Fluctuations in the interplanetary electric potential and energy coupling between the solar-wind and the magnetosphere
IInternational Symposium on Solar-Terrestrial Physics
ASI Conference Series, 2012, Vol. 00, pp 1–5Edited by P. B. Rao, N. Gopalswamy, S. S. Hasan & P. Subramanian
Fluctuations in the interplanetary electric potential andenergy coupling between the solar wind and themagnetosphere
Badruddin ∗ and O. P. M. Aslam Department of Physics, Aligarh Muslim University, Aligarh-202002, India.
Received — ; accepted —
Abstract.
We utilize solar rotation average geomagnetic index ap and vari-ous solar wind plasma and field parameters for four solar cycles 20-23. Weperform analysis to search for a best possible coupling function at 27-daytime resolution. Regression analysis using these data at di ff erent phases ofsolar activity (increasing including maximum / decreasing including min-imum) led us to suggest that the time variation of interplanetary electricpotential is a better coupling function for solar wind-magnetosphere coup-ling. We suspect that a faster rate of change in interplanetary electric poten-tial at the magnetopause might enhance the reconnection rate and energytransfer from the solar wind into the magnetosphere. The possible mech-anism that involves the interplanetary potential fluctuations in influencingthe solar wind-magnetosphere coupling is being investigated. Keywords : solar-terrestrial physics– solar wind-magnetosphere coupling–geomagnetic activity– interplanetary electric potential
1. Introduction
In the area of solar-terrestrial physics, one of the key problems is to investigate themechanism of energy transfer from the solar wind into the magnetosphere. It is gen-erally believed that the basic parameter leading to geomagnetic disturbances is thesouthward component of the interplanetary magnetic field ( − B z ) and / or the duskwardcomponent of the interplanetary electric field E y = − V × B z (see e.g. ?? ; ?? ; ?? ; ?? ; Badruddin & Singh 2009; Kane 2010; Alves, Echer & Gonzalez 2011; Singh & ∗ email: [email protected] a r X i v : . [ a s t r o - ph . S R ] D ec Badruddin & O. P. M. Aslam
Figure 1.
Time variation (27-day solar rotation average) of various interplanetary plasma / fieldparameters with geomagnetic ap index during increasing including maximum phase of solarcycle 23. Badruddin 2012; Yermolaev et al. 2012 and references therein). In spite of the suc-cess of the so called Dungey mechanism some e ff ort (e.g. Murayama 1982; Gupta& Badruddin 2009; Joshi et al. 2011) has gone into looking for other parametersthat might correlate better with geomagnetic activity. Geomagnetic activity beinginfluenced by irregularities in the solar wind and interplanetary magnetic field, andenhanced dynamic pressure (Murayama 1982; Srivastava & Venkatakrishnan 2002;Xie et al. 2008; Dwivedi, Tiwari & Agrawal 2009; Singh & Badruddin 2012) havebeen suggested. But a unique relationship is still lacking which may ultimately leadto understand the intensity of geomagnetic disturbances under di ff erent solar windconditions.
2. Analysis and results
We have divided a complete solar cycle into two parts; (i) increasing including max-imum and (ii) decreasing including minimum phases. We consider the time variationsin di ff erent parameters at solar rotation time scale, both during increasing includingmaximum and decreasing including minimum phases of solar cycle 23. These timevariations in various interplanetary plasma / field parameters ( V, B , B z , E y , BV / BV ) are compared with time variations of geomagnetic parameter ap during increas-ing including maximum phase (see Figure 1). From these figures, it appears that, atthis time scale, BV follows in a better way the time variation of ap , as compared toother parameters V, B , B z , E y and BV / ap during increasing includ-ing maximum phase of solar cycle 23. Therefore, a quantitative analysis has beendone by linear regression method, not only during increasing including maximumphase of solar cycle 23, but also during similar phases of solar cycles 20, 21, and22. The rate of change of the ap index with various plasma / field parameters ( ∆ I / ∆ P ) nterplanetary electric potential and energy couplingnterplanetary electric potential and energy coupling
We have divided a complete solar cycle into two parts; (i) increasing including max-imum and (ii) decreasing including minimum phases. We consider the time variationsin di ff erent parameters at solar rotation time scale, both during increasing includingmaximum and decreasing including minimum phases of solar cycle 23. These timevariations in various interplanetary plasma / field parameters ( V, B , B z , E y , BV / BV ) are compared with time variations of geomagnetic parameter ap during increas-ing including maximum phase (see Figure 1). From these figures, it appears that, atthis time scale, BV follows in a better way the time variation of ap , as compared toother parameters V, B , B z , E y and BV / ap during increasing includ-ing maximum phase of solar cycle 23. Therefore, a quantitative analysis has beendone by linear regression method, not only during increasing including maximumphase of solar cycle 23, but also during similar phases of solar cycles 20, 21, and22. The rate of change of the ap index with various plasma / field parameters ( ∆ I / ∆ P ) nterplanetary electric potential and energy couplingnterplanetary electric potential and energy coupling Figure 2.
