F. K. H. Chun

An overview of the early November 1993 geomagnetic storm

This paper describes the development of a major space storm during November 2-11, 1993. We discuss the history of the contributing high-speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high-speed flow which prolonged the storm and the near-Earth manifestations of the storm. The 8-day storm period was unusually long; the result of a high-speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near-Earth environment was in a state of tumult. A super-dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100% and F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200°K within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was ∼190 PJ, with 30% of that generated within 24 hours of storm onset.

Polar cap index as a proxy for hemispheric Joule heating

The polar cap (PC) index measures the level of geomagnetic activity in the polar cap based on magnetic perturbations from overhead ionospheric currents and distant field-aligned currents on the poleward edge of the nightside auroral oval. Because PC essentially measures the main sources of energy input into the polar cap, we propose to use PC as a proxy for the hemispheric Joule heat production rate (JH). In this study, JH is estimated from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. We fit hourly PC values to hourly averages of JH. Using a data base approximately three times larger than studies, we find a quadratic relationship between JH and PC, differentiated by season. A comparison during the November 1993 storm interval with earlier reported methods using the AE index and the cross polar cap potential, shows that the PC-based Joule heating estimate is as equally accurate. Thus the single station PC index appears to provide a quick estimate of, and is an appropriate proxy for, the hemispheric Joule heating rate.

Plasmaspheric depletion and refilling associated with the September 25, 1998 magnetic storm observed by ground magnetometers at L = 2

The plasmaspheric mass density at L ≃ 2 was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to ≃ 1/4 of the pre-storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as L = 2, which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern.

The evolution of field‐aligned currents as a function of substorm phase

ISEE 1 and 2 measurements of magnetic changes across nightside field-aligned currents at mid altitudes in the inner magnetosphere are examined to determine the average ionospheric extrapolated characteristics and properties of region 1 and 2 currents as a function of substorm phase. The properties of these currents studied include current intensity, density, layer width, and velocity and are consistent with earlier studies at lower altitudes. In general, region 1 and 2 currents behave differently during substorm growth and expansion, but are comparable in the recovery phase. During a substorm, region 1 current intensity ranges from 0.4 to 0.6 A/m and peaks during the expansion phase. Region 2 intensity varies from 0.15 to 0.35 A/m and reaches a maximum during recovery. The density of region 2 currents remains essentially steady at 1.4 μA/m², while region 1 decreases from 5 to 1.3 μA/m². The width of these current sheets throughout a substorm remains in the range from 100 to 500 km. The velocity of these sheets is for the most part inwards with speeds from 50 to 200 m/s. Even though these current sheet crossings were not originally selected as substorm associated events, they all occurred during one of the phases of a substorm. This indicates that substorms play a major role in the generation of nightside region 1 and 2 currents detected by the ISEE spacecraft. These results are consistent with a model in which these currents flow to couple the outer magnetosphere and the ionosphere in response to the stresses applied by both dayside and nightside reconnection. Region 1 currents are responsive to variations in the stress caused by both reconnection regions, whereas region 2 currents are responsive to only nightside reconnection.

Field‐aligned currents in the inner magnetosphere: Control by geomagnetic activity

The occurrence rate of field-aligned currents measured in the Earths inner magnetosphere by ISEE 1 and 2 is examined for three levels of geomagnetic activity, |AL| ≤ 35 nT, 35 nT 135 nT, using observations from 180 orbits during the period from January 1978 to February 1980. Maps of the occurrence rate of these currents in L-value/LT space are provided at each of the three levels of activity. There is a clear distinction of the field-aligned current (FAC) occurrence rate between the three levels of activity. At low activity, FACs are observed at high L values and near dawn/dusk and midnight regions, with the dawn/dusk FACs perhaps being associated with the viscous solar wind-magnetospheric interaction along the flanks of the magnetopause. At intermediate levels of activity, FACs are observed with greater frequency at lower L values and throughout a larger range of local times, indicating internal stress buildup in the magnetosphere. When auroral activity levels are high, magnetospheric internal stresses become even greater, leading to widespread occurrence of FACs in the Earths inner magnetosphere. Field-aligned currents in the magnetosphere show as much range in strength as the currents flowing in the auroral ionosphere and vary in concert with these currents.

