Solar wind-magnetosphere energy input functions

A new formula for the solar wind-magnetosphere energy input parameter, P/sub i/, is sought by applying the constraints imposed by dimensional analysis. Applying these constraints yields a general equation for P/sub i/ which is equal to rho V/sup 3/l/sub CF//sup 2/F(M/sub A/,theta) where, rho V/sup 3/ is the solar wind kinetic energy density and l/sub CF//sup 2/ is the scale size of the magnetosphere's effective energy ''collection'' region. The function F which depends on M/sub A/, the Alfven Mach number, and on theta, the interplanetary magnetic field clock angle is included in the general equation for P/sub i/ in order to model the magnetohydrodynamic processes which are responsible for solar wind-magnetosphere energy transfer. By assuming the form of the function F, it is possible to further constrain the formula for P/sub i/. This is accomplished by using solar wind data, geomagnetic activity indices, and simple statistical methods. It is found that P/sub i/ is proportional to (rho V/sup 2/)/sup 1/6/VBG(theta) where, rho V/sup 2/ is the solar wind dynamic pressure and VBG(theta) is a rectified version of the solar wind motional electric field. Furthermore, it is found that G(theta), the gating function which modulates the energy input to the magnetosphere, is well represented by a ''leaky'' rectifier function such as sin/sup 4/(theta/2). This function allows for enhanced energy input when the interplanetary magnetic field is oriented southward. This function also allows for some energy input when the interplanetary magnetic field is oriented northward. 9 refs., 4 figs

Superposed epoch analysis of magnetospheric substorms using solar wind, auroral zone, and geostationary orbit data sets

A primary goal in solar wind-magnetosphere studies is to understand how and what role the solar wind plays in controlling the temporal sequence of events during substorms at many widely separated regions within the magnetosphere. Here, the average, correlated behavior of definitive solar wind, auroral zone, and geostationary orbit parameters during isolated substorms is examined. High time resolution (1 min) measurements of two solar wind quantities: B/sub z/ and VB/sub s/, two auroral electrojet indices: AE and AL, and three parameters which define the energetic (30 to 300 keV) electron distribution at geostationary orbit from 13 events are analyzed by using the superposed epoch technique. The zero epoch time used to organize the analyses were defined by the time of energetic electron injection at geostationary orbit. The average variations of the auroral zone and geostationary orbit parameters in relation to the solar wind are discussed in context of the three phase (growth, expansion, and recovery) model of substorms. Notably, we find an approximate 6 minute time lag of expansion phase onset at geostationary orbit relative to time of expansion phase onset in the auroral zone. A possible explanation for this time lag is briefly discussed. 14 refs., 8 figs., 1 tab

Linear filters as a method of real-time prediction of geomagnetic activity

Important factors controlling geomagnetic activity include the solar wind velocity, the strength of the interplanetary magnetic field (IMF), and the field orientation. Because these quantities change so much in transit through the solar wind, real-time monitoring immediately upstream of the earth provides the best input for any technique of real-time prediction. One such technique is linear prediction filtering which utilizes past histories of the input and output of a linear system to create a time-invariant filter characterizing the system. Problems of nonlinearity or temporal changes of the system can be handled by appropriate choice of input parameters and piecewise approximation in various ranges of the input. We have created prediction filters for all the standard magnetic indices and tested their efficiency. The filters show that the initial response of the magnetosphere to a southward turning of the IMF peaks in 20 minutes and then again in 55 minutes. After a northward turning, auroral zone indices and the midlatitude ASYM index return to background within 2 hours, while Dst decays exponentially with a time constant of about 8 hours. This paper describes a simple, real-time system utilizing these filters which could predict a substantial fraction of the variation in magnetic activity indices 20 to 50 minutes in advance

Application of dimensional analysis to the problem of solar wind-magnetosphere energy coupling

The constraints imposed by dimensional analyses are used to find how the solar wind-magnetosphere energy transfer rate depends upon interplanetary parameters. The analyses reported here assume that only magnetohydrodynamic processes are important in controlling the rate of energy transfer. The study utilizes ISEE-3 solar wind observations, the AE index, and U/sub T/ from three 10-day intervals during the IMS: Simple linear regression and histogram techniques are used to find the value of the MHD coupling exponent, ..cap alpha.., which is consistent with observations of magnetospheric response. Once ..cap alpha.. is estimated, the form of the solar wind energy transfer rate is obtained by substitution into an equation of the interplanetary variables whose exponents depend upon ..cap alpha... 7 references, 6 figures, 1 table

Evaluation of the total magnetospheric energy output parameter, U/sub T/

Over the last few years the relationship between U/sub T/, the magnetospheric energy consumption or output rate, and epsilon, a commonly used solar wind-magnetosphere energy input function, has been explored in some detail. Very high correlations between U/sub T/ and epsilon are found during periods of strong activity, and by using linear prediction filtering techniques a ''delta-function'' impulse response was found for filter elements representing essentially zero delay. In light of these remarkable results, the derivation of U/sub T/ for these intervals is re-examined. We find that U/sub T/ is dominated in each event interval by the term containing tau/sub R/, the ring current decay time, and that when tau/sub R/ is defined as a function of epsilon the ''delta-function'' impulse response is present. If a constant tau/sub R/ is assumed, the delta-function part of the filter disappears completely. Thus, this delta-function, which has been taken as being indicative of the directly driven component is an artifact of the earlier analysis, and it is due to the dependence of U/sub T/ on epsilon. Our results imply that until U/sub T/ can be derived independently from epsilon, these two quantities cannot be compared in a meaningful way, and that results obtained in previous studies are not valid