摘要 :
One of the longstanding questions of space science is: How does the Earth's magnetosphere generate auroral arcs? A related question is: What form of energy is extracted from the magnetosphere to drive auroral arcs? Not knowing the...
展开
One of the longstanding questions of space science is: How does the Earth's magnetosphere generate auroral arcs? A related question is: What form of energy is extracted from the magnetosphere to drive auroral arcs? Not knowing the answers to these questions hinders our ability to determine the impact of auroral arcs on the magnetospheric system. Magnetospheric mechanisms for driving quiescent auroral arcs are reviewed. Two types of quiescent arcs are (1) low-latitude non-Alfvénic (growth-phase) arcs magnetically connecting to the electron plasma sheet and (2) high-latitude arcs magnetically connecting near the plasma-sheet boundary layer. The reviews of the magnetospheric generator mechanisms are separated for the two types of quiescent arcs. The driving of auroral-arc currents in largescale computer simulations is examined. Predicted observables in the magnetosphere and in the ionosphere are compiled for the various generator mechanisms.
收起
摘要 :
Heavy (O+) ion energization and field-aligned motion in and near the ionosphere are still not well understood. Based on observations from the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) Enhanced Polar Outflow Probe...
展开
Heavy (O+) ion energization and field-aligned motion in and near the ionosphere are still not well understood. Based on observations from the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) Enhanced Polar Outflow Probe at altitudes between 325 km and 730 km over 1 year, we present a statistical study (24 events) of ion heating and its relation to field-aligned ion bulk flow velocity,low-frequency waves, and field-aligned currents. The ion temperature and field-aligned bulk flow velocity are derived from 2-D ion velocity distribution functions measured by the suprathermal electron imager (SEI) instrument. Consistent ion heating and flow velocity characteristics are observed from both the SEI and the rapid-scanning ion mass spectrometer instruments. We find that transverse O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to very narrow regions (~2 km across B),is more likely to occur in the downward current region and is associated with broadband extremely low frequency (BBELF) waves. These waves are interpreted as linearly polarized perpendicular to the magnetic field. The amount of ion heating cannot be explained by frictional heating, and the correlation of ion heating with BBELF waves suggests that significant wave-ion heating is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Surprisingly, the majority of these heating events (17 out 24) are associated with core ion downflows rather than upflows. This may be explained by a downward pointing electric field in the low-altitude return current region.
收起
摘要 :
High‐latitude ionospheric plasma convection plays a fundamental role in determining many processes in the terrestrial ionosphere. Electric Field Instruments on the European Space Agency's three polar‐orbiting Swarm satellites me...
展开
High‐latitude ionospheric plasma convection plays a fundamental role in determining many processes in the terrestrial ionosphere. Electric Field Instruments on the European Space Agency's three polar‐orbiting Swarm satellites measure ionospheric ion drift velocities at about 500?km altitude using thermal ion imager energy/angle‐of‐arrival electrostatic analyzers. Recently, European Space Agency released horizontal cross‐track components of these drifts, calibrated at high latitudes. This paper concerns the validation of the Swarm horizontal cross‐track ion drift measurements. All available Swarm‐A and Swarm‐B 2?Hz data between November 2015 and July 2017 were used and the climatology of high‐latitude ion convection was constructed and examined. Results were compared to corresponding climatology obtained from the Weimer 2005 empirical convection electric field model under different interplanetary magnetic field and solar wind conditions in the northern and southern hemispheres, separately. The ion drift data sometimes exhibit large offsets at middle latitudes. However, following a recalibration of the drifts using a refinement of the offset removal, the Swarm cross‐track ion drift climatology agrees reasonably well statistically with the Weimer 2005 model, and properly responds to the changing geospace environment. The two results agree within about 200?m/s (root‐mean‐square deviation), however the correlations are higher for southward interplanetary magnetic field and in the northern hemisphere ( r swarm‐A ?=?0.84, r swarm‐B ?=?0.77), for which the corresponding magnitudes of Swarm‐A and Swarm‐B drifts are ~14% and ~33% larger than the model estimates, respectively. The convection patterns seen in the revised Swarm horizontal cross‐track drift velocities are more structured than those obtained using the model, but overall no significant systematic errors are present.
收起
摘要 :
We investigate mechanisms of wave particle heating of ionospheric O~+ ions resulting from broadband extremely low frequency (BBELF) waves using numerical test particle simulations that take into account ion-neutral collisions, in ...
