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Local-time variations of geomagnetic disturbances during intense geomagnetic storms and possible association with their interplanetary causes
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Advances in Space Research 51 (2013) 1924–1933 www.elsevier.com/locate/asr
Local-time variations of geomagnetic disturbances during intense geomagnetic storms and possible association with their interplanetary causes Alicia L. Clu´a de Gonzalez ⇑, Walter D. Gonzalez INPE-CP 515, Sa˜o Jose´ dos Campos, SP, Brazil Available online 10 November 2012
Abstract In the present paper the local-time variations in the disturbance of the geomagnetic-field horizontal component (H) for eight intense geomagnetic storms that occurred during the descending phase of solar cycle 23 have been analyzed. The study was based on the plot of contour lines of the H-depletion intensity in the plane local time versus universal time (LT–UT maps) with the objective of observing how the morphology and evolution of the ring current is mapped into the surface of the Earth in presence of intense geomagnetic storms. The criterion for the selection of the events was a peak Dst below about – 200 nT. The values of the horizontal geomagnetic field were obtained from six mid-latitude stations, with 1 min resolution, and the disturbances were computed by a process similar to that for obtaining the Sym-H geomagnetic index. By means of a cubic spline, the results were interpolated to all longitudes with 6 min in LT spacement. The observed contour maps show, as expected, that the region of largest depletion in H are situated around dusk, as a consequence of the formation of a partial ring current, mainly located in the noon-dusk-midnight hemisphere. However, H disturbances are also observed around midnight and, to a less extend, at the noon-dawn sector. In order to detect a prevalent pattern for the behavior of the geomagnetic-disturbance distribution, a statistical analysis was done by means of occurrence histograms, for different levels of the relative intensity of the storm as a function of local time. The relative intensity for each event was defined as a parameter varying between 0 and 1, with 0 (1) corresponding to the maximum (minimum) horizontal field disturbance during that particular event. Although this analysis does not show the temporal evolution of the disturbance, it confirms the above conclusions about its LT distribution. When only the main phases of the storms are considered in the statistics, the basic differences are the dawn peak is lightly shifted towards noon and that the noon-dawn contribution becomes larger. The observed distributions were tentatively associated with the corresponding interplanetary causes of the events. Since among the eight considered storms four are associated to magnetic cloud with a shock interplanetary structure (sMC) and three to sheath regions followed by a magnetic cloud (SH/MC), the statistical study was also performed for the two subsets of storms separately. For the first group (sMC) the LT distribution looks very similar to that of the whole set of storms. On the other hand, it was observed that for the last group (SH/MC) the peak around midnight was not present. This result might be a consequence that these type of storms are probably not associated to the presence of substorms. Ó 2012 Published by Elsevier Ltd. on behalf of COSPAR. Keywords: Solar-terrestrial relations; Intense geomagnetic storms; Geomagnetic disturbances
1. Introduction The time evolution of geomagnetic storms is usually monitored by the indices Dst (Disturbance storm time) ⇑ Corresponding author.
E-mail address: [email protected] (A.L. Clu´a de Gonzalez). 0273-1177/$36.00 Ó 2012 Published by Elsevier Ltd. on behalf of COSPAR. http://dx.doi.org/10.1016/j.asr.2012.10.029
and Sym-H (Symmetric disturbance field), provided by the (WDC-Kyoto, 2012). These indices are obtained by a worldwide network of ground-based mid-latitude magnetometers and have a time resolution of 1 hour and 1 minute, respectively. (Iyemori, 1990; Kyoto University, 2012). As it is well known, a geomagnetic storm, is characterized by a decrease in the horizontal component of the
A.L. Clu´a de Gonzalez, W.D. Gonzalez / Advances in Space Research 51 (2013) 1924–1933
geomagnetic field at the surface of the Earth, H, as a consequence of the formation of the so called “ring current” (Gonzalez et al., 1994; McPherron, 1997; Tsurutani et al., 1997; Valdivia et al., 1999; Kamide et al., 1998; Kozyra and Liemohn, 2003; Soraas et al., 2006). The ring current is originated in the longitudinal drift of charged particles trapped in the Earth’s magnetosphere, resulting from the energy coupling between the solar wind and the magnetospheric fields. Ideally, the ring current is a toroidal westward current at the Earth’s equatorial plane, at radial distances of 4–7 RE . However, long-time in situ observations have shown that in fact it presents a complex spatial structure as well as temporal variations, determined by the effectiveness of that energy coupling. The ring current is mainly constituted by positive ions, although according to some authors the electron contribution could reach up to 25% of the storm-time ring current energy contents (Frank, 1967; Liu et al., 2005). Besides the lost of charged particles produced by a variety of wave particle interactions, the existence of field aligned currents (or Birkeland currents), which cause the coupling between the magnetosphere and the ionosphere, is responsible for the large inhomogeneity observed in the ring current (i. e. Potemra, 1988; Baumjohann and Treumann, 1997). According to experimental results (i. e. Le et al., 2004), besides the main westward ring current at 4–7 RE , there is an inner current that flows eastward at 3 RE for all levels of geomagnetic disturbances. Furthermore, while a portion of the current encircles completely the earth, another part stays in a limited portion of the torus, forming what is called a “partial ring current”. During moderate magnetic storms, the total partial ring current reaches 3 MA while the total symmetric ring current is about 1 MA. Therefore, the partial ring current contributes dominantly to the decrease in the Dst index. The partial ring current is mainly located in the dusk sector and is closed by field-aligned currents, which are maximum at local times around dawn and dusk i.e. (Li et al., 2011). Since Dst and Sym-H are global geomagnetic indices, in the sense that they do not take into account the variations with local time, we propose in this paper to see how the morphology and evolution of the ring current is mapped into the surface of the Earth in the presence of intense geomagnetic storms. Pointing to this goal, we have used the data from six low latitude geomagnetic stations (see Table 1) for time intervals for which very intense storms
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that occurred during the descending phase of solar cycle #23, as listed in Table 2. The disturbance in the horizontal component of the geomagnetic field, H, was plotted by means of LT–UT maps, using a similar method to that developed by Clauer and McPherron (1974), as described in the next section. Based on the contours obtained for each storm we analyze the statistical distribution of the disturbances as a function of local time (Section 3). 2. Data analysis The present analysis deals with the intense geomagnetic storms listed in Table 2, which occurred during the descending phase of solar cycle #23. In this table, besides the date and peak values of the Dst index, the interplanetary structures most probably associated with their origin are given, according to the Echer et al. (2008). For each of the events, the analysis was based on the geomagnetic observations from the six stations shown on Table 1, which have been selected based on a homogeneous longitudinal distribution. Some of these observatories are the source for the derivation of the Dst index (Iyemori, 1990 and the booklets of the Data Analysis Center for Geomagnetism and Space Magnetism of the Kyoto University, 2012). The procedure is basically the same developed by Clauer and McPherron (1974) and adapted by Clu´a de Gonzalez et al. (2004), as described below. The data for H (with 1 min resolution) are downloaded from the site of the World Data Center, Kyoto. Using the five quietest days in the month as a reference, the disturbance in the horizontal field, DH , is computed at each point, in a basically similar way to that used for the computation of the geomagnetic index Sym-H (WDC-Kyoto, 2012). The bottom panels of Figs. 1a–f show the disturbance for the intervals listed in Table 2. The two curves plotted there give, respectively, the 1 min WDC Sym-H index (black solid line) and the computed Sym-H value (red dashed line). The latter is the average of the disturbances obtained from the six stations considered in the present analysis. The rest of the panels in those figures give, from top to bottom, the value of the z-component of the interplanetary magnetic field, Bz (GSM), the dynamic proton pressure of solar wind, as reported at the CDAweb site for the WIND satellite (Space Physics Data Facility, 2012), and the Dst index (WDC-Kyoto, 2012), respectively.