Scatter plot and best-fit linear curve between ap and BV during (a) increasingincluding maximum phases, (b) decreasing including minimum phases and (c) complete cycles20-23 combined. Table 1.
Rate of change of ap with various solar wind parameters ( ∆ I / ∆ P ) and correlationcoe ffi cient ( R ) during increasing including maximum phases of solar cycles 20, 21, 22 and 23. Parameters Solar cycle 20 Solar cycle 21 Solar cycle 22 Solar cycle 23 ∆ I / ∆ P R ∆ I / ∆ P R ∆ I / ∆ P R ∆ I / ∆ P RB (nT) 3.44 0.57 2.81 0.55 3.99 0.74 3.77 0.68 B z (nT) − . − . − . − . − . − . − . − . V (km s − ) 0.08 0.69 0.10 0.78 0.12 0.77 0.10 0.77 P (n Pa) 8.08 0.35 5.41 0.54 7.43 0.80 7.62 0.53 E y − ) BV / − ) BV . − . E − . E − . E − . E − − ) VB . E − . E − . E − . E − ffi -cients between them are tabulated in Table 1, during increasing including maximumphases of solar cycles 20, 21, 22 and 23. From Table 1 we observe that, out of variousplasma / field parameters, the best correlation is found with the rate of change of inter-planetary electric potential ( BV ) with time, consistently during increasing includingmaximum phases of all four solar cycles 20, 21, 22 and 23.Similar correlative analysis during decreasing including minimum phases of solarcycles 20, 21, 22 and 23 was also done and the values of the rate of change of geomag-netic index ap with various parameters ( ∆ I / ∆ P ) and the corresponding correlationcoe ffi cients were calculated. We found, in this case also, that out of all parametersconsidered here, the rate of change of the interplanetary electric field ( BV ) with timebest correlates with the ap during decreasing including minimum phases of almost all Badruddin & O. P. M. Aslam the four cycles 20, 21, 22 and 23. However, fluctuations in the interplanetary elec-tric field occur in a much shorter time scale. Therefore we extended our analysis to3-hourly resolution data especially for increasing including maximum and decreasingincluding minimum phases of solar cycle 23. We find that, at this shorter time scaletoo, the correlation coe ffi cient ( R ) between ap and BV is high ( R = R = ap with BV for increasing includingmaximum phase [Figure 2 (a)], decreasing including minimum phase [Figure 2 (b)]and combined periods [Figure 2 (c)] with values of slope ( ∆ I / ∆ P ) and R are shownin Figure 2. From these figures we can give a best-fit relationship between ap and BV as; ap = . × − ( BV ) − .
83. This relation may be useful for estimating thegeomagnetic activity level from the values of solar / interplanetary parameters B and V .
3. Conclusions
The time variability of interplanetary electric potential at the magnetopause appearsto be an important parameter for solar wind-magnetosphere coupling. When this vari-ability in interplanetary potential is su ffi ciently large, it appears to increase the re-connection rate between the solar wind and terrestrial magnetosphere, significantlyincreasing the geoe ffi ciency of the solar wind. We suspect that, in addition to theduskward electric field, enhanced fluctuations in the interplanetary electric potentialat the magnetopause is the most likely additional e ff ect that leads to enhanced coup-ling between the solar wind and the terrestrial magnetosphere, significantly increasingthe geoe ff ectiveness of the solar wind. However, this hypothesis needs to be testedwith high time resolution measurements of in situ interplanetary data. References
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