Variability of Solar Wind Dynamic Pressure with Solar Wind Parameters During Intense and Severe Storms

Transcription

1 The African Review of Physics (213) 8: Variability of Solar Wind Dynamic Pressure with Solar Wind Parameters During Intense and Severe Storms B. O. Adebesin 1,*, S. O. Ikubanni 1, J. S. Kayode 1 and B. J. Adekoya 2 1 Department of Physics, College of Science & Engineering, Landmark University, Omu Aran, Kwara State, Nigeria 2 Department of Physics, Olabisi Onabanjo University, Ago-Iwoye, Ogun State, Nigeria The solar wind dynamic pressure (P) variability with plasma flow speed (V) and the Interplanetary Magnetic Field (IMF) Bz for the storm s initial and main phases, as well as during peak plasma flow speed value periods, have been presented. Nine intense and eleven severe storms were used for our analysis. The strong P versus V relationship during the storm s initial phase was linked to the accelerated ring current build-up. The weak P versus V relationship during storms main phase was attributed to ring current energy losses, which may have originated from different sources. The solar wind dynamic pressure have a strong relationship with the southward IMF Bz during the main phase of a storm, typically when the plasma flow speed value is very large, exceeding the km/s value, and the Dst < -2 nt. An enhancement in P during enhanced solar wind speed flow and under a steady flow southward Bz was observed, which always led to intense storm. 1. Introduction Geomagnetic storms are characterized by a sudden form of enhancement in the ring current that circulates around the Earth. According to Gonzalez et al. ([1] and references therein), this current mode of transportation is O +, protons, and electrons in the 1-3 kev energy range when drifting. It is well known that the energy transfer mechanism from the solar wind to the magnetosphere for magnetic storms is magnetic reconnection between the interplanetary magnetic field (IMF) and Earth s magnetic field, where the interplanetary dawn-dusk electric field is given by V x Bz [1], V is the solar wind speed and Bz is the IMF southward component. However, Gonzalez et al. [2] had raised several questions as to the detailed causes of very intense magnetic storms. These storms are with unusually high velocities or the ones with unusual high magnetic fields, or both. This suggests that both Bz and the solar wind speed are very important in the understanding of the dynamics of magnetic storms. Gonzalez and Tsurutani [3] have shown that a southward IMF < - 1 nt (> mv/m) for T > 3 hours is typically needed for the creation of an intense (Dst < -1 nt) magnetic storm. On the other hand, the earth s magnetosphere is a highly dynamic structure that responds quite dramatically to changes in the dynamic pressure of solar wind and the orientation of the IMF. These * f_adebesin@yahoo.co.uk variations of solar wind dynamic pressure are known to affect the energy and momentum transfer from the solar wind to the magnetosphereionosphere system. In this respect, the rise time and the duration of the pressure perturbation are two important factors. For short rise times strong transient perturbations are observed both in the magnetosphere and in the ionosphere ([4] and references therein). Subsequently, the duration of a pressure perturbation determines whether the effects will be localized or global. If the duration is long enough to engulf most of the magnetosphere in the solar wind region of enhanced/reduced pressure, the pressure variation causes a typical global increase of the geomagnetic field strength measured at the geostationary orbit, and on the ground at equatorial and middle latitudes. The present paper is therefore aimed at investigating the variability of the dynamic pressure with the IMF) and plasma flow speed during the initial and main phases of geomagnetic storms, as well as during peak plasma flow velocities under intense and severe (very intense) storm conditions. Adebesin [] had investigated the roles played by both interplanetary and geomagnetic parameters in the generation of intense (-2 nt peak Dst < -1 nt) and very intense (peak Dst < -2 nt) classes of storms. He argued that all very intense storms are likely to have a plasma flow speed greater than km/s within the storm interval, but not all flow speeds greater than km/s are very intense storms.