Table 1 Geographic and geomagnetic coordinates of observatories used for data analysis. Station name
ABB code
Geographic latitude
Geographic longitude
Geomagnetic latitude
Geomagnetic longitude
Magnetic local time (*)
Alibag Kakioka Honolulu Boulder San Juan Hermanus
ABG KAK HON BOU SJG HER
18.6 36.2 21.3 40.1 18.4 34.4
72.9 140.2 202.0 254.8 293.9 19.2
9.9 26.6 21.5 48.7 29.1 33.7
145.8 207.8 268.6 319.0 5.2 82.7
18:55 15:04 11:08 7:21 4:12 23:48
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Table 2 List of analized geomagnetic storms [adapted from Echer et al., 2008]. Event
Date
Minimum Dst value (nT)
Associated interplanetary structure
October, 2003a October, 2003b November, 2003 July, 2004 November, 2004a November, 2004b May, 2005 August, 2005
October 29–30, 2003 October 30–31, 2003 November 20–21,2003 July 26–28, 2004 November 7–8,2004 November 9–11,2004 May 15–19, 2005 August 23–26, 2005
353 383 422 197 373 289 263 216
SH/MC SH sMC SH/MC sMC SH/MC sMC sMC
As seen in these figures, the gross behavior shown by the computed and the reported Sym-H curves is very similar. However, some discrepances are observed between both curves. These discrepances (of less than 2% around the peak of Dst) are probably due to the different data sources used for the computation, as well as to the latitude corrections applied by the WDC. The points given by vertical lines (labeled by capital letters) correspond to times of minimum Dst. The study of the LT variations of the horizontal geomagnetic field at low latitudes, DH , was performed in the following way. For each UT minute, the value of DH was computed at each station as a function of the corresponding LT, in the way described above. The symbols in Fig. 2 shows the respective values of DH for each of the six stations in the case of event of November 2003, at the instants marked as A and B in Fig. 1b. Then, the local time interval 00–24 was divided in a 6 min spaced grid and filled up with the values resulting from a Spline interpolation, using the IDL’s Spline subroutine with sigma ¼ 1. In order to keep the circular symmetry in this interpolation, the figures corresponding to the two stations closest to noon were repeated at both sides of local noon. The result of this interpolation for the event of November 2003, at instants A and B of Fig. 1b, are respectively shown by the solid and the dotted lines in Fig. 2. If N p is the number of points (UT-minutes) in a given interval, the above described process leads to a matrix of N p 240 elements for each of the considered intervals. These matrices are used to plot the contour maps, which give the disturbance in the horizontal component of the geomagnetic field in the LT–UT plane. Fig. 3a–h shows the LT-UT map for the time intervals listed in Table 2. The color bars at the right of each map give the respective intensity levels of the disturbance, with black and blue1 colors corresponding to the most negative values (largest depression). The points A; B, . . .of Fig. 1a–f are also indicated on abscissae of these maps. The horizontal lines give particular values of LT: Midnight (solid line at LT = 00), Dawn (red dashed line) and Dusk (dashed black line). In order to present in more detail the behavior of DH around the Dst peaks, only portions of the intervals given 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
in Table 2 were selected for the maps of Fig. 3a–h. As these contour maps show, there is a high degree of inhomogeneity in the H-depletion, which becomes masked by the Dst or Sym-H indices due to the averaging process used for their computation. In general, the maps present a minimum around dusk at the instant of minimum Dst, as a consequence of the existence of the partial ring current. In order to better visualize the LT distribution of the disturbance, the contour maps were also displayed in a polar representation as shown by Fig. 4a–h. In these polar maps, the radial coordinate is proportional to UT, with the starting point given by to , at the center, and the end at t1 , at the circle boundary. The azimuthal angle represents LT, with noon at the top (Sunward direction). The other circumferences plotted in the maps, which correspond to a given value of UT, are those for the points A; B, . . .of previous figures (white lines), and for six-hour intervals in UT (dotted black lines). The dusk depletion is clearly seen for the events July, 2004, and August, 2005 (Fig. 4d and h). For the event November, 2004a (Fig. 4e), the disturbance decays with longitude, from a maximum at dusk reaching a longitudinal extension of almost 360°, at the instant labeled by A (November 8, 6:00 UT). For November 2004b (Fig. 4f), the maximum depletion is located at the dusk-noon sector. For October, 2003b (Fig. 4b), there is a maximum depletion around dusk, but also a considerable disturbance around dawn at the time of minimum Dst, labeled by C (October 30, 22:00 UT). For October, 2003a (Fig. 4), a large depletion in the dusk-midnight sector occurs before the Dst peak, B, (October 30, 00:00 UT). Afterwards, during the recovery phase, large depletions are also observed in the dusk-noon sector as well as around dawn, although with less intensity. For the event of November, 2003, the maximum depletion regions are located mainly around midnight and to a lesser extension before noon. Finally, for the event of May, 2005, the maximum depletion region is located around midnight. 3. Statistical study of the LT distribution of the horizontal geomagnetic field disturbances As stated in the Introduction section, it is generally accepted that the largest disturbances in the H component of the geomagnetic field occur at the dusk and evening region (i. e. Clauer and McPherron, 1980 and references
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Fig. 1. From top to bottom: z-component of the interplanetary magnetic field in GSM coordinates [Szabo], ion dynamic pressure [Olgivie]; Dst index [WDC-Kyoto] Sym-H index according to WDC and computed in this paper. The points shown by vertical lines and labeled by capital letters correspond to times of minimum Dst (see the text).