展开
We investigate mechanisms of wave particle heating of ionospheric O~+ ions resulting from broadband extremely low frequency (BBELF) waves using numerical test particle simulations that take into account ion-neutral collisions, in order to explain observations from the Enhanced Polar Outflow Probe (e-POP) satellite at low altitudes (~400 km) (Shen et al., 2018, https://doi.org/10.1002/ 2017JA024955). We argue that in order to reproduce ion temperatures observed at e-POP altitudes, the most effective ion heating mechanism is through cyclotron acceleration by short-scale electrostatic ion cyclotron (EIC) waves with perpendicular wavelengths λ_⊥ ≤ 200 m. The interplay between finite perpendicular wavelengths, wave amplitudes, and ion-neutral collision frequencies collectively determine the ionospheric ion heating limit, which begins to decrease sharply with decreasing altitude below approximately 500 km, where the ratio νc/f_(ci) becomes larger than 10~(-3), νc and fci denoting the O~+-O collision frequency and ion cyclotron frequency. We derive, both numerically and analytically, the ion gyroradius limit from heating by an EIC wave at half the cyclotron frequency. The limit is 0.28λ_⊥. The ion gyroradius limit from an EIC wave can be surpassed either through adding waves with different λ_⊥ or through stochastic "breakout," meaning ions diffuse in energy beyond the gyroradius limit due to stochastic heating from large-amplitude waves. Our two-dimensional simulations indicate that small-scale (<1 km) Alfvén waves cannot account for the observed ion heating through trapping or stochastic heating.
收起
摘要 :
We investigate low-energy (<10 eV) ion upflows (mainly O~+) within the cleft ion fountain (CIF) using conjunctions of the Enhanced Polar Outflow Probe (e-POP) satellite, the DMSP F16 satellite, the SuperDARN radar, and the Resolute Bay Incoherent Scatter Radar North (RISR-N). The SEI instrument on board e-POP enables us to derive ion upflow velocities from the 2-D images of ion distribution functions with a frame rate of 100 images per second, and with a velocity resolution of the order of 25 m/s. We identify three cleft ion fountain events with very intense (>1.6 km/s) ion upflow velocities near 1000 km altitude during quiet geomagnetic activity (Kp < 3). Such large ion upflow velocities have been reported previously at or below 1000 km, but only during active periods. Analysis of the core ion distribution images allows us to demonstrate that the ion temperature within the CIF does not rise by more than 0.3 eV relative to background values, which is consistent with RISR-N observations in the F region. The presence of soft electron precipitation seen by DMSP and lack of significant ion heating indicate that the ion upflows we observe near 1000 km altitude are primarily driven by ambipolar electric fields. DC field-aligned currents (FACs) and convection velocity gradients accompany these events. The strongest ion upflows are associated with downward current regions, which is consistent with some (although not all) previously published results. The moderate correlation coefficient (0.51) between upflow velocities and currents implies that FACs serve as indirect energy inputs to the ion upflow process....
展开
We investigate low-energy (<10 eV) ion upflows (mainly O~+) within the cleft ion fountain (CIF) using conjunctions of the Enhanced Polar Outflow Probe (e-POP) satellite, the DMSP F16 satellite, the SuperDARN radar, and the Resolute Bay Incoherent Scatter Radar North (RISR-N). The SEI instrument on board e-POP enables us to derive ion upflow velocities from the 2-D images of ion distribution functions with a frame rate of 100 images per second, and with a velocity resolution of the order of 25 m/s. We identify three cleft ion fountain events with very intense (>1.6 km/s) ion upflow velocities near 1000 km altitude during quiet geomagnetic activity (Kp < 3). Such large ion upflow velocities have been reported previously at or below 1000 km, but only during active periods. Analysis of the core ion distribution images allows us to demonstrate that the ion temperature within the CIF does not rise by more than 0.3 eV relative to background values, which is consistent with RISR-N observations in the F region. The presence of soft electron precipitation seen by DMSP and lack of significant ion heating indicate that the ion upflows we observe near 1000 km altitude are primarily driven by ambipolar electric fields. DC field-aligned currents (FACs) and convection velocity gradients accompany these events. The strongest ion upflows are associated with downward current regions, which is consistent with some (although not all) previously published results. The moderate correlation coefficient (0.51) between upflow velocities and currents implies that FACs serve as indirect energy inputs to the ion upflow process.
收起
摘要 :
Airglow patches have been recently associated with channels of enhanced antisunward ionospheric flows propagating across the polar cap from the dayside to nightside auroral ovals. However, how these flows maintain their localized ...
展开
Airglow patches have been recently associated with channels of enhanced antisunward ionospheric flows propagating across the polar cap from the dayside to nightside auroral ovals. However, how these flows maintain their localized nature without diffusing away remains unsolved. We examine whether patches and collocated flows are associated with localized field-aligned currents (FACs) in the polar cap by using coordinated observations of the Swarm spacecraft, a polar cap all-sky imager, and Super Dual Auroral Radar Network (SuperDARN) radars. We commonly (66% of cases) identify substantial FAC enhancements around patches, particularly near the patches' leading edge and center, in contrast to what is seen in the otherwise quiet polar cap. These FACs have densities of 0.1–0.2 μA/m~(-2) and have a distribution of width peaking at ~75 km. They can be approximated as infinite current sheets that are orientated roughly parallel to patches. They usually exhibit a Region 1 sense, i.e., a downward FAC lying eastward of an upward FAC. With the addition of Resolute Bay Incoherent Scatter radar data, we find that the FACs can close through Pedersen currents in the ionosphere, consistent with the locally enhanced dawn-dusk electric field across the patch. Our results suggest that ionospheric polar cap flow channels are imposed by structures in the magnetospheric lobe via FACs, and thus manifest mesoscale magnetosphere-ionosphere coupling embedded in large-scale convection.
