A comparison of mean density and microscale density fluctuations in a CME at 10 R_⊙

Abstract

[1] We have observed intensity scintillation (IPS) of the radio source 0854 + 201 at 8 GHz on August 2, 2000 during the passage of a coronal mass ejection (CME) across the line of sight. The source was at a distance of 10 R over the north solar pole. Simultaneous observations with the LASCO C3 instrument allow us to model the mean density N e and the microscale density fluctuations dN e within the CME. We find that N e increased by a factor of 2.18 but dN e increased by only 1.76, so the ratio dN e /N e is 19% smaller than in the pre-CME slow wind. During the passage of the CME a short burst of enhanced turbulence doubled the IPS variance but was not visible in the C3 images. This was likely caused by a thin flux tube crossing the line of sight. Detailed modeling indicates that the diameter of the tube was 41,000 km and its density was 14.5 times the CME density. Observations[2] Intensity scintillation at 8 GHz is ''weak'' at 10 R , so it provides a linear measure of the density fluctuations with spatial scales of the order of 50 km. With the LASCO C3 instrument [Brueckner et al., 1995] it is possible to make a direct comparison between these ''microscale'' fluctuations and the mean density. Both observations involve line of sight integrals which cannot be directly inverted, but a useful comparison can be made by fitting a simple CME model to the two data sets simultaneously. The comparison can be made with respect to the pre-CME slow wind without requiring absolute calibration of the IPS or the coronagraph.[3] The structure of the CME is shown clearly in Figure 1. This is a grey-scale version of the summary images available on the SOHO web site (http://sohowww.nascom.nasa.gov/). A description of the image processing is available on the LASCO web site (http://lasco-www.nrl.navy.mil/idl_pros/ help_synoptic.html).[4] Eight C3 images taken during the passage of the CME are shown with the same processing in Figure 2. A 3-hour average of the pre-event corona is used as the slow wind background reference. These images were used to estimate the CME velocity using the running difference technique described by Sheeley et al. [1997]. Height-time plots at the position angle of the radio source show that the velocity of the leading edge was 546 ± 26 km/s and the velocity in the center of the CME was 444 ± 21 km/s. As the CME is relatively fast, one might expect to see some compression at the leading edge.[5] To obtain an estimate of the integrated mean electron density from the C3 images, we must separate the K (electron) component from the F (dust) component. This is done using polarization measurements because the polarization of the K and F components are very different. We assume that the F component is independent of the CME and calculate it from the pre-CME reference. We know P F / B F = 0.002 at 10 R [Mann, 1992]. We calculated P K /B K using well-known expressions [van de Hulst, 1950;Billings, 1966;Hayes et al., 2001]. At 10 R the familiar coronal density models are not valid, so we used two spherically symmetric s...

Internal structure of a coronal mass ejection revealed by Akatsuki radio occultation observations

“…Although they did not have phase measurements, the electron density was estimated from the scintillation index on the assumption that the fractional density variation at small scales is almost constant. This relationship was roughly confirmed by simultaneous IPS and SOHO/LASCO observations [e.g., Lynch et al , ]. Considering that the turbulence level can change significantly inside CMEs, the phase variation seen in our radio occultation measurements can provide more reliable estimates of the electron column density variation.…”

Simultaneous interplanetary scintillation and Heliospheric Imager observations of a coronal mass ejection

“…The geometry of the HI‐1A field‐of‐view and the IPS raypath in this observation make the 16 May 2007 CME a particularly suitable case study for comparing the HI and IPS results. Combined IPS and white‐light coronal CME observations have been made before [e.g., Lynch et al , 2002] but this is the first time that IPS signatures of an interplanetary CME have been directly and unambiguously associated with a specific region seen in white‐light images. The observations are of particular interest in that they reveal the small‐scale structure present within CME fronts, and can potentially provide new information on interaction between CMEs.…”

Turbulence and Anomalous Resistivity inside Near-Earth Magnetic Clouds

“…Their findings suggest that Alfvénic fluctuations can contribute significantly to the local heating of the ICME plasma. While these studies concentrate on the magnetic field fluctuations inside ICMEs close to or beyond the Earth, Lynch, Coles, & Sheeley (2002) address the density fluctuations inside a CME at 10 R . They studied the interplanetary scintilation (IPS) of the radio source 0854+201 at 8 GHz during the passage of a CME along the line of sight.…”