SM11C-01
From which side is the Ionosphere-Magnetosphere coupling processes driven?
Temporal and spatial ambiguities are difficult to resolve in single point measurements. However, the different travel times of ions and electrons can give information of the time history of a flux tube. We apply this technique to interpret FAST observations from the upward current region of the auroral oval. Assuming the classical quasi-static picture of multiple U-shape potential structures along the same flux tube, these observations indicate that the potential structure at the lowest altitude is the first to be created or the last to disappear. An improved understanding of how the U-shape potential structures develop can gain a better understanding of the dynamical Ionosphere-Magnetosphere interaction.
SM11C-02
Transverse ion heating, field-aligned electron acceleration, and parallel electric fields in the auroral acceleration region: Modeling several FAST events
Many of the ions in the magnetosphere originate in the ionosphere, whence they are extracted by wave heating perpendicular to the magnetic field. Much of this ion heating occurs in regions where electrons are also accelerated along the magnetic field, and the differing anisotropies lead to a charge separation which is balanced by a parallel electric field.a Using a recently developed model which includes turbulent heating,b,c we investigate the distribution of parallel electric fields in several events measured with the FAST satellite. We investigate the effects of different model closures on the predicted parallel electric fields. The goal of the research is to develop a physics-based module of ion outflow to include in global models of the magnetosphere. a Alfvén, H., and C.-G. Fälthammar (1963), Cosmical Electrodynamics: Fundamental Principles, Clarendon Press, Oxford. b Jasperse, J. R., et al. (2006), Phys. Plasmas 13, 072903. c Jasperse, J. R., et al. (2006), Phys. Plasmas 13, 112902.
SM11C-03
Diffuse, Monoenergetic, and Broadband (wave) Aurora: Relative Importance
Auroral precipitation can be divided into three broad categories. The mono-energetic peak aurora, in which
most of the energy flux occurs in a relatively narrow electron energy range, is perhaps the most studied.
Fairly good evidence associates mono-energetic peak aurora (often occurring in misleadingly named
"inverted V" events) with quasi-static electric fields. In recent years, better appreciation of wave aurora has
arisen. In wave aurora, the electron acceleration is distributed over a range of energies. Finally the direct
dumping of plasma sheet electrons and ions produces the diffuse, or unaccelerated aurora.
Twenty-two years of DMSP electron and ion data (about 60 satellite years) were surveyed, and each
spectrum sorted into one of these three categories. For example, electron precipitation with most of the
energy flux in one or two DMSP channels, and with signs of acceleration (energy flux above 10**8 eV/cm2 s
str eV) were classified as mono-energetic, whereas if the acceleration occurred in 3 or more channels, it was
considered broadband acceleration. Spectra which had neither the signature of broadband nor
monoenergetic aurora were treated as diffuse aurora – with both electron and ion contributions included. As
a result, we are able for the first time to report on the relative importance of these three types of precipitation
over a wide variety of circumstances. The effects of UV insolation are incorporated and discussed. Data is
functionally fitted to solar wind conditions rather than being binned into coarse Kp categories.
http://sd-www.jhuapl.edu/Aurora
SM11C-04
A magnetospheric dynamo to explain quasi-stationary acceleration of plasma in discrete auroral arcs as observed by Cluster and DMSP spacecraft.
Auroral field lines couple the ionosphere with the magnetosphere via field-aligned currents. The nature of the generator initiating this current system is still under debate and several mechanisms have been proposed. In particular, we discuss a model that couples a magnetospheric boundary and the evening polar ionosphere. The model uses a non-linear current-voltage relationship for the upward current and solves the current continuity equation at the top of the ionosphere. It results in producing discrete auroral arcs, intense upward field-aligned currents and regions of enhanced Pedersen conductance. In the dusk sector Cluster identifies outflowing ion beams and bipolar electric field signatures at the interface between two plasma regions with different macroscopic properties. We estimate the orientation and spatial scale of the plasma interface for different events as well as the field aligned current density and parallel potential drop. In particular we show a detailed comparison of the model results with experimental data taken during the 28 April 2001 conjunction between Cluster (above the acceleration region) and DMSP spacecraft (passing trough Region 1 field-aligned currents embedding a discrete auroral arc) discussed by Vaivads et al. (GRL, 2003). Using the discontinuity parameters as observed in-situ by Cluster, the model fairly reproduces the characteristics of the arc both at Cluster and DMSP altitude. This study shows evidence of a quasi-stationary acceleration of auroral electrons by a field aligned potential drop sustained by the convergent electric field at a magnetospheric boundary.
