The spatial distributions of different ion species are useful indicators for plasma sheet dynamics. In this statistical study based on 7 years of Cluster observations, we establish the spatial ...distributions of oxygen ions and protons at energies from 274 to 955 keV, depending on geomagnetic and solar wind (SW) conditions. Compared with protons, the distribution of energetic oxygen has stronger dawn‐dusk asymmetry in response to changes in the geomagnetic activity. When the interplanetary magnetic field (IMF) is directed southward, the oxygen ions show significant acceleration in the tail plasma sheet. Changes in the SW dynamic pressure (Pdyn) affect the oxygen and proton intensities in the same way. The energetic protons show significant intensity increases at the near‐Earth duskside during disturbed geomagnetic conditions, enhanced SW Pdyn, and southward IMF, implying there location of effective inductive acceleration mechanisms and a strong duskward drift due to the increase of the magnetic field gradient in the near‐Earth tail. Higher losses of energetic ions are observed in the dayside plasma sheet under disturbed geomagnetic conditions and enhanced SW Pdyn. These observations are in agreement with theoretical models.
Key Points
The spatial distributions of energetic O+ and H+ are established
Effective inductive acceleration is located at the near‐Earth duskside
Higher energetic ion losses in the day plasma sheet under disturbed conditions
Magnetospheric plasma sheet ions drift toward the Earth and populate the ring current. The ring current plasma pressure distorts the terrestrial internal magnetic field at the surface, and this ...disturbance strongly affects the strength of a magnetic storm. The contribution of energetic ions (>40 keV) and of heavy ions to the total plasma pressure in the near‐Earth plasma sheet is not always considered. In this study, we evaluate the contribution of low‐energy and energetic ions of different species to the total plasma pressure for the storm observed by the Cluster mission from 27 September until 3 October 2002. We show that the contribution of energetic ions (>40 keV) and of heavy ions to the total plasma pressure is ≃76–98.6% in the ring current and ≃14–59% in the magnetotail. The main source of oxygen ions, responsible for ≃56% of the plasma pressure of the ring current, is located at distances earthward of XGSE ≃ −13.5 RE during the main phase of the storm. The contribution of the ring current particles agrees with the observed Dst index. We model the magnetic storm using the Space Weather Modeling Framework (SWMF). We assess the plasma pressure output in the ring current for two different ion outflow models in the SWMF through comparison with observations. Both models yield reasonable results. The model which produces the most heavy ions agrees best with the observations. However, the data suggest that there is still potential for refinement in the simulations.
Plain Language Summary
Magnetospheric plasma sheet ions drift toward the Earth and populate the ring current. The ring current plasma pressure distorts the terrestrial internal magnetic field at the surface and strongly affects the strength of a magnetic storm. The contribution of energetic ions and of heavy ions to the total plasma pressure in the near‐Earth plasma sheet is not always considered. In this study, we evaluate the input of these components for the storm observed from 27 September until 3 October 2002 using observations by the Cluster mission. We compare the results with simulations from the Space Weather Modeling Framework which take into account ionospheric ion outflow. We show that neglecting the contribution of energetic ions and of heavy ions to the total plasma pressure can lead to the pressure underestimations of 76–98.6% in the ring current and 14–59% in the magnetotail. We find that it is important to consider heavy ions, especially ionospheric oxygen, and include the energetic part of the ion distribution in the simulations of the ring current and the magnetotail during the magnetic storm.
Key Points
The contribution of ions of different species to the total plasma pressure in the near‐Earth magnetosphere is estimated
The ability of the Space Weather Modeling Framework to reproduce the plasma pressure during a magnetic storm is tested
The main source of oxygen ions is located at distances closer than XGSE = −13.5 RE during the main phase
The demand for spatial climate data in digital form has risen dramatically in recent years. In response to this need, a variety of statistical techniques have been used to facilitate the production ...of GIS-compatible climate maps. However, observational data are often too sparse and unrepresentative to directly support the creation of high-quality climate maps and data sets that truly represent the current state of knowledge. An effective approach is to use the wealth of expert knowledge on the spatial patterns of climate and their relationships with geographic features, termed 'geospatial climatology', to help enhance, control, and parameterize a statistical technique. Described here is a dynamic knowledge-based framework that allows for the effective accumulation, application, and refinement of climatic knowledge, as expressed in a statistical regression model known as PRISM (parameter-elevation regressions on independent slopes model). The ultimate goal is to develop an expert system capable of reproducing the process a knowledgeable climatologist would use to create high-quality climate maps, with the added benefits of consistency and repeatability. However, knowledge must first be accumulated and evaluated through an ongoing process of model application; development of knowledge prototypes, parameters and parameter settings; testing; evaluation; and modification. This paper describes the current state of a knowledge-based framework for climate mapping and presents specific algorithms from PRISM to demonstrate how this framework is applied and refined to accommodate difficult climate mapping situations. A weighted climate-elevation regression function acknowledges the dominant influence of elevation on climate. Climate stations are assigned weights that account for other climatically important factors besides elevation. Aspect and topographic exposure, which affect climate at a variety of scales, from hill slope to windward and leeward sides of mountain ranges, are simulated by dividing the terrain into topographic facets. A coastal proximity measure is used to account for sharp climatic gradients near coastlines. A 2-layer model structure divides the atmosphere into a lower boundary layer and an upper free atmosphere layer, allowing the simulation of temperature inversions, as well as mid-slope precipitation maxima. The effectiveness of various terrain configurations at producing orographic precipitation enhancement is also estimated. Climate mapping examples are presented.