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Fig. 2. Smoothed curve obtained for the H-variation as a function of LT, for the event of November 2003.
therein, Hakkinen et al., 2003; Soraas et al., 2006; Li et al., 2011). However, as discussed in the previous section, the LT–UT maps for the eight strong storms under consideration show that actual behavior of the geomagnetic disturbances for these cases may be rather complex. In order to show this in a quantitative way, we have performed the following statistical study. Since each of the geomagnetic storms has a different amplitude, the first step was to normalize the variations to the [0, 1] range. For this, we introduce an intensity parameter z given by: z¼
H H min H max H min
ð1Þ
where H max and H min are, respectively, the maximum and the minimum values of the horizontal geomagnetic field for the storm under consideration, and H the value of the computed field at a given point in the LT–UT plane. According to Eq. (1), z ¼ 0 means H ¼ H min , or maximum disturbance, while z ¼ 1. corresponds to H ¼ H max , no disturbance. For each of the analized events, the LT axis is divided in one hour intervals. Since the points are 6 min spaced in LT, each of these intervals contains 10 points. Furthermore, within these 1 h-LT intervals, 10 levels of the intensity parameter, z, were considered (0:0 6 z < 0:1, 0:1 6 z < 0:2, . . .). In this way, 10 10 cells are obtained for each of the considered events. Counting the number of percent occurrences in each of these cells, a picture of
the contribution to the geomagnetic-disturbance intensity as a function of the LT is obtained. Figs. 5a-h show the result of the above described statistics for each of the UT intervals given in Figs. 3a–h and 4a–h, for the two lowest levels of intensity parameter z; 0:0 6 z < 0:1 and 0:1 6 z < 0:2, which correspond to maximum disturbance. The distribution of the depletion intensity shown by the histogram of each event is in agreement with the conclusions reached in previous section although, of course, these type of graphics do not show the temporal evolution of the disturbance. In Fig. 6 the averages obtained from Figs. 5a–h, as a function of LT, are given also in a percent scale. The vertical lines in this plot, give the standard deviation from the average derived from the eight values corresponding to each 1 h-LT interval. As seen in this figure, in spite of the large error lines, the predominance of the dusk maximum depletion is observed, together with a less important contribution around midnight. Fig. 7 shows the result of this statistics when only the main phases of the storms are considered, also for 0:0 6 z < 0:1 and 0:1 6 z < 0:2. The dusk peak appears to be shifted towards the dusk-noon sector, showing that the partial ring current is mainly situated at this sector during the main phase of the storms. On the other hand, indications of disturbances around midnight and the dawnnoon sector also appear. Furthermore, a finer study was done by separating the events according to the associated interplanetary structure assigned in Table 2. Two groups of events were formed. One is for storms driven by magnetic clouds with a shock (sMC cases), and is composed composed by four events. The other for is the storms driven by sheath regions followed by a magnetic cloud (SH/MC cases) and is composed of 3 events. Since the event October, 2003b, does not correspond to any of these groups, it was taken apart from this study. The statistics based on these two subsets are respectively shown in Figs. 8a and 9b, also for the largest levels of disturbance (0:0 6 z < 0:1 and 0:1 6 z < 0:2). The vertical lines give the standard deviations at each local time for the samples of four and three events, respectively. Although clearly the results based on these small samples can not be conclusive, some differences in the distributions seem to appear. For the subset associated to sMC structures (Fig. 8a), with four out of eight events, the distribution looks very similar to that of all the events (Fig. 7). On the other hand, for the SH/MC related subset (Fig. 9b) the disturbances are mostly localized at the dusk-noon sector and, to a less extend, also at the dawn-noon sector but with no incidences around midnight. As a matter of fact, this conclusion should be confirmed by a study based on a larger sample of intense geomagnetic storms. 4. Discussion and conclusions The study of the LT–UT distribution of the horizontal geomagnetic field component for the eight intense geomag-
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12
Dawn
00
Dusk
06
12
18
00
06
UT
12
18
00
06
UT
12
Dawn
00
Dusk
12
06
18
00
12
06
UT
12
00
06
12
18
00
06
UT
12
Dawn
00
Dusk
18
00
06
12
18
UT
12
06
12
18
00
06
12
18
00
06
12
18
UT
12
Dawn
00
Dusk
09
12
15
18
12
18
00
06
12
18
UT
Fig. 3. LT–UT maps for the geomagnetic storms listed in Table 2.
netic storms of the descending phase of solar cycle #23, shows that the disturbance is mostly concentrated on the midnight-dusk-noon sectors, as would be expected from the related literature (i. e. Clauer and McPherron, 1980 and references therein, Soraas et al., 2006; Li et al., 2011). Nevertheless, the differences observed in the LT
distribution of the geomagnetic disturbance from case to case lead to the conclusion that there is not a clear recurrent pattern for this distribution, for the considered set of events. It can be said that in average for the eight considered events, the maximum disturbance regions are mainly located at the dusk-noon sector, with some incidence also
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Fig. 4. Polar representation of the LT–UT maps for selected intervals of the events in Table 2.
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Fig. 5. Intensity of H-depletion as a function of LT, as described by intensity parameter z (see the text for definition of z). The histograms give the percent occurrences during 1 h-LT, for 0:0 < z < 0:1 and 0:1 < z < 0:2, for the UT intervals given in Figs. 3a–h and 4a–h.
