Engineering topics

Fillingim, M. O.

Publications and source records attributed to Fillingim, M. O..

18 records

NASA NTRS2009

Hemispheric Asymmetries in Substorm Recovery Time Scales

Previous statistical observations have shown that the recovery time scales of substorms occurring in the winter and near equinox (when the nighttime auroral zone was in darkness) are roughly twice as long as the recovery time scales for substorms occurring in the summer (when the nighttime auroral region was sunlit). This suggests that auroral substorms in the northern and southern hemispheres develop asymmetrically during solstice conditions with substorms lasting longer in the winter (dark) hemisphere than in the summer (sunlit) hemisphere. Additionally, this implies that more energy is deposited by electron precipitation in the winter hemisphere than in the summer one during substorms. This result, coupled with previous observations that have shown that auroral activity is more common when the ionosphere is in darkness and is suppressed when the ionosphere is in daylight, strongly suggests that the ionospheric conductivity plays an important role governing how magnetospheric energy is transferred to the ionosphere during substorms. Therefore, the ionosphere itself may dictate how much energy it will accept from the magnetosphere during substorms rather than this being an externally imposed quantity. Here, we extend our earlier work by statistically analyzing the recovery time scales for a large number of substorms observed in the conjugate hemispheres simultaneously by two orbiting global auroral imagers: Polar UVI and IMAGE FUV. Our current results are consistent with previous observations. The recovery time scales are observed to be longer in the winter (dark) hemisphere while the auroral activity has a shorter duration in the summer (sunlit) hemisphere. This leads to an asymmetric energy input from the magnetosphere to the ionosphere with more energy being deposited in the winter hemisphere than in the summer hemisphere.

Fillingim, M. O.↗

NASA NTRS2000

Coincident POLAR/UVI and WIND Observations of Pseudobreakups

Using POLAR/UVI global images, we have identified a period of successive minor auroral activations during which WIND was making a perigee pass (X approx. - 11Re). These auroral brightenings are interpreted to be pseudobreakups due to the lack of global expansion. Large magnetic field fluctuations and high earthward ion velocity moments measured by the WIND spacecraft show a nearly one-to-one correspondence with the auroral intensifications. Analysis of the plasma parameters indicates that there is no difference in the behavior of the plasma during pseudobreakups as compared to substorm expansive phase onset. Inspection of the ion distribution functions during high velocity moment events reveals the presence of a two component plasma. The particles contributing to the large mean velocities are energetic ions from approx. 2-27 keV. We conclude that pseudobreakups are the ionospheric signature of high velocity moment events.

Fillingim, M. O.↗

NASA NTRS1999

Dynamics of the Auroral Luminosity Boundary of the Polar Cap During Substorms

The area of the polar cap during substorms has been measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) conditions. Changes in the poleward boundary of auroral luminosity have been analyzed in relation to substorm phase and IMF orientation. Reconnection models of flux transport into the polar cap during the substorm growth phase, and loss from the polar cap during the expansion phase, provide a framework by which these UVI observations can be analyzed. By comparison of the observations with the model predictions we can determine to what extent these models accurately predict the polar cap dynamics, and also where anomalous behavior calls for a new understanding of the dynamics beyond what these models provide. It was found that the polar cap boundary near noon and midnight usually shifted down in latitude by 1-2 degrees and 3-4 degrees respectively, increasing the area of the polar cap during the substorm growth phase as predicted. However, this growth phase phenomenon also unexpectedly occurs independently of the IMF Bz component, as shown for a substorm on January 9, 1997. The polar cap area also increased due to motion of the dawn and dusk aurora to lower latitudes, although the latitudinal shifts were asymmetric, not always concurrent, and continued well into the substorm expansion phase. The polar cap area decreased immediately following the expansion phase due to the poleward motion of the aurora on the nightside, consistent with the model prediction. What is not explained by the models is that the poleward auroral boundary in the nightside region sometimes reached very high latitudes (greater than 80 degrees MLat) greatly decreasing the polar cap area, independent of the magnitude of the substorm.

Brittnacher, M. J.↗

NASA NTRS1999

Global Observations of Poleward Moving Aurora on the Dayside

Auroral arcs found at high latitude that move poleward from the nominal dayside oval, also known as poleward moving auroral forms been extensively studied from ground-based all-sky camera and meridian scanning photometric measurements. These auroral forms are thought to be he ionospheric signature of dayside reconnection processes at the magnetopause and therefore important for determining the relationship between the solar wind interplanetary magnetic field (IMF) and the location and size of the reconnection region. The large-scale picture of these dayside phenomenon derived from satellite imagery, however, has not been fully developed. Observations from the Polar Ultraviolet Imager (UVI) have sufficient time and spatial resolution to examine the longitudinal extent and motion of high-latitude arcs that extend across several hours of magnetic local time in the dayside aurora. We discuss the size and evolution of moving dayside aurorae in relation to the solar wind IMF orientation. We show also that the intensity of these auroral features is related to both solar wind pressure pulses and nightside auroral intensifications.

