on cosmic ray decreases, geomagnetic storms and cmes · 2015-07-02 · 33rd international cosmic...
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33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013THE ASTROPARTICLE PHYSICS CONFERENCE
On cosmic ray decreases, geomagnetic storms and CMEsI.PARNAHAJ1,2 , K.KUDELA1, M.KANCIROVA1,2 , B.PASTIRCAK1
1 IEP SAS Kosice, Slovakia2 Faculty of Science, UPJS, Kosice, Slovakia
Abstract: Although geomagnetic storms are usually accompanied by Forbush decreases (FD), there exist FDswithout clear sudden Dst depression and events with Dst depression not accompanied by FDs. We prepared thecatalogue using the FD catalogue [15,16], data from NASA OMNIWEB, combined with the CME list and list ofICMEs. Preliminary results of statistical study on the relation between the amplitude of FDs and characteristics ofsolar, interplanetary (solar wind, IMF) and geomagnetic activity are presented.
Keywords: Cosmic ray, Forbush decrease, geomagnetic storm, CME, ICME, Halo CME.
1 IntroductionNeutron monitors (NM) detect sometimes a rapid decreasein the GCR named Forbush decreases (FD, [1]). FD isrelated to solar activity, its overall rate tends to follow ∼ 11-year sunspot cycle and usually following the interplanetarycounterparts of a coronal mass ejection (CME) passing theEarth’s orbit. FDs are usually accompanied by geomagneticstorms, but the associated geomagnetic activity (measurede.g. by Dst, description in [4]) is not exactly matching theFD profile (e.g. [2,3]). In recent years the FDs and thevariations of the observed CR near Earth are announced tobe connected with CMEs, especially halo CMEs (e.g. [5-7]). It was found [8] recently that the primary contributorsto FDs observed in high rigidity CRs are the Earth-directedCMEs. Authors [9] described CR modulation in relationto indices of solar activity and heliospheric parametersusing the CME-index. Interplanetary coronal mass ejections(ICMEs) are interplanetary counterparts of Earth-directedCMEs, which expand and propagate in interplanetary space.The relationship between CMEs and ICMEs is not veryclear. Common signatures for ICMEs are described e.g. in[10]. ICMEs and their relationship to the FDs have beendescribed in several publications. The [11] summarizedthe response of GCR to the passage of ICMEs and theirassociated shocks during the period 1995-2009. Propertiesof the ICMEs and solar activity levels during the minimafollowing solar cycle 22 and 23 were examined in [12].Recently [13] analyzed 59 ICMEs, but only 25% of themwere associated to a FD. It was found that the magneticclouds (MCs) produce deeper FDs than ejecta. In [14]the authors studied the relative geoeffectiveness of theICMEs and have compared them with associated solarand plasma/field properties. Halo events are some CMEspropagating towards the Earth. As they loom larger andlarger they appear to envelop the Sun, forming a halo around.Halo CMEs are fast and wide on the average, associatedwith flares, more energetic with average speed 1000 km/s,and have an apparent width of 360◦. Partial halos havewidth between 120◦ and 360◦ [5]. Halo CMEs and magneticclouds (MCs) play important role in producing geomagneticstorms. Condition for geomagnetic storm is that ICMEmust contain southward magnetic field component. FDs arecaused by the combination of shocks and CMEs, connectedto the turbulent structure behind the shocks, and to the
enhanced magnetic field and loop-like field configuration ofthe CMEs [7]. CMEs produce major disturbances in solarwind and interplanetary magnetic field (IMF).
Here we use the data from catalogue of FDs, combinedwith the catalogues of CMEs and ICMEs, and presentpreliminary results of the statistical study targeted on FD -geomagnetic activity - CME (ICME) characteristics.
2 DataData on FD are used from catalogue [15,16]. Data consist of6594 events. Out of them 1115 FDs have amplitude > 2%.We use 250 FDs from 1996 to 2006. Data of CME are takenfrom LASCO CME list [17]. From that list 392 Halo CMEswere selected, but only 96 of them were associated with theICME and shock that caused the disturbance at Earth. Dataof Near-Earth Interplanetary CMEs are taken from ACElist [18]. Some information about interplanetary shockswere taken from [19,20]. Final catalogue was prepared bycombining the three mentioned catalogues. In the summaryonly 25,2% (63 of 250 FDs) are associated with halo CMEs,the other 74,8% were identified as ”non Halo” CME. Varietyof relations between FDs and geomagnetic disturbances isillustrated in Figure 1.
