A DISSERTATION FOR THE DEGREE OF DOCTOR SCIENTIARUM High Spatial and Temporal Resolution Auroral Imaging Trond S. Trondsen November 1998 DEPARTMENT OF PHYSICS Faculty of Science University of Tromsø High Spatial and Temporal Resolution Auroral Imaging Trond S. Trondsen November 1998 University of Tromsø _________________ 1998 1 Abstract High Spatial and Temporal Resolution Auroral Imaging by Trond S. Trondsen Dr. Scient. in Cosmic Geophysics University of Tromsø Professor Asgeir Brekke, Chair A versatile auroral imaging system capable of high spatial and temporal resolution imaging at low light levels has been constructed and fielded. The Portable Auroral Imager (PAI) uses as primary detector a third-generation image intensified charge-coupled device (CCD) television camera. A secondary, wider field of view camera provides, along with resident all-sky cameras, important context measurements. Data are recorded onto high-resolution video tape at a rate of 30 frames per second, each individual video frame receiving an accurate time stamp immediately prior to recording. The field data is later digitized and analyzed in the laboratory using a personal computer based image digitizer and modern image processing workstations. The PAI system is designed primarily for narrow field auroral observations, although the modular design allows the system to be fully customized to meet changing scientific needs. The design, characterization, and operation of the instrument are described in some detail for the purpose of aiding others in emulating or improving upon the concept. The PAI was fielded during two-week-long campaigns in the winters of 1994, 1995, and 1997. The resulting data contained impressive examples of the rich variety of evening and midnight sector short-scale auroral phenomena. Presented in this thesis are four separate phenomenological surveys pertaining to the small-scale aurora, namely, surveys of, (a) very thin auroral forms (less than 100 m widths), (b) “black” au- roral forms, (c) “asymmetric” multiple arcs, and (d) auroral vortices, also known as “curls.” Some significant new findings are contained in these surveys. Throughout, it will also be demonstrated that the widely used concepts of “auroral forms,” “auroral arcs,” “diffuse aurora,” and “discrete aurora” are very dependent on the scale size involved, and thus also on the characteristics of the observer’s particular imaging instrumentation. Such terms should thus be used in the relevant fora only with extreme caution, especially when associating the terms with specific meanings in auroral theories. Original photograph by Jan Curtis. Fine-scale structures within auroral arcs are real and they are unexplained. Joe Borovsky, 1995 iii To my wife Elizabeth. iv Contents List of Figures vi List of Tables ix 1 Introduction 1 I Instrumentation 6 2 The Portable Auroral Imager 7 2.1 The Field Component . 8 2.1.1 Camera Head . 11 2.1.2 Optics . 14 2.1.3 Time-Code Generator . 15 2.1.4 Video Cassette Recorder . 17 2.2 The Laboratory Component . 17 2.2.1 Hardware . 18 2.2.2 Software . 20 2.2.3 Public Outreach . 23 3 Design Considerations 25 3.1 Scientific Considerations . 25 3.1.1 Angular Resolution . 26 3.1.2 Temporal Resolution . 30 3.1.3 Sensitivity and Dynamic Range . 32 3.2 Practical Considerations . 34 3.2.1 Data Recording Convenience . 34 3.2.2 Interface Requirements . 37 3.2.3 EMI Requirements . 40 3.2.4 Environmental Requirements . 41 3.2.5 Transportability Requirements . 41 4 Sensitivity and Resolution: A Closer Look 43 4.1 Some Radiometric Concepts . 43 4.1.1 Source Radiance . 43 4.1.2 Detector Irradiance . 44 4.2 Sensitivity . 46 4.2.1 Photon Noise . 46 v 4.2.2 Electron Multiplication Noise . 48 4.2.3 Dark Noise . 48 4.2.4 Read Noise . 49 4.2.5 Threshold of Detection . 49 4.2.6 Intensifier Noise Factor . 53 4.3 Spatial Resolution . 55 4.3.1 The Modulation Transfer Function . 55 4.3.2 Photon Noise . 57 4.3.3 Limiting Low Light Level Resolution . 60 5 Imager Characterization 63 5.1 Field of View . 63 5.2 Sensitivity . 65 5.3 Spatial Resolution . 67 II Observations 73 6 Introduction to Observations 74 6.1 The Observing Site . 74 6.2 Instrument Pointing . 75 6.3 Data Coverage . 76 7 Thin Auroral Forms 79 7.1 Introduction . 79 7.2 Observations and Discussion . 81 8 Black Aurora 86 8.1 Introduction . 86 8.2 Observations . 88 8.2.1 Eastward Drifting Black Patches and Arc Segments . 88 8.2.2 Black Arcs . 95 8.2.3 Vortex Formation on Black Arcs . 98 8.3 Discussion . 106 9 Asymmetric Multiple Auroral Arcs 108 9.1 Introduction . 108 9.2 Observations . 110 9.3 Discussion . 115 10 Small-Scale Spatially Periodic Distortions of Auroral Forms 118 10.1 Introduction . 118 10.2 Observations . 120 10.2.1 Wavelength . 121 10.2.2 Speed . 123 10.2.3 Lifetime . 124 10.2.4 Dimensions . 126 10.3 Discussion . 128 11 Conclusion 134 A Determining the Geocentric Latitude and Longitude of an Image Pixel 136 vi List of Figures 1.1 The distribution of auroral-arc thicknesses as measured by Maggs and Davis [1968]. 3 2.1 Schematic showing the field component of the PAI. 8 2.2 Photograph of Instrument Case I of Table 2.2. 10 2.3 Three widely different fields of view of the same auroral scene. 11 2.4 Schematic of the PAI field component primary channel signal chain. 11 2.5 Third-generation intensifier tube responsivity. 13 2.6 PAI image data showing SMPTE time code. 16 2.7 Facsimile of PAI log sheet. 18 2.8 Schematic showing the PAI laboratory component. 19 2.9 Sample output from the PAI Fourier domain analysis and filtering tool. 22 2.10 Stellar fields with superimposed coordinate grids. 24 3.1 Simple objective lens geometry. 27 3.2 Schematic of the homemade PAI gain control interface electronics. 39 4.1 Optical system represented by a single lens in front of a detector. 45 4.2 Calculated signal-to-noise ratios vs. emission rate. 52 4.3 Calculated SNR vs. continuum emission rate. 52 4.4 Modeled horizontal MTF for the PAI third-generation ICCD camera. 56 4.5 Photon limited resolution as a function of column emission rate. 60 4.6 Modeled spatial resolution as a function of column emission rate, taking MTF as well as photon statistics into account. 61 4.7 Limiting spatial resolution at auroral altitudes using the telescope. 62 4.8 Limiting spatial resolution at auroral altitudes using the /1.8 lens. 62 4.9 Limiting spatial resolution at auroral altitudes using the /1.4 lens. 62 5.1 Ephemerides and stellar data used to determine total field of view. 64 5.2 Calculated and measured signal-to-noise ratios vs. emission rate. 66 5.3 Three-dimensional plot of the imager’s point spread function. 68 5.4 The PAI ICCD point.