Planetary and Space Science 78 (2013) 52–63

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Planetary and Space Science

0032-06

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n Corr

Academ

E-m

journal homepage: www.elsevier.com/locate/pss

The magnetic local time distribution of ring current duringthe geomagnetic storm

X.D. Zhao a,b,n, A.M. Du a, W.Y. Xu a

a Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, Chinab Institute of Geophysics, China Earthquake Administration, Beijing, China

a r t i c l e i n f o

Article history:

Received 6 September 2012

Received in revised form

18 January 2013

Accepted 21 January 2013Available online 29 January 2013

Keywords:

Ring current

Storm

Magnetic local time

Magnetic field

33/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.pss.2013.01.008

esponding author at: Institute of Geolog

y of Sciences, Beijing, China. Tel.: þ86 1346

ail addresses: zhaoxudong@cea-igp.ac.cn, zxd

a b s t r a c t

The magnetic local time distribution of the ring current during 879 geomagnetic storms (identified by

SYMHo�30 nT) in the 23rd solar cycle (1996–2006) was investigated by using 23 mid-low latitude

ground-based magnetometers. The storms are divided into eight different classes with a step of 20 nT

for the statistical analysis. For each class, the dusk side events, for which the H component minimum

located in the dusk sector is mostly corresponding to the UT of minimum SYMH index, are about 59.5%

of the total events. Whereas the noon side events are about 20.0%, the night side events are about 18.7%,

and the dawn side events are about 1.8%. The H component distributions with MLT indicate that the

magnetic field disturbance during the magnetic storm events is not only related to the symmetrical ring

current, but also to the other current, mainly the partial current. A further statistical study of the dusk

side events shows that both the symmetric and partial ring currents enhance accompanied by the

increase in the storm class during the main phase. And the partial ring current makes a greater

contribution to the main phase of the storm. Referring to the interplanetary parameters, the distinction

of the solar wind velocity Vx is more obvious than the interplanetary magnetic field Bz for the dusk side

events in different classes. The comparisons between dusk side and other side events in the same class

indicate that besides the solar wind velocity Vx, the interplanetary magnetic field By also affects on the

disturbance of ring current on the ground in MLT.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The ring current which circles the Earth is generally thought tobe symmetric in the equatorial plane and distributes in certainlatitude range regions (Stormer, 1911). The location and width ofring current vary with the geomagnetic activity from 2 to 10RE

in the geocentre (Xu, 2009). The ring current consists of mediumenergetic particles ranging from 20 to 200 keV drifting around theEarth (Liu, 2005). There are westward (�4–7RE) and eastwardring currents (�3RE), and the westward is the dominating feature.The horizontal (H) component of the magnetic field whichparallels to the dipole axis at mid and low latitude regions hasmade a distinct decrease in the ring current.

The magnitude of storms is directly proportional to the energy ofthe ring current particles. Observation from ground stations is aneffective measure to monitor the occurrence of the storms. Usingthe horizontal component of four stations (Hermanus, Kakioka,Honolulu and San Juan ) in the middle and low latitude, the Dst

ll rights reserved.

y and Geophysics, Chinese

6693365.

9801@163.com (X.D. Zhao).

index is derived (Rangarajan, 1989). Dst index is continuously usedas a function of UT. Its variation could clearly indicate the occur-rences of magnetic storms and their severity. However, the Dstindex is based on the assumption that the ring current is axiallysymmetrical and has no local time dependence. The Dst index is theaverage of axial components and reflects the geomagnetic effectsfrom the symmetrical ring current (Liemohn et al., 2001). But thering current is not always symmetrical (Du et al., 2005, 2008, 2011).It is obviously asymmetric in local time during the main phase of thestorms (Chapman and Bartels, 1940). Even during quiet times, thesymmetric and partial ring currents are similar in strength, �0.5 MA(Le et al., 2004).