Brittnacher, M. J.↗

NASA NTRS1999

Magnetotail Plasma Signatures of Pseudobreakups and Substorms

Using Polar/UVI global images, we have identified a period of successive minor auroral activations during which WIND was making a perigee pass through the near-Earth magnetotail. On the basis of images, these auroral brightenings are interpreted to be pseudobreakups due to the lack of significant global expansion. Large magnetic by the WIND spacecraft show a nearly one-to-one correspondence auroral intensifications. During intervals of large field auroral brightenings, energized ions have an Earthward velocity energized electrons generally remain isotropic. Closer inspection ion distribution functions indicate that the high velocity moments are not due to convective flows. Rather, the plasma is composed of a component and a stagnate cold component. We also trace the observed by WIND backwards in time to determine the source regions for the particles. Based upon these observations, we find that to zeroth order there is no difference in the behavior of the plasma during as compared to substorm expansive phase events.

Fillingim, M. O.↗

NASA NTRS1998

Global Ultraviolet Imaging of the Aurora from Space

Global observation of the aurora by the Ultraviolet Imager (UVI) on the Polar spacecraft has provided both the benefit of placing ground and space-based observations in the context of auroral activity as well as the ability to make quantitative measurements of important parameters that characterize energy transfer to the ionosphere. The UVI images have provided simultaneously the timing of substorm onsets, the location of auroral boundaries, the polar cap area, and changes in the intensity of auroral activity at all local times. Increased accuracy in the measurement of energy flux and characteristic energy of the precipitating electrons in conjunction with auroral precipitation models are now available at high time resolution over many hours through the use of narrow-band far ultraviolet filters on the UVI. We will discuss how ultraviolet imaging of the aurora from space has provided fresh insight into processes such as substorm energy loading and deposition, substorm triggering, and solar wind control of substorm dynamics.

Brittnacher, M. J.↗

NASA NTRS1998

Polar Cap Area and Boundary Motion During Substorms

The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) orientations during three substorms in January 1997. We measured changes in the polar cap area and motion of the poleward and equatorward boundary of the auroral oval as determined by UVI images. It was found that the polar cap boundary is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight and the fading of luminosity below the instrument sensitivity threshold. Generally these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. A new feature is that the polar cap region clears of precipitation during the substorm growth phase, which expands the size of the polar cap but may not necessarily be related to an expansion of the open flux. We present a new finding that the increase in polar cap area prior to onset and the decrease in the area following it are independent of the strength of the southward IMF component. For one case the polar cap area increased while the southward component of the IMF was no less than -0.5 nT. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

Brittnacher, M.↗

NASA NTRS1998

What the Polar Cap Tells Us about the Substorm Growth Phase

The polar cap region in the 30 to 60 minute period prior to the onset of the auroral substorm has been examined using global images from the Polar Ultraviolet Imager (UVI) to look for observational evidence of processes related to the substorm growth phase. In particular, the area of the polar cap has been measured to determine changes in its size in relation to the orientation of the interplanetary magnetic field (IMF). It was found that the size of the polar cap region increases during the growth phase even if the IMF has no southward component. Three phenomena have been observed to produce the increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense auroral precipitation in the polar cap region. The first phenomenon has been considered to be a result of the growth of the tail lobe magnetic field and the second is related to the thinning of the plasma sheet. Both of these have been supported by in situ observational evidence and are consistent with current models of substorm development. However, the third phenomenon appears to be unrelated to the first two and does not appear to be the result of opening of the polar cap flux tubes to the solar wind IMF. This reduction of auroral precipitation provides evidence of a growth phase process, or change in auroral precipitation processes, that is not explained by current substorm models.

Brittnacher, M. J.↗

NASA NTRS1998

Coincident UVI and Wind Observations of Pseudo-Breakups

Using images taken by the Ultraviolet Imager (UVI) onboard the Polar spacecraft, we identify periods of pseudo-breakup activity coincident with perigee passes of the WIND spacecraft through the magnetotail. Previous studies have shown that from both observations on the ground and in the magnetotail there is very little difference phenomenologically between substorm onset and pseudo-breakups except for the degree of localization and the absence of global expansion. This raises the question of what prevents a pseudo-breakup from expanding globally. For the time intervals studied, we find a high correlation between pseudo-breakups and short-lived particle flux enhancements in the magnetotail. The velocity distribution of the plasma during some of these flux enhancements are indicative of bursty bulk flows.

