Abstract We investigate the statistical, dual‐spacecraft correlations of field‐aligned current (FAC) signatures between two Swarm spacecraft. For the first time, we infer the orientations of the current sheets of FACs by directly using the maximum correlations obtained from sliding data segments. The current sheet orientations are shown to broadly follow the mean shape of the auroral boundary for the lower latitudes and that these are most well ordered on the dusk side. Orientations at higher latitudes are less well ordered. In addition, the maximum correlation coefficients are explored as a function of magnetic local time and in terms of either the time shift ( δt ) or the shift in longitude ( δlon ) between Swarms A and C for various filtering levels and choice of auroral region. We find that the low‐latitude FACs show the strongest correlations for a broad range of magnetic local time centered on dawn and dusk, with a higher correlation coefficient on the dusk side and lower correlations near noon and midnight. The positions of maximum correlation are sensitive to the level of low‐pass filter applied to the data, implying temporal influence in the data. This study clearly reflects the two different domains of FACs: small‐scale (some tens of kilometers), which are time variable, and large‐scale (>50 km), which are rather stationary. The methodology is deliberately chosen to highlight the locations of small‐scale influences that are generally variable in both time and space. We may fortuitously find a potential new way to recognize bursts of irregular pulsations (Pi1B) using low‐Earth orbit satellites.
Abstract Using the plasma data of Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite and the NRLMSISE‐00 atmospheric model, we examined the semiannual and solar activity variations of the daytime plasma and neutral composition densities in the ionosphere‐plasmasphere transition region (~670–710 km). The results demonstrate that the semiannually latitudinal variation of the daytime oxygen ions (O + ) is basically controlled by that of neutral atomic oxygen (O), whereas the latitude distributions of the helium and hydrogen ions (He + and H + ) do not fully depend on the neutral atomic helium (He) and hydrogen (H). The summer enhancement of the heavy oxygen ions is consistent with the neutral O enhancement in the summer hemisphere, and the oxygen ion density has significantly the summer‐dense and winter‐tenuous hemispheric asymmetry with respect to the dip equator. Although the winter enhancements of the lighter He + and H + ions are also associated with the neutral He and H enhancements in the winter hemisphere, the high‐density light ions (He + and H + ) and electrons ( e − ) mainly appear at the low and middle magnetic latitudes (| λ | < 50°). The equatorial accumulations of the light plasma species indicate that the light charged particles (He + , H + , and e − ) are easily transported by some equatorward forces (e.g., the magnetic mirror force and centrifugal force). The frequent Coulomb collisions between the charged particles probably lead to the particle trappings at different latitudes. Moreover, the neutral composition densities also influence their ion concentrations during different solar activities. From the low‐ F 10.7 year (2007–2008) to the high‐ F 10.7 year (2004–2005), the daytime oxygen ions and electrons increase with the increasing neutral atomic oxygen, whereas the daytime hydrogen ions tend to decrease with the decreasing neutral atomic hydrogen. The helium ion density has no obvious solar activity variation, suggesting that the generation (via the neutral He photoionization) and loss (via the charge exchange with neutral nitrogen N 2 and/or the recombination with electrons) of the daytime He + ions are comparable during different solar activities.
[1] During the interval from 06:15 to 07:30 UT on 24 August 2005, the Chinese Tan-Ce 1 (TC1) satellite observed the multiple responses of the near-Earth magnetotail to the combined changes in solar wind dynamic pressure and interplanetary magnetic field (IMF). The magnetotail was highly compressed by a strong interplanetary shock because of the dynamic pressure enhancement (∼15 nPa), and the large shrinkage of magnetotail made a northern lobe and plasma mantle move inward to the position of the inbound TC1 that was initially in the plasma sheet. Meanwhile, the dynamic pressure fluctuations (∼0.5–3 nPa) behind the shock drove the quasi-periodic oscillations of the magnetopause, lobe-mantle boundary, and geomagnetic field at the same frequencies: one dominant frequency was around 3 mHz and the other was around 5 mHz. The quasi-periodic oscillations of the lobe-mantle boundary caused the alternate entries of TC1 into the northern lobe and the plasma mantle. In contrast to a single squeezed or deformed magnetotail by a solar wind discontinuity moving tailward, the compressed and oscillating magnetotail can better indicate the dynamic evolution of magnetotail when solar wind dynamic pressure increases and fluctuates remarkably, and the near-Earth magnetotail is quite sensitive even to some small changes in the solar wind dynamic pressure when it is highly compressed. Furthermore, it is found that a considerable amount of oxygen ions (O+) appeared in the lobe after the southward turning of IMF.
