Magnetospheric Energy Research Articles

Challenges to Understanding the Earth's Ionosphere and Thermosphere

AbstractWe discuss, in a limited way, some of the challenges to advancing our understanding and description of the coupled plasma and neutral gas that make up the ionosphere and thermosphere (I‐T). The I‐T is strongly influenced by wave motions of the neutral atmosphere from the lower atmosphere and is coupled to the magnetosphere, which supplies energetic particle precipitation and field‐aligned currents at high latitudes. The resulting plasma dynamics are associated with currents generated by solar heating and upward propagating waves, by heating from energetic particles and electromagnetic energy from the magnetosphere and by the closure of the field‐aligned currents applied at high latitudes. These three contributors to the current are functions of position, magnetic activity, and other variables that must be unraveled to understand how the I‐T responds to coupling from the surrounding regions of geospace. We have captured the challenges to this understanding in four major themes associated with coupling to the lower atmosphere, the generation and flow of currents within the I‐T region, the coupling to the magnetosphere, and the response of the I‐T region reflected in the neutral and plasma density changes. Addressing these challenges requires advances in observing the neutral density, composition, and velocity and simultaneous observations of the plasma density and motions as well as the particles and field‐aligned current describing the magnetospheric energy inputs. Additionally, our modeling capability must advance to include better descriptions of the processes affecting the I‐T region and incorporate coupling to below and above at smaller spatial and temporal scales.

Relationship between Solar Wind—Magnetosphere Energy and Eurasian Winter Cold Events

The profound impact of solar irradiance variations on the decadal variability of Earth’s climate has been investigated by previous studies. However, it remains a challenge to quantify the energetic particle precipitation (EPP) influence on the surface climate, which is an emerging research topic. The solar wind is a source of magnetospheric EPP, and the total energy input from the solar wind into Earth’s magnetosphere (Ein) shows remarkable interdecadal and interannual variability. Based on the new Ein index, this study reveals a significant interannual relationship between the annual mean Ein and Eurasian cold extremes in the subsequent winter. Less frequent cold events are observed over Eurasia (primarily north of 50°N) following the higher-than-normal Ein activity in the previous year, accompanied by more frequent cold events over northern Africa, and vice versa. This response pattern shows great resemblance to the first empirical orthogonal function of the variability of cold extremes over Eurasia, with a spatial correlation coefficient of 0.79. The pronounced intensification of the positive North Atlantic Oscillation events and poleward shift of the North Atlantic storm track associated with the anomalously higher Ein favor the anomalous extreme atmospheric circulation events, and thus less frequent extreme cold temperatures over northern Eurasia on the interannual time scale. It is further hypothesized that the wave-mean flow interaction in the stratosphere and troposphere is favorable for the connection of Ein signals to tropospheric circulation and climate in the following winter.

An Explicit IMF B Dependence on Solar Wind‐Magnetosphere Coupling

AbstractPresently, all empirical coupling functions quantifying the solar wind—magnetosphere energy—or magnetic flux conversion assume that the coupling is independent of the sign of the dawn‐dusk component (B ) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF B effect on the solar wind‐magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF B is found to on average lead to a larger polar cap than when IMF B is negative during otherwise similar conditions. This explicit IMF B effect is found to reverse when the Earth's dipole is inclined in the opposite direction (northern summer) and is consistently observed from both hemispheres. We interpret the different responses of the polar cap size due to the sign of IMF B to likely be a result of differences in the dayside reconnection rate.

Instability of twisted magnetar magnetospheres

ABSTRACT We present 3D force-free electrodynamics simulations of magnetar magnetospheres that demonstrate the instability of certain degenerate, high energy equilibrium solutions of the Grad–Shafranov equation. This result indicates the existence of an unstable branch of twisted magnetospheric solutions and allows us to formulate an instability criterion. The rearrangement of magnetic field lines as a consequence of this instability triggers the dissipation of up to 30 per cent of the magnetospheric energy on a thin layer above the magnetar surface. During this process, we predict an increase of the mechanical stresses on to the stellar crust, which can potentially result in a global mechanical failure of a significant fraction of it. We find that the estimated energy release and the emission properties are compatible with the observed giant flare events. The newly identified instability is a candidate for recurrent energy dissipation, which could explain part of the phenomenology observed in magnetars.

