A record of the geomagnetic field on the ground sometimes shows smooth daily variations on the order of a few tens of nano teslas. These daily variations, commonly known as Sq, are caused by electric ...currents of several
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flowing on the sunlit side of the E-region ionosphere at about 90–150 km heights. We review advances in our understanding of the geomagnetic daily variation and its source ionospheric currents during the past 75 years. Observations and existing theories are first outlined as background knowledge for the non-specialist. Data analysis methods, such as spherical harmonic analysis, are then described in detail. Various aspects of the geomagnetic daily variation are discussed and interpreted using these results. Finally, remaining issues are highlighted to provide possible directions for future work.
We 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.
Plain Language Summary
The ionosphere is the region of Earth's upper atmosphere made up of a mixture of charged and neutral gases between approximately 50 and 1,000 miles (80–1,600 km) above the Earth's surface. Sandwiched between the lower atmosphere and the magnetosphere, the ionosphere reacts to weather and climate near the Earth's surface and to eruptions and sunspot activity on the Sun. The ionosphere absorbs the harmful radiation from the Sun and determines the fidelity of all radio communication, navigation, and surveillance transmissions through it. It is part of a complex, coupled system that changes on scales from meters to the planetary radius, and from seconds to decades. Understanding how the behavior of this region is controlled, by internal interactions and by the external regions to which it is coupled, is the preeminent challenge for the next generation of scientists. These challenges in understanding Earth's ionosphere are associated with deciphering the many changes in neutral and plasma density and their relationships to the coupling with the Earth's lower atmosphere, the generation and flow of currents within the region, and the coupling to the magnetosphere. Addressing these challenges requires advances in observing the composition and dynamics of the neutral particles and simultaneous observations of the charged particles, as well as the particles and field‐aligned current describing the coupling of the ionosphere to the magnetosphere. Additionally, our modeling capability must advance to include better descriptions of the processes affecting the ionosphere and thermosphere region and to incorporate coupling with the regions below and above at smaller spatial and temporal scales.
Key Points
Winds and currents dependent on external drivers and internal processes need improved descriptions
Coupling to the magnetosphere should include hemispheric differences in energy and mass flow
Formation and evolution of multiscale structures require detailed investigation
Observations show that equatorial ionospheric vertical drifts during solar minimum differ from the climatology between late afternoon and midnight. By analyzing WACCM‐X simulations, which reproduce ...this solar cycle dependence, we show that the interplay of the dominant migrating tides, their propagating and in situ forced components, and their solar cycle dependence impact the F‐region wind dynamo. In particular, the amplitude and phase of the propagating migrating semidiurnal tide (SW2) in the F‐region plays a key role. Under solar minimum conditions, the SW2 tide propagate to and beyond the F‐region in the winter hemisphere, and consequently its zonal wind amplitude in the F‐region is much stronger than that under solar maximum conditions. Furthermore, its phase shift leads to a strong eastward wind perturbation near local midnight. This in turn drives a F‐region dynamo with an equatorial upward drift between 18 and 1 hr local times.
Plain Language Summary
The vertical ion motion in the equatorial ionosphere plays a key role in the space weather. Satellite observations found that such vertical motion during periods with low solar activity can be quite different from the known climatology, and the cause is not clear. Using a whole atmosphere general circulation model, WACCM‐X, we are able to reproduce the pattern of the vertical ion motion similar to that observed during low activity solar cycle periods. By analyzing the model results, we find that the relative significance of the different atmosphere tidal wave components and its variation with solar activity contribute to the solar dependence of the vertical ion motion. The propagating altitudes of tide with 12‐hr period, as well as where and when the tidal wind becomes large, are of particular importance.
