Alexeev I.I., Kalegaev V.V.
Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia
Y.I.Feldstein, L.I.Gromova
IXMIRAN, 142092 Troitsk, Moscow Region, Russia
A.Grafe
GeoForschungsZentrum, Potsdam, Adolf Schmidt Observatorium, Lindenstrasse 7, D-14823 Niemegk, Germany
Complex study of satellite and ground based data was carried out to estimate the contribution of various sources of geomagnetic disturbances during magnetic storm. The auroral oval and polar cap boundaries were determined using the DMSP electron fluxes precipitation data as well as data of magnetic observatories longitudinal chain. The solar wind and AL index data were used to calculate the time dependences of the paraboloid model parameters. The total energy of ring current particles (ions with energy of 1.5-300 keV/q, AMPTE /CCE mission) was calculated for each ring current intersection in the course of the magnetic storm. As a result of our analysis, we have received description of dynamics of the magnetic fields of the symmetric ring current and the tail current in the course of magnetic storm.
During the quiet time, the tail current contribution to the horizontal field at the Earth's surface has a normal value about 15-20 nT (many different papers report this value, see, e.g., [Sibeck and Tsyganenko, 1994]). However during the strong disturbances the extremely large enhancement of geotail magnetic flux is detected [Kaufman, 1987; Fennell et al., 1993]. The main source of this perturbation is the intense radially localized currents in the inner part of the geotail current sheet arising during strong disturbances. These currents together with the closure currents at the dayside magnetopause form the current loops which are located close to the equator. These current circuits produce the magnetic field in the inner magnetosphere like as the ring current. During strongly disturbed periods when the plasma sheet moves toward the Earth and the stand-off distance decrease as well, the intense currents in the inner part of geotail may produce the magnetic field perturbation at the Earth's surface comparable with perturbation produced by the ring current.
The magnetospheric magnetic field model capable to describe dynamic changes in the Earth's magnetosphere will be using. The independently obtained experimental data will serve as input parameters. It is supposed, that the planetary distribution of the magnetic field is due to the following sources:
(1) the dipole field;
(2) the magnetic field of the Chapman-Ferraro currents at the magnetopause (screening the dipole field);
(3) the magnetic field of the ring current;
(4) the magnetic field of the magnetospheric tail current system;
(5) the magnetic field at the magnetopause, which screens the ring current effect.
The paraboloid model [Alexeev, 1978; Alexeev et al., 1996] allows a description of the disturbance caused by each of these components of the magnetospheric magnetic field separately. This is very important for calculation of dynamic processes, since different magnetospheric sources of the magnetic field change with different characteristic times. The input parameters to the new magnetospheric magnetic field model (distance to the subsolar point, distance to the inner boundary of the tail current sheet, ring current intensity, etc.) can be fitted using obervational data, both from ground- based and satellites observations.
Let us calculate the magnetic field on the Earth's surface using independent experimental data of the magnetospheric disturbances on November 23-27, 1986.
Figure 1 show the available data to evaluate the input parameters of the model. Figures 1a,1b,1c shows King's catalogue data of the solar wind parameters in the interval November 23-27, 1986. These data allow calculation of the time dependence of the geocentric distance R1 to the subsolar point based on [Roelof and Sibeck, 1993] results. The AL index shown in Figure 1d was determined by data of auroral observatories.
We suppose that equatorward boundary of auroral oval corresponds to the inner edge of the geotail current sheet. The position of the equatorward boundary of auroral oval at noon and midnight were calculated using data of DMSP F06 and F07 auroral oval crossings during considered intervals. The polar oval equatorial boundary radius and shifts realting the north pole were calculated. Mapping of the midnight equatorial boundary along the dipole magnetic field line allow us to determine the distance to the geotail plasma sheet inner edge R2.
In Figure 1f the Dst index is presented. It was constructed using the data of the longitudinal chain of the 8 low-latitude stations for a magnetic storm November 23-27, 1986.
First of them F0 corresponds the slow adiabatic geotail evolutiond during the quite period. Inspite of the dimensional parameters and geomagnetic tail magnetic field variability it remain equal to constant [Rostoker and Scone, 1993]. We use the value
F0 = 3.7 * 10**8 Wb
accordance with [Stern and Alexeev, 1988]. Another term is connected with substorm activity and proportional to the (AL. For the paraboloid model of the magnetosphere it reads
Fs = -AL/7 * (pi R1**2)/2 * sqrt(2 R2/R1 + 1)
In [Lopez and von Rosenvinge, 1993; Alexeev et al., 1996] the relation of auroral activity and geotail magnetic field intensity was discussed.
