R P Lepping A Szabo and K W Ogilvie (Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD 20771, 301-286-5413) C E DeForest (SOHO MDI/Stanford Univ. Stanford, CA 94305)
In studies that attempt to relate interplanetary magnetic clouds to photospheric phenomena many important questions remain and some relevant quantities are not yet well estimated. We consider two such quantities: (1) the typical characteristic magnetic field intensity (Bs) of the photospheric flux tube (or effective flux tube) that develops into a magnetic cloud at 1 AU and (2) the flux tube's effective radius (Rs) assuming cylindrical symmetry. [We refer to an effective flux tube at the Sun and a flux rope (or magnetic cloud) at 1 AU for clarity of identification and because a 'flux tube' is less restrictive at the Sun.] Twenty nine magnetic clouds seen at 1 AU are examined; these comprise 11 examples from WIND data and the remainder from IMP-8, earlier IMP's, ISEE-3, and HEOS. Employing a simple 7 parameter constant alpha force free flux rope model (where J = alpha * B) to determine the properties of the magnetic clouds at 1 AU, in particular their diameters (Do = 2Ro) and axial magnetic field strengths (Bo), we are able to estimate the total magnetic flux along the typical cloud, i.e., normal to its cross-section. The expression for the total magnetic flux, Fo, from the model is 0.432 Bo pi Ro^2. For the 29 events the average Do is 0.27 +/- 0.10(1 sigma) AU and the average Bo is 17.7 +/- 6.3 nT giving an average magnetic flux of Fo = 9.85 x 10^10 G km^2, which we will consider to be representative of a typical case. Assuming conservation of flux between the magnetic cloud and this ideal cylindrical solar flux tube, we have Fo = F(Sun) = Bs pi Rs^2, where Bs and Rs refer to the effective solar flux tube; this, of course, holds only during an interval when supposed field-line connection between the photospheric flux tube and the magnetic cloud occurs. We will investigate the implications of Rs = (Fo/pi Bs)^1/2. For example, if Bs = 100 G, a not unreasonable field intensity in the vicinity of an active region in the photosphere, we see that Rs = 1.8 X 10^4 km, which is only 1.3% of the Sun's radius. We will estimate the degree of reasonableness for values of Rs around this one for the radii of such flux tubes. The nature of such a structure will be discussed, based on recent SOHO observations.
TOP