Braneworld effects in plasma magnetosphere of a slowly rotating magnetized neutron star

Results of our previous paper [B. V. Turimov, B. J. Ahmedov and A. A. Hakimov, Phys. Rev. D 96 (2017) 104001] show that the effects of brane charges are not negligible in the magnetic field of the magnetized neutron star, in particular at the surface of the star, and increasing the value of brane tidal charges causes an increases in the value of surface magnetic field of magnetized neutron star, that is why it is important to consider the effects of braneworlds on energetic processes in the plasma magnetosphere of the neutron star. In this paper, we have obtained the analytical expression for Goldreich–Julian (GJ) charge density in braneworlds for inclined neutron star by solving Maxwell’s equations and found that the value of GJ charge density decreases in braneworlds. The analytical expression for scalar potential in the polar cap region of the neutron star has also been obtained. It is shown that the values of the parallel accelerating electrical fields increase with the increase of the value of the tidal charge near the surface of the neutron star. The influence of braneworlds on pair production condition on the surface of the neutron star and magnetospheric energy losses due to electromagnetic radiations have also been studied. We have shown how radiation beam becomes narrow due to the effects of braneworlds by studying the particle’s trajectory in the polar cap region in the [Formula: see text]–[Formula: see text] ([Formula: see text].) plane. Numerical calculations for particle motion in the polar cap region show that accelerating distance of charged particle increases up to its maximum value in braneworld in comparison with that in GR, due to additional gravitating behavior of tidal charges.

Solar-wind-magnetosphere energy influences the interannual variability of the northern-hemispheric winter climate.

Solar irradiance has been universally acknowledged to be dominant by quasi-decadal variability, which has been adopted frequently to investigate its effect on climate decadal variability. As one major terrestrial energy source, solar-wind energy flux into Earth's magnetosphere (Ein) exhibits dramatic interannual variation, the effect of which on Earth's climate, however, has not drawn much attention. Based on the Ein estimated by 3D magnetohydrodynamic simulations, we demonstrate a novelty that the annual mean Ein can explain up to 25% total interannual variance of the northern-hemispheric temperature in the subsequent boreal winter. The concurrent anomalous atmospheric circulation resembles the positive phase of Arctic Oscillation/North Atlantic Oscillation. The warm anomalies in the tropic stratopause and tropopause induced by increased solar-wind–magnetosphere energy persist into the subsequent winter. Due to the dominant change in the polar vortex and mid-latitude westerly in boreal winter, a ‘top-down’ propagation of the stationary planetary wave emerges in the Northern Hemisphere and further influences the atmospheric circulation and climate.

Fabry‐Perot Interferometer Observations of Thermospheric Horizontal Winds During Magnetospheric Substorms

AbstractThe high‐latitude ionosphere‐thermosphere system is strongly affected by the magnetospheric energy input during magnetospheric substorms. In this study, we investigate the response of the upper thermospheric winds to four substorm events by using the Fabry‐Perot interferometer at Tromsø, Norway, the International Monitor for Auroral Geomagnetic Effects magnetometers, the EISCAT radar, and an all‐sky camera. The upper thermospheric winds had distinct responses to substorm phases. During the growth phase, westward acceleration of the wind was observed in the premidnight sector within the eastward electrojet region. We suggest that the westward acceleration of the neutral wind is caused by the ion drag force associated with the large‐scale westward plasma convection within the eastward electrojet. During the expansion phase, the zonal wind had a prompt response to the intensification of the westward electrojet (WEJ) overhead Tromsø. The zonal wind was accelerated eastward, which is likely to be associated with the eastward plasma convection within the substorm current wedge. During the expansion and recovery phases, the meridional wind was frequently accelerated to the southward direction, when the majority of the substorm WEJ current was located on the poleward side of Tromsø. We suggest that this meridional wind acceleration is related to a pressure gradient produced by Joule heating within the substorm WEJ region. In addition, strong atmospheric gravity waves during the expansion and the recovery phases were observed.

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

AbstractSaturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short‐lived ∼1‐hr quasiperiodic high‐energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time‐resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low‐frequency waves and instead directly caused by recurrent small‐scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent‐back to a more dipolar configuration. This manifests as a sawtooth‐shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.

