“…In lines emitted from ions with temperatures ranging from the lower transition region (20,000 K) through the corona (106 K), loops have been directly observed, or inferred to exist, above photospheric magnetic bipoles (Pre s & Phillips 1999 ;Kankelborg et al 1996Kankelborg et al , 1997Falconer et al 1996Falconer et al , 1998Dowdy 1993 ;Fontenla et al 1989 ;Mariska 1986 ;Bonnet 1980) for a wide range of size scales. Recent observations with high temporal and spatial resolution show that many of these loop structures are transient, dynamically evolving structures (see, for example, Kjeldseth-Moe & Brekke 1998 ;Yun et al 1998 ;Wang et al 1997 ;Korendyke et al 1995 ;Strong 1994). Nevertheless, a large number of loop structures have been observed to persist for periods of time that are long, compared to radiative and conductive cooling timescales for a fully ionized plasma, without undergoing dramatic changes in either their luminosities or structures (see, for example, Craig & McClymont 1981 ;Veseckey, Antiochos, & Underwood 1979 ;Gerassimenko et al 1978).…”