Thomas Jerome Immel

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

[1] During 2001 and 2002, when the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE-FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (AMLT) is strongly correlated with the IMF clock angle (θ C ) and a linear fit, ΔMLT = -0.017c C + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

Dayside enhancements of thermospheric O/N2 following magnetic storm onset

One frequently observed effect of thermospheric storms is the reduction of atomic oxygen relative to molecular nitrogen at high and middle latitudes. These composition changes lead to a decrease in thermospheric O I 130.4-nm emissions in the sunlit hemisphere. Such decreases have been observed by various satellite-based instruments including the Dynamics Explorer 1 (DE 1) Spin-Scan Auroral Imager. In contrast, this paper focuses on enhancements of the terrestrial 130.4-nm dayglow emission observed with DE 1. Following the onset of a geomagnetic storm at 1610 UT on February 5, 1983, an increase (>20%) in the O I 130.4-nm emission was observed at middle latitudes in the morning sector of the Southern Hemisphere. The increased O I 130.4-nm emission indicates an increase of atomic oxygen relative to molecular nitrogen. The brightness enhancement coincided with the passage of a large-scale gravity wave that was observed in measurements from a global network of ionosondes. The global FUV images and ionosonde observations are complemented by a TIMEGCM simulation of the February 1–6 period, which provides a framework for combining the ionosonde and DE 1 observations. The primary mechanisms for the FUV variations considered here are increases in the relative abundance of atomic oxygen in the morning sector caused by (1) the large-scale gravity wave launched by the onset of magnetic activity and (2) corotation of previously affected parcels onto the dayside. The investigation leads to a physical interpretation of the observed FUV features in terms of the gravity wave effects and a transient Hadley circulation cell. This work represents the first detection of a large-scale gravity wave from an orbiting FUV imager.

The effect of non-migrating tides on the morphology of the equatorial ionospheric anomaly: seasonal variability

Recent observations of the low-latitude F-region ionosphere at times near equinox have shown that it varies with a predominant zonal wavenumber-four pattern in a fixed local-time frame. It has been shown that this pattern corresponds well to the non-migrating diurnal eastward wavenumber-three atmospheric tide (DE3) at E-region altitudes simulated by the Global Scale Wave Model (GSWM). Here we present details of the morphology of the F-region ionosphere from TIMED GUVI with simultaneous observations of the non-migrating diurnal tides at E-region altitudes from TIMED SABER. For the case of equinox (March 2002), the correspondence of the SABER and GUVI observations confirms the relationship previously established using the GSWM simulations. There is also a wavenumber-one signature that is present which may be related to the semi-diurnal westward wavenumber-three, possibly in conjunction with changes in the magnetic field with longitude. During July 2002, when the amplitude of the DE3 maximizes, the amplitude of the wavenumber-four pattern in the F-region ionosphere intensifies. There is also evidence of a strong wavenumber-three pattern in the F-region ionosphere, which can be attributed to the strong diurnal eastward wavenumber-two tide during this period. During January 2003, the amplitude of all non-migrating components observed by SABER are either small or asymmetric and the ionosphere does not display either a wavenumber-three or -four pattern. During both solstice periods, a strong wavenumber-one is seen that is attributed to the offset of the subsolar point and the geomagnetic equator that maximizes at solstice, possibly in conjunction with other geomagnetic effects. During all seasons, significant hemispheric asymmetries in the airglow wavenumber spectra are seen. The combined GUVI and SABER observations presented here demonstrate that the large-scale periodic longitudinal structure of the F-region ionosphere responds significantly to changes in the forcing by non-migrating diurnal tides at E-region altitudes.

Simulated variability of the high‐latitude thermosphere induced by small‐scale gravity waves during a sudden stratospheric warming

We present the results of the first investigation of the influence of small-scale gravity waves (GWs) originating in the lower atmosphere on the variability of the high-latitude thermosphere during a sudden stratospheric warming (SSW). We use a general circulation model that incorporates the spectral GW parameterization of Yigit et al. (2008). During the warming, the GW penetration into the thermosphere and resulting momentum deposition rates increase by up to a factor of 3–6 in the high-latitude thermosphere. The associated temporal variability of GW dynamical effects at ~250 km are enhanced by up to a factor of ~10, exhibiting complex geographical variations. The peak magnitude of the GW drag temporal variability locally exceeds the mean GW drag by more than a factor of 2. The small-scale thermospheric wind variability is larger when GW propagation into the thermosphere is allowed compared to the case when thermospheric GW effects are absent. These results suggest that GW-induced variations during SSWs constitute a significant source of high-latitude thermospheric variability.

Upward propagating tidal effects across the E- and F-regions of the ionosphere

Recent far-ultraviolet (FUV) observations of Earth have shown the remarkable spatial correspondence between the amplitude of non-migrating atmospheric tides originating in the troposphere and the density and morphology of the nighttime equatorial ionospheric anomaly (EIA). This is likely a result of the modulation of the E-region dynamo electric field in daytime by the tidal winds. FUV observations around the time of the vernal equinox of 2002 show that the signature of tidal influence, the wave-4 periodicity in the separation and density of the two EIA bands, itself exhibits significant temporal variability. Here, we seek to understand this variability, and whether (or not) it is linked to variations in the strength of the upward-propagating tides. This study relies on tidal measurements provided by the global observations from the TIMED-SABER instrument that measures the temperature variations in the MLT associated with the upward-propagating tides. TIMED-GUVI provides F-region density measurements concurrent to the MLT temperature retrievals. It is found that the atmospheric and ionospheric zonal wave-4 signatures very nearly covary over a 30-day period, strongly supporting the theory that the influence of the the diurnal eastward 3 (DE3) tide originating in the troposphere extends to the F-layer of the ionosphere. Additionally, a 6-day periodicity in the power of the ionospheric wave-4 signature is found that may originate with the tide’s interaction with longer period planetary waves.

