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«Longitudinal Expansion of Compressional Disturbances at Middle Latitudes on Ground during CME Events T. Nikolova and D. Teodosiev Space Research ...»

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WDS'07 Proceedings of Contributed Papers, Part II, 13–21, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS

Longitudinal Expansion of Compressional Disturbances

at Middle Latitudes on Ground during CME Events

T. Nikolova and D. Teodosiev

Space Research Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria.

P. Nenovski and I. Blagoeva

Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria.

Abstract. Two strong magnetic storms observed during CME events in November 2001 and 2003 are considered in this study. Magnetic field data from L’Aquila (Italy), Bay St Louis and Boulder (USA), Alibag (India) and other observatories are analyzed where amplitude, possible time lag and expansion characteristics of the geomagnetic field variations among observatories at middle and low latitudes are examined. Special attention to the long-range interactions of the magnetic field disturbances during strong geomagnetic storms is paid. A strong long-range interaction (Brownian motion) of the geomagnetic disturbances is discovered and the differences that emerge are assigned to the observed time lag between magnetic observatories.

Introduction During the main phase of the geomagnetic storms a longitudinal asymmetry in the disturbance of the Earth’s magnetic field on ground at low and middle latitudes emerges. This asymmetry is characterized by depleted magnitudes in the horizontal, H-component and centered in the dusk and evening sectors (Iyemori and Rao, 1996; Ebihara and Ejiri, 2000; Gonzalez et al., 2004). It is generally accepted that enhanced magnetospheric convection caused by long lasting, strong southward interplanetary magnetic field (IMF) has principal role in the ring current intensification (Gonzalez et al., 1994; Tsurutani and Gonzalez, 1997) and it is responsible for the depletion in the H component.

Ion flow along open trajectories in the dusk and afternoon sectors before encountering the dayside magnetopause are responsible for producing highly asymmetric ring current (Liemohn et al, 2001).

The asymmetric ring current extends from evening to noon, up to the early morning during moderate solar wind conditions (Francia et al., 2004).

Other studies have revealed close connection of the ring current intensity and the plasma sheet density (Daglis and Kozyra, 2002). Analyzing the development of geomagnetic storms Kozyra (2001) has found that plasma sheet density drops significantly limit the energy input in the inner part of the Earth’s magnetosphere. It is suggested that plasma sheet drops just follow the occurrence of major substorms during these storms.

Study of the geomagnetic variations at equatorial and middle latitude regions is thus very important in terms of understanding the storm-substorm development mechanisms. The asymmetric ring current properties have been studied in detail considering spatial and temporal (UT vs LT) development of the magnetic field disturbances observed at low and middle magnetic observatories (Francia et al., 2004).

In the present study the temporal and spatial properties of magnetic disturbances occurred during extreme coronal mass ejection-Earth’s magnetosphere interaction events are studied from another point of view. Dynamic properties of the H-depletion caused by asymmetric ring current enhancements can be studied by Detrended Fluctuation Analysis (DFA) (Kantelhardt et al., 2002).

This approach can delineate long-range correlations in the geomagnetic disturbances at different magnetic observatories. The DFA scaling exponent could thus enlighten possible differences of dynamic properties of the magnetic storm in longitude and latitude and be a measure of the time evolution of the storm magnetic field disturbances. Geomagnetic field signatures from the digital records of time resolutions of one minutes provide a good estimate of the storm energy transfer process from interplanetary to auroral and/or to the low-latitude ionosphere.

The present analysis is based on nine low and middle latitude geomagnetic observatory data.

These observatories are listed in Table with the respective geodetic and magnetic coordinates. Two events of strong geomagnetic activity were selected for years 2001 and 2003. The storms are on 4-7

NIKOLOVA ET AL.: EXPANSION OF COMPRESSIONAL DISTURBANCES

November 2001 and on 19-21 November 2003.

Solar wind (SW) and interplanetary magnetic field (IMF) data are taken from ACE satellite. Two cases of strong geomagnetic storms caused by coronal mass ejection (CME) events were examined in detail.