Key Points
Upward equatorial vertical ion drift near midnight under solar minimum conditions reproduced by WACCM‐X
Modulation of F‐region dynamo by propagating semidiurnal tide is much stronger during solar minimum
Tidal phase change in equatorial F‐region during solar minimum shifts upward drift toward midnight
Variability of the midlatitude ionosphere and thermosphere during the 2009 and 2013 sudden stratosphere warmings (SSWs) is investigated in the present study using a combination of Constellation ...Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations and thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulations. Both the COSMIC observations and TIME‐GCM simulations reveal perturbations in the F region peak height (hmF2) at Southern Hemisphere midlatitudes during SSW time periods. The perturbations are ∼20–30 km, which corresponds to 10–20% variability of the background mean hmF2. The TIME‐GCM simulations and COSMIC observations of the hmF2 variability are in overall good agreement, and the simulations can thus be used to understand the physical processes responsible for the hmF2 variability. Through comparison of simulations with and without the migrating semidiurnal lunar tide (M2), we conclude that the midlatitude hmF2 variability is primarily driven by the propagation of the M2 into the thermosphere where it modulates the field‐aligned neutral winds, which in turn raise and lower the F region peak height. Though there are subtle differences, the consistency of the behavior between the 2009 and 2013 SSWs suggests that variability in the Southern Hemisphere midlatitude ionosphere and thermosphere is a consistent feature of the SSW impact on the upper atmosphere.
Key Points
Variability in midlatitude hmF2 is observed and modeled during 2009 and 2013 SSWs
Variability is larger in the Southern Hemisphere
The hmF2 variability is due to modulation of the field‐aligned neutral winds by the M2 lunar tide
The strength and structure of the Earth's magnetic field is gradually changing. During the next 50 years the dipole moment is predicted to decrease by
3.5%, with the South Atlantic Anomaly expanding, ...deepening, and continuing to move westward, while the magnetic dip poles move northwestward. We used simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model to study how predicted changes in the magnetic field will affect the climate of the thermosphere-ionosphere system from 2015 to 2065. The global mean neutral density in the thermosphere is expected to increase slightly, by up to 1% on average or up to 2% during geomagnetically disturbed conditions (
). This is due to an increase in Joule heating power, mainly in the Southern Hemisphere. Global mean changes in total electron content (TEC) range from
3% to +4%, depending on season and UT. However, regional changes can be much larger, up to about
35% in the region of
45°S to 45°N and 110°W to 0°W during daytime. Changes in the vertical
drift are the most important driver of changes in TEC, although other plasma transport processes also play a role. A reduction in the low-latitude upward
drift weakens the equatorial ionization anomaly in the longitude sector of
105-60°W, manifesting itself as a local increase in electron density over Jicamarca (12.0°S, 76.9°W). The predicted increase in neutral density associated with main magnetic field changes is very small compared to observed trends and other trend drivers, but the predicted changes in TEC could make a significant contribution to observationally detectable trends.
We use the CESM2‐Whole Atmosphere Community Climate Model, to study the importance of ozone in the vertical coupling between lower and upper atmosphere during sudden stratospheric warmings (SSWs). ...During SSWs, the build up of stratospheric ozone concentrations at tropical latitudes and its increased asymmetrical distribution carries the potential to affect the generation of migrating and nonmigrating semidiurnal solar tides. Much of the upper atmospheric variability associated with SSWs is known to be driven by large changes in the vertically propagating semidiurnal migrating (SW2) and nonmigrating (SW1 and SW3) solar tides. In this study, we investigate the effect of stratospheric ozone variability during SSWs on these solar tides. For this purpose, a case study of the 2009 SSW event is carried out using the WACCM with two distinct simulation setups. In the first setup, the ozone concentrations are interactively calculated in the model and resemble the ozone observations during the 2009 SSW event, while in the second setup, the ozone concentrations are specified using zonal mean values. We constrain both of the simulations to the Modern‐Era Retrospective Analysis for Research and Applications‐2 reanalysis so that the background atmosphere through which the solar tides propagate are almost identical in each case. Following the onset of the SSW, we find that in the vicinity of the peak enhancements of SW1, SW2, and SW3 in the mesosphere‐lower thermosphere (MLT), the amplitudes of these semidiurnal solar tides are approximately about 15–50% larger for the simulation with interactive ozone as compared with the one with prescribed ozone, indicating that the stratospheric ozone variability plays an important role in driving semidiurnal solar tidal changes during SSWs.