Responses of Nitrogen Oxide to High‐Speed Solar Wind Stream in the Polar Middle Atmosphere

AbstractDuring high‐speed solar wind stream (HSS) events, energetic electrons from the Earth's inner magnetosphere transfer solar wind energy to the high‐latitude upper atmosphere, which may affect chemical compositions in the region. We conduct a study on the production of nitrogen oxides (NOx) in the polar middle atmosphere by energetic electron precipitation (EEP) during HSS events in the period of international polar year 2007–2008 northern winter. During this period, the geomagnetic activity was generally quiet and there were no major solar events, which indicates that the EEPs were mostly associated with HSS events. The electron flux immediately increases with the onset of HSS events and remains elevated during the passage of the events. The estimation of the directly produced NOx by EEPs was attempted by using the correlation between NOx and dynamic tracers such as CO and CH4. It was found that the direct effect of EEPs on NOx reaches down to about 55‐km altitude and the amount is estimated to be about 2 ppbv. This result indicates that the variations of polar stratospheric NOx in winter are mostly associated with dynamical processes such as vertical transport and horizontal mixing. We also found that the middle atmospheric O3 depletion during HSS events seems to be related to the EEP‐induced NOx at least in the uppermost stratosphere in the polar region.

Tomographic Estimation of Exospheric Hydrogen Density Distributions

AbstractFor the past decade, the Lyman‐alpha detectors on board National Aeronautics and Space Administration's Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission have obtained routine measurements of solar Lyman‐α photons (121.6 nm) resonantly scattered by atomic hydrogen (H) in the terrestrial exosphere. These data have been used to derive global three‐dimensional (3‐D) models of exospheric H density beyond 3 RE, which are needed to understand various aspects of aeronomy and heliophysics, such as atmospheric chemistry and energetics, magnetospheric energy dissipation, ion‐neutral coupling, and atmospheric evolution through gravitational escape. These empirical distributions are obtained through parametric fitting of assumed functional forms that have little observational justification, thus limiting confidence in conclusions drawn from analysis of the resulting exospheric structure. In this work, we present a new means of global 3‐D reconstruction of exospheric H density through tomographic inversion of the scattered H Lyman‐α emission. Our approach avoids the conventional dependence on ad hoc parametric formulations and, based on the case studies reported here, appears to enable a more accurate characterization of the global structure of the H density in the outer exosphere. We evaluate the bounds of technique feasibility using simulated TWINS data and report new geophysical insights gained from applying this promising new approach to an example of actual TWINS data.

Influence of Solar Wind Plasma Parameters on the Intensity of Isolated Magnetospheric Substorms

Parameters of the interplanetary magnetic field and solar wind plasma during periods of 163 isolated substorms have been studied. It is shown that the solar wind velocity V and plasma density N remain approximately constant for at least 3 h before substorm onset Т o and 1 h after Т o . On average, the velocity of the solar wind exhibits a stable trend toward anticorrelation with its density over the whole data array. However, the situation is different if the values of V and N are considered with respect to the intensity of substorms observed during that period. With the growth of substorm intensity, quantified as the maximum absolute value of AL index, an increase in both the solar wind plasma velocity and density, at which these substorms appear, is obsreved. It has been found that the magnitude of the solar wind dynamic pressure P is closely related to the magnetosphere energy load defined as averaged values of the Kan–Lee electric field EKL and Newell parameter dΦ/dt averaged for 1 h interval before Т o . The growth of the dynamic pressure is accompanied by an increase in the load energy necessary for substorm generation. This interrelation between P and values of EKL and dΦ/dt is absent in other, arbitrarily chosen periods. It is believed that the processes accompanying increasing dynamic pressure of the solar wind result in the formation of magnetosphere conditions that increasingly impede substorm generation. Thus, the larger is P, the more solar wind energy must enter the Earth’s magnetosphere during the period of the growth phase for substorm generation. This energy is later released during the period of the substorm expansion phase and creates even more intense magnetic bays.

High‐Latitude Neutral Density Structures Investigated by Utilizing Multi‐Instrument Satellite Data and NRLMSISE‐00 Simulations

