to visualize the emergence and propagation direction of coronal mass ejections (CMEs) in three dimensions in the heliosphere; thus, it is now possible to test the suggested mechanisms. One simultaneous FD event observed on 18 February 2011 may have been caused by a CME heading directly toward the Earth, which was observed on 15 February 2011 by the STEREO mission. Therefore, the simultaneity of FD events is proven to be a useful analysis tool in understanding the geoeffectiveness of solar events such as interplanetary CMEs and IP shocks.     

May 2005 Halo CMEs and Galactic Cosmic Ray Flux Changes at Earth’s Orbit

The pressure corrected hourly data from the global network of cosmic ray detectors, measurements of the interplanetary magnetic field (IMF) intensity (B) at Earth’s orbit and its components B _x , B _y , B _z (in the geocentric solar ecliptic coordinates) are used to conduct a comprehensive study of the galactic cosmic ray (GCR) intensity fluctuations caused by the halo coronal mass ejection of 13 May 2005. Distinct differences exist in GCR timelines recorded by neutron monitors (NMs) and multidirectional muon telescopes (MTs), the latter respond to the high rigidity portion of the GCR differential rigidity spectrum. The Forbush decrease (FD) onset in MTs is delayed (～5 h) with respect to the onset of a geomagnetic storm sudden commencement (SSC) and a large pre-increase is present in MT data before, during, and after the SSC onset, of unknown origin. The rigidity spectrum, for a range of GCR rigidities (≤200 GV), is a power law in rigidity (R) with a negative exponent (γ=?1.05) at GCR minimum intensity, leading us to infer that the quasi-linear theory of modulation is inconsistent with observations at high rigidities (>1 GV); the results support the force field theory of modulation. At present, we do not have a comprehensive model for the FD explaining quantitatively all the observational features but we present a preliminary model listing physical processes that may contribute to a FD timeline. We explored the connections between different phases of the FD and the power spectra of IMF components but did not find a sustained relationship.     

Analyzing the Solar Proton Event of 22 October 1989, Using the Method of Spectrographic Global Survey

Using ground-level observations of cosmic-ray (CR) intensities from a worldwide network of stations during the ground-level enhancement (GLE) of 22–23 October 1989, variations of the particle distribution function in all phases of the event were investigated.It is shown that time intensity profiles of 2–4 GV rigidity particles differ greatly from those of higher-energy particles. After the high-energy particle intensity attains a maximum, there is an abrupt decrease in intensity below the background level, followed by the phase of slow recovery to background values. The angular distribution of high-energy particles across the celestial sphere shows, along with an increased intensity, also regions with decreased relative background intensity.To explain the detected phenomenon, it is concluded that it is necessary to consider the solar CR propagation process in the heliosphere not in terms of the movement of particles in the external electromagnetic field but with proper account of the self-consistency of fields with the particle distribution function.     

An Anisotropic Cosmic-Ray Enhancement Event on 07-June-2015: A Possible Origin

A usual event, called anisotropic cosmic-ray enhancement (ACRE), was observed as a small increase (\({\leq}\,5\%\)) in the count rates of polar neutron monitors during 12?–?19 UT on 07 June 2015. The enhancement was highly anisotropic, as detected only by neutron monitors with asymptotic directions in the southwest quadrant in geocentric solar ecliptic (GSE) coordinates. The estimated rigidity of the corresponding particles is \({\leq}\,1\) GV. No associated detectable increase was found in the space-borne data from the Geostationary Operational Environmental Satellite (GOES), the Energetic and Relativistic Nuclei and Electron (ERNE) on board the Solar and Heliospheric Observatory (SOHO), or the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instruments, whose sensitivity was not sufficient to detect the event. No solar energetic particles were present during that time interval. The heliospheric conditions were slightly disturbed, so that the interplanetary magnetic field strength gradually increased during the event, followed by an increase of the solar wind speed after the event. It is proposed that the event was related to a crossing of the boundary layer between two regions with different heliospheric parameters, with a strong gradient of low-rigidity (\({<}\,1\) GV) particles. It was apparently similar to another cosmic-ray enhancement (e.g., on 22 June 2015) that is thought to have been caused by the local anisotropy of Forbush decreases, with the difference that in our case, the interplanetary disturbance was not observed at Earth, but passed by southward for this event.     

Sectorial anisotropy of solar cosmic rays

A hitherto unobserved sectorial pattern of anisotropy that was limited to a narrow and stable region was displayed by the ground level event (GLE) of January 24, 1971. For the entire 11/2 h interval following onset before isotropy set in, the anisotropy was limited to a 10° cone centered about 60° from the spiral magnetic field line. It is also the first solar particle event for which it is possible, by analytical procedures based upon a theoretical propagation model, to distinguish between two rival candidates for the parent flare.This research was sponsored by the National Science Foundation and Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract No. F19628-70-C-0190, but the report does not necessarily reflect endorsement by the sponsor.     

Cosmic-Ray Variations During the Two Greatest Bursts of Solar Activity in the 23rd Solar Cycle

During two extreme bursts of solar activity in March–April 2001 and October–November 2003, the ground-based neutron monitor network recorded a series of outstanding events distinguished by their magnitude and unusual peculiarities. The important changes that lead to increased activity initiated not with the sunspot appearance, but with the large-scale solar magnetic field reconfiguration. A series of strong and moderate magnetic storms and powerful proton events (including ground-level enhancements, GLE) were registered during these periods. The largest and most productive in the 23rd solar cycle, active region 486, generated a significant series of solar flares among which the 4 November 2003 flare (X28/3B) was the most powerful X-ray solar event ever observed. The fastest arrival of the interplanetary disturbance from the Sun (after August 1972) and the highest solar wind velocity and IMF intensity were recorded during these events. Within 1 week, three GLEs of solar cosmic rays were registered by the neutron monitor network (28 and 29 October and 2 November 2003). In this work, we perform a tentative analysis of a number of the effects seen in cosmic rays during these two periods, using the neutron monitor network and other relevant data.     

Solar cycle distribution of strong solar proton events and the related solar-terrestrial phenomena

We investigated the solar cycle distribution of strong solar proton events (SPEs, peak flux ≥1000 pfu) and the solar-terrestrial phenomena associated with the strong SPEs during solar cycles 21–23. The results show that 37 strong SPEs were registered over this period of time, where 20 strong SPEs were originated from the super active regions (SARs) and 28 strong SPEs were accompanied by the X-class flares. Most strong SPEs were not associated with the ground level enhancement (GLE) event. Most strong SPEs occurred in the descending phases of the solar cycles. The weaker the solar cycle, the higher the proportion of strong SPES occurred in the descending phase of the cycle. The number of the strong SPEs that occurred within a solar cycle is poorly associated with the solar cycle size. The intensity of the SPEs is highly dependent of the location of their source regions, with the super SPEs (≥20000 pfu) distributed around solar disk center. A super SPE was always accompanied by a fast shock driven by the associated coronal mass ejection and a great geomagnetic storm. The source location of strongest GLE event is distributed in the well-connected region. The SPEs associated with super GLE events (peak increase rate ≥100%) which have their peak flux much lower than 10000 pfu were not accompanied by an intense geomagnetic storm.