The partial ring current makes great contribution to the storms.Many simulation works indicate that some energetic particles ofring current could pass through the inner magnetosphere on opendrift paths intersecting the dayside magnetopause (Takahashi et al.,1990; Fok et al., 1991; Wolf and Spiro, 1997; Liemohn et al., 2001;Daglis, 2001). A 3-D model designed by Takahashi and Iyemori(1989) indicates that the particles of ring current could escape fromthe dusk side magnetopause. The statistical analysis for theupstream energetic particles in the bowshock and magnetosheathalso suggests that the ring current particles could escape from themagnetosphere (Christon et al., 2000; Keika et al., 2004). Liemohn

Table 1Location of ground stations.

Station

code

Geographical

latitude (deg.)

Geographical

longitude (deg.)

Geographical

latitude (deg.)

Geographical

longitude (deg.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–63 53

et al. (2001) point out that the energy of the storm’ main phase isprimarily from the energetic particles on the open drift paths,namely the contributions from the partial ring current. Le et al.(2004) propose that the partial ring current exhibits most drasticintensification as the level of disturbances increases and contributesdominantly to the decrease of the Dst index.

The asymmetric part of the magnetic storms has already beenmuch studied. Iyemori (1990) uses the magnetic field observa-tions from the middle latitude stations to study the asymmetry ofthe disturbance from ring current during the storms. Fok et al.(1996) develop a three dimensional ring current model todescribe the day–night asymmetry of ion distributions in ringcurrent. Grafe (1999) suggests that it is the partial ring currentthat only exists during the storms. Li et al. (2009) compares theobservations from different ground stations to reveal the depen-dence of the storm-time amplitude on longitude and latitude.

In this paper, the Earth’s magnetic field measurement bymiddle and low latitude ground stations for 11 years fromINTERMAGNET is used to statistically analyze the ring currentdistribution for different magnitude storms. We amass amplestorm events using the superposed epoch method to characterizethe ring current distribution with the magnetic local time (MLT)at the minimum of the SYMH index for each storm class. In thesections below we report on our analysis.

SJG 18.1 293.8 27.9 10.4

VSS �22.4 316.4 �17.7 23.1

KOU 5.2 307.3 8.8 23.4

ASC �7.9 345.6 �9.8 56.1

MBO 14.4 343.0 3.2 57.8

SPT 39.5 355.6 32.2 71.9

TAM 22.8 5.5 5.5 78.3

SUA 44.7 26.3 39.6 99.4

TAN �18.9 47.3 �28.8 116.5

ABG 18.6 72.9 11.9 145.3

AAA 43.2 76.9 38.5 149.3

PHU 21.0 106.0 14.0 177.8

BMT 40.3 116.2 34.6 188.8

GUA 13.6 144.9 6.1 215.8

MMB 43.9 144.2 37.1 215.5

CTA �20.1 146.3 �29.2 220.4

MID 28.2 182.6 24.8 249.9

API �13.8 188.2 15.6 262.5

HON 21.3 202.0 21.4 269.9

PPT 17.6 210.4 �16.7 285.2

TUC 32.2 249.2 39.8 314.5

DLR 29.3 259.2 38.7 326.5

TEO 19.8 260.8 29.1 329.2

2. The data and method

The ground station geomagnetic field component (H) which isparallel to the dipole axis is used to analyze the disturbance fromring current during the storms. In each event, we select eightstations evenly located in the geomagnetic longitude to calculatethe geomagnetic field component (H). Fig. 1 shows the stationswhich we used in our work and Table 1 gives the locationinformation for the stations.

The observation data from each station firstly subtracts thesolar quiet daily variation (Sq) and the main field. The averageSq variation and the main field are determined from the geomag-netic data for the internationally selected five quietest days ofeach month. Using the fast Fourier transformation (FFT), theaverage data of five quiet days could be separated by differentorder harmonic waves. The summation of the first six orderharmonic waves could represent the Sq variation and the mainfield. After eliminating the Sq variation and the main field from

Fig. 1. Distribution of the geomagnetic stations used in the work. Abscissa axis is the

triangle in the map.

the observation data, the disturbance components X and Y ingeomagnetic observation coordinates are obtained.