Fillingim, M. O.↗

NASA NTRS1998

Unloading Versus Driven Processes Derived from Auroral Energy Deposition and Polar Cap Size

The intensity of far ultraviolet auroral emissions at all local times during the three substorm phases has been monitored by the Ultraviolet Imager (UVI) on the Polar spacecraft for many substorms. Changes in the energy flux and characteristic energy of the precipitating electrons can be derived from these observations by modeling of the spectral emission processes. The global and local energy deposition is a new parameter that can be used in substorm studies since it provides a measure of energy transfer from the tail to the ionosphere due to precipitating electrons at a time resolution of three minutes. The polar cap area and area of auroral emissions can also be determined at high time resolution during substorms from the UVI images. An example of a substorm that appears to be driven by solar wind dynamic pressure alone will be presented. The polar cap area and other parameters do not indicate a growth phase prior to substorm onset. In another example, the slow growth phase followed by a very rapid increase in energy deposition during the expansion phase will be shown. This substorm was preceded by a southward IMF orientation. In these two examples, the role the solar wind in determining polar cap area is discussed. The time development of the area of auroral emissions is also discussed in relation to substorm phase and energy deposition. If the auroral emissions occur on closed field lines then the area of auroral emissions may provide an indication of changes in the thickness of the plasma sheet during each substorm phase.

Brittnacher, M. J.↗

NASA NTRS1998

Global Ultraviolet Imaging of the Aurora from Space

Global observation of the aurora by the Ultraviolet Imager (UVI) on the Polar spacecraft has provided both the benefit of placing-ground and space-based observations in the context of auroral activity as well as the ability to make quantitative measurements of important parameters that characterize energy transfer to the ionosphere. The UVI images have provided simultaneously the the timing of substorm onsets, the location of auroral boundaries, the polar cap area, and changes in the intensity of aurora] activity at all local times. Increased accuracy in the measurement of energy flux and characteristic energy of the precipitating electrons in conjunction with auroral precipitation models are now available at high time resolution over many hours through the use of narrow-band far ultraviolet filters on the UVI. We will discuss how ultraviolet imaging of the aurora from space has provided fresh insight into processes such as substorm energy loading and deposition, substorm triggering, and solar wind control of substorm dynamics.

Brittnacher, M. J.↗

NASA NTRS1998

Observations of the Polar Cap Area During Substorms

The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager for different interplanetary magnetic field orientations during three substorms in January 1997. We present a new finding that the increase in polar cap area prior to onset and the decrease in the area following it are independent of the strength of the southward component. For one case the polar cap area increased while the southward component of the IMF was no less that -0.5 nT. It was also found that the polar cap boundary is determined by auroral images is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight and the fading of luminosity below the instrument sensitivity threshold. Generally these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

Brittnacher, M. J.↗

NASA NTRS1998

Solar Wind-Magnetosphere Coupling Influences on Pseudo-Breakup Activity

Pseudo-breakups are brief, localized aurora[ arc brightening, which do not lead to a global expansion, are historically observed during the growth phase of substorms. Previous studies have demonstrated that phenomenologically there is very little difference between substorm onsets and pseudo-breakups except for the degree of localization and the absence of a global expansion phase. A key open question is what physical mechanism prevents a pseudo-breakup form expanding globally. Using Polar Ultraviolet Imager (UVI) images, we identify periods of pseudo-breakup activity. Foe the data analyzed we find that most pseudo-breakups occur near local midnight, between magnetic local times of 21 and 03, at magnetic latitudes near 70 degrees, through this value may change by several degrees. While often discussed in the context of substorm growth phase events, pseudo-breakups are also shown to occur during prolonged relatively inactive periods. These quiet time pseudo-breakups can occur over a period of several hours without the development of a significant substorm for at least an hour after pseudo-breakup activity stops. In an attempt to understand the cause of quiet time pseudo-breakups, we compute the epsilon parameter as a measure of the efficiency of solar wind-magnetosphere coupling. It is noted that quiet time pseudo-breakups occur typically when epsilon is low; less than about 50 GW. We suggest that quiet time pseudo-breakups are driven by relatively small amounts of energy transferred to the magnetosphere by the solar wind insufficient to initiate a substorm expansion onset.

Fillingim, M. O.↗

NASA NTRS1998

Characteristics of Dynamic Activity in the Dayside Aurora

Long term global monitoring of the aurora by the Ultraviolet Imager (UVI) on the Polar spacecraft has enabled observation of auroral activity under various conditions of solar wind input. UVI is particularly suited to dayside imaging, especially in sunlit conditions during the northern hemisphere summer, owing to its solar blind narrow band filters. Several types of activity have been observed in the dayside aurora when observed on a global scale: enhancement of the auroral precipitation beginning at local noon and traveling along the flanks toward midnight observed in connection with solar wind shock fronts, regions of bright arcs traveling toward midnight that may be associated with boundary waves, and'break-up' like events in the high latitude midday region sometimes concurrent with a theta aurora. We will present several examples of dayside activity and discuss the possible mechanisms for these phenomena.

