Y.-J. Su

Longitudinal and seasonal dependence of nighttime equatorial plasma density irregularities during solar minimum detected on the C/NOFS satellite

[1]xa0During the night in the F region about the equator, plasma density depletions form, causing scintillation. In April 2008, the Communications/Navigation Outage Forecasting System (C/NOFS) satellite developed by the Air Force Research Laboratory was launched to predict ionospheric scintillation. Using its Planar Langmuir Probe (PLP), C/NOFS is capable of measuring in situ ion density within the F region over the equator. Plasma irregularities are found regularly during the night. We examine how these irregularities depend on longitude, latitude, and season. The most significant observations from this study are longitudinal structures in which these irregularities most frequently occur. Since similar structure has been found in diurnal tides, we conclude that lower atmospheric tides may play a strong role in determining the amplitude of equatorial irregularities, at least during low solar minimum conditions when the presented observations were made. We propose that this link is likely related to the generation of zonal electric fields by the E-region dynamo.

Zonal drift of plasma particles inside equatorial plasma bubbles and its relation to the zonal drift of the bubble structure

[1]xa0It has been observed that the zonal drift velocity of equatorial plasma bubbles is generally eastward. However, it has not been well understood whether the zonal drift of plasma bubbles is the same as the ambient plasma drift and what process causes differences in the drift velocities of the ambient plasma and bubbles. In this study we analyze the ion drift velocities measured by the Defense Meteorological Satellites Program and ROCSAT-1 satellites and the electric fields measured by the Communications/Navigation Outage Forecasting System (C/NOFS) satellite in the presence of equatorial spread F. We find that the zonal drift velocity of the plasma particles inside plasma bubbles is significantly different from the ambient plasma drift. The relative zonal velocity of the ions inside the depletion region with respect to the ambient plasma is generally westward. In most cases it can be as high as several hundreds of meters per second. The plasma bubbles detected by the C/NOFS satellite in the midnight-dawn sector are still growing, and the polarization electric field inside the postmidnight bubbles is much stronger than the electric field in the ambient plasma. We suggest that the zonal drift velocity of the plasma particles inside the depletion region is driven by polarization electric field. When a plasma bubble is tilted, the E × B drift velocity caused by the polarization electric field has an upward component and a zonal component. Because of the zonal motion of the plasma particles inside the bubble, the eastward drift velocity of the bubble structure is faster than the ambient plasma drift for a west-tilted bubble and slower than the ambient plasma drift for an east-tilted bubble.

Energy coupling during the August 2011 magnetic storm

Abstract : We present results from an analysis of high-latitude ionosphere-thermosphere (IT) coupling to the solar wind during a moderate magnetic storm which occurred on 5 6 August 2011. During the storm, a multipoint set of observations of the ionosphere and thermosphere was available. We make use of ionospheric measurements of electromagnetic and particle energy made by the Defense Meteorological Satellite Program and neutral densities measured by the Gravity Recovery and Climate Experiment satellite to infer (1) the energy budget and (2) timing of the energy transfer process during the storm. We conclude that the primary location for energy input to the IT system may be the extremely high latitude region. We suggest that the total energy available to the IT system is not completely captured either by observation or empirical models.

Formation of the inner electron radiation belt by enhanced large-scale electric fields

A 2-dimensional bounce-averaged test particle code was developed to examine trapped electron trajectories during geomagnetic storms with the assumption of conservation of the first and second adiabatic invariants. The March 2013 storm was selected as an example because the geomagnetic activity KP index sharply increased from 2+ to 7- at 6:00UT on March 17. Electron measurements with energies between 37 and 460 keV from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard Van Allen Probes (VAP) are used as initial conditions prior to the storm onset, and served to validate test particle simulations during the storm. Simulation results help to interpret the observed electron injection as non-diffusive radial transport over a short distance in the inner belt and slot region based on various electric field models, although the quantitative comparisons are not precise. We show that electron drift trajectories are sensitive to the selection of electric field models. Moreover, our simulation results suggest that the actual field strength of penetration electric fields during this storm is stronger than any existing electric field model, particularly for Lu2009≤u20092.