Key Points
The stratospheric ozone variability effect on SW2 amplitudes in the vicinity of the peak SW2 enhancements can be up to 15–25% at MLT heights
Results suggest that enhanced QSPW1 in the NH before the onset of 2009 SSW may lead to large variability of SW1 and SW3 in the MLT of SH
Before the SSW onset, the SW1 and SW3 variability in the SH seems to be related to the nonlinear interaction between SW2 and QSPW1
The influence of atmospheric planetary waves on the occurrence of irregularities in the low latitude ionosphere is investigated using Whole Atmosphere Community Climate Model with ...thermosphere‐ionosphere eXtension (WACCM‐X) simulations and Global Observations of the Limb and Disk (GOLD) observations. GOLD observations of equatorial plasma bubbles (EPBs) exhibit a ∼6–8 day periodicity during January–February 2021. Analysis of WACCM‐X simulations, which are constrained to reproduce realistic weather variability in the lower atmosphere, reveals that this coincides with an amplification of the westward propagating wavenumber‐1 quasi‐six day wave (Q6DW) in the mesosphere and lower thermosphere (MLT). The WACCM‐X simulated Rayleigh‐Taylor (R‐T) instability growth rate, considered as a proxy of EPB occurrence, is found to exhibit a ∼6‐day periodicity that is coincident with the enhanced Q6DW in the MLT. Additional WACCM‐X simulations performed with fixed solar and geomagnetic activity demonstrate that the ∼6‐day periodicity in the R‐T instability growth rate is related to the forcing from the lower atmosphere. The simulations suggest that the Q6DW influences the day‐to‐day formation of EPBs through interaction with the migrating semidiurnal tide. This leads to periodic oscillations in the zonal winds, resulting in periodic variability in the strength of the prereversal enhancement, which influences the R‐T instability growth rate and EPBs. The results demonstrate that atmospheric planetary waves, and their interaction with atmospheric tides, can have a significant impact on the day‐to‐day variability of EPBs.
Key Points
A ∼6‐day oscillation occurs in observed equatorial plasma bubbles (EPBs) during January 2021
Analysis of simulations reveals that the ∼6‐day oscillation in EPBs is due to the quasi‐six day planetary wave
Planetary waves influence EPBs through modulation of the semidiurnal tide and the prereversal enhancement
Oscillation of the Ionosphere at Planetary‐Wave Periods Forbes, J. M.; Maute, A.; Zhang, X. ...
Journal of geophysical research. Space physics,
September 2018, 2018-09-00, 20180901, Letnik:
123, Številka:
9
Journal Article
Recenzirano
Odprti dostop
F‐region ionospheric oscillations at planetary‐wave (PW) periods (2–20 days) are investigated, with primary focus on those oscillations transmitted to the ionosphere by PW modulation of the ...vertically propagating tidal spectrum. Tidal effects are isolated by specifically designed numerical experiments performed with the National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model for October 2009, when familiar PW and tides are present in the model. Longitude versus day‐of‐month perturbations in topside F‐region electron density (Ne) of order ±30–50% at PW periods occur as a result of PW‐modulated tides. At a given height, these oscillations are mainly due to vertical oscillations in the F layer of order ±15–40 km. These vertical movements are diagnosed in terms of changes in the F2‐layer peak height, ΔhmF2, which are driven by the vertical projections of E × B drifts and field‐aligned in situ neutral winds. E × B drifts dominate at the magnetic equator, while the two sources play more equal roles between 20° and 40° magnetic latitudes in each hemisphere. The in situ neutral wind effect arises from vertical propagation of PW‐modulated tides, whereas the E × B drifts originate from dynamo‐generated electric fields produced by the E‐region component of the same wind field; the former represents a new coupling mechanism for production of ionospheric oscillations at PW periods. Roughly half the above‐mentioned variability in Ne and hmF2 is associated with zonally symmetric (S0) oscillations, which contribute at about half the level of low‐level magnetic activity during October 2009. The thermosphere‐ionosphere‐electrodynamics general circulation model simulates the S0 oscillations in Ne observed from the CHAMP satellite well during this period and reveals that S0 oscillations in E × B play a significant role in driving S0 oscillations in ΔhmF2, in addition to neutral winds.