2 The African Review of Physics (213) 8:19 12 Subsequently, this paper is also set to establish these with the dynamic pressure of the solar wind. The basic classifications of magnetic storms on the basis of the Dst index using the measurements (after [6]) are highlighted in Table 1. It was shown that very-intense (i.e., severe and great) storms take just about % of the list, weak storms (44%), moderate (32%) and strong (i.e., intense) 19%. Table 1: Classification of magnetic storms on the basis of the Dst index using the measurements (after [6]). Class Number % Dst Range (nt) Weak Moderate Strong (i.e., intense) Severe (very-intense) Great 6 1 < Data, Plots and Results This paper investigates the variability of solar wind dynamic pressure on the plasma flow speed IMF Bz during the initial and main phases of geomagnetic storms, as well as during peak plasma flow speed values. Twenty magnetic storms were considered. Nine of these were intense, while the remaining eleven were severe (very intense). We also ascertain the variability influence of the dynamic pressure for both classes of storms. The interplanetary and solar wind parameters data used are the 1-hour averages from NASA NSSDC OMNI data set. These parameters include hourly resolutions of the IMF Bz, solar wind plasma and some solar and geomagnetic activity indices. The Pearson correlation coefficient was used for the interpretation. Two of the storm events are double steps (i.e., 3- Oct., 21 and 2 Oct., 1989 storms) thereby having two minimum downward peak values for Dst (nt). The corresponding storm dates and their interplanetary and solar wind parameters values are highlighted in Table 2 (a and b). The plot interval spans five days (the storm day, two days before and two days after the storm). On the whole, only four storm dates are discussed extensively and separately in this section because of space consideration two intense storm dates (Figs. 2 and 4), and two severe storm dates (Figs. 1 and 3). The other storm observational values are also summarized in Table 2 (a and b). The vertical line PP across the entire plots marks Storm Sudden Commencement (SSC), line QQ marks point of minimum peak Dst value, and RR marks point of maximum flow speed (i.e., Vmax). Dst(nT) Flow speed V (km/s) Dynamic Pressure (npa) P Q Electric field (mv/m) P' Q' R' Fig.1: Plot of solar wind and magnetic parameters for 4-8 April,2. R

3 Table 2a: Corresponding intense storm dates with interplanetary and solar wind parameters values (Note: N/A stands for unavailability of data during the period). STORM ONSET VALUES MAIN PHASE VALUES AT PEAK V (km/s) VALUES Storm Date Plot interval Peak Dst (nt) Step P (npa) V (km/s) P (npa) V (km/s) Dst (nt) P (npa) V (km/s) Dst (nt) 1-Oct Sept-3 Oct -176 Single N/A Apr Apr -187 Single Oct Oct -148 Double Sep Sept -181 Single Oct Oct -268 Double Mar Mar Single N/A N/A Sept Sept Single Oct Oct Single Sept Sept-1 Oct -224 Single Dec Dec -146 Single Table 2b: Same as in Table 2a, but for severe storms. STORM ONSET VALUES MAIN PHASE VALUES AT PEAK V (km/s) VALUES Storm Date Plot interval Peak Dst (nt) Step P (npa) V (km/s) P (npa) V (km/s) Dst (nt) P (npa) V (km/s) Dst (nt) 6-Apr Apr -287 Single Nov Nov -34 Single N/A N/A. N/A N/A N/A Apr Apr -271 Single Jul July -32 Single N/A N/A May May -263 Single. 1. N/A N/A Oct Oct -22 Double N/A N/A N/A N/A N/A Nov Nov Single Oct Oct Single N/A N/A N/A Mar Mar-2 Apr Single Oct Oct- 1 Nov -33 Single N/A N/A Jul July -31 Single The African Review of Physics (213) 8:19 121