AbstractThis study investigates various types of neutral density features developed in the cusp region during magnetically active and quiet times. Multi‐instrument Challenging Minisatellite Payload data provide neutral density, electron temperature, neutral wind speed, and small‐scale field‐aligned current (SS‐FAC) values. Gravity Recovery and Climate Experiment neutral density data are also employed. During active times, cusp densities or density spikes appeared with their underlying flow channels (FCs) and enhanced SS‐FACs implying upwelling, fueled by Joule heating, within/above FCs. Both the moderate nightside cusp enhancements under disturbed conditions and the minor dayside cusp enhancements under quiet conditions developed without any underlying FC and enhanced SS‐FACs implying the role of particle precipitation in their development. Observations demonstrate the relations of FCs, density spikes, and upwelling‐related divergent flows and their connections to the underlying (1) dayside magnetopause reconnection depositing magnetospheric energy into the high‐latitude region and (2) Joule heating‐driven disturbance dynamo effects. Results provide observational evidence that the moderate nightside cusp enhancements and the minor dayside cusp enhancements detected developed due to direct heating by weak particle precipitation. Chemical compositions related to the dayside density spike and low cusp densities are modeled by Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar Extended 2000. Modeled composition outputs for the dayside density spike's plasma environment depict some characteristic upwelling signatures. Oppositely, in the case of low dayside cusp densities, composition outputs show opposite characteristics due to the absence of upwelling.

Variation of Solar Wind Parameters During Intense Geomagnetic Storms

Geomagnetic disturbances are caused by enhanced solar wind magnetospheric energy coupling process. The principal cause of geomagnetic disturbance is the magnetic reconnection that establishes an electrodynamical coupling between the solar wind plasma and magnetosphere. Around solar maximum, the main structures emanating from the sun are sporadic Coronal Mass Ejection (CMEs) and their interplanetary counterparts (ICMEs). During the descending and minimum solar cycle phases, coronal holes occur more often. They appear as dark regions confined to Solar poles during the solar maximum but expand in size and moves toward the solar equator during the descending phase. In this work, we have taken three different geomagnetic storms during solar maxima. For the interpretation of events, we used interplanetary solar wind data and geomagnetic indices. These satellite data and Dst indices (ranging from -100nT to above) are interpreted by using the method of cross correlation. The values of Bz found approximately 20nT, -50nT and -20nT respectively. Similarly, the value of Dst is -250nT, -400nT and -300nT which shows very intense effect. Likewise, the correlation coefficient we obtained from this research work strongly suggest that interplanetary magnetic field Bz has strong impact for the cause of geomagnetic storms.The Himalayan Physics Vol. 6 & 7, April 2017 (80-85)

MAVEN observations of tail current sheet flapping at Mars

AbstractThe Martian magnetotail is a complex regime through which atmospheric particles are lost to space. Our current understanding of Mars' tail continues to develop with the comprehensive particle and field data collected by Mars Atmosphere and Volatile EvolutioN (MAVEN). In this work, we identify periods when MAVEN encounters multiple current sheet crossings through a single tail traversal in order to understand tail dynamics. We apply an analysis technique that has been developed and validated by using multipoint measurements in order to separate the spatial and temporal properties associated with current sheet flapping. Events are classified into periods of steady flapping, due to a global motion of the current sheet, and kink‐like flapping, resulting from localized wave propagation along the tail current sheet. Out of 106 periods during which multiple current sheet crossings were observed, 20 were due to steady flapping and 10 from kink‐like flapping. A majority of the kink‐like events resulted from waves propagating in the opposite direction of the solar wind convection electric field, regardless of their location in the tail, unlike at Earth and Venus. This finding suggests that possible magnetosphere energy sources, whereby plasma is accelerated and removed from the Martian environment, are not located in the central magnetotail; rather, these waves may be driven by a source located at the tail flank based on the direction of the solar wind electric field. Therefore, by identifying potential sources of impulsive energy release in the tail, we may better understand mechanisms that drive atmospheric loss at Mars.

Comparative planetary nitrogen atmospheres: Density and thermal structures of Pluto and Triton