Magnetosphere on May 11, 1999, the day the solar wind almost disappeared: II. Magnetic pulsations in space and on the ground

Simultaneous observations by Wind and IMP-8 in the upstream region on May 11, 1999, when the solar wind density was well below its usual values and the IMF was generally weakly northward, indicate there were upstream waves present in the foreshock, but wave power was an order of magnitude weaker than usual due to an extremely weak bow shock and tenuous solar wind plasma. Magnetic pulsations in the magnetosphere have been observed in the magnetic field data from Polar and at mid-latitude ground stations. By comparing May 11 with a control day under normal solar wind conditions and with a similar foreshock geometry, we find that the magnetosphere was much quieter than usual. The Pc 3–4 waves were nearly absent in the dayside magnetosphere both at Polar and as seen at mid-latitude ground stations even through the foreshock geometry was favorable for the generation of these waves. Since the solar wind speed was not unusual on this day, these observations suggest that it is the Mach number of the solar wind flow relative to the magnetosphere that controls the amplitude of Pc 3–4 waves in the magnetosphere.

Sino-Magnetic Array at Low Latitudes (SMALL) including initial results from the sister sites in the United States

Abstract The Sino-Magnetic Array at Low Latitudes (SMALL) is an ongoing project for the development of a two-dimensional (2-D) ground-based magnetometer array in China. The ultimate array will consist of 24 stations spanning over 3 hours in local time in the low-latitude (L

Results from an experiment that collected visible-light polarization data using unresolved imagery for classification of geosynchronous satellites

In order to protect critical military and commercial space assets, the United States Space Surveillance Network must have the ability to positively identify and characterize all space objects. Unfortunately, positive identification and characterization of space objects is a manual and labor intensive process today since even large telescopes cannot provide resolved images of most space objects. The objective of this study was to collect and analyze visible-spectrum polarization data from unresolved images of geosynchronous satellites taken over various solar phase angles. Different collection geometries were used to evaluate the polarization contribution of solar arrays, thermal control materials, antennas, and the satellite bus as the solar phase angle changed. Since materials on space objects age due to the space environment, their polarization signature may change enough to allow discrimination of identical satellites launched at different times. Preliminary data suggests this optical signature may lead to positive identification or classification of each satellite by an automated process on a shorter timeline. The instrumentation used in this experiment was a United States Air Force Academy (USAFA) Department of Physics system that consists of a 20-inch Ritchey-Chrétien telescope and a dual focal plane optical train fed with a polarizing beam splitter. Following a rigorous calibration, polarization data was collected during two nights on eight geosynchronous satellites built by various manufacturers and launched several years apart. When Stokes parameters were plotted against time and solar phase angle, the data indicates that a polarization signature from unresolved images may have promise in classifying specific satellites.

Spectral Measurements of Geosynchronous Satellites During Glint Season

Operational communication geosynchronous satellites are typically large structures with solar panels that maintain a stable attitude relative to the Earth and Sun. During the equinox periods, the geometry of the satellite, the sun, and an Earth observer is favorable for a condition to occur called a glint, or a specular reflection off the satellite. It is presumed that the glint is caused by the solar panels because those structures are solar oriented. This phenomenon was previously observed and reported using broadband photometry techniques. In this paper, glint observations from two satellites are presented, Wildblue-1 and DirecTV-12, as measured across the visible spectrum using slitless spectroscopy techniques. It is clear from the results that the wavelength where the glint maximum occurs is shifted toward the blue or shorter wavelength end of the spectrum relative to the time periods before and after the glint. It is also clear from the data that the spectral reflectance during the glint is less lik...

Slitless spectroscopy of geosynchronous satellites

Abstract. A preliminary investigation into the use of slitless spectroscopy for characterization of geosynchronous satellites is described. A 100  line/mm diffraction grating is used as the dispersing device, and the spectral data obtained are compared to a model with good results. A method used to collect and calibrate slitless spectral observations accounting for pixel to wavelength conversion, pixel response as a function of wavelength, and solar features is presented. Observations of several geosynchronous satellites throughout a night reveal reflectance with noticeable and different profiles indicating that slitless spectroscopy offers the potential for another modality for identifying and discriminating satellites.