Fig. 6. Average of percent occurrences in the histograms of Figs. 5a–h. The vertical lines show the standard deviation derived from those 8 histograms.
around midnight. A similar result is obtained when the analysis is restricted to the main phases of the storms although, in this case, the dusk peak looks lightly shifted to the noon side and a lower peak in the dawn-noon sector is shown up. As an intent to see if there is an influence of the interplanetary origin of the storms in the resulting LT distribution of the geomagnetic disturbance, the events were classified according to their interplanetary origin in two groups. One for the storms driven by magnetic cloud with
a shock (sMC), and the other for storms driven by sheath regions followed by a magnetic cloud (SH/MC). For the first subset (with four out of eight cases), the behavior of the distribution during the main phase of the storms looks very similar to that of the total set, while for the second subset (with three out of eight cases), the peak around midnight is not observed. Since the size of the considered samples is too small, the difference in the distribution of the geomagnetic disturbance according to the origin of the storms found
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Fig. 7. Similar to Fig. 6 but only considering the main phase of each geomagnetic storms of Table 2.
Fig. 8. Similar to Fig. 7 for geomagnetic storms associated to sMC interplanetary structures.
Fig. 9. Like Fig. 8a for SH/MC events.
here may not be conclusive. Therefore, we are working on the the extension of the present analysis to the whole solar cycle #23 in order to find out if this distinction really exists. Furthermore, since there are some indications that storms driven by sMC develop substorms, while the the SH/MC do not, a complementary analysis will be undertaken in order to find out a possible correlation between the LT distribution of the geomagnetic disturbance and the existence or not of substorms along with each storm. Acknowledgements The authors would like to thank “Fundo de Desenvolvimento Cientı´fico e Tecnolo´gico” and the “Conselho
Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico” (Grants PQ-342734/2008-2 and PQ-300321/2005-8) of Brazil. The data for solar wind parameters were provided by K. Ogilvie and A. Szabo, at NASA , GSFC and CDAWeb. References Baumjohann, W., Treumann, R.A. Basic Space Plasma Physics. Imperial College Press, London, 1997. Clauer, C.R., McPherron, R.L. Mapping the local time development of magnetospheric substorms using mid-latitude magnetic observations. J. Geophys. Res. 79, 2811–2820, http://dx.doi.org/10.1029/ JA079i019p02811, 1974. Clauer, C.R., McPherron, R.L. The relative importance of the interplanetary electric field and magnetospheric substorms on partial ring current development. J. Geophys. Res., 6747–6759, http://dx.doi.org/ 10.1029/JA085iA12p06747, 1980.
A.L. Clu´a de Gonzalez, W.D. Gonzalez / Advances in Space Research 51 (2013) 1924–1933 Clu´a de Gonzalez, A.L., Mendes da Costa, A., Gonzalez, W.D. Ring current space time inhomogeneities observed in association with intense geomagnetic storms. Geofı´sica Internacional 43, 205–215, 2004. Echer, E., Gonzalez, W.D., Tsurutan, B.T., Clu´a de Gonzalez, A.L. Interplanetary conditions causing intense geomagnetic storms (Dst 6 100 nT) during solar cycle 23 (1996–2006). J. Geophys. Res. 113, 1–6, http://dx.doi.org/10.1029/2007JA012744. A05221, 2008. Frank, L.A. On the extraterrestrial ring current during geomagnetic storms. J. Geophys. Res. 72, 3753–3767, http://dx.doi.org/10.1029/ JZ072i015p03753, 1967. Gonzalez, W.D., Joselyn, J.A., Kamide, Y., Kroehl, H.W., Rostoker, G., Tsurutani, B.T., Vasyliunas, V.M. What is a geomagnetic storm? J. Geophys. Res. 99, 5771–5792, 1994. Hakkinen, L.V.T., Pulkkinen, T.I., Pirjola, R.J., Nevanlinna, H., Tanskanen, E.I., Turner, N.E. Seasonal and diurnal variation of geomagnetic activity: revised dst versus external drivers. J. Geophys. Res. 108, 2003. Iyemori, T. Storm-time magnetospheric currents inferred from midlatitude geomagnetic field variations. J. Geomag. Geolectr. 42, 1249, 1990. Kamide, Y., Baumjohann, W., Daglis, I.A., Gonzalez, W.D., Grande, M., Joselyn, J.A., McPherron, R., Phillips, J., Reeves, E.G.D., Rostoker, G., Sharma, A.S., Singer, H.J., Tsurutani, B.T., Vasyliunas, V.M. Current understanding of magnetic storms: storm/substorm relationship, invited review. J. Geophys. Res. 103, 17705–17728, 1998. Kozyra, J., Liemohn, M. Ring current energy input and decay. Space Sci. Rev. 109, 105–131, http://dx.doi.org/10.1023/B:SPAC.0000007516. 10433.ad, 2003. Kyoto University: Data analysis center for geomagnetism and space magnetism. High-Time Resolution Geomagnetic Indices.. WDC for Geomagnetism, Faculty of Science, Kyoto University, Japan.
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Le, G., Russell, C.T., Takahashi, K. Morphology of the ring current derived from magnetic field observations. Ann. Geophys. 22, 1267– 1295, 2004. Li, H., Wang, C., Kan, J.R. Contribution of the partial ring current to the symh index during magnetic storms. J. Geophys. Res. 116, 1–12, 2011. Liu, S., Chen, M.W., Roeder, J.L., Lyons, L.R., Schulz, M. Relative contribution of electrons to the stormtime total ring current energy content. Geophys. Res. Lett. 32, 1–5, http://dx.doi.org/10.1029/ 2004GL021672, 2005. McPherron, R.L. The role of substorms in the generation of magnetic storms, in: Tsurutani, B.T., Gonzalez, W.D., Kamide, Y., Arballo, J.K. (Eds.), Magnetic Storms, Geophys. Monogr. Ser., vol. 98. Amer. Geophys. Un., Washington, pp. 131–147, 1997. Potemra, T.A. Birkeland currents in the earth’s magnetosphere. Astrophys. Space Sci. 144, 155–169, 1988. Soraas, F., Sorbo, M., Aarsnes, K., Evans, D.S. Ring current behavior inferred from ground magnetic and space observations, in: Tsurutani, B., McPherron, R., Gonzalez, W., Lu, G., Sobral, J.H.A., Gopalswamy, N. (Eds.), Recurrent Magnetic Storms: Corotating Solar Wind Streams, Geophys. Monogr. Ser., vol. 167. Amer. Geophys. Un., Washington, pp. 85–95, 2006. Space physics data facility – coordinated data analysis web (cdaweb)., 2012.