SM11C-05
Relationship Between UV Auroral Events and the Magnetic Storm Main Phase Development
This paper will present results from a statistical study of auroral events and their relationship to the development of the main-phase magnetospheric storm. Using global auroral images obtained by the Polar satellite and the SYMH index we have analyzed the occurrence and properties of optical auroral events and the main phase of a total of 16 magnetic storms with good average imaging coverage throughout the development period. We find that classical Akasofu type optical substorms rarely occur during the main phase of magnetic storms and hence conclude that they are not required for the development of the ring current. We have identified 64 auroral events and found that these are randomly scattered throughout the storm main phase development. For these events the average typical development time from onset to maximum intensity is about half that of classical isolated substorms and they develop at much lower latitudes. We will present results from this study and show individual events as well as results from superposed epoch analysis of the auroral disturbances, their characteristics and their relationship to the storm main phase evolution.
SM11C-06
Electric Field Statistics and Modulation Characteristics of Bursty Langmuir Waves Observed in the Cusp
The Twin Rockets to Investigate Cusp Electrodynamics (TRICE) were launched Dec. 10, 2007, from Andoya into the active cusp over Svalbard. The high-frequency electric field waveform receiver on the higher flying payload detected Langmuir waves concentrated at 900-1145 km on the downleg. The waves occurred in bursts having durations between tens of ms and a few hundred ms. From analysis of selected sample intervals, it is estimated that the rocket encountered more than 1000 such bursts ranging in amplitude from about 1 mV/m, near the detection threshold, to nearly 1000 mV/m, though only a few bursts approached this upper bound. Waveform analysis showed that the waves were modulated at kHz to tens of kHz frequencies. The modulation was not monochromatic but consisted of many nonstationary frequency components giving rise to a complex envelope waveform. The modulated waves were sufficiently oversampled to clearly observe 180-degree phase shifts when the envelopes of the modulations passed through zero, as expected if a superposition of modulating waves is responsible. Dynamic spectra of the bursts showed that the bandwidth of each event was related to the modulation frequencies. The dynamic spectra also revealed a repeatable pattern whereby the highest Langmuir wave frequency and largest bandwidth occurred in the center of each burst, with frequency and bandwidth decreasing toward either side of each burst. For the conditions of the observations (fpe ≤ fce/2), Langmuir waves convert to whistler mode waves, and the observed pattern may be explained by trapping of the Langmuir/whistler waves in density enhancements. The observed wave electric fields were statistically analyzed in two ways: First, the distribution of the absolute values of all of the electric field measurements was determined, hereafter referred to as the "waveform distribution"; second, the envelope of the Langmuir waves was extracted and the distribution of this field was determined, hereafter referred to as the "envelope distribution". The waveform distribution showed a characteristic E+1 dependence at low electric fields. Bandpass-filtering the waves with progressively narrower passbands preserved the E+1 dependence but with a sharper cutoff at the high-electric field end, approaching the analytical expectation for a rectified sine wave. The envelope distribution showed a steeper power-law index at low electric fields. Averaging the waveforms by factors of 10-300 produced waveform distributions with a power index comparable to that of the envelope distribution, as expected since averaging by 10 corresponded to half the wave period so that the resulting statistics were dominated by the envelope. Averaging by factors greater than about 300, corresponding to averaging times exceeding about half of the modulation period, led to waveform distributions that appeared Gaussian although they were not well defined due to the small number of points. The relation between the waveform and envelope distributions appeared to be well explained by recent analytical predictions confirmed by numerical calculations [Cairns et al., Statistics of Waveform and Envelope Fields: Theory, Simulations and Initial Applications to TRICE Data, EOS Trans. Am. Geophys. Union, 2008, this issue]. The observed envelope distributions also appear consistent with theory [Kletzing et al., Determination of the Envelope Distribution for Langmuir Waves in the Topside Ionosphere, EOS Trans. Am. Geophys. Union, 2008, this issue].
