R. Elsen

Initial response and complex polar cap structures of the aurora in response to the January 10, 1997 magnetic cloud

On January 10th, 1997, a magnetic cloud originating at the Sun was incident on the Earth. The initial disturbance to the magnetosphere, as reflected in the activities of the aurora, was measured by the Ultraviolet Imager on the Polar Spacecraft. During this event we have observed the development of several unusual unique auroral forms that to our knowledge are unexplained in current models and theories. The observations were made on a global scale with unprecedented spatial and temporal resolution. The first activation of the aurora at local noon occurred within minutes of the arrival of the shock at 0107 UT. The substorm onset was observed at 0334 UT. During the intervening time significant polar cap precipitation occurred.

A dayside auroral energy deposition case study using the Polar Ultraviolet Imager

In this letter, we report preliminary results from a study of dayside auroral energy deposition during quiet times using global auroral images acquired by the Ultraviolet Imager experiment on the Polar spacecraft. Solar wind plasma and interplanetary magnetic field measurements from the Wind spacecraft and Kp values were used to characterize the state of the magnetosphere. The auroral oval was observed for a two-hour period in spring and summer during relatively quiet times (KP ≃ 0 to 1). We find that, although the nightside energy deposition rate varied by an order of magnitude to as low as 1 × 1016 erg s−1 (1 gigawatt), the dayside was much less variable and remained between 4 and 10 × 1016 erg s−1.

Analysis of auroral morphology : Substorm precursor and onset on January 10, 1997

The solar wind interaction with the geomagnetic field is studied using global auroral images obtained by the Ultraviolet Imager (UVI) on Polar. We study the dynam,cs of the poleward and equatorward boundaries of the auroral oval in response to the solar wind IMF on January 10, 1997 using a neural network algorithm to perform an automated morphological analysis. Poleward and equatorward boundaries identified by the algorithm demonstrate a clear growth motion with the southward turning of the IMF and growth and poleward expansion at substorm onset. The area poleward of the oval (polar cap) is found to increase in size coincident with thesouthward turning of the IMF Bz component at 0220 UT and peaks at substorm onset at 0334 UT. The area of the oval, however, decreases continuously throughout the period of the polar cap area increase with a slight recovery observed during the substorm onset. These observations are consistent with the concept that magnetospheric dynamics are directly driven by the solar wind-geomagnetic field interactions.

The auroral oval boundaries on January 10 1997: A comparison of global magnetospheric simulations with UVI images

We present the results of a global magnetospheric simulation of the initial period of the January 10 - 11, 1997 magnetic cloud event. Distinct magnetospheric boundaries derived from the model are mapped down to the ionosphere and compared to UVI images of the auroral oval from 1:00 to 4:30 UT. The convection reversal boundary, which coincides with the maximum Region I currents, tends to generally match the UVI equatorward boundary. This boundary is almost always poleward of the boundary separating Region 1 and Region 2 currents. The separatrix between open and closed magnetic field lines as mapped in the model matches the poleward boundary of the UVI images very well during quiet periods. During dynamic periods, however, when the separatrix can move several degrees in latitude in some sectors, the poleward boundary of the Region 1 currents matches the UVI images better.

Does the UVI on Polar Detect Cosmic Snowballs

If 20 to 40 ton cosmic snowballs pelt Earth as claimed by Frank and Sigwarth [1997a], dark pixels will be produced in the 130.4 nm images of dayglow obtained by the Ultraviolet Imager (UVI) on the Polar spacecraft. Examination of the UVI images has revealed that dayglow images are indeed spotted with single and multiple dark pixels. But is a snowball the only explanation for these dark pixels? To learn more about the dark pixels, we have examined the calibration images obtained from the same camera just before the instrument was launched. We find that dark pixels similar to those in dayglow images also exist in calibration images. This strongly indicates that the source of the dark pixels is instrumental. For further verification, a statistical analysis found the dark pixels from dayglow and calibration images have nearly identically shaped occurrence patterns. We have also looked for evidence of spacecraft “wobble” which demonstrates that the source of a bright or dark feature in the images is external to the camera, but found none for dark pixels. Finally, we studied the bright streaks that frequently appear in UVI images, sometimes comet-like in appearance. These trails are ionization tracks produced by cosmic rays or other penetrating energetic particles interacting with our camera. We conclude that the source of the dark pixels in dayglow images is internal to the camera system and there is no scientific evidence from UVI that snowballs pelt Earth.

Comparison of dark pixels observed by VIS and UVI in dayglow images

Frank and Sigwarth [1997a] claim that the dark pixels observed in dayglow images obtained by the Earth sensor of the Visible Imaging System (VIS) are due to bombardment of Earth by 20 to 40 ton cosmic snowballs. We have independently studied the VIS data and compared the dark pixels from the VIS images to those obtained from the overlapping images from the Ultraviolet Imager (UVI). We find the occurrence distributions of the dark pixels, single and multiple, from VIS and UVI are nearly identical. The distributions also do not show any altitude dependence. A search for evidence of spacecraft “wobble” motion, whose presence would indicate that the source is external to the camera, has found that pairs of dark pixel clusters are uniformly distributed in orientation and no preference is observed in the wobble direction. Instrument artifacts as the source of the dark pixels is the most likely explanation for these results. The conclusion of this study is that neither VIS nor UVI provide any scientific evidence that the dark pixels are geophysical.

IMF induced changes to the nightside magnetotail: A comparison between WIND/Geotail/IMP 8 observations and modeling

A 3-D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross-section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.

Far-ultraviolet observations of the neutral comae of Comet Hale-Bopp (C/1995 01) near perihelion

The Ultraviolet Imager on the Polar spacecraft observed the neutral oxygen and carbon comae of Comet Hale-Bopp (C/1995 01) at far-ultraviolet wavelengths near perihelion on March 31 and April 2, 1997. The OI (1304) coma was circularly symmetric with a diameter of about 5 million km. The coma was smaller in the longer wavelength (1400-1600 A and 1600-1800 A) bands, which is probably due to fluorescence of neutral carbon at 1561 A and 1657 A. The production rate for O calculated from the observed OI (1304) flux was (9.8 ± 2.5) x 10 30 . Using this value the estimated production rate for OH was (8.9±2.5) x 10 30 and for H 2 O was (1.0 ± 0.3) × 10 31 .