4 The African Review of Physics (213) 8: April, 2 storm Fig. 1 depicts the solar and geomagnetic observations of the storm of 6 April 2, spanning 4-8 April. This storm originates from an interplanetary shock wave, which is caused by solar Coronal Mass Ejection (CME) as signified on the plot of the solar wind flow speed on panel three of Fig. 3. The solar wind shoots up from about 37 km/s around 14UT of 6 April to almost 6 km/s around 19UT of the same day. Immediately after the shock, as represented by the vertical line PP, the interplanetary magnetic field Bz turns southward and thereafter intensifies because of a compression of the sheath region, maintaining the posture for close to 16 hours. This makes the Dst reach its minimum peak value of approximately - 287nT. However, no ejecta structure is observed after the shock, indicating that perhaps the probing satellite did not actually intercept the middle of the structure (e.g., [7]). It should be noted that sudden outburst of the solar wind dynamic pressure aggravated the rising in the value of the electric field, which rose to its maximum level around this time. This also coincides with the Bz symmetry pointing southward as well as the increase in plasma flow speed. However, as the B z (southward pointed) value reaches its negative downward peak, the associated geomagnetic activity enhances. 22 September, 1999 magnetic activity The response plot to this storm is highlighted in Fig. 2. It extends from 2-24 September. The Bz plot shows a southward turning from UT on 21 September to around 2UT on 22 September to a first minimum value of -7.nT indicating that the IMF has experienced over 2 hours of southward turning. Thereafter, it experiences northward turning up till around 17UT on 22 September. However, the Storm Sudden Commencement (SSC), which is indicative of the arrival of a shock in the interplanetary medium, must have triggered the depression of Dst beginning from 18UT and subsequently resulted in the sharp southward turning of Bz at 18UT of the same day to a minimum peak value of -1nT. At about 2UT, Bz had rotated northward again attaining a value of -4nT, and then continued in the northward direction again throughout 23 and 24. The plasma flow pressure plot had no effect until around 1UT on 21 September, rising to a value of ~18. npa. This increase, according to Adebesin ([] and references therein), signifies the arrival of a shock in the interplanetary medium. The flow speed plot shows a moderate speed stream from UT on 21 September to 12UT 22 on September. The stream peaked with a value of 4km/s at 13UT on 22 September. Subsequently, a steady value was maintained and then increased to its second peak value of 9km/s at 2UT on 22 September. It is obvious that throughout 21 September till 18UT on 22 September, the flow speed never attained the km/s, in which case it could never have met the criterion of fast solar winds. However, it is worth mentioning that a geomagnetic storm could still occur at the solar wind speed shown in the plot. The Dst plot signifies that beginning from 18UT on 22 September Dst was depressed sharply to a downward minimum peak value of - 18nT around 19UT. Thereafter, Dst recovers rather gradually throughout 23 September. The dusk-ward electric field plot shows that starting from UT on 21 September through 1UT on 22 September, the electric fields were less than.mv/m. but as from 19:UT on 22 September, it began a gradual increase getting to a value of 1.m V/m two hours later. Hence, these electric field conditions which gave Bz > 1nT are indicative of an intense storm. It was also observed that the dynamic pressure experiences its peak value at the instance the interplanetary medium experiences shock through the ring current intensification. This also coincides with the point of maximum flow speed value, as indicated by the line RR. 8 September, 22 storm The response plot for 8 September, 22 was as shown in Fig. 3. The plot spans 6-1 September. Observe that around 17UT on 7 September, when the IMF Bz recorded its minimum peak value of -22nT, the plasma density, plasma flow speed and the solar wind dynamic pressure recorded their respective first enhancement values, indicating a shock before the storms sudden commencement. The abrupt northward Bz rotation at 22UTon 8 September is reflective of the recovery state of the storm through 9 September. Note that the electric field value is not up to. mv/m, up till 16:UT on 7 September, confirming that Bz < 1nT. However, beyond this hour, the electric field began to increase attaining a peak value of 12.m V/m at 17UT, decrease a while then begins to rise again getting to another significant value of 8. mv/m on 7 September. The first electric field peak value coincides with a rise in plasma flow speed up to a value of 8km/s. An increase in the solar wind/magnetosphere coupling efficiency was