SM11C-07
Determination of the Envelope Distribution for Langmuir Waves in the Topside Ionosphere
The modulations of Langmuir waves envelopes are an essential input to calculations of wave amplitudes used in Stochastic Growth Theory. Reported here are statistics of the distributions of amplitudes of Langmuir waves taken from multiple snapshots of Langmuir wave packets observed on the TRICE high-flyer sounding rocket launched in December, 2007. For each case, wave amplitudes are determined using the Hilbert transform and are then binned to produce a probability distribution of amplitudes. These distributions are then fit to a functional form of P(W) = WA exp(-W/Wth) where W is the electric field energy density. This functional form is suggested by the form for thermal waves used in stochastic growth theory. Taking more than 25 cases, the TRICE data consistently show values of A that are near 1 and values of Wth that vary widely. Values of A near one suggest that the Langmuir waves are essentially one dimensional in nature. Examples of the waveforms and distributions are presented. The implications of these measurements are discussed with respect to theory.
SM11C-08
Statistics of Waveform and Envelope Fields: Theory, Simulations and Initial Applications to TRICE Data
Plasma waves in space are almost invariably bursty and widely variable in amplitude, motivating statistical approaches such as stochastic growth theory. Recent wave experiments on rockets moving through Earth's auroral regions, as well as the STEREO and Wind spacecraft, have sufficient time resolution to measure the waveform as well as the envelope field. Typically, however, experiments measure the envelope field averaged over long times compared with the wave period. Four sets of new contributions are presented. First, analytic theory is used to predict the distribution of waveform fields for a single mode with known distribution of envelope fields. The distribution P(log Ew) of waveform fields Ew is shown to be proportional to the rectified field Ewa with a ≈ 1.0 for a number of special cases of the distribution P(log Ee) of envelope field Ee. This form arises due to P(log Ew) being proportional to an integral over P(log Ee) that has a square-root singularity in Ee2. Numerical calculations confirm and extend this prediction to wide range of envelope distributions. Second, ensembles of stochastically-driven waves are simulated and the distributions P(log Ew) and P(log Ee) calculated. While small differences exist between the case of a single mode and multiple modes, it is found in general that the results are independent of the product of the wave frequency and decorrelation time. Of importance here is that the distributions P(log Ew) are found to be power-law with index ≈ 1.0 at low Ew, consistent with the analytic prediction. Moreover, the envelope distribution is found to be well fit by the form P(log Ee) ∝ Ee2 exp(- Ee2 / Eth2). This form applies to one- dimensional thermal waves and now, unexpectedly, also to waves driven stochastically near marginal stability. Third, initial calculations show that averaging (boxcar and sliding averages, whether linear or logarithmic) over multiple wave periods leads to both the envelope and waveform distributions being well fitted by lognormal distributions. Fourth, initial comparisons are made with Langmuir-like waves observed in Earth's cusp region by the TRICE rocket. It appears that the foregoing analytic and numerical calculations explain semi-quantitatively the power-law form and index near 1.0 for the waveform distribution of unaveraged fields, the functional form of the envelope distribution of unaveraged fields, and the transition of the waveform and envelope distributions towards lognormal forms with averaging over multiple wave periods. The waves appear consistent with stochastic growth. The theory and simulation results extend stochastic growth theory to measurements on timescales less than or close to the wave period.