In each storm event, the disturbance field components fromevery ground station are transferred to the geocentric dipolecoordinates using the formula.

H0

¼ X cos aþ Y sin a ð1Þ

where, X and Y are the magnetic field components which point tothe geographic north and east in the geomagnetic observationcoordinates respectively, and a is the angle between the geo-graphic meridian and geomagnetic meridian. H’points to thegeomagnetic north in the geocentric dipole coordinates. Thedisturbance component, H, could be calculated from the formula.

H¼H0

cos j ð2Þ

For each ground station, the denominator is the cosine of thecorrected geomagnetic latitude j. So, H is parallel to the dipole axis.

longitude and vertical axis is the latitude. Stations are represented by the black

Fig. 2. Geomagnetic component H distribution of eight stations for four storm events. In each event, the upper three panels express AE index, SYMH index from WDC-C2,

Kyoto and the H components of eight stations variation with UT. The bottom two panels show H component of the eight stations along with MLT. (For interpretation of the

references to color in this figure, the reader is referred to the web version of this article.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–6354

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–63 55

Using the above equations, the observation data of eightground stations located in the even longitude regions are calcu-lated. According to the different level storms, the disturbancefield for each level along the geomagnetic local time is analyzed.

3. The data analysis

3.1. The ground station observations for case studies

To analyze the ring current geomagnetic effects in differentmagnetic local time, four storm events from SYMH¼�46 nT toSYMH¼�188 nT are selected (shown in Fig. 2(a)–(d)). For eachevent, the upper three panels show the AE index, SYMH indexfrom WDC-C2, Kyoto and the H components of eight stationsvariation with UT. The different vertical lines in the upper threepanels represent the different UT time in the event. Among these,the four different color real lines mark the UT time in the mainphase of the storm and the four different color dashed lines markthe UT time in the recovery phase of the storm. In each event, theblue real line indicates the UT time corresponding to the SYMHindex minimum. The bottom two panels show the H componentof the eight stations variation with MLT. The different color andshape lines are the fitted curves of the observation points (markedby the black dots) corresponding to the different UT time in themain phase and the recovery phase is represented by the verticallines in the upper three panels.

In Fig. 2, the SYMH index indicates the storm events that takeplace. The AE index indicates the substorms occurrence. Thereis often a rapid succession of substorms during the main phaseof magnetospheric storms (Fok et al., 1999). According to themagnetic field observation of stations in the middle and lowlatitude, the H component shows much difference in the differentMLT regions during the main phase of storms (the different colorreal curves in the fourth panel of each event). Especially, the MLTdifference of H component is most obviously corresponding to thetime of the SYMH index minimum for all the four storm events(marked by the blue vertical line in the third panel and the bluecurve in the fourth panel). However, during the recovery phaseof the storms, the MLT difference of H component diminishesgradually (the different color dotted curves in the fifth panelof each event). In the later stage of the recovery phase for eachstorm event, the MLT difference of H component for observationsalmost disappears. It could be seen in the fifth panel of each event

Fig. 3. Event number and percentage dist

that the blue dotted curve becomes nearly flat in all the MLTregions. The case studies show that the partial current may causea major effect on the MLT difference of H component during themain phase of storm events. But in the recovery phase of thestorm, the effect of partial current is not obvious. Whether thischaracter fits for more events or not, a statistical analysis fordifferent SYMH class events is needed.

3.2. Statistics analysis

Using the SYMH index of WDC-C2, 879 storm events areselected during the 23rd solar cycle (1996–2006). According tothe minimum SYMH index of each event, the storm events aredivided into eight classes: �30 nT4SYMH4�50 nT, �50 nT4SYMH4�70 nT, �70 nT4SYMH4�90 nT, �90 nT4SYMH4�110 nT, �110 nT4SYMH4�130 nT, �130 nT4SYMH4�150,�150 nT4SYMH4�170 and SYMHo�170 nT. The event num-ber and percentage distributions are shown in Fig. 3.