5 The African Review of Physics (213) 8: projected immediately after an abrupt increase in solar wind dynamic pressure during steady southward IMF configuration. Bz orientation was southward for more than 24 hours. The efficiency increase was observed even when the solar wind electric field was reduced after the pressure front, indicating that the sudden increase in pressure enhanced magnetospheric convection and partially balanced the effects of the decreasing electric field. Dst(nT) Flow speed V (km/s) Dynamic Pressure (npa) PRQ Dst(nT) Flow speed V (km/s) Dynamic Pressure (npa) Electric field (mv/m) P R Q P' R' Q' Fig.3: Response plot for 6-1 September, 22. Electric field (mv/m) P'R'Q' Fig.2: Interplanetary and solar wind parameters response plot for 2-24 September, Dst recorded its first minimum value of -142nT around 18UT on 7 September, rotates northward shortly, and then southward again reaching a minimum peak value of -18nT around UT on 8 September. Thereafter, it begins to recover until around 21UT when a sudden decrease was noticed again to a value of -78nT and then continued with the recovery process. The two minima peak values observed is indicative of a magnetic shock in the interplanetary medium. This is so because these two points coincide with a significant southward turning of Bz at this exact period. However, the occurrence of a new major particle injection leads to a further development of the ring current with Dst index increasing for the

6 The African Review of Physics (213) 8: second time. We may thus assume the presence of both sheath field and the magnetic cloud field, and argue that both the sheath field and the cloud field have the proper orientation, and there is magnetic reconnection from both phenomena resulting in a double storm. This is so, as it is most likely that the first step of the storm was caused by the sheath Bz, while the second was from the second magnetic cloud field July, 2 Storm The response plots of geomagnetic and interplanetary parameters for July 2 are illustrated in Fig. 4. According to Dal Lago et al. [7], this storm event is regarded as the Bastilla event, in which case it consists of an interplanetary shock driven by a magnetic cloud, whose intense magnetic field rotates from south to north smoothly. While the Bz is pointing southward, it causes a very intense fall in the Dst value, reaching its minimum peak value of -31 nt. Immediately after the shock, there was a sudden rise in the plasma dynamic pressure, as indicated by line PP, as well as an increase in the flow speed to ~88 km/s. On 13 July, the solar wind shows a fairly flat, although high, speed. The dynamic pressure goes from over 6 npa near 18UT to around 2 npa towards the end of the day. On 14 July, the plasma parameters stay fairly flat until about 13UT when there is a clear forward shock with a speed increase to over 7 km/s. This is followed by a sudden plasma pressure increase near 17UT. At the end of the day, the dynamic pressure is around 17 npa. On 1 July, there is a declining speed until a large forward shock arrives near 14 UT. This shock is clearly identified by the abrupt and strong speed increase from about 6 km/s to over 9 km/s. The dynamic pressure reaches about 43 npa. At 16UT there is a further increase in the speed. Unfortunately, there is a tracking gap after 2 UT. On 16 July the speed continues to be quite high with an interesting and substantial decrease occurring at about 14UT and lasting until 21UT. During this decrease the density also falls, resulting in a dynamic pressure drop to below 1 npa. The pressure afterwards is about 6. npa. Dst(nT) Flow speed V (km/s) Dynamic Pressure (npa) Electric field(mv/m) P R Q P' R' Q' Fig.4: Response of interplanetary and solar wind parameters for 13-17July, Discussion Solar wind dynamic pressure enhancements can significantly compress the Earth s magnetosphere and lead to global changes in the magnetospheric and ionospheric currents [8]. Recently, Shi et al. [9] concluded that pressure enhancements also cause further intensification of the storm time preexisting partial ring current (PRC), provided that the IMF Bz has been southward for a while before the onset of the pressure enhancements. Wang et al. [1] (and references therein) observed that the ring