One of the elements of the monitoring of the magnetospheric energy balance is the energy of the ring current. Using the total energy of the plasma particles carrying the current it is possible to calculate the intensity of the disturbance associated with the ring current. The AMPTE/CCE measurements inside the ring current allow us to estimate the total particle energies during the intervals under consideration. The Dessler-Parker-Scopke equation
BR / Bs = 2K/3 Um,
allow to calculate the ring current distribution into Dst. (Here BR is the magnetic perturbation due to ring current at the Earth's centre, Bs - dipole field at the Earth's equator, K - total energy of ring current particles, and Um - energy of the dypole field outside the Earth).
The paraboloid model input parameters are presented in the Figure 2. We can see the significant growth of the geotail lobe magnetic flux associated with high auroral activity in the Earth's ionosphere. According to [Alexeev et al., 1992, 1996] these variation are responsible for the fast Dst changes in the disturbed magnetosphere. We can see also the significant reducing of the dimensional parameters of the magnitosphere R1 and R2, associated with the growth of solar wind dynamical pressure and earthward shift of the inner boundary of the plasma sheet in the magnetospheric tail. As a consequence of this "compression" the geotail related part of the Earth's measured magnetic perturbation can reach the significant values.
The results of calculation are presented in the Figure 3. The fast changes in Dst profoles correspond to the geotail lobe magnetic flux enhancements. The ring current and geomagnetic tail contributions to Dst are approximately of one order.
Our global dynamical model connects phenomena in the solar wind to processes occurring in the Earth's magnetosphere, ionosphere, and upper atmosphere. The actual experimental data used to compute the large-scale magnetospheric current system contributions to Dst show that the paraboloid model is enough reliable.
The calculations carried out by paraboloid model using data of on-ground, AMPTE/CCE and DMSP F06, F07 spacecraft measurements show that Dst is not only ring current related index. The other magnetospheric magnetic field sources (geotail current system, mainly) are responsible for the significant part of this perturbation, as well as for the fast changes of Dst profile.
The ring current and geomagnetic tail contributions to Dst are approximately of one order. We can see also from the Figure 3 a reasonable fit between the model calculations and Dst.
Acknowledgement. We appreciate very much to M. Greenspan for AMPTE/CCE ions energy measurements data inside the ring current.
REFERENCES
Alexeev, I. I., Regular magnetic field in the Earth's magnetosphere, (in Russian), Geomagn. Aeron., 18, 656, 1978.
Alexeev, I. I., V. V. Kalegaev, and Ya. I. Feldstein, Modelling of the magnetic field in a strongly disturbed magnetosphere, Geomagn. Aeron., (in Russian), 32, 8, 1992.
Alexeev, I. I., E. S. Belenkaya, V. V. Kalegaev, Ya.I. Feldstein, and A. Grafe, Magnetic Storms and Magnetospheric Currents, J. Geophys. Res., 101, 7737-7749, 1996.
Fennel, J. F., Roeder, J. L., Spence, H. E., Grande, M. and Livi, S., CRRESS observations of local morning and evening flux drop out events, EOS Suppl., 1993, 74, N 43, p.303.
Kaufmann, T. G. Substorm currents: Growth phase and onset, J. Geophys. Res., 92, 7471, 1987.
Lopez, R. E., T. von Rosenvinge, A statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude, J. Geophys. Res., 98, 3851, 1993.
Roelof, E.C., and D.G.Sibeck, The magnetopause shape as a bivariate function of IMF Bz and solar wind dynamics pressure, J. Geophys. Res., 98, 21421, 1993.
Rostoker, G., and S. Skone, Magnetic flux mapping consideration in the auroral oval and the Earth's magnetotail, J. Geophys. Res., 98, 1377, 1993.
Stern, D.P., and Alexeev, I.I., Where do field lines go in the quiet magnetosphere, Rev. Geophys., 26, 782, 1988.
Figure 1. (a) solar wind velocity, (b) solar wind density, and (c) Bz component of IMF, (d) AL index, (e) midnight equatorward boundary of the auroral oval, (f) Dst index for the magnetic storm of 23-27 November 1986.
Figure. 2. Magnetic flux via the magnetosphere tail lobes " [Wb] (a), distance to the earthward boundary of current sheet in the tail R2 [RE] (b), distance to the subsolar magnetopause point R1 [RE] (c) for the magnetic storm of 23-27 November 1986.
Figure 3. a) Model of the magnetopause currents magnetic field BCF [nT] - marked by triangles, magnetic field of tail current BT [nT] - marked by asterisks, calculated in the framework of paraboloid magnetospheric model, the magnetic field of the ring current BR [nT] - marked by circles, calculated from the total energy of the ring current particles measured by AMPTE/CCE CHEM device using Dessler-Parker-Scopke equation during the magnetic storm of 23-27 November 1986.
b) Comparison of Dst (heavy solid line) and the total modelled magnetic field Bmod (fine line) on the Earth's surface for the magnetic storm of 23-27 November 1986.
4. CONCLUSIONS
Figure Captions
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