The weak storm events (SYMH4�50 nT) are about 55.7% of thetotal events. And the medium (�50 nT4SYMH4�110 nT) andintensive storm events (SYMHo�110 nT) are about 35% and 9.3%of the total events respectively. Though the numbers of the stormevents become fewer with the class for stronger storms, they couldalso indicate the general feature of each storm class.

Using the superposed epoch method, the storm events in eachclass are overlapped along with the magnetic local time (MLT) to beanalyzed. Fig. 4 shows the statistical results for the storm events ineight classes corresponding to the minimum SYMH index of eachevent. The different symbols represent that the minimum H compo-nent locates in different MLT sections. The gray circle denotes that theminimum H component is in the dusk side (dusk side events: 15–21MLT). And the gray cross is for dawn side (dawn side events: 03–09MLT), and the black square is for noon side (noon side events: 09–15MLT), the black asterisk is for night side (night side events: 21–3 MLT).

The events in which the minimum H component locates in thedusk side are the most in each storm class, about 59.5% of the totalevents. Whereas the noon side one is about 20.0%, the night side oneis about 18.7%, and the dawn side one is about 1.8%. The H

component of dusk side events shows a relatively regular exhibitionwith an extreme value in the dawn MLT sector and another extremevalue in the dusk MLT sector. The dusk side events are the most

ributions for different storm classes.

Fig. 4. Statistics results for the storm events of eight classes corresponding to the SYMH index minimum. In each class, the gray circle denotes that the minimum H

component is in the dusk side (15–21 MLT); the gray cross is for dawn side (03–09 MLT); the black square is for noon side (09–15 MLT); the black asterisk is for night side

(21–3 MLT). The numbers of the events for each side are shown in the top right corner.

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–6356

typical in all the storm classes. So this kind of event is to be furtherstudied.

In Fig. 5, the event numbers of each class is marked at the topright corner. The black line is the fitted curve for the medianvalues which are selected in every 2 h of MLT for each storm class.The application of median value is done to avoid the singularityeffect from some particular storm events. The fitted curve couldrepresent the general feature of the dusk side storm events ineach class.

The observations of the ground stations (gray circles) show thatthere are obvious differences in the magnetic field component H

distributions along with MLT corresponding to the minimum SYMHindex. Comparison with all the storm event classes, the fitted curvesshow the similar configuration like sine curves. There are twoextreme values in the fitted curves with a maximum one in thedawn side and a minimum one in the dusk side. The reason for this

feature may be the westward partial current in the dusk side. Boththe symmetric ring current and westward partial current depress themagnetic field component H observed by ground stations. And theexistence of the partial ring current makes the H componentdistribution uneven in the MLT regions.

Though the extreme values in the dawn and dusk sides of thecurves are the major features for the storm events, the extreme valuesalso indicate some obvious distinctions in different classes. Fig. 6shows the extreme values’ variation with the storm classes. The graydots and black circles in the figure are the extreme values for thedawn and dusk sides respectively, and the average minimum SYMHof storm events is used to represent the magnitude of each stormclass (the abscissa axis in the figure). Accompanying with storm classgrowth, the dawn side extreme value decreases from �8.8 nT(average minimum SYMH¼�30.0 nT) to �113.8 nT (average mini-mum SYMH¼�197.9 nT); the dusk side extreme value varies from

Fig. 5. Statistics results for the storm events in which the minimum H component is located in the dusk side corresponding to the SYMH index minimum. In each class, the

gray circle denotes the H component observed by stations and the black line is the fitted curve for the observations.

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–63 57

nearly �48.0 nT (average minimum SYMH¼�30.0 nT) to nearly�271.2 nT (average minimum SYMH¼�197.9 nT). It is obvious thatboth the dawn and dusk side extreme values decrease along with theincrease in the storm classes. The dawn side extreme values are largerthan the corresponding average minimum SYMH and the dusk sideextreme values are smaller than the corresponding averageminimum SYMH.