Redistribution of the Stormtime Ionosphere and the Formation of a Plasmaspheric Bulge

Plasmasphere drainage plumes resulting from the erosion of the plasmasphere boundary layer by disturbance electric fields have been identified from both ground and space. Here we describe a localized enhancement of total electron content (TEC) seen at the base of the erosion plume, on field lines mapping into the outer plasmasphere. Observations suggest that this enhanced TEC results from a poleward redistribution of post-noon sector low latitude ionospheric plasma during the early stages of a strong geomagnetic disturbance. Ground based and low- altitude observations with GPS TEC, incoherent scatter radar, and DMSP in situ observations provide details and a temporal history of the evolution of such events. Seen from space by IMAGE EUV, the region of enhanced TEC appears as a pronounced brightening in the inner plasmasphere. IMAGE FUV provides complementary images at lower altitude of this inner-plasmasphere feature, showing that it is associated with localized enhancement in the vicinity of the equatorial anomaly peak. These effects are especially pronounced over the Americas, and we suggest that this results from a strengthening of the equatorial ion fountain due to undershielded (penetrating) electric fields in the vicinity of the South Atlantic magnetic anomaly. The enhanced low-latitude features, seen both from the ground and from space, corotate with the Earth once they are formed. The high-TEC plasma in these regions contributes to the intensity of the erosion plumes arising in the American sector during strong disturbance events.

Ionospheric redistribution during geomagnetic storms

[1]The abundance of plasma in the daytime ionosphere is often seen to grow greatly during geomagnetic storms. Recent reports suggest that the magnitude of the plasma density enhancement depends on the UT of storm onset. This possibility is investigated over a 7year period using global maps of ionospheric total electron content (TEC) produced at the Jet Propulsion Laboratory. The analysis confirms that the American sector exhibits, on average, larger storm time enhancement in ionospheric plasma content, up to 50% in the afternoon middle-latitude region and 30% in the vicinity of the high-latitude auroral cusp, with largest effect in the Southern Hemisphere. We investigate whether this effect is related to the magnitude of the causative magnetic storms. Using the same advanced Dst index employed to sort the TEC maps into quiet and active (Dst<−100 nT) sets, we find variation in storm strength that corresponds closely to the TEC variation but follows it by 3–6h. For this and other reasons detailed in this report, we conclude that the UT-dependent peak in storm time TEC is likely not related to the magnitude of external storm time forcing but more likely attributable to phenomena such as the low magnetic field in the South American region. The large Dst variation suggests a possible system-level effect of the observed variation in ionospheric storm response on the measured strength of the terrestrial ring current, possibly connected through UT-dependent modulation of ion outflow.

Start‐to‐end global imaging of a sunward propagating, SAPS‐associated giant undulation event

We present high-time resolution global imaging of a sunward propagating giant undulation event from start to finish. The event occurred on November 24, 2001 during a very disturbed storm interval. The giant undulations began to develop at around 13UT and persisted for approximately 2 hours. The sunward propagation speed was on the order of 0.6 km/s (relative to SM coordinate system). The undulations had a wavelength of {approx} 750 km, amplitudes of {approx} 890 km and produced ULF pulsations on the ground with a period of {approx} 1108s. We show that the undulations were associated with SAPs flows that were caused by the proton plasma sheet penetrating substantially farther Earthward than the electron plasma sheet on the duskside. The observations appear to be consistent with the development of a shear flow and/or ballooning type of instability at the plasmapause driven by intense SAPS-associated shear flows.

Evidence of Tropospheric Effects on the Ionosphere

A new paradigm in upper atmospheric and ionospheric physics has begun to emerge, starting with discoveries from observations taken with the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite in 2002. These discoveries [Immel et al., 2006] show that the entire ionosphere, sometimes referred to as “the inner edge of space,” regularly responds to the tropospheric weather systems below. The likely mechanism for effectively carrying tropospheric energy upward to the edge of space to modify the ionosphere is the generation and upward propagation of large-scale waves, known as atmospheric tides. The coupling of these tides to the ionosphere is inferred from the strong similarity in longitudinal structure of the tidal winds and the ionospheric densities at low latitudes. However, the appearance of this behavior in the upper ionosphere is surprising because the tides that are forced by tropospheric weather are believed to dissipate at the base of the ionosphere and are expected to have little effect at higher altitudes.

ULF Waves Associated with Enhanced Subauroral Proton Precipitation

Several types of sub-auroral proton precipitation events have been identified using the Spectrographic Imager (SI) onboard the NASA-IMAGE satellite, including dayside subauroral proton flashes and detached proton arcs in the dusk sector. These have been observed at various levels of geomagnetic activity and solar wind conditions and the mechanism driving the precipitation has often been assumed to be scattering of protons into the loss cone by enhancement of ion-cyclotron waves in the interaction of the thermal plasmaspheric populations and more energetic ring current particles. Indeed, recent investigation of the detached arcs using the MPA instruments aboard the LANL geosynchronous satellites has shown there are nearly always heightened densities of cold plasma on high-altitude field lines which map down directly to the sub-auroral precipitation. If the ion-cyclotron instability is a causative mechanism, the enhancement of wave activity at ion-cyclotron frequencies should be measurable. It is here reported that magnetic pulsations in the Pcl range occur in the vicinity of each of 4 detached arcs observed in 2000-2002, though with widely varying signatures. Additionally, longer period pulsations in the Pc5 ranges are also observed in the vicinity of the arcs, leading to the conclusion that a bounce-resonance of ring-current protons with the azimuthal Pc5 wave structure may also contribute to the detached precipitation.