Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1071–1075 www.elsevier.com/locate/jastp Further studies of the Perkins stability during Space Weather Month Michael C. Kelley ∗ , Jonathan J. Makela 1 , Michael N. Vlasov, Avik Sur School of Electrical and Computer Engineering, Cornell University, 318 Rhodes Hall, Ithaca, NY 14853, USA Received 2 May 2002; received in revised form 17 June 2003; accepted 28 July 2003 Abstract A simple integral equation describing the height of the mid-latitude ionosphere is presented and compared with observations and a more sophisticated model. At any instant, the ionosphere moves toward an equilibrium altitude, if it exists, which is determined by a combination of southward wind and eastward electric eld. If no equilibrium exists, the layer simply falls at its terminal velocity or at a wind or electric eld-aided rate. Including a simple recombination model provides an even more complete characterization of the nighttime ionosphere. By comparing the measured and predicted content, an estimate of the ux from the plasmasphere is possible. c 2003 Elsevier Ltd. All rights reserved. Keywords: Perkins stability; Midlatitude ionosphere; Ionospheric modeling 1. Introduction Until recently, ionospheric science has been based solely on experiment, theory, and simulation/modeling. To move into a new phase characterized by the term Space Weather, this triad must be expanded to include data assimilation. This is not simply a matter of making larger or even bet- ter physical models—a new way of thinking is required. It is also important to revisit the basic questions that we are asking. In this series of papers (Makela et al., 2003; Sojka et al., 2003; Vlasov et al., 2003), we record some rst steps in data assimilation based on measurements made during Space Weather Month, September 1999. September 1999 was the second of the Coordinated Ionosphere Campaigns (CIC99). Kelley et al. (2000) and companion papers reported on the rst CIC. In September ∗ Corresponding author. Tel.: +1-607-255-7425; fax: +1-607- 255-6236. E-mail address: mikek@ece.cornell.edu (M.C. Kelley). 1 Now at the E.O. Hulburt Center for Space Research, Code 7607, the Naval Research Laboratory, Washington, DC 20375, USA. 1999 the World Days were extended by a separate Arecibo proposal, leading to a total of 8 out of 9 consecutive nights of coverage. Nighttime data for the whole campaign are summarized in Fig. 1 with relevant interplanetary and other geophysical parameters presented in Fig. 2. There was a period of moderate magnetic activity pre- ceding a major storm beginning on September 12. The full World Day period (September 15–17) is described in Makela et al. (2003), who report on the very complex electrical behavior in this period, using both Jicamarca and Arecibo data. Prompt penetrating and disturbance dynamo elds were detected and a new feature, seem- ingly related to the y-component of this IMF, was iden- tied in the Jicamarca data (Kelley and Makela, 2002). Vlasov et al. (2003) use analytical theory to show that plasma ow from the plasmasphere into the ionosphere is required to explain the maintenance of the F layer on the night of September 16–17, 1999. Sojka et al. (2003) show that an assimilative model which does not include this high ux will create erroneous ow elds to force compliance with the assimilated data stream. Valida- tion of such models, using complete data sets such as those available from incoherent scatter radars, is thus found to be important indeed. 1364-6826/$ - see front matter c 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2003.07.001