7 The African Review of Physics (213) 8:19 12 current injection rate would increase during a period of enhanced solar wind dynamic pressure. Moreover, his investigations from the most recent OMNI data set, then ( ), suggests that the dynamic pressure plays an essential role in controlling the injection of the ring current, especially during strong magnetic storms. It was, however, concluded that the strength of the ring current injection is proportional to the solar wind dynamic pressure with a power index of.2 during southward IMF. This implies that the ring current injection increases when the magnetosphere is more compressed by high solar wind dynamic pressure. This assertion, therefore, makes it more interesting to probe into the present study of the variability of the dynamic pressure (a ring current signature) on the mentioned parameters in the previous section during the different phases of geomagnetic activities. With respect to variability, the correlation coefficient between the dynamic pressure and the flow speed during the initial phase of the entire twenty geomagnetic disturbances under analysis is.7, which is rather strong (Fig. a). The strong correlation is as a result of energetic particles coming from the Sun as part of the solar wind, which are free to enter the magnetosphere, and after a period of storage, are injected into the ring current of the system; of which the majority of these particles are densely populated. This would cause an increase in the magnitude of the plasma flow speed, and subsequently causing an increase in the dynamic pressure. Ionosphreric particles had also been observed to contribute to the ring current and can even become the dominant source during main phase of major magnetic storms. The correlation between the dynamic pressure and the IMF Bz is about. (Fig. b). The correlation activity is categorized based on the following range: very strong (>.6), Good (.-.64), weak (.3-.49), very weak ( ) and irrelevant (<.1). During the main phase of the entire storms (i.e., the period during which the Dst reaches its peak minimum value), the dynamic pressure variability with the flow speed is very weak (.17), see Fig. 6a. This is attributed to energy losses. The correlation between the dynamic pressure and the IMF Bz here is.32 (which is also weak) and.7 for P-Dst relation (Fig. 6c). 1 (a) 1 (b) V(km/s) y = 13.4x C = y = -.418x C =.148 Fig.: Correlation plot for dynamic pressure versus (a) V and (b) Bz during storm onset for all storms.

8 The African Review of Physics (213) 8: V(km/s) (a) y = x C = (b) y = x C = Dst (nt) (c) y = x C =.737 Fig.6: Correlation plot for dynamic pressure versus (a) V, (b) Bz and (c) Dst during storm main phase for all storms. Regarding the correlation between the P and the flow speed when the plasma flow speed is maximum during the storm interval, a weak relationship occurred between the two, as well as with Dst (Fig. 7), with an insignificant value. However, a rather interesting behavior is the strong (.) relationship between P and Bz. A better explanation for these is the geomagnetic activity of 6 April, 2 (Fig. 1) in which a sudden outburst of the solar wind dynamic pressure aggravated the southward turning of the Bz symmetry, as well as the increase in flow speed. The Bz turned southward and became intensified because of a compression of the sheath region, remaining like that for close to 14 hours, which in turn makes the Dst reach its minimum peak value of approximately -287nT. After the pressure pulse arrives the interplanetary magnetic field, the Dst strength increases and the Bz begins to have strong fluctuations in the north-south direction. V(km/s) (a) y =.37x C = (c) (b) y = x C = Dst (nt) y = x C =.283 Fig.7: Same as in Fig. 6, but for peak flow speed value period for all storms.

9 The African Review of Physics (213) 8: On the basis of intense and severe storms, it was observed that during the storm initial phase (Table 3), the P-V relation during intense class of storms is around.3, which is rather weak. A corresponding value for severe storms shows a very strong.8 correlation. In like manner, the corresponding correlation in the dynamic pressure versus IMF Bz during the initial phase of both the intense and severe storms are.73 and.4, respectively. For the main phase, the corresponding correlation for P versus V, and P versus Bz, respectively, during intense storms are weak. On the other hand, severe storms recorded.6 and.74, respectively, for the same period. At the point of maximum flow speed (Vmax) during the storm period, the correlation of P versus V, and P versus Bz during intense storms are also low. In the case of severe storms under similar conditions, the values are.34 and.66, respectively. This observed.66 value shows a very strong relationship between the dynamic pressure and the IMF Bz during high solar wind speed period, and the reason from this is explained from Gonzalez et al. [11] point of view. According to them, the intense interplanetary magnetic fields can be thought of as being associated with essentially two parts of a high-speed stream, the intrinsic ejecta (called driver gas fields), and the shocked and compressed fields and plasma due to the collisions of the high-speed stream with the slower solar wind preceding it. In the latter case, the compression is related to the strength of the shock and thus to the speed of the high-speed stream relative to the upstream (slow) solar wind. The higher the relative velocity, the stronger is the shock and the field compression. However, if the shock runs into a trailing portion of a high speed stream, preceding it, exceptionally high magnetic fields may result, hence the reason for the high correlation value observed. Table 3: Observed correlation coefficient values for intense and very-intense storm conditions. Storm Initial Phase Storm Main Phase Peak Flow Speed Period P vs. Bz P vs. V P vs. Bz P vs. V P vs. Dst P vs. Bz P vs. V P vs. Dst Severe storms Intense storms The dynamic pressure versus Dst relationship recorded high values during the main phase of the geomagnetic activities;.48 for intense and.6 for severe storms. Xie et al. [12], had investigated the effects of solar wind dynamic pressure P, and preconditioning in over 8 large magnetic storms (Dst < 1 nt) occurring during solar cycle 23, and concluded that there is always an increase in the Dst peak value when there is a large enhancement of the dynamic pressure during the main phase of a storm. 4. Conclusion We have presented solar wind dynamic pressure variability with the plasma flow speed and the IMF Bz for storms initial and main phase, as well as during peak plasma flow speed value periods. Twenty geomagnetic activities were considered. P versus V attained a significant correlation percentage (7%) only during the storm s initial phase. This may not be unconnected with the intensification of the accelerated ring current. The dynamic pressure variability with the flow speed is weak during the main phase, and was attributed to ring current energy losses, which may have originated from either charge exchange and Coulomb scattering or resonant interactions with plasma waves. P versus Bz correlation here is also very strong for maximum plasma flow speed value period, P versus V recorded weak relationship. On the contrary, the P versus Bz recorded a strong correlation. For intense and severe storms, a good relationship existed between P and Bz only during the initial phase of intense class of storms. The severe storms also showed good relationship between P and V (above.64) during storm s initial and main phases. It was also observed that the solar wind dynamic pressure is highly geoeffective with the southward IMF Bz during the main phase of a storm, mostly when the plasma flow speed value is very large. Acknowledgments Many thanks to NSSDC s OMNI database ( for the data used in this study.