Observations on 4–7 November 2001 Figure 1 presents data from ACE satellite for an event of strong geomagnetic storm occurred on 4-7 November 2001. On Figure 1 the following parameters are given (from upper to bottom): total IMF magnetic field B, Bx, By, Bz (in GSM system), solar wind proton number density (n/cc), and solar wind velocity in GSE system): Vx,Vy, and Vz. The full halo CME ejected on 4th November 2001 compresses the Earth’s magnetosphere on 6th November at 01:50 UT. The IMF turned southward and the Interplanetary Magnetic Field (IMF) Bz magnitude reaches −76nT at maximum main phase. The sources of this geomagnetic storm at Earth are strong dawn-to-dusk electric fields associated with the passage of southward directed IMF. The southward IMF Bz (of magnitude 5 nT) persists for a period up to 3 hours.





Fig. 1. Figure presents data from ACE satellite for an event of strong geomagnetic storm on 4-6 November 2001. From top to bottom: total IMF magnetic field B, Bx, By, Bz (in GSM system), solar wind proton number density (n/cc), and solar wind velocity in GSE system): Vx,Vy, and Vz. The CME ejected on 4-th compresses the Earth’s magnetosphere on 6th November at 01:50 UT. The Bz magnitude reaches -76 nT at maximum main phase.

Observations on November 2003 Another very strong geomagnetic storm occurred on 19-21 November 2003. Figure 2 demonstrates that the solar wind plasma parameters (V, T и N) do not differ considerably during this

month, while the IMF magnitude B and Bz component are changed considerably on 20-21 November:

B 50 nT and Bz is up to –50 nT and appear as a main source of this exceptional geomagnetic storm.

NIKOLOVA ET AL.: EXPANSION OF COMPRESSIONAL DISTURBANCES

The solar wind parameter behavior suggests that the disturbance on 20 November refers to magnetic cloud, the plasma β parameter diminishes and the solar wind density variation on 21 November is a consequence of plasma rarefaction following the fast and dense solar wind [Usmanov et al., 2000], Fig. 2. Figure presents data from ACE satellite for an event of very strong geomagnetic activity on November 2003. From top to bottom: total IMF magnetic field B, Bx, By, Bz (in GSM system), solar wind proton number density (n/cc), and solar wind velocity in GSE system): Vx,Vy, and Vz. The CME compresses the Earth’s magnetosphere on 20th November and the storm started between 07:00-08:00 UT. The solar wind velocity suddenly increases from 450 to 700 km/s and more, the IMF magnitude B increases (due to IMF By increase), the IMF Bz component gets firstly positive values (20 nT), it occurs on 20 Nov before 12 UT. After 12 UT the IMF Bz component becomes negative (reaches values of -50 nT) for more than 12 hours.

Ground-based geomagnetic observations One minute digital data from World Data Center for Geomagnetism, Kyoto for magnetic field data set are taken for consideration. Table 1 yields the geodetic and geomagnetic coordinates of the magnetic observatories whose data measurements are used for comparison. A row of low-latitude (0°° degrees) and mid-latitude (at ~ 40° degrees) magnetic observatories equispaced in longitude were chosen.

For comparison, three specific times were designated: the storm onset, the peak of the storm (corresponding to the amplitude minimum) and secondary peak (if exists).

The next figure (Figure 3) depicts variations of the Earth’s magnetic field amplitude B during another strong magnetic storm occurred on 4-7 November 2001. On this figure the disturbances of the geomagnetic field magnitude respectively for each from six mid and low latitude observatories listed in Table are given. The vertical thin lines show the mentioned specific times designated by A, B and C and correspond to the storm onset (A), the secondary peak (B) and the peak of the storm, respectively

NIKOLOVA ET AL.: EXPANSION OF COMPRESSIONAL DISTURBANCES

(similar specific times have been used by Clua de Gonzalez et al, 2004). At the storm onset (line A) an initial (visible) jump in magnitude that occurs simultaneously at all observatories is observed. After this jump the Earth’s magnetic field magnitude decreases, the secondary peak (line B) is not always visible and after this the peak of storm (line C) emerges. These peaks go practically simultaneously irrespectively at all six observatories.

–  –  –

Fig.3. The figure depicts variations of the Earth’s magnetic field amplitude B during strong magnetic storm occurred on 4-7 November 2001. The six plots give respectively the disturbances of the total geomagnetic field for each of the six mid and low latitude observatories listed in Table 1. The vertical thin lines show specific times designated by A, B and C and correspond to the storm onset (A), the secondary peak (B) and the peak of the storm, respectively. After an initial (visible) increase in magnitude that occurs simultaneously at all observatories, the Earth’s magnetic field decrease at L’Aquila and Bay St. Louis (with ~ 120 degrees difference in longitude) goes practically simultaneously irrespectively.