Both the symmetric and partial ring currents contribute to themain phase of the storm. In order to investigate the partial ringcurrent effect the relative variation between the dawn and duskside extreme values is studied. Fig. 7 shows the different (d)variations of the extreme values with the storm classes. d isdetermined by the following formula.

d¼ Eda�Edu ð3Þ

where Edaand Edu are the dawn and dusk side extreme values ofthe fitted curves for each storm class respectively.

The black dots in Fig. 7 represent the differences in the extremevalues. The abscissa axis is the average SYMH of each storm class. It isclear that the dusk side extreme value is much less than the dawnside one. The minimum difference is about 38.5 nT corresponding tothe average SYMH �29.0 nT. And the maximum difference is about157.4 nT corresponding to the average SYMH �197.9 nT. It is morenotable that there is an obvious linearity variation of the extremevalues difference with the decreasing average SYMH. The differencebecomes larger when accompanied with the storm class growth. Theresults shown in Figs. 5–7 may indicate that the storm events notonly relate to the symmetrical ring current, but also to the partial ringcurrents. Along with the increasing storm class, both the symmetricaland partial ring currents enhance. But the partial ring current has

Fig. 6. Extreme values variation with the storm classes. The gray dots and black circles are for dawn and dusk side extreme values of the fitted curves in Fig. 5 respectively.

The abscissa axis is the average SYMH index of each storm class.

Fig. 7. Difference between dusk and dawn side extreme values variation with the storm classes. The black dots represent the extreme values difference and the abscissa

axis is the average SYMH index of each storm class.

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–6358

a larger growth. This means that the partial ring current makes agreat contribution to the main phase of the storm. And the higher thestorm class is the more effect the partial ring current takes.

3.3. The analysis of interplanetary parameters

In the statistical analysis, 879 storms are divided into eight classes.In each class, the events are sorted by dusk, dawn, noon and nightside events. The interplanetary parameters play an important role inthe development of the ring current for the storms. Whether there aresome differences in the interplanetary parameters or which para-meters have more effects on the distribution of ring current indifferent kinds of events, the comparisons between some events aremade. The interplanetary parameters used in this paper are all fromACE satellite, and the data have made the time shift to thebowshock nose.

For the dusk side events, several events in different classes arecompared. As an example shown in Fig. 8, two events are demon-strated. According to the UT time corresponding to the minimumSYMH (marked by the blue vertical line), the duration of these twoevents is normalized in UT time. In the upper seven panels, theinterplanetary magnetic field, the solar wind velocity, the number

density of the proton, AE index and SYMH index are shown. In eachpanel, the black line is for the event 19971011 and the blue line is forthe event 20011124. The bottom three panels are the H componentvariation with UT and MLT corresponding to different UT in the mainphase and recovery phase for the events 19971011 (left) and20011124 (right), as shown in Fig. 2.

It is very obvious that the H component of these two events isasymmetric in the main phase of the events. However in the recoveryphase, this asymmetry gradually diminishes. This is consistent withthe analysis of the case and statistical study in the previous sections.Comparing with the interplanetary parameters of the two events(shown in the upper five panels), the most distinct one is the solarwind velocity Vx. The velocity of solar wind is a key factor for theenergy transmission from the solar to the magnetosphere duringthe storm events. The Vx of event 20011124 is much higher(the difference is about 400 km/s) than that of event 19971011. Theminimum SYMH of event 20011124 is about �234 nT while that ofevent 20011124 is about �139 nT. From these two events, it isindicated that the solar wind velocity plays an important role indifferent storm classes for the dusk side events. The comparison ofthese two events is just an example of several events. From ourcomparisons, more than 63% events have similar results.