Advances in Space Research 51 (2013) 1924–1933 www.elsevier.com/locate/asr
Local-time variations of geomagnetic disturbances during intense geomagnetic storms and possible association with their interplanetary causes Alicia L. Clu´a de Gonzalez ⇑, Walter D. Gonzalez INPE-CP 515, Sa˜o Jose´ dos Campos, SP, Brazil Available online 10 November 2012
Abstract In the present paper the local-time variations in the disturbance of the geomagnetic-field horizontal component (H) for eight intense geomagnetic storms that occurred during the descending phase of solar cycle 23 have been analyzed. The study was based on the plot of contour lines of the H-depletion intensity in the plane local time versus universal time (LT–UT maps) with the objective of observing how the morphology and evolution of the ring current is mapped into the surface of the Earth in presence of intense geomagnetic storms. The criterion for the selection of the events was a peak Dst below about – 200 nT. The values of the horizontal geomagnetic field were obtained from six mid-latitude stations, with 1 min resolution, and the disturbances were computed by a process similar to that for obtaining the Sym-H geomagnetic index. By means of a cubic spline, the results were interpolated to all longitudes with 6 min in LT spacement. The observed contour maps show, as expected, that the region of largest depletion in H are situated around dusk, as a consequence of the formation of a partial ring current, mainly located in the noon-dusk-midnight hemisphere. However, H disturbances are also observed around midnight and, to a less extend, at the noon-dawn sector. In order to detect a prevalent pattern for the behavior of the geomagnetic-disturbance distribution, a statistical analysis was done by means of occurrence histograms, for different levels of the relative intensity of the storm as a function of local time. The relative intensity for each event was defined as a parameter varying between 0 and 1, with 0 (1) corresponding to the maximum (minimum) horizontal field disturbance during that particular event. Although this analysis does not show the temporal evolution of the disturbance, it confirms the above conclusions about its LT distribution. When only the main phases of the storms are considered in the statistics, the basic differences are the dawn peak is lightly shifted towards noon and that the noon-dawn contribution becomes larger. The observed distributions were tentatively associated with the corresponding interplanetary causes of the events. Since among the eight considered storms four are associated to magnetic cloud with a shock interplanetary structure (sMC) and three to sheath regions followed by a magnetic cloud (SH/MC), the statistical study was also performed for the two subsets of storms separately. For the first group (sMC) the LT distribution looks very similar to that of the whole set of storms. On the other hand, it was observed that for the last group (SH/MC) the peak around midnight was not present. This result might be a consequence that these type of storms are probably not associated to the presence of substorms. Ó 2012 Published by Elsevier Ltd. on behalf of COSPAR. Keywords: Solar-terrestrial relations; Intense geomagnetic storms; Geomagnetic disturbances
1. Introduction The time evolution of geomagnetic storms is usually monitored by the indices Dst (Disturbance storm time) ⇑ Corresponding author.
E-mail address: [email protected] (A.L. Clu´a de Gonzalez). 0273-1177/$36.00 Ó 2012 Published by Elsevier Ltd. on behalf of COSPAR. http://dx.doi.org/10.1016/j.asr.2012.10.029
and Sym-H (Symmetric disturbance field), provided by the (WDC-Kyoto, 2012). These indices are obtained by a worldwide network of ground-based mid-latitude magnetometers and have a time resolution of 1 hour and 1 minute, respectively. (Iyemori, 1990; Kyoto University, 2012). As it is well known, a geomagnetic storm, is characterized by a decrease in the horizontal component of the
A.L. Clu´a de Gonzalez, W.D. Gonzalez / Advances in Space Research 51 (2013) 1924–1933
geomagnetic field at the surface of the Earth, H, as a consequence of the formation of the so called “ring current” (Gonzalez et al., 1994; McPherron, 1997; Tsurutani et al., 1997; Valdivia et al., 1999; Kamide et al., 1998; Kozyra and Liemohn, 2003; Soraas et al., 2006). The ring current is originated in the longitudinal drift of charged particles trapped in the Earth’s magnetosphere, resulting from the energy coupling between the solar wind and the magnetospheric fields. Ideally, the ring current is a toroidal westward current at the Earth’s equatorial plane, at radial distances of 4–7 RE . However, long-time in situ observations have shown that in fact it presents a complex spatial structure as well as temporal variations, determined by the effectiveness of that energy coupling. The ring current is mainly constituted by positive ions, although according to some authors the electron contribution could reach up to 25% of the storm-time ring current energy contents (Frank, 1967; Liu et al., 2005). Besides the lost of charged particles produced by a variety of wave particle interactions, the existence of field aligned currents (or Birkeland currents), which cause the coupling between the magnetosphere and the ionosphere, is responsible for the large inhomogeneity observed in the ring current (i. e. Potemra, 1988; Baumjohann and Treumann, 1997). According to experimental results (i. e. Le et al., 2004), besides the main westward ring current at 4–7 RE , there is an inner current that flows eastward at 3 RE for all levels of geomagnetic disturbances. Furthermore, while a portion of the current encircles completely the earth, another part stays in a limited portion of the torus, forming what is called a “partial ring current”. During moderate magnetic storms, the total partial ring current reaches 3 MA while the total symmetric ring current is about 1 MA. Therefore, the partial ring current contributes dominantly to the decrease in the Dst index. The partial ring current is mainly located in the dusk sector and is closed by field-aligned currents, which are maximum at local times around dawn and dusk i.e. (Li et al., 2011). Since Dst and Sym-H are global geomagnetic indices, in the sense that they do not take into account the variations with local time, we propose in this paper to see how the morphology and evolution of the ring current is mapped into the surface of the Earth in the presence of intense geomagnetic storms. Pointing to this goal, we have used the data from six low latitude geomagnetic stations (see Table 1) for time intervals for which very intense storms
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that occurred during the descending phase of solar cycle #23, as listed in Table 2. The disturbance in the horizontal component of the geomagnetic field, H, was plotted by means of LT–UT maps, using a similar method to that developed by Clauer and McPherron (1974), as described in the next section. Based on the contours obtained for each storm we analyze the statistical distribution of the disturbances as a function of local time (Section 3). 2. Data analysis The present analysis deals with the intense geomagnetic storms listed in Table 2, which occurred during the descending phase of solar cycle #23. In this table, besides the date and peak values of the Dst index, the interplanetary structures most probably associated with their origin are given, according to the Echer et al. (2008). For each of the events, the analysis was based on the geomagnetic observations from the six stations shown on Table 1, which have been selected based on a homogeneous longitudinal distribution. Some of these observatories are the source for the derivation of the Dst index (Iyemori, 1990 and the booklets of the Data Analysis Center for Geomagnetism and Space Magnetism of the Kyoto University, 2012). The procedure is basically the same developed by Clauer and McPherron (1974) and adapted by Clu´a de Gonzalez et al. (2004), as described below. The data for H (with 1 min resolution) are downloaded from the site of the World Data Center, Kyoto. Using the five quietest days in the month as a reference, the disturbance in the horizontal field, DH , is computed at each point, in a basically similar way to that used for the computation of the geomagnetic index Sym-H (WDC-Kyoto, 2012). The bottom panels of Figs. 1a–f show the disturbance for the intervals listed in Table 2. The two curves plotted there give, respectively, the 1 min WDC Sym-H index (black solid line) and the computed Sym-H value (red dashed line). The latter is the average of the disturbances obtained from the six stations considered in the present analysis. The rest of the panels in those figures give, from top to bottom, the value of the z-component of the interplanetary magnetic field, Bz (GSM), the dynamic proton pressure of solar wind, as reported at the CDAweb site for the WIND satellite (Space Physics Data Facility, 2012), and the Dst index (WDC-Kyoto, 2012), respectively.
