Auroral oval dynamics: Comparison of APEX and DMSP observations

Auroral oval dynamics: Comparison of APEX and DMSP observations

Adv. Space Res. Vol. 22. No. 9, pp. 1341-1344, 1998 Q 1998COSPAR. Published by Elscvier Science Ltd. Ail rights resctvcd Pergamon Printed in Great...

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Adv. Space Res. Vol. 22. No. 9, pp. 1341-1344,

1998

Q 1998COSPAR. Published by Elscvier Science Ltd. Ail rights resctvcd

Pergamon

Printed in Great Britain 0273-I 177/98 $19.00 + 0.00 PII: SO273-I 177(98)00183-S

AURORAL OVAL DYNAMICS: DMSP OBSERVATIONS J. Safrinkova,

COMPARISON

OF APEX

AND

Z. NBmeEek, J. Merka

Faculty of Mathematics and Physics, Charles University,

V HoleJoviEkdch 2, 18000 Prague 8, Czech Republic

ABSTRACT The present paper deals with a detailed study of dynamics of the polar cap boundaries. The study involves data from two satellites of the APEX project, and data from three DMSP satellites. The APEX satellites are moving along an elliptical polar orbit and provide charged particle measurements at different altitudes (up to 3000 km), the DMSP satellites are orbiting in an altitude of 835 km. Different regions of the polar oval are identified using the characteristics of the precipitating particles in the energy range 0.1- 30 keV which have been registered onboard all satellites. A fast equatorward shift of the auroral oval, which can reach up to 0.2 degrees per minute during the geomagnetic substorms, is characterized by notable redistribution of the precipitating flow between the discrete and diffuse auroras in favour of the diffuse part. A comparison of the energy of the precipitating particles in different altitudes allows US to estimate the value of sn electrostatic potential jump between these two altitudes. Q1998 COSPAR. published by Elsevier Science Ltd. INTRODUCTION The distribution of the energy of the auroral plasma (hundreds of eV to several keV) in the magnetosphere is among the most important characteristics of the global magnetospheric morphology. At present, the large-scale structure and dynamics of particle precipitation in the ionosphere and the issue of mapping these regions to the outer magnetosphere is under active discussion (Feldstein et d, 1995). The linear relationships of the equatorward shift of the nightside auroral oval position and the decrease of the IMF Bs magnitude along with the increase of geomagnetic activity are established. The equatorial boundary position of the night oval is far less sensitive than the polar cusp region to small-scale variations of IMF Es and weak substorms (e.g., Kamide and Akasofu, 1974). Craven and Frank (1990) reported a rapid poleward motion of the poleward boundary and a less rapid equatorward motion of the equatorward boundary of the nightside oval during the expansion phase of the substorms. Global oval dynamics has been mostly studied by optical observations (Elphistone et al., 1995) because a similar study of the precipitating particles needs real multipoint measurements. The comparison of the optical imaging of the oval with simultaneous in situ particle measurements was made by Newell et al. (1992). All studies of the oval location take the equatorward boundary position as the measure because the poleward boundary seems to be much less stable and subjected to fast random changes. Our study is baaed on the simultaneous measurements of precipitating particles in the northern hemisphere by three DMSP spacecraft and by one APEX satellite in the beginning of 1992. The satellite configuration allows us to determine the position of the nightside auroral oval at different heights and thus to test the models of the magnetospheric magnetic field and to study the field aligned potential drop between two altitudes. INSTRUMENTATION The data analyzed in this paper have been obtained by the charged particle spectrometers onboard the APEX (Active Plasma Experiment) satellite. The charged particle package consisted of several electrostatic analyzers which measured electron and ion fluxes in 16 energy steps placed logarithmically in the range from 0.2 to 25keV. The spectrometers had narrow angular characteristics and they were directed into different directions to obtain optimal pitch angle coverage. A more complete description has been published in Nemecek et ol. (1999). To study the global structures we have combined our measurements with the data from three DMSP satellites. The spectrometers onboard provide the electron/ion distribution once per second in the energy range from 0.03 to 30 keV in twenty logarithmically spaced steps. The detector aperturea are always pointed toward the local zenith. For our purpose, we have used the automatically processed data to classify the precipitating particles into seven different regions according to their probable source (Newell et al., 1991), or the full energy spectra. 1341

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EXPERIMENTALRESULTS We have chosen from a relatively -6:OOand N in Figure 1. A negative during magnetosheath.