收起
摘要 :
The European Space Agency’s Swarm constellation can measure electric field, magnetic field, and plasma density on board multiple satellites at altitudes of about 500 km. Based on the data set at high latitudes, we estimate Poynti...
展开
The European Space Agency’s Swarm constellation can measure electric field, magnetic field, and plasma density on board multiple satellites at altitudes of about 500 km. Based on the data set at high latitudes, we estimate Poynting flux and ionospheric reflection coefficients of Alfvén waves with scale sizes of about 10–100 km. The reflection coefficients are generally higher surrounding the cusp and auroral regions than in the polar cap and higher in the summer than in the winter hemisphere. In the summer (winter) hemisphere the reflection coefficients generally peak on the dayside (nightside). Distributions of the reflection coefficients are not controlled by those of in situ plasma density. Poynting flux of the Alfvén waves maximizes surrounding the cusp and near-midnight auroral region with magnitudes approaching 1 mW/m~2, which are consistent with previous magnetospheric observations. The observed Poynting flux is downward on average for both hemispheres, and the magnitudes do not exhibit clear hemispheric asymmetry.
收起
摘要 :
We present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to the magnetic field in the topside (900-1,500 km) ionosphere. Based on measurements from the suprathermal elec...
展开
We present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to the magnetic field in the topside (900-1,500 km) ionosphere. Based on measurements from the suprathermal electron imager (SEI) onboard the Enhanced Polar Outflow Probe satellite over several months, we identify 30 events (28 in the dayside cusp) of enhanced suprathermal electron fluxes peaking at ±90? pitch angles, with energies spanning from tens of eV to 325 eV, the upper limit of the SEI measurement. These events take place with a time duration of the order of 0.1 s, corresponding to a spatial scale of less than 1 km across the magnetic field. They are associated with parallel suprathermal electron bursts and upward currents. The correlation between perpendicular and parallel suprathermal electrons suggests a scenario in which downward bursts and the associated Alfvén waves that drive them provide a free energy source to destabilize plasma waves at electron frequencies, which in turn heat electrons in the perpendicular direction. This is a new energy dissipation process for cusp suprathermal electron precipitation in the topside ionosphere.
收起
摘要 :
This is the first statistical survey of field fluctuations related with medium-scale traveling ionospheric disturbances (MSTIDs), which considers magnetic field, electric field, and plasma density variations at the same time. Midl...
展开
This is the first statistical survey of field fluctuations related with medium-scale traveling ionospheric disturbances (MSTIDs), which considers magnetic field, electric field, and plasma density variations at the same time. Midlatitude electric fluctuations (MEFs) and midlatitude magnetic fluctuations (MMFs) observed in the nighttime topside ionosphere have generally been attributed to MSTIDs. Although the topic has been studied for several decades, statistical studies of the Poynting flux related with MEF/MMF/MSTID have not yet been conducted. In this study we make use of electric/magnetic field and plasma density observations by the European Space Agency's Swarm constellation to address the statistical behavior of the Poynting flux. We have found that (1) the Poynting flux is directed mainly from the summer to winter hemisphere, (2) its magnitude is larger before midnight than thereafter, and (3) the magnitude is not well correlated with fluctuation level of in situ plasma density. These results are discussed in the context of previous studies.
收起
摘要 :
We provide insight into the vertical distribution of multi-scale scintillation-inducing irregularities in the low-latitude ionosphere. In four sets of novel experiments, we sampled altitudes from 330 to 1,280 km in the 18–24 magn...
展开
We provide insight into the vertical distribution of multi-scale scintillation-inducing irregularities in the low-latitude ionosphere. In four sets of novel experiments, we sampled altitudes from 330 to 1,280 km in the 18–24 magnetic local time (MLT) sector using the Swarm Echo GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O) GPS receiver with its antenna oriented toward zenith. In order to identify multi-scale irregularities both above and at the satellite's position, we utilize high-sample-rate GAP-O amplitude and phase measurements along with a measurement of net current on the surface of the imaging and rapid-scanning ion mass spectrometer sensor on board, which serves as a proxy for density variations. By calculating the rate of change of total electron content index using two sets of GPS parameter choices, we are able to sample irregularities as small as 160 m, which is comparable to or smaller than the Fresnel scale responsible for scintillation-inducing irregularities. During one campaign, we find that amplitude scintillations on the GPS signal coincide with strong in-situ small-scale density irregularities in 32% of cases, indicative of a broad irregularity region extending from the satellite's position to hundreds of kilometers above. Also, we show that large-scale ionospheric disturbances (larger than 80 km) occur predominantly below 500 km, and down to the 330 km perigee of Swarm Echo in the 18–21 MLT sector. In contrast, small-scale variations of total electron content are detected at all MLTs between 18 MLT and magnetic midnight and at all altitudes sampled in this experiment. However, they are more frequent in the 22–24 MLT range.
收起
原文获取