Fig. 8. Comparison between dusk side events. The upper seven panels are the interplanetary parameters from ACE satellite, AE index and SYMH index. The black line is for

event 19971011 and the blue line is for event 20011124. These two events are normalized in UT time. The bottom three panels are the H component variation with UT and

MLT corresponding to the different UT for events 19971011 (left) and 20011124 (right). (For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–63 59

Also, the comparisons between dusk side and dawn sideevents for the same class are made. In the most event compar-isons, the distinct interplanetary parameters are the interplane-tary magnetic component By and solar wind velocity Vx. Theexample is shown in Fig. 9. The description in the figure is same asFig. 8. Two super storms are selected for comparison. The storm20010331 is the dusk side event and 20031120 is the dawn sideevent. The minimum SYMH of these two events are less than�400 nT. Corresponding to the UT of the minimum SYMH, thedifference of solar wind velocity Vx is about 81 km/s, and theinterplanetary magnetic component By is opposite in direction.

In Fig. 10, two storms 19980130 and 20030511 are selected tomake comparison between dusk side and noon side events for thesame class. Similar to Fig. 8, the black lines in the figure is forevent 19980130 and the blue line is for event 20030511. Duringthe main phase of the two storm events, the minimum H

component is located in the dusk side (event 19980130) and

noon side (event 20030511) for MLT distribution (shown in theninth panel). The minimum SYMH of these two events is nearly�60 nT. Corresponding to the UT of the minimum SYMH, themain difference in interplanetary parameters is seen in the solarvelocity. The Vx of these two events are about �352.5 km/s and�562.9 km/s. The interplanetary magnetic field By also shows anobvious distinction in direction with about 1.39 nT and �4.98 nT.

The comparison between dusk side and night side events isshown in Fig. 11. In the first seven panels, the black linesrepresent the storm event 19970110 for which the minimum H

component is located in the dusk side. The blue lines indicate theevent 20010327 for which the minimum H components located inthe night side. The SYMH index denotes that these two events aremedium storms with the minimum SYMH index nearly �80 nT.Corresponding to the UT of minimum SYMH index, the inter-planetary parameters have less difference except for the inter-planetary magnetic field By and the solar wind velocity Vx. By

Fig. 9. Comparison between dusk side and dawn side events The description in the figure is the same as Fig. 8. The black line in the figure is for the dusk side event

20010331 and the blue line is for the dawn side event 20031120. (For interpretation of the references to color in this figure legend, the reader is referred to the web version

of this article.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–6360

of these two events is opposite in direction (about �6.2 nT and14.4 nT). The discrepancy of Vx is more obvious, about 170 km/s(�445.8 km/s and �617 km/s).

According to the comparisons of the dusk side events indifferent classes, the solar wind velocity Vx makes a greatcontribution to the storm magnitude. For the southward inter-planetary magnetic field, the energy from solar wind could betransferred to the magnetosphere through reconnection. Theenergy is related to the direction of interplanetary magnetic fieldand solar wind velocity. In our comparisons, the distinction of thesolar wind velocity Vx is more obvious than the interplanetarymagnetic filed Bz for the dusk side events in different classes.

The comparisons between dusk side and the other side eventsin the same class indicate that besides the solar wind velocity Vx,the interplanetary magnetic filed By also affects the disturbance ofring current on the ground in MLT. The solar wind velocity Vx

relates to the amount of the energy transferring from solar windto the magnetosphere. And the interplanetary magnetic field By

acts on the direction of the energy transmission. The combinedaction of these two parameters leads to the difference of partialring current distribution in MLT.

4. Discussions and conclusions

The distribution of ring current accompanied by MLT corre-sponding to the SYMH index minimum of the storm event isanalyzed in this paper. Eight stations which are located evenly inthe geomagnetic longitude are selected for each storm event. Theground station geomagnetic field component which is parallel tothe dipole axis is used to study the disturbance from the ringcurrent.