3 ResultsUsing the updated catalogue of FDs with high statisticscovering the FDs since 1957 until 2012 [15,16], the scatterplots in Figure 2 show the relations of FDs to minimum Dstand to interplanetary magnetic field and solar wind speed.
By selecting halo CMEs (H, 63) and ”non halo CMEs”(NH, 187), the cumulative distribution of correspondingFDs is different (not shown here): FDs with amplitude <4% for H events are found in 37% of cases, while for NHin 82% cases. Similar difference is at higher thresholdsof FDs. Slightly higher r is for relation between FD andDst for the H events than for NH ones (Figure 3). Table 1summarizes r values for linear correlation between FD andsolar wind speed (Vsw) and module of IMF (B). The samesummary is listed for minimum Dst. H and NH events areseparated. Significance of differences in r for H and NHevents is marked as p (one-tailed) using z statistics valuesobtained from [21].
Halo CMEs33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
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Figure 1: Two examples of different relations between CR decerases and geomagnetic activity. Both events are related tohalo CMEs. While the CR decrease in right panel (6.3.- 21.3.2012) is accompanied by Dst depression (followed by otherdepressions corresponding to sharp solar wind speed increases and partial CR decreases in the long lasting recovery phase),the CR event in left panel (11.4.-21.4.2013) is not associated with clear Dst depression. Hourly data of IMF and solar winddownloaded from omniweb nasa, Dst from Kyoto center, and NM data from normalized to unity for the first day fromeuropean NMs with different geomagnetic cut-off rigidity (Oulu, Moscow and LS).
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Figure 2: Left: scatter plot of the amplitude of FD vs minimum Dst reached for each event. Right panels show thedependence of the FD amplitude and of minimum Dst versus solar wind velocity and module of IMF B. The linearcorrelation coefficients ( r ), number of points and the best linear fits are shown. When using B.v2, the correlation is notsignificantly increasing.
RH NH RNH NNH Z pFD vs Dst -0,40 63 -0,28 187 -0,96 0,1685
FD vs VSW 0,63 61 0,39 183 2,16 0,0154FD vs B 0,49 62 0,33 183 1,34 0,0901
Dst vs VSW -0,53 61 -0,28 183 -1,99 0,0233Dst vs B -0,79 62 -0,62 183 -2,31 0,0104
Table 1: Table indicates tendency to better ”binding” of both FD and Dst in minimum with IMF B and solar wind speed forhalo events than for ”non-halo” ones. R is linear correlation coefficient.
Halo CMEs33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
Using the values of the CME speed, acceleration of CMEas well as ICME speed (ACE list), the amplitude of corre-sponding FDs have been checked for various dependences(Figure 4).
Figure 3: scatter plot of FD amplitude versus Dst in mini-mum of the events for cases H and NH. Larger spread ofthe Dst for H than that for NH especially for FD < 5 %.
4 Concluding remarksAlthough CR decreases (FDs) pronounce significant cor-relation with geomagnetic disturbances (GD measured byDst in minimum) as well as with the IMF module and solarwind speed, mutual relations of GDs and FDs have rathercomplicated character and differ in individual events. ForCME halo events the correlations have tendency to increase.Rather clear connection of FD amplitude with CME speedand with acceleration is seen for halo events. Further anal-ysis require to split the data according to shocks, bidirec-tional electron streaming events and magnetic clouds, toassume the characteristics of the parent flares, and includeCR data from NMs at different cut-offs.
Acknowledgment:VEGA grant agency, project 2/0040/13 isacknowledged for support. We thank to Dr. E. Eroshenko for pro-viding FD catalogue. The CME catalog used is generated andmaintained at the CDAW Data Center by NASA and The CatholicUniversity of America in cooperation with the Naval ResearchLaboratory. SOHO is a project of international cooperation be-tween ESA and NASA.
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Figure 4: Scatter plots of amplitudes of FDs correspondingto H events vs CME speed, ICME speed (ACE list), solarwind speed from FD catalogue and acceleration.
Halo CMEs33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
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http://www.ssg.sr.unh.edu/mag/ace/ACElists/ICMEtable.html[19] http://umtof.umd.edu/pm/FIGS.HTML[20] http://www.ssg.sr.unh.edu/mag/ace/ACElists/obs list.html[21] http://www.vassarstats.net/rdiff.html