Key Points
Ionospheric oscillations occur at planetary‐wave (PW) periods (2–20 days) due to PW modulation of the upward‐propagating tidal spectrum
A new mechanism for PW coupling in the atmosphere‐ionosphere system is discovered
Zonally symmetric oscillations account for roughly half the ionospheric perturbations at PW periods
Accurate magnetic field measurements at ground and low‐Earth orbit (LEO) are crucial to describe Earth's magnetic field. One of the challenges with processing LEO magnetic field measurements to study ...Earth's magnetic field is that the satellite flies in regions of highly varying ionospheric currents, which needs to be characterized accurately. The present study focuses on ionospheric current systems due to gravity and plasma pressure gradient forcing and aims to provide guidance on the estimation of their magnetic effect at LEO altitudes with the help of numerical modeling. We assess the diamagnetic approximation that estimates the magnetic signal of the plasma pressure gradient current. The simulations indicate that the diamagnetic effect should not be removed from LEO magnetic observations without considering the gravity current effect, as this will lead to an error larger than the magnetic signal of these currents. We introduce and evaluate a method to capture the magnetic effect of the gravity‐driven current. The diamagnetic and gravity current approximations ignore the magnetic effect from currents set up by the induced electric field. The combined gravity and plasma pressure gradient magnetic effect tends to cancel above the F region peak; however, between approximately 300 km and the peak it exhibits a significant height and latitudinal variation with magnitudes up to 8 nT. During solar minimum the combined magnetic signal is less than 1 nT above 300 km. In addition to the solar cycle dependence, the magnetic signal strength varies with longitude (approximately by 50%) and season (up to 80%) at solar maximum.
Plain Language Summary
Accurate magnetic field measurements at ground and low‐Earth orbit (LEO) are crucial to describe Earth's magnetic field. One of the challenges with processing LEO magnetic field measurements to study Earth's magnetic field is that the satellite flies in regions of highly varying ionospheric currents. The ionospheric signals need to be removed from the measurements to isolate the signal from the Earth's magnetic field. Therefore, it is crucial to describe the ionospheric current accurately. The present study focuses on the ionospheric current systems due to gravity and plasma pressure gradient forcing and aims to provide guidance on the estimation of their magnetic effect at LEO altitudes. We assess the diamagnetic approximation which estimates the magnetic signal of the plasma pressure gradient current. And we introduce and evaluate a method to capture the magnetic effect of the gravity‐driven current. Both approximations simplify the current system which leads to an error. The combined gravity and plasma pressure gradient magnetic effect tends to cancel above approximately 500 km; however, between approximately 300 and 500 km it exhibits a significant height and latitudinal variation. The combined magnetic signal is less than 1 nT above 300 km during solar minimum and up to 9 nT during solar maximum.
Key Points
Magnetic effect of plasma pressure gradient current should not be removed from LEO data without considering the gravity current
Approximations to capture the magnetic effect of gravity and plasma pressure gradient current are assessed
Above the F region peak the magnetic signal of the two currents is small, but below it is larger with a strong height and latitude variation
The Ionospheric Connection Explorer, or ICON, is a new NASA Explorer mission that will explore the boundary between Earth and space to understand the physical connection between our world and our ...space environment. This connection is made in the ionosphere, which has long been known to exhibit variability associated with the sun and solar wind. However, it has been recognized in the 21st century that equally significant changes in ionospheric conditions are apparently associated with energy and momentum propagating upward from our own atmosphere. ICON’s goal is to weigh the competing impacts of these two drivers as they influence our space environment. Here we describe the specific science objectives that address this goal, as well as the means by which they will be achieved. The instruments selected, the overall performance requirements of the science payload and the operational requirements are also described. ICON’s development began in 2013 and the mission is on track for launch in 2018. ICON is developed and managed by the Space Sciences Laboratory at the University of California, Berkeley, with key contributions from several partner institutions.
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