Table 1 Geographic and geomagnetic coordinates of observatories used for data analysis. Station name
ABB code
Geographic latitude
Geographic longitude
Geomagnetic latitude
Geomagnetic longitude
Magnetic local time (*)
Alibag Kakioka Honolulu Boulder San Juan Hermanus
ABG KAK HON BOU SJG HER
18.6 36.2 21.3 40.1 18.4 34.4
72.9 140.2 202.0 254.8 293.9 19.2
9.9 26.6 21.5 48.7 29.1 33.7
145.8 207.8 268.6 319.0 5.2 82.7
18:55 15:04 11:08 7:21 4:12 23:48
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Table 2 List of analized geomagnetic storms [adapted from Echer et al., 2008]. Event
Date
Minimum Dst value (nT)
Associated interplanetary structure
October, 2003a October, 2003b November, 2003 July, 2004 November, 2004a November, 2004b May, 2005 August, 2005
October 29–30, 2003 October 30–31, 2003 November 20–21,2003 July 26–28, 2004 November 7–8,2004 November 9–11,2004 May 15–19, 2005 August 23–26, 2005
353 383 422 197 373 289 263 216
SH/MC SH sMC SH/MC sMC SH/MC sMC sMC
As seen in these figures, the gross behavior shown by the computed and the reported Sym-H curves is very similar. However, some discrepances are observed between both curves. These discrepances (of less than 2% around the peak of Dst) are probably due to the different data sources used for the computation, as well as to the latitude corrections applied by the WDC. The points given by vertical lines (labeled by capital letters) correspond to times of minimum Dst. The study of the LT variations of the horizontal geomagnetic field at low latitudes, DH , was performed in the following way. For each UT minute, the value of DH was computed at each station as a function of the corresponding LT, in the way described above. The symbols in Fig. 2 shows the respective values of DH for each of the six stations in the case of event of November 2003, at the instants marked as A and B in Fig. 1b. Then, the local time interval 00–24 was divided in a 6 min spaced grid and filled up with the values resulting from a Spline interpolation, using the IDL’s Spline subroutine with sigma ¼ 1. In order to keep the circular symmetry in this interpolation, the figures corresponding to the two stations closest to noon were repeated at both sides of local noon. The result of this interpolation for the event of November 2003, at instants A and B of Fig. 1b, are respectively shown by the solid and the dotted lines in Fig. 2. If N p is the number of points (UT-minutes) in a given interval, the above described process leads to a matrix of N p 240 elements for each of the considered intervals. These matrices are used to plot the contour maps, which give the disturbance in the horizontal component of the geomagnetic field in the LT–UT plane. Fig. 3a–h shows the LT-UT map for the time intervals listed in Table 2. The color bars at the right of each map give the respective intensity levels of the disturbance, with black and blue1 colors corresponding to the most negative values (largest depression). The points A; B, . . .of Fig. 1a–f are also indicated on abscissae of these maps. The horizontal lines give particular values of LT: Midnight (solid line at LT = 00), Dawn (red dashed line) and Dusk (dashed black line). In order to present in more detail the behavior of DH around the Dst peaks, only portions of the intervals given 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
in Table 2 were selected for the maps of Fig. 3a–h. As these contour maps show, there is a high degree of inhomogeneity in the H-depletion, which becomes masked by the Dst or Sym-H indices due to the averaging process used for their computation. In general, the maps present a minimum around dusk at the instant of minimum Dst, as a consequence of the existence of the partial ring current. In order to better visualize the LT distribution of the disturbance, the contour maps were also displayed in a polar representation as shown by Fig. 4a–h. In these polar maps, the radial coordinate is proportional to UT, with the starting point given by to , at the center, and the end at t1 , at the circle boundary. The azimuthal angle represents LT, with noon at the top (Sunward direction). The other circumferences plotted in the maps, which correspond to a given value of UT, are those for the points A; B, . . .of previous figures (white lines), and for six-hour intervals in UT (dotted black lines). The dusk depletion is clearly seen for the events July, 2004, and August, 2005 (Fig. 4d and h). For the event November, 2004a (Fig. 4e), the disturbance decays with longitude, from a maximum at dusk reaching a longitudinal extension of almost 360°, at the instant labeled by A (November 8, 6:00 UT). For November 2004b (Fig. 4f), the maximum depletion is located at the dusk-noon sector. For October, 2003b (Fig. 4b), there is a maximum depletion around dusk, but also a considerable disturbance around dawn at the time of minimum Dst, labeled by C (October 30, 22:00 UT). For October, 2003a (Fig. 4), a large depletion in the dusk-midnight sector occurs before the Dst peak, B, (October 30, 00:00 UT). Afterwards, during the recovery phase, large depletions are also observed in the dusk-noon sector as well as around dawn, although with less intensity. For the event of November, 2003, the maximum depletion regions are located mainly around midnight and to a lesser extension before noon. Finally, for the event of May, 2005, the maximum depletion region is located around midnight. 3. Statistical study of the LT distribution of the horizontal geomagnetic field disturbances As stated in the Introduction section, it is generally accepted that the largest disturbances in the H component of the geomagnetic field occur at the dusk and evening region (i. e. Clauer and McPherron, 1980 and references
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Fig. 1. From top to bottom: z-component of the interplanetary magnetic field in GSM coordinates [Szabo], ion dynamic pressure [Olgivie]; Dst index [WDC-Kyoto] Sym-H index according to WDC and computed in this paper. The points shown by vertical lines and labeled by capital letters correspond to times of minimum Dst (see the text).
