Dumbovic, M.

Leer, Kristoffer

Rodriguez, L.

Sudar, D.

Vennerstrøm, Susanne

Veronig, A.

Vrsnak, B.

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Semi(?) annual variations of geo-magnetic stormsLeer, Kristoffer; Dumbovic, M.; Vennerstrøm, Susanne; Veronig, A.; Rodriguez, L.; Sudar, D.; Vrsnak, B.Publication date:2012Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Leer, K., Dumbovic, M., Vennerstrøm, S., Veronig, A., Rodriguez, L., Sudar, D., & Vrsnak, B. (2012). Semi(?)annual variations of geo-magnetic storms. Poster session presented at 9th European Space Weather Week,Brussels, Belgium.Data setTable 1 gives the statistics of the two data sets. The divi-sion is based on both sunspot number and also keeping the two time periods equally long.Semi(?)-annual variation of geo-magnetic stormsK. Leer1, M. Dumbovic2, S. Vennerstrom1, A. Veronig3, L. Rodriguez4, D. Sudar2, B. Vrsnak21) DTU Space, Technical University of Denmark , Denmark2) HVAR Observatory, Faculty of Geodesy - University of Zagreb (HVAR), Croatia3) University of Graz (UNIGRAZ), Austria4) Royal Observatory of Belgium (ROB), BelgiumThis work has received funding from the European Commission FP7 Project COMESEP (263252)IntroductionIt has long been known that for instance the H-component of Earth magnetic field and indices like Kp and Ap vary semi-annually (Mursula et al 2005) peaking in March and Septem-ber. When forecasting the geo-effectiveness of Coronal Mass Ejections (CMEs) it is important to understand if the time of year is influencing the risk for a geomagnetic storm.  This study is a part of understanding how these variations should be treated when forecasting geo-effectiveness of CMEs. CMEs and storms in the period 1997-2010 are here considered. Based on sunspot number two subgroups have been identified, one for solar maximum (1997-2003) and one for solar minimum (2004-2010). A list of storms in the entire time span has been created and compared to the SOHO/LASCO catalogue (CDAW).DECFEBMARCHAUGJANJULSEPMAYOCTJUNNOVAPR02040608010010 14 18 22 26average	  |Dst|num	  of	  events0204060801001 2 3 4 5 6 7 8 9 10 11 12average	  |Dst|MONTHReferencesMursula, K. and A. Karinen (2005), Explaining and cor-recting excessive semiannual variation in the Dst index, Geophysical Res. Lett. 32, L14107, doi: 10.1029/2005GL023132CDAW - weblink: http://cdaw.gsfc.nasa.gov "This CME catalog is generated and maintained at the CDAW Data Center by NASA and The Catholic University of Amer-ica in cooperation with the Naval Research Laboratory. SOHO is a project of international cooperation between ESA and NASA."ParameterSolar Max (1997-2003)Solar Min (2004-2010)Total CMEs (v>200 km/s) 7261 5730Total Storms (dst<-30 nT) 317 134CMEs/day 3.32 2.67Storms/day 0.145 0.0610CME Speed (km/s) 541 546Ap peak 82.7 80.7Ap mean 58.5 60.4Mean DST (nT) -77 -750 2 4 6 8 10 120.030.060.090.120.150.180.210.240.270.3Storms with DST less than −30 nT  and CMEs (v > 200)Year: 1997−2003Storms per dayMonth0.511.522.533.544.55CMEs per day0 2 4 6 8 10 120.020.040.060.080.10.120.140.160.180.2Storms with DST less than −30 nT  and CMEs (v > 200)Year: 2004−2010Storms per dayMonth0.511.522.533.544.55CMEs per day1995 2000 2005 2010050100150200250Sunspot number in SOHO area YearSunspot numberResultsThe smallest events have been removed from the data sets, so that only CMEs with speed greater than 200 km/s and storms with dst less than -30 nT are considered. For each month in the two time intervals the number of events in each month has been counted and plotted in figure 2 and 3. The reduced χ2 has been calculated in order to test wether the number of events is evenly distributed over each month. The number of CMEs has been corrected for SOHO downtime and given as CMEs/day.Figure 2: The monthly distribution of CMEs and storms in solar maximum.Prob (χ2=5.1)=82% (CMEs)Prob (χ2=133)=0% (Storms)Figure 3: The monthly distribution of CMEs and storms in solar minimum.Prob (χ2=2.4)=98% (CMEs)Prob (χ2=59.4)=0% (Storms)Figure 1: Sunspot number i SOHO areaDiscussion The CME rate is even with a 5% significance level evenly dis-tributed over each month. The storms on the other hand have variations over time. It is interesting to see that the distribu-tion varies during the solar cycle. In solar maximum we find most storms in october. It can be discussed wether there is a peak in spring as well or if there only is a seasonal variation.During solar minimum there is no clear maximumAlternative approachAnother data set has been created to test the results.  211 CMEs has been randomly selected in the time 1996-2010. For each an average dst has been found regardless wether the CME was geo-effective or not. The norm of the DST is plotted in figure 4. It is seen that the distribution of DST is very similar to the distributions of storms found in fig 2.Conclusion•CME rate has no seasonal variation•Geomagnetic storm distribution vary both over the year and solar cycle•Is there an annual variation and not semi-annual variation?Figure 4: DST distribution over the year in the periode 1996-2010. Dst peaks only once per year in this study.AbstractThe distribution of Coronal Mass Ejections and geomagnetic storms are studied during solar cycle 23. There is no seasonal variation of CMEs but there is an annual variation of storms. It is suggested that the dependence can be annual and also dependent on solar activity. Table 1: Overview of data setsPresented at European Space Weather Week, Brussels 5-9. november 2012

Semi(?) annual variations of geo-magnetic storms

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Semi(?) annual variations of geo-magnetic storms

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