Quiet Time Magnetosphere Research Articles

The role of high-resolution geomagnetic field models for investigating ionospheric currents at low Earth orbit satellites

Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate ionospheric currents on a global scale and to make in situ measurements of F region currents. High-precision geomagnetic satellite missions are also able to detect ionospheric currents during quiet-time geomagnetic conditions that only have few nanotesla amplitudes in the magnetic field. An efficient method to isolate the ionospheric signals from satellite magnetic field measurements has been the use of residuals between the observations and predictions from empirical geomagnetic models for other geomagnetic sources, such as the core and lithospheric field or signals from the quiet-time magnetospheric currents. This study aims at highlighting the importance of high-resolution magnetic field models that are able to predict the lithospheric field and that consider the quiet-time magnetosphere for reliably isolating signatures from ionospheric currents during geomagnetically quiet times. The effects on the detection of ionospheric currents arising from neglecting the lithospheric and magnetospheric sources are discussed on the example of four Swarm orbits during very quiet times. The respective orbits show a broad range of typical scenarios, such as strong and weak ionospheric signal (during day- and nighttime, respectively) superimposed over strong and weak lithospheric signals. If predictions from the lithosphere or magnetosphere are not properly considered, the amplitude of the ionospheric currents, such as the midlatitude Sq currents or the equatorial electrojet (EEJ), is modulated by 10–15 % in the examples shown. An analysis from several orbits above the African sector, where the lithospheric field is significant, showed that the peak value of the signatures of the EEJ is in error by 5 % in average when lithospheric contributions are not considered, which is in the range of uncertainties of present empirical models of the EEJ.

Remote observations of ion temperatures in the quiet time magnetosphere

Ion temperature analysis of the first energetic neutral atom images of the quiet -time, extended magnetosphere provides evidence of multiple regions of ion heating. This study confirms the existence of a dawn -dusk asymmetry in ion temperature predicted for quiescent magnetospheric conditions by Spence and Kivelson (1993) and demonstrates that it is an inherent magnetospheric feature.

Evolution of remotely measured inner magnetospheric ion temperatures during a geomagnetic storm

[1] Previous studies have demonstrated that with the Medium Energy Neutral Atom instrument aboard the IMAGE spacecraft it is possible to remotely measure the ion temperature of the terrestrial magnetosphere during periods of strong geomagnetic activity. However, neutral atom imaging of the magnetosphere during quiet intervals is more difficult. In this work we show that by mapping neutral atom fluxes obtained over many days of observation to an equatorial plane fixed in geocentric solar magnetospheric coordinates, it is possible to construct neutral atom images of the quiet time magnetosphere. Enhanced neutral fluxes in the energy range of 1–70 keV/nucleon are observed in the quiet time premidnight region. A superposed-epoch analysis of multiple storm intervals also permits imaging of the ion temperature structure as a function of time through a geomagnetic storm. Using nearly 40 geomagnetic storms to produce average ion temperature maps as a function of storm phase, we find that there are significant differences between the spatial distribution of neutral fluxes and ion heating during the main phase of storms. Pronounced ion heating (up to 12 keV) is observed on the dayside during storm main phase from 5 to 8 Earth radii. During the early recovery phase, the ion temperature on the dayside drops to approximately 9 keV, and a colder region of approximately 6.5 keV persists near predawn. In the late recovery phase of the storm the ion temperature throughout the inner magnetosphere appears to relax to a nearly uniform 8 keV.

Ion temperature imaging of the Earth's magnetosphere

Detection of neutral atom emission from hot plasmas has evolved to the point where it is now possible to image hot plasmas in neutral atom emission. In this paper, we review the qualitative and quantitative spatially resolved information that can be obtained from neutral atom images, e.g., hot ion transport and ion temperatures. A challenging aspect of neutral atom imaging of the earth's quiet time magnetosphere is that the combination of small neutral fluxes and spacecraft motion necessitates the development of algorithms capable of summing neutral emission obtained from different vantage points over many months to obtain statistically significant images. The image summing algorithm and typical summed images are also presented in this work.