NIKOLOVA ET AL.: EXPANSION OF COMPRESSIONAL DISTURBANCES

Fig. 4. The figure depicts variations of the Earth’s magnetic field amplitude B during strong magnetic storm occurred on 19-21 November 2003. The nine plots give the disturbances of the total geomagnetic field for each of the nine mid and low latitude observatories listed in Table 1. The vertical thin lines show again specific times. Line A (as in Fig. 3) corresponds to the storm onset that coincides for all observatories; line B corresponds to the peak of storm that occurs at BSL and BOU, while line C corresponds to the peak of the storm that occurs at AAE and ABG observatories. Thus, the Earth’s magnetic field decrease at maximum main phase occurs at different time at different observatories. A maximum lag of 125 minutes emerges. The peak at the Honolulu observatory lies within this lag.

Figure 4 depicts variations of the Earth’s magnetic field amplitude B during very strong magnetic storm on 20-21 November 2003. The nine plots give the disturbances of the total geomagnetic field for each of the nine middle and low latitude observatories listed in Table. The vertical thin lines show again the specific times. Line A (as in Fig. 3) corresponds to the storm onset that again coincides for all observatories; line B corresponds to the main peak of storm that occurs at BSL and BOU observatories, while line C corresponds to the peak of the storm that occurs at AAE and ABG observatories. In Boulder the magnetic field amplitude decreases to −323.5 nT, in Alibag – to −557.3 nT. A maximum lag of 125 minutes emerges between BOU and ABG observatories. Within this largest lag is the disturbance peak at the Honolulu observatory. Note that during this very strong geomagnetic storm, the Earth’s magnetic field decrease at maximum main phase occurs at different time at different observatories.

In order to understand further the mechanism of the observed temporal and spatial characteristics of the geomagnetic disturbance penetration in longitude, Detrended Fluctuation Analysis (DFA) approach (that yields the fractal dimension D) is applied to the geomagnetic field data set. DFA is a well-established method for determining data scaling (self similarity) behavior in the presence of possible trends without knowing their origin and shape (Kantelhardt et al, 2001). In difference to conventional, e.g. power spectrum analysis, the DFA permits detection of intrinsic dynamical features, e.g. long-range correlations, embedded in non-stationary time series. For long-range correlated signals the power spectral density P would behave as a power-law of the frequency f, that satisfies P(f) ~ f-β.

NIKOLOVA ET AL.: EXPANSION OF COMPRESSIONAL DISTURBANCES

Thus its slope should be constant and usually denoted by β index. For example, the well-known flicker noise, also known as 1/f-like noise (β = 1), is specific characteristic and presents a balance between the randomness of white noise (β = 0) and the much smoother Brownian motion (β = 2). The exponent β is related to the DFA scaling index α by relation β = 2α −1. Thus, the value of the DFA scaling index α resulting from a least-square fit to a straight line, reveals a presence, or not, of long-range correlations in time series under study. In particular, the case α = 1/2 represents the absence of long-range correlations. This analysis applied to geomagnetic data is described in paper by Nenovski et al, submitted to the WDS ’07 proceeding). When a 1/f-like noise appears the DFA scaling index α is around 1 (β = 1). Should DFA index α becomes greater than 1, there is a non-stationary, random walk like motion. When α is equal to 3/2, a Brownian motion occurs.

Figure 5 depicts the fluctuation functions obtained from the DFA applied to geomagnetic disturbances on 19-23 November 2003 from two observatories: Bay St Louis (USA) and L’Aquila (Italy). Different values of the DFA index α (α = 3−D) are found. At BSL the DFA index α is equal to 1.47, at AQU – 1.14. This means an emergence of different long-range interactions at both the observatories during the strong magnetic storm event occurred on 19-21 November 2003. Note that in the case of magnetic storm (occurred on 4-7 November 2001) the DFA scaling indices at BSL and AQU are practically equal (DFA index α = 1.33-1.37, D = 1.63-1.67).

In order to verify the proper fractal dimension of the compressive magnetic disturbances during magnetic storm on 20-21 November 2003, data set of additional observatories are analyzed (Fig. 6).



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