for our detail study the geomagnetic disturbance which occurred on April 3, 1992 and which started long period of the weak geomagnetic activity. The evolution of the auroral oval boundaries between 18 : 00 MLT complete with the Kp and D ST indexes and the magnetic field data are summarized plot of the IMF Bs component shows that this component is positive for first five hours and is following 4 hours. The IMF data after 9:00 UT are not reliable because IMP-8 is probably in the

The event starts from very quiet geomagnetic conditions (Kp = O+, IMF Bs positive, and DST = -10). The auroral oval is relatively broad, about 17’ of latitude, and its poleward edge reaches 85O of latitude for both MLT. The position of the auroral oval indicated in Fig. 1 by hatched aress has been determined from the DMSP measurements, i. e. in the altitude of about N 830 km. Despite IMF Bz being positive up to 5:lO UT the auroral oval moves rapidly toward the equator. This fluctuation can be caused by the slight increase of the solar wind dynamic pressure (not shown) which continues up to 9:00 UT. After this time no solar wind data are available. The increase of the geomagnetic activity between 3:00 UT and 16:00 UT is connected with the intensification of the ring current intensity as documented by the DST index. The position of the auroral oval boundaries determined at higher altitude from the APEX data corresponds well with the boundaries determined from the DMSP measurements. The dots stand for the same boundaries determined by the APEX observations at altitudes between 2500 and i3000km and mapped to the DMSP altitude using the Tsyganenko model (Tsyganenko, 1990). The small difference on the equatorward boundary seems to have a systematic character which can be attributed either to the insufficiency of the magnetic field model or to the presence of external forces which shift the particles in the latitudinal direction. The electron and ion energy distributions measured simultaneously during low geomagnetic activity (until 07:OOUT) by the APEX and the DMSP satellites are very similar and peak at the same energy in corresponding latitudes. It indicates no potential difference between the satellites. The pitch-angle distribution measured at the altitude of about h 2300 km in the discrete aurora region exhibits features which are typical for the presence of a field aligned potential jump above this altitude - accelerated electron population, upgoing electrons scattered at the atmosphere, and reflected back by an electrostatic potential (Hoffman, 1993). The sequence of dynamic spectrograms in Figure 2 shows the evolution of the precipitating flows during the disturbed period (16:45 - 18:30 UT). The measurements were taken at two altitudes - 835 km (top and bottom panels) and N 2300 km (middle panels). The DMSP spectrogram shows an ion population with an energy of N 4 keV at 16:49 16:50 UT. We suggest that it is the same population which is registered by APEX at 17:20 - 17:24 UT but at an energy N 2 keV. The second high energy ion population (- 20 keV) in the APEX ion spectrogram is not seen by DMSP at 1649 - 1650 UT but it is probably the same as that registered by DMSP at 18:32 - 18:34 UT. The energy of this population is too high with respect to the APEX and DMSP energy range and thus the shift of the energy cannot be reliably determined. The energy of the precipitating electrons in both DMSP spectrograms does not exceed IkeV, but the energy of the APEX electrons is about 6 keV at the maximum of energy flux. The difference in particle energies registered at different altitudes indicates the presence of an electrostatic potential jump. A value of the potential difference is about 2 kV and corresponds to a field aligned electric field of N 1 - 2 mV/m. It is not a big value but the direction is opposite to that usually expected for the auroral field aligned potential drop (Hoffman, 1993). The pitch-angle distributions plotted in Figure 3 for the time interval from 17:24 - 17:25 UT again indicate the presence of a potential drop decelerating the electrons. The electron distribution exhibits a large proportion of the upgoing electrons which cannot be reflected by the magnetic mirror and, on the other hand, the ion distribution shows a depletion at pitch-angles which correspond to the magnetic mirroring. From the presence of the trapped electron population at pitch angles N 60° and N 120° the retarding potential above the APEX altitude can be deduced. DISCUSSION AND CONCLUSION We have presented one example of simultaneous APEX-DMSP measurements but a study of other similar cases indicates that the described features are typical. A good correlation between the DST index and the position of the oval equatorial boundary has been noted by Meng (1984) and suggests that the intensification of the ring current can be the source of the boundary motion. However, the poleward boundary starts its equatorial motion before Bz becomes negative as is demonstrated by Figure 1. This boundary probably either follows the slight continuous increase of the solar wind dynamic pressure or moves due to the decrease of the IMF BZ component (but not to negative values) which occurs in this time interval. The poleward boundary is a projection of the outer part of the plasma sheet boundary layer and thus should be sensitive to the fluctuation of the solar wind parameters and to the

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Fig. 2: A comparison of the DMSP and APEX dynamic spectrograms of the precipitating particles. (The positions of both satellites are following: at 16:49 UT (DMSP) - MLT = 20:54, ALT = 835 Inn, INL - 69’, at 17:ZO UT (APEX) - MLT = &0:12, ALT = 2411, INL - 55“, at 18:32 UT (DMSP) - MLT = 21:00, ALT = 835 km, INL - 67“. The gray scale in the APEX spectrograms (middle panels) is in counts but it corresponds to the DMSP scale.