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Fig. 2. Smoothed curve obtained for the H-variation as a function of LT, for the event of November 2003.
therein, Hakkinen et al., 2003; Soraas et al., 2006; Li et al., 2011). However, as discussed in the previous section, the LT–UT maps for the eight strong storms under consideration show that actual behavior of the geomagnetic disturbances for these cases may be rather complex. In order to show this in a quantitative way, we have performed the following statistical study. Since each of the geomagnetic storms has a different amplitude, the first step was to normalize the variations to the [0, 1] range. For this, we introduce an intensity parameter z given by: z¼
H H min H max H min
ð1Þ
where H max and H min are, respectively, the maximum and the minimum values of the horizontal geomagnetic field for the storm under consideration, and H the value of the computed field at a given point in the LT–UT plane. According to Eq. (1), z ¼ 0 means H ¼ H min , or maximum disturbance, while z ¼ 1. corresponds to H ¼ H max , no disturbance. For each of the analized events, the LT axis is divided in one hour intervals. Since the points are 6 min spaced in LT, each of these intervals contains 10 points. Furthermore, within these 1 h-LT intervals, 10 levels of the intensity parameter, z, were considered (0:0 6 z < 0:1, 0:1 6 z < 0:2, . . .). In this way, 10 10 cells are obtained for each of the considered events. Counting the number of percent occurrences in each of these cells, a picture of
the contribution to the geomagnetic-disturbance intensity as a function of the LT is obtained. Figs. 5a-h show the result of the above described statistics for each of the UT intervals given in Figs. 3a–h and 4a–h, for the two lowest levels of intensity parameter z; 0:0 6 z < 0:1 and 0:1 6 z < 0:2, which correspond to maximum disturbance. The distribution of the depletion intensity shown by the histogram of each event is in agreement with the conclusions reached in previous section although, of course, these type of graphics do not show the temporal evolution of the disturbance. In Fig. 6 the averages obtained from Figs. 5a–h, as a function of LT, are given also in a percent scale. The vertical lines in this plot, give the standard deviation from the average derived from the eight values corresponding to each 1 h-LT interval. As seen in this figure, in spite of the large error lines, the predominance of the dusk maximum depletion is observed, together with a less important contribution around midnight. Fig. 7 shows the result of this statistics when only the main phases of the storms are considered, also for 0:0 6 z < 0:1 and 0:1 6 z < 0:2. The dusk peak appears to be shifted towards the dusk-noon sector, showing that the partial ring current is mainly situated at this sector during the main phase of the storms. On the other hand, indications of disturbances around midnight and the dawnnoon sector also appear. Furthermore, a finer study was done by separating the events according to the associated interplanetary structure assigned in Table 2. Two groups of events were formed. One is for storms driven by magnetic clouds with a shock (sMC cases), and is composed composed by four events. The other for is the storms driven by sheath regions followed by a magnetic cloud (SH/MC cases) and is composed of 3 events. Since the event October, 2003b, does not correspond to any of these groups, it was taken apart from this study. The statistics based on these two subsets are respectively shown in Figs. 8a and 9b, also for the largest levels of disturbance (0:0 6 z < 0:1 and 0:1 6 z < 0:2). The vertical lines give the standard deviations at each local time for the samples of four and three events, respectively. Although clearly the results based on these small samples can not be conclusive, some differences in the distributions seem to appear. For the subset associated to sMC structures (Fig. 8a), with four out of eight events, the distribution looks very similar to that of all the events (Fig. 7). On the other hand, for the SH/MC related subset (Fig. 9b) the disturbances are mostly localized at the dusk-noon sector and, to a less extend, also at the dawn-noon sector but with no incidences around midnight. As a matter of fact, this conclusion should be confirmed by a study based on a larger sample of intense geomagnetic storms. 4. Discussion and conclusions The study of the LT–UT distribution of the horizontal geomagnetic field component for the eight intense geomag-
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12
Dawn
00
Dusk
06
12
18
00
06
UT
12
18
00
06
UT
12
Dawn
00
Dusk
12
06
18
00
12
06
UT
12
00
06
12
18
00
06
UT
12
Dawn
00
Dusk
18
00
06
12
18
UT
12
06
12
18
00
06
12
18
00
06
12
18
UT
12
Dawn
00
Dusk
09
12
15
18
12
18
00
06
12
18
UT
Fig. 3. LT–UT maps for the geomagnetic storms listed in Table 2.
netic storms of the descending phase of solar cycle #23, shows that the disturbance is mostly concentrated on the midnight-dusk-noon sectors, as would be expected from the related literature (i. e. Clauer and McPherron, 1980 and references therein, Soraas et al., 2006; Li et al., 2011). Nevertheless, the differences observed in the LT
distribution of the geomagnetic disturbance from case to case lead to the conclusion that there is not a clear recurrent pattern for this distribution, for the considered set of events. It can be said that in average for the eight considered events, the maximum disturbance regions are mainly located at the dusk-noon sector, with some incidence also
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Fig. 4. Polar representation of the LT–UT maps for selected intervals of the events in Table 2.
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Fig. 5. Intensity of H-depletion as a function of LT, as described by intensity parameter z (see the text for definition of z). The histograms give the percent occurrences during 1 h-LT, for 0:0 < z < 0:1 and 0:1 < z < 0:2, for the UT intervals given in Figs. 3a–h and 4a–h.
