Magnetosphere Response to Impulse Space Weather Events: PC, AE, and SymH Indices Relationship

1. Magnetosphere response to impulse space weather events: relationships between PC, AE and SymH indicesO. Troshichev and D.SormakovArctic and Antarcrtic Research Institute, St.Petersburgolegtro@aari.ru The Solar-Terrestrial Physics Symposium (STP14) York University, Toronto, Canada, July 9-13, 2018 1

2. Introduction Classic magnetic storm includes two energizing parts and a subsequent recovery. The first part consists of a sudden commencement (SC) and an initial phase, which are result of change in compression of the magnetosphere following the passage of a discontinuity (such as a shock front) propagating in the solar wind. A rise time of SC (impulsive increase produced in the geomagnetic H-component) is one to six minutes and an amplitude of several tens of nT. The initial phase produced by currents flowing on magnetopause (DCF) typically lasts few hours, during which the field remains compressed by the solar wind pressure increase following the discontinuity. The second phase is the main phase. It is depression of the magnetic field produced by a ring current (DR), which is developed in the inner magnetosphere. Since solar wind discontinuities usually involve changes in both pressure and the interplanetary magnetic field (IMF) direction, magnetic storms typically show both compression and inflation effects. The summary effect of both contributors, which is maximal in the equatorial region, is evaluated by the 1-hour Dst index or 1-min SymH index depicting a longitudinally averaged magnetic field variation at low latitudes [Sugiura, 1976]. In course of magnetic storms developing the DR current effect far exceeds the DCF current effect, that is why the effect of impulse space weather events (interplanetary shocks) can be revealed only during the initial phase. In this study we examine relationships between the magnetic activity indices (PC, AL, SymH) and corresponding changes in the solar wind parameters (solar wind dynamic pressure Pd, southward IMF component, solar wind velocity Vsw and interplanetary electric field Ekl) during the initial phase produced by shock front propagating in solar wind.

3. PC index: Physical backgrounds The variable solar wind coupling with the geomagnetic field constantly generates the “magnetospheric field-aligned electric currents” flowing along the geomagnetic field lines [Langel, 1975; McDiarmid et al.,1977; Iijima & Potemra, 1982; Bythrow & Potemra, 1983]. The currents are distributed along the poleward boundary of the auroral zone (Region 1 FAC) and flow into the polar ionosphere on the dawn side and flow out of the ionosphere on the dusk side of the auroral zone. These currents are responsible for the cross-polar cap potential difference and ionospheric currents producing the polar cap magnetic disturbances [Troshichev and Tsyganenko, 1979]. PC index has been introduced [Troshichev and Andrezen, 1985; Troshichev et al., 1988] to characterize the polar cap magnetic activity produced by the interplanetary electric field EKL [Kan and Lee, 1979] EKL=Vsw*(By2+Bz2)1/2sin2θ/2 where Vsw – solar wind speed, By, Bz –azimuthal and vertical IMF components. РС index is determined as a value of the EKL-produced magnetic disturbances at the near-pole stations (Thule and Vostok) with allowance for UT time, season and hemisphere.

4. Resolutions of XXII Scientific Assembly of International Geomagnetism and Aeronomy Association (12th IAGA), Merida, Меxico, August 2013 No. 3: Polar Cap (PC) index IAGA, noting that polar cap magnetic activity is not yet described by existing IAGA geomagnetic indices, considering that the Polar Cap (PC) index constitutes a quantitative estimate of geomagnetic activity at polar latitudes and serves as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling, emphasising that the usefulness of such an index is dependent on having a continuous data series, recognising that the PC index is derived in partnership between the Arctic and Antarctic Research Institute (AARI, Russian Federation) and the National Space Institute, Technical University of Denmark (DTU, Denmark) recommends use of the PC index by the international scientific community in its near-real time and definitive forms, and urges that all possible efforts be made to maintain continuous operation of all geomagnetic observatories contributing to the PC index. Therein lies the principal distinction of the PC index from various coupling functions (which are characteristics of the solar wind arriving to the Lagrange point L1) and from AL and Dst indices (which are characteristics of the energy realized in form of magnetospheric substorm and magnetic storms). 4

5. Experimental data indicative of the PC index as a proxy of the solar wind energy incoming into the magnetosphere In course of magnetospheric substorms and magnetic storms the PC index strongly follows the time evolution of interplanetary electric field EKL (correlation R > 0.5 in 98% of events) with delay time ΔT ~20-30 min. The value of delay time ΔT is controlled by the EKL growth rate (dEKL/dt). Development of magnetospheric substorms and magnetic storms is generally preceded by the PC index growth. Magnetospheric substorms start as soon as the PC index exceeds the threshold level PC=1.5 mV/m, irrespective of the substorm growth phase duration and type of substorm (isolated or extended). Intensity of magnetic disturbances in the auroral zone (AL index) before and after the substorm sudden onset is linearly related to PC value. Steady exceeding the threshold level PC=1.5 mV/m is necessary and sufficient condition for the storm beginning, like to case of magnetic substorms; In course of magnetic storms the SymH index generally follows the time evolution of the 30-min smoothed PC index irrespective of type and intensity of magnetic storms. In case of classic storms the maximal depression of geomagnetic field (i.e magnetic storm intensity Dstmin), follows, with delay ~60 min,the maximal value of smoothed PC (PCmax),the values PCmax and Dstmin being connected by linear relationship. The low (R<0.50) or negative correlation between EKL and PC was observed in ~ 10% of examined events suggesting that the solar wind fixed by ACE did not encounter the magnetosphere in these cases. Therefore, the PC index might be useful to monitor the space weather andreal state of the magnetosphere and to keep check whether or not the solar wind fixed in Lagrange point L1 actually encounters the magnetosphere

