PC index as a standard of magnetospheric disturbances in the auroral zone

PC index as a standard of magnetospheric disturbances in the auroral zone Oleg Troshichev, Arctic and Antarcrtic Research Institute, St.Petersburg olegtro@aari.ru The Solar-Terrestrial Physics Symposium (STP1) York University, Toronto, Canada, July 9-13, 18 1

Outline 1. Introduction. Physical backgrounds for PC index 3. PC index and development of magnetospheric substorms. Relationship between PC index and magnetic storms 5. Relationship between PC and interplanetary electric field 6. PC index as a verifier of the actual coupling of the Earth s magnetosphere with solar wind fixed by ACE spacecraft. 7. PC index as a standard of power for disturbances in the auroral zone 8. Conclusion

Introduction: solar wind and state of the magnetosphere Solar wind parameters n, v, P SW, B Z, B Y, B T Indicators of state of the magnetosphere: AE, Dst, etc indices Coupling function State of magnetosphere is determined by the solar wind energy incoming into the magnetosphere in course of the solar wind-magnetosphere coupling. Estimation of the solar wind geoefficiency ( coupling function ) is based on measurements of the solar wind parameters out of the magnetosphere, usually on board ACE spacecraft spaced 1.5 M km apart the Earth in the Lagrange point L1. These data can provide information on space weather changes about -5 minutes in advance. State of the magnetosphere is evaluated by such indicators of magnetic activity as 3 Dst/SymH indices (magnetic storms) and AL/AE indices (auroral substorms), and etc.

. Physical backgrounds for PC index: magnetic activity in the polar caps responds to the solar wind geoeffective variations 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, 198; 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 crosspolar cap potential difference and ionospheric currents producing the polar cap magnetic disturbances [Troshichev and Tsyganenko, 1979]. PC index was introduced [Troshichev and Andrezen, 1985; Troshichev et al., 1988] to characterize the polar cap magnetic activity produced by the interplanetary electric field E KL [Kan and Lee, 1979] E KL =Vsw*(By +Bz ) 1/ sin θ/ where Vsw solar wind speed, By, Bz azimuthal and vertical IMF components. РС index is determined as a value of the E KL -produced magnetic disturbances at the near-pole stations (Thule and Vostok) with allowance for UT time, season and hemisphere.

Resolutions of XXII Scientific Assembly of International Geomagnetism and Aeronomy Association (1th IAGA), Merida, Меxico, August 13 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 nearreal 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 5 storms).

3. PC index and development of magnetospheric disturbances: substorms are preceded by the PC index growth (Substorms are separated in groups by value of РС in time of substorm sudden onset) Moment of the substorm onset (SO) is taken as a key date (T=). Thin red lines present a run of PC and AL in course of individual events. Thick black line presents variation of mean PC and AL values for each group. Development of magnetospheric substorms is generally preceded by the PC index growth.

Relationship between AL and PC indices Number of events 35 3 5 15 1 5 Expanded substorms (N=118) Isolated substorms (N=19) 6 8 PC, mv/m Sudden onset magnitude, nt -5-1 -15 - -5-3 -35 - -5 1 3 5 6 7 8 9 1 11 1 Growth phase duration, min Magnetospheric substorms generally start as soon as the PC index exceeds the threshold level PC=1 mv/m, irrespective of the substorm growth phase duration and type of substorm (isolated or extended). Occurrence. of substorms reaches maximum under conditions of PC =1.5 mv/m Duration of growth phase does not affect the substorm onset PC T=, intensity. mv/m Magnitude of the substorm sudden onset (SO) demonstrates tendency to decrease with the growth phase length increase

Relationship between AL and PC indices Before SO After SO AL, nt PC, mv/m PC, mv/m PC, mv/m Intensity of magnetic disturbances in the auroral zone (AL index) before and after the substorm sudden onset is linearly related to PC value. Slope coefficient characterizing linear linkage between PC and AL sharply increases after the substorm onset as an evident consequence of the auroral particle precipitation giving rise to the ionospheric conductivity and powerful westward auroral electrojet in course of the substorm expansive phase.

. Relationship between the PC index and magnetic storms Criteria used to choose magnetic storms for the analysis: (1) depression of magnetic field during storm should be larger than Dst = -3nT, and () magnetic storm duration should be longer than 1 hours. The depression advent is regarded as a beginning of the storm main phase, and the maximum of depression (Dst min ) is regarded as a magnetic storm intensity. Magnetic storms are divided by their intensity into 5 categories: 3-6nT, 6-9nT, 9-1nT, 1-nT, -nt. Basing on these criteria, 9 magnetic storms were separated for the epoch of solar activity maximum (1998-15) with a maximal storm intensity varying in the range from Dst = -3nT to Dst = -nt.

PC index, Ekl field (mv/m) 8 6 Classical storms (short growth phase) Three basic types of the magnetic storm development identified by the PC index behavior PC index Ekl field - -16-8 8 16 3 8 6 Classical storms Composite storms (long growth phase) Pulsed storms Combined storms PC index Ekl field - -16-8 8 16 3 8 6 - PC index Ekl field 16 3 8 6 8 6 - PC index Ekl field -8 8 16 3 AL index (nt) - - -6-8 -1-1 -1-16 -8 8 16 3 - - -6-8 -1-1 - - -6-8 -1-1 -1-1 -16-8 8 16 3 - - -6-8 -1-1 -1 16 3 8 6-8 8 16 3 SymH iindex (nt) - - -6-8 -1-16 -8 8 16 3 Storm duration, hours 3 July 15 - - -6-8 31 October 5-1 -16-8 8 16 3 Storm duration, hours - - -6-8 -1 8 November 1999 16 3 8 6 Storm duration, hours - - -6-8 -1 6 September 1-8 8 16 3 Storm duration, hours Three basic types of magnetic storms have been determined taking peculiarities of the PC index behavior as a principle: - classical storms, which demonstrate the main phase with one clearly expressed maximum of depression (%), - pulsed storms, which contain a series of periodically repeating depressions, lasting during many hours (7%), - composite storms, which are superposition, with different weight, of classical and pulsed storms (31%). It is significant that the time evolution of Dst index in all cases is predetermined by the PC index behavior.

Relation of different types of the magnetic storms to the solar wind drivers (ICME and CIR) Analysis of magnetic storms for period of 1998-15 was carried out basing on the following catalogues: I. Richardson and H.Cane List of Near-Earth Interplanetary Coronal Mass Ejections (ICME) for 1997-15 II. L. Jian List of Stream Interaction Regions (SIR) for 1995-9 Classical storms are related mainly to Interplanetary Coronal Mass Ejections (59%); powerful magnetic storms (Dst>1 nt) are generated exclusively by ICME. Pulsed storms are related (6%) to Stream Interaction Regions (SIRs) or Corotating Interaction Regions (CIRs); it is evident in case of weak magnetic storms (Dst<9 nt), Combined storms are produced by joint action of ICME (3%) and SIR (%), the weak storms (Dst<6 nt) being mainly related to SIR, the strong storms (Dst>1 nt) being related to ICME.

Distribution of the classic, pulsed and composite storms over years Number of events 15 1 5 N=9 Classic Pulsed Composite 1998 6 8 1 1 1 16 The classic and composite storms were observed throughout the whole period, with maximum rate (N 1 in year) in 1998, -, 5, 11-13, 15 and minimum rate (N<3) in 7-9. The pulsed storms reached a peak incidence (N 1) in 3 and 8, being absent in 9.

Time evolution of PC index and development of classic storms (SymH index) Magnetic storms were separated onto 5 categories according to their intensity. Development of classical magnetic storm is determined by time evolution of the PC index: Magnetic storm starts (T=) when PC index steadily exceeds the threshold level PC~1.5 mv/m. Magnetic storm continues till PC index stays higher the threshold level, as a result, the storm growth phase duration is determined by time period with PC>1.5 mv/m.

Time evolution of PC index and development of composite storms SymH, nt PC index, mv/m SymH, nt PC index, mv/m 1 8 6 - -16-8 8 16 3 - - -6-8 -1-1 -16-8 8 16 3 1 8 6 - -16-8 8 16 3 - - -6-8 -1 1 9 8 7 6 5 3 1-1 - -16-8 8 16 3 N=1 N=1 N=15 N=6 N=9-1 -16-8 8 16 3 (a) Storm intensity SymHMIN = - (3-6) nt Growth phase -8 hours Growth phase 8-1 hours Growth phase 1-16 hours Growth phase 16- hours - - -6-8 -1-1 -16-8 8 16 3 1 8 6 - -16-8 8 16 3 - - -6-8 -1-1 -16-8 8 16 3 1 8 6 - -16-8 8 16 3 - - -6-8 -1-1 -16-8 8 16 3 1 8 6 - -16-8 8 16 3 - - -6-8 -1-1 -16-8 8 16 3 1 8 6 - -16-8 8 16 3 - - -6-8 -1-1 -16-8 8 16 3 Storm duration, hours Storm duration, hours Storm duration, hours Storm duration, hours (b) Storm intensity SymHMIN = - (6-9) nt Growth phase 6-8 hours Growth phase 8-1 hours Growth phase 1-16 hours Growth phase 16- hours N=7 N=9 N=6 1 8 6 - -8 8 16 3 - - -6-8 -1-1 -8 8 16 3 Storm duration, hours Storm duration, hours Storm duration, hours Storm duration, hours In case of composite storms Dst variation is in progress as long as the PC index remains above the threshold level (PC=1.5 mv/m), the clear minimum SymH value being observed, like the classic storms. The mean PC and SymH values for each composed storm category demonstrate the clearly defined regularity: the maximum depression of geomagnetic field (SymHMIN) is predetermined by the maximum PC value (PCMAX).

Storm intensity (SymHMIN) as a response to PCMAX value The maximal depression of magnetic field (storm intensity) follows to maximum value of the smoothed РС index. Intensity of storms (Dst MIN ) is linearly related to preceding maximal PC value (PC MAX ): the higher the PCmax value, the larger is magnetic storm intensity (Dstmin). Delay times ΔT in response of SymHMIN to the PCMAX occurrence lie in the range from 3 to 18 minutes. Value of time ΔT seems to be slightly dependent on the storm growth phase duration (the longer the growth phase, the larger is delay time), however, this tendency is too slight to be the controlling factor. The mechanisms determining the actual ΔT values are not clear.

5. Relationship between PC and interplanetary electric field E KL Relationship between PC and EKL were analyzed for substorm and storm events examined in the previous sections. Values PC and EKL were smoothed using the 15 min averaging running window. Substorms were subdivided into isolated and expanded ones, and relationship between PC and EKL was examined during 1-hour interval preceding the substorm onset, the moment of substorm sudden onset (SO) being taken as a zero time (T). Besides, events were separated when PC and EKL demonstrated the coordinated behavior during -hour interval preceding sibstorm onset, the moment of the sudden rise of EKL being taken as a zero date T. These events were examined as coordinated events irrespective of type of substorm (isolated or expanded). Correlation between PC and EKL in these cases was calculated within the interval from T 3 to T+3 min. The correlation was calculated with variable shifts of EKL relative to PC. A value of shift providing the maximal correlation between EKL was identified for each event as the delay time ΔT in response of the PC index to EKL variations. In case of magnetic storms the relation between PC and EKL was examined in course of entire storm events.

Delay time in response of PC index to different solar wind parameters (solar wind velocity Vsw, interplanetary magnetic field (IMF), EKL field) AL, nt PCN/PCS, mv/m E KL, mv/m ΔΤ ΔΤ, min ΔΤ, min Correlation between ΔΤ and the solar wind parameters averaged for 1-hour preceding the substorm onset 6 8 1 1 min (a) (b) (c) (d) There is slight tendency to ΔT decrease when the solar wind velocity increases. There is slight tendency to ΔT decrease when the IMF turns from northward (BZN) to southward (BZS). Таngential IMF component BT=(BY +BZ ) 1/ does not affect the ΔΤ value. The solar wind dynamic pressure Pd does not affect the ΔΤ value PC index follows to change in the interplanetary electric field EKL with delay time (ΔΤ), which does not depend on separate solar wind parameters, such as the solar wind velocity (Vsw), dynamic pressure (Psw), and vertical (Bz), azimuthal (BY) and tangential (BT) IMF components.

5 3 1-1 - 5 3 1-1 Relationship between PC and interplanetary electric field E KL Delay time in response of PC index to the EKL increase is examined All substorm events were separated into groups according to ΔT value. The moment of beginning of the E KL growth was taken as o zero time T=. PC, mv/m E KL, mv/m ΔΤ = 13-15 min - -3 - -1 1 3 Time, min 5 3 1-1 - 5 3 1-1 ΔΤ = 19-1 min 1 min min - -3 - -1 1 3 Time, min Thin lines variations of Ekl and PC values in course of concrete events. Thick lines variation of average Ekl and PC vakues. Vertical lines sudden onsets of the Ekl and appropriate PC growth...5.1.15..5.3.35 de KL /dt, mv/m Delay time ΔΤ and the electric field growth rate (dekl/dt) are linearly connected. Delay time (ΔΤ) in response of PC to the solar wind changes is determined not by some separate solar wind parameters but the growth rate of the interplanetary electric field (dekl/dt). Delay time, min 3 5 15 1 5 ΔΤ vs. de KL /dt ΔT = 57.1 39.1*(dEkl/dt) R= -.9 Interplanetary shocks are denoted by rhombus.

6. PC index as verifier of the solar wind parameters fixed by ACE spacecraft in the Lagrange point L1 Correspondence between the PC index and EKL field variations, typical of conditions of magnetospheric substorms, sometimes is fully destroyed. Sun IMF lines Earth It seems to be reasonable since the estimated values of EKL field are based on measurements of the solar wind parameters on board ACE spacecraft far in front of the magnetosphere. As a result, the actual EKL field affecting the magnetosphere can be quite distinct from EKL fixed in the Lagrange point by ACE spacecraft. Indeed, in ~1.5% of the substorm events the correlation between EKL and PC was sufficiently high (R>5%), but delay time ΔΤ between the corresponding changes in the estimated Ekl field and PC index was zero or even negative (up to 1 minutes) In such cases we have to allow that substorms started just before the EKL impact on magnetosphere. In actuality it indicates that the solar wind fixed in the Lagrange point L1 was propagated with acceleration in the space. In 13% of the substorm events the correlation between the PC and EKL field was poor or even negative, in spite of the good correspondence between the PC and AL indices in course of these events. It means that solar wind fixed in these cases on board ACE spacecraft (at the distance of ~ 1.5 million km upstream of the Earth) did not encounter the Earth s magnetosphere at all (owing to large curve of the IMF lines). Thus, PC index can be successfully used to verify that the solar wind fixed in the point of Lagrange L1 is just the solar wind which actually impact on the magnetosphere.

Examples of events with negative (a) and zero (b) delay times in response of PC index to changes in E KL field The moment of the substorm sudden onset is marked by vertical black line. The moment of sharp E KL field increase reduced to magnetopause is marked vertical red line. It is evident that in these cases (~% of events) we deal with incorrect evaluation of time required for E KL field transportation from the ACE spacecraft position to magnetopause.

6. PC index as a standard of power for different magnetospheric disturbances Magnetospheric disturbances include a wide range of phenomena and processes in the space and ionosphere. This section demonstrates that high-latitude ionospheric and magnetic disturbances can be calibrated with the PC value. The maps of spatial temporal distributions of ionosospheric and magnetic disturbances occurring in system of geomagnetic coordinate were presented in system of geographical coordinate for the moment of 6 UT (Grinvich) time for different levels of PC.

Auroral absorption as function of PC index value. РС =.7 mv/m PC = 1. mv/m PC = 1.8 mv/m 18 UT 6 UT

Auroral (electron and ions) precipitation as function of PC index value. Maps of the of energy density (erg/смsec) distribution РС < 1.5 mv/m 1.5 < PC < mv/m mv/m < PC Critical frequencies for sporadic ionospheric layer Es in winter season РС < 1.5 mv/m РС > 1.5 mv/m

Conclusions PC index as an indicator of the solar wind energy input into the magnetosphere AE, Dst, etc indices as indicators of the magnetosphere state : The experimental facts are indicative of the PC index as the ground-based indicator of the solar wind energy input into the magnetosphere. The PC index might be useful as a means for monitoring the space weather, nowcasting the magnetosphere state, fitting the solar wind-magnetosphere coupling function, standard calibrating of power for different disturbances in the auroral zone, and checking whether or not the solar wind fixed in Lagrange point L1 actually encounters the magnetosphere. The sets of data on PC index for 1997-17 and the current PCN and PCS indices calculated on-line by magnetic data from stations Thule and Vostok are presented 11 at web site: http://pcindex.org

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