Abstract Van Allen Probe observations indicate that whistler‐mode hiss waves below 1 kHz are absorbed at low altitudes near magnetic equator. The lowest cutoff frequency of equatorial hiss is close to the gyrofrequency of hydrogen ions. The lowest cutoff altitude of global hiss is extracted when its occurrence rate is equal to 0.005 on the plane of altitude ( L in R E ) and magnetic local time (MLT). By fitting the lowest cutoff altitude of global hiss, we constructed the empirical model of the lowest cutoff altitude of equatorial hiss under geomagnetically quiet (AE < 200 nT) and active (AE ≥ 200 nT) conditions. The enhanced substorm activities reduce the lowest cutoff altitude of hiss waves on the dawnside (MLT ∼ 1–5 hr), whereas the lowest cutoff altitude of the dayside hiss is nearly fixed at ∼1.1 R E (MLT ∼ 6–20 hr). From the dayside to the nightside (MLT ∼ 0–6 hr and 20–24 hr), the lowest cutoff altitude of equatorial hiss raises gradually from 1.1 R E to 1.4 R E .
Abstract We report a rare event of intense plasmaspheric hiss and chorus waves simultaneously observed at the same L shell but different magnetic local times by Van Allen Probes and Magnetospheric Multiscale. Based on the measured waves and electron distributions, we calculate the bounce‐averaged diffusion coefficients and subsequently simulate the temporal evolution of electron distributions. The simulations show that the dynamics of tens to hundreds of keV electrons are jointly controlled by hiss and chorus. The dynamics of MeV electrons are dominantly controlled by hiss near the loss cone but by chorus at intermediate to large pitch angles. The simulated electron distributions driven by combined diffusion can reproduce the majority of the observations. Our results provide a direct observational evidence that hiss and chorus can simultaneously occur at the same electron drifting shells due to the irregular plasmasphere and highlight the importance of their combined effect on electron dynamics.
Abstract Previous observations indicate that there are mainly unstructured plasmaspheric hiss in the high‐density plasmasphere, whereas structured whistler mode chorus waves with the lowest frequency of 0.1 f ce ( f ce is the electron gyrofrequency) are observed mostly outside the plasmapause. Here we observed ultrawideband rising‐tone chorus waves with frequencies extending to lower hybrid resonance frequency ( f LHR ~ 101 Hz) in a dawnside high‐density region (magnetic local time [MLT] ~ 7 hr and N e ~ 75 cm −3 ) inside the oscillating plasmapause. The ultrawideband chorus waves have also typical two‐band structures separated by a power gap at 0.5 f ce , but their lowest frequency ( f LHR ~ 0.023 f ce ) in the high‐density region is much smaller than that of the normal chorus waves (>0.1 f ce ) in the low‐density trough ( N e < 40 cm −3 ). The Poynting fluxes of the waves indicate that the ultrawideband chorus waves are excited near the magnetic equator. By comparing the linear wave growth rate to the nonlinear growth rate, we found that the ultrawideband chorus waves are probably amplified through the nonlinear excitation mechanism.
