SM54A-01 16:00h
Comparison of local velocity measurements to the cross-polar-cap potential under steady southward IMF conditions.
In a previous study [Bristow et al., 2004], SuperDARN estimates of the cross-polar-cap potential (CCP) were compared to the variability that would be expected from the observed interplanetary magnetic field (IMF). The study showed that there was approximately twice as much variability as would be predicted using empirical formulae relating the CCP to the IMF. This study extends the earlier work by examining SuperDARN velocity estimates averaged over local time - latitude sectors. The distributions of observed velocities are examined, as are correlations between the velocity observations and the CCP estimates. As would be expected, it is found that the velocity distributions are broadest near noon and midnight. Also, it is found that variations in the CCP can be linked to localized velocity surges.
SM54A-02 16:15h
A statistical comparison of the AMIE derived and DMSP-IES particle drift velocities.
The assimilative mapping of ionospheric electrodynamics (AMIE) technique utilizes a wide range of electrodynamics measurements to determine high latitude maps of the electric potential, electron particle precipitation (average energy and total energy flux), and ionospheric conductance (Hall and Pedersen). AMIE does this by conducting least squares fit to the difference between the data and a background model. This fit is then added to the background model. This allows for a very stable technique with even minimal amounts of data. The background models are typically statistical models that are driven by the solar wind and interplanetary magnetic field or the hemispheric power index. This study presents results of a statistical validation of the AMIE technique with respect to determined electric fields Specifically, we compare the AMIE derived particle drift velocities to DMSP-IES measurements during extended periods in 1998. We present the results of the comparison binned by MLT, IMF, and latitude and do statistical comparisons. This result is applicable to those using AMIE to describe the ionosphere in particular when coupled to MHD magnetospheric representations for example.
SM54A-03 16:30h
Ionospheric Prompt Penetration Electric Fields: Comparison of First-principle Solutions With Observations
Prompt penetration of magnetospheric convection electric fields to the mid- and low-latitude ionosphere is caused by transient changes in the shielding capacity of region-2 Birkeland currents, reconfiguration of inner magnetospheric magnetic field, and other dynamic processes ultimately originating in the solar wind. During magnetically disturbed times, these electric fields cause substantial redistribution of ionospheric electron densities, and, in the equatorial region, are a major parameter controlling linear growth rates and non-linear evolution of spread-F plasma instabilities. In this paper, we use the global numerical model BATSRUS-RCM, a global MHD code with an inner magnetospheric module, to study time propagation of prompt penetration electric fields in response to idealized changes in the solar wind conditions. For the first time, we are able to compute, from first principles, solutions that link changes in the solar wind parameters (e.g., IMF strength and orientation) to the local-time dependent electric fields at the equator of the Earth. These theoretical solutions will be compared with a number of case studies, relating cases of prompt-penetration electric fields observed with the Jicamarca Observatory incoherent scatter radar to changes in the solar wind conditions.
SM54A-04 16:45h
Drift of Auroral Absorption and Ionospheric Convection
We develop a method for quasi-continuous monitoring of the particle precipitation regions using the cosmic noise absorption (CNA) data from an imaging riometer. The method does not require vast computational resources nor does it involve excessive manual sifting through the data. The horizontal absorption drift velocity is estimated from the regression lines to positions of the maxima in the two-dimensional absorption intensity images for each individual region of enhanced CNA (absorption patches). The absorption drift velocity estimates from a 7x7-beam Imaging Riometer for Ionospheric Studies (IRIS) in Kilpisjarvi are compared with the electrojet plasma flows deduced from magnetic perturbations recorded by the Kilpisjarvi magnetometer as well as with the tristatic ion drift velocities in the F region (for one event) measured by the EISCAT radar facility within the IRIS field of view (FoV). A reasonable agreement was found between the directions of absorption drift and ionospheric convection both in point-by-point comparisons and in terms of direction reversal timings. The absorption patches of lower intensity appear to have smaller drift velocities and to be associated with weaker magnetic perturbations. Based on our observations, we interpret the relatively slow motions of the auroral absorption as associated with the ExB drift of the entire magnetic flux tube as opposed to the gradient-curvature drift of energetic electrons injected into the ionosphere at the substorm onset. Since the absorption intensity in beam 16 of IRIS, the beam closest to that of EISCAT at 90 km, was found to correlate well with the height-integrated Hall conductivity due to particle precipitation inferred from the EISCAT density measurements, we also assess the conductivity gradient effects on the agreement between the convection measurements by different techniques by considering the gradients of the absorption intensity within the IRIS FoV.
SM54A-05 17:00h
Meaning of Ionospheric Joule Heating
The possibility of relating the electric current in the ionosphere to the electric field in the frame of reference either of the neutral atmosphere (as commonly done) or of the plasma raises the question: which should be used in calculating Joule heating? The energy equations for the plasma and for the neutral medium, including collision effects, can be combined with momentum equations to separate energy transfer into work done and heating (this is {\em not} the same as separation of energy content into bulk-flow kinetic energy and thermal energy). Heating/dissipation processes can be unambiguously identified, showing that true electromagnetic (Joule) heating rate is given by ${\bf J}\cdot{\bf E}$ in the frame of reference of the plasma. The major part of energy dissipation associated with ${\bf J}$ and ${\bf E}$ in the ionosphere is heating by collisions between plasma and neutrals, proportional to the square of the difference in their bulk flow velocities and distributed approximately equally between the two. The conventionally calculated ionospheric Joule heating as ${\bf J}\cdot{\bf E}$ in the frame of reference of the neutral atmosphere equals the sum of total heating rate of plasma (only a small part of which is true Joule heating) plus total heating rate of neutrals, plus a term equal to work done on plasma in the neutral-atmosphere frame of reference; the latter is assumed negligible, with ${\bf J}\times{\bf B}/c$ balanced entirely by plasma-neutral collisions and with no net work done on the plasma. This assumption implies that all the magnetic stresses exerted on the plasma at ionospheric heights are taken up by the local neutral medium; hence the dynamics of the neutral atmosphere play an important role in magnetosphere/ionosphere interactions.
