Instruments:pcv

From CedarWiki

Jump to: navigation, search

Vostok (Antarctica) Polar Cap Index

Contents


Contact Person

Web Page

ACKNOWLEDGMENTS

The Vostok Polar Cap Index was provided by the Arctic and Antarctic Research Institute of St. Petersburg, Russia via the CEDAR Database. (Please send a courtesy copy of any publications using the Vostok PCI to Dr. Oleg Troshichev.).

NOTES

01/29/03: Hourly data from Oct 2001 to Dec 2002, and 1 min data from May-Dec 2002 (plotted) will be added to the DB soon.

Data Description

The Vostok ground magnetometer station is located in Antarctica (-78.463 S, 106.826 E) and has been operated by the Arctic and Antarctic Research Institute of St. Petersburg since 1958. The analog data have been digitized for 1978 and 1979, and for 1983-1995. A 1-sec digital magnetometer was installed in 1996. The station is part of the former Soviet Union's scientific exploration of Antarctica, which started in 1957 during the International Geophysical Year (IGY). It is located near the center of the Antarctic ice sheet, about 3488 m above mean sea level, and about 1250 km from McMurdo and about 1500 km from Mirny on the coast.

Vostok is located in the polar cap at a high magnetic latitude. In 1978, the magnetic field at the surface was about 60950 nT, with an inclination angle of -77.682 degrees, a declination angle of -11.835 degrees, and an apex magnetic location of (-83.30, 53.56). In 2001, the magnetic field at the surface was about 59470 nT, with an inclination angle of -76.807 degrees, a declination angle of -12.019 degrees, and an apex magnetic location of (-83.31, 55.11). The apex magetic location of -83.3 is very close to the corrected geomagnetic (cgm) location of -83.4 cgmlat used for Vostok.

Magnetometer stations sense the currents flowing in the ionosphere. Magnetic activity in the polar cap is caused mainly by changes in the IMF southward component (Bz) and the solar wind velocity. Numerous investigations have showed that the IMF southward component drives ionospheric convection over the polar caps, which is sensed by ground magnetometers responding to the two-cell system of currents flowing in the ionosphere. As the Earth rotates under this two-cell current system, a near-pole station is always located under the sun-aligned part of the system (i.e. under a transpolar current).

The transpolar current allows a Polar Cap magnetic activity Index (PCI) to be derived from a single near-pole station by simply calculating a magnetic horizontal component disturbance along the dawn-dusk meridian. The new magnetic activity index PC was introduced and studied by Troshichev and Andrezen [1985] and Troshichev et al. [1988] using data from two near-pole stations at Thule (Greenland, 85.4 cgmlat) and Vostok (Antarctica, -83.4 cgmlat).

The algorithm to derive the Vostok PC index is based on a statistical analysis of the relationship between the IMF and the magnetic disturbances observed at near-pole stations. Since the stations are located under the sunward transpolar current, the magnetic perturbations at such a station will be approximately in the duskward direction. The latter is slightly modified because the two-cell equivalent current system is skewed with respect to the noon-midnight meridian. The transverse magnetic perturbation caused by the transpolar current is:

  • DHpc(nT) = DH(nT) * sin(gamma) +/- DD(nT) * cos(gamma)
    • gamma = glon +/- Decl + UTdeg + phi
    • DH = deviation in the H magnetic field component from the quiet level in nT
    • DD = deviation in the D magnetic field component from the quiet level, converted from angles to nT
    • Decl = is the station's average declination angle in degrees
    • glon = geographic longitude in degrees
    • UTdeg = Universal Time (UT) hour expressed in degrees (UTdeg = UT(hr) * 15 deg/hr)
    • phi = an angle between the transpolar current and the noon-midnight meridian for a given UT

The "plus" sign is for the south pole (Vostok) while the "minus" sign if for the north pole (Thule or Eureka). The quiet levels are determined from quiet days from a particular month, and the method of determining the quiet level differs in the calculation of PCI at Vostok and Thule. For Vostok, a quiet daily variation from quiet days of the previous month is used, except for the month of March when the sun sets where the quiet level must be recalculated from the quiet days during the month of March. The final PC Vostok values are thus nearly real time except during March when the final values are adjusted after the end of the month. For Thule, a constant daily quiet level is used which is an interpolation between the nighttime quiet levels from two consecutive winter periods to characterize the secular variation. Daily quiet variations are ignored.

The "true" transpolar current direction (phi) is derived through a linear correlation analysis relating the IMF and the horizontal perturbations projected onto various directions. The direction that shows the maximum correlation with the IMF parameter is then used for derivation of the PC index. Troshichev and Andrezen [1985] found that the highest correlation with the PC index was with the "merging electric field" (Em) in the magnetosphere suggested by Kan and Lee [1979], where

  • Em (mV/m) = 1.e-3 * Vsw * sqrt(By**2+Bz**2) * [sin(theta/2)]**2
    and
  • Vsw = solar wind velocity in km/s
  • By = IMF azimuthal component in nT
  • Bz = IMF vertical component in nT
  • theta = angle between the Earth's magnetic field and the IMF

The correlation is optimal when there is about a 20 minute (satellite to earth transit time) delay between the IMF/SW parameters and the polar cap disturbances. The magnitude of DHpc indicates the response of the ground magnetic activity to the interplanetary electric field imparted to the magnetosphere.