Fig. 6. Average of percent occurrences in the histograms of Figs. 5a–h. The vertical lines show the standard deviation derived from those 8 histograms.
around midnight. A similar result is obtained when the analysis is restricted to the main phases of the storms although, in this case, the dusk peak looks lightly shifted to the noon side and a lower peak in the dawn-noon sector is shown up. As an intent to see if there is an influence of the interplanetary origin of the storms in the resulting LT distribution of the geomagnetic disturbance, the events were classified according to their interplanetary origin in two groups. One for the storms driven by magnetic cloud with
a shock (sMC), and the other for storms driven by sheath regions followed by a magnetic cloud (SH/MC). For the first subset (with four out of eight cases), the behavior of the distribution during the main phase of the storms looks very similar to that of the total set, while for the second subset (with three out of eight cases), the peak around midnight is not observed. Since the size of the considered samples is too small, the difference in the distribution of the geomagnetic disturbance according to the origin of the storms found
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Fig. 7. Similar to Fig. 6 but only considering the main phase of each geomagnetic storms of Table 2.
Fig. 8. Similar to Fig. 7 for geomagnetic storms associated to sMC interplanetary structures.
Fig. 9. Like Fig. 8a for SH/MC events.
here may not be conclusive. Therefore, we are working on the the extension of the present analysis to the whole solar cycle #23 in order to find out if this distinction really exists. Furthermore, since there are some indications that storms driven by sMC develop substorms, while the the SH/MC do not, a complementary analysis will be undertaken in order to find out a possible correlation between the LT distribution of the geomagnetic disturbance and the existence or not of substorms along with each storm. Acknowledgements The authors would like to thank “Fundo de Desenvolvimento Cientı´fico e Tecnolo´gico” and the “Conselho
Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico” (Grants PQ-342734/2008-2 and PQ-300321/2005-8) of Brazil. The data for solar wind parameters were provided by K. Ogilvie and A. Szabo, at NASA , GSFC and CDAWeb. References Baumjohann, W., Treumann, R.A. Basic Space Plasma Physics. Imperial College Press, London, 1997. Clauer, C.R., McPherron, R.L. Mapping the local time development of magnetospheric substorms using mid-latitude magnetic observations. J. Geophys. Res. 79, 2811–2820, http://dx.doi.org/10.1029/ JA079i019p02811, 1974. Clauer, C.R., McPherron, R.L. The relative importance of the interplanetary electric field and magnetospheric substorms on partial ring current development. J. Geophys. Res., 6747–6759, http://dx.doi.org/ 10.1029/JA085iA12p06747, 1980.
A.L. Clu´a de Gonzalez, W.D. Gonzalez / Advances in Space Research 51 (2013) 1924–1933 Clu´a de Gonzalez, A.L., Mendes da Costa, A., Gonzalez, W.D. Ring current space time inhomogeneities observed in association with intense geomagnetic storms. Geofı´sica Internacional 43, 205–215, 2004. Echer, E., Gonzalez, W.D., Tsurutan, B.T., Clu´a de Gonzalez, A.L. Interplanetary conditions causing intense geomagnetic storms (Dst 6 100 nT) during solar cycle 23 (1996–2006). J. Geophys. Res. 113, 1–6, http://dx.doi.org/10.1029/2007JA012744. A05221, 2008. Frank, L.A. On the extraterrestrial ring current during geomagnetic storms. J. Geophys. Res. 72, 3753–3767, http://dx.doi.org/10.1029/ JZ072i015p03753, 1967. Gonzalez, W.D., Joselyn, J.A., Kamide, Y., Kroehl, H.W., Rostoker, G., Tsurutani, B.T., Vasyliunas, V.M. What is a geomagnetic storm? J. Geophys. Res. 99, 5771–5792, 1994. Hakkinen, L.V.T., Pulkkinen, T.I., Pirjola, R.J., Nevanlinna, H., Tanskanen, E.I., Turner, N.E. Seasonal and diurnal variation of geomagnetic activity: revised dst versus external drivers. J. Geophys. Res. 108, 2003. Iyemori, T. Storm-time magnetospheric currents inferred from midlatitude geomagnetic field variations. J. Geomag. Geolectr. 42, 1249, 1990. Kamide, Y., Baumjohann, W., Daglis, I.A., Gonzalez, W.D., Grande, M., Joselyn, J.A., McPherron, R., Phillips, J., Reeves, E.G.D., Rostoker, G., Sharma, A.S., Singer, H.J., Tsurutani, B.T., Vasyliunas, V.M. Current understanding of magnetic storms: storm/substorm relationship, invited review. J. Geophys. Res. 103, 17705–17728, 1998. Kozyra, J., Liemohn, M. Ring current energy input and decay. Space Sci. Rev. 109, 105–131, http://dx.doi.org/10.1023/B:SPAC.0000007516. 10433.ad, 2003. Kyoto University: Data analysis center for geomagnetism and space magnetism. High-Time Resolution Geomagnetic Indices.
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Le, G., Russell, C.T., Takahashi, K. Morphology of the ring current derived from magnetic field observations. Ann. Geophys. 22, 1267– 1295, 2004. Li, H., Wang, C., Kan, J.R. Contribution of the partial ring current to the symh index during magnetic storms. J. Geophys. Res. 116, 1–12, 2011. Liu, S., Chen, M.W., Roeder, J.L., Lyons, L.R., Schulz, M. Relative contribution of electrons to the stormtime total ring current energy content. Geophys. Res. Lett. 32, 1–5, http://dx.doi.org/10.1029/ 2004GL021672, 2005. McPherron, R.L. The role of substorms in the generation of magnetic storms, in: Tsurutani, B.T., Gonzalez, W.D., Kamide, Y., Arballo, J.K. (Eds.), Magnetic Storms, Geophys. Monogr. Ser., vol. 98. Amer. Geophys. Un., Washington, pp. 131–147, 1997. Potemra, T.A. Birkeland currents in the earth’s magnetosphere. Astrophys. Space Sci. 144, 155–169, 1988. Soraas, F., Sorbo, M., Aarsnes, K., Evans, D.S. Ring current behavior inferred from ground magnetic and space observations, in: Tsurutani, B., McPherron, R., Gonzalez, W., Lu, G., Sobral, J.H.A., Gopalswamy, N. (Eds.), Recurrent Magnetic Storms: Corotating Solar Wind Streams, Geophys. Monogr. Ser., vol. 167. Amer. Geophys. Un., Washington, pp. 85–95, 2006. Space physics data facility – coordinated data analysis web (cdaweb).