Control of the innermost electron radiation belt by large‐scale electric fields

Electron measurements from the MagEIS instruments on Van Allen Probes, for kinetic energies ∼100 to 400xa0keV, show characteristic dynamical features of the innermost ( L≲1.3) radiation belt: rapid injections, slow decay, and structured energy spectra. There are also periods of steady or slowly increasing intensity and of fast decay following injections. Local time asymmetry, with higher intensity near dawn, is interpreted as evidence for drift shell distortion by a convection electric field of magnitude ∼0.4xa0mV/m during geomagnetically quiet times. Fast fluctuations in the electric field, on the drift time scale, cause inward diffusion. Assuming they are proportional to changes in Kp, the resulting diffusion coefficient is sufficient to replenish trapped electrons lost by atmospheric scattering. Major electric field increases cause injections by inward electron transport. An injection associated with the June 2015 magnetic storm is consistent with an enhanced field magnitude ∼5xa0mV/m. Subsequent drift echoes cause spectral structure.

Assimilative modeling of observed postmidnight equatorial plasma depletions in June 2008

Abstract : The Communications/Navigation Outage Forecasting System (C/NOFS) satellite observed large-scale density depletions at postmidnight and early morning local times in the Northern Hemisphere summer during solar minimum conditions. Using electric field data obtained from the vector electric field instrument (VEFI) as input, the assimilative physics-based model (PBMOD) qualitatively reproduced more than 70% of the large-scale density depletions observed by the Planar Langmuir Probe (PLP) onboard C/NOFS. In contrast, the use of a climatological specification of plasma drifts in the model produces no plasma depletions at night. Results from a one-month statistical study found that the large-scale depletion structures most often occur near longitudes of 60 deg, 140 deg, and 330 deg, suggesting that these depletions may be associated with nonmigrating atmospheric tides, although the generation mechanisms of eastward electric fields at postmidnight local times are still uncertain. In this paper, densities obtained from both assimilation and climatology for the entire month of June 2008 are compared with PLP data from C/NOFS and the Challenging Minisatellite Payload (CHAMP), as well as special sensor ionospheric plasma drift/scintillation meter (SSIES) measurements from the Defense Meteorological Satellite Program (DMSP) satellites. Our statistical study has shown that, on average, the densities obtained by the PBMOD, when it assimilates VEFI electric fields, agree better with observed background densities than when PBMOD uses climatological electric fields.

SCATHA measurements of electron decay times at 5 < L ≤ 8

[1]xa0The electron decay timescale (τ) is well known to be associated with radiation belt loss processes. Knowledge of τ is important for understanding pitch angle and radial diffusion mechanisms. Previous studies reported decay timescales from the inner belt to geosynchronous orbits; however, relatively few statistical studies have been focused on the region beyond the traditional outer belt. In this paper, a systematic calculation of electron decay times at 5 1 day. τ is examined as a function of energy, pitch angle, L shell, Kp, and AE during magnetically disturbed periods when Dst ≤ −50 nT. Results show that τ increases with increasing electron energy at L 6.6. This suggests radial transport as the dominant effect at L > 6.6. Additionally, τ decreases with increasing L-shell. This dependence has the strongest correlation and is seen in all energies and pitch angles. However,τ has no systematic dependence with pitch angle suggesting that pitch angle diffusion also plays a key role in the electron loss process. Based on our results, τ can be expressed as a function of energy and L, and coefficients are provided for a two-variable fit. Surprisingly,τ is slightly longer for higher activity cases at L < 6.6, which is inconsistent with the current radial or pitch angle diffusion models. Global effective decay times on the timescale of days place an upper bound on the true loss timescale.