Field line resonances in quiet and disturbed time three‐dimensional magnetospheres

Numerical solutions for field line resonances (FLRs) in the magnetosphere are presented for three‐dimensional equilibrium magnetic fields represented by two Euler potentials as B = ∇ψ × ∇α, where ψ is the poloidal flux and α is a toroidal angle‐like variable. The linearized ideal MHD equations for FLR harmonics of shear Alfvén waves and slow magnetosonic modes are solved for plasmas with the pressure assumed to be isotropic and constant along a field line. The coupling between the shear Alfvén waves and the slow magnetosonic waves is via the combined effects of geodesic magnetic field curvature and plasma pressure. Numerical solutions of the FLR equations are obtained for a quiet time magnetosphere as well as a disturbed time magnetosphere with a thin current sheet in the near‐Earth region. The FLR frequency spectra in the equatorial plane as well as in the auroral latitude are presented. The field line length, magnetic field intensity, plasma beta, geodesic curvature, and pressure gradient in the poloidal flux surface are important in determining the FLR frequencies. In general, the computed shear Alfvén FLR frequency based on the full MHD model is larger than that based on the commonly adopted cold plasma model in the βeq > 1 region. For the quiet time magnetosphere, the shear Alfvén resonance frequency decreases monotonically with the equatorial field line distance, which reasonably explains the harmonically structured continuous spectrum of the azimuthal magnetic field oscillations as a function of the dipole L shell in the L ≤ 8.8 RE region. However, the FLR frequency spectrum for the disturbed time magnetosphere with a near‐Earth thin current sheet is substantially different from that for the quiet time magnetosphere for R > 6 RE, mainly due to shorter field line length due to magnetic field compression by solar wind, reduced magnetic field intensity in the high β current sheet region, azimuthal pressure gradient and geodesic magnetic field curvature.

Low‐altitude convection, precipitation, and current patterns in the baseline magnetosphere

Three states of the magnetosphere may be identified from polar auroral and magnetic activity: the active auroral oval state, the active polar cap state, and the quiet state. The quiet time magnetosphere occurs when the energy transfer from the solar wind has been maintained at a minimum level for several hours. The conditions for minimal energy transfer occur for low solar wind velocity and for small IMF B and Bz. The quiet magnetosphere is characterized by minimal, but nonzero, cross‐cap electric field potentials (6–20 kV) and, therefore, low earthward convection velocities in the plasma sheet. The auroral oval shrinks to a circle of radius 20° geomagnetic latitude (MLAT) offset, as in active times, toward midnight. Diffuse auroral precipitation is continuous throughout these periods but has low average energy and number flux. Discrete arcs are weak (subvisual) and multiple, often extending to extremely high latitudes, particularly on the dayside of the cap. The cusp lies between 77° and 84° MLAT at noon. The total particle energy flux into both hemispheres is ∼ 10 GW. Region 1 and region 2 field‐aligned current systems frequently are not observed during extended periods of magnetic quiet. However, small‐scale current systems, which are associated with the weak discrete arcs, are still seen. Cusp currents in the sunlit hemisphere remain strong. Of particular interest in modeling the quiet magnetosphere are the following topics: the entry and transport of cusp particle populations, stable convection in the plasma sheet, and the closure of tail lobe magnetic field lines.

Origin of the plasma sheet

The plasma sheet is a region of hot plasma (∼107 °K) that surrounds the earth, filling the distant magnetosphere and separating the northern and southern lobes of the magnetospheric tail; its origin is one of the basic outstanding problems of magnetospheric physics. A critical review of the available observations indicates that the plasma sheet source must provide roughly 1025–1026 particles/s to balance known loss mechanisms operating in the quiet time magnetosphere and that an adiabatic or quasi‐adiabatic (ΔE ∝ E) acceleration mechanism must provide most of the observed particle energy. These constraints together with auroral helium ion measurements indicate that the plasma sheet particles must originate in the solar wind rather than in the terrestrial ionosphere. The available hypotheses for a solar wind source are examined, and it is concluded that a two‐step process of magnetopause diffusion plus electrostatic acceleration is the simplest source mechanism that is consistent with the available observations.