Brittnacher, M. J.↗

NASA NTRS1998

Global Auroral Energy Deposition during Substorm Onset Compared with Local Time and Solar Wind IMF Conditions

The global images made by the Ultraviolet Imager (UVI) aboard the IASTP/Polar Satellite are used to derive the global auroral energy deposited in the ionosphere resulting from electron precipitation. During a substorm onset, the energy deposited and its location in local time are compared to the solar wind IMF conditions. Previously, insitu measurements of low orbiting satellites have made precipitating particle measurements along the spacecraft track and global images of the auroral zone, without the ability to quantify energy parameters, have been available. However, usage of the high temporal, spatial, and spectral resolution of consecutive UVI images enables quantitative measurement of the energy deposited in the ionosphere not previously available on a global scale. Data over an extended period beginning in January 1997 will be presented.

Spann, J. F.↗

NASA NTRS1997

Global Auroral Energy Deposition Derived from Polar UVI Images

Quantitative measurement of the transfer of energy and momentum to the ionosphere from the solar wind is one of the main objectives of the ISTP program. Global measurement of auroral energy deposition derived from observations of the longer wavelength LBH band emissions made by the Ultraviolet Imager on the Polar spacecraft is one of the key elements in this satellite and ground-based instrument campaign. These "measurements" are inferred by combining information from consecutive images using different filters and have a time resolution on the average of three minutes and are made continuously over a 5 to 8 hour period during each 18 hour orbit of the Polar spacecraft. The energy deposition in the ionosphere from auroral electron precipitation augments are due to Joule heating associated with field aligned currents. Assuming conjugacy of energy deposition between the two hemispheres the total energy input to the ionosphere through electron precipitation can be determined at high time resolution. Previously, precipitating particle measurements along the tracks of low altitude satellites provided only local measurements and the global energy precipitation could be inferred through models but not directly measured. We use the UVI images for the entire month of January 1997 to estimate the global energy deposition at high time resolution. We also sort the energy deposition into sectors to find possible trends, for example, on the dayside and nightside, or the dawn and dusk sides.

Fillingim, M. O.↗

NASA NTRS1997

Comparisons of Solar Wind Coupling Parameters with Auroral Energy Deposition Rates

Measurement of the global rate of energy deposition in the ionosphere via auroral particle precipitation is one of the primary goals of the Polar UVI program and is an important component of the ISTP program. The instantaneous rate of energy deposition for the entire month of January 1997 has been calculated by applying models to the UVI images and is presented by Fillingim et al. In this session. A number of parameters that predict the rate of coupling of solar wind energy into the magnetosphere have been proposed in the last few decades. Some of these parameters, such as the epsilon parameter of Perrault and Akasofu, depend on the instantaneous values in the solar wind. Other parameters depend on the integrated values of solar wind parameters, especially IMF Bz, e.g. applied flux which predicts the net transfer of magnetic flux to the tail. While these parameters have often been used successfully with substorm studies, their validity in terms of global energy input has not yet been ascertained, largely because data such as that supplied by the ISTP program was lacking. We have calculated these and other energy coupling parameters for January 1997 using solar wind data provided by WIND and other solar wind monitors. The rates of energy input predicted by these parameters are compared to those measured through UVI data and correlations are sought. Whether these parameters are better at providing an instantaneous rate of energy input or an average input over some time period is addressed. We also study if either type of parameter may provide better correlations if a time delay is introduced; if so, this time delay may provide a characteristic time for energy transport in the coupled solar wind-magnetosphere-ionosphere system.

Elsen, R.↗

NASA NTRS1997

Global Auroral Energy Deposition Compared with Magnetic Indices

Measurement of the global rate of energy deposition in the ionosphere via auroral particle precipitation is one of the primary goals of the Polar UVI program and is an important component of the ISTP program. The instantaneous rate of energy deposition for the entire month of January 1997 has been calculated by applying models to the UVI images and is presented by Fillingim et al. in this session. Magnetic indices, such as Kp, AE, and Dst, which are sensitive to variations in magnetospheric current systems have been constructed from ground magnetometer measurements and employed as measures of activity. The systematic study of global energy deposition raises the possibility of constructing a global magnetospheric activity index explicitly based on particle precipitation to supplement magnetic indices derived from ground magnetometer measurements. The relationship between global magnetic activity as measured by these indices and the rate of total global energy loss due to precipitation is not known at present. We study the correlation of the traditional magnetic index of Kp for the month of January 1997 with the energy deposition derived from the UVI images. We address the question of whether the energy deposition through particle precipitation generally matches the Kp and AE indices, or the more exciting, but distinct, possibility that this particle-derived index may provide an somewhat independent measure of global magnetospheric activity that could supplement traditional magnetically-based activity indices.

Brittnacher, M. J.↗