Both atmospheres of Pluto and Neptune's largest satellite Triton have cold surfaces with surface gravitational accelerations and atmospheric surface pressures of comparable magnitude. To study their atmospheres we have updated Zhu et al. (2014) model for Pluto's atmosphere by adopting Voigt line profiles in the radiation module with the latest spectral database and extended the model to Triton's atmosphere by including additional parameterized heating due to the magnetospheric electron transport and energy deposition. The CH4 mixing ratio profiles play central roles in differentiating the atmospheres of Pluto and Triton. On Pluto the surface CH4 mole fraction is in the range of 0.3-0.8%, sufficiently high to ensure that it is well mixed in the lower atmosphere and not subject to photochemical destruction. Near the exobase CH4 attains comparable density to N2 due to gravitational diffusive separation and escapes at 500 times the N2 rate (= 1 × 1023 N2s−1). In Triton's atmosphere, the surface CH4 mole fraction is on the order of 0.015%, sufficiently low to ensure that it is photochemically destroyed irreversibly in the lower atmosphere and that N2 remains the major species, even at the exobase. With solar EUV power only, Triton's upper thermosphere is too cold and magnetospheric heating, approximately comparable to the solar EUV power, is needed to bring the N2 tangential column number density in the 500–800km range up to values derived from the Voyager 2 UVS observations (Broadfoot et al., 1989). Due to their cold exobase temperatures relative to the gravitational potential energy wells that N2 resides in, atmospheric escape from Triton and Pluto is not dominated by N2 Jeans escape but by CH4 from Pluto and H, C, N and H2 from Triton. The atmospheric thermal structure near the exobase is sensitive to the atmospheric escape rate only when it is significantly greater than 2×1027amus−1, above which enhanced Jeans escape and larger radial velocity adiabatically cools the atmosphere to a lower temperature. Finally we suggest that Pluto's thermosphere is a cold ∼ 70K due to ablation of H2O molecules from the influx of dust grains detected by New Horizons Student Dust Counter.

Influence of ion outflow in coupled geospace simulations: 1. Physics‐based ion outflow model development and sensitivity study

AbstractWe describe a coupled geospace model that includes causally regulated ion outflow from a physics‐based ionosphere/polar wind model. The model two‐way couples the multifluid Lyon‐Fedder‐Mobarry magnetohydrodynamics (MHD) model to the ionosphere/polar wind model (IPWM). IPWM includes the H+ and O+ polar wind as well as a phenomenological treatment of energetic O+ accelerated by wave‐particle interactions (WPI). Alfvénic Poynting flux from the MHD simulation causally regulates the ion acceleration. The wave‐particle interactions (WPI) model has been tuned and validated with comparisons to particle‐in‐cell simulations and empirical relationships derived from Fast Auroral Snapshot satellite data. IPWM captures many aspects of the ion outflow that empirical relationships miss. First, the entire coupled model conserves mass between the ionospheric and magnetospheric portions, meaning the amount of outflow produced is limited by realistic photochemistry in the ionosphere. Second, under intense driving conditions, the outflow becomes flux limited by what the ionosphere is capable of providing. Furthermore, the outflows produced exhibit realistic temporal and spatial delays relative to the magnetospheric energy inputs. The coupled model provides a flexible way to explore the impacts of dynamic heavy ion outflow on the coupled geospace system. Some of the example simulations presented exhibit internally driven sawtooth oscillations associated with the outflow, and the properties of these oscillations are analyzed further in a companion paper.

On the reduced geoeffectiveness of solar cycle 24: A moderate storm perspective

AbstractThe moderate and intense geomagnetic storms are identified for the first 77 months of solar cycles 23 and 24. The solar sources responsible for the moderate geomagnetic storms are indentified during the same epoch for both the cycles. Solar cycle 24 has shown nearly 80% reduction in the occurrence of intense storms whereas it is only 40% in case of moderate storms when compared to previous cycle. The solar and interplanetary characteristics of the moderate storms driven by coronal mass ejection (CME) are compared for solar cycles 23 and 24 in order to see reduction in geoeffectiveness has anything to do with the occurrence of moderate storm. Though there is reduction in the occurrence of moderate storms, the Dst distribution does not show much difference. Similarly, the solar source parameters like CME speed, mass, and width did not show any significant variation in the average values as well as the distribution. The correlation between VBz and Dst is determined, and it is found to be moderate with value of 0.68 for cycle 23 and 0.61 for cycle 24. The magnetospheric energy flux parameter epsilon (ε) is estimated during the main phase of all moderate storms during solar cycles 23 and 24. The energy transfer decreased in solar cycle 24 when compared to cycle 23. These results are significantly different when all geomagnetic storms are taken into consideration for both the solar cycles.