10 The African Review of Physics (213) 8: References [1] W. D. Gonzalez; J. A. Joselyn, Y. Kamide, H. W. Kroehl, G. Rostoker, B. T. Tsurutani and V. M. Vasyliunas, J. Geophys. Res. 99, (A4), 771 (1994). [2] W. D. Gonzalez, B. T. Tsurutani, Y. Kamide, A. L. CIlia de Gonzalez, A. Dal Lago and J. K. Arballo Physics of Space Plasmas, No 1 (1998). [3] W. Gonzalez and B. T. Tsurutani, Planet, Space Sci. 3, 111 (1987). [4] Igino Coco, Ermanno Amata, Maria Federica Marcucci, Danila Ambrosino and Simon G. Shepherd, Hindawi Publishing Corporation International Journal of Geophysics Volume 211, Article ID 2714, (211); doi:1.11/211/2714 [] B. O. Adebesin, Acta. Geod. Geoph. Hungarica 43, 383 (28); [DOI: 1.16/A Geod ] [6] C. A. Loewe and G. W. Prolss, J. Geophys. Res. 12, 1429 (1997). [7] A. Dal Lago, W. D. Gonzalez, A. L. Clua de Gonzalez and L. E. A. Vieira, J. Atmos. Sol. Terre. Phys. 63, 41 (24). [8] E. Zesta, H. J. Singer, D. Lummerzheim, C. T. Russell, L. R. Lyons and M. J. Brittnacher, The effect of the January 1, 1997, pressure pulse on the magnetosphere-ionosphere current system, in Magnetospheric Current Systems, Geophys. Monogr. Ser., Vol. 118, edited by S. Ohtani et al., pp , AGU, Washington, D. C. (2): [9] Y. Shi, E. Zesta, L. R. Lyons, A. Boudouridis, K. Yumoto and K. Kitamura J. Geophys. Res. 111, A1216 (26); doi: 1.129/2JA1132 [1] C. B. Wang, J. K. Chao and C.-H. Lin, J. Geophys. Res. 18 (A9), 1341 (23); doi:1.129/23ja 981 [11] W. D. Gonzalez, B. T. Tsurutani, A. L. C. Gonzalez, E. J. Smith, F. Tang and S. I. Akasofu, J. Geophys. Res. 94, 883 (1992). [12] H. Xie, N. Gopalswamy, O. C. St. Cyr and S. Yashiro, Geophys. Res. Lett. 3, L6S8 (28); doi:1.129/27gl Received: 16 January, 212 Accepted: 26 April. 213