The case study for four storms with minimum SYMH rangingfrom �46 nT to �188 nT shows that the H component of stationsmanifests much asymmetry in different MLT regions during themain phase of the storm events. But during the recovery phase

Fig. 10. Comparison between dusk side and noon side events The description in the figure is the same as Fig. 8. The black line in the figure is for the dusk side event

19980130 and the blue line is for the noon side event 20030511. (For interpretation of the references to color in this figure legend, the reader is referred to the web version

of this article.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–63 61

of the storm events, the asymmetry gradually diminishes and theH component is nearly even in all the MLT regions.

879 storm events with minimum SYMH less than �30 nTduring 1996–2006 are selected for the statistical analysis. Theevents are divided into eight different classes and the superposedepoch method is used for the storm events analysis. According tothe minimum H component location, the storm events in eachclass are sorted by dusk, dawn, noon and night side events. Thedusk side events are about 59.5% of the total events; the noon sideevents are about 20.0%; the night side events are about 18.7%; andthe dawn side events are about 1.8%.

For the dusk side events which are the most of the total events,the H component of observations shows that there are twoextreme values in the dawn and dusk sides. As the magnitudeof the storm class increases, both the dawn and dusk extremevalues of H component decrease dramatically. Also, the differencebetween these two extreme values enhances with the increasing

storm class. It indicates that both the symmetric and partial ringcurrents affect the main phase of the storm. But the partialring current has a larger growth. This means that the partial ringcurrent makes a greater contribution to the main phase ofthe storm. And the higher the storm class is the more effect thepartial ring current has. Le et al. (2004) indicate that the partialring current has the most drastic intensification as the level ofdisturbances increases, and it is mainly the partial ring currentthat contributes to the varying depressions of the Earth’s surfacemagnetic field. Liemohn et al. (2001) point that . R% of the mainphase magnetic field depression is from the asymmetric compo-nent of the ring current, while, Grafe (1999) states that only theasymmetric ring current exists during the storms. In our results,the symmetric and the partial ring currents coexist at theminimum SYMH of the storm events. The symmetric ring currentcauses the magnetic field component H to decrease in all MLTregions and the partial ring current depresses the magnetic field

Fig. 11. Comparison between dusk side and night side events The description in the figure is the same as Fig. 8. The black line in the figure is for the dusk side event

19970110 and the blue line is for the night side event 20010327. (For interpretation of the references to color in this figure legend, the reader is referred to the web version

of this article.)

X.D. Zhao et al. / Planetary and Space Science 78 (2013) 52–6362

component H in the dusk side. But during the recovery phase ofthe storm, both the symmetric and partial ring current decrease.And the partial ring current diminishes more quickly. The ringcurrent could decrease through the charge exchange with theneutral atmosphere, interactions with plasma waves, and theCoulomb collisions with the plasmasphere (Fok et al., 2001).

The analysis of interplanetary parameters of storm eventsindicates that both the solar wind velocity Vx and the interplane-tary magnetic filed By affect the MLT distribution of ring current.The solar wind velocity Vx relates to the energy from solar windentering the magnetosphere, while the interplanetary magneticfield By controls the transmission direction.

The magnetic field observation from the ground stations iscontinuous and steady. It includes all the storm events and adaptsto make the statistical analysis for the storm study. The result in thispaper is the average effects of the storm events for each storm class.It is the general feature for the ring current distributions during thestorm events. Much more details are needed for further study. Also,this paper only shows some examples for comparison of interplane-tary parameters. The statistical study of interplanetary parameterswill be made in the future work.

Acknowledgments

This research was supported by NSFC (41174122, 41031066), theChinese Academy of Sciences (KZZD-EW-01-3) and the National KeyBasic Research Program of China (2012CB825604). The SYMH and AE

data from WDC-C2 are downloaded from http://wdc.kugi.kyoto-u.ac.jp/index.html. The magnetic field data of the ground stations is fromINTERMAGNET (http://www.intermagnet.org/). The interplanetaryparameters data is from ACE satellite (http://omniweb.gsfc.nasa.gov/form/sc_merge_min1.html).

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