Ices on Mercury: Chemistry of volatiles in permanently cold areas of Mercury's north polar region

Observations by the MESSENGER spacecraft during its flyby and orbital observations of Mercury in 2008–2015 indicated the presence of cold icy materials hiding in permanently-shadowed craters in Mercury's north polar region. These icy condensed volatiles are thought to be composed of water ice and frozen organics that can persist over long geologic timescales and evolve under the influence of the Mercury space environment. Polar ices never see solar photons because at such high latitudes, sunlight cannot reach over the crater rims. The craters maintain a permanently cold environment for the ices to persist. However, the magnetosphere will supply a beam of ions and electrons that can reach the frozen volatiles and induce ice chemistry. Mercury's magnetic field contains magnetic cusps, areas of focused field lines containing trapped magnetospheric charged particles that will be funneled onto the Mercury surface at very high latitudes. This magnetic highway will act to direct energetic protons, ions and electrons directly onto the polar ices. The radiation processing of the ices could convert them into higher-order organics and dark refractory materials whose spectral characteristics are consistent with low-albedo materials observed by MESSENGER Laser Altimeter (MLA) and RADAR instruments. Galactic cosmic rays (GCR), scattered UV light and solar energetic particles (SEP) also supply energy for ice processing. Cometary impacts will deposit H2O, CH4, CO2 and NH3 raw materials onto Mercury's surface which will migrate to the poles and be converted to more complex CHNOS-containing molecules such as aldehydes, amines, alcohols, cyanates, ketones, hydroxides, carbon oxides and suboxides, organic acids and others. Based on lab experiments in the literature, possible specific compounds produced may be: H2CO, HCOOH, CH3OH, HCO, H2CO3, CH3C(O)CH3, C2O, CxO, C3O2, CxOy, CH3CHO, CH3OCH2CH2OCH3, C2H6, CxHy, NO2, HNO2, HNO3, NH2OH, HNO, N2H2, N3, HCN, Na2O, NaOH, CH3NH2, SO, SO2, SO3, OCS, H2S, CH3SH, even BxHy. Three types of radiation processing mechanisms may be at work in the ices: (1) Impact/dissociation, (2) Ion implantation and (3) Nuclear recoil (hot atom chemistry). Magnetospheric energy sources dominate the radiation effects. Total energy fluxes of photons, SEPs and GCRs are all around two or more orders of magnitude less than the fluxes from magnetospheric energy sources (in the focused cusp particles). However, SEPs and GCRs cause chemical processing at greater depths than other particles leading to thicker organic layers. Processing of polar volatiles on Mercury would be somewhat different from that on the Moon because Mercury has a magnetic field while the Moon does not. The channeled flux of charged particles through these magnetospheric cusps is a chemical processing mechanism unique to Mercury as compared to other airless bodies.

High‐latitude energy input and its impact on the thermosphere

AbstractThis paper presents a quantitative assessment of high‐latitude energy input and its partitioning in the polar cap by synthesizing various space and ground‐based observations during the 17 January 2005 geomagnetic storm. It was found that Joule heating is the primary form of magnetospheric energy input, especially during active times when the hemispheric‐integrated Joule heating can be an order of magnitude larger than the hemispheric‐integrated auroral power. Most of magnetospheric energy is dissipated in the auroral zone rather than in the polar cap. On average, only about 22–25% of the total hemispheric energy input is dissipated into the polar cap region bordered by the convection reversal boundary (CRB) and the poleward auroral flux boundary (FXB). The impact of high‐latitude energy input was also investigated to unveil the causal relationship between Joule heating and the formation of polar cap mass density anomalies. Our numerical simulation demonstrated that thermosphere dynamics readily redistributes composition, temperature, and mass through upwelling and atmospheric gravity waves. The polar cap mass density anomalies observed by the CHAMP satellite during the storm were largely a result of large‐scale atmospheric gravity waves. Therefore, an increase in local thermospheric mass density does not necessarily mean there is direct energy input.

Driving ionospheric outflows and magnetospheric O+ energy density with Alfvén waves

AbstractWe show how dispersive Alfvén waves observed in the inner magnetosphere during geomagnetic storms can extract O+ ions from the topside ionosphere and accelerate these ions to energies exceeding 50 keV in the equatorial plane. This occurs through wave trapping, a variant of “shock” surfing, and stochastic ion acceleration. These processes in combination with the mirror force drive field‐aligned beams of outflowing ionospheric ions into the equatorial plane that evolve to provide energetic O+ distributions trapped near the equator. These waves also accelerate preexisting/injected ion populations on the same field lines. We show that the action of dispersive Alfvén waves over several minutes may drive order of magnitude increases in O+ ion pressure to make substantial contributions to magnetospheric ion energy density. These wave accelerated ions will enhance the ring current and play a role in the storm time evolution of the magnetosphere.