In the quasi‐linear approximation, we study electron acceleration process generated by whistler‐mode and compressional ULF (fast mode waves) turbulences near the Earth's synchronous orbit. The results show that the whistler‐mode turbulence (0.1 f ce ≤ f ≤ 0.75 f ce ) can accelerate substorm injection electrons with several hundreds of keV through wave‐particle gyroresonant interaction and hence may play an important role in the electron acceleration during substorms. The compressional ULF turbulence (2–15 mHz) can accelerate both lower‐energy background electrons (<30 keV) and substorm injection electrons (∼30–300 keV) through the transit‐time damping mechanism. So the compressional ULF turbulence acceleration mechanism is important during both substorms and quiet times. The compressional ULF turbulence accelerates substorm injection electrons more effectively than whistler‐mode turbulence. The combined electron acceleration by whistler‐mode and ULF turbulences is most effective and can cause the number density of the relativistic electrons increase largely within about 8 hours. Substorms can offer both substorm injection electrons and strong turbulences, and therefore large flux enhancement events of relativistic electrons (≥1 MeV) always occur during substorm time. For magnetic storms that are composed of a series of substorms, extremely large flux enhancement events of the relativistic electrons can thus occur.
1. The first four files contain the data of the density and temperature of neutral atmospheric compositions. They are calculated through the NRLMSISE-00 atmospheric model (Picone et al., 2002).2. The fifth file introduces the variable names of each column in the data files of neutral atmospheric compositions.
Abstract. In this paper, we report significant evidence for preseismic ionospheric anomalies in total electron content (TEC) of the global ionosphere map (GIM) and plasma density appearing on day 2 before the 17 July 2006 M7.7 south of Java earthquake. After distinguishing other anomalies related to the geomagnetic activities, we found a temporal precursor around the epicenter on day 2 before the earthquake (15 July 2006), which agrees well with the spatial variations in latitude–longitude–time (LLT) maps. Meanwhile, the sequences of latitude–time–TEC (LTT) plots reveal that the TECs on epicenter side anomalously decrease and lead to an anomalous asymmetric structure with respect to the magnetic equator in the daytime from day 2 before the earthquake. This anomalous asymmetric structure disappears after the earthquake. To further confirm these anomalies, we studied the plasma data from DEMETER satellite in the earthquake preparation zone (2046.4 km in radius) during the period from day 45 before to day 10 after the earthquake, and also found that the densities of both electron and total ion in the daytime significantly increase on day 2 before the earthquake. Very interestingly, O+ density increases significantly and H+ density decreases, while He+ remains relatively stable. These results indicate that there exists a distinct preseismic signal (preseismic ionospheric anomaly) over the epicenter.
This paper studied statistically the joint responses of magnetic field and relativistic (>0.5 MeV) electrons at geosynchronous orbit to 201 interplanetary perturbations during 6 years from 2003 (solar maximum) to 2008 (solar minimum). The statistical results indicate that during geomagnetically quiet times ( H SYM > −30 nT, and AE < 200 nT), ~47.3% changes in the geosynchronous magnetic field and relativistic electron fluxes are caused by the combined actions of the enhancement of solar wind dynamic pressure ( P d ) and the southward turning of interplanetary magnetic field (IMF) (Δ P d > 0.4 nPa and IMF B z < 0 nT), and only ~18.4% changes are due to single dynamic pressure increase (Δ P d > 0.4 nPa, but IMF B z > 0 nT), and ~34.3% changes are due to single southward turning of IMF (IMF B z < 0 nT, but |Δ P d | < 0.4 nPa). Although the responses of magnetic field and relativistic electrons to the southward turning of IMF are weaker than their responses to the dynamic pressure increase, the southward turning of IMF can cause significant dawn‐dusk asymmetric perturbations that the magnetic field and relativistic electron fluxes increase on the dawnside (LT ~ 00:00–12:00) but decrease on the duskside (LT ~ 13:00–23:00) during the quiet times. Furthermore, the variation of relativistic electron fluxes is adiabatically controlled by the magnitude and elevation angle changes of magnetic field during the single IMF southward turnings. However, the variation of relativistic electron fluxes is independent of the change in magnetic field in some magnetospheric compression regions during the solar wind dynamic pressure enhancements (including the single pressure increases and the combined external perturbations), indicating that nonadiabatic dynamic processes of relativistic electrons occur there.
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