Published in the Proceedings of the Fourth International Conference on Substorms, Hamanako, Japan, March 9-13, 1998.
ELECTRON PRECIPITATION CAUSED BY CONVECTION INTO THE LARGER LOSS CONE ON LOWER-LATITUDE FIELD LINES
W. Calvert
Iowa City, Iowa, 52245, USA
Abstract. As a consequence of the increasing size of the loss cone on lower-latitude field lines, a weak band of aurora is predicted to occur at the high-latitude edge of the auroral zone during the inward convection that supplies electrons to the auroral zone during a substorm.
1. Introduction
As shown in Figure 1, the electrons that cause the aurora during a substorm can be accounted for by the inward convection that is driven by the cross-tail electric field in the near-Earth plasma sheet, where the dotted line in this figure illustrates the time that it takes for this convection to occur, as previously analyzed by Ejiri [1978]. The diffuse and discrete aurora during a substorm can then be attributed to scattering into the loss cone inside the electron acceleration region, as discussed by Calvert [1995, 1997].
Figure 1. Schematic illustration of the aurora during a substorm, showing as a dotted line the inward convection that supplies electrons to auroral zone field lines during a substorm, substorm onset, the diffuse aurora prior to substorm onset, the latitudinal expansion of the discrete aurora during sub storm expansion, and the aurora that is discussed in this paper that occurs at the high-latitude edge of the auroral zone.
The inward convection that supplies electrons to the auroral zone during a substorm is therefore found to take about an hour in order to reach the magnetic latitudes where substorm onset occurs, by which time the loss cone for electron precipitation into the ionosphere ought to be almost com pletely empty as a result of precipitation into the ionosphere at higher latitudes in the auroral zone. This model for the source of the electrons that cause the aurora therefore also requires scattering into the loss cone in order to account for the aurora during a substorm.
On the other hand, since it contributes to the aurora, it is also relevant to calculate the precipitation flux that empties the loss cone during the inward convection that supplies the electrons to the auroral field lines on which the aurora occurs. The purpose of this paper is therefore to calculate the precipi tation flux that is caused by the inward convection that empties the loss cone as a result of the cross-tail electric field in the near-Earth plasma sheet.
2. Convection into the Loss Cone
The convection velocity of an electron that convects inward from the tail of the Earth's magnetosphere is given by
where E and Bo are the electric and magnetic fields in the near-Earth plasma sheet. Because it occurs perpendicular to the magnetic field, except for the increase in energy that occurs inside the electron acceleration region, the pitch angle and energy of an electron do not change significantly during the convection that supplies the electrons to the auroral zone during a substorm. As a result, because of the short electron bounce period on the closed magnetic field lines on which the aurora occurs, the electron precipitation that empties the loss cone during this inward convection occurs as a result of the larger size of the loss cone on lower-latitude field lines, as shown in Figure 2.
Figure 2. Loss cone at the equator as a function of the magnetic latitude in the auroral zone, where e and m are the electron charge and mass, and V is the electric potential of the electron acceleration region.
The precipitation flux that is caused by inward convection can then be calculated by integrating the flux that is given by Equation (1) times the difference in the solid angle of the loss cone as a function of the magnetic latitude in the auroral zone. The result of this calculation is then shown in Figure 3, for a dipole magnetic field, a cross-tail electric field of 1 mV/m perpendicular to the magnetic meri dian in the midnight sector of the Earth's magnetosphere, a 1 keV average energy of an isotropic electron velocity distribution at the high-latitude edge of the auroral zone, and 10 kV for the electric potential of the electron acceleration region.
Figure 3. Electron precipitation caused by convection into the loss cone by the cross-tail electric field in the near-Earth plasma sheet.
The predicted precipitation therefore varies from only about 107 electrons/cm2 sec at the high- latitude edge of the auroral zone, to less than 105 electrons/cm2 sec near the low-latitude edge where substorm onset occurs. Since the electron precipitation during a substorm is found to be in the range of 107 to 109 electrons/cm2 sec, this model cannot account for more than about 1% of the electron precipitation during a substorm. Although this model cannot account for the aurora during substorm expansion, it can explain a weak band of aurora that is predicted to occur at the high latitude edge of the auroral zone, as shown in Figure 1.
3. Conclusions
A weak band of aurora is predicted to occur at the high-latitude edge of the auroral zone as a result of the inward convection that is driven by the cross-tail electric field in the near-Earth plasma sheet. As a consequence, there are five kinds of aurora that can be attributed to inward convection and scat tering into the loss cone, consisting of (1) the diffuse aurora prior to substorm expansion, as shown in Figure 1, (2) the discrete aurora during a substorm that is shown in Figure 1, (3) the aurora that is discussed in this paper, (4) the aurora that occurs outside discrete arcs during substorm expansion, as discussed in another paper at this meeting, and (5) the aurora that can also occur above the high- latitude edge of the auroral zone as a result of a filled loss cone at the top of the acceleration region, as previously calculated by Calvert [1998].
Acknowledgments. This work was supported in part by NASA contract NAS5-96020. Useful discussions with M. Ejiri of the Polar Research Institute in Tokyo, Japan, are also gratefully acknowledged.
References
Calvert, W., An explanation for auroral structure and the triggering of auroral kilometric radiation, J. Geophys. Res., 100, 14,887-14,894, 1995.
Calvert, W., Uji Lectures on the Aurora, copyright W. Calvert, RASC, Kyoto University, Uji, Japan, August 1, 1997.
Calvert, W., Predicted electron precipitation for a filled loss cone at the top of the auroral electron acceleration region, Geophys. Res. Lett., 25, 13-16, 1998.
Ejiri, M., Trajectory traces of charged particles in the magnetosphere, J. Geophys. Res., 83, 4798- 4810, 1978.
Correspondence: W. Calvert, 219 Friendship Street, Iowa City, Iowa, 52245, USA; email: calvert@fyiowa.infi.net.