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Fig. 1: Evolution of the geomagnetic disturbance on April 3, 1992 (from top to bottom: IMF modulus, IMF Bz component, DST index, Kr indez, the aurora1 oval position for two magnetic local times (- 6 : 00 and - 18 : 00 MLT) horizontally hatched area corresponds to the difise aurora, vertically hatched area corresponds to the discrete aurora, dots - the high altitude determination of the oval position by the APEX).

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Fig. 3: The energy/pitch-angle distribution of the charged particles observed at 2300 km altitude during disturbed geomagnetic period ( Kp N 6).( The horizontal axis is labeled in degrees of the pitch-angle.)

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which follows each change of the IMF direction.

Another interesting feature of the polar oval expansion is a redistribution between two, clearly distinguishable, types of precipitation. The initial contraction of the auroral oval width is caused by the narrowing of the discrete aurora region. The following equatorward shift of the equatorial boundary is a consequence of the expansion of the diffuse aurora region. These two effects result in the fact that the boundary between these two types of precipitation is more stable than the oval itself during the expansion phase of the substorm. This boundary is generally accepted as a boundary between the regions with closed and opened field lines. Its stability during the initial phase of the substorm indicated that the small changes of the magnetic field configuration which are connected with the equatorward movement of the polar edge,of the aurora1 oval did not change the magnetic field configuration equatorward of this boundary. The position of the auroral oval boundaries determined at different altitudes is consistent with the concept of auroral oval precipitation along magnetic field lines if an appropriate model of the magnetic field is used for the mapping. A comparison of the energies of the precipitating particles at two altitudes indicates no remarkable change from the APEX to DMSP altitudes during the period of a low geomagnetic activity. However, if the geomagnetic activity is high the energy of electrons registered at high altitude is significantly higher than that observed at lower altitude. Since the ion energy increases with decreasing altitude the observed changes of the energy of precipitating particles can be explained by an electrostatic potential jump between the observational points. The source of this potential jump is not clear but may be connected either with the higher energy or with the higher flux of the precipitating particles during the disturbed geomagnetic periods. ACKNOWLEDGEMENTS. 202/97/1122

The present work was supported by the Czech Grant Agency under Contract

and by the Charles University Grant Agency, Contract

No. 23/1997.

No. 205/96/1575

and

The authors are grateful to P. T. Newell for

the DMSP data.

REFERENCES Craven, J. D., and L. A. Frank, Diagnosis of auroral dynamics using global auroral imaging with emphasis on large scale evolutions, in: Aurora1 Physics, 249 (1990). Elphistone, R. D., et al., Observations in the vicinity of substorm onset: Implications for the substorm process, J. Geophys. Res., 100, 7937 (1995). Feldstein, Y. I., P. T. Newell, I. Sandahl, J. Woch, S. V. Leontjev, and V. G. Vorobjev, Structure of auroral precipitation during a theta aurora from multisatellite observations, J. Geophys. Res., 100, 17429 (1995). Hoffman, R. A., From balloons to chemical releases - what do charged particles tell us about the auroral potential region?, in: Aurora1 Plasma Dynamics, Geophysical Monograph 80, AGU, I33 (1993). Kamide, Y., and S.-I. Akasofu, Latitudinal cross section of the auroral electrojet and its relation to the interplanetary magnetic field polarity, J. Geopha. Res., 79, 3755 (1974). Meng, C.-I., Dynamic variation of the auroral oval during intense magnetic storms, J. Geophys. Res., 89, 227 (1984). Nemecek, Z., J. Safrankova, L. Prech, P. Holub, J. Merka, Dynamics of the polar cap boundaries: Multipoint measurements, Adv. Space Res., 18, (8)131 (1996). Newell, P. T., S. Wing, C.-I. Meng, V. Sigillito, The auroral oval position, structure, and intensity of precipitation from 1984 onward: An automated on-line data base, J. Geophys. Res., 96, 5877 (1991). Newell, P. T., C.-I. Meng, R. E. Huffman, Determining the source region of auroral emissions in the prenoon oval using coordinated polar BEAR UV-imaging and DMSP particle measurements, J. Geophys. Res., 97, 12245 (1992). Tsyganenko, N. A., Quantitative models of the magnetospheric magnetic field, Space Sci. Rev., 54, 75 (1990).