Energetic ion distributions in the near‐Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and ...losses in the near‐Earth region. In this study, 11 years of energetic proton and oxygen observations (> ~274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near‐Earth region. The dawn‐dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near‐Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are influenced by the dawn‐dusk interplanetary magnetic field (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm‐related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near‐Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause significantly affect the energetic ion population in the magnetosphere. We also believe that the influence of the dawn‐dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.
Key Points
We present the statistical observation of energetic ion distributions in the dayside magnetosphere and near‐Earth plasma sheet
The dawn‐dusk asymmetry of the distribution shows strong IMF dependence
The location of magnetic reconnection at the magnetopause influences the dawn‐dusk asymmetry
In this work, we review the results of observations by the Cluster/Research with Adaptive Particle Imaging Detectors (RAPID) energetic charged particle detector (∼>40 keV–∼1 MeV). The origin of ...energetic ions in the region upstream of the Earth's bow shock is compared under different solar wind conditions. We give a brief overview of the acceleration mechanisms of charged particles in the plasma sheet. Effective acceleration is associated with (multiple) interaction(s) of charged particles with multiscale magnetic structures and/or electromagnetic fluctuations, which are the consequences of reconnection or other plasma instabilities. The necessity of such acceleration mechanisms to reproduce the distributions of energetic particles on closed magnetic field lines is discussed. We review empirical models describing the distributions of charged particles. Several aspects of the dynamics of energetic particles during substorms and their influence on the dynamics of magnetic storms are shown. We advise to consider including the energetic particles at >40 keV in calculations of the plasma temperature and pressure during these dynamic processes.
Plain Language Summary
A fundamental scientific question is how plasma in the universe is heated up and accelerated. Understanding acceleration at supernovae shocks, of cosmic jets or laboratory plasmas still has many open questions. Near‐Earth space environment is an excellent laboratory to investigate plasma dynamics and to reveal the fundamental laws it obeys. Energetic plasmas are also hazardous for space satellites and play a key role in space weather. It is, therefore, necessary to study the energization of space plasmas, their distribution and consequences on the magnetospheric dynamics. In situ observations in the near‐Earth space by the Cluster satellites and the energetic particle detector, Research with Adaptive Particle Imaging Detectors (RAPID), reveal new insights in plasma acceleration, unexpected features in its distributions, and effects on substorm and geomagnetic storm dynamics. These observations also help space weather applications to determine the level of energetic particle intensities.
Key Points
Energetic particles at energies >40 keV have to be considered for the plasma temperature and pressure calculations
Effective acceleration is due to interaction of charged particles with multiscale magnetic structures and/or electromagnetic fluctuations
Direction of Interplanetary Magnetic Field leads to ion distribution asymmetries between Northern and Southern hemispheres
We study the acceleration of energetic electrons during magnetotail reconnection by using Cluster simultaneous measurements of three‐dimensional electron distribution functions, electric and magnetic ...fields, and waves in a thin current sheet. We present observations of two consecutive current sheet crossings where the flux of electrons 35–127 keV peaks within an interval of tailward flows. The first crossing shows the signatures of a tailward moving flux rope. The observed magnetic field and density indicate that the flux rope was very dynamic, and a comparison with numerical simulation suggests a crossing right after coalescence of smaller flux ropes. The second crossing occurs within the ion diffusion region. The flux of electrons is largest within the flux rope where they are mainly directed perpendicular to the magnetic field. At the magnetic separatrices, the fluxes are smaller, but the energy spectra are harder and electrons are mainly field aligned. Reconnection electric fields EY ∼ 7 mV/m are observed within the diffusion region, whereas in the flux rope, EY are much smaller. Waves around lower hybrid frequency do not show a clear correlation with energetic electrons. We interpret the field‐aligned electrons at the separatrices as directly accelerated by the reconnection electric field in the diffusion region, whereas we interpret the perpendicular electrons as trapped within the flux rope and accelerated by a combination of betatron acceleration with nonadiabatic pitch‐angle scattering. Our observations indicate that thin current sheets during dynamic reconnection are important for in situ production of energetic electrons and that simultaneous measurements of electrons and electromagnetic fields within thin sheets are crucial to understand the acceleration mechanisms.