SM54A-06 17:15h
The Ion Aurora and Its Seasonal Variations
Recent studies have shown that intense discrete aurora, auroral kilometric radiation, upflowing ion beams, and downward directed electric fields are more intense in the winter hemisphere than in the summer. This is particularly true of the dusk to midnight sector where intense electron aurora are most common. Here, we use one solar cycle of DMSP satellite particle data to investigate the seasonality of the ion aurora. The ion aurora proves to be approximately equal in the summer and winter hemispheres in the dusk-midnight sector (with the summer hemisphere favored by 0 to 4 percent). However in the MLT hours from midnight to dawn, the ion precipitating energy flux is 15-40 percent higher in winter than in summer. The absolute magnitude of the ion effect is smaller than was found for discrete electron aurora (which show a 3-fold difference between winter and summer). The seasonal behavior of the ions may reflect the observation that diverging electric fields, which accelerate ions downward, are found mainly postmidnight, and are stronger in the winter. The relative weakness of the seasonal effects in ions may reflect their high average energy (many tens of keV), which is substantially larger than typical of electric potentials found in the auroral circuit. Ions in the dusk to midnight sector are most intense equatorward of the region of discrete aurora, and are thus probably not much affected by the seasonality of the field-aligned electric fields which exist there (and which are of the sense to retard ion precipitation). Interestingly, ion average energies are higher in the winter hemisphere than in the summer hemisphere at all local times, regardless of whether or not energy fluxes are enhanced.
http://sd-www.jhuapl.edu/Aurora
SM54A-07 17:30h
Meso-scale and small-scale conjugacy of discrete and pulsating auroras observed with TV cameras
Syowa Station in Antarctica and Japanese stations in Iceland provide an ideal set of observatories to study geomagnetically conjugate optical auroras. A campaign of auroral conjugate observations using all-sky TV cameras has been carried out since 1984 during the equinox periods when simultaneous optical observations are possible under enough darkness in both hemispheres. In this paper we will focus on conjugacy studies of meso-scale and small-scale discrete and pulsating auroras. On 26 September 2003, a fine conjugate auroral substorm was observed at the conjugate-pair observatories Syowa and Tjornes. The observations covered the whole event, from the growth phase through the expansion phase to the recovery phase. Auroral breakup occurred at relatively low latitudes, after which the active auroral region expanded rapidly poleward. Subsequently, north-south structured auroral forms appeared from higher latitudes and extended to lower latitudes several times. During this event, meso-scale discrete auroras, including both east-west and north-south structured auroral forms, showed quite good conjugacy in shape, movement, and luminosity variations. This made it possible to trace the conjugate point with high time- and spatial resolution. On the other hand, small-scale discrete auroras gave no indication of conjugacy. For example, the rotation speed of vortices of curl-type aurora was higher in the southern hemisphere than in the northern hemisphere. Furthermore, pulsating auroras, which occurred during the recovery phase of the auroral substorm, showed good conjugacy in their meso-scale structure and shape, but the pulsation ON/OFF signature of the small-scale aurora appeared independent in the two hemispheres. This failure in conjugacy suggests that the generation region is not located in the equatorial region of the magnetosphere. We will discuss these meso-scale and small-scale auroral features in order to clarify how the IMF parameters and/or ionosphere-magnetosphere coupling processes control the conjugacy of the aurora.
SM54A-08 17:45h
Enhanced Plasma Expansion in Auroral Plasma Cavities in the Upward Current Region
The existence of deep density cavities extending from topside ionosphere to altitudes of several Earth's radii is now well established from several satellite observations. Observations from FAST and Polar also show that double layers form in the cavity at both low and high altitudes in the upward current plasma. Such double layers accelerate electrons downward and ionospheric ions upward. Since the early observations of ion beams from S3-3 at altitudes of about one RE , the Earth's radius, the supply of cold ionospheric ions to the double layers at such altitudes for large parallel accelerations forming in beams has been a problem. Suggestions for the supply range from wave-particle interaction resulting into ion heating at low altitudes to accelerations in different layers. We further suggest here that the ion supply could be facilitated by expansion of the ionospheric plasma. This is specially so because of the deep density cavity in combination with the generation of secondary and backscattered (SBS) electrons by the primary precipitating energetic electrons. The SBS electrons generate a warms electron population, which enhances the process of plasma expansion. The heating of the cold ionospheric electrons as well as of the SBS electrons by waves generated by the primary electrons further enhances the plasma expansion. The transport of the ionospheric plasma to high altitudes by means of the enhanced plasma expansion does not only supply ions for the ion beam formation at high altitudes, but it also generates an ion beam at low altitudes as well as supplies the trapped electron population needed for the formation of double layers at mid and high altitudes. We demonstrate these assertions by means of one-dimensional particle-in-cell simulations of long auroral flux tubes.