Dissipation Range Observations in Interplanetary Magnetic Clouds

In two earlier papers Leamon et al. [1998a,b] examined the properties of the dissipation range for interplanetary magnetic fluctuations at 1 AU. In the first paper they focused on 33 1-hour samples of open field line measurements chosen without any regard for context other than being sufficiently well-behaved for sufficient time to yield good spectra. All 33 intervals were chosen from WIND/MFI measurements in the solar wind near 1 AU. They found that the dissipation range typically set in at frequencies slightly greater than the proton cyclotron frequency, had consistently steeper forms than the associated inertial range spectra with power law indexes generally between -3 and -5, were consistently more compressive than the inertial range, and possessed wave vectors more nearly field aligned than in the inertial range. In the second paper they chose to examine intervals from within a single magnetic cloud. They found that the cloud spectra showed generally less steepening in the dissipation range than did the open field line examples. Inertial range fluctuations were significantly less compressive in the cloud examples. Very little energy was seen to reside with wave vectors parallel to the mean magnetic field in either the inertial or dissipation ranges. We have examined 30 additional magnetic clouds observed by ACE in order to develop a more statistically significant characterization of magnetic cloud dissipation range spectra near 1 AU. We find that the Leamon results characterize frequently observed aspects of cloud spectra, but that they constitute a common example within a range of possible results. In an effort to better understand the in situ heating of magnetic clouds, we present the statistics we have gathered and compare these results with typical open field line observations. Leamon et al., JGR, A103, 4775--4787, 1998a Leamon et al., GRL, 25, 2505--2509, 1998b