The composition of ions plays a crucial role for the fundamental plasma properties in the terrestrial magnetosphere. We investigate the oxygen‐to‐hydrogen ratio in the near‐Earth magnetosphere from ...−10 RE < XGSE < 10 RE. The results are based on seven years of ion flux measurements in the energy range ∼10 keV to ∼955 keV from the RAPID and CIS instruments on board the Cluster satellites. We find that (1) hydrogen ions at ∼10 keV show only a slight correlation with the geomagnetic conditions and interplanetary magnetic field changes. They are best correlated with the solar wind dynamic pressure and density, which is an expected effect of the magnetospheric compression; (2) ∼10 keV O+ ion intensities are more strongly affected during disturbed phase of a geomagnetic storm or substorm than >274 keV O+ ion intensities, relative to the corresponding hydrogen intensities; (3) In contrast to ∼10 keV ions, the >274 keV O+ions show the strongest acceleration during growth phase and not during the expansion phase itself. This suggests a connection between the energy input to the magnetosphere and the effective energization of energetic ions during growth phase; (4) The ratio between quiet and disturbed times for the intensities of ion ionospheric outflow is similar to those observed in the near‐Earth magnetosphere at >274 keV. Therefore, the increase of the energetic ion intensity during disturbed time is likely due to the intensification and the effective acceleration of the ionospheric source. In conclusion, the energization process in the near‐Earth magnetosphere is mass dependent and it is more effective for the heavier ions.
Key Points
Response of the O+ and H+ to the geomagnetic and solar wind changes
The strongest energetic O+ acceleration is during growth phase
O+ at lower energies is strongly affected by storms and substorms
We study energetic spectra of H+, He+, and O+ ion fluxes in the energy range ≥130 keV measured by Cluster/Research with Adaptive Particle Imaging Detectors (RAPID) instruments during 37 intervals of ...the tailward bulk flow propagation in the near‐Earth tail (at X ≤ −19 RE). In all events from our database, the plasmoid‐like magnetic structures with the superimposed low‐frequency magnetic and electric field fluctuations were observed along with the tailward bulk flows. The plasmoid‐like structures were associated with the enhancements of energetic ion fluxes and the hardening of energy spectra of H+ and He+ ion components in 80% of events and of O+ ion component in 64% of events. The hardening of energy spectra was more pronounced for heavy ions than for protons. The analysis of the magnetic structures and power spectral density (PSD) of the magnetic and electric field fluctuations from our database revealed the following factors favorable for the ion energization: (1) the spatial scale of a plasmoid should exceed the thermal gyroradius of a given ion component in the neutral plane inside the plasmoid; (2) the PSD of the magnetic fluctuations near the gyrofrequency of a particular ion component should exceed ~ 50.0 nT2/Hz for oxygen ions; while the energization of helium ions and protons takes place for much lower values of the PSD. The kinetic analysis of ion dynamics in the plasmoid‐like magnetic configuration similar to the observed one with the superimposed turbulence confirms the importance of ion resonant interactions with the low‐frequency electromagnetic fluctuations for ion energization inside plasmoids.
Key Points
Spectra of energetic H+, He+, O+ fluxes were studied tailward of X line
Strong ion acceleration occurs in turbulent region inside plasmoids
Ions are accelerated due to resonant interaction with EM fluctuations
We investigate and compare Cluster observations of electron dynamics in different locations of the ion diffusion region for magnetic reconnection in the Earth's magnetotail. On the basis of the 2‐D ...reconstructed magnetic field map from Cluster 1 (C1), we pinpoint that the observed Hall field is ∼6000 km (∼9 ion inertial lengths) away from the magnetic X‐point, and reveal that C3 was the one in closest proximity to the X‐point at the time when the reconnection jet reversal was simultaneously seen by three spacecraft, namely, C1, C3, and C4. No evidence is found for strong wave emission and energetic electron enhancement near to the X‐point, as compared to that within the diffusion region. We find that (1) the Hall current loop is mainly carried by the low‐energy, field‐aligned counterstreaming electrons; (2) a flat‐top distribution in phase space density is a common feature for Hall‐related electrons; (3) an enhancement of energetic electrons is observed together with the presence of the flat‐top electrons; and (4) electromagnetic wave emission is enhanced within the diffusion region. Two different regions of field‐aligned counterstreaming (FC) electrons are identified: one is associated with the Hall current loop (i.e., the electron flow reversal) while another one stays at the edge of the loop. Interestingly, observations show that at the transition between the two FC regions, the waves seem to suppress the energetic electrons but to promote the flat‐top electrons.
Key Points
We study electron dynamics in different locations of the ion diffusion region
We find no enhancements of energetic electrons and wave emissions near X‐point
We pinpoint spacecraft locations using 2‐D Hall‐MHD reconstruction technique
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