High-Latitude Neutral Mass Density Maxima

Recent studies have reported that thermospheric effects due to solar wind driving can be observed poleward of auroral latitudes. In these papers, the measured neutral mass density perturbations appear as narrow, localized maxima in the cusp and polar cap. They conclude that Joule heating below the spacecraft is the cause of the mass density increases, which are sometimes associated with local field-aligned current structures, but not always. In this paper we investigate neutral mass densities measured by accelerometers on the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) spacecraft from launch until years 2010 (CHAMP) and 2012 (GRACE), approximately 10xa0years of observations from each satellite. We extract local maxima in neutral mass densities over the background using a smoothing window with size of one quarter of the orbit. The maxima have been analyzed for each year and also for the duration of each set of satellite observations. We show where they occur, under what solar wind conditions, and their relation to magnetic activity. The region with the highest frequency of occurrence coincides approximately with the cusp and mantle, with little direct evidence of an auroral zone source. Our conclusions agree with the “hot polar cap” observations that have been reported and studied in the past.

Reply to comment by C. E. J. Watt and R. Rankin on: Role of dispersive Alfvén waves in generating parallel electric fields along the Io-Jupiter fluxtube

[1] In our recent paper [Jones and Su, 2008] (hereinafter referred to as JS) we discuss the effect of dispersive Alfven waves (DAW) in relation to observed particle acceleration at the Io-Jupiter auroral footprint using a two-fluid model. Watt and Rankin [2009] (hereinafter WR) assert that a kinetic wave model should be used when discussing the region separating the inertial and kinetic Alfven regimes. We agree with this analysis, but point out that the same fundamental conclusions are reached with both approaches. The transition between the two regimes should occur within the Io torus, and parallel electric fields within the torus are smaller than the fields in the lower-density region outside the torus. [2] We wish to thank WR for calling attention to the important work done in kinetic Alfven wave theory by Lysak and Lotko [1996] and Lysak [1998] and how it relates to our recent paper (JS). We agree with point 1 of WR, that using the kinetic DAW theory does provide a more complete description of dispersive Alfven wave (DAW) behavior at the transition between inertial and kinetic regions (b ’ me/mi). In JS we chose to use an analytic two-fluid model of DAW [Streltsov et al., 1998; Stasiewicz et al., 2000] to study relative magnitudes of Alfvenic Ek/E? along the entire Io-Jupiter fluxtube. The kinetic approach relies on numeric solutions of the plasma dispersion function Z(z) as done by Lysak and Lotko [1996]. While this does give a more accurate calculation of Ek in the transition region, it is cumbersome in regions where b is not close to me/mi. [3] The fundamental conclusions of JS remain valid whether considering the two-fluid treatment of the aforementioned kinetic treatments. For the assumed initial wave parameters of f and k? the ratio of Ek/E? at higher latitudes of the Io-Jupiter fluxtube will be much larger than within the Io torus (see JS Figure 1f and WR Figure 1a). [4] We also agree with point 2 of WR, that the uncertainty about the values of k? in the torus region make it difficult to predict what the amplitudes of DAW Ek will be. The model values of k? considered in JS were motivated by previous studies of Su et al. [2006] and Hess et al. [2007] and the common assumption of k? / B. It was not our intent to state what k? or Ek should be in the torus but rather what the relative amplitudes of Ek may be for a wide range of parameters both inside and out of the torus. As stated by WR and discussed in JS, increasing k? within the torus will generate larger theoretical values of Ek (see JS Figure 5). However, unless k? is modified as it propagates out of the torus, model values of Ek at higher latitudes will still be two orders of magnitude larger than within the torus. [5] In closing, we appreciate the interest WR have shown in our research and thank them for providing the community with this additional analysis. The model discussed in JS shows that for reasonable models of the Io-Jupiter plasma parameters, the transition between the inertial and kinetic DAW regimes should occur within the torus. Both kinetic and fluid approaches show that theoretical parallel Alfvenic electric fields along the Io-Jupiter fluxtube should be larger at high latitudes than within the torus.