6. Correlation between PC and the solar wind parameters (BZ, Vx, EKL) in course of magnetic storms Correlation between the PC and IMF Bz (R>0.5) is observed in 397 storm events of 429 (92.5%), with typical delay time ΔΤ=20-30 min in response of PC to Bz changes. Correlation between PC and the solar wind velocity VX R>0.5 is observed only in 86 events of 429 (20%). The best connection is between PC and EKL: correlation coefficients R>0.5 take place in 422 storm events of 429 (98.4%).

7. Correlation between PC and AL/SymH indices in course of magnetic storms Correlation between the EKLfield and AL index (R>0.5) is observed in 411 storm events of 429 (96%). Correlation between the PC and AL indices is always better than R=0.5, in 72% of storms the correlation coefficients being higher than 0.75. The AL index responds to the PC index changes with delay times ΔΤ=0÷10 min; Correlation between the PC and SymH indices (R>0.5) is observed in 81% of storm events) being better than between EKL and SymH (in 68%of storm events).

8. Method of the analysis Selection of magnetic storms events with distinctive initial phase was carried out by data on the solar wind dynamic pressure (Pd), obtained from the GSFC/SPDF OMNI/Web interface at http://omniweb.gsfc.nasa.gov. The SC magnitude was identified as a Pd increaseby value more than 10 nP within 5 minutes. The events with gaps in the solar wind data were excluded from the analysis if the gaps were in excess of 30% of the event data series Sets of 1-min data on the PC, AL and SymH indices were used in the analysis. The PC index, as a mean value of the appropriate PC indices in the northern and southern hemispheres, was taken. To release effects of the Pd impulses themselves the only magnetic storms events, developing against the background of the relatively quiet conditions, lasting during one hours or more prior SC, were only examined. As a result, the total number of the analyzed events for the period of 1998-2016 turned out to be N=84. • The actual moment of the solar wind contact with magnetosphere was determined by sharp jump in the SymH index, the moment of this jump was identified as SC moment (T0). • All separated events were examined at interval of T0±40 minutes, where is a moment of sharp increase in Sym H index. ..

9. Relationship between actual SC moments (derived from SymH data) and SC moments estimated from the Pd measurements on board ACE spacecraft • Solar wind dynamic pressure Pd was evaluated by data on solar wind parameters measured on board ACE spacecraft in the point of libration (at the distance of ~ 1.5 million km upstream of the Earth) reduced to the magnetopause. • The actual contact of the fixed solar wind with the magnetosphere can differ from estimated time, if the shock front was propagated with acceleration or deceleration. • Comparison of actual and estimated SC moments demonstrates that both options can meet in reality. Time, min

10. Initial phase under conditions of southward IMF Bz<0 and growing Vsw Magnetic disturbances in the auroral zone (AL index) and PC index reach maximum values under conditions of southward IMF (Bzmean~-3 nT) and growing solar wind velocity (mean Vsw increases from 200 to 500 km/c during initial phase).

11. Initial phase under conditions of southward IMF Bz<0 and steady Vsw Under conditions of southward IMF (Bzmean~-3 nT) and steady solar wind velocity (Vswmean~380km/c) during initial phase the intensity of magnetic disturbances in auroral zone (AL index) sharply decreases (~150 km/sec) for the same values of dynamic pressure.

12. Initial phase under conditions of northward IMF Bz>0 and growing Vsw Under conditions of norththward IMF (Bzmean~ 4 nT) and growing solar wind velocity (Vswmean~475km/c) the mean intensity of magnetic disturbances in auroral zone (AL index) during initial phase negligibly increases (from 50 to 100 nT).

13. Initial phase under conditions of northward IMF Bz>0 and steady Vsw Under conditions of norththward IMF (Bzmean~ 4 nT) and steady solar wind velocity (Vswmean~475km/c) the mean intensity of magnetic disturbances in auroral zone (AL index) remains on zero level during initial phase (in spite of action of same Pd impulse).

14. Total relationships between Pd, SymH, PC and AL during initial phase Conclusions: Intensity of magnetospheric disturbances is strongly dependent on the IMF polarity and the solar wind velocity variability and can be quiet different for the same power of the pressure impulses (Pd). The disturbances are maximal for conditions of southward IMF and growing solar wind velocity Vsw and fall to minimum under conditions of northward IMF and steady Vsw (with the same power of the solar wind dynamic pressure Pd!!!). It means that the pressure shocks themselves are not promote (or insignificantly promote) the solar wind energy input into the magnetosphere which is controlled mainly by the interplanetary electric field EKL~Vsw*(BY2+BZ2)1/2 anddisplayed by the PC index.

15. Thank you for attention! . PC web site: http://pcindex.org