The optimal directions (phi) vary with UT and season [Troshichev et al., 1988; Vennerstroem et al., 1991]. To take into account the diurnal and seasonal variations, the projected horizontal perturbation (DHpc) have been normalized with respect to the parameter Em as:

  • DHpc(nT) = alpha * Em(mV/m) + beta
  • PC(dimensionless) = eta * (DHpc(nT) - beta) / alpha
    where
  • alpha, beta = regression coefficients in UT and month
  • eta = 1 mV/m, normalization to make PCI dimensionless

The original alpha and beta regression coefficients and phi angles were calculated separately for Vostok and Thule using data from 1977 to 1979 for every UT hour and every month (24x12 arrays). (See the description in Vennerstroem et al., 1994). The Vostok regression coefficients and phi angles were recalculated in 1997 using the original 1978-1979 data for every 5 minutes of UT and every month (288x12 arrays). These 5-min regression coefficients are interpolated inbetween every 5-min to calculate 1-min Vostok PC indices, while the original 60-min Thule regression coefficients are interpolated and smoothed between every hour to calculate the 1-min Thule PC indices. The greater time frequency of these coefficients for the Vostok PCI in combination with the greater time frequency and daily variation in the calculation of the quiet levels removed from the Vostok data result in a larger range of positive and negative values for the Vostok 1-min PCI compared to the Thule 1-min PCI. For 15-min indices, the differences are not great. The recommended basic reference is Troshichev et al [1988].

The dimensionless PC index is roughly equivalent to the merging electric field Em at the magnetopause in mV/m if there were a perfect correlation between the two parameters. Em varies between near zero and about 30 mV/m, while the PC index is mostly positive, but can be negative. Large positive values of the PC index indicate a disturbed state of of the magnetosphere (large merging electric field Em) whereas low or negative PC index values corresponds to a quiet magnetosphere.

The Vostok PC index is available starting in 1978. The 15-minute averages are available in 1978 and 1979, and in 1983-1991. Starting in 1992 there are 1-minute values. Hourly averages are available for both periods. Gaps in the record are from 1980-1982; February and April 1993 (Perestroika); November 1993 through December 1994 (station closed); 21 January to 31 December 1996 (station closed). The 1993 data are of questionable quality and users should contact Dr. Troshichev before using them.

Please use the following acknowledgement when using Vostok PC index data: "The PC index from Vostok station (Antarctica) used in this paper was provided by R. Y. Lukianova and O. A. Troshichev (http://www.aari.nw.ru)". Please provide reprints of refereed papers in international journals to maintain the list of the Vostok PC index related publications at the Arctic and Antarctic Research Institute in St. Petersburg.

Descriptions of this polar cap index may be found in:

J. R. Kan and L. C. Lee, Energy coupling function and solar wind-magnetosphere cynamo, Geophys. Res. Lett., 6, 577-580, 1979.
O. A. Troshichev and G. V. Andrezen, The relationship between interplanetary quantities and magnetic activity in the wouthern polar cap, Planet. Space Sci., 33, 415-419, 1985.
O. A. Troshichev, V. G. Andrezen, S. Vennerstroem, and E. Friis-Christensen, Magnetic activity in the Polar Cap - a new index, Planet. Space Sci., 36, 1095-1102, 1988.
S. Vennerstroem, E. Friis-Christensen, O. A. Troshichev, and V. G. Andrezen, Comparison between the polar cap index, PC, and the auroral electrojet indices AE, AL, and AU, J. Geophys. Res., 96, 101-113, 1991.
S. Vennerstroem, E. Friis-Christensen, O. A. Troshichev and V. G. Andrezen, Geomagnetic Polar Cap (PC) Index 1975-1993, Report UAG-103, WDC-A for Solar-Terrestrial Physics, NOAA/NGDC, Boulder, Colorado, 274 pp, 1994.

Summary Plots for Vostok Polar Cap Incex

Plots are for every 60 days of polar cap indices calculated from Vostok, Antarctica. The 15-min values are plotted between 1978-1991, and the 1-min values are plotted from 1992 to the present.

Summary Plots for 15-min Vostok Polar Cap Index

Summary Plots for 1-min Vostok Polar Cap Index

Both 1-min and 1-hour data are available for this period.

-Revised 29 Jan 2003 by Barbara Emery

Retrieved from "http://cedarweb.hao.ucar.edu